Institute of Anatomy, University of Zurich, CH-8057 Zurich, Switzerland
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ABSTRACT |
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The final adjustment of renal sodium and calcium excretion is achieved by the distal nephron, in which transepithelial ion transport is under control of various hormones, tubular fluid composition, and flow rate. Acquired or inherited diseases leading to deranged renal sodium and calcium balance have been linked to dysfunction of the distal nephron. Diuretic drugs elicit their effects on sodium balance by specifically inhibiting sodium transport proteins in the apical plasma membrane of distal nephron segments. The identification of the major apical sodium transport proteins allows study of their precise distribution pattern along the distal nephron and helps address their cellular and molecular regulation under various physiological and pathophysiological settings. This review focuses on the topological arrangement of sodium and calcium transport proteins along the cortical distal nephron and on some aspects of their functional regulation. The availability of data on the distribution of transporters in various species points to the strengths, as well as to the limitations, of animal models for the extrapolation to humans.
thiazide-sensitive sodium chloride cotransporter; amiloride-sensitive epithelial sodium channel; epithelial calcium channel; rat; mouse
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INTRODUCTION |
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IN THE LAST DECADE, THE MAJOR salt and water transport proteins in the renal distal nephron have been identified, and their precise intrarenal localizations were explored by immunomethods or by in situ hybridization. The resulting data confirmed that the inventory of the transport proteins in the distal nephron identified so far is the same in all investigated mammalian species (rabbit, rat, mouse, and human). However, the transporter topology along the distal convolution shows subtle species differences, the unawareness of which might have been the cause for occasional discrepancies in the interpretation of experimental data. Furthermore, under altered functional conditions the abundance and extension along the distal nephron and intracellular localization of transporter proteins change. The studies also unveiled the strikingly consistent link between the given inventory of apical transport proteins along the distal nephron and the fine structure of the respective cells. This points to the potential of structural approaches in investigating functional mechanisms in distal nephron electrolyte transport in vivo.
The detailed physiology of the transporters and their involvement in human pathophysiology has been discussed in depth in excellent recent reviews (33, 44, 53, 95, 107, 110, 114, 138). Here, we will review the distributions in the distal nephron of the major sodium, calcium, and water transport proteins and regulation of their related functions, in correlation with the structural organization of the distal nephron, with the main emphasis on the distal convolution.
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CORTICAL DISTAL NEPHRON |
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Microanatomic Organization of the Cortical Distal Nephron
The anatomic definition of the distal nephron takes into account exclusively those tubular portions that originate from the metanephric blastema: the distal straight part [thick ascending limb of Henle's loop (TAL)], located in the medullary rays, and the convoluted portion, the "distal convolution," located in the cortical labyrinth. The functionally defined "distal nephron" also includes, besides the TAL and the distal convolution, the cortical collecting duct (CCD), which embryologically originates from the most peripheral branchings of the ureteral bud.Microanatomically, the distal convolution of superficial nephrons is a simple tube, opening into the most peripheral extensions of a CCD (70). In contrast, distal tubules of deeper nephron generations merge to so-called "arcades" (103). The extent of nephron fusion and the ratio between nephrons draining individually or through an arcade into a CCD vary among and within species (35, 103). The arcades ascend within the cortical labyrinth, proximate to the cortical radial vessels, before they open into a CCD within the medullary rays (35, 57). The number of nephrons drained by each CCD averages 11 in the human kidney (98), 6 in rabbits (58) and rats (69), and 5 in mice (71).
The present conventionally used subdivision of the distal convolution into the so-called "distal convoluted tubule" (DCT) and the "connecting tubule" (CNT; including arcades) is based on more or less quite obvious structural differences along the distal convolution. They were initially observed by Schweigger-Seidel in 1865 (118) and disclosed in microdissected preparations of kidneys from rabbits, humans, mice, sheep, cats, pigs, cattle, and dolphins in astounding detail by Peter and Inouye in 1909 (103). Some 70 years later, Peter's light microscopic observations were confirmed by detailed electron microscopic studies in rabbits (58) and extended to rats (29).
In all species, the TAL epithelium changes at various distances downstream of the macula densa abruptly to the DCT, which is the initial segment of the distal convolution. In rabbits, the entire length of the DCT epithelium is composed by one cell type, the DCT cells. An exceedingly high density of mitochondria, encased in narrow palisade-like-arranged, interdigitated lateral cell processes, characterizes them. The DCT cells are abruptly replaced by the "CNT" cells, which by light and electron microscopy appear "lighter" and display among other irregularly arranged basolateral plasma membrane infoldings and fewer mitochondria than do DCT cells (58). Intercalated cells do not show up before the transition to the CNT and continue all along the CNT and CCD (58). In deep and intermediate nephrons, the change from the DCT to the CNT epithelium regularly occurs a few cells before fusion of two tubules; hence the arcades are entirely made up of CNT epithelium (58). The transition from the CNT to the CCD is given by the abrupt substitution of CNT by CCD cells (principal cells). Basolateral membrane infoldings in CCD cells are restricted to the most basal cell portion, and the few mitochondria are normally situated in the cytoplasm above the infoldings (58).
In rats and mice, the situation is different. On the basis of serial 1-µm sections, Crayen and Thoenes (29) reconstructed the distal convolution of a superficial rat nephron, from its beginning shortly downstream of the macula densa to the first confluence with another tubule. They distinguished by light microscopy and ultrastructure a total of four cell types; types 1-3 correspond to DCT, CNT, and CCD (principal) cells and type 4 to intercalated cells. Type 1 (DCT) cells exclusively comprised the first part of the tubular portion. In the direction of the flow, these cells became intermingled with intercalated cells. Then, type 2 cells progressively replaced type 1 cells, and type 3 cells progressively replaced type 2 cells. Furthermore, cell height, basolateral cell membranes, and mitochondrial density of cell types 1-3 gradually decreased in flow direction. Because of the gradual structural changes, Crayen and Thoenes concluded that in rats segmentation of the distal convolution can be made only arbitrarily. A decade later, the lack of sharp segment borders in the rat distal convolution was confirmed by Dorup (35), and excellent ultrastructural descriptions of each of the four cell types were given by Madsen and Tisher (83). Dorup (35) also reported, in addition to the gradual segment transitions, ultrastructural distinctions between the DCT cells in the early and late part of the DCT. These were corroborated by the observation of the abundant presence of caveolin in late DCT cells (17), but not in early DCT cells, consistent with the structural abundance of caveolae on the basolateral plasma membranes of cells in this tubular region (57).
In mice (Fig. 1), the distal convolution
seems to be similarly organized as it is in rats; however, systematic
ultrastructural studies are not available. Ultrastuctural descriptions
of the transitional regions in the human distal convolution are lacking as well.
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Distribution of Transport Proteins Along the Distal Nephron
The following apical transport systems have been shown by immunomethods and/or in situ hybridization to be confined in the kidney exclusively to the distal nephron: the bumetanide-sensitive sodium-2 chloride potassium cotransporter (NKCC2) (62, 82, 97), the thiazide-sensitive sodium-chloride cotransporter (NCC) (3, 96, 105), the amiloride-sensitive epithelial sodium channel (ENaC) (37), the recently discovered epithelial calcium channel (ECaC1; CaT2; TRPV5) (54, 101), and the vasopressin-sensitive water channel [aquaporin-2 (AQP2)] (66, 94). The basolateral sodium/calcium exchanger (NCX) (96, 108) and the plasma membrane Ca-ATPase (PMCA) (13, 14), as well as the cytoplasmic calcium-binding protein calbindin D28k (10, 13, 112) are, in contrast to other nephron portions, particularly abundant in some parts of the distal nephron.Mapping the apical transport systems along the distal nephron of rabbits (77), rats (26, 55, 116), mice (19, 65, 78), and humans (11) revealed their serial arrangement. NKCC2 is confined to the TAL, including the macula densa (62, 82, 97), and distinguishes this segment from all others and in all species. Salt subtraction from the tubular fluid via NKCC2 in the (water impermeable) TAL is the precondition for urinary concentration. Solute reabsorption by the subsequent distal convolution is the premise for fine-tuning of renal electrolyte excretion. It proceeds by the concerted action of the apical transporters NCC, ENaC, ECaC1, and AQP2, as well as others not mentioned in this context. Of these transporters, NCC is without exception the most upstream transporter in the distal convolution and replaces the NKCC2 exactly at the structural transition from the TAL to the DCT (3, 96, 97, 105) (Fig. 1, a-c).
What differs markedly among the species is the site of start-off along
the distal convolution of ENaC, ECaC1, and AQP2 expression. In the
rabbit kidney (Fig. 2, top; Fig. 4, a and
b),
immunostaining for ENaC starts where in situ hybridization for NCC mRNA
stops (77). NCC immunostaining, made with antibodies
directed against a synthetic peptide, is replaced abruptly by ECaC1
immunostaining (Fig. 4, a and
b; Loffing
J and Loffing D, unpublished observations). Immunostaining with
a monoclonal antibody against a metolazone-binding protein continues
beyond the stop of the in situ signal for some distance into the CNT
(3), where it colocalizes with ECaC1 (52).
The abrupt start of coexpression with ENaC of the vasopressin-sensitive water channel AQP2 (Fig. 3, a and b) sharply
marks the beginning of the CCD (77), which structurally is
discernible by abrupt replacement of CNT cells by CCD cells (principal
cells). The basolateral NCX is immunohistochemically detectable
exclusively in the CNT (3, 108), and calbindin
D28k is intermediate in the DCT, rises sharply with the
beginning of the CNT, and continues somewhat more weakly along the CCD
(77).
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The congruency of structural segmentation and distribution pattern of apical transporters along the rabbit distal nephron are explicit. The distribution pattern of apical transporters along the rat distal convolution mirrors the above-described lack of sharp structural transitions in this species. DCT cells in this late portion differ in some structural aspects from those in the early portion (see above) and coexpress, in addition to NCC, the apical channels ENaC (116) and ECaC1 (55). Furthermore, they display a very high abundance of cytoplasmic calbindin D28k (76, 105) and basolateral NCX (96). In male Sprague-Dawley rats, the fractional length of the late NCC-displaying tubular portion, defined by its high abundance of basolateral NCX, has been estimated to amount to ~20% of the total NCC-positive portion (26).
A further significant difference between rats and rabbits concerns the distribution of AQP2 in the distal nephron. In rats, immunostaining for AQP2 has been detected to start with the break-off of NCC-staining (77). Occasionally, AQP2-positive cells may be intermingled even with the last few NCC-positive cells. This observation agrees with the former structural data on intermingling of cell types along the distal convolution (29, 35). Quantitative estimation by Western blot analysis of AQP2 in preparations of isolated rat CNT and CCD revealed ~60% AQP2 abundance in the CNT from that in the CCD (66), consistent with weaker AQP2 immunostaining along the CNT than in the CCD of rats (26, 66, 77). The CCD cells in the successive segment coexpress ENaC and AQP2, but not ECaC1. In the cortex of male Sprague-Dawley rats, the fractional volumes of DCT, CNT, and CCD correspond to ~1:1:1 (26).
In mice, the distribution of NCC, ENaC, ECaC1, and AQP2 (Fig. 1,
c and f; Fig 2, middle; Fig
3, c and d; Fig. 4, c and
d) resembles that in rats. In
the late part of the DCT, the apical transporters NCC, ENaC, and ECaC1
are coexpressed (78) and, additionally, the most
downstream NCC-positive cells might even coexpress AQP2 (Loffing J,
unpublished observations). A significant abundance of
cytoplasmic calbindin D28k, basolateral NCX (19, 78), and PMCA (78) has also been demonstrated by
immunomethods in the early portion of the DCT. Nevertheless, a marked
jump in abundance of these latter proteins was consistently observed at the sites where, in addition to NCC, coexpression of ENaC and ECaC1
began (Fig. 5) (78). The
functional link between apical ECaC1 and NCX and PMCA is further
evidenced by the parallel decreases in apical ECaC1 and basolateral NCX
and PMCA immunostaining along the CNT and their simultaneous cessation
at the histotopographically recognized CCD
(78). Single cells, coexpressing ECaC1, NCX, PMCA,
and calbindin D28k, are occasionally interspersed in
the CCD epithelium (78). In female C57/BL6 mice, the
fractional volumes of early DCT, late DCT, and CNT correspond to
~2:1:2 (75).
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In humans, the tubular portion with overlap of NCC and ENaC is rather short. NCC characterizes ~30-35% of profiles of the distal convolution and ENaC ~70-75% (11). AQP2 does not directly supersede the NCC, but the initial ~15% of the CNT [at least the portions before the fusion with arcades display ENaC alone (Fig. 3, e and f)] (11). The distribution of ECaC1 along the human nephron has not been assessed by immunostaining so far. Similarly to mice, NCX, PMCA, and calbindin D28k are traceable to varying extents even in the initial portion of the DCT, and, in pronounced difference from all other species, continue in significant abundance along the cortical collecting duct (11).
Definition of the Distal Nephron Based on Distribution of Transport Proteins
The clear-cut structural and functional organization of the rabbit distal nephron constitutes a simple model, which aids in an understanding of the more complex distal nephron organization in rats, mice, and humans (Fig. 6). In all four mammalian species analyzed so far, of the proteins discussed in this article, NKCC2 is the most upstream apical salt transporter and unequivocally defines the TAL. The succeeding salt transporter is the NCC, the onset and end of which in the distal convolution of all species are clearly discernible, and which defines the DCT. The next apical ion transporters in the series are ENaC and ECaC1. In rabbits, they abruptly replace NCC in the apical membrane. In rats, mice, and humans, ENaC and ECaC1 seem to be "pushed" more or less upstream along the distal convolution into the NCC-displaying late portion of the DCT, giving rise to a portion with apical coexpression of NCC, ENaC, and ECaC1. Accordingly, in these latter species the DCT can be further subdivided into an early and a late portion. Besides the coexpression with NCC of the apical transporters ENaC (11, 79, 116) and ECaC1 (55, 78), the second portion differs from the first, e.g., by the presence of intercalated cells (35, 83), by very prominent cytoplasmic calbindin D28k (76, 105), by basolateral NCX (96) and PMCA (78), as well as by discrete structural differences (35). The conspicuous onset of basolateral NCX immunostaining in the NCC-positive segment of the rat (96) prompted the groups of Bachmann (3) and Ellison (40) to propose the subdivision of the DCT into DCT1 (NCX negative) and DCT2 (NCX positive). According to this criterion, neither in mice (19) nor in humans could such subdivision of the DCT be made. The onset of traceability of ENaC and ECaC1, as far as has been investigated to date, coincides approximately with the marked start (rat) (96) or rise (mouse) (78) in NCX, PMCA, and calbindin D28k immunostaining. The coexpression of NCC and ENaC has been used as well to distinguish the late (~DCT2) from the early (only NCC expressing) DCT (~DCT1) (78). That the late part of the DCT varies in length among species, probably also among species strains, possibly along with age, sex, and other factors, has been emphasized in an editorial comment by Wade (134).
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The end of NCC immunostaining can be used in all species to mark the beginning of the CNT. In rabbits, it comes along with the onset of ENaC and ECaC1 and basolateral NCX immunostaining and in rats and mice with onset of additional AQP2 coexpression (see Fig. 6). In all species, the presence of AQP2 and the histotopographical location of the tubule in the medullary ray (or in superficial nephrons the proximity to the renal capsule) defines a CCD. The overall length of the distal convolution (DCT and CNT) can be conveniently estimated by double immunostaining for NCC and NCX (26, 96) or NCC and calbindin D28k (11).
Relevance of Specific Transporter Topology
The specific sequence of electrolyte transporters along the distal nephron probably guarantees sodium recovery under a large range of physiopathological situations. It seems to imply the possibility of partially compensating for inadequate salt reabsorption by adapting salt reabsorption via other and differentially regulated salt transport systems in downstream tubular portions. For instance, impairment of NaCl reabsorption in the TAL, by whatever means, drastically increases the NaCl load in the downstream segments of the TAL, which respond with higher salt reabsorption rates, and over longer periods, with increased transport capacity and associated epithelial hypertrophy (70). Such compensatory mechanisms along the nephron are thought to occur also under impaired sodium uptake due to mutations of genes for some transport proteins (e.g., NKCC2 in Barrter syndrome, NCC in Gitelman syndrome) (49, 107).Although at first glance the species differences in the distribution pattern of transporters appear to be rather trivial, their functional relevance might be substantial. The definite (rabbit) or gradual (rat, mouse, human) changes in the distribution of specific transport pathways along the distal nephron are also reflected in respective distribution patterns of sensitivities to various peptide hormones (87). For instance, the sites of vasopressin sensitivity along the distal nephron, assessed in preparations of isolated distal tubules from rabbit, rat, mouse, and human kidneys (87), match precisely in all cases the respective sites of AQP2 occurrence. The upstream shifting of vasopressin-sensitive water retrieval into the ENaC-displaying CNT may be functionally relevant for at least two reasons. It might favor ENaC-mediated sodium entry into the cells, by creating a favorable gradient for sodium entry by water subtraction from the (salt diluted) tubular fluid in the CNT. The differential hormonal control of AQP2 and ENaC activity opens subtle and complex possibilities for regulatory interactions. Furthermore, the AQP2 upstream shifting of vasopressin-regulated water subtraction in the CNTs might go along with a considerable gain in the overall water reabsorption (66).
Thus the final result of solute excretion depends not only on the presence and abundance of given transport proteins in the kidney but also on their species-specific topological arrangement.
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CONTROL OF TRANSPORT ACROSS APICAL PROTEINS |
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The differences in the specific distribution patterns of transport proteins are probably evolutionary responses to species-specific living conditions. Evidently, within this given frame there is room for individual adaptation to the actual needs. Immunomethods and/or morphology can disclose the nephron sites in which, under specific functional conditions, in vivo adaptive changes took place.
Regulation of NCC-Mediated Salt Reabsorption
NCC mediates electroneutral NaCl uptake into the DCT cells. Because renal NCC is confined to the DCT, data on NCC derived from whole-organ homogenates truly reflect corresponding changes in NCC abundance in this segment. As long as specific anti-NCC antibodies were not available, abundance of NCC was often assessed by its specific binding to metolazone, a thiazide-like diuretic (5). Changes in metolazone binding protein, NCC abundance, or NCC mRNA have been observed under many different pathophysiological or clinical situations (for a review, see Refs. 47, 67, and 107). What might be the common denominator under the differential conditions, controlling specifically apical NCC abundance and NCC-mediated cellular salt uptake?Tubular sodium load and flow rates. Some data from the time when neither [3H]metolazone-binding studies nor NCC antibodies were available suggested that tubular salt load and/or flow rate in the DCT directly controls sodium reabsorption in the DCT. Rabbits under chronic high-sodium, reduced-potassium diets (with respective chronic very low endogenous plasma levels of aldosterone) revealed marked hypertrophy of the DCT epithelium, indicating chronically upregulated transport capacity in the DCT epithelium (59). Micropuncture studies in rats under furosemide-induced impairment of NaCl reabsorption (with simultaneous replacement of salt and fluid loss) confirmed that the marked increases in surface basolateral plasma membrane and in mitochondrial volume in the distal convolution cells mirror correspondingly increased sodium reabsorption rates (40, 60, 122). In situ hybridization (96) as well as RNAse protection assays (140) disclosed upregulation of mRNA for NCC under the given conditions. Others were unable to detect any increase for NCC mRNA (1, 88). Indications for NCC mRNA-independent regulation of NCC protein abundance, e.g., by mechanisms that control protein translation and/or stability, have been obtained in some studies (1, 137), but so far intracellular processing and trafficking of NCC in vivo have not been addressed. Studies in rats under the above experimental conditions demonstrated the involvement of IGF-1 and IGF binding protein 3 in the hypertrophy of the DCT epithelium (68).
Steroid hormones. mineralocorticoids and glucocorticoids. In rabbits, drastically increased endogenous aldosterone levels associated with chronic low-sodium, high-potassium intake did not increase the transport machinery in the DCT but did markedly in the CNT (59). Adaptive structural and functional hypertrophy in the DCT epithelium after furosemide-induced inhibition of salt reabsorption in the TAL occurred in adrenalectomized rats with clamped plasma levels of mineralocorticoid and glucocorticoid hormones to similar extents as in intact rats (60, 122). These collected observations suggested that changes in salt delivery and flow rate by themselves might be sufficient to initiate modulation of salt transport rates in the DCT. Nevertheless, several studies advocated the implication of aldosterone in the control of apical NCC abundance and/or NCC-mediated salt reabsorption. In rat kidneys, low dietary sodium intake, which promotes rises of endogenous aldosterone plasma levels, as well as exogeneous aldosterone application increased the abundance of NCC protein, as assessed by Western blotting and immunohistochemistry (64). Increased binding of [3H]metolazone to kidney homogenates was observed after exogenously applied aldosterone (22) but not with dietary sodium restriction (21). Velazquez et al. (128) observed that the low electroneutral sodium transport activity, measured in vivo in the early distal convolution of adrenalectomized rats, can be restored to normal and further increased by replacement of aldosterone and/or high doses of glucocorticoids (128). The increase in NCC-mediated sodium transport and NCC protein after furosemide treatment or dietary sodium restriction was significantly lower when aldosterone receptors were blocked by spironolactone (1, 93). This experimental setting provided indirect evidence for the role of aldosterone in the observed changes; however, the possible contribution of systemic or upstream renal parameters to the given observations has not been experimentally excluded.
MINERALOCORTICOID RECEPTORS AND 11Peptide hormones. The presence of receptors for peptide hormones such as calcitonin, PTH, and isoproterenol had been shown indirectly by the increases in cAMP activity after application of these hormones to morphologically defined, microdissected distal tubule segments (87). Blakely et al. (12) reported increased [3H]metolazone binding in kidney homogenates of calcitonin-treated rats.
Angiotensin II is involved in the control of arterial blood pressure and whole body sodium homeostasis. Independently from its effect on adrenal aldosterone secretion, angiotensin II stimulates sodium reabsorption in the distal tubule (135). It has been discussed that the hypertrophy in the DCT under increased salt load of the DCT (after furosemide treatment, combined with high salt and water load) might be ascribed, in part, to an effect of angiotensin II (6). The role of angiotensin II in NCC regulation can be deduced from recent experiments in mice with targeted disruption of the angiotensin II type 1 (AT1) receptor (18). Unlike wild-type mice, these AT1 knockout mice lack increases in NCC protein abundance in response to a low dietary sodium intake. Thus in the rodent DCT part of the adaptation to low dietary sodium intake is possibly mediated by increased angiotensin II levels.Inhibition of NCC-mediated transport. Thiazide diuretics are frequently used in the treatment of hypertension. Thiazides inhibit the uptake of NaCl into the DCT cell by binding to NCC. Prolonged reduction of sodium entry into the cells results in chronically lower transport rates, structurally reflected by lowering of the epithelium with a reduction of the active-salt transporting machinery (70). In the DCT of mice, treatment with thiazides is indeed associated with lowering of DCT epithelium (Valderrabano V and Loffing J, unpublished observations), and NCC knockout mice have a marked hypotrophy of the DCT epithelium (117). However, treatment of rats with metolazone for 3 days or with hydrochlorothiazide induced massive apoptosis exclusively in the early DCT (76). In rats treated continuously for 2 and 4 wk with metolazone, the epithelium of the early DCT was markedly simplified, displaying fewer basolateral membranes and mitochondria, and de- and regenerating cells. These observations suggest chronically lower transport rates and corroborate former data by Morsing et al. (89), who measured that post-chronic in vivo blockade of NaCl transport by thiazides reduced transport capacity of rat distal tubules, despite a significantly increased number of thiazide binding sites. The authors pointed out that increases in the number of thiazide receptors are not necessarily synonymous with increases in transport activity. The different effects of thiazides on the DCT in rats (apoptosis) and mice (hypotrophy) once again emphasize species differences, even between closely related species.
Regulation of ENaC-Mediated Sodium Transport
The second major sodium transporter in the distal convolution is ENaC. ENaC is the major player in aldosterone-regulated sodium reabsorption, not only in the kidneys but also in other organs (e.g., distal colon, salivary glands) (131). In the kidney, all ENaC-positive tubule portions have been subsumed under the term "aldosterone-sensitive distal nephron" (ASDN) (81), which comprises the CNT and CD in all species and includes, in some species at least, the late part of the DCT (see above). The essential role of ENaC in control of salt and volume homeostasis is highlighted by the fact that some hereditary forms of severe salt-sensitive arterial hypertension (Liddle's syndrome) and severe renal salt wasting (pseudohypoaldosteronism type I) are related to gain- and loss-of-function mutations, respectively, of ENaC genes (for a review, see Ref. 110).Sodium uptake into the cell via ENaC is electrogenic and can be measured by corresponding sodium currents that are inhibited by amiloride. ENaC-related sodium transport favors potassium secretion, which proceeds most probably via the apical renal outer medulla potassium channel ROMK, coexisting with ENaC in the same cells (136). Putative potassium reabsorption by intercalated cells, which are regularly interspersed among the ENaC-expressing cells, might modify the rigid link between sodium reabsorption via ENaC and K secretion via ROMK.
ENaC is composed of three subunits (,
,
) (110).
All three subunits have been located along the ASDN (37),
although in differential imunnohistochemical abundance and
intracellular sites (50). Interestingly,
-ENaC, but not
- and
-ENaC, has been evidenced by RT-PCR in microdissected
rabbit DCTs; however, amiloride-sensitive sodium currents could not be
detected in this segment (129). Expression of
-ENaC
mRNA has been also revealed by in situ RT-PCR in the mouse TAL and DCT
(25).
Regulation of renal sodium transport via ENaC could intervene 1) directly at the level of single channels in the membrane, by changing the channel gating kinetics; in vitro studies in amphibian cells and heterologous expressions systems suggested this possibility (for a review, see Refs. 38 and 48), but to our knowledge, such effects have not been described so far for the ASDN in vivo; or 2) at the level of density of functional apical channels. This can result either from altered synthesis rates of ENaC subunits and/or from altered exo- and endocytosis rates of already present ENaC subunits.
Role of synthesis of ENaC subunits.
Although the exact stoichiometry of the subunits in the channel is
still debated, there seems to be consensus that the -subunit plays a
pivotal role in the assembling of functionally active channels
(120). In the kidney, primarily the abundance of
-ENaC appears to be regulated by aldosterone. Exogenous aldosterone application increases the abundance of
-ENaC at the mRNA and protein
level (84, 123). Dietary sodium restriction, which increases endogenous aldosterone production, has been shown to induce
-ENaC in some (84, 85, 140) but not all studies
(109, 123). Based on previous studies in Xenopus
laevis A6 cells, it has been proposed that the induction of
-ENaC might be a prerequiste for the apical translocation of ENaC
(86). Sufficient availability of
-ENaC may allow full
assembly of ENaC channels and their subsequent release from the
endoplasmic reticulum and delivery to the cell surface. The induction
of
-ENaC (at least in nonadrenalectomized rats) in response to
aldosterone, however, is rather small, and it is conceivable that in
vivo the induction of
-ENaC alone does not account for the apical
targeting of all three ENaC subunits. Interestingly, Nielsen and
co-workers (93) recently reported that MR inhibition by
spironolactone blunts the induction of
-ENaC but does not prevent
the apical redistribution of all three ENaC subunits in response to a
low-sodium diet. Short-term adaptation of the kidneys to 24 h of
dietary sodium restriction apparently does not involve significant
upregulation of
-ENaC (85). However, amiloride-sensitive renal sodium reabsorption was shown to be significantly increased after 4 h of sodium restriction
(46).
Regulation of ENaC activity by redistribution of ENaC subunits. In rodent kidneys, dietary sodium restriction causes an immunohistochemically traceable redistribution of ENaC subunits from intracellular compartments toward the apical cell surface (79, 84). These observations correlate well with previous patch-clamp studies in isolated rat collecting ducts by Pacha and co-workers (99), who recorded increases in the number of open sodium channels in the apical plasma membrane after 1 wk of dietary sodium restriction (99). The increases in active channels were in parallel to increases in endogenous plasma aldosterone levels (99) and were observable even within 15 h after sodium restriction (45). Immunohistochemistry revealed that in adrenalectomized rats within less than 4 h after one single aldosterone injection, the apical ENaC density increased (81). The rapid upregulation of apical ENaC activity and abundance indicates that such changes might be relevant not only for the long-term adaptation of renal sodium excretion but also in the adaptation to circadian variations of dietary sodium intake (45).
The molecular mechanisms underlying the rapid accumulation of ENaC in the apical cell surface are not well understood. Data suggest involvement of the serum and glucocorticoid-regulated kinase SGK1, which is an aldosterone-induced protein (20, 90). SGK1 induction by aldosterone is clearly dose dependent (119) and occurs even under small variations in endogenous plasma aldosterone levels provoked by alterations in dietary sodium intake (56). Coexpression of ENaC with SGK1 in X. laevis oocytes manifestly stimulates ENaC activity (20, 90) and cell surface abundance (32). The latter is thought to be related to SGK1-dependent phosphorylation and inactivation of the ubiquitinligase Nedd4-2 that downregulates ENaC (31, 121). The importance and regulation of SGK1 and Nedd4-2 under various experimental conditions are summarized in several recent reviews (61, 73, 80, 100, 120). The induction of SGK1 by aldosterone, detectable by immunomethods exclusively in the ENaC-positive DCT2, CNT, and CD cells (81), precedes the apical targeting (81) and functional activation of ENaC (9). An in vivo role of SGK1 in ENaC regulation can be deduced from recent findings made in SGK1 knockout mice (141). However, the salt-loosing phenotype of the SGK1 knockout mice is mild compared with that of MR knockout mice (7) or that of theAxial differences in ENaC density.
Under standard conditions of sodium intake, all three subunits are
traceable in the apical membranes only in the initial parts of the ASDN
(77, 79). Farther downstream in particular, - and
-subunits seem to vanish from the apical membrane and become increasingly prominent in intracellular compartments (50, 77, 79,
84).
Sites of Calcium Transport in the Distal Nephron
Former micropuncture experiments and studies in isolated tubules of rabbits attributed active, transcellular movement of calcium to the distal segments downstream of the macula densa (reviewed in Refs. 44, 53, and 107). The observation that, in humans, the tubular portion with a high abundance of the calcium-extruding proteins PMCA and NCX, and calbindinD28k, is much longer than in rabbits, rats, and mice (see Fig. 6) suggests that, in humans, relevant transcellular calcium transport may occur over a much longer tubular portion than in laboratory animals, i.e., all along the distal convolution and also in the cortical collecting duct.The specific apical calcium channels in the distal tubules remained
elusive for a long time. Verapamil (dihydropyridine)-sensitive (cardiac
L-type) calcium channels have been implicated in transcellular calcium
movements (4, 72). In an immortalized mouse DCT cell line,
antisense oligonucleotides directed against the 1c- or the
3-subunits of verapamil-sensitive calcium channels
inhibited the rise in intracellular calcium concentrations in response
to PTH or chlorothiazide (4). Targeted disruption of the
3-subunit in gene-modified mice blunted (in vivo) the
hypocalciuric action of thiazides (8). These data could
support the hypothesis that verapamil-sensitive calcium channels indeed
may be implicated in transcellular calcium transport in the distal
nephron. However, the predominantly cytoplasmic and basolateral
distribution pattern of
1c-subunits of the cardiac
L-type calcium channel is more compatible with a role of these channels
in intracellular and membrane signaling processes rather than in
transcellular calcium movement (142). The epithelial
calcium channel ECaC1 exhibits highly selective calcium permeability,
is activated by hyperpolarization, and yet is insensitive to verapamil
(53). A homologous channel was identified from rat kidney
by Peng et al. (101) and called "calcium transporter
2" (CaT2) (101), to distinguish it from the apical
calcium channel CaT1 in intestine described earlier (102).
Interestingly, immunohistochemical ECaC1 traceability in the kidney of
rabbits (52), rats (55), and mice
(78) goes precisely in parallel with immunohistochemical
abundance of PMCA and NCX in distal nephron portions. Particularly well
evident is the axial decrease in ECaC1 localization in the apical
membrane from the most upstream ECaC1-positive portions toward
cytoplasmic domains in portions further downstream in mice
(78) and the parallel decreases in basolateral NCX and
PMCA (Fig. 5). These observations suggest a parallel reduction of
calcium transport rates. Whether the intracellulary localized ECaC1
molecules are recruited to the apical plasma membrane, and in response
to which stimulus, is unknown as yet. Like NCC (see above), ECaC1 also
appears to be upregulated by estrogens (127). Consistent
with a role of ECaC1 in regulated transcellular calcium transport is
the response in vivo at the mRNA and protein level to vitamin
D3 (55) and the reduced ECaC1 expression in
kidneys of 25-hydroxyvitamin D3-1-hydroxylase knockout
mice (51). These observations, taken together, speak in
favor of ECaC1 as the key apical calcium channel in renal transcellular calcium transport in vivo (53). However, the notion that
NCX and other proteins involved in transcellular calcium movement are
detectable, although in comparably lower abundance, in distal sites
lacking ECaC1 suggests that these tubular portions and other apical
transporters may contribute to some calcium homeostasis as well.
Interaction of sodium and calcium transport along the distal convolution. Costanzo and Windhager (27, 28) demonstrated in a series of microperfusion experiments that the acute application of thiazides or amiloride has a direct stimulatory effect on transcellular calcium reabsorption in the renal distal convolution. The obvious inverse relationship of sodium and calcium transport in the distal convolution has been explained by two rationales. 1) Ion transport inhibition by thiazide and amiloride hyperpolarizes the cells, and thereby activates calcium channels within the apical plasma membrane (44). The hyperpolarization of the cells is thought to be either indirectely related to the lowered chloride entry via NCC, causing an enhanced chloride influx across chloride channels, or is directly related to impaired sodium entry via ENaC, respectively. 2) The reduced apical sodium entry lowers intracellular sodium concentration and thus may increase the driving force for basolateral Na/Ca exchange (44). These mechanisms could explain the hypocalciuric effect of thiazide diuretics and amiloride, as well as the reduced urinary calcium excretion found in patients with loss-of-function mutations for NCC (Gitelman syndrome) (44, 115). However, alternative explanations for the hypocalciuria seen under these conditions are conceivable. For example, reduced urinary calcium excretion may be secondary to extracellular volume contraction, contributing to enhanced sodium and calcium reabsorption in the proximal tubules (16, 139).
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CONCLUDING REMARKS |
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The cortical distal nephron is the site of fine regulation of renal electrolyte excretion. Along this nephron portion, three different salt transporters are arranged in series: the two cotransporters NKCC2 and NCC, and ENaC. In addition, within the distal convolution ENaC is consistently coexpressed (as far as is known) with ECaC1, and in some species also with the vasopressin-sensitive water channel AQP2. Because the transport activity of each of these transporters is differentially controlled, the serial arrangement might guarantee, teleologically speaking, optimal electrolyte recovery under a vast range of environmental conditions.
Although the inventory and the basic sequence of transporters along the distal cortical nephron are the same in kidneys of the investigated mammalian species, there exist, however, subtle differences that might be functionally relevant. The distinctions pertain to the extent of coexpression of NCC with ENaC and ECaC1, ranging from not existent in the rabbit to considerable in the mouse, and to the lengths of overlap of ENaC and AQP2. The species-specific distributions of transport proteins along the distal nephron coincide with the respective structural organization, either with sharp (rabbit) or with more or less progressive (mouse, rat human) structural segment transitions in the distal convolution.
These species differences present some problems. One is a semantic one regarding criteria for subdivision of the distal convolution. The segments DCT, CNT, and CCD are clearly demarcated by structure and congruent distribution of transport proteins only in rabbits. The more or less gradual structural changes and the partial coexpressions of transporters in the other species make segmentation a matter of definition. Thus interpretation of functional data from the distal nephron needs precise criteria defining the investigated nephron portion.
A presumably more challenging problem is implicated in the functional consequences of the different species-specific topological arrangements of the transporters. Any maneuver affecting a given transporter activity in the distal nephron will result in slightly different secondary changes along the nephron and in the final urinary electrolyte excretion pattern in each species. These differences might be amplified by the exceedingly functional plasticity of the distal convolution, in particular of the DCT, triggered by the manifold factors mentioned in this article and probably by many others not discussed here.
Thus despite the large congruence among the species with respect to the occurrence of transport proteins in the distal convolution, the distinct differences in the topology of transporters along the distal convolution limit direct extrapolation of data from one species to the other. Discrepancies between the urinary excretion pattern of humans suffering from a defined gene defect for a given transport protein and genetically engineered mouse models with the respective defect might lie, in part, in the different species-specific organizations of the distal convolution and of the kidney.
![]() |
ACKNOWLEDGEMENTS |
---|
The authors thank Dr. D. Loffing-Cueni for contributing Fig. 4.
![]() |
FOOTNOTES |
---|
The recent studies of the authors were supported by Swiss National Science Foundation Grants 31-47742.96 (to B. Kaissling) and 32-061742.00 (to J. Loffing and B. Kaissling).
Address for reprint requests and other correspondence: B. Kaissling, Univ. of Zurich, Institute of Anatomy, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland (E-mail: bkaissl{at}anatom.unizh.ch or jloffing{at}anatom.unizh.ch).
10.1152/ajprenal.00217.2002
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Abdallah, JG,
Schrier RW,
Edelstein C,
Jennings SD,
Wyse B,
and
Ellison DH.
Loop diuretic infusion increases thiazide-sensitive Na+/Cl-cotransporter abundance: role of aldosterone.
J Am Soc Nephrol
12:
1335-1341,
2001
2.
Almeida, AJ,
and
Burg MB.
Sodium transport in the rabbit connecting tubule.
Am J Physiol Renal Fluid Electrolyte Physiol
243:
F330-F334,
1982
3.
Bachmann, S,
Velazquez H,
Obermüller N,
Reilly RF,
Moser D,
and
Ellison DH.
Expression of the thiazide-sensitive Na-Cl cotransporter by rabbit distal convoluted tubule cells.
J Clin Invest
96:
2510-2514,
1995[ISI][Medline].
4.
Barry, ELR,
Gesek FA,
Yu ASL,
Lytton J,
and
Friedman PA.
Distinct calcium channel isoforms mediate parathyroid hormone and chlorothiazide-stimulated calcium entry in transporting epithelial cells.
J Membr Biol
161:
55-64,
1998[ISI][Medline].
5.
Beaumont, K,
Vaughn DA,
and
Fanestil DD.
Thiazide diuretic drug receptors in rat kidney: identification with (3H)metolazone.
Proc Natl Acad Sci USA
85:
2311-2314,
1988[Abstract].
6.
Beck, FX,
Ohno A,
Müller E,
Seppi T,
and
Pfaller W.
Inhibition of angiotensin-converting enzyme modulates structural and functional adaptation to loop diuretic-induced diuresis.
Kidney Int
51:
36-43,
1997[ISI][Medline].
7.
Berger, S,
Bleich M,
Schmid W,
Cole TJ,
Peters J,
Watanabe H,
Kriz W,
Warth R,
Greger R,
and
Schutz G.
Mineralocorticoid receptor knockout mice: pathophysiology of Na+ metabolism.
Proc Natl Acad Sci USA
95:
9424-9429,
1998
8.
Bernardo, JF,
Sneddon WB,
and
Friedman PA.
Mice lacking the calcium channel 3 subunit are refractory to chlorothiazide (Abstract).
J Am Soc Nephrol
12:
27A,
2001.
9.
Bhargava, A,
Fullerton MJ,
Myles K,
Purdy TM,
Funder JW,
Pearce D,
and
Cole TJ.
The serum- and glucocorticoid-induced kinase is a physiological mediator of aldosterone action.
Endocrinology
142:
1587-1594,
2001
10.
Bindels, RJ,
Hartog A,
Timmermans A,
and
Van Os CH.
Immunocytochemical localization of calbindin-D28k, calbindin-D9k and parvalbumin in rat kidney.
Contrib Nephrol
91:
7-13,
1991[Medline].
11.
Biner, HL,
Arpin-Bott MP,
Loffing J,
Wang X,
Knepper M,
Hebert SC,
and
Kaissling B.
Human cortical distal nephron: distribution of electrolyte and water transport pathways.
J Am Soc Nephrol
13:
836-847,
2002
12.
Blakely, P,
Vaughn DA,
and
Fanestil DD.
Amylin, calcitonin gene-related peptide, and adrenomedullin: effects on thiazide receptor and calcium.
Am J Physiol Renal Fluid Electrolyte Physiol
272:
F410-F415,
1997
13.
Borke, JL,
Caride A,
Verma AK,
Penniston JT,
and
Kumar R.
Plasma membrane calcium pump and 28-kDa calcium binding protein in cells of rat kidney distal tubules.
Am J Physiol Renal Fluid Electrolyte Physiol
257:
F842-F849,
1989
14.
Borke, JL,
Minami J,
Verma A,
Penniston JT,
and
Kumar R.
Monoclonal antibodies to human erythrocyte membrane Ca2+-Mg2+ adenosine triphosphatase pump recognize an epitope in the basolateral membrane of human kidney distal tubule cells.
J Clin Invest
80:
1225-1231,
1987[ISI][Medline].
15.
Bostonjoglo, M,
Reeves WB,
Reilly RF,
Velazques H,
Robertson N,
Litwack G,
Morsing P,
Dorup J,
Bachmann S,
and
Ellison DH.
11-Hydroxysteroid dehydrogenase, mineralocorticoid receptor, and thiazide-sensitive Na-Cl cotransporter expression by distal tubules.
J Am Soc Nephrol
9:
1347-1358,
1998[Abstract].
16.
Breslau, N,
Moses AM,
and
Weiner IM.
The role of volume contraction in the hypocalciuric action of chlorothiazide.
Kidney Int
10:
164-170,
1976[ISI][Medline].
17.
Breton, S,
Lisanti MP,
Tyszkowski R,
McLaughlin M,
and
Brown D.
Basolateral distribution of caveolin-1 in the kidney: absence from H+-ATPase-coated endocytic vesicles in intercalated cells.
J Histochem Cytochem
46:
205-214,
1998
18.
Brooks, HL,
Allred AJ,
Beutler KT,
Coffman TM,
and
Knepper MA.
Targeted proteomic profiling of renal Na+ transporter and channel abundances in angiotensin II type 1a receptor knockout mice.
Hypertension
39:
470-473,
2002
19.
Campean, V,
Kricke J,
Ellison D,
Luft FC,
and
Bachmann S.
Localization of thiazide-sensitive Na+-Cl cotransport and associated gene products in mouse DCT.
Am J Physiol Renal Physiol
281:
F1028-F1035,
2001
20.
Chen, SY,
Bhargava A,
Mastroberardino L,
Meijer OC,
Wang J,
Buse P,
Firestone GL,
Verrey F,
and
Pearce D.
Epithelial sodium channel regulated by aldosterone-induced protein sgk.
Proc Natl Acad Sci USA
96:
2514-2519,
1999
21.
Chen, Z,
Vaughn DA,
Beaumont K,
and
Fanestil DD.
Effects of diuretic treatment and of dietary sodium on renal binding of 3H-metolazone.
J Am Soc Nephrol
1:
91-98,
1990[Abstract].
22.
Chen, Z,
Vaughn DA,
Blakely P,
and
Fanestil DD.
Adrenocortical steroids increase renal thiazide diuretic receptor density and response.
J Am Soc Nephrol
5:
1361-1368,
1994[Abstract].
23.
Chen, Z,
Vaughn DA,
and
Fanestil DD.
Influence of gender on renal thiazide diuretic receptor density and response.
J Am Soc Nephrol
5:
1112-1119,
1994[Abstract].
24.
Chraibi, A,
Vallet V,
Firsov D,
Hess SK,
and
Horisberger JD.
Protease modulation of the activity of the epithelial sodium channel expressed in Xenopus oocytes.
J Gen Physiol
111:
127-138,
1998
25.
Ciampolillo, F,
McCoy DE,
Green RB,
Karlson KH,
Dagenais A,
Molday RS,
and
Stanton BA.
Cell-specific expression of amiloride-sensitive, Na+-conducting ion channels in the kidney.
Am J Physiol Cell Physiol
271:
C1303-C1315,
1996
26.
Coleman, RA,
Wu DC,
Liu J,
and
Wade JB.
Expression of aquaporins in the renal connecting tubule.
Am J Physiol Renal Physiol
279:
F874-F883,
2000
27.
Costanzo, LS.
Comparison of calcium and sodium transport in early and late rat distal tubules: effect of amiloride.
Am J Physiol Renal Fluid Electrolyte Physiol
246:
F937-F945,
1984
28.
Costanzo, LS,
and
Windhager EE.
Calcium and sodium transport by the distal convoluted tubule of the rat.
Am J Physiol Renal Fluid Electrolyte Physiol
235:
F497-F506,
1978.
29.
Crayen, ML,
and
Thoenes W.
Architecture and cell structures in the distal nephron of the rat kidney.
Cytobiologie
17:
197-211,
1978[ISI][Medline].
30.
Davidoff, M,
Caffier H,
and
Schiebler TH.
Steroid hormone binding receptors in the rat kidney.
Histochemistry
69:
39-48,
1980[ISI][Medline].
31.
Debonneville, C,
Flores SY,
Kamynina E,
Plant PJ,
Tauxe C,
Thomas MA,
Munster C,
Chraibi A,
Pratt JH,
Horisberger JD,
Pearce D,
Loffing J,
and
Staub O.
Phosphorylation of Nedd4-2 by Sgk1 regulates epithelial Na+ channel cell surface expression.
Embo J
20:
7052-7059,
2001
32.
De la Rosa, AD,
Zhang P,
Naray-Fejes-Toth A,
Fejes-Toth G,
and
Canessa CM.
The serum of glucocorticoid kinase sgk increases the abundance of epithelial sodium channels in the plasma membrane of Xenopus oocytes.
J Biol Chem
274:
37834-37839,
1999
33.
Delpire, E,
and
Mount DB.
Human and murine phenotypes associated with defects in cation-chloride cotransport.
Annu Rev Physiol
64:
803-843,
2002[ISI][Medline].
34.
Djelidi, S,
Fay M,
Cluzeaud F,
Escoubet B,
Eugene E,
Capurro C,
Bonvalet JP,
Farman N,
and
Blot-Chabaud M.
Transcriptional regulation of sodium transport by vasopressin in renal cells.
J Biol Chem
272:
32919-32924,
1997
35.
Dorup, J.
Ultrastructure of three-dimensionally localized distal nephron segments in superficial cortex of the rat kidney.
J Ultrastruct Mol Struct Res
99:
169-187,
1988[ISI][Medline].
36.
Doucet, A,
and
Katz AI.
Mineralocorticoid receptors along the nephron: [3H]aldosterone binding in rabbit tubules.
Am J Physiol Renal Fluid Electrolyte Physiol
241:
F605-F611,
1981
37.
Duc, C,
Farman N,
Canessa CM,
Bonvalet JP,
and
Rossier BC.
Cell-specific expression of epithelial sodium channel ,
, and
subunits in aldosterone-responsive epithelia from the rat: localization by in situ hybridization and immunocytochemistry.
J Cell Biol
127:
1907-1921,
1994[Abstract].
38.
Eaton, DC,
Malik B,
Saxena NC,
Al-Khalili OK,
and
Yue G.
Mechanisms of aldosterone's action on epithelial Na+ transport.
J Membr Biol
184:
313-319,
2001[ISI][Medline].
39.
Ecelbarger, CA,
Kim GH,
Terris J,
Masilamani S,
Mitchell C,
Reyes I,
Verbalis JG,
and
Knepper MA.
Vasopressin-mediated regulation of epithelial sodium channel abundance in rat kidney.
Am J Physiol Renal Physiol
279:
F46-F53,
2000
40.
Ellison, DH,
Velàzques H,
and
Wright FS.
Adaptation of distal convoluted tubule of the rat.
J Clin Invest
83:
113-126,
1989[ISI][Medline].
42.
Farman, N,
and
Rafestin-Oblin ME.
Multiple aspects of mineralocorticoid selectivity.
Am J Physiol Renal Physiol
280:
F181-F192,
2001
43.
Farman, N,
Vandewalle A,
and
Bonvalet JP.
Aldosterone binding in isolated tubules. II. An autoradiographic study of concentration dependency in the rabbit nephron.
Am J Physiol Renal Fluid Electrolyte Physiol
242:
F69-F77,
1982
44.
Friedman, PA.
Mechanisms of renal calcium transport.
Exp Nephrol
8:
343-350,
2000[ISI][Medline].
45.
Frindt, G,
Masilamani S,
Knepper MA,
and
Palmer LG.
Activation of epithelial Na channels during short-term Na deprivation.
Am J Physiol Renal Physiol
280:
F112-F118,
2001
46.
Frindt, G,
McNair T,
Dahlmann A,
Jacobs-Palmer E,
and
Palmer LG.
Epithelial Na channels and short-term renal response to salt deprivation.
Am J Physiol Renal Physiol
283:
F717-F726,
2002
47.
Gamba, G.
Molecular biology of distal nephron sodium transport mechanisms.
Kidney Int
56:
1606-1622,
1999[ISI][Medline].
48.
Garty, H,
and
Palmer LG.
Epithelial sodium channels: function, structure, and regulation.
Physiol Rev
77:
359-396,
1997
49.
Greger, R.
Physiology of renal sodium transport.
Am J Med Sci
319:
51-62,
2000[ISI][Medline].
50.
Hager, H,
Kwon TH,
Vinnikova AK,
Masilamani S,
Brooks HL,
Frøkiær J,
Knepper MA,
and
Nielsen S.
Immunocytochemical and immunoelectron microscopic localization of -,
-, and
-ENaC in rat kidney.
Am J Physiol Renal Physiol
280:
F1093-F1106,
2001
51.
Hoenderop, JG,
Dardenne O,
Van Abel M,
Van Der Kemp AW,
Van Os CH,
St-Arnaud R,
and
Bindels RJ.
Modulation of renal Ca2+ transport protein genes by dietary Ca2+ and 1,25-dihydroxyvitamin D3 in 25-hydroxyvitamin D3-1-hydroxylase knockout mice.
FASEB J
16:
1398-1406,
2002
52.
Hoenderop, JG,
Hartog A,
Stuiver M,
Doucet A,
Willems PH,
and
Bindels RJ.
Localization of the epithelial Ca2+ channel in rabbit kidney and intestine.
J Am Soc Nephrol
11:
1171-1178,
2000
53.
Hoenderop, JG,
Nilius B,
and
Bindels RJ.
Molecular mechanism of active Ca2+ reabsorption in the distal nephron.
Annu Rev Physiol
64:
529-549,
2002[ISI][Medline].
54.
Hoenderop, JG,
van der Kemp AW,
Hartog A,
van de Graaf SF,
van Os CH,
Willems PH,
and
Bindels RJ.
Molecular identification of the apical Ca2+ channel in 1,25-dihydroxyvitamin D3-responsive epithelia.
J Biol Chem
274:
8375-8378,
1999
55.
Hoenderop, JGJ,
Müller D,
van der Kemp AWCM,
Hartog A,
Suzuki M,
Ishibashi K,
Imai M,
Sweep F,
Willems PHGM,
van Os CH,
and
Bindels RJM
Calcitriol controls the epithelial calcium channel in kidney.
J Am Soc Nephrol
12:
1342-1349,
2001
56.
Hou, J,
Speirs HJ,
Seckl JR,
and
Brown RW.
Sgk1 gene expression in kidney and its regulation by aldosterone: spatio-temporal heterogeneity and quantitative analysis.
J Am Soc Nephrol
13:
1190-1198,
2002
57.
Kaissling, B,
and
Kriz W.
Morphology of the loop of Henle, distal tubule, and collecting duct.
In: Handbook of Physiology. Renal Physiology. Bethesda, MD: Am Physiol Soc, 1992, sect. 8, vol. I, chapt. 3, p. 109-168.
58.
Kaissling, B,
and
Kriz W.
Structural analysis of the rabbit kidney.
In: Advances in Anatomy, Embryology and Cell Biology, edited by Brodal A,
Hild W,
van Limborgh J,
Ortmann R,
Schiebler TH,
Töndury G,
and Wolf E.. Berlin: Springer-Verlag, 1979, p. 1-123.
59.
Kaissling, B,
and
Le Hir M.
Distal tubular segments of the rabbit kidney after adaptation to altered Na- and K-intake. I. Structural changes.
Cell Tiss Res
224:
469-492,
1982[ISI][Medline].
60.
Kaissling, B,
and
Stanton BA.
Adaptation of distal tubule and collecting duct to increased sodium delivery. I. Ultrastructure.
Am J Physiol Renal Fluid Electrolyte Physiol
255:
F1256-F1268,
1988
61.
Kamynina, E,
and
Staub O.
Concerted action of ENaC, Nedd4-2, and Sgk1 in transepithelial Na+ transport.
Am J Physiol Renal Physiol
283:
F377-F387,
2002
62.
Kaplan, MR,
Plotkin MD,
Lee WS,
Xu ZC,
Lytton J,
and
Hebert SC.
Apical localization of the Na-K-Cl cotransporter, rBSC1, on rat thick ascending limbs.
Kidney Int
49:
40-47,
1996[ISI][Medline].
63.
Katz, AI,
Doucet A,
and
Morel F.
Na-K-ATPase activity along the rabbit, rat and mouse nephron.
Am J Physiol Renal Fluid Electrolyte Physiol
237:
F114-F120,
1979
64.
Kim, GH,
Masilamani S,
Turner R,
Mitchell C,
Wade JB,
and
Knepper MA.
The thiazide-sensitive Na-Cl cotransporter is an aldosterone-induced protein.
Proc Natl Acad Sci USA
95:
14552-14557,
1998
65.
Kim, J,
Kim YH,
Cha JH,
Tisher CC,
and
Madsen KM.
Intercalated cell subtypes in connecting tubule and cortical collecting duct of rat and mouse.
J Am Soc Nephrol
10:
1-12,
1999
66.
Kishore, BK,
Mandon B,
Oza NB,
DiGiovanni SR,
Coleman RA,
Ostrowski NL,
Wade JB,
and
Knepper MA.
Rat renal arcade segment expresses vasopressin-regulated water channel and vasopressin V2 receptor.
J Clin Invest
97:
2763-2771,
1996
67.
Knepper, MA,
and
Brooks HL.
Regulation of the sodium transporters NHE3, NKCC2 and NCC in the kidney.
Curr Opin Nephrol Hypertens
10:
655-659,
2001[ISI][Medline].
68.
Kobayashi, S,
Clemmons DR,
Nogami H,
Roy AK,
and
Venkatachalam MA.
Tubular hypertrophy due to work load induced by furosemide is associated with increases of IGF-1 and IGFBP-1.
Kidney Int
47:
818-828,
1995[ISI][Medline].
69.
Kriz, W.
The architectonic and functional structure of the rat kidney.
Z Zellforsch Mikrosk Anat
82:
495-535,
1967[ISI][Medline].
70.
Kriz, W,
and
Kaissling B.
Structural organization of the distal nephron.
In: The Kidney: Physiology and Pathophysiology, edited by Seldin DW,
and Giebisch G.. Philadelphia, PA: Lippincot, 2000, p. 587-654.
71.
Kriz, W,
and
Koepsell H.
The structural organization of the mouse kidney.
Z Anat Entwickl-Gesch
144:
137-163,
1974[ISI][Medline].
72.
Lajeunesse, D,
Bouhtiauy I,
and
Brunette MG.
Parathyroid hormone and hydrochlorothiazide increase calcium transport by luminal membrane of rabbit distal nephron segments through different pathways.
J Endocrinol
134:
35-41,
1994.
73.
Lang, F,
and
Cohen P.
Regulation and physiological roles of serum- and glucocorticoid-induced protein kinase isoforms.
Sci STKE
2001:
RE17,
2001[Medline].
74.
Lim, SK,
Won YJ,
Lee HC,
Huh KB,
and
Park YS.
A PCR analysis of ERalpha and ERbeta mRNA abundance in rats and the effect of ovariectomy.
J Bone Miner Res
14:
1189-1196,
1999[ISI][Medline].
75.
Loffing, J,
Aregger F,
Pietri L,
Bloch-Faure M,
Schultheiss P,
Shull G,
Meneton P,
and
Kaissling B.
NCC(/
) mice lack the distal convoluted tubule 1 (Abstract).
Kidney Blood Press Res
23:
22A,
2000.
76.
Loffing, J,
Loffing-Cueni D,
Hegyi I,
Kaplan MR,
Hebert SC,
Le Hir M,
and
Kaissling B.
Thiazide treatment of rats provokes apoptosis in distal tubule cells.
Kidney Int
50:
1180-1190,
1996[ISI][Medline].
77.
Loffing, J,
Loffing-Cueni D,
Macher A,
Hebert SC,
Olson B,
Knepper MA,
Rossier BC,
and
Kaissling B.
Localization of epithelial sodium channel and aquaporin-2 in rabbit kidney cortex.
Am J Physiol Renal Physiol
278:
F530-F539,
2000
78.
Loffing, J,
Loffing-Cueni D,
Valderrabano V,
Klausli L,
Hebert SC,
Rossier BC,
Hoenderop JG,
Bindels RJ,
and
Kaissling B.
Distribution of transcellular calcium and sodium transport pathways along mouse distal nephron.
Am J Physiol Renal Physiol
281:
F1021-F1027,
2001
79.
Loffing, J,
Pietri L,
Aregger F,
Bloch-Faure M,
Ziegler U,
Meneton P,
Rossier BC,
and
Kaissling B.
Differential subcellular localization of ENaC subunits in mouse kidney in response to high- and low-Na diets.
Am J Physiol Renal Physiol
279:
F252-F258,
2000
80.
Loffing, J,
Summa V,
Zecevic M,
and
Verrey F.
Mediators of aldosterone action in the renal tubule.
Curr Opin Nephrol Hypertens
10:
667-675,
2001[ISI][Medline].
81.
Loffing, J,
Zecevic M,
Feraille E,
Kaissling B,
Asher C,
Rossier BC,
Firestone GL,
Pearce D,
and
Verrey F.
Aldosterone induces rapid apical translocation of ENaC in early portion of renal collecting system: possible role of SGK.
Am J Physiol Renal Physiol
280:
F675-F682,
2001
82.
Lytle, C,
Xu JC,
Biemesderfer D,
and
Forbush B, III.
Distribution and diversity of Na-K-Cl cotransport proteins: a study with monoclonal antibodies.
Am J Physiol Cell Physiol
269:
C1496-C1505,
1995
83.
Madsen, K,
and
Tisher CC.
Structural-functional relationships along the distal nephrons.
Am J Physiol Renal Fluid Electrolyte Physiol
250:
F1-F15,
1986
84.
Masilamani, S,
Kim GH,
Mitchell C,
Wade JB,
and
Knepper MA.
Aldosterone-mediated regulation of ENaC ,
, and
subunit proteins in rat kidney.
J Clin Invest
104:
R19-R23,
1999[ISI][Medline].
85.
Masilamani, S,
Wang X,
Kim GH,
Brooks H,
Nielsen J,
Nielsen S,
Nakamura K,
Stokes JB,
and
Knepper MA.
Time course of renal Na-K-ATPase, NHE3, NKCC2, NCC, and ENaC abundance changes with dietary NaCl restriction.
Am J Physiol Renal Physiol
283:
F648-F657,
2002
86.
May, A,
Puoti A,
Gaeggeler HP,
Horisberger JD,
and
Rossier BC.
Early effect of aldosterone on the rate of synthesis of the epithelial sodium channel alpha subunit in A6 renal cells.
J Am Soc Nephrol
8:
1813-1822,
1997[Abstract].
87.
Morel, F.
Sites of hormone action in the mammalian nephron.
Am J Physiol Renal Fluid Electrolyte Physiol
240:
F159-F164,
1981
88.
Moreno, G,
Merino A,
Mercado A,
Herrera JP,
Gonzàlez-Salazar J,
Correa-Rotter R,
Hebert SC,
and
Gamba G.
Electroneutral Na-coupled cotransporter expression in the kidney during variations of NaCl and water metabolism.
Hypertension
31:
1002-1006,
1998
89.
Morsing, P,
Velazquez H,
Wright FS,
and
Ellison DH.
Adaptation of distal convoluted tubule of rats. II. Effects of chronic thiazide infusion.
Am J Physiol Renal Fluid Electrolyte Physiol
261:
F137-F143,
1991
90.
Naray-Fejes-Toth, A,
Canessa C,
Cleaveland ES,
Aldrich G,
and
Fejes-Toth G.
Sgk is an aldosterone-induced kinase in the renal collecting duct. Effects on epithelial Na+ channels.
J Biol Chem
274:
16973-16978,
1999
91.
Narikiyo, T,
Kitamura K,
Adachi M,
Miyoshi T,
Iwashita K,
Shiraishi N,
Nonoguchi H,
Chen LM,
Chai KX,
Chao J,
and
Tomita K.
Regulation of prostasin by aldosterone in the kidney.
J Clin Invest
109:
401-408,
2002
92.
Nicco, C,
Wittner M,
DiStefano A,
Jounier S,
Bankir L,
and
Bouby N.
Chronic exposure to vasopressin upregulates ENaC and sodium transport in the rat renal collecting duct and lung.
Hypertension
38:
1143-1149,
2001
93.
Nielsen, J,
Kwon TH,
Masilamani S,
Beutler K,
Hager H,
Nielsen S,
and
Knepper MA.
Sodium transporter abundance profiling in kidney: effect of spironolactone.
Am J Physiol Renal Physiol
283:
F923-F933,
2002
94.
Nielsen, S,
DiGiovanni SR,
Christensen EI,
Knepper MA,
and
Harris HW.
Cellular and subcellular immunolocalization of vasopressin-regulated water channel in rat kidney.
Proc Natl Acad Sci USA
90:
11663-11667,
1993[Abstract].
95.
Nielsen, S,
Frøkiær J,
Marples D,
Kwon TH,
Agre P,
and
Knepper MA.
Aquaporins in the kidney: from molecules to medicine.
Physiol Rev
82:
205-244,
2002
96.
Obermüller, N,
Bernstein P,
Velazquez H,
Reilly R,
Moser D,
Ellison DH,
and
Bachmann S.
Expression of the thiazide-sensitive Na-Cl cotransporter in rat and human kidney.
Am J Physiol Renal Fluid Electrolyte Physiol
269:
F900-F910,
1995
97.
Obermüller, N,
Kunchaparty S,
Ellison DH,
and
Bachmann S.
Expression of the Na-K-2Cl cotransporter by macula densa and thick ascending limb cells of rat and rabbit nephron.
J Clin Invest
98:
635-640,
1996
98.
Oliver, J.
Nephrons and Kidneys. New York: Harper & Row, 1968.
99.
Pacha, J,
Frindt G,
Antonian L,
Silver RB,
and
Palmer LG.
Regulation of Na channels of the rat cortical collecting tubule by aldosterone.
J Gen Physiol
102:
25-42,
1993[Abstract].
100.
Pearce, D.
The role of SGK1 in hormone-regulated sodium transport.
Trends Endocrinol Metab
12:
341-347,
2001[ISI][Medline].
101.
Peng, JB,
Chen XZ,
Berger UV,
Vassilev PM,
Brown EM,
and
Hediger MA.
A rat kidney-specific calcium transporter in the distal nephron.
J Biol Chem
275:
28186-28194,
2000
102.
Peng, JB,
Chen XZ,
Berger UV,
Vassilev PM,
Tsukaguchi H,
Brown EM,
and
Hediger MA.
Molecular cloning and characterization of a channel-like transporter mediating intestinal calcium absorption.
J Biol Chem
274:
22739-22746,
1999
103.
Peter, K.
Untersuchungen über Bau und Entwicklung der Niere. Jena, Germany: Fischer, 1909, vol. 1.
104.
Peti-Peterdi, J,
Warnock DG,
and
Bell PD.
Angiotensin II directly stimulates ENaC activity in the cortical collecting duct via AT1 receptors.
J Am Soc Nephrol
13:
1131-1135,
2002
105.
Plotkin, MD,
Kaplan MR,
Verlander JW,
Lee WS,
Brown D,
Poch E,
Gullans SR,
and
Hebert SC.
Localization of the thiazide sensitive Na-Cl cotransporter, rTSC1, in the rat kidney.
Kidney Int
50:
174-183,
1996[ISI][Medline].
106.
Reif, MC,
Troutman SL,
and
Schafer JA.
Sodium transport by rat cortical collecting tubule: effects of vasopressin and deoxycorticosterone.
J Clin Invest
77:
1291-1298,
1986[ISI][Medline].
107.
Reilly, RF,
and
Ellison DH.
Mammalian distal tubule: physiology, pathophysiology, and molecular anatomy.
Physiol Rev
80:
277-313,
2000
108.
Reilly, RF,
Shugrue CA,
Lattanzi D,
and
Biemesderfer D.
Immunolocalization of the Na+/Ca2+ exchanger in rabbit kidney.
Am J Physiol Renal Fluid Electrolyte Physiol
265:
F327-F332,
1993
109.
Renard, S,
Voilley N,
Bassilana F,
Lazdunski M,
and
Barbry P.
Localization and regulation by steroids of the ,
and
subunits of the amiloride-sensitive Na+ chanel in colon, lung and kidney.
Pflügers Arch
430:
299-307,
1995[ISI][Medline].
110.
Rossier, BC,
Pradervand S,
Schild L,
and
Hummler E.
Epithelial sodium channel and the control of sodium balance: interaction between genetic and environmental factors.
Annu Rev Physiol
64:
877-897,
2002[ISI][Medline].
112.
Roth, J,
Brown D,
Norman AW,
and
Orci L.
Localization of the vitamin D-dependent calcium-binding protein in mammalian kidney.
Am J Physiol Renal Fluid Electrolyte Physiol
243:
F243-F252,
1982
113.
Satlin, LM,
Sheng S,
Woda CB,
and
Kleyman TR.
Epithelial Na+ channels are regulated by flow.
Am J Physiol Renal Physiol
280:
F1010-F1018,
2001
114.
Schafer, JA.
Abnormal regulation of ENaC: syndromes of salt retention and salt wasting by the collecting duct.
Am J Physiol Renal Physiol
283:
F221-F235,
2002
115.
Scheinman, SJ,
Guay-Woodford LM,
Thakker RV,
and
Warnock DG.
Genetic disorders of renal electrolyte transport.
N Engl J Med
340:
1177-1187,
1999
116.
Schmitt, R,
Ellison DH,
Farman N,
Rossier BC,
Reilly RF,
Reeves WB,
Oberbäumer I,
Tapp R,
and
Bachmann S.
Developmental expression of sodium entry pathways in rat distal nephron.
Am J Physiol Renal Physiol
276:
F367-F381,
1999
117.
Schultheis, PJ,
Lorenz JN,
Meneton P,
Niema ML,
Riddle TM,
Flagella M,
Duffy JJ,
Doetschman T,
Miller ML,
and
Shull GE.
Phenotype resembling Gitelman's syndrome in mice lacking the apical Na+-Cl cotransporter of the distal convolute tubule.
J Biol Chem
273:
1-6,
1998
118.
Schweigger-Seidel, F.
Die Nieren der Menschen und der Säugetiere in ihrem feineren Baue.
In: Handbuch der mikroskopischen Anatomie des Menschen. Harn- und Geschlechtsapparat, edited by von Möllendorff W.. Berlin: Springer, 1930, vol. vol. VII/1, p. 1-328.
119.
Shigaev, A,
Asher C,
Latter H,
Garty H,
and
Reuveny E.
Regulation of sgk by aldosterone and its effects on the epithelial Na+ channel.
Am J Physiol Renal Physiol
278:
F613-F619,
2000
120.
Snyder, PM.
The epithelial Na+ channel: cell surface insertion and retrieval in Na+ homeostasis and hypertension.
Endocr Rev
23:
258-275,
2002
121.
Snyder, PM,
Olson DR,
and
Thomas BC.
Serum and glucocorticoid-regulated kinase modulates Nedd4-2-mediated inhibition of the epithelial Na+ channel.
J Biol Chem
277:
5-8,
2002
122.
Stanton, BA,
and
Kaissling B.
Adaptation of distal tubule and collecting duct to increased Na delivery. II. Na+ and K+ transport.
Am J Physiol Renal Fluid Electrolyte Physiol
255:
F1269-F1275,
1988
123.
Stokes, JB,
and
Sigmund RD.
Regulation of rENaC mRNA by dietary NaCl and steroids: organ, tissue, and steroid heterogeneity.
Am J Physiol Cell Physiol
274:
C1699-C1707,
1998
124.
Stumpf, WE,
Sar M,
Narbaitz R,
Reid FA,
DeLuca HF,
and
Tanaka Y.
Cellular and subcellular localization of 1,25-(OH)2-vitamin D3 in rat kidney: comparison with localization of parathyroid hormone and estradiol.
Proc Natl Acad Sci USA
77:
1149-1153,
1980[Abstract].
125.
Tomita, K,
Pisano JJ,
and
Knepper MA.
Control of sodium and potassium transport in the cortical collecting duct of the rat.
J Clin Invest
76:
132-136,
1985[ISI][Medline].
126.
Vallet, V,
Chraibi A,
Gaeggeler HP,
Horisberger JD,
and
Rossier BC.
An epithelial serine protease activates the amiloride-sensitive sodium channel.
Nature
389:
607-610,
1997[ISI][Medline].
127.
Van Abel, M,
Hoenderop JG,
Dardenne O,
St Arnaud R,
Van Os CH,
Van Leeuwen HJ,
and
Bindels RJ.
1,25-Dihydroxyvitamin D3-independent stimulatory effect of estrogen on the expression of ECaC1 in the kidney.
J Am Soc Nephrol
13:
2102-2109,
2002
128.
Velazquez, H,
Bartiss A,
Bernstein P,
and
Ellison DH.
Adrenal steroids stimulate thiazide-sensitive NaCl transport by rat renal distal tubules.
Am J Physiol Renal Fluid Electrolyte Physiol
270:
F211-F219,
1996
129.
Velazquez, H,
Silva T,
Andujar E,
Desir GV,
Ellison DH,
and
Greger R.
The distal convoluted tubule of rabbit kidney does not express a functional sodium channel.
Am J Physiol Renal Physiol
280:
F530-F539,
2001
130.
Verlander, JW,
Tran TM,
Zhang L,
Kaplan MR,
and
Hebert SC.
Estradiol enhances thiazide-sensitive NaCl cotransporter density in the apical plasma membrane of the distal convoluted tubule in ovarectomized rats.
J Clin Invest
101:
1661-1669,
1998
131.
Verrey, F,
Hummler E,
Schild L,
and
Rossier BC.
Control of Na+ transport by aldosterone.
In: The Kidney: Physiology and Pathophysiology (3rd ed.), edited by Seldin W,
and Giebisch G.. Philadelphia, PA: Lippincott Williams & Wilkins, 2000.
132.
Vio, CP,
and
Figueroa CD.
Subcellular localization of renal kallikrein by ultrastructural immunocytochemistry.
Kidney Int
28:
36-42,
1985[ISI][Medline].
133.
Vuagniaux, G,
Vallet V,
Jaeger NF,
Pfister C,
Bens M,
Farman N,
Courtois-Coutry N,
Vandewalle A,
Rossier BC,
and
Hummler E.
Activation of the amiloride-sensitive epithelial sodium channel by the serine protease mCAP1 expressed in a mouse cortical collecting duct cell line.
J Am Soc Nephrol
11:
828-834,
2000
134.
Wade, JB.
Distribution of transporters along the mouse distal nephron: something old, something borrowed, something new.
Am J Physiol Renal Physiol
281:
F1019-F1020,
2001
135.
Wang, T,
and
Giebisch G.
Effects of angiotensin II on electrolyte transport in the early and late distal tubule in rat kidney.
Am J Physiol Renal Fluid Electrolyte Physiol
271:
F143-F149,
1996
136.
Wang, W,
Hebert SC,
and
Giebisch G.
Renal K+ channels: structure and function.
Annu Rev Physiol
59:
413-436,
1997[ISI][Medline].
137.
Wang, XY,
Masilamani S,
Nielsen J,
Kwon TH,
Brooks HL,
Nielsen S,
and
Knepper MA.
The renal thiazide-sensitive Na-Cl cotransporter as mediator of the aldosterone-escape phenomenon.
J Clin Invest
108:
215-222,
2001
138.
Warnock, DG.
Renal genetic disorders related to K+ and Mg2+.
Annu Rev Physiol
64:
845-876,
2002[ISI][Medline].
139.
Weinman, EJ,
and
Eknoyan G.
Chronic effects of chlorothiazide on reabsorption by the proximal tubule of the rat.
Clin Sci Mol Med
49:
107-113,
1975[ISI][Medline].
140.
Wolf, K,
Castrop H,
Riegger GA,
Kurtz A,
and
Kramer BK.
Differential gene regulation of renal salt entry pathways by salt load in the distal nephron of the rat.
Pflügers Arch
442:
498-504,
2001[ISI][Medline].
141.
Wulff, P,
Vallon V,
Huang DY,
Völkl H,
Yu F,
Richter K,
Jansen M,
Schlünz M,
Klingel K,
Loffing J,
Kauselmann G,
Bösl MR,
Lang F,
and
Kuhl D.
Impaired renal Na+ retention in the sgk1-knockout mouse.
J Clin Invest
110:
1263-1268,
2002
142.
Zhao, PL,
Wang XT,
Zhang XM,
Cebotaru V,
Cebotaru L,
Guo G,
Morales M,
and
Guggino SE.
Tubular and cellular localization of the cardiac L-type calcium channel in rat kidney.
Kidney Int
61:
1393-1406,
2002[ISI][Medline].