Department of Physiology, Tulane University School of Medicine, New Orleans, Louisiana 70112, and Department of Physiology, Medical College of Georgia, Augusta, Georgia 30912-3000
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ABSTRACT |
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In the last 10-15 years, interest in the physiological role of P2 receptors has grown rapidly. Cellular, tissue, and organ responses to P2 receptor activation have been described in numerous in vivo and in vitro models. The purpose of this review is to provide an update of the recent advances made in determining the involvement of P2 receptors in the control of renal hemodynamics and the renal microcirculation. Special attention will be paid to work published in the last 5-6 years directed at understanding the role of P2 receptors in the physiological control of renal microvascular function. Several investigators have begun to evaluate the effects of P2 receptor activation on renal microvascular function across several species. In vivo and in vitro evidence consistently supports the hypothesis that P2 receptor activation by locally released extracellular nucleotides influences microvascular function. Extracellular nucleotides selectively influence preglomerular resistance without having an effect on postglomerular tone. P2 receptor inactivation blocks autoregulatory behavior whereas responsiveness to other vasoconstrictor agonists is retained. P2 receptor stimulation activates multiple intracellular signal transduction pathways in preglomerular smooth muscle cells and mesangial cells. Renal microvascular cells and mesangial cells express multiple subtypes of P2 receptors; however, the specific role each plays in regulating vascular and mesangial cell function remains unclear. Accordingly, the results of studies performed to date provide strong support for the hypothesis that P2 receptors are important contributors to the physiological regulation of renal microvascular and/or glomerular function.
extracellular nucleotides; microcirculation; afferent arteriole; calcium signaling; mesangial cells; diadenosine polyphosphates; kidney; purinoceptors
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INTRODUCTION |
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P2 RECEPTORS ARE EMERGING as important elements by which vascular responses to physiological and pathophysiological stimuli are mediated and maintained (1, 4, 20, 26, 30, 33, 64, 76, 83, 92, 94, 111, 116, 135). Development of this field of study has been slow due to the lack of selective probes with which to manipulate receptor activation and signaling. However, in recent years, significant advances have been made, in part, through the development of more selective investigational tools, such as expression cloning and knockout models (34, 70, 108, 135). Application of these approaches has clarified our understanding of the pharmacology of P2 receptors. However, determination of the specific roles P2 receptors play in mammalian physiology or pathophysiology still remains to be established.
As interest in P2 receptor physiology has grown, so has interest in the role P2 receptors play in the physiology and pathophysiology of renal function. With this renewed interest has come a new appreciation for the roles extracellular adenine nucleotides, like ATP, can play in regulating or modulating renal function. In the last 5 years, investigators have provided compelling evidence that extracellular nucleotides, working through activation of P2 receptors, have a significant impact on renal microvascular function, mesangial cell function, and renal epithelial transport. The latter concept is the focus of a companion review in the present issue of this journal (150). P2 receptor activation has been implicated in regulating preglomerular resistance and in mediating renal microvascular autoregulatory behavior. Locally released ATP has a direct paracrine and/or autocrine effect, modulating renal epithelial transporters and tubular epithelial channels to influence tubular fluid composition. Although the specific roles for extracellular nucleotides and their receptors in the kidney have not been firmly established, it is clear that locally released ATP may play a significant role in the regulation of renal hemodynamics and tubular function. The purpose of this review is to summarize the present literature pertaining to the effect of P2 receptor activation on renal microvascular function and to detail the signal transduction mechanisms involved.
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PURINOCEPTOR CLASSIFICATION |
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Purinoceptors are membrane-bound receptors that are divided into
two distinct families These families are summarized in Fig. 1 (1, 4, 20, 30, 56, 57, 64, 92,
116, 118, 120, 135). P1 receptors respond to adenosine and AMP
and are further divided into four major subtypes, identified as
A1, A2A, A2B and A3
receptors (56, 57, 89, 118, 120, 135). A1 receptors are thought to induce vasoconstriction by inhibiting adenylate cyclase activity, thus reducing the formation of cAMP in
vascular smooth muscle. A2A and A2B receptors
evoke vasodilation by stimulating adenylate cyclase activity and
increasing the formation of cAMP in vascular smooth muscle. The more
recently described A3 receptors are thought to exert their
physiological effects through activation of calcium signaling pathways
and perhaps inhibition of cAMP accumulation (88, 89, 135).
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Functional effects of A1 and A2 receptors have been described in the kidney and include direct actions on preglomerular and postglomerular microvascular function (2, 5, 22, 27, 36, 61, 83, 91, 98, 99, 104, 105, 109, 124, 133, 158, 172), protection from ischemic damage (37, 38, 50), endotoxic shock (115), renin secretion (66, 99, 110, 122, 124, 143, 153), and in mediating autoregulatory adjustments in preglomerular resistance (66, 121, 132, 143, 162). The role of A3 receptors in the kidney remains unclear (107).
P2 receptors (Fig. 1) were originally described by Burnstock (18,
19) as purinergic receptors, reflecting the fact that the
original description pertained to the release of ATP as a neurotransmitter from peripheral sympathetic nerves (18,
19). Later, the term "P purinergic receptor" was revised to
"P purinoceptor," and the nomenclature for identification of P1 and
P2 receptors was standardized to "P2 receptors" by the
International Union of Pharmacology (55, 56). Presently,
P2 receptors are divided into two major families, labeled P2X and P2Y
(Figs. 1 and 2), based on the facts that
distinct genes code for these receptors, there are marked differences
in receptor structure between the two families, and unique signal
transduction pathways are invoked after receptor activation (1,
4, 20, 30, 55, 56, 64, 92, 116, 135).
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P2X receptors (Fig. 2) comprise multiple subtypes that can form homomeric complexes or exist as heteromeric complexes that combine with other P2X receptor subtypes. The latter results in hybrid receptors that exhibit modified pharmacological, biophysical, and electrophysiological properties compared with those of each constituent monomeric form of the receptor (1, 4, 30, 48, 64, 92, 116, 135). A single P2X receptor possesses two membrane-spanning domains linked by a large extracellular loop and with the NH2- and COOH-terminal tails exposed to the cytoplasm. These receptors function as ligand-gated channels and can be profoundly influenced by the presence of cations (48, 135, 163). For example, high concentrations of Mg2+ or Ca2+ generally inhibit P2X receptor currents, whereas cations like Zn2+, Cu2+, and Cd2+ can either potentiate or inhibit cation currents in a P2X receptor-specific manner (1, 4, 30, 64, 92, 116, 135). On activation by ATP, the nonselective cation channel, which is intrinsic in the receptor structure, is opened, allowing Na+ and Ca2+ to pass from the extracellular fluid to the intracellular cytoplasm (39, 42, 135). Potassium ions can also pass through this channel (39, 42, 135). Presently, there are approximately seven P2X receptor subtypes (P2X1-7) that have been described on the basis of pharmacological assessment of receptor activity and of receptor cloning and expression studies (48, 116, 135). Investigators have also reported the cloning of the gene for P2XM and P2X8 receptors that are derived from skeletal muscle; however, more work needs to be completed before their places in the P2X receptor family can be determined (11, 67, 112).
A splice variant of the P2X2 receptor isolated from the rat cerebellum has also been described (16, 135, 151). This receptor differs from other P2X2 receptors by the absence of a 69-residue sequence from the COOH terminal of the receptor protein. The role of this receptor is presently unknown. However, because of the potential physiological alterations that splicing could represent, more attention should be focused on the in vivo study of these receptors rather than relying on data obtained from reconstituted receptor models.
As mentioned above, it has recently been determined that some P2X
receptors can form heteromultimers by combining with other P2X subtypes
(48, 116). On the basis of experiments conducted in
heterologous expression systems, receptor pairs formed from P2X1 and P2X5
(P2X1/P2X5), P2X2 and
P2X3 (P2X2/P2X3), and
P2X4 and P2X6
(P2X4/P2X6) receptors have been described.
P2X7 receptors do not coimmunoprecipitate with any other
known P2X receptor; however, P2X5 receptors
coimmunoprecipitate with all other known P2X receptor subtypes
(116). Although the physiological role of the heteromeric
P2 receptors remains to be determined, the potential for adding a new
level of sophistication and selectivity to the regulatory influences of
P2 receptors is quite interesting, because the behavior of each
heteromeric receptor is modified according to the respective
pharmacological properties of the individual subunits. For example,
homomeric P2X1 receptors are activated by
,
-methylene-ATP and desensitize rapidly, and P2X5 receptors are insensitive to
,
-methylene-ATP. In contrast, the P2X1/P2X5 heteromer exhibits a sustained
current in response to
,
-methylene-ATP that does not readily
desensitize (116). The unique combination of properties
that can be achieved through the formation of heteromeric P2X receptors
confers potentially new levels of physiological or pathophysiological
control that still need to be elucidated.
P2Y receptors (Fig. 2) are composed of seven transmembrane-spanning domains and are regulated by G proteins (1, 4, 32, 97, 135). In most systems, activation of P2Y receptors is associated with activation of phospholipase C and the subsequent mobilization of calcium from inositol 1,4,5-triphosphate (IP3)-sensitive intracellular stores. ATP has also been reported to stimulate cAMP formation in some cell types through a variety of mechanisms and intermediates (32). For example, published reports indicate that P2 receptor stimulation can induce cAMP accumulation through adenosine-mediated A2 receptor activation after ATP hydrolysis and through the autocrine actions of P2 receptor-mediated prostaglandin E2 formation (32). The P2Y11 receptor has been suggested to couple directly to both phospholipase C and adenylate cyclase in Chinese hamster ovary cells (31, 32). The mammalian family of P2Y receptors includes the P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11 receptor subtypes (Fig. 1) on the basis of responses to pharmacological agonists and antagonists and from receptor cloning and expression studies. Other putative P2Y receptors have been described in other species or cell types; however, verification of the receptor structures as that of unique P2Y receptors remains to be established (32, 135).
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P2 RECEPTOR AGONISTS AND ANTAGONISTS |
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Rapid advances in the field of P2 receptor pharmacology and
physiology have been hampered by the lack of selective pharmacological tools. Ralevic and Burnstock (135) have compiled an
excellent review of the tools presently in use as P2 receptor agonists
and antagonists. In their description, they highlight the primary uses
of these compounds, discuss the utility of these compounds against
different P2 receptors, and indicate the limitations each compound
presents. None of the agonists or antagonists presently in use
discriminates very definitively between P2X and P2Y receptors. This
leads to considerable confusion in the pharmacological characterization of P2 receptors from one investigator to the next, and from one tissue
or cell type to the next. Nevertheless, some aspects of P2 receptor
pharmacology can be deciphered by using these probes. Some of the most
common agonists used are ,
-methylene-ATP and
,
-methylene-ATP, which are stable analogs of ATP. These agents are considered essentially inactive on P2Y receptors but exhibit fairly
high selectivity for P2X receptors, in particular, the P2X1
and P2X3 receptor subtypes. In contrast, P2Y
receptor-mediated responses can best be ascertained by using ADP,
ADP
S, and UTP, which are weakly active or nearly inactive at P2X
receptors. It is essential to remember, however, that because a
significant lack of receptor subtype specificity exists with some
agonists, cautious interpretation of data obtained with these probes is important.
P2 receptor antagonists should also be used with careful consideration of their limitations (135). Some of the most commonly used antagonists that are reasonably selective for P2 receptors, but do not readily discriminate between P2X and P2Y subtypes, include suramin, a symmetrical 3'-urea of 8-(benzamido)naphthalene-1,3,5-trisulfonic acid that is a derivative of suramin, pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), reactive blue 2, and reactive red. Antagonists with some specificity for P2X receptors include 8, 8'-[carbonylbis(imino-4,1-phenylenecarbonylimino-4,1-phenylenecarbonylimino)]bis(1,3,5-naphthalene- trisulfonic acid) and pyridoxal phosphate-6-azophenyl-2',5'-disulfonic acid (iso-PPADS). The interested reader is referred to the review by Ralevic and Burnstock (135) for a more thorough discussion of these agents.
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P2 RECEPTOR DISTRIBUTION WITHIN THE KIDNEY |
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There are numerous reports describing the expression and distribution of P2 receptors within the kidney or in renal tissues (3, 24-26). Although this review will not provide a comprehensive listing of these reports, it will endeavor to provide a reasonable representation of the ubiquitous expression of P2 receptors by different segments of the nephron and renal vasculature. P2 receptors are expressed on various segments of the renal vasculature (3, 10, 25, 44, 77, 101, 102, 169), microvasculature (3, 25, 61, 80, 83, 85, 171), glomeruli (17, 24, 72, 125, 126), and cells of the glomerular mesangium (59, 62, 63, 71, 73, 96, 128, 129, 131, 139, 146, 148, 155). In addition, P2 receptors have been shown to be expressed by proximal tubular epithelial cells (3, 23, 41, 90, 156, 175, 176), distal tubule cells (3, 9, 136), and loop of Henle cells and collecting duct cells (3, 8, 24, 87, 95, 123, 154, 156). For a more thorough discussion of this subject, readers are referred to the review by Schwiebert and Kishore (150) that appears in this issue of the journal. The P2X and P2Y receptor families are present on renal tissue, and some cells may express receptors from both families. For example, the vascular smooth muscle of afferent arterioles appears to express both P2X and P2Y receptors (80, 82). Investigations continue in an effort to establish the role that P2 receptors play in regulating renal vascular and tubular function. Those investigations have resulted in the emergence of many interesting possibilities for P2 receptor involvement in the regulation of renal microvascular function, glomerular hemodynamics, renal tubular function, and even in the regulation of urinary bladder function (26). The remainder of this review will focus on the renal microvascular effects of P2 receptor activation and discuss the possible role P2 receptors may play in the physiology of renal microvascular control.
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RENAL VASCULAR RESPONSE TO ATP |
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ATP concentrations in human arterial and venous plasma have been measured to be ~0.19 (0.38 µM) and 0.7 µg/ml (1.38 µM), respectively (53, 54). Much of the ATP could derive from red blood cells (35), endothelial cells (12, 13, 127), or platelets (6, 54). The interaction of circulating ATP with endothelial or vascular smooth muscle cells could play an important role in the regulation of vascular tone (35, 43, 49, 97). Exposure of renal microvascular smooth muscle to extracellular nucleotides can occur by administration of ATP into the vascular lumen. Alternatively, it can occur by direct application of ATP to the exposed vascular smooth muscle from the adventitial surface. Intrarenal infusion of ATP will deliver it directly into the vascular space and will result in its interaction with the endothelium before gaining access to the underlying vascular smooth muscle cells. Interaction of ATP with endothelial P2 receptors can result in the generation of endothelium-derived vasoactive factors that can influence the renal microvascular response. Adventitial administration allows the nucleotide to interact directly with receptors on the vascular smooth muscle cells. The order of the interaction may be an important contributor to the overall renal hemodynamic response to P2 agonists.
The impact of infused ATP or P2 agonists on renal blood flow or renal
perfusion pressure is dependent on a number of factors, including the
species being studied, the type of agonist infused, the ambient
vascular tone, and the experimental approach being used. Infusion of
ATP directly into the renal artery of the isolated perfused rat kidney
evokes vasoconstriction under basal tone conditions and both
vasoconstriction and vasodilation when renal vascular resistance is
elevated (44, 49, 167, 168). At basal tone, infused ATP or
the P2X agonist ,
-methylene-ATP induces a sustained concentration-dependent vasoconstriction (28, 44, 49, 167, 168); however, ATP-mediated responses observed in kidneys
perfused at high tone are less consistent. Eltze and Ullrich
(44) increased renal vascular resistance with
norepinephrine and reported that infusion of the P2X agonist
,
-methylene-ATP consistently evoked a renal vasoconstriction
(Fig. 3). Infusion of low doses of ATP induced vasodilation with a vasoconstrictor response occurring only at
higher infusion doses (44). Curiously, when Vargas and co-workers (167) increased renal vascular tone with
phenylephrine, low-dose ATP infusion led to a renal vasoconstriction,
with vasodilation only occurring in response to infusion of high doses
of ATP. The reason for these conflicting observations is unclear, but
the data do suggest that the renal vascular response to infused ATP may
be influenced by ambient renal vascular tone.
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It is interesting to note that isolated segments of human and rabbit
renal arteries, preconstricted with either norepinephrine or
prostaglandin F2, relaxed on intraluminal exposure to ATP (137, 138). Subsequent studies revealed that the
vasodilation occurred through P2Y receptor-mediated generation of
nitric oxide and through the local generation of adenosine and
activation of adenosine-sensitive P1 receptors (138). In a
separate study, ATP-mediated vasoconstriction of rat afferent
arterioles was found to be augmented during adenosine receptor blockade
(85). The potential interaction between the P2 receptor
system and the adenosine-sensitive P1 receptor system has not been
carefully examined; however, the close association between the two
receptor families and the prevalence of both receptor families in the
cardiovascular system (52, 97, 118, 119, 135, 142) provide
an interesting opportunity for a local integration of coregulatory
roles between P1 and P2 receptor-dependent responses.
In the isolated perfused rabbit kidney, ATP infusion leads to a modest
vasoconstriction (113), whereas in the dog kidney, infusion of ATP produces vasodilation by stimulating the synthesis and
release of nitric oxide (101, 102). Infusion of P2Y
receptor-selective agonists, such as 2-methylthio ATP, UTP, or
adenosine 5'-O-(3-thiotriphosphate) (ATPS) at
concentrations up to 10 µM, also leads to nitric oxide-dependent vasodilation in isolated, perfused rat kidneys (28, 44),
but greater concentrations lead to vasoconstriction (Fig. 3)
(44). In addition, intrarenal infusion of 2-methylthio ATP
or ATP into the isolated perfused rat kidney stimulates renin secretion
(29), but similar infusion of the P2X agonist
,
-methylene-ATP did not effect renin secretion (29).
These data suggest that luminally delivered P2Y agonists will interact
with P2 receptors expressed by intrarenal endothelial cells, resulting
in a nitric oxide-dependent relaxation of the intrarenal
microvasculature and a decrease in renal vascular resistance. P2Y
receptor-mediated vasodilation is converted to vasoconstriction during
inhibition of nitric oxide synthesis, whereas P2X receptor-mediated
vasoconstrictor responses are augmented (28, 44). These
data suggest that intrarenal microvascular smooth muscle and
endothelium express P2X and P2Y receptors; however, the results of
these studies do not reveal the specific intrarenal microvascular
segments that are responsible for agonist-mediated alterations in renal
vascular resistance.
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ADVENTITIAL DELIVERY OF P2 AGONISTS |
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ATP released from sympathetic nerves travels through the interstitial fluid adjacent to the varicosities and binds to postsynaptic P2 receptors located on vascular smooth muscle, where it influences vascular tone (15, 21, 40, 134, 149). ATP can also be released from smooth muscle cells (127, 157), epithelial cells (60, 68, 159), and perhaps even macula densa cells (7). ATP released from such cell types would also move through the interstitial fluid space before reaching P2 receptors on vascular smooth muscle cells or other cell types to induce appropriate physiological or pathophysiological responses. In the kidney, that response may be to provide local paracrine regulation of preglomerular renal vascular resistance via activation of P2 receptors expressed by the renal microvasculature (111).
The distribution of P2X1 receptors along the renal
vasculature was addressed by Chan and co-workers (25).
Autoradiographic assessment of radiolabeled (3H-labeled)
,
-methylene-ATP revealed a distribution of P2 receptors along
afferent arterioles and interlobular arteries, but no binding was
visible along efferent arterioles. In the same study,
immunohistochemical assessment for P2X1 receptors, using an
antibody selective for P2X1 receptors, revealed strong
positive staining along all segments of the preglomerular
microvasculature, whereas no visible staining was evident along
efferent arterioles or glomeruli (25). These data provide
strong support for the expression of P2X1 receptors by
preglomerular vascular smooth muscle cells.
Figure 4 illustrates the results of
experiments conducted to determine the responsiveness of each
preglomerular and postglomerular segment to ATP (85). The
responsiveness of each segment was assessed at four concentrations of
ATP applied in succession. These data reveal that only the arteries and
arterioles comprising the preglomerular renal circulation respond to
ATP administration (85). Of these segments, only the
afferent arteriole exhibits a sustained vasoconstriction in response to
ATP concentrations below 10 µM. Arcuate and interlobular arteries
respond with a transient vasoconstriction that subsides within 2-3
min of exposure. Notably, efferent arteriolar diameter remains
unchanged during ATP administration. Weihprecht et al.
(171) also reported that ATP concentrations below 10 µM
evoked a significant vasoconstriction of isolated rabbit afferent
arterioles. These data define the functional responses of the renal
microvasculature to P2 receptor activation and are in excellent
agreement with the distribution of P2X1 receptors
determined by immunohistochemistry and autoradiography (25).
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As described above, the absence of highly selective agonists and
antagonists creates a problem with regard to identification of the
types of receptors expressed in a given tissue. Conventional pharmacological approaches to this problem include determination of
rank order potency profiles using a variety of P2X and P2Y receptor
agonists. This approach was used to determine the receptor subtypes
possibly contributing to the afferent arteriolar response to P2
receptor activation, and the data for the sustained responses are
illustrated in Fig. 5 (80).
Administration of the P2X-selective agonists, ,
-methylene-ATP or
,
-methylene-ATP, yielded rapid biphasic vasoconstrictor responses
consistent with the expression of P2X receptors on afferent arterioles.
A typical response to
,
-methylene-ATP is illustrated in Fig.
6. The afferent arteriolar diameter
response to these P2X agonists began with a rapid initial vasoconstriction that gradually declined to a smaller but sustained vasoconstriction (80). The afferent arteriolar
vasoconstriction elicited by the endogenous ligand, i.e., ATP, closely
paralleled the response observed after administration of the P2X
agonist
,
-methylene-ATP (80, 171). Desensitization
is a prominent characteristic of P2X1 and P2X3
receptors (116, 135). Accordingly, it is interesting to
note the declining effectiveness of subsequent addition of higher
concentrations of
,
-methylene-ATP to evoke greater
vasoconstriction as would be observed in response to a single
application of a high concentration. These and other data (81) suggest that the response to
,
-methylene-ATP
quickly desensitized. Given that P2X1 receptors are heavily
expressed along the afferent arteriole (25), it is
reasonable to conclude that P2X1 receptors participate
significantly in the afferent arteriolar response to ATP and P2X
agonists
,
-methylene-ATP and
,
-methylene-ATP.
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Administration of the P2Y agonists 2-methylthio-ATP, ADP, UDP, UTP, or
ATPS in the juxtamedullary nephron preparation yielded markedly
different results (Fig. 5) (78, 80). In contrast to
experiments performed in the isolated perfused kidney
(44), 2-methylthio ATP, ADP, and UDP induced only very
slight vasoconstrictions. UTP and ATP
S were less potent agonists
compared with ATP or the P2X agonists but, at concentrations of 10 and
100 µM, these agents elicited very large monophasic
vasoconstrictions. The afferent arteriolar diameter usually closed
during exposure to 100 µM concentrations of UTP and ATP
S, and
afferent arteriolar blood flow ceased. Removal of the agonists from the
bathing medium resulted in a rapid reversal of the vasoconstriction and
restoration of afferent arteriolar blood flow (80). UDP
had no significant effect on afferent diameter. The pattern of the
afferent arteriolar vasoconstrictor response to these agents is in
qualitative agreement with the results obtained in the isolated
perfused kidney (44) with a few notable exceptions. In the
concentration ranges used in the juxtamedullary nephron preparation, no
significant vasodilatory response is observed even though the renal
vasculature is under significant endogenous tone. This may reflect a
difference in the concentrations tested as the vasodilatory responses
in the isolated perfused kidney experiments were observed at agonist
concentrations several orders of magnitude lower than were used in the
juxtamedullary nephron experiments. In addition, the isolated perfused
kidney experiments measure the whole kidney hemodynamic response as a
reflection of the response of the entire renal vasculature to the
infused agents, whereas the juxtamedullary nephron approach examines
the responses of individual microvascular segments of a finite nephron population.
Taken together, these agonist response experiments suggest that at
least two distinct P2 receptors are found on afferent arterioles of the
rat kidney. One receptor belongs to the P2X family and responds
strongly to the P2X agonists ,
-methylene-ATP or
,
-methylene-ATP. Given the existing data, it is likely that this
receptor is a P2X1 receptor subtype. A second receptor
probably belongs to the P2Y receptor family and is sensitive to UTP and
ATP
S. Activation of either receptor will evoke a sustained,
concentration-dependent vasoconstriction of afferent arterioles. In
addition, assessment of the disparate response patterns observed
between experiments performed in the isolated perfused kidney and in
the juxtamedullary nephron technique suggests that P2 receptor
activation may evoke more complex regional alterations in renal
microvascular function and could result in zonal changes in vascular
resistance and renal perfusion.
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P2 RECEPTOR-MEDIATED CALCIUM SIGNALING IN RENAL MICROVASCULAR SMOOTH MUSCLE |
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Calcium plays a major role in agonist-mediated afferent arteriolar vasoconstrictor responses, autoregulatory responses, and tubuloglomerular feedback responses (111). P2 receptor activation has also been shown to activate varied calcium signaling mechanisms involving multiple calcium influx pathways as well as mobilization of calcium from intracellular stores (111). P2X receptor stimulation activates a nonselective cation current, which can directly increase intracellular calcium concentration (39, 135). Furthermore, the net influx of cations can depolarize the plasma membrane and activate voltage-dependent calcium channels. Alternatively, P2Y receptors have been shown to be coupled to phospholipase C and generate IP3-dependent mobilization of calcium from intracellular stores (39, 135). Given the diversity of effects that may be elicited via calcium-dependent mechanisms, it is important to assess the calcium signaling pathways utilized by afferent arteriolar smooth muscle cells in responding to P2 receptor stimulation.
P2X receptor channels conduct calcium down its steep concentration
gradient into the cell. Therefore, calcium influx from the
extracellular medium is an important component of P2X receptor-mediated responses. Removal of calcium from the extracellular medium prevents the afferent arteriolar vasoconstriction normally evoked by
,
-methylene-ATP (84). Replenishing the extracellular
calcium concentration to the physiological range also restores the
,
-methylene-ATP-mediated afferent arteriolar vasoconstriction
(84). These findings demonstrate that calcium influx from
the extracellular medium is an essential component in the
,
-methylene-ATP-mediated afferent arteriolar vasoconstrictor
response; however, they do not identify the influx pathway involved.
Many agonists activate L-type calcium channels to stimulate
vasoconstriction of afferent arterioles (111). Therefore,
the role of L-type calcium channels in the afferent arteriolar response to P2 receptor activation was assessed by using different P2X and P2Y
agonists. As shown in Fig. 6, P2X receptor stimulation with
,
-methylene-ATP produces a biphasic vasoconstrictor response under control conditions (78, 80, 84). Typical responses include a rapid initial vasoconstriction that reaches a maximum value
within 20-30 s, followed by a partial recovery to a stable diameter significantly smaller than control. Removal of
,
-methylene-ATP from the bathing solution results in a prompt and
complete recovery to the control diameter. Blockade of L-type calcium
channels with felodipine (Fig. 6) or diltiazem evokes a rapid
vasorelaxation, implicating L-type calcium channel activity in
maintaining ambient afferent arteriolar tone. Subsequent introduction
of
,
-methylene-ATP, with continued blockade of L-type calcium
channels, results in attenuation of the rapid initial vasoconstriction
by ~50-60% and abolition of the sustained vasoconstrictor
response (84). Preliminary data reveal that stimulation of
P2Y receptors with UTP evokes a more monophasic vasoconstriction that
is only slightly attenuated by calcium channel blockade with diltiazem
(79). Calcium channel blockade abolishes afferent
arteriolar responses to low concentrations of ATP (0.1-1 µM) and
significantly attenuates the responses evoked by higher concentrations
of ATP (79). These data suggest that calcium influx
through voltage-dependent calcium channels plays a central role in the
afferent arteriolar vasoconstriction elicited by P2X receptor
activation by P2X receptor-selective agonists and by low concentrations
of ATP. The data also suggest that calcium channels are not the major
mechanism invoked in vasoconstrictor responses evoked by P2Y receptor
activation or by higher concentrations of ATP.
Subsequent studies were performed using single microvascular smooth
muscle cells that were freshly isolated from rat preglomerular microvasculature. Intracellular calcium signaling in response to P2
receptor activation was examined by using the calcium-sensitive fluorescent probe fura 2. The effect of selective P2 receptor stimulation on the intracellular calcium concentration was determined under control conditions, during calcium channel blockade, and during
exposure of the cells to nominally calcium-free conditions (Fig.
7). Those studies revealed that
stimulation of P2Y receptors with UTP stimulated a biphasic increase in
intracellular calcium concentration. The magnitude and time course of
the calcium response was nearly identical compared with similarly
treated cells bathed in calcium-free medium (Fig. 7) and also during
calcium channel blockade with diltiazem (82). These data
indicate that the renal microvascular smooth muscle cell response to
UTP involves an increase in intracellular calcium that arises primarily
by mobilizing calcium from intracellular stores. Preliminary data
indicate that P2X receptor activation with ,
-methylene-ATP
stimulates an increase in intracellular calcium concentration that is
highly dependent on the influx of extracellular calcium (Fig. 7). The
mechanism of the calcium influx pathway remains to be determined, but
preliminary studies suggest that it is Ni2+ sensitive and
can be significantly attenuated by calcium channel blockade
(173). The dependency of P2X receptor activation on the
influx of calcium in these cells is consistent with the role of calcium
influx noted for P2X receptor-mediated afferent arteriolar vasoconstriction (79, 84, 173).
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Exposure of cells to a high concentration of ATP results in an elevation of intracellular calcium concentration that depends partly on the influx of calcium through L-type calcium channels and partly on the mobilization of calcium from intracellular stores (82, 86). ATP administration elicits a biphasic increase in intracellular calcium concentration that is similar in magnitude and time course to that elicited by an equimolar concentration of UTP (Fig. 7). However, unlike the example with UTP, removal of calcium from the extracellular bathing medium reduces the magnitude of the response to ATP by ~50%. Nearly identical results are obtained when ATP is administered during calcium channel blockade (82, 86).
Conflicting results, using isolated rabbit afferent arterioles exposed to ATP or UTP, have been reported (61). ATP administration increases intracellular calcium concentration through mechanisms that appear to be entirely dependent on calcium influx. No evidence of a calcium mobilization component was observed. In addition, UTP treatment has no detectable effect on intracellular calcium concentration in similarly treated arterioles. These data suggest that there might be significant species variation in the P2 receptor population expressed by afferent arterioles of rabbit and rat kidney or that P2 receptors on these arterioles may involve alternative signal transduction pathways.
Nevertheless, these data establish that stimulation of P2 receptors on preglomerular vascular smooth muscle increases intracellular calcium concentration through multiple signal transduction pathways, possibly involving multiple P2 receptor subtypes. In rat renal microvascular smooth muscle cells, P2 receptor activation can increase calcium by stimulating calcium mobilization from intracellular stores and by stimulating calcium influx via voltage-dependent calcium influx pathways. In addition, these data lend further support to the hypothesis that afferent arteriolar smooth muscle cells express both P2X and P2Y receptor subtypes that utilize distinct calcium signaling mechanisms to influence afferent arteriolar function.
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DIADENOSINE POLYPHOSPHATES |
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Diadenosine polyphosphates are a unique group of naturally occurring nucleotide agonists that also activate P2 receptors (52, 117, 135, 142). The original descriptions of diadenosine polyphosphates in human physiology appeared in the literature in the early 1980s and described the release of diadenosine polyphosphates from human platelets (51, 100). Those early studies led investigators to suggest that diadenosine phosphates may play an important physiological role in the control of platelet aggregation. In the ensuing years, several structurally distinct and biologically active diadenosine polyphosphates have been described (142, 161, 164-166).
Diadenosine polyphosphates exist as two adenosine molecules linked by a variable number of phosphate groups. The number of phosphates is indicated by the number substituted for the X in the formula APXA. Therefore, diadenosine pentaphosphate is noted as AP5A. The number of phosphates present can have a significant impact on the biological activity of each diadenosine polyphosphate species (52, 58, 161, 164-166). The vascular response can also vary markedly depending on the specific vascular tissue being studied, the ambient vascular tone of the sample tissue or vascular bed, and the status of the endothelium (52, 142). Also important is the type of P2 receptor expressed by the tissue being studied (174).
Presently, there are several reports describing the effect of these agents on renal hemodynamics and renal function (69, 93), the renal microvasculature (58, 141, 161, 164-166), mesangial cells (96, 139, 144, 160), and cortical collecting duct cells (140). Bolus intravenous injection of AP4A, AP5A, or AP6A into the anesthetized rat resulted in a rapid and transient decline in renal blood flow that was coincident with a transient decline in heart rate, cardiac output, and mean arterial pressure (93). Assessment of renal function after intravenous injection of AP6A increased urine flow and sodium excretion, whereas AP3A or AP4A reduced urine flow and sodium excretion (69). These changes in renal function were observed without any detectable effect on glomerular filtration rate.
Intrarenal infusion of AP4A, AP5A, or AP6A into isolated perfused kidneys increases renal perfusion pressure, suggesting that these agents increase renal vascular resistance (164-166). The vasoconstriction can be observed at agonist concentrations as low as 1.0 nM (164). The renal vasoconstriction could be attenuated with the P2 receptor antagonist PPADS (164-166) and was completely blocked by combining PPADS with the P1 receptor blocker 8-cyclopentyl-1,3-dipropylxanthine (165). This observation suggests that the vasoconstrictor response is mediated partly through activation of P2X receptors and partly through activation of adenosine-sensitive P1 receptors. Large-caliber renal resistance vessels between 200 and 250 µm respond to AP4A, AP5A, and AP6A with a concentration-dependent contraction (161). The magnitude of the response was similar for each agent. The vasoconstriction evoked by AP4A could be attenuated by removal of calcium from the extracellular medium and was blocked by the calcium channel blocker nifedipine or by the nonselective P2 receptor antagonist PPADS (161).
Subsequent experiments revealed that AP3A and AP5A exert differential effects on the microvasculature of the hydronephrotic kidney (58). These agents induced transient vasoconstriction of interlobular arteries and afferent arterioles but had little effect on efferent arteriolar diameter. The vasoconstrictor response to AP3A was completely inhibited during adenosine A1 receptor blockade and was attenuated during adenosine A2 receptor blockade or with P2 receptor blockade, suggesting that P1 receptors play a major role in the response to this diadenosine polyphosphate. Vasoconstrictor responses elicited by AP5A were attenuated slightly by adenosine receptor blockade but were more sensitive to P2 receptor blockade with PPADS. These data suggest that AP5A-mediated vasoconstriction in this model is predominately mediated through P2 receptor activation (58). Therefore, it seems clear that diadenosine polyphosphates can directly alter renal microvascular diameter and resistance and that the length of the polyphosphate chain is an important consideration in determining the mechanism by which diadenosine polyphosphates influence microvascular function. The complex nature of the possible interactions between diadenosine polyphosphates and P1 and P2 receptors will certainly require considerable more study to resolve.
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WHAT PHYSIOLOGICAL ROLE DO RENAL P2 RECEPTORS FULFILL? |
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This important question is not likely to be answered in the near future. The reason for this is the wide distribution of P2 receptors throughout the kidney (3, 24, 111). P2 receptors are found on renal microvascular smooth muscle cells and endothelial cells, glomerular epithelial cells, proximal tubular cells, cultured mesangial cells, and cultured epithelial cells. Efforts are underway in several laboratories to determine the role each plays in specific tissues, but the complete picture will take time to develop.
Neural Control
One possibility is that P2 receptors serve as postsynaptic receptors that bind ATP released from sympathetic nerve terminals (3, 134, 149). Schwartz and Malik (149) have reported that low-frequency renal nerve stimulation increased renal vascular resistance through a mechanism that was insensitive toMesangial Cells
Glomerular epithelial cells and mesangial cells also respond to P2 receptor stimulation, suggesting a possible role for P2 receptor-mediated regulation of glomerular function. The majority of the older data, as well as more current reports, are consistent with the presence of a P2Y2 receptor subtype on mesangial cells, although the presence of P2X7 receptors has also been suggested (146). Extracellular ATP and UTP induce contraction of cultured rat mesangial cells (124) in concert with an elevation of intracellular calcium concentration (62, 71, 124, 155). The increase in intracellular calcium arises from the mobilization of calcium from intracellular stores and from the influx of calcium from the extracellular medium (62, 124). Calcium channel blockade with verapamil failed to alter the magnitude or time course of ATP-mediated increases in intracellular calcium concentration, suggesting that calcium influx occurs through a mechanism independent of voltage gated L-type calcium channels (124).ATP and UTP have also been reported to exhibit equal potency in stimulating phosphatidylcholine hydrolysis through activation of phospholipase D (131). Pretreatment with pertussis toxin significantly attenuates IP3 and 1,2-diacylglycerol (DAG) generation (129). Inhibition of protein kinase C enhances IP3 and DAG formation (129), whereas enhancement of protein kinase C activity with phorbol 12-myristate 13-acetate enhances phospholipase D activity (131). A recent report indicated that P2 receptor activation by ATP or UTP inhibited the activation of adenylate cyclase activity (145). This inhibition could only be observed during measures designed to elevate adenyl cyclase activity above baseline. No effect of these agents on baseline cAMP formation could be detected (71, 145). These data suggest that regulatory G proteins and protein kinase C are important modulatory pathways in the mesangial cell response to P2 receptor activation by ATP. The role of cAMP in the response is less clear.
Exposure of cultured mesangial cells to ATP or UTP appear to have differing effects on the magnitude and time course of changes in membrane potential (71, 124). Patch-clamp experiments reveal that ATP stimulated a transient membrane depolarization (71, 124), whereas UTP stimulated a sustained depolarization (71). Similarly, during the depolarization phase, ATP induced a transient increase in whole cell conductance, whereas UTP evoked a sustained increase in whole cell conductance (71). Reduction of the extracellular chloride concentration augmented the depolarization in response to ATP and UTP, suggesting that chloride channels participate in the depolarization response (71, 124). Interestingly, the depolarization induced by ATP can be inhibited by the nonselective P2 receptor blockers suramin and reactive blue 2, but the response to UTP is unaltered. These data suggest that responses elicited by ATP and UTP may arise from different P2 receptors.
Diadenosine polyphosphates are also known to influence mesangial cell function and thus may play a role in the control of glomerular hemodynamics. Exposure of mesangial cells to diadenosine polyphosphates stimulates membrane depolarization through activation of chloride channels and nonselective cation channels (96). Consistent with membrane depolarization and activation of nonselective cation channels, AP3A is reported to stimulate a sustained elevation in intracellular calcium concentration and mesangial cell contraction when external calcium is in the normal range but only a transient elevation in intracellular calcium concentration when external calcium is removed (139). In contrast, AP4A, AP5A, and AP6A appear to increase intracellular calcium concentration exclusively by stimulating calcium influx through voltage-gated L-type calcium channels (160). Whether these conflicting data reflect agonist-specific calcium signaling pathways or are due to experimental variation is not clear. Nevertheless, elevation of intracellular calcium is a primary response of mesangial cells challenged with diadenosine polyphosphates. Most of these responses appear to be sensitive to P2 receptor blockade with suramin or PPADS but are insensitive to adenosine A1 receptor blockade, suggesting that the responses arise through selective activation of P2 receptors (160).
The physiological or pathophysiological role of P2 receptors expressed by cultured mesangial cells remains to be determined; however, a couple of conflicting possibilities have recently been proposed. P2 receptor activation has been implicated in mesangial cell proliferation (73-75, 147, 148) and also in triggering apoptosis and necrosis (146). Exposure of mesangial cells to ATP stimulated a significant elevation in cell number, thymidine incorporation, and prostaglandin E2 production (129, 148). Similar treatment with UTP resulted in a smaller proliferative response. The mesangial cell response to ATP involves the mitogen-activated protein kinase cascade (73) and the stress-activated protein kinase cascade (74, 75). In another study, a 90-min exposure of mesangial cells to extracellular ATP caused an increase in DNA fragmentation and an increase in tumor suppressor p53 protein, which is thought to regulate apoptosis (146). Further studies implicated activation of the P2X7 receptor in the apoptotic response to ATP and confirmed the presence of P2X7 receptor-associated pore formation in the presence of ATP. Collectively, these studies suggest a possible role for mesangial P2 receptors in inflammatory cell proliferation and apoptosis (130, 147).
Vascular Smooth Muscle Cell Proliferation
The mitogenic actions of extracellular ATP have been known since 1992 (170), and these observations have been confirmed by many others using different approaches and cell systems (45, 45-47, 65, 103, 170). ATP has been shown to stimulate DNA synthesis (45-47, 170) and protein synthesis (45, 47) in vascular smooth muscle cells cultured from rat aorta and vena cava and human subcutaneous arteries and veins. Stimulation of the proliferative response appears to involve activation of P2Y receptors and is reported to occur in response to a wide variety of nucleotide agonists, including ATP, UTP, UDP, ADP, and AP4A (45-47, 65, 103). P2Y receptor activation initiates a broad series of intracellular events, culminating in cell growth and proliferation. Included in the signaling pathways appear to be crucial steps for tyrosine kinase (45, 46) and mitogen-activated protein kinase (45, 65). Similar studies examining the potential role of extracellular nucleotides to stimulate, or regulate, preglomerular vascular smooth muscle cell growth have not been performed. Nevertheless, the implication is intriguing that locally released adenine nucleotides could participate in renal microvascular remodeling, under physiological or pathophysiological conditions.Autoregulatory Control
The kidney has the unique ability to regulate renal blood flow and glomerular filtration rate at near-constant levels over a wide range of arterial pressures. The phenomenon of renal blood flow autoregulation has been recognized for many years; however, our understanding of the specific mechanisms by which it occurs remains incomplete. It is known that autoregulatory alterations in renal vascular resistance involve sustained adjustments in preglomerular resistance and that most of the resistance change occurs at the afferent arteriole. Interestingly, ATP-mediated renal vasoconstriction occurs exclusively at the preglomerular microvasculature, with afferent arterioles exhibiting sustained responses (Fig. 4) (85). Autoregulatory adjustments in afferent arteriolar diameter are sensitive to calcium channel blockade (Fig. 6) as are ATP-mediated afferent arteriolar responses (84). Therefore, experiments were performed to test the postulate that afferent arteriolar P2 receptors play an important role in mediating renal autoregulatory responses.Studies were conducted in vitro using the juxtamedullary nephron technique in the isolated rat kidney and examined the effect of P2 receptor inactivation on autoregulatory behavior (81). P2 receptor inactivation was accomplished by imposing P2 receptor desensitization or saturation and by pharmacological blockade. The results of those studies revealed that inactivation of P2 receptors by receptor desensitization abolished pressure-mediated autoregulatory reductions in afferent arteriolar diameter (81). An alternative approach to inactivation of the P2 receptor influence on autoregulatory responses was implemented by clamping the extracellular ATP concentration by adding high concentrations of ATP to the exogenous medium. Under these conditions, the influence of endogenously released ATP would be overwhelmed by the exogenous ATP present in the bathing medium. Under ATP-clamp conditions, pressure-mediated afferent arteriolar autoregulatory responses were completely blocked (81). Interestingly, this inhibition of autoregulatory behavior was obtained despite the fact that afferent arteriolar responsiveness to other non-P2 vasoconstrictor agonists such as KCl, angiotensin II, or norepinephrine was well preserved (81).
In another series of experiments, we examined the effect of
pharmacological inactivation of P2 receptors on autoregulatory behavior
(Fig. 8). Autoregulatory responsiveness
was examined before and during exposure of the vasculature to the
nonselective P2 receptor antagonists PPADS or suramin
(81). Under control conditions, elevation of renal
perfusion pressure in 30-mmHg increments from 100 to 130 and 160 mmHg
resulted in appropriate pressure-mediated reductions in afferent
arteriolar diameter. However, during blockade of P2 receptors with
PPADS, the pressure-mediated vasoconstrictor responses were abolished
and afferent arteriolar diameter remained unchanged. Suramin
administration consistently increased baseline afferent arteriolar
diameter. Similar to the effect of PPADS on autoregulatory behavior, P2
receptor blockade with suramin resulted in a marked attenuation
of the pressure-mediated vasoconstrictor responses (81).
Therefore, each experimental manipulation designed to selectively
inactivate afferent arteriolar P2 receptors simultaneously blocked
normal afferent arteriolar autoregulatory behavior. The confirmation
that afferent arterioles retained normal responsiveness to other
vasoconstrictor stimuli supports the contention that the functional
integrity of the contractile apparatus remained intact during P2
receptor inactivation but that the ability of the arteriole to respond
to a pressure stimulus was impaired. These fundamental observations are
consistent with selective inhibition of the autoregulatory response by
P2 receptor inactivation and strongly implicate P2 receptor activation
as an essential step in pressure-mediated autoregulatory adjustments in
afferent arteriolar diameter. These data also suggest that ATP may be
constitutively released, which underscores the necessity to quantify
how much ATP is being released into the interstitial fluid under basal and stimulated conditions.
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Complimentary studies were performed in vivo, in the dog kidney, where
the extracellular ATP concentration was clamped at a high level while
the effect of alterations in renal arterial pressure on renal blood
flow and glomerular filtration rate was examined (101). In
these experiments (Fig. 9), the kidney
was pretreated with the nitric oxide synthase inhibitor
nitro-L-arginine to eliminate the vasodilation that
accompanies infusion of ATP into the renal artery of the dog
(102). As shown in Fig. 9, renal blood flow was well
autoregulated over renal arterial pressures ranging from ~75 to 150 mmHg under control conditions (101). During the ATP-clamp
period, renal blood flow declined proportionally with each decrement in
renal perfusion pressure (Fig. 9). Control experiments confirmed that
impairment of autoregulatory responsiveness during the ATP clamp could
not be attributed to a nonspecific ATP-dependent elevation in renal
vascular resistance, because a similar reduction in autoregulatory
efficiency was not observed when renal vascular resistance was
increased to a similar extent with norepinephrine. Therefore, selective
inhibition of a P2 receptor-dependent signaling pathway appears to lead
to simultaneous impairment of autoregulatory behavior. Taken together,
the in vitro and in vivo evidence obtained from two different species,
under different experimental conditions, strongly support the
hypothesis that renal microvascular P2 receptors play a central role in
mediating pressure-dependent autoregulatory adjustments in afferent
arteriolar diameter.
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Tubuloglomerular Feedback
Autoregulation of renal blood flow and glomerular filtration rate occurs through the combined influences of myogenic- and tubuloglomerular feedback (TGF)-dependent mechanisms. Regulation of preglomerular resistance through TGF-dependent influences involves alterations in afferent arteriolar diameter in response to signals transmitted from the macula densa. Although there is general agreement that the TGF mechanism is an important contributor to the overall regulation of renal hemodynamics, identification of the "signal" that links macula densa function with alterations in preglomerular resistance has not been accomplished.Recent data assessing the effect of P2 receptor inactivation on
autoregulation support the hypothesis that extracellular ATP activates
afferent arteriolar P2 receptors to simulate autoregulatory adjustments
in preglomerular resistance. If these changes in afferent diameter
occur as a consequence of TGF-mediated signals, then pressure-mediated
stimulation of afferent arteriolar vasoconstriction should occur in
response to an increase in the concentration of ATP in the interstitial
fluid of the renal cortex. Nishiyama et al. (114)
performed experiments in the dog, using the dialysis tubing technique
of Siragy and Linden (152), in an attempt to address this
hypothesis. Experiments were designed to measure changes in renal
cortical interstitial ATP concentration in response to alterations in
renal arterial pressure. The concentration of ATP in the
interstitial fluid was estimated by measuring the ATP concentration
equilibrium in dialysate collected from microdialysis tubing implanted
in the renal cortex. The results of those studies revealed that
the concentration of ATP measured in the dialysate, equilibrated
with renal cortical interstitial fluid, varied in direct proportion
with renal arterial pressure (Fig. 10,
left). Additionally, the change in interstitial ATP
concentration was also significantly correlated with pressure-mediated
changes in renal vascular resistance (114). Interestingly,
whereas changes in interstitial ATP concentration correlated directly
with changes in renal arterial pressure, no relationship was observed
between renal arterial pressure and interstitial adenosine
concentration (114). These data are consistent with the
hypothesis that acute elevations in renal arterial pressure stimulate
an elevation in renal interstitial ATP concentration leading to an
elevation in renal vascular resistance. The data do not support a
significant role for interstitial adenosine in mediating autoregulatory
adjustments in renal vascular resistance.
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In similar experiments, Nishiyama et al. (114) assessed the effect of manipulating the TGF mechanism on interstitial ATP concentration. The TGF mechanism was stimulated by administering acetazolamide to increase distal tubular solute delivery. Alternatively, TGF activity was blocked by administering furosemide (Fig. 10, right). As predicted by the TGF hypothesis, increasing TGF signaling by increasing distal tubular solute delivery resulted in a significant elevation of the renal interstitial ATP concentration (114). Subsequent inhibition of TGF signals with furosemide evoked a rapid decline in interstitial ATP concentration to values below those obtained under control conditions.
More recent preliminary data from Bell et al. (7) suggest the possible detection of ATP release from the basolateral aspect of macula densa. This release was measured by using a novel biosensor designed to detect P2 receptor activation after exposure of macula densa cells to an osmotic stimulus (7). When the biosensor, expressing P2 receptors, was placed adjacent to the basolateral surface, detectable membrane currents were obtained. In contrast, similar responses were not detected when the biosensor was moved a considerable distance away from the macula densa cells. Although these data are highly preliminary, they do mark a clever and novel approach to determining whether macula densa cells are capable of controlled release of ATP into the pericellular space in response to a stimulus. These correlative data are consistent with the hypothesis that activation of TGF signals from the macula densa stimulates ATP release into the renal cortical interstitial fluid, where it activates P2 receptors on afferent arterioles to induce vasoconstriction.
Using an in vivo micropuncture approach, Mitchell and Navar
(106) demonstrated that infusion of ATP, or the P2X
agonist ,
-methylene-ATP, into the peritubular capillary
circulation attenuated TGF responsiveness (106). In those
studies, an ATP clamp was imposed by infusing high concentrations of
these P2 agonists into the peritubular circulation while the
TGF-mediated stop-flow pressure response to increases in late proximal
tubule perfusion rate was being measured. Under control conditions,
increasing late proximal perfusion rate from 0 to 40 nl/min caused a
marked reduction in stop-flow pressure (Fig.
11), consistent with TGF-mediated
afferent arteriolar vasoconstriction. However, during simultaneous
peritubular capillary infusion of P2 agonists, the stop-flow pressure
response to an identical increase in proximal perfusion rate was
markedly attenuated (Fig. 11, left). Similar responses could
not be obtained during administration of adenosine (Fig. 11,
right) or the nonselective adenosine receptor blocker 1,3 dipropyl-8-p-sulfophenylxanthine (106). These
data suggest that P2 rather than P1 receptors are responsible for the
noted reduction in stop-flow pressure during P2 agonist infusion and
support the hypothesis that P2 receptor activation is a central
mechanism involved in TGF-mediated autoregulatory adjustments in
afferent arteriolar resistance.
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FINAL PERSPECTIVES |
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More questions than answers are being generated as investigators continue to probe the involvement of P2 receptors in various physiological and/or pathophysiological conditions. In the course of past investigations, much has been learned about the pharmacology of P2 receptor agonists and antagonists and about the varied nature of P2 receptors that mediate biological responses. These studies have given rise to new ideas regarding the possible roles these receptors might play in regulating aspects of organ and tissue function. However, from a physiological perspective, many important questions remain before a serious case can be made that P2 receptors are integrally involved in regulating organ or tissue function. The postulate that P2 receptors are involved in the physiological regulation of renal microvascular function and autoregulatory behavior is comparatively new and not firmly established. Nevertheless, the data presently available provide compelling support for the involvement of P2 receptors in regulating renal microvascular function. The nature of the nucleotide signal remains to be identified. Is it an adenine nucleotide or nucleoside, or could it be one of several diadenosine polyphosphate compounds? What role is played by pyrimidine nucleotides like UTP and UDP? What role, if any, does adenosine play in these responses? What is the source of the extracellular messenger, and what is the mechanism for its release to the interstitial fluid? What role, if any, does the macula densa play in the P2 receptor-dependent regulation of afferent arteriolar diameter? What receptor subtype or subtypes are involved in the microvascular and/or glomerular response to extracellular nucleotides. Are the functional receptors on the microvasculature in the monomeric form, or are they perhaps expressed as heteromeric receptor complexes that might confer unique, tissue-specific properties for the selective regulation of tissue function? With continued effort, answers to these and other questions will be obtained. Progress in this field awaits the development of new and better receptor agonists and antagonists with which to selectively probe receptor function and the intracellular signaling pathways involved. Adoption of new molecular approaches promises to provide a greater understanding of the pharmacology of these receptors and lead to the development of engineered animal models with which to assess the roles of discreet receptor subtypes. Regardless of the limitations presently faced, the challenges that lie ahead are not insurmountable. Persistent efforts will undoubtedly yield new insights into the role that P2 receptors play in regulating renal tubular and microvascular function. With these new insights, the renal P2 receptor system may emerge as a major new mechanism responsible for the physiological regulation of renal function.
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ACKNOWLEDGEMENTS |
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The author thanks Anthony K. Cook, Bao Thang Pham, Elizabeth A. LeBlanc, Alan C. Schroeder, and Steven M. White for excellent technical assistance. The author also thanks Drs. John D. Imig and Robert A. Johnson for critical review of the manuscript.
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FOOTNOTES |
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The author has been supported by National Institute of Diabetes and Digestive and Kidney Diseases Grants R29-DK-44628 and RO1-DK-44628, Grants-in-Aid from the Louisiana affiliate of the American Heart Association, and an Established Investigator Award from the National American Heart Association. Earlier work was supported by the National Kidney Foundation. The studies on the renal microvascular response to P2 receptor stimulation were performed by the author while in the Department of Physiology at the Tulane University School of Medicine.
Address for reprint requests and other correspondence: E. W. Inscho, Dept. of Physiology, Medical College of Georgia, 1120 15th St., Augusta, GA 30912-3000 (E-mail: einscho{at}mail.mcg.edu).
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