1 Nephrology and Hypertension Services and 2 Diabetes Unit, Hadassah University Hospital, Jerusalem, Israel 91120
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Psammomys obesus lives in an arid environment and feeds on saltbush. When animals are fed a laboratory diet, urine osmolarity drops. To explore the mechanism(s) of water conservation, we measured renal function, kidney solute content, Na-K-ATPase activity, and mRNA in several groups: group I (saltbush diet, 18 g/day, 4.2 g protein); group II (laboratory diet, 10 g/day, 1.8 g protein); and group III, the same as group I, and group IV, the same as group II, both plus a 1-day fast. Urine osmolarity was 2,223 ± 160, 941 ± 144, 1,122 ± 169 and 648 ± 70.9 mosM in groups I, II, III, and IV, respectively. Tissue osmolarities in cortex, outer medulla, and inner medulla, respectively, were 349 ± 14, 644 ± 63, and 1,152 ± 34 µosM/mg tissue in group I; 317 ± 24, 493 ± 17, and 766 ± 60 µosM/mg tissue in group II; 335 ± 6, 582 ± 15, 707 ± 35 µosM/mg tissue in group III; and 314 ± 18, 490 ± 22, and 597 ± 29 µosM/mg tissue in group IV. There were no differences in Na-K-ATPase activity and mRNA in cortex and in medulla between groups I and II, whereas in group III Na-K-ATPase activity and mRNA increased in cortex and outer medulla. These results suggest a key role for urea in corticomedullary osmotic gradient of Psammomys. The absence of differences in Na-K-ATPase activity and mRNA between groups I and II despite differences in tissue sodium concentrations is consistent with Na-K-ATPase-independent Na absorption. Increased Na-K-ATPase activity and mRNA in fasting suggest transition to Na-K-ATPase- dependent Na transport.
sodium-potassium-adenosinetriphosphatase; mRNA of - and
-subunits
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
THE FAT SAND RAT, Psammomys obesus, lives in the most arid areas of Sahara-Arabian deserts, from Algeria to Sudan in North Africa eastward to Arabia. In Israel, it lives in the Negev, the Judean desert, and the "Arava" (7, 26, 27). Psammomys differs from other desert geribillids in that it is diurnally active above ground all year round. The other geribillids are nocturnal animals and remain in burrows during the day (10). Psammomys is able to thrive to reproduce and to grow when feeding entirely on leaves and stems of plants belonging to the Chenopodiaceae family. In the Israeli deserts, it feeds solely on Atriplex halimus, the saltbush (3).
A. halimus has a high moisture content and thus provides
much of the needed water. It is also readily available throughout the
year. Because the burrows of the Psammomys are at the base of this plant, little energy is expended for foraging. In addition, no
other rodents are feeding on A. halimus. The A. halimus saltbush is low in energy content and as such has a
relatively low efficiency of energy for maintenance when consumed by
Psammomys (15). Thus the animal consumes large
amounts of this plant for survival. The saltbush has a high nitrogen
and electrolyte content (Table 1).
Psammomys scrapes the salt-coated outer layer of the leaves with its teeth before consuming them (14, 15), thus
removing many of the electrolytes.
|
The distinctive and specialized features of Psammomys lifestyle has aroused much interest among physiologists. The structural organization of Psammomys kidney has revealed that, contrary to what has been earlier reported in the literature, that most nephrons have long loops of Henle (18, 24), this species has short (66%) and long (34%) nephrons alike (13). The ultrastructure of the thin limbs of Henle of these short-looped and long-looped nephron segments reveals specialized structure and function (1, 2). From micropuncture studies, it seems that in Psammomys the NaCl and urea addition to the thin descending limb of Henle's loop plays a major part in the concentration mechanism (4, 5).
It is widely accepted that Na-K-ATPase, the enzymatic equivalent of the Na-K pump, which is present in a high concentration in the kidney, is responsible for the active transport of sodium and potassium across the tubular epithelium (16, 23). The importance of this enzyme in creating the hyperosmolarity of the medulla is well established. Thus the highest activity of Na-K-ATPase was found in the thick ascending limb of Henle's loop (11, 16, 22).
Psammomys feeds on a saltbush diet, excretes urine with high
osmolarity, and, when kept in the laboratory and fed a laboratory diet,
its urine osmolarity falls dramatically. The above findings raised many
questions regarding the effect of environmental factors, and
principally dietary composition and consumption of food, on renal
concentrating ability in Psammomys. More specifically, we faced the question of what components of the diet may play a key role
in the urinary concentrating ability: is it the salt content, nitrogen
content, or caloric intake? The present study was undertaken to further
explore and define in more detail the renal mechanism(s) of the
adaptation of Psammomys to its natural arid environment. Similarly, we find it very important to define the alteration that
takes place when the animals are removed from their natural environment
and are kept in the laboratory. For this purpose, we employed in the
present study dietary manipulations including fasting for 24 h but
allowed free access to water. This may allow us to delineate in more
detail the dependence of the renal mechanism of urine concentration on
dietary regimens. Renal function, aldosterone levels, kidney solute
content, Na-K-ATPase activity, and mRNA of - and
-subunits of
Na-K-ATPase were measured in Psammomys fed a saltbush
(natural) diet, a laboratory diet, and in fasting animals fed both diets.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Animal experiments were approved by the Institutional Animal Welfare Committee.
P. obesus pups of the Hebrew University strain were weaned to different diets: A. halimus (SB; group I); laboratory diet (ALD; group II); saltbush diet plus 1-day fast (SB-FA; group III); and laboratoy diet plus 1-day fast (ALD-FA; group IV). The compositions of the diets are given in Table 1.
The animals were kept individually for 6 wk and were fed as above. The saltbush leaves that served as the native diet were picked from their natural habitat, the Dead Sea area. In the fasted groups, the animals were fed their suitable diet before entering a 24-h fast. All groups drank tap water ad libitum.
By the end of the 6-wk period, the day before the experiments the animals were housed individually in metabolic cages, and groups I and II were given access to their respective diets. Fluid and food intake and 24-h urine excretion were measured. Blood was drawn from the bifurcation of the aorta under light ether anesthesia.
All groups were subdivided into three subsections. In one, the kidneys
were removed for solute content determination. In the second, the
kidneys were removed, immediately decapsulated, weighed, and kept on
ice. Slices of cortex, outer medulla, and inner medulla were cut and
pooled separately for enzyme preparation. In the third, the kidneys
were excised and dissected into cortex, outer medulla, and inner
medulla for measurement of cellular mRNA levels of - and
-subunits of Na-K-ATPase.
Kidney solute content determination.
Psammomys were exsanguinated from the bifurcation of the
aorta under light ether anesthesia. The kidneys were removed, and their
water and solute composition were determined by the method of Levitin
et al. (19). Briefly, slices of cortex, outer medulla, and
inner medulla were dissected and processed. Electrolyte and osmolar
content were determined from the tissue extracts, and urea was
calculated by subtracting measured electrolytes from tissue osmolality:
osmoles (2 Na+ + 2 K+). We also
measured urea content directly and found no differences between the
measured and calculated urea concentrations.
Preparation of microsomes.
Preparation of the microsomal ATPase was carried out essentially
according to Jørgensen and Skou (12).
Psammomys tissues pooled from at least four animals were
homogenized in 10 vol of a medium containing 0.25 M sucrose and 2 mM
EDTA buffered with 5 mM Tris · HCl to a pH of 7.4-7.5. The
homogenate was centrifuged at 7,000 g for 15 min; the
supernatant was decanted and centrifuged at 48,000 g for 40 min. The pellet was resuspended in an equal volume of the above
solution and again homogenized in 10 vol of desoxycholate 0.1%
containing 2 mM EDTA and 25 mM Tris · HCl (pH 7.0). After
incubation at 37°C for 30 min, the suspension was centrifuged at
25,000 g for 30 min. The pellet was suspended in the above
sucrose-EDTA-Tris. This final suspension was frozen at 20°C until assayed.
Assay of ATPase. ATPase activity was determined by the amount of Pi released during incubation at 37°C in a shaking, thermostatic bath, according to Gutman et al. (9). All assays were run in duplicate. The Pi release was measured with and without K+ in the medium. The standard incubation medium consisted of (in mM) 100 NaCl, 10 KCl, 4 MgCl2, and 4 ATP. Enzymatic activity was stopped by the addition of 10% trichloroacetic acid. Pi was determined according to the method of Fiske and Subbarow (6). Enzymatic protein was assayed according to Lowry et al. (20). Na-K-ATPase was estimated as the difference in Pi release in the medium without and with K+.
Measurement of cellular mRNA levels (Northern blots).
RNA was extracted from Psammomys kidney slices, and the
levels of mRNA for - and
-subunits of Na-K-ATPase were measured by Northern blots. After extraction with Tri Reagent kit (Molecular Research Center, Cincinnati, OH), RNA (10 µg) was denatured and ethidum bromide was added to each sample at a concentration of 0.1 mg/ml. The samples were size fractionated by electrophoresis in 1%
agarose gels containing formaldehyde and transferred to nylon membrane
(Gene Screen; New England Nuclear Research Products, Boston, MA) by
diffusion blotting. The integrity of the RNA and the uniformity of RNA
transfer to the membrane were determined by ultraviolet (UV)
visualization of the ribosomal RNA bands of the gels and the filters.
The filters were fixed by UV cross-linking. Membrane strips were
hybridized for 16-20 h with 32P-labeled cDNA fragments
corresponding to
-Na-K-ATPase and
-Na-K-ATPase under stringent
conditions. The radioactive probe was prepared with a Rediprime DNA
labeling kit (Amersham). The hybridizations were performed with
Pst I/EcoR I fragment of the
-subunit of Na-K-ATPase (nucleotide 3060-3636) and EcoR I fragment
of
-subunit of Na-K-ATPase (nucleotide 343-1600). Membranes
were washed and autoradiographed by standard procedures. Bound cDNA
probes were removed by 1-2 min of boiling in 1× standard sodium
citrate+0.1% sodium dodecyle sulfate, and the same membranes were
hybridized with a control probe synthesized from a cloned fragment of
-actin cDNA. The abundance of this cDNA/RNA species was independent
of any of the treatments described in this study. Bindings were
quantified by phosphorimaging (Fujix, BHS 1000) and expressed as the
ratio of intensities obtained by hybridizing the same stripe with the cDNA studied and
-actin cDNA, respectively, as control gene. Each
result was confirmed by repeating the Northern hybridization with four
different RNA preparations.
Chemical determinations. Blood, urine, and tissue extracts were analyzed for sodium, potassium, and osmolality. Blood and urine samples were also analyzed for creatinine, whereas blood was taken also for aldosterone level determinations. Sodium and potassium were analyzed by flame photometry (Instrumentation Laboratory). Osmolality was measured by a Fiske osmometer (Fiske Associates). Urea concentration was determined by automated enzymatic ultraviolet test, urease/glutamate dehydrogenase (GLDH) method, and creatinine concentration was determined by an automated picric acid method, with autoanalyzer of Cobas Mira (Hoffmann-La Roche and Limited Diagnostica, Basel, Switzerland). Aldosterone levels in the plasma were determined by radioimmunoassay (Coat-A-Count, Diagnostic Products, Los Angeles, CA).
Creatinine clearance and urinary excretion of sodium and potassium were calculated. Data are presented as statistical evaluation among the four groups. Results were presented as means ± SE for the enzymatic assay, four determinations from different membrane preparations, each pooled from four animals. Results between individual groups were compared by a nonpaired Student's t-test with a modified level of significance according to the Bonferroni method (8). All reagents were purchased from Sigma, St. Louis, MO. ![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Food intake of Psammomys fed the saltbush and
laboratory diet is given in Table 2. The
data on food intake are normalized to animal food intake per animal.
There is a marked difference in fresh material intake of SB animals
compared with ALD animals (18.10 ± 0.64 vs. 10.92 ± 1.14 g/day, P < 0.001). Total protein intake of SB animals
was 43% higher than in ALD animals (2.39 ± 0.085 compared with
1.68 ± 0.17 g/day, P < 0.005). Water intake, however, did not differ between the two groups, whether fed or fasting. In group I it was 10.2 ± 3.5 ml/24 h,
in group II 10.3 ± 1.52 ml/24 h, in group
III 12.2 ± 3.14 ml/24 h, and in group IV
14.5 ± 3.2 ml/24 h.
|
The NaCl content of the natural saltbush was obviously strikingly higher than that of laboratory diet.
Blood aldosterone, sodium, potassium, and creatinine levels in the
experimental groups are given in Table 3.
This is the first report on aldosterone levels in Psammomys
fed on their native diet. Aldosterone levels were significantly lower
(2.20 ± 0.57 ng%) in Psammomys fed on their native
saltbush diet compared with animals fed the laboratory diet (37 ± 7.3, P < 0.001). In ALD-FA, aldosterone levels
remained elevated (30.4 ± 7.9 ng%), whereas in the SB-FA animals
fed their native food, the saltbush leaves, it rose markedly compared
with fed animals (14.2 ± 5.38 ng%, P < 0.05).
|
It is of interest that hypokalemia was observed in fasting animals,
groups III and IV. This could reflect the
kaliuretic effect of aldosterone in the absence of dietary intake of
potassium. The serum sodium of the fasted animals of both groups was
significantly lower compared with fed animals (P < 0.001). Blood creatinine and creatinine clearance (see also Tables
4 and 5)
remained unchanged, indicating the lack of effect of fasting on
glomerular function.
|
|
Metabolic data of SB and SB-FA Psammomys are listed in Table 4. Creatinine clearance did not differ between fed and fasted animals. Reduction by 50% was observed in the urine osmolarity of the SB-FA animals (1,122 ±169 compared with 2,223 ± 160 mosM, P < 0.001). This reduction was not associated with a significant decrease in the diuresis of these animals. Marked natriuresis and kaliuresis were found in Psammomys eating saltbush compared with those fasting.
Metabolic data for ALD and ALD-FA Psammomys are presented in Table 5. Again, as in the SB and SB-FA animals, there were no differences in the creatinine clearance between fed and fasted animals maintained on a laboratory diet. A decrease in urinary osmolarity in ALD-FA animals was observed but did not reach a statistically significant difference.
Renal solute composition.
Renal solute composition in Psammomys fed with saltbush and
laboratory diets and in the fasted animals is presented in Table 6 and Fig.
1, where Kan represents anions associated
with K.
|
|
Cortex. In the cortex, osmolarity and electrolyte and urea contents are very similar among SB and ALD animals or in ALD-FA. By contrast, in the SB-FA animals (group III), the sodium content increased and urea content decreased (Table 6, Fig. 1).
Outer medulla. In the outer medulla of ALD and ALD-FA animals (groups II and IV, respectively), the osmotic gradient is decreased compared with in SB and SB-FA animals (groups I and III, respectively). The sodium and urea contents in ALD and ALD-FA animals were lower compared with SB animals. The osmolarity of the outer medulla of SB-FA animals (group III) is still high, with sodium content even higher than that found in SB animals (group I) (249 ± 13 compared with 209 ± 25 µosM/mg tissue, P not significant). At the same proportion, the urea content in this group of animals is decreased compared with fed animals (180 ± 14 and 309 ± 30 µosM/mg, respectively, P < 0.005). Thus, in the face of reduced tissue osmolarity, there is a change in the solute composition featuring decreased urea and increased NaCl content in SB-FA animals (group III).
It appears, therefore, that in SB-FA animals the decrement in urea is partly replaced with an increment in salt content, which helps preserve the tissue osmolarity.Inner medulla. There is marked reduction in the osmotic gradient of the inner medulla of ALD-FA Psammomys compared with all other groups of animals. In this group, the osmolarity and sodium and urea contents were decreased compared with the other three groups (Fig. 1, Table 6).
The marked differences of the inner medulla osmolarity among the different groups are in sodium and urea content. The osmolarity in the inner medulla of the SB-FA Psammomys (group III) accrued from the high sodium content compared with in ALD-FA animals (whereas the urea content is similar).Na-K-ATPase activity.
Na-K-ATPase activity in the cortex, outer medulla, and inner medulla of
the four groups is depicted in Fig. 2.
There was no difference in enzyme activity between both fed animal
groups I and II.
|
Na-K-ATPase gene expression.
Na-K-ATPase gene expression in animals on the different diets is
presented in Fig. 3 for the -subunit
and in Fig. 4 for the
-subunit.
|
|
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The present study examined the renal function, aldosterone levels,
kidney solute content, Na-K-ATPase activity, and gene expression of its
- and
-subunits in Psammomys. The studies were
conducted both in animals nourished with native and with laboratory
diets, both in fed and fasting animals. As expected, urinary osmolarity correlated directly with the magnitude of kidney tissue osmotic gradient. It is noteworthy that the osmotic concentration gradient was
determined by the type of diet. Thus animals that were fed native
saltbush leaves had a higher osmotic tissue concentration gradient than
those that were fed with a laboratory diet.
A saltbush diet (A. halimus) is high in water and nitrogen (Table 1) (15, 26) but low in energy content, which makes Psammomys consume large quantitites for maintenance of energy balance. Thus this plant creates a paradox for Psammomys. On one hand, Psammomys receives much preformed water from the saltbush, but, on the other hand, the high nitrogen content of the plant requires much water for excretion.
Animals fed on a saltbush diet consumed significantly more protein than animals fed a laboratory diet. The animals fed a saltbush diet consumed 2.39 ± 0.85 g/day of protein compared with laboratory-diet-fed animals that consumed 1.18 ± 0.17 g/day of protein. Thus the nitrogen content in the saltbush diet is 40% higher than in the laboratory diet (Table 2).
The osmolarity of the outer medulla of saltbush-fed animals is high compared with animals fed a laboratory diet. This was accounted for by a 40% increase in NaCl and 20% rise in urea content (Fig. 1, Table 6).
Although animals fed on native saltbush exhibited a medullary osmotic concentration that was 1.5 times higher than that in laboratory-diet-fed animals, surprisingly no significant difference was found between the two groups with regard to Na-K-ATPase activity.
The differences in medullary osmolarity without parallel changes in Na-K-ATPase activities lend support to the thesis that the mechanism which produces these disparities in osmotic gradients of Psammomys kidney medulla is primarily a Na-K-ATPase-independent sodium absorption mechanism.
Thus it may be proposed that in the animals fed saltbush the high dietary protein content with higher urea concentration in the medulla may account for a Na-K-ATPase-independent sodium absorption mechanism that can reflect a passive mechanism of urinary concentration in Psammomys (17, 25).
A model of a passive urinary concentrating mechanism, which assigns a major role for urea, was published by Stephenson (25) and Kokko and Rector (17). Urea in the medullary interstitium extracts salt-free water from the descending limb, causing the concentration of NaCl in the descending limb to rise above that in the surrounding interstitium, setting the stage for the passive outward movement of NaCl from the thin ascending limb and removing the necessity for an energy-requiring transepithelial pump in the thin ascending limb.
In agreement with above observations respective to Na-K-ATPase activity
no differences were observed between the two subgroups (I
and II) with regard to gene expression of - and
-subunits of sodium pump (Figs. 3 and 4).
It is of interest that Ohtaka et al. (21) found that urea
does not increase Na-K-ATPase 1- and
1-subunit mRNA accumulation in primary cultures of inner medullary collecting duct cells of the
rat. On the other hand, they observed upregulation of Na-K-ATPase
1-
and
1- subunit mRNA by hyperosmolarity induced by poorly permeating
solutes other than urea. This indicated that an increase in osmolarity
per se does not induce Na-K-ATPase. These findings may be pertinent to
our results that failed to show changes in enzyme activity despite
changes in tissue osmolarity.
To further assess the role of dietary nitrogen in urinary concentrating
mechanism, we used a different experimental manipulation. The animals
were not fed for 24 h, thus depriving them of their source of
urea. No significant changes in tissue osmolarity of kidney cortex were
noticed between fed and fasted animals consuming the same diet. The
composition of solutes in SB-FA animals (group III),
however, was changed; urea was replaced with equivalent osmoles of
sodium (Fig. 1, Table 6). These alterations in solute composition were
associated with a commensurate increase in Na-K-ATPase activity (Fig.
2). The osmolarity in the outer medulla of SB-FA animals (group
III) was higher than that in ALD-FA animals (group IV).
This difference between the two fasted groups was primarily due to
increased salt content in the former group. Furthermore, the salt
content in the SB-FA animals was even higher than that in SB animals.
The Na-K-ATPase activity changed accordingly (Fig. 2). Moreover, the
gene expression of - and
-subunits in the cortex and in outer
medulla increased significantly in SB-FA animals (Figs. 3-5).
There was no change in Na-K-ATPase activity in ALD-FA animals
(group IV), and the increase in -subunit failed to reach
statistical significance (Fig. 3). We cannot rule out, however, the
possibility that longer fasting of these animals could eventually
increase the enzyme activity and its gene expression.
As opposed to outer medulla, the inner medulla tissue osmolarity decreased in the fasting state both in saltbush- and laboratory diet-consuming animals. This fall in osmolarity reflected decreases in both sodium and urea content. This phenomenon can be explained by the anatomic differences between the outer and inner medullary portions of Henle's loop (2, 4, 13). In the outer medullary segments, the abundance of sodium pumps plays an important role in driving sodium transport, whereas in the inner medullary portions that mainly consist of thin limbs, sodium absorption is mediated mainly by Na-K-ATPase-independent mechanisms. This is reflected in very low enzyme activity measured in the inner medulla and the presumed dependence of sodium transport on a passive mechanism related to urea (17, 25). Therefore, presumably the shortage of dietary nitrogen in the fasting state reduced the urea content (Table 6, Fig. 1) necessary for Na-K-ATPase independent sodium reabsorption, resulting in a decline in tissue osmolarity.
Taken together, these results suggest that intergroup differences in dietary protein may account for the increased urea content of the medulla and the higher tissue and higher urine osmolarity in SB animals (group I) compared with ALD animals (group II). Thus urea could play a key role in the corticomedullary gradient in Psammomys. The absence of differences in Na-K-ATPase activity and its gene expression between groups I and II despite marked differences in renal tissue sodium and urea concentrations is consistent with predominance of Na-K-ATPase-independent sodium absorption mechanism in Psammomys kidney. By contrast, in SB-FA animals (group III), the observed increases in Na-K-ATPase activity and in its gene expression suggest transition to a Na-K-ATPase-dependent sodium transport mechanism. This change may help renal concentrating ability in the face of reduced urea supply due to food and protein deprivation.
![]() |
FOOTNOTES |
---|
Address for reprint requests and other correspondence: P. Scherzer, Nephrology and Hypertension Services, PO Box 12000, Jerusalem, Israel 91120 (E-mail: popovtzer{at}Hadassah.org.il.).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 18 January 2000; accepted in final form 21 August 2000.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Barrett, JM,
Kriz W,
Kaissling B,
and
de Rouffignac C.
The ultrastructure of the nephrons of the desert rodent (Psammomys obesus) kidney. I. Thin limb of Henle of short-looped nephrons.
Am J Anat
151:
487-498,
1978[ISI][Medline].
2.
Barrett, JM,
Kriz W,
Kaissling B,
and
de Rouffignac C.
The ultrastructure of the nephrons of the desert rodent (Psammomys obesus) kidney. II. Thin limbs of Henle of long-looped nephrons.
Am J Anat
151:
499-514,
1978[ISI][Medline].
3.
Degen, AA,
Kam M,
and
Jungrav D.
Energy requirements of fat sand rats (Psammomys obesus) and their efficiency of utilization of the salt-bush Atriplex halimus for maintenance.
J Zool
215:
443-452,
1988[ISI].
4.
De Rouffignac, C,
and
Morel F.
Micropuncture study of water, electrolytes and urea movements along the loops of Henle in Psammomys.
J Clin Invest
48:
474-486,
1969[ISI][Medline].
5.
De Rouffignac, C,
Morel F,
Moss N,
and
Roinel N.
Micropuncture study of water and electrolyte movements along the loop of Henle in Psammomys with special reference to magnesium, calcium and phosphorus.
Pflügers Arch
344:
309-326,
1973[ISI][Medline].
6.
Fiske, CH,
and
Subbarow Y.
The colorimetric determination of phosphorus.
J Biol Chem
66:
375-400,
1925
7.
Frenkel, G,
and
Kraicir PF.
Metabolic pattern of sand rats (Psammomys obesus) and rats during fasting.
Life Sci
11:
209-222,
1972[ISI].
8.
Godfrey, K.
Comparing the means of several groups.
New Engl J Med
313:
1450-1456,
1985[Abstract].
9.
Gutman, Y,
Hochman S,
and
Wald H.
The differential effect of Li on microsomal ATPase in cortex, medulla and papilla of the rat kidney.
Biochim Biophys Acta
298:
284-290,
1973[ISI][Medline].
10.
Ilan, M,
and
Yom-Tov Y.
Dial activity pattern of a diurnal desert rodent Psammomys obesus.
J Mammal
71:
66-69,
1990[ISI].
11.
Jørgensen, PL.
Structure, function and regulation of Na-K-ATPase in the kidney.
Kidney Int
29:
10-20,
1986[ISI][Medline].
12.
Jørgensen, PL,
and
Skou JC.
Preparation of highly active (Na+-K+)-ATPase from the outer medulla of rabbit kidney.
Biochem Biophys Res Commun
37:
39-46,
1969[ISI][Medline].
13.
Kaissling, B,
de Rouffignac C,
Barrett JM,
and
Kriz W.
The structural organization of the kidney of the desert rodent Psammomys obesus.
Anat Embryol
148:
121-143,
1975[ISI][Medline].
14.
Kam, M,
and
Degen AA.
Water electrolyte and nitrogen balances of fat sand rats (Psammomys obesus) when consuming the saltbush Atriplex halimus.
J Zool
215:
453-462,
1988[ISI].
15.
Kam, M,
and
Degen AA.
Efficiency of use of saltbush (Atriples halimus) for growth by fat sand rats (Psammomys obesus).
J Mammal
70:
485-493,
1989[ISI].
16.
Katz, AI,
and
Epstein FH.
The role of sodium potassium-activated adenosine triphosphatase in the reabsorption of sodium by the kidney.
J Clin Invest
46:
1999-2011,
1967[ISI][Medline].
17.
Kokko, JP,
and
Rector FC, Jr.
Countercurrent multiplication system without active transport in inner medulla.
Kidney Int
2:
214-223,
1972[ISI][Medline].
18.
Lechène, C,
Corby C,
and
Morel F.
Distributions des néphrons accessible á la surface du rein en fonction de la longueur de leur anse de Henle chez le rat, le hamster, le mèrion et la Psammomys.
C R Acad Sci Série D
262:
1126-1129,
1966[ISI].
19.
Levitin, H,
Goodman A,
Pigeon G,
and
Epstein FH.
Composition of the renal medulla during water diuresis.
J Clin Invest
41:
1145-1151,
1962[ISI].
20.
Lowry, OH,
Rosebrough NJ,
Farr AL,
and
Randall RJ.
Protein measurement with the Folin-phenol reagent.
J Biol Chem
193:
265-275,
1951
21.
Ohtaka, A,
Muto S,
Nemoto J,
Kowakami K,
Magano K,
and
Asano V.
Hyperosmolarity stimulates Na-K-ATPase gene expression in inner medullary collecting duct cells.
Am J Physiol Renal Fluid Electrolyte Physiol
270:
F728-F738,
1996
22.
Schmidt, U,
and
Dubach UC.
Activity of (Na+K+)-stimulated adenosintriphosphatase in the rat nephron.
Pflügers Arch
306:
219-226,
1969[ISI][Medline].
23.
Skou, JC.
Enzymatic basis for active transport of Na+ and K+ across cell membranes.
Physiol Rev
45:
596-617,
1965
24.
Sperber, J.
Studies on the mammalian kidney.
Zool Bidrag Uppsala
22:
249-431,
1944.
25.
Stephenson, JL.
Concentration of urine in a central core model of the renal counterflow system.
Kidney Int
2:
85-94,
1972[ISI][Medline].
26.
Ziv, E,
Kalman R,
Hershkop K,
Barash V,
Shafrir E,
and
Bar-On H.
Insulin resistance in the NIDDM model Psammomys obesus in the normoglycemia, normoinsulinemic state.
Diabetology
139:
1269-1275,
1996.
27.
Ziv, E,
and
Shafrir E.
Psammomys obesus: nutritionally induced NIDDM-like syndrome on a "thrifty gene" background.
In: Lessons From Animal Diabetes, edited by Shafrir E.. London: Smith-Gordon, 1995, p. 285-300.