Renal tubular sites of increased phosphate transport and NaPi-2 expression in the juvenile rat

To determine the tubular sites and mechanisms involved in enhanced renal phosphate (P(i)) reabsorption seen in the juvenile animal, renal micropuncture experiments were performed in acutely thyroparathyroidectomized adult (>14 wk old) and juvenile (4 wk old) male Wistar rats fed either a normal P(i) diet (NPD, 0.6% P(i)) or low P(i) diet (0.07% P(i)) for 2 days, in the presence and absence of parathyroid hormone (PTH). P(i) reabsorption was greater in proximal convoluted (PCT) and straight tubules (PST) of the juvenile compared with adult rats fed NPD, whether or not PTH was present. These findings were consistent with a greater P(i) uptake in brush-border membrane (BBM) vesicles from both superficial (SC) and outer juxtamedullary (JMC) cortices of juvenile animals. Western blot analysis revealed a 2- and 1.8-fold higher amount of NaPi-2 protein in the SC and JMC, respectively, in juvenile rats. Immunofluorescence microscopy also indicated that NaPi-2 protein expression was present in the proximal tubule (PT) BBM to a greater extent in juvenile rats. Dietary P(i) restriction in juvenile rats resulted in a significant increase in P(i) reabsorption in the PCT and PST segments. NaPi-2 expression in the PT BBM was also increased, as was the expression of intracellular NaPi-2 protein. These studies indicate that P(i) reabsorption in both the PCT and PST segments of the renal tubule contributes to the attenuated response to PTH in the normal juvenile animal. In addition, dietary P(i) restriction in the juvenile rat upregulates BBM NaPi-2 expression, which is associated with a further increase in proximal tubular P(i) reabsorption.

Evidence for induction of a phosphate appetite in juvenile rats

This study examined whether dietary phosphate (Pi) restriction stimulates an appetite for Pi in the juvenile rat, which normally has a high metabolic Pi demand for growth. Juvenile Wistar rats were placed in individual cages with unrestricted access to tap water and a low (LPD, 0.02% Pi) or normal Pi diet (NPD, 0.6% Pi) for 7 days. On day 8, both groups of rats were given unlimited access to a solution of 0.3 M potassium phosphate water (PiH2O) for 8 additional days. Rats fed LPD consumed 70-100% more PiH2O then those rats fed NPD (P < 0.001). The increase in PiH2O intake resulted in a marked rise in the growth rate of rats fed LPD during days 8-15. A similar Pi intake was inducible after only 2 days of LPD and was associated with significant reductions in both plasma and cerebrospinal fluid (CSF) Pi levels; these levels remained low throughout Pi restriction, despite a significant PiH2O intake. Furthermore, the renal adaptation to enhance Pi reabsorption (TmPi) during Pi deprivation remained elevated despite enhanced PiH2O intake. Replenishment with a high-Pi diet rapidly quenched the PiH2O appetite and was associated with restoration of both plasma and CSF Pi levels. These findings suggest that an appetite for Pi can be induced in juvenile rats, perhaps through lowered plasma and CSF Pi levels. This behavioral response may serve as an additional mechanism to maintain an adequate supply of Pi necessary for growth and development of the animal.

Changes in renal phosphate reabsorption in the aged rat

Depletion of inorganic phosphate (Pi) reserves occurs frequently in aged animals and can result in diminished bone mineralization and osteoporosis. This altered Pi balance results from a reduction in intestinal Pi absorption and an elevation in renal Pi excretion. Since the kidney plays a central role in maintaining Pi homeostasis, we tested whether the increased phosphaturia seen with aging is a consequence of changes in the intrinsic tubular capacity to reabsorb Pi (TmPi). Male Wistar rats (12-, 18-, and 24-months-old) were acutely thyroparathyroidectomized (TPTX) and prepared for renal clearance studies in the presence and absence of fixed levels of parathyroid hormone (synthetic PTH-(1-34), 1 U/kg/min). The maximum capacity for Pi transport (TmPi) was assessed by infusion of Pi at progressively higher rates (0-6 micromol/min) to increase the filtered load of Pi and facilitate the determination of the TmPi. TmPi declined significantly with age (3.51 +/- 0.12 vs 3.04 +/- 0.19 vs 2.30 +/- 0.18 micromol/ml, for 12-, 18-, and 24-month-old rats, respectively, P < 0.05) in TPTX rats. Administration of PTH markedly reduced the TmPi in all age groups. Although the TmPi attained was similar among the age groups (1.15 +/- 0.13 vs 1.15 +/- 0.06 vs 1.03 +/- 0.09 micromol/ml, for 12-, 18-, and 24-month-old rats, respectively), the magnitude of the reduction in the presence of PTH declined from 67% in 12-month-old rats to 62% and 55% in 18- and 24-month-old rats, respectively. These results demonstrate that aging is associated with a PTH-independent decrease in the intrinsic capacity of the kidney to reabsorb phosphate. Further, the kidney of the aged rat can respond to a pharmacological dose of PTH with appropriate reductions in the TmPi although the magnitude of the response declines with age.

Impaired nitric oxide-mediated renal vasodilation in rats with experimental heart failure: role of angiotensin II

BACKGROUND

Congestive heart failure (CHF) is associated with a decrease in renal perfusion. Because endothelium-derived NO is important in the regulation of renal blood flow (RBF), we tested the hypothesis that an impairment in the NO system may contribute to the decrease in RBF in rats with experimental CHF.

METHODS AND RESULTS

Studies were performed in rats with experimental high-output CHF induced by aortocaval (AV) fistula and sham-operated controls. In controls, incremental doses of acetylcholine (ACh, 1 to 100 microg x kg(-1) x min(-1)) increased RBF and caused a dose-related decrease in renal vascular resistance (RVR). However, the increase in RBF and decrease in RVR were markedly attenuated in rats with CHF. Likewise, the effects of ACh on urinary sodium and cGMP excretion were also diminished in CHF rats, as was the renal vasodilatory effect of the NO donor S-nitroso-N-acetylpenicillamine (SNAP). These attenuated responses to endothelium-dependent and -independent renal vasodilators in CHF rats occurred despite a normal baseline and stimulated NO2+NO3 excretion and normal expression of renal endothelial NO synthase (eNOS), as determined by eNOS mRNA levels and immunoreactive protein. Infusion of the NO precursor L-arginine did not affect baseline RBF or the response to ACh in rats with CHF. However, administration of the nonpeptide angiotensin II receptor antagonist A81988 before ACh completely restored the renal vasodilatory response to ACh in CHF rats.

CONCLUSIONS

This study demonstrates that despite a significant attenuation in the NO-related renal vasodilatory responses, the integrity of the renal NO system is preserved in rats with chronic AV fistula. This impairment in NO-mediated renal vasodilation in experimental CHF appears to be related to increased activity of the renin-angiotensin system and may contribute further to the decrease in renal perfusion seen in CHF.

Temporal changes in insulin-like growth factor I, c-fos, and c-jun gene expression during hyperplastic kidney growth in weanling rats

We have previously determined that compensatory renal growth (CRG) during the initial 24-48 h after uninephrectomy (UNX) is GH independent in weanling animals, but associated with significant increases in insulin-like growth factor I (IGF-I) and IGF-I receptor gene expression. The purpose of the present study was to determine the temporal sequence of molecular and cellular events that occur at various time points (1, 6, 12, 18, 24, 48, and 72 h post-UNX) during this early period of accelerated renal growth in the weanling (21- to 25-day-old) rat. Rapid and sustained increases in steady state renal IGF-I receptor and IGF-I messenger RNA (mRNA) were observed at 1 and 6 h, respectively, and remained elevated in the remnant kidneys until 72 h post-UNX. The mRNAs for the early response genes, c-fos and c-jun, were not induced in the remnant kidneys from weanling rats until between 12-18 h, but were also sustained through 48 h post-UNX. Increases in remnant kidney DNA content and [3H]thymidine incorporation also occurred from 18-48 h post-UNX and returned to baseline levels by 72 h post-UNX, indicating that the hyperplastic response in the weanling remnant kidney occurs over a discrete period early after UNX. Neither IGF-I nor early response genes were elevated in kidneys from adult animals, which exhibited only hypertrophic renal growth at those early time points after UNX. These findings suggest that early CRG in the weanling rat is associated with rapid increases in IGF-I mRNA followed by a rise in c-fos and c-jun gene expression and a mitogenic response. Furthermore, when the mRNA levels of IGF-I and early response genes returned to baseline levels, mitogenic growth stopped, and slower prolonged hypertrophic renal growth ensued.

Does the Heart's Hormone, ANP, Help in Congestive Heart Failure?

In congestive heart failure, increased secretion of atrial natriuretic peptide (ANP) is an important compensatory mechanism that unloads the failing heart and promotes renal salt and water excretion. However, activation of opposing sodium-retaining factors, particularly the renin-angiotensin system, reduces renal responsiveness to ANP and shifts the cardiovascular system to a state of decompensation.

Morphometric analysis of atrial natriuretic peptide-containing granules in atriocytes of rats with experimental congestive heart failure

The morphometric characteristics of atrial natriuretic peptide-containing granules were studied in atrial myoendocrine cells of rats with aorto-caval fistula, an experimental model of congestive heart failure. A total of 6680 granules of control and aorto-caval rats were analyzed by a computerized image analysis system that evaluated the number and sectioned surface area of granules and their subcellular location. Compared with control animals, rats with congestive heart failure displayed a slight increase in the number of peripheral granules, adjacent to the sarcolemma, but not centrally located in the Golgi areas. The mean sectioned surface area of granules in rats with congestive heart failure was about 50% of that in controls, both in the right and left atria. Rats with aorto-caval fistula had a higher percent of small granules and lower percent of large granules compared with controls. The data demonstrate different morphometric characteristics in atrial natriuretic peptide-containing granules in atriocytes in rats with experimental congestive heart failure; this may reflect the enhanced synthesis and release of atrial natriuretic peptide in heart failure.

Early increase in pulsatile growth hormone release after unilateral nephrectomy in adult rats

Removal of one kidney results, within days, in accelerated growth of the remaining kidney. However, the mechanisms that underlie this compensatory renal hypertrophic response, particularly in the early time period following nephrectomy, are not understood. In this study we tested the hypothesis that removal of one kidney leads to a change in the pulsatile release of growth hormone (GH), which facilitates compensatory renal growth. Adult Wistar rats were implanted with Silastic cannulas in jugular veins and underwent either unilateral nephrectomy (UNX) or sham operation. Plasma levels of GH were determined 24 and 48 h after sham operation or UNX. Blood samples were taken every 20 min over a 6-h period from conscious, unrestrained animals. Pulsatile GH release was markedly elevated 24 h after UNX in both the amplitude of the surges as well as in the duration of release. Peak GH levels after 24 h were three- to fourfold higher in UNX rats compared with sham controls (417 +/- 75 vs. 119 +/- 23 ng/ml, P < 0.05). However, this enhanced release of GH appeared to be of short duration and began declining by 48 h post-UNX (peak level of 227 +/- 37 ng/ml, P < 0.05 vs. both 24 h UNX and sham controls). To examine whether this rise in GH release post-UNX contributed to the compensatory renal growth, rats underwent UNX and were immediately treated with an antagonist to GH-releasing factor (GRF-AN; i.e., [N-Ac-Tyr1,D-Arg2]GRF-(1-29) amide, 200 micrograms/kg twice daily), and the effects on GH release and renal growth were determined. Administration of GRF-AN significantly suppressed the increase in GH release post-UNX and was associated with a significant attenuation in renal growth 48 h post-UNX in GRF-AN-treated rats (8.7 +/- 2.6% vs. 22.7 +/- 3.0% in UNX controls, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Diuretic-natriuretic actions and pressor effects of big-endothelin (1-39) in phosphoramidon-treated rats

The effects of phosphoramidon, a metalloproteinase inhibitor, on the pressor and renal actions of big-endothelin (BET), the precursor of porcine Endothelin-1 (ET), was studied in rats. In control rats, BET (0.3, 1.0, and 3.0 nmol/kg) elicited a marked increase in mean arterial blood pressure (from 110 +/- 7 to 105 +/- 7, 120 +/- 8, 147 +/- 6 mm Hg, respectively), and a prominent, dose-dependent, diuretic and natriuretic response (fractional sodium excretion (FENa) increased from 0.4 +/- 0.2 to 0.8 +/- 0.2, 3.1 +/- 0.1, and 8.5 +/- 1.7%, respectively). Pretreatment with phosphoramidon (10 mg/kg + 0.25 mg/kg/min) completely abolished the increase in blood pressure induced by BET, but the diuretic-natriuretic effects were only partially inhibited (FENa increased from 2.0 +/- 0.9 to 3.7 +/- 1.5, 3.9 +/- 1.3, and 4.3 +/- 1.2%, respectively, P < 0.05). Rats treated with phosphoramidon only had no natriuresis over time (FENa changed from 1.9 +/- 0.5 to 2.3 +/- 0.3, 1.6 +/- 0.4, 1.7 +/- 0.6 respectively, P--NS). The data suggest that, unlike the vascular type of the enzyme, the renal endothelin converting enzyme is relatively insensitive to phosphoramidon. Further, diuresis and natriuresis can be induced by BET in the absence of any pressor effect.

Regulation of renal glomerular and papillary ANP receptors in rats with experimental heart failure

Rats with aortocaval (A-V) fistula, an experimental model of congestive heart failure (CHF), display high circulating atrial natriuretic peptide (ANP) levels and a markedly blunted natriuretic response to ANP infusion. The present study was designed to evaluate whether alterations in renal ANP receptors may contribute to renal hyporesponsiveness to ANP in experimental CHF. Densities (Bmax) and dissociation constants (Kd) of both the biologically active (ANPA) and clearance receptors (ANPC) were evaluated in glomerular and papillary membranes from A-V fistula rats (n = 18) and sham-operated controls (n = 20). ANPA and ANPC receptor subtypes were assayed by displacement of 125I-ANP-(99-126) bound to glomerular or papillary membranes by increasing concentrations of unlabeled ANP-(99-126) or des-(18-22)-ANP-(4-23), an analogue which binds only to ANPC. Seven days after the operation, rats with A-V fistula displayed avid sodium retention, elevated plasma renin activity, and approximately a 10-fold increase in plasma ANP levels. Bmax of total ANP binding sites was significantly decreased in rats with A-V fistula compared with controls (220 +/- 61 vs. 399 +/- 88 fmol/mg protein, P < 0.05). The decrease was mainly due to a reduction in ANPA receptor density (51 +/- 10 vs. 110 +/- 15 fmol/mg protein, P < 0.05) with no change in receptor affinity. Likewise, a significant reduction in the density of ANPA (23 +/- 5 vs. 64 +/- 10 fmol/mg protein, P < 0.05) with no change in receptor affinity was observed in papillary membranes of rats with A-V fistula.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a growth hormone-releasing factor antagonist on compensatory renal growth, insulin-like growth factor-I (IGF-I), and IGF-I receptor gene expression after unilateral nephrectomy in immature rats

We have recently reported that pulsatile GH secretion is elevated 24 h after unilateral nephrectomy (UNX) in adult rats. In addition, suppression of the increase in GH with an antagonist to GH-releasing factor (GRF-AN) significantly attenuated compensatory renal growth (CRG) in adult rats. The present study examined the role of GH in CRG in immature animals. Pulsatile GH release was determined 24 h post-UNX in immature (26-28 days of age) sham-operated and UNX male Wistar rats. In contrast to the adult UNX rats, no increase in GH secretion was seen in the immature UNX rats compared with that in the controls. When pulsatile GH release was suppressed with GRF-AN, there was preferential growth of the remnant kidney despite the attenuated gain in whole body weight. In addition, insulin-like growth factor-I (IGF-I) and IGF-I receptor mRNA levels were elevated 3-fold in the remnant kidneys of GRF-AN-treated rats, despite the suppression of pulsatile GH release. These findings suggest that the initial phase of CRG is GH independent in the immature rat and, further, that CRG is associated with an increase in IGF-I and IGF-I receptor gene expression that is independent of episodic GH secretion.

Altered expression of insulin-like growth factor-I (IGF-I) and IGF receptor genes after unilateral nephrectomy in immature rats

There is a developmental difference in the initial phase of compensatory renal growth (CRG) following unilateral nephrectomy (UNX), in that CRG is GH-dependent in adult rats and GH-independent in immature rats. Furthermore, CRG in immature rats is associated with an increase in renal IGF-I mRNA, an effect not seen in adult rats. In this study we have examined the age-related differences in expression of the insulin-like growth factor-I (IGF-I) and IGF-II genes as well as in IGF-I and IGF-II receptors and membrane binding after UNX. Immature (22-24 days of age) and adult (4 months of age) male Wistar rats underwent a sham operation or left UNX and were killed 24 or 48 h later. Levels of mRNA for IGF-I and IGF-II and their receptors were determined in the left (control) and right (compensated) remnant kidneys using solution hybridization/RNase protection assays. Steady state levels of IGF-I mRNA as well as IGF-I receptor and IGF-II/mannose-6-phosphate receptor mRNAs were increased 3- to 4-fold in immature remnant kidneys, but not in adult kidneys. The findings related to IGF-I gene expression were confirmed by in situ hybridization to immature and adult kidney slices. The increase in IGF-I gene expression in the immature remnant kidneys was localized to the thick ascending limbs of the loops of Henle. Furthermore, in concert with the changes in mRNA levels, membrane binding studies showed significant increases in specific binding to IGF-I in cortical membranes and increases in specific binding to IGF-II in whole kidney membranes from immature, but not adult, rats. Thus, these findings demonstrate that the initial phase of CRG in the immature rat is associated with increased renal IGF-I gene expression as well as enhanced specific renal binding of IGF-I and IGF-II to plasma membranes and support the notion that this period of rapid renal growth in the immature UNX rat may involve the paracrine influence of the IGFs.

Suppression of growth hormone release restores phosphaturic response to PTH in immature rats

Immature rats display a blunted rise in urinary phosphate but not adenosine 3',5'-cyclic monophosphate (cAMP) excretion in response to parathyroid hormone (PTH), perhaps as a consequence of the increased demand for phosphate during growth. Because a major driving force for growth is growth hormone (GH), and in view of the fact that GH has been shown to promote renal phosphate retention in the immature animal, it is possible that GH may attenuate the phosphaturic effect of PTH. The objective of this study was to determine whether suppression of pulsatile GH release, during administration of a synthetic peptide antagonist to GH-releasing factor, i.e., [N-acetyl-Tyr1-D-Arg2]-GRF-(1-29)-NH2 (GRF-AN), alters the renal response to increasing doses of PTH (1.5-15.0 micrograms.100 g-1.h-1) in the acutely thyroparathyroidectomized immature rat. Baseline fractional excretion of phosphate (FEPi), before administration of PTH, was negligible in all groups (less than 0.05%). Infusion of PTH resulted in an attenuated rise in FEPi in immature control rats compared with adult control rats (from 3.8 +/- 1.4% at lowest PTH dose to 16.7 +/- 3.1% at highest dose in immature rats vs. 21.1 +/- 3.5 to 31.9 +/- 4.4% in adult rats, P less than 0.05). In contrast, immature rats treated for 2 days with GRF-AN (100 micrograms/kg, twice daily) displayed an enhanced phosphaturic response (FEPi from 12.0 +/- 4.2 to 42.9 +/- 3.7%, P less than 0.05) compared with immature control rats, which was not different from that observed in control adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide and renal cGMP in rats with experimental heart failure

Rats with chronic aortocaval (AV) fistula, an experimental model of congestive heart failure, display high plasma levels of atrial natriuretic factor (ANF) and a blunted natriuretic response to ANF infusion. We previously reported that rats with AV fistula either develop progressive sodium retention (urinary sodium excretion, UNaV less than 100 microeq/24 h) or compensate (UNaV greater than 1,200 microeq/24 h). To gain further insight into the mechanism of renal hyporesponsiveness to ANF, we evaluated the effect of ANF on renal guanosine 3',5'-cyclic monophosphate (cGMP) production in sham-operated control rats and in the two groups of rats with AV fistula. Infusion of synthetic ANF-(99-126) (at either 10 or 50 micrograms.kg-1.h-1) resulted in a reduced fractional sodium excretion (P less than 0.05) in both compensated rats (0.7 +/- 0.2 and 7.9 +/- 1.6%) and sodium-retaining rats (0.3 +/- 0.1 and 0.5 +/- 0.1%) compared with controls (8.5 +/- 1.2 and 13.7 +/- 2.3% for low and high doses, respectively). Similarly, urinary cGMP excretion corrected by glomerular filtration rate (UcGMPV/GFR) during low-dose ANF infusion was significantly reduced (P less than 0.05) in both groups with AV fistula (compensated: 39 +/- 10 pmol/ml; sodium-retaining: 55 +/- 13 pmol/ml) compared with controls (115 +/- 16 pmol/ml). During high-dose ANF infusion, compensated rats, but not sodium-retaining rats, displayed a significant increase in UcGMPV/GFR. The differences in UcGMPV/GFR are probably not due to variations in urine flow because furosemide infusion to a separate group of rats with AV fistula increased urine flow approximately eightfold but did not increase UcGMPV/GFR.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal IGF-1 mRNA levels are enhanced following unilateral nephrectomy in immature but not adult rats

The increase in IGF-1 gene expression following unilateral nephrectomy (UNX) in adult rats is controversial. In this study we have examined whether developmental differences exist in the effect of UNX on IGF-1 gene expression. Immature (23 days) and adult (4 months) Wistar rats underwent a sham operation or left UNX, and were sacrificed 24 or 48 hrs later. IGF-1 mRNA levels were determined in left (control) and right (compensated) kidneys using solution hybridization/RNase protection assays. By 48 hrs post-UNX, remnant kidneys had grown 20 +/- 1% in adult rats (P less than 0.05), and 69 +/- 5% in immature rats (P less than 0.05). IGF-1 mRNA levels were not increased in the adult compensated kidneys at either 24 or 48 hrs post-UNX. In contrast, kidneys from immature rats 24 and 48 hrs post-UNX had an average 4-fold increase (P less than 0.05) in exon 1 IGF-1 mRNA levels, and an average 3-fold increase (P less than 0.05) in exon 2 mRNA levels. Thus, these findings suggest that there is an age-dependent difference in the effects of UnX on IGF-1 gene expression, and provide the first evidence that IGF-1 gene expression increases following unilateral nephrectomy in immature rats.

Regulation of renal phosphate reabsorption during development: implications from a new model of growth hormone deficiency

It has been hypothesized that the high rate of renal phosphate (Pi) reabsorption in the immature animal is a consequence of the increased demand for Pi associated with the rapid rate of growth. Although growth hormone (GH) has been proposed to play a role in this process, investigations of the relationship between GH, growth and the renal Pi transport have been hampered by the lack of methods available to specifically alter circulating GH levels. This review summarizes the findings from recent studies using a newly developed peptidic antagonist to GH-releasing factor (GRF-AN) as a method of specifically inhibiting GH release. Systemic injection of GRF-AN was effective in suppressing the pulsatile release of GH, and in significantly attenuating the rate of growth, in both immature and adult rats. However, the inhibition of growth was associated with a reduction in net Pi retention only in immature rats, resulting in a doubling in the urinary excretion of Pi. GRF-AN treatment of immature rats lead to a decrease in the maximum tubular capacity to transport Pi-down to the level seen in adult rats. However, GRF-AN treatment did not alter renal Pi reabsorption in adult rats. We conclude that chronic administration of an antagonist to GRF in rats provides a new model of GH deficiency with which to study the interrelationships between growth, GH and other physiological systems. Furthermore, the findings suggest that the pulsatile release of GH, directly or indirectly, contributes to the high rate of renal Pi reabsorption in young, growing animals and may play a critical role in regulating Pi homeostasis during development.

Effect of converting-enzyme inhibition on renal response to ANF in rats with experimental heart failure

Increased activity of the renin-angiotensin system is thought to play a major role in the pathogenesis of salt retention and edema formation in congestive heart failure. The present study evaluates the effects of chronic inhibition of angiotensin-converting enzyme on the response to infusion of exogenous atrial natriuretic factor (ANF) in salt-retaining rats with chronic arteriovenous (a-v) fistula, an experimental model of high-output congestive heart failure. Administration of ANF in incremental doses (5-50 micrograms.kg-1.h-1) to Inactin-anesthetized, sham-operated control rats resulted in dose-dependent increases in urine flow, sodium excretion, and glomerular filtration rate, and significant decreases in mean arterial blood pressure. These effects of atrial peptide were markedly attenuated in salt-retaining rats with a-v fistula. However, chronic oral treatment with the angiotensin-converting-enzyme inhibitor enalapril restored the natriuretic response of sodium-retaining rats with a-v fistula to high doses of ANF. At a dose of 50 micrograms.kg-1.h-1, fractional excretion of Na (FENa) in a-v fistula rats given enalapril was 4.0 +/- 0.5%, which was significantly greater than that in a-v fistula rats without enalapril (0.5 +/- 0.4%, P less than 0.05) and not different from the response in sham-control rats (4.9 +/- 0.7%). The improvement in the natriuretic response after enalapril was not associated with a significant increase in GFR and occurred despite a decrease in mean arterial pressure. Moreover, chronic enalapril treatment did not significantly alter the plasma levels of immunoreactive ANF in either the sham controls or in the rats with a-v fistula.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal adaptation to changes in dietary phosphate during development

The present study tested the hypothesis that the influence of dietary phosphate (Pi) on the renal handling of Pi changes during development. We evaluated whether variations in the dietary Pi content would alter the tubular capacity of Pi reabsorption [Max RPi/glomerular filtration rate (GFR)] in immature rats, which have a relatively greater Max RPi/GFR compared with adult rats. Then we examined the response of immature and adult Pi-deprived rats to dietary Pi replenishment. Studies were performed in acutely thyroparathyroidectomized Wistar rats at three different stages of development: immature (3- to 4-wk old), young (6- to 7-wk old), and adult (12- to 13-wk old). Animals were fed either low (0.07%)-, normal (0.7%), or high (1.8%)-phosphate diet (LPD, NPD, and HPD, respectively) for 4 days and were then prepared for renal clearance experiments to determine the Max RPi/GFR. On all dietary regimens, the Max RPi/GFR was highest in immature rats and decreased progressively with age. When fed LPD, immature rats, with an already elevated rate of phosphate transport, displayed a remarkable 68 +/- 13% increase in the Max RPi/GFR (from 5.58 +/- 0.29 to 9.47 +/- 0.76 mumol/ml, P less than 0.01). This was significantly greater than the 38 +/- 3% increase observed in adult rats (from 3.50 +/- 0.18 to 4.81 +/- 0.09 mumol/ml). Conversely, in response to HPD, the decrease in the Max RPi/GFR was smallest in immature rats (-42 +/- 2%) compared with the decrement in either young (-54 +/- 3%) or adult (-61 +/- 6%) rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphate Handling by the Kidney During Development: Immaturity or Unique Adaptations for Growth?

Renal retention of phosphate during development is not simply an expression of limited excretory capacity due to functional immaturity but rather is the direct result of unique adaptations that enhance the tubular reabsorption of phosphate in the neonate to meet its demand for phosphate during growth.

Antagonist to GH-releasing factor inhibits growth and renal Pi reabsorption in immature rats

Compared with adult rats, the immature rat has an enhanced tubular capacity for phosphate reabsorption, which presumably facilitates the growth process. Since the main driving force for growth is thought to be the pulsatile release of growth hormone, we examined the possibility that the adaptation in phosphate handling by the immature kidney is promoted by growth hormone (GH). To address this issue, we used a synthetic peptide antagonist to GH-releasing factor (GRF-AN) that we have shown blocks episodic GH secretion, and attenuates somatic growth. Immature male Wistar rats (4-5 wk of age) were catheterized with Silastic jugular cannulas and placed in metabolic cages. The rats were injected intravenously with either saline or GRF-AN (100 micrograms/kg) twice daily for 4 days. On the 4th day, they were prepared for renal clearance experiments to assess the maximum capacity for phosphate transport (TmPi). In animals treated with GRF-AN, there was an attenuated gain in body weight over 4 days of treatment (5 +/- 2 vs. 23 +/- 2% in saline controls, P less than 0.01). The suppressed growth was associated with a doubling of daily urinary phosphate excretion, and a reduction in the TmPi (3.3 +/- 0.1 vs. 4.6 +/- 0.3 mumol/ml in controls, P less than 0.01). A single injection of the antagonist to a separate group of immature rats did not alter TmPi. Thus injections of this new antagonist to GH-releasing factor over a 4-day period inhibit the pulsatile release of GH and significantly attenuate growth. The decline in growth of the immature rat was associated with a decrease in the renal capacity for phosphate reabsorption, down to levels seen in normal adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Restoration of a phosphaturic response to parathyroid hormone in the immature rat

Recent studies have shown that immature rats display a diminished sensitivity to the phosphaturic effects of parathyroid hormone (PTH), and that the responsiveness to PTH increases with age. The attenuated phosphaturia may reflect an inability of the neonate to respond to the hormone because of functional immaturity of the developing kidney. Alternatively, PTH may actually inhibit tubular phosphate reabsorption in the neonate but, due to other phosphate conservation mechanisms, no phosphaturia occurs. Our objective was to determine whether a phosphaturic response to PTH would be elicited in immature rats during infusion of moderate amounts of phosphate (Pi). Clearance experiments were performed on 26 acutely thyroparathyroidectomized immature Wistar rats (3-5 wk of age) fed a normal Pi diet (0.63%). In response to infusion of either Pi (1 mumol/min.100 g) (group I) or PTH (8.3 ng/min.100 g) (group II) alone, the fractional excretion of phosphate rose minimally (from 0.01 +/- 0.01% to 4.9 +/- 1.9% and from 0.12 +/- 0.12% to 2.9 +/- 1.4% for groups I and II, respectively). However, when Pi and PTH were combined either Pi first followed by PTH (group III) or PTH first followed by Pi (group IV), the fractional excretion of Pi rose dramatically (from 0.01 +/- 0.01 to 21.8 +/- 3.5% and from 0.04 +/- 0.04 to 27.7 +/- 3.3% for groups III and IV, respectively). A significant increase in urinary cAMP excretion occurred during infusion of PTH even when Pi excretion was minimal, but there was no further increase in urinary cAMP during the combined infusion of Pi and PTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of pulsatile growth hormone (GH) secretion and somatic growth in immature rats with a synthetic GH-releasing factor antagonist

We previously reported that systemic administration of the recently described GRF peptide antagonist (N-Ac-Tyr1,D-Arg2)GRF-(1-29)-NH2 to adult male rats would suppress the pulsatile release of GH. In the present study, we have sought to determine whether this same antagonist would be efficacious in immature male rats to block spontaneous GH secretion and, as a result, retard several parameters of somatic growth. Indwelling Silastic catheters were placed into the jugular veins of immature male rats (120-140 g) at 29 days of age. After a recovery period of 48 h, beginning at 1000 h, 100-400 micrograms/kg GRF antagonist or its vehicle (controls) were injected iv immediately after withdrawing an initial blood sample from conscious undisturbed animals. Subsequent samples were obtained every 20 min until 1520 h. Red blood cells were resuspended in a restorative volume of saline and reinjected after each blood sample. Results showed that both doses of antagonist prevented the two major periods of episodic GH release observed in controls. For example, mean plasma GH (+/- SEM; nanograms per ml) at 1120 h was 9.0 +/- 2.7 in antagonist-treated rats and 37.1 +/- 5.1 in controls (P less than 0.05). Mean plasma GH (+/- SEM) at 1340 h was 10.8 +/- 3.7 in antagonist-treated rats and 38.8 +/- 9.6 in controls (P less than 0.05). Injection of 400 micrograms/kg of the structurally related VIP antagonist (N-Ac-Tyr1,D-Phe2)GRF-(1-29)-NH2, iv failed to suppress spontaneous GH release. GRF antagonist (100 micrograms/kg) was next administered twice daily iv for 4 days to 31-day-old rats in metabolic cages. This treatment essentially arrested the normal rapid body weight gain, significantly suppressed increases in body and tail lengths, and reduced increases in heart and kidney weights (P less than 0.01). Food intake and fecal output were unchanged by antagonist treatment and, therefore, did not contribute to the observed effects. These results support the idea that a number of tissues and organs are stimulated by the pulsatile secretion of GH and that a peptidic GRF receptor antagonist is useful in blocking episodic GH release in immature animals. As a consequence, this specific antagonist is effective in suppressing numerous aspects of somatic growth.

Developmental changes in the tubular capacity for phosphate reabsorption in the rat

The need for young, immature animals to maintain positive phosphate balance for growth is well known. However, whether this process involves changes in the intrinsic capacity of the kidney to reabsorb phosphate is not clear. In the present study, the maximum capacity of phosphate reabsorption [Max RPi/glomerular filtration rate (GFR)] was measured in four groups of rats at different stages of development, from weanling to adulthood (3-4, 5-6, 10-14, and 52 wk of age). Clearance experiments were performed in acutely thyroparathyroidectomized (TPTX) rats in the presence and absence of fixed levels of parathyroid hormone (synthetic PTH-(1-34), 1 U.kg-1.min-1). Max RPi/GFR was determined with progressive infusions of phosphate (0-6 mumol Pi/min) that raised the filtered load of phosphate. Max RPi/GFR in TPTX 3- to 4- and 5- to 6-wk-old rats (5.55 +/- 0.36 and 4.28 +/- 0.18 mumol/ml, respectively) was significantly greater than in the corresponding 52-wk-old rats (3.51 +/- 0.13 mumol/ml, P less than 0.05). PTH decreased the Max RPi/GFR in all age groups. However, the developmental pattern was maintained, with the highest levels present in the youngest rats (2.79 +/- 0.25 mumol/ml, P less than 0.05) compared with the other age groups (1.92 +/- 0.23, 1.35 +/- 0.11, and 1.15 +/- 0.13 mumol/ml for 5- to 6-, 10- to 14-, and 52-wk-old rats, respectively). These results demonstrate that the tubular capacity for phosphate reabsorption per milliliter GFR is enhanced in immature rats and progressively decreases with age. This PTH-independent adaptation in young rats may contribute to the renal retention of phosphate during growth.

Hormonal determinants of sodium excretion in rats with experimental high-output heart failure

The present study evaluates the inter-relationship between the alteration in atrial natriuretic factor (ANF) and the renal handling of Na in rats with chronic aortocaval (a-v) fistula, an experimental model of congestive heart failure. Balance studies in these animals showed two distinct patterns of Na excretion: some rats developed progressive Na retention [urinary sodium excretion (UNaV) less than 100 mueq/24 h], whereas others compensated and returned to normal Na balance (UNaV greater than 1,200 mueq/24 h). Base-line plasma ANF levels were equally elevated in Na-retaining and compensated rats with a-v fistula (588 +/- 70 vs. 621 +/- 114 pg/ml, P, NS). However, the response of the two groups to exogenous administration of synthetic rat ANF-(101-126) in incremental doses varied greatly. ANF infusion increased the fractional Na excretion (FENa) in compensated animals from 0.12 +/- 0.03 to 2.6 +/- 0.5%, whereas the rise in FENa in Na-retaining animals was markedly blunted (0.11 +/- 0.06 to 0.89 +/- 0.35%). A similar pattern of ANF action was observed on the glomerular filtration rate and urine flow. The blunted response to ANF in the Na-retaining animals was associated with a marked increase in plasma renin activity (PRA) (35.6 +/- 6.9 vs. 4.5 +/- 0.7 ng ANG I.ml-1.h-1 in sham control rats, P less than 0.05) and plasma aldosterone levels (729.3 +/- 28.2 vs. 42.6 +/- 18.4 ng/dl in sham control rats, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dexamethasone on segmental phosphate reabsorption in phosphate-deprived rats

These experiments were designed to test the hypothesis that avid phosphate reabsorption by the pars recta accounts for the resistance to the phosphaturic effects of acute dexamethasone (DEX) and parathyroid hormone (PTH) infusions in rats fed a low-phosphate diet. Acute infusion of DEX [0.4 mg/(kg X h)] increased the fractional delivery of phosphate (FDPi) to the late proximal tubule from 7.1 +/- 2.1 to 14.4 +/- 3.5%, whereas FDPi to the early distal tubule and urine were not different. PTH alone [1 U/(kg X min)] increased FDPi to the late proximal tubule from 4.0 +/- 1.1 to 15.7 +/- 3.7%, whereas FDPi to the early distal tubule or urine was not different. The combination of DEX and PTH further increased FDPi to the late proximal tubule (32.7 +/- 6.4%) and resulted in an increase in fractional excretion of phosphate (FEPi), in spite of the fact that the FDPi to the early distal tubule was not significantly increased. The increased delivered load of phosphate to the pars recta following inhibition of phosphate transport in superficial proximal convoluted tubules resulted in a comparable increase in phosphate reabsorption in the pars recta, based on linear regression analysis, in rats fed low-phosphate diet but not in rats fed normal phosphate diet. These results demonstrate that acute infusion of DEX or PTH inhibits fractional phosphate reabsorption in the superficial proximal tubule but does not result in an increase in FEPi due at least in part to avid phosphate reabsorption in the superficial pars recta in rats fed low-phosphate diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthetic atrial natriuretic factor decreases renal tubular phosphate reabsorption in rats

Atrial natriuretic factor (ANF), a family of peptides isolated from cardiac atria, has marked effects on sodium excretion. A synthetic 26 amino acid sequence of ANF peptide has also been shown to be phosphaturic. However, it is difficult to assess whether the phosphaturia is due to changes in tubular reabsorption of phosphate without control of filtered load of phosphate. In the present study, the hypothesis that ANF peptide decreases tubular phosphate reabsorption was tested by using graded phosphate infusions of 0, 1, 2, and 3 mumol/min in thyroparathyroidectomized rats. Further, reabsorbed phosphate was similarly assessed in rats infused with parathyroid hormone (PTH) to allow comparison with a known phosphaturic hormone. ANF peptide decreased reabsorbed phosphate compared with saline controls (2.72 +/- 0.28 mumol/ml GFR compared with 3.35 +/- 0.35, P less than 0.05) but not as much as a maximally phosphaturic dose of PTH (2.04 +/- 0.13 mumol/ml GFR). We conclude that synthetic ANF peptide decreases tubular phosphate reabsorption in vivo.

Developmental changes in the phosphaturic response to parathyroid hormone in the rat

The need for young, immature rats to maintain positive phosphate balance for growth is well recognized. However, whether this process is associated with a resistance to the phosphaturic effect of parathyroid hormone (PTH) is not clear. In these experiments we examined the effect of PTH on urinary phosphate and cAMP excretion in rats at 3, 6, 12, and 20 wk of age. Clearance experiments were performed in acutely thyroparathyroidectomized (TPTX) rats fed a normal phosphate diet (0.86%). Basal fractional excretion of phosphate (FEPi) was low in all TPTX rats (less than 1%). The phosphaturic response to a high dose of PTH (1 U X kg-1 X min-1) increased with development (from 4 to 29%). The responses to increasing doses of PTH demonstrated a decrease in sensitivity to PTH in 6- compared with 20-wk-old rats. Urinary cAMP excretion (either per milliliter glomerular filtrate or per gram kidney weight) following PTH was not different among 6-, 12-, and 20-wk-old rats, thus demonstrating a dissociation between the increase in phosphate excretion and cAMP excretion. These results indicate that the phosphaturic response to PTH is blunted in immature, acutely TPTX rats and that the phosphaturia increases progressively with development.

Tubular capacity for phosphate reabsorption in superficial and deep nephrons

The present studies were performed to determine the capacity for phosphate reabsorption in superficial and deep nephron proximal tubules in vivo. Micropuncture experiments were performed in 20 acutely thyroparathyroidectomized (TPTX) Munich-Wistar rats fed a normal phosphate diet (0.7%). Four groups were infused with differing amounts of phosphate (0,2,4, or 6 mumol/min) to increase the filtered phosphate load. The sites selected for micropuncture were the superficial early distal tubule and the deep nephron loop of Henle, which reflect fractional phosphate delivery (FDPi%) from superficial and deep nephron proximal tubules, respectively. In response to phosphate infusions, plasma phosphate increased from 3.03 +/- 0.09 to 7.01 +/- 0.58 mM, and fractional phosphate excretion rose from 2 +/- 1 to 58 +/- 5%. FDPi% increased from both superficial (14 +/- 1 to 58 +/- 2%) and deep nephron proximal tubules (4 +/- 1 to 27 +/- 5%) but always remained lower from deep nephrons, reflecting more avid reabsorption by deep nephron proximal tubules. The maximal rate of phosphate reabsorption (max RPi/SNGFR) in the superficial proximal tubule was significantly less than in the deep nephron proximal tubule (3.2 +/- 0.4 vs. 5.1 +/- 0.1 pmol/nl). In seven of the phosphate-infused rats, parathyroid hormone (PTH, 33 U/kg bolus; 1 U X kg-1 X min-1) was added to the infusion following the initial collections. In the presence of PTH, the RPi/SNGFR was significantly lower in deep than in superficial proximal tubules (0.4 +/- 0.5 vs. 1.6 +/- 0.4 pmol/nl). Thus, the maximum capacity for phosphate reabsorption was greater in deep than in superficial nephrons in TPTX rats. Furthermore, in the presence of phosphate infusions, PTH inhibited phosphate reabsorption to a greater extent in deep than in superficial proximal tubules.

Renal handling of phosphate during acute respiratory acidosis and alkalosis in the rat

Clearance experiments were performed in acutely thyroparathyroidectomized rats to evaluate the renal handling of phosphate during respiratory acidosis (R ACID) and alkalosis (R ALK) in rats fed either a normal (0.7%) or low (0.07%) phosphate diet for 4 days. Different acid-base states were achieved by varying the mixture of carbon dioxide in the inspired air. Each group received graded infusions of phosphate to control for differences in plasma phosphate (PPi) and to determine the maximum transport capacity of phosphate reabsorption (TmPi/GFR). In rats fed a normal phosphate diet, PPi and the fractional excretion of phosphate (FEPi) were significantly greater in R ACID than in R ALK. However, there were no differences between R ACID and R ALK when FEPi was evaluated as a function of the PPi, and values for TmPi/GFR during R ACID were not different from those during R ALK. In rats fed low phosphate diet, PPi during R ACID was significantly greater than during R ALK, yet FEPi was less than 1% in all groups due to an adaptive increase in TmPi/GFR. Further, the TmPi/GFR was similar irrespective of the acid-base state. We conclude that acute respiratory acid-base changes do not alter the intrinsic capacity of the kidney to reabsorb phosphate.

Adenosine-induced decrease in renin release: dissociation from hemodynamic effects

Adenosine has been reported to produce a biphasic renal blood flow (RBF) response (vasoconstriction followed by a return of flow to control level) and a decrease in glomerular filtration rate (GFR) when infused into the kidney. Intrarenal adenosine infusion also leads to a decrease in renin release. By altering the hemodynamic response to adenosine, we sought to determine whether the decrease in renin release depends on vascular or filtration-induced events. In nine dogs with nonfiltering kidneys, adenosine infusion (3 X 10(-7) mol/min) resulted in a biphasic RBF response and an inhibition of renin release (309 +/- 53 vs. 71 +/- 26 ng ANG I/min). In 11 dogs treated with verapamil (10 micrograms X kg-1 X min-1) no vasoconstriction or decrease in GFR occurred; however, renin release was inhibited by adenosine (1,300 +/- 159 vs. 534 +/- 225 ng ANG I/min). In a third group of nine dogs whose ureteral pressure was raised to 80 cmH2O, adenosine infusion produced a sustained vasoconstriction and an inhibition of renin release (3,086 +/- 1,144 vs. 328 +/- 130 ng ANG I/min). These experiments, in which the renin release effects of adenosine are dissociated from the hemodynamic effects, lead us to conclude that the inhibition of renin release produced by adenosine does not depend either on the vascular or filtration-induced effects of adenosine.

Nephron heterogeneity of phosphate reabsorption: effect of parathyroid hormone

We evaluated the response of superficial and deep nephron proximal tubules to PTH in thyroparathyroidectomized (TPTX) rats fed a normal phosphate diet (0.7%). As phosphate reabsorption is not detectable in the ascending limb of the loop of Henle, fractional phosphate delivery (FDPi%) to the superficial early distal tubule and papillary loop of Henle reflects delivery from superficial and deep nephron proximal tubules, respectively. Re-collection micropuncture experiments were performed in nine acutely TPTX rats before and after the infusion of PTH (33 U/kg bolus; 1 U X kg-1 X min-1). In response to PTH, fractional phosphate excretion increased from 3.3 to 26.2% (P less than 0.05). FDPi% was less from the deep than from the superficial proximal tubule (5.7 vs. 15.7%, P less than 0.05) prior to PTH, indicating enhanced phosphate reabsorption by deep compared with superficial proximal tubules. During PTH infusion, FDPi% was increased in both nephron groups compared with control (P less than 0.05), but there were no differences in phosphate delivery between deep (28.0%) and superficial (29.7%) proximal tubules. We conclude that in acutely volume-expanded TPTX rats, infusion of a pharmacologic dose of PTH decreases phosphate reabsorption in both superficial and deep nephrons. Furthermore, the heterogeneity of FDPi% from deep compared with superficial proximal tubules seen in TPTX rats is absent during PTH infusion.

Nephron sites of action of nicotinamide on phosphate reabsorption

The administration of nicotinamide results in urinary phosphate excretions similar to those obtained with pharmacologic doses of parathyroid hormone (PTH). Free-flow micropuncture was performed to localize the nephron site(s) of inhibition of phosphate reabsorption by nicotinamide or PTH in thyroparathyroidectomized (TPTX) rats stabilized on a normal or low phosphate diet. In rats fed a normal phosphate diet phosphaturia was observed following either nicotinamide or PTH treatment. Nicotinamide inhibited phosphate reabsorption in the loop of Henle (pars recta) but not in the accessible proximal tubule. PTH inhibited phosphate reabsorption in both the accessible proximal tubule and the pars recta. In phosphate deprivation, the phosphaturic response to either nicotinamide or PTH was blunted. Although phosphate reabsorption was markedly inhibited in the accessible proximal tubule with both nicotinamide and PTH, subsequent reabsorption in the loop of Henle and distal tubule blunted the phosphaturia. We conclude that nicotinamide primarily inhibits phosphate reabsorption by the pars recta in rats fed a normal phosphate diet, whereas it inhibits phosphate reabsorption by the proximal convoluted tubule in rats fed a low phosphate diet. Furthermore, avid reabsorption of phosphate in the pars recta accounts for the resistance to the phosphaturic effect of nicotinamide or PTH seen in rats fed a low phosphate diet.

Role of hydrostatic and oncotic pressures in renal sodium reabsorption

Physical factors, and renal interstitial hydrostatic pressure in particular, have an important effect on sodium excretion by the kidney. Changes in hydrostatic and oncotic pressures in the peritubular microcirculation may have effects on proximal tubule reabsorption under some, but not all, circumstances. In regard to control of sodium excretion, the loop of Henle may be a particularly important segment which is sensitive to transepithelial hydrostatic pressure changes. There is little evidence to support an effect of physical factors on sodium reabsorption by the distal tubule. The collecting tubule may be another pressure-sensitive site; however, changes in sodium reabsorption by deep nephrons in the kidney may account for changes that have been attributed to the collecting duct. Changes in intrarenal pressure may be an important link in the regulation of sodium excretion, particularly in pathological circumstances, such as the exaggerated natriuresis of hypertension and the sodium retention seen in congestive heart failure.

Adaptation of deep and superficial nephrons to changes in dietary phosphate intake

Dietary phosphate intake is now recognized to be a primary regulator of renal phosphate excretion. However, the nephron sites involved in the adaptation to changes in dietary phosphate are unclear. We tested the hypothesis that deep and superficial nephrons respond differently to changes in dietary phosphate by comparing fractional phosphate delivery (FDP%) from proximal tubules of both nephron populations. Because phosphate reabsorption is not detectable in the ascending loop of Henle, FDP% to the superficial early distal tubule and papillary loop of Henle reflect delivery from superficial and deep nephron proximal tubules, respectively. Micropuncture experiments were performed in 17 acutely TPTX rats fed either a low (0.07%) or a high (1.8%) phosphate diet for 4 days prior to the experiment. In low phosphate diet, fractional phosphate excretion was 0.93 +/- 0.26%. FDP% was 7.5 +/- 0.5 and 9.1 +/- 2.2% from superficial and deep nephron proximal tubules, respectively (P greater than 0.05). In high phosphate diet, fractional phosphate excretion was 29.6 +/- 5.0%. FDP% was significantly greater from superficial proximal tubules, 33.9 +/- 4.6%, compared with that from deep nephron proximal tubules, 14.0 +/- 2.7% (P less than 0.05). We conclude that significant adaptation of phosphate reabsorption in response to changes in dietary phosphate intake occurs in the superficial but not in the deep nephron proximal tubule in acutely TPTX volume-expanded rats. In addition, the presence of distal phosphate reabsorption was not evident in high phosphate diet but must occur in low phosphate diet.

Tubular capacity of phosphate transport in phosphate-deprived rats: effects of nicotinamide and PTH

The present study tested the hypothesis that nicotinamide and/or parathyroid hormone (PTH) would reverse the resistance of phosphate-deprived rats to the phosphaturic effects of phosphate infusions by reducing the tubular capacity for phosphate transport. The response to progressively increasing rates of phosphate infusion (1, 2, and 3 mumol/min) was evaluated in thyroparathyroidectomized (TPTX) rats fed low phosphate diet (0.07%) for 4 days and treated with either vehicle or nicotinamide 2 h before the experiment. Following phosphate infusions, phosphate excretion was unchanged in vehicle-treated rats but increased progressively in nicotinamide-treated rats to 60% of the final rate of infusion. In a second series conducted in the presence of exogenous PTH, vehicle-treated rats responded to phosphate infusions by increasing phosphate excretion to 80-90% of the rate of infusion. In the presence of both nicotinamide and PTH, the rate of phosphate excretion matched the rate of infusion. Vehicle-treated rats displayed the highest reabsorptive rates without reaching a transport maximum, whereas a lower maximum reabsorptive rate was evident in nicotinamide-treated rats. However, in both groups given PTH, not only was the transport maximum lower but it declined with further increases in filtered load. We conclude that nicotinamide and PTH reverse the effects of dietary phosphate deprivation by decreasing the tubular capacity for phosphate transport.

Restoration of phosphaturia in phosphate-deprived rats in the presence of both parathyroid hormone and phosphate infusion

The present study tested the hypothesis that the combination of parathyroid hormone and phosphate infusion would be phosphaturic in phosphate-deprived rats. Clearance experiments were performed in Sprague-Dawley rats fed a low-phosphate diet for 4 days. The animals were first given a phosphate infusion and then administered parathyroid hormone. The reverse experiment was also performed in a second group of rats by giving the hormone first followed by a phosphate infusion. There was no increase in phosphate excretion in response to phosphate infusion or parathyroid hormone given alone. However, the combination of parathyroid hormone and phosphate infusions, given in either order, was phosphaturic in phosphate-deprived rats.

Chloride concentration gradient in newborn dogs in the presence of distal nephron blockade

Renal tubular Na+, Cl-, and H2O reabsorption was determined in 14 newborn dogs, 3--29 days of age, and in three adult dogs. In all animals NaCl reabsorption beyond the proximal tubule was blocked with ethacrynic acid (2 mg/kg) and amiloride (2.4 mg/kg). During distal blockade, fractional reabsorption of NaCl and water in both newborns and adults was approximately 70%, and there was a urine-to-plasma chloride gradient equal to 1.34 +/- 0.01, indicating that the proximal tubules of the newborn as well as those of the adult can generate a transtubular Cl- gradient. Upon administration of acetazolamide (50 mg/kg), there was a dramatic increase in excretion of Na+, Cl-, HCO3-, and water, and a decrease in the transtubular chloride gradient. After acetazolamide, the degree of inhibition of HCO3 reabsorption was well correlated with that of Na+, (r = 0.77) or Cl- (r = 0.74), and Na+ or Cl- inhibition exceeded that of HCO3-. In the newborn animal, the ratio of inner-to outer cortical nephron function is high at birth and declines rapidly during the first few weeks of life. However, there was no correlation between age and changes in either electrolyte excretion or in the transtubular chloride gradient. Therefore, the newborn dog possesses Cl- permselective tubules in the inner cortex that, in the presence of intact HCO3- reabsorption, are capable of establishing a functional transtubular Cl- gradient contributing to NaCl reabsorption.