Renal Phosphate Transport Research Articles

Mechanism of stimulation of renal phosphate transport by 1,25-dihydroxycholecalciferol

Vitamin D has been shown to stimulate renal phosphate transport and to alter membrane phospholipid composition. The present studies examine the possibility that the effects of 1,25(OH) 2D 3 on phosphate transport are related to its effects on membrane lipids. Arrhenius plots, which relate maximum rates of sodium dependent phosphate uptake into brush-border membrane vesicles to temperature were constructed. Phosphate transport was studied using brush-border membrane vesicles from normal, vitamin D-deficient, and physiologically replete (15 pmol/100 g body weight per 24 h) rats. These plots were triphasic with characteristic, lipid-dependent, slopes ( M 1, M 2, M 3 ) representing activation energies and transition temperatures ( T 1, T 2 ). Physiologic 1,25(OH) 2D 3 repletion normalized these plots by stimulating phosphate transport at all temperatures, increasing T 2 from 18 ± 0.7 to 23.5 ± 0.9°C and decreasing M 2 and M 3 from −5.8 ± 0.2 and −10.2 ± 0.4 to −4.5 ± 0.4 and −7.7 ± 0.3, respectively. Pharmacologic (1.2 nmol/100 g per 3 h) 1,25(OH) 2D 3 treatment resulted in a change in the Arrhenius plot of phosphate transport to a biphasic one with a transition temperature of 30°C. This effect was not blocked by cycloheximide. The Arrhenius plots of glucose transport were triphasic and unchanged with vitamin D repletion. These data support a liponomic mechanism of action for 1,25(OH) 2D 3 on phosphate transport.

Effects of Phosphate and Calcium Infusion on Renal Phosphate Transport in the Dog

The effect of phosphate infusion on renal tubular handling of calcium and phosphate was examined in dogs which had been thyroparathyroidectomized (TPTX) immediately prior to the studies. Phosphate infusions in TPTX animals caused a small decrease in total and ultrafilterable plasma calcium, and decreased phosphate reabsorptive capacity in the proximal tubule and loop segment. Infusion of CaCl2 during phosphate loading to offset the fall in plasma calcium prevented the reduction in proximal phosphate reabsorptive capacity. However, between the proximal and distal sampling site, the reduction in phosphate reabsorptive capacity could not be prevented by CaCl2 administration. These data are consistent with the presence of two phosphate transport systems; one in the early proximal tubule, modulated by changes in plasma calcium level, and a second in the loop segment, which is independent of calcium. While the data suggest that the depression of proximal phosphate reabsorption during phosphate infusion may be secondary to the fall in plasma calcium concentration, they do not exclude a direct effect of infused phosphate on proximal phosphate reabsorption that may be antagonized by an opposing direct effect of the calcium infusion.

Phosphate uptake by kidney epithelial (LLC-PK 1) cells

Phosphate uptake by the cultured kidney epithelial cell (LLC-PK 1) was studied. The uptake was Na + dependent, saturable with respect to phosphate and Na +, and energy dependent. The characteristics of the cell uptake system resembled the properties of phosphate transport in the kidney. Parathyroid hormone, dibutyryl cyclic AMP, and forskolin decreased Na +-dependent phosphate uptake. These agonists did not affect Na +-dependent α-methylglucoside uptake. Vasopressin and isoproterenol, which do not affect renal phosphate transport, did not inhibit phosphate uptake by the cell. These findings suggest that the cultured cell system may be a useful experimental model for studies of renal phosphate transport and its regulation.

Parathyroid hormone effect on chicken renal brush-border membrane phosphate transport.

Renal clearance studies showed that parathyroidectomy (PTX) stimulated renal PO4 reabsorption within 90 min in 3-wk-old chicks. Subsequent parathyroid hormone (PTH) infusion caused net PO4 secretion. Kidneys were taken for brush-border membrane (BBM) preparations 3 h after PTX or sham operations and 1 h after PTH or saline injections, thus yielding four categories of PTH status: PTX/saline, sham/saline, PTX/PTH, and sham/PTH. Efficacy of PTX and PTH was confirmed by examination of serum Ca concentrations. A 100 mM NaSCN gradient, out greater than in, caused concentrative PO4 uptake by BBM from sham/saline animals. Concentrative uptake was not produced by 100 mM KSCN, out greater than in; pH 7.4in vs. pH 5.4out; or 100 mM NaC1, out = in. Removal of endogenous PTH (PTX/saline) significantly stimulated Na-dependent PO4 uptake compared with sham/saline. PTH infusion significantly depressed Na-dependent PO4 uptake in PTX/PTH and sham/PTH groups compared with sham/saline and PTX/saline. Na-dependent, concentrative glucose uptake was present and unchanged by PTH. Kinetic analysis, based on 5-s uptakes, showed that both apparent affinity and maximal velocity (Vmax) for Na-dependent PO4 transport were decreased by PTH treatment. The data suggest that PTH influences avian renal PO4 excretion at least in part through a Na-dependent PO4 transport system in proximal tubule BBM.

Effects of 1,25-dihydroxycholecalciferol on phosphate transport in vitamin D-deprived rats.

To clarify the role of vitamin D in renal phosphate transport, weanling rats were fed a vitamin D-deficient diet containing 1.8% calcium and 1.2% phosphorus. After 5-6 wk, the rats were normocalcemic, normophosphatemic, and had normal levels of PTH. Assays of vitamin D metabolites revealed undetectable plasma levels of 25(OH)D, and 1,25(OH)2D levels of 92 +/- 16 pg/ml in partially vitamin D-depleted (PVDD) rats and 169 +/- 58 pg/ml in normal rats. PVDD rats had increased phosphate excretion, both absolute and fractional, and a decrease in Na+ gradient-dependent Pi transport in proximal tubular brush border membrane vesicles (BBMV) prepared from their kidneys. Vitamin D repletion of PVDD rats with 1,25(OH)2D3, 15 pmol/100 g body wt, decreased fractional excretion of Pi from 22.6 +/- 1.9 to 13.5 +/- 1.3%; the latter values were similar to normal rats. Repletion with 1,25(OH)2D3 also increased Na+-dependent phosphate transport in BBMV from 322 +/- 24 pmol X mg protein-1 X 15 s-1 in BBMV from PVDD rats to 698 +/- 70 pmol X mg protein-1 X 15 s-1. Repletion with larger doses of 1,25(OH)2D3 produced hypercalcemia and hyperphosphatemia from intestinal absorption, an increase in phosphate excretion, and a blunted response of Pi transport to 1,25(OH)2D3. Prevention of hyperphosphatemia by dietary adjustments allowed full expression of the stimulatory effects of 1,25(OH)2D3 on Pi transport. These later data may partially explain the inhibitory effects reported in prior studies in which plasma Pi was not controlled and the larger doses of 1,25(OH)2D3 administered.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone-induced phosphate excretion following preequilibration with 32P.

Parathyroid hormone (PTH) stimulates a secretory component of avian renal inorganic phosphate (Pi) transport, but the secreted Pi does not appear to be derived directly from peritubular (plasma) Pi. In the present study, experiments were conducted to determine whether differences in parathyroid status during 32P infusion influenced entry of the isotope into the Pi secretory pool. Fractional excretion values for Pi (FEPi) and 32P (FE32P) were compared in normal and parathyroidectomized (PTX) anesthetized birds that had been preinfused with 32P for 0, 90, and 240 min before PTH infusion. The results demonstrate that in the absence of exogenous PTH, FEPi is identical to FE32P in normal and PTX birds, reflecting full equilibration of 32P with excreted Pi under these conditions; and, regardless of the duration of 32P preequilibration or the parathyroid status of the experimental animals, exogenous PTH always causes FEPi to exceed FE32P. It is concluded that the Pi secretory pool is inaccessible to 32P under conditions that should markedly alter cellular Pi influx and efflux.

The biochemical modifications of the brush border membrane induced by vitamin D and parathyroid hormone in their actions on phosphate transport.

Since the development of techniques for the preparation and isolation of brush border membrane vesicles (BBMV),1,2 from the luminal cell membrane of renal tubular epthelial cells considerable progress has been made in our understanding of renal tubular cell phosphate transport. A sodium dependent co-transport system capable of moving phosphate uphill against an electrochemical gradient has been characterized in BBMV. The stoichiometry of the carrier mechanism remains controversial. According to one report, it is either 2Na+:HPO 4 −2 or Na+:H2PO4−, dependent upon the charge of the phosphate species present in the bathing fluid, and phosphate transfer is electroneutral.3 However, others have reported that at pH 6 the stoichiometry is 2Na:H2PO4− and the carrier may be electrogenic at low pH.4,5 Carrier activity increases as the pH of the medium is increased in the range of 6.0–8.5.3–5 Thus, a mechanism of secondary active phosphate transport in the renal tubular brush border membrane (BBM) has been partially characterized.

Intracellular processes that affect renal phosphate transport.

Studies of phosphate transport in the proximal renal tubule have focused recently on interactions with intracellular processes, especially oxidative metabolism and gluconeogenesis. Intracellular inorganic phosphate is essential for oxidative phosphorylation and participates in mitochondrial respiration. Gluconeogenesis liberates phosphate. This review will summarize our recent observations on changes in phosphate transport with metabolic inhibitors, metabolic substrates and changes in gluconeogenesis.KeywordsProximal TubuleMitochondrial RespirationMetabolic InhibitorPhosphate TransportCortical TubuleThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Influence of acute potassium loading on renal phosphate transport in the rat kidney.

This study examined 1) whether potassium-induced depression of phosphate excretion is a parathyroid hormone-dependent phenomenon, and 2) whether such stimulation of tubular phosphate reabsorption capacity involves increased phosphate reabsorption in the distal tubule. Potassium was infused into intact rats (25 mumol X min-1 X kg-1) during stepwise addition of phosphate to the infusion and led to a significant drop in phosphate excretion; this effect was abolished in thyroparathyroidectomized (TPTX) animals. In intact rats the maximal tubular Pi reabsorption per milliliter of glomerular filtrate (max TRPi/ml GF) was significantly higher in the potassium group (2.54 +/- 0.06 mumol/ml GF) compared with the control group (2.31 +/- 0.06 mumol/ml GF) (means +/- SE). In TPTX rats no difference in max TRPi/ml GF was observed: 3.44 +/- 0.07 and 3.49 +/- 0.07 mumol/ml GF during potassium and sodium infusion, respectively. Free-flow micropuncture was carried out on superficial distal tubules of intact rats and fluid samples were analyzed for [3H]inulin and phosphorus (electron microprobe). Phosphorus delivery into the distal tubule was similar in control and potassium-loaded rats. Whereas net phosphorus reabsorption along the distal tubule was absent in the control group, intravenous potassium administration stimulated distal phosphorus reabsorption. potassium stimulates renal phosphate reabsorption capacity, an effect that is abolished after TPTX. The potassium effect on phosphate occurs along the distal tubule.

Influence of vitamin D status and chronic administration on the renal tubular effects of 1,25-dihydroxy vitamin D3.

Previous studies from this laboratory have demonstrated that the acute infusion of the vitamin D3 derivative, 1,25-dihydroxy vitamin D3 (1,25(OH)2D3) is antiphosphaturic when administered with small "permissive" amounts of parathyroid hormone (PTH). To determine the effect of chronic administration of this metabolite, studies were performed in both vitamin D-deficient (-D) and vitamin D-fed (+D) rats which had been pretreated with 1,25(OH)2D3 for 6 days and then infused with either the metabolite alone, the metabolite with a small "permissive" dose of parathyroid hormone (PTH) or neither. The results indicate that in the -D animals, 1,25(OH)2D3 infusion alone results in antiphosphaturia without the presence of PTH. However, in the +D rats, PTH was required, despite 1,25(OH)2D3 pretreatment, for the acute infusion of the metabolite to enhance phosphate reabsorption. Thus, both vitamin D and parathyroid status are important in determining the effect of 1,25(OH)2D3 on the renal tubular transport of phosphate.

Inhibition of renal brush border phosphate transport and stimulation of renal gluconeogenesis by cyclic AMP and parathyroid hormone

The aims of the study were to determine whether 8-bromo-cyclic AMP (8BcAMP) in vivo mimics the inhibitory action of parathyroid hormone (PTH) on phosphate transport across the brush border membrane (BBM) of the renal proximal tubule, and to examine whether changes in BBM transport are accompanied by changes in the rate of renal gluconeogenesis. Thyroparathyroidectomized dogs were anesthetized and equilibrated, and control urine collections were obtained prior to removing the left kidney. Subsequent intravenous infusion of 8BcAMP at 50 mg/hr for 2 hr increased fractional excretion of phosphate from 4 ± 1 (controls) to 29 ± 4% (P < 0.001) without changing glomerular filtration. In BBM vesicles isolated from the renal cortex, the initial Na −-dependent transport of phosphate was decreased from 747 ± 135 (controls) to 564 ± 126 pmoles per mg per 0.25 min after 8BcAMP (P < 0.025), but Na −-independent phosphate uptake and Na −-dependent L-proline uptake were not changed significantly. Renal gluconeogenesis in the same animals was increased from 2.5 ± 0.3 (controls) to 5.3 ± 0.5 μmoles glucose per g tissue per hr after infusion of 8BcAMP (P < 0.001). Infusion of PTH, like 8BcAMP, inhibited BBM phosphate transport and stimulated renal gluconeogenesis. We conclude that the inhibitory action of cyclic AMP and PTH on BBM phosphate transport is accompanied by stimulation of gluconeogenesis which suggests, indirectly, that changes in gluconeogenesis may be part of the intracellular mechanism for regulating BBM phosphate uptake in response to certain stimuli.

Parathyroid hormone: effects of the 3-34 fragment in vivo and vitro.

The biologically active fragment ofparathyroid hormone, consisting of residues 1-34, and its in vitro antagonist, fragment 3-34, were administered separately or in combination to chronically thyroparathyroidectomized dogs. These fragments were also studied in vitro with dog renal cortical membranes. Fragment 3-34 inhibited the stimulation of adenylate cyclase by fragment 1-34 in vitro, but had no agonist or antagonistic effects on renal phosphate transport in vivo.

Renal transport of phosphate during phosphate loading in the fresh-water turtle

The net renal transport of phosphate was measured in fresh-water turtlesChrysemys picta orC. scripta elegans before and during acute elevation of plasma phosphate by intravenous phosphate infusion. In parathyroid-intact (sham PTX) animals, net secretion or net reabsorption of phosphate could be observed during control infusions. With increasing plasma phosphate net secretion was no longer seen, and net reabsorption increased, eventually reaching a reabsorptive maximal transport rate (Tmax) of −1.5 μmoles/ml GFR at plasma concentrations of 4.0–5.0 mmoles/l. A similar pattern was observed in chronically parathyroidectomized (chronic PTX, 7–14 days) animals, but net reabsorption in this group was higher at all plasma levels, and net secretion was never observed. Tmax in chronic PTX animals was estimated at −2.0 μmoles/ml GFR. Chronic PTX animals treated with mammalian parathyroid extract (100 IU, intramuscularly, three days prior to infusion experiments) showed a reversal of the effects of PTX on net phosphate transport. Net reabsorption was reduced and net secretion was observed at both low and high plasma phosphate concentrations. These results demonstrate that net renal phosphate transport in the intact fresh-water turtle is characterized by a reabsorptive Tmax. The demonstration of net phosphate secretion in these animals requires the presence of parathyroid hormone. The secretory component of phosphate transport may be either a transepithelial flux of phosphate which is saturated at very low plasma phosphate concentrations, or a process distinct from a peritubular-to-luminal transport, and thus independent of plasma phosphate concentration.

Mechanism of glucocorticoid effect on renal transport of phosphate.

We explored whether glucocorticoid administration, a known stimulus of renal gluconeogenesis (GNG), could decrease avid inorganic phosphate (Pi) reabsorption in rats stabilized on low-phosphorus diet (LPD). Rats adapted to LPD were injected with the glucocorticoid (GCD) triamcinolone acetonide (1.25 or 2.5 mg.100 g body wt-1.day-1 ip) for 2 days; they showed a profound increase in urinary excretion of Pi during the injection period. In clearance studies GCD increased the clearance and fractional excretion of Pi but did not change the filtered load of Pi. Initial "uphill" Na+-gradient (Nao+ greater than Nai+)-dependent uptake of 32Pi by luminal brush-border membrane (BBM) vesicles prepared from renal cortex of rats treated with GCD was markedly (greater than 40%) decreased compared with control rats; Na+-gradient-dependent uptake of D-[3H]glucose was not diminished. At the "equilibrium" time interval, measured at 120 min, BBM vesicles from control and GCD-treated rats did not differ in the uptake of 32Pi or D-[3H]glucose. With kinetic analysis, BBM from GCD-treated rats showed a marked decrease (-40%) in the maximum velocity (Vmax) of initial Na+-dependent 32Pi uptake, but the apparent affinity of the BBM transport system for Pi (apparent Km = 0.078 mM Pi) was not different from that of controls. Alkaline phosphatase specific activity was much lower (-40%) in BBM from GCD-treated rats compared with controls, but the activities of three other BBM enzymes (maltase, leucine aminopeptidase, and gamma-glutamyl transferase) were not different. The addition of triamcinolone to BBM in vitro had no effect on either Na+-dependent uptake of 32Pi or alkaline phosphatase activity. The rate of GNG from alpha-ketoglutarate was significantly increased in cortical slices from GCD-treated rats adapted to LPD. Also, the NAD+-to-NADH ratio was higher in the renal cortex of GCD-treated rats, although the total content of NAD [NAD+ + NADH] was not different from controls. Renal excretory, BBM, and metabolic changes elicited controls. Renal excretory, BBM, and metabolic changes elicited by GCD treatment were similar in intact and thyroparathyroidectomized rats. Phosphaturia elicited in rats fed LPD by GCD administration in vivo appears to be at least in part due to a decreased capacity of luminal BBM of proximal tubules for decreased capacity of luminal BBM of proximal tubules for Na+-dependent uptake of Pi. Although the causal relationship between observed parameters is not established, our results are compatible with the interpretation that an increase in the rate of renal GNG, perhaps via action of NAD+ on BBM (J. Clin. Invest. 67: 1347-1360, 1981), decreases luminal uptake and reabsorption of Pi in proximal tubules.

Effect of respiratory alkalosis on renal phosphate excretion.

Respiratory alkalosis induced hypophosphatemia and hypophosphaturia in intact animals. The present studies evaluated the effect of respiratory alkalosis on tissue phosphate distribution and renal phosphate transport in the presence and absence of parathyroid hormone (PTH). Respiratory alkalosis decreased plasma phosphate concentration and increased phosphate concentrations in muscle and liver. It decreased fractional phosphate excretion (FEPi) from 6.1 +/- 1.4 to 0.6 +/- 0.2%. In thyroparathyroidectomized (TPTX) rats infused with 20 mM phosphate, respiratory alkalosis decreased FEPi from 15.0 +/- 0.9 to 5.5 +/- 0.1%. PTH or dibutyryl cAMP administration produced a phosphaturia that was blunted by respiratory alkalosis. The phosphaturic response to PTH was also blunted in hypocapnic rats in which alkalosis was prevented by infusion of HCl. We conclude that respiratory alkalosis increases phosphate uptake by muscle, which largely accounts for the hypophosphatemia. The kidney response with increased phosphate reabsorption independent of plasma and kidney phosphate concentrations and with refractoriness to the phosphaturic effect of PTH. This refractoriness to the phosphaturic effect of PTH is due to decreased PCO2 rather than to the concomitant extracellular alkalosis.

Dosis-response-relationship between bovine parathyroid hormone (b-PTH (1-34)-Beckman) and the renal handling of inorganic phosphate in acutely parathyroidectomized rats

A. Frick, M. Neuweg and I. Durasin Among the different parathyroid hormones (human, bovine, porcine etc.) we examined the synthetic bovine parathyroid hormone (b-PTH(l-34)-Beck/nan) in clearance experiments in the rat. It was our aim to find out the dosis which is necessary to normalize the renal transport of inorganic phosphate (Pi) in short-term experiments (2 h) in acutely parathyroidectomized animals.

Acute effects of a "physiological" dose of 1,25-dihydroxy vitamin D3 on renal phosphate transport.

The renal phosphate transport response of thyroparathyroidectomized, vitamin D-deficient rats to the infusion of 1,25-dihydroxy vitamin D3 (1,25 D3) was studied with and without the simultaneous administration of a small (or "permissive") non-phosphaturic amount of bovine parathyroid hormone (bPTH). Although phosphate excretion (UPV) was unaltered by the infusion of either 0.1 U (= .0025 microgram or 6 pmoles) of 1,25 D3 or 0.2 U bPTH per hour for 6 hours, their combined administration reduced UPV from 14.8 +/- 1.6 to 10.3 +/- 1.2 microgram/min. (P less than .05). There were no alterations in inulin excretion. These data verify that: 1) 1,25 D3 is antiphosphaturic in this experimental setting in a very low dose which may represent a "physiological" amount of the metabolite; and 2) to enhance phosphate transport, the 1,25 D3 requires the presence of a small ("permissive") amount of PTH.

Renal clearance of phosphate and clacium in the fresh-water turtle: Effects of parathyroid hormone

The renal clearances of phosphate and calcium were studied in fresh-water turtles,Chrysemys picta andC. scripta elegans, at various times after either parathyroidectomy (PTX) or parathyroid hormone (PTH) administration. The relative clearance of phosphate\({{C_{PO_4 } } \mathord{\left/ {\vphantom {{C_{PO_4 } } {C_{In} }}} \right. \kern-\nulldelimiterspace} {C_{In} }}\) fell from a mean value greater than one, to values near zero following PTX. This effect occurred over a period of 5–7 days, and was accompanied by increased values of net tubular phosphate reabsorption (TxPO4/ml GFR). There were no consistent changes in the relative clearance or tubular transport rates of calcium after PTX. Administration of mammalian PTH to chronic PTX animals had no effect for up to 2 h on any of the measured variables. However, beginning one day after a single intramuscular injection of 100 units of PTH,\({{C_{PO_4 } } \mathord{\left/ {\vphantom {{C_{PO_4 } } {C_{In} }}} \right. \kern-\nulldelimiterspace} {C_{In} }}\) increased significantly over vehicle-injected controls. This phosphaturic effect reached a maximal level at 3 days after the hormone injection. PTH administration caused a slight, but not significant increase in the relative clearance of calcium at 5 and 7 days. It was concluded that the fresh-water turtle maintains a bidirectional renal phosphate transport system, which is under the control of the level of circulating parathyroid hormone. These species do not appear to possess a PTH-sensitive renal calcium conserving mechanism, such as is found in mammals.

Sodium gradient-dependent phosphate transport in renal brush border membrane vesicles. Effect of an intravesicular greater than extravesicular proton gradient.

A H+ gradient (intravesicular greater than extravesicular), in the absence of a Na+ gradient (extravesicular greater than intravesicular) stimulated phosphate uptake by renal brush border membrane vesicles and provided the driving force to effect the transient accumulation of phosphate against its concentration gradient. The H+ gradient-dependent uptake of phosphate had an absolute requirement for Na+. The rates of uptake and peak accumulation were functions of the delta pH and the concentration of H+ in the intravesicular medium. The H+ gradient-energized Na+-phosphate cotransport system was not affected by valinomycin- or carbonyl cyanide p-fluoromethoxyphenylhydrazone-induced ion diffusion potentials. Therefore, it was independent of the membrane potential, i.e. an electroneutral process. Amiloride, which inhibited the H+-Na+ exchange reaction and prevented the efflux of H+ from the intravesicular medium, enhanced the uptake of phosphate. A model is proposed by which the H+ gradient mediates the uphill transport of phosphate. It is suggested that a similar process may operate in more physiologically intact preparations and may provide one mechanism by which acid-base balance regulates renal phosphate transport.

Possible role of nicotinamide adenine dinucleotide as an intracellular regulator of renal transport of phosphate in the rat.

In these experiments we investigated whether NAD could serve as an intracellular modulator of the brush border membrane (BBM) transport of inorganic phosphate (Pi). NAD, both oxidized (NAD+) and reduced (NADH) form, inhibited the Na+-dependent uptake of 32Pi in the concentration range of 10-300 microM NAD when added in vitro to BBM vesicles isolated from rat kidney cortex, but did not inhibit BBM uptake of D-[3H]glucose or BBM uptake of 22Na+. Neither nicotinamide (NiAm) nor adenosine alone influenced BBM uptake of 32Pi. NAD had a similar relative effect (percent inhibition) in BBM from rats stabilized on low Pi diet (0.07% Pi), high Pi diet (1.2% Pi), or normal Pi diet (0.7% Pi). Subsequently, we examined the renal effects of changing the tissue NAD level in vivo. Rats stabilized on low Pi diet were injected intraperitoneally with NiAm (0.25-1.0 g/kg body wt); urinary excretions of Pi (UPiV), of fluid, and of other solutes were measured before and after NiAm injection, then renal cortical tissue nucleotide content was determined, and a BBM fraction was isolated for transport measurements. In BBM from NiAm-treated rats, the Na+-dependent uptake of 32Pi was decreased, but BBM uptake of D-[3H]glucose and BBM uptake of 22Na+ were not changed. NiAm injection elicited an increase in NAD+ (maximum change, 290%), a lesser increase in NADH (maximum change, +45%), but no change in the content of ATP or cyclic AMP in the renal cortex. Na+-dependent BBM uptake of 32Pi ws inversely correlated with NAD+ content in renal cortex (r = -0.77 +/- 0.1; P less than 0.001) and with UPiV (r = -0.67 +/- 0.13; P less than 0.01). NAD+ in renal cortex was positively correlated with UPiV (r = 0.88 +/- 0.05; P less than 0.001). Injection of NiAm elicited a marked increase in UPiV, but no change in excretions of creatinine or K+, or in urine flow; excretion of Na+ and Ca declined. NiAm injection caused similar renal responses, in normal and in thyroparathyroidectomized rats, as well as in rats on normal Pi diet and low Pi diet. We conclude that NAD can serve as an intracellular modulator (inhibitor) of Na+-dependent transport of Pi across the renal luminal BBM and across the proximal tubular wall by its direct interaction with BBM. We propose that at least some hormonal and/or metabolic stimuli elicit phosphaturia by increasing NAD+ in cytoplasm of proximal tubular cells.