The acute effect of 25-hydroxycholecalciferol on renal handling of phosphorus. Evidence for a parathyroid hormone-dependent mechanism

Vitamin D: an overview of its role in skeletal muscle physiology in children and adolescents

Many children may have insufficient serum concentrations of vitamin D, which could prevent optimal muscle development and function. Vitamin D deficiency in animal models results in negative effects on muscle fiber structure and calcium/phosphorus handling, suggesting an integral role of vitamin D in skeletal muscle function. While there is a dearth of data in humans, the available evidence demonstrates a positive association between vitamin D status and muscle function. This review focuses on the important role of vitamin D in muscle function in children and adolescents who live in North American regions where exposure to ultraviolet B radiation is limited and who are thus at increased risk for vitamin D insufficiency. The effects of vitamin D on muscle cell proliferation and differentiation, muscle fiber structure, and calcium and phosphorus handling are discussed. Moreover, the roles of vitamin D and the vitamin D receptor and their genomic and nongenomic actions in muscle function are explored in depth. Future research should aim to establish a vitamin D status consistent with optimal musculoskeletal development and function in young children.

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

The effect of 1-alpha-hydroxycholecalciferol on the renal handling of phosphate in parathyroidectomized man

In a previous investigation by our group it was suggested that the stimulating effect of 1-alpha-hydroxycholecalciferol (1-alpha-OH-D3) on the tubular reabsorption of phosphate is mediated via the parallel suppression of the parathyroid hormone (PTH). A direct effect of 1-alpha-OH-D3 on the renal tubule could however not be completely excluded. Therefore, the effect of 1-alpha-OH-D3 was studied in 5 totally parathyroidectomized patients, in whom concomitant suppression of PTH would not occur. TmP/GFR, i.e. the ratio between the maximal tubular reabsorption of phosphate (TmP) and the glomerular filtration rate (GFR), was used as an indicator of the renal handling of phosphate. Estimation of TmP/GFR was performed 1) when the patients were vitamin D depleted and hypocalcemic, and 2) after 14-27 days of treatment with 1-alpha-OH-D3 to obtain stable normocalcemia. In patients with absent parathyroid function, no effect of 1-alpha-OH-D3 on TmP/GFR could be demonstrated. It is therefore concluded that 1-alpha-OH-D3 exhbits no antiphosphaturic effect in the absence of PTH and that the previously demonstrated antiphosphaturic effect of 1-alpha-OH-D3 in man is mediated via a concomitant suppression of PTH.

PHOSPHORUS METABOLISM AND MANAGEMENT IN CHRONIC KIDNEY DISEASE: Phosphate Metabolism in the Setting of Chronic Kidney Disease: Significance and Recommendations for Treatment

Phosphorus is an essential mineral that plays a crucial role in cell structure and metabolism. In living organisms, phosphorus exists surrounded by four oxygen atoms to form phosphate (PO(4)). Within cells, PO(4) regulates enzymatic activity and serves as an essential component of nucleic acids, adenosine triphosphate, and phospholipid membranes. Outside cells, PO(4) primarily resides in bone and teeth as hydroxyapatite. A small amount of inorganic PO(4) circulates in serum, with levels balanced by gastrointestinal intake, renal excretion, and a set of specific hormones. Under normal conditions, PO(4) is excreted through the kidneys. Among patients with end stage renal disease (ESRD) receiving chronic dialysis, circulating PO(4) levels typically rise to levels well above the normal laboratory range. Higher serum PO(4) levels are strongly associated with arterial calcification and mortality in this setting. Among predialysis patients with chronic kidney disease (CKD), phosphaturic hormones enhance renal PO(4) excretion to maintain serum PO(4) levels within the high-normal laboratory range. Recently, high-normal serum PO(4) levels have been associated with cardiovascular (CV) events and mortality among individuals who have CKD and among those who have normal kidney function. This review discusses PO(4) metabolism in the context of CKD, examines associations of PO(4) levels with adverse outcomes in the CKD setting, and suggests treatment strategies for moderating serum PO(4) levels.

Vitamin D reduces renal NaPi-2 in PTH-infused rats: complexity of vitamin D action on renal P(i) handling

Acute administration of dihydroxycholecalciferol [1,25(OH)(2)D(3)] blunts phosphaturia and increases urinary cAMP excretion in parathyroid hormone (PTH)-infused parathyroidectomized (PTX) rats. Because chronic administration of 1,25(OH)(2)D(3) enhances the phosphaturic response to exogenous parathyroid hormone despite blunting of urinary cAMP excretion, we have examined the expression of the renal cortex type II Na-P(i) cotransporter (NaPi-2) mRNA and protein in 1) chronic PTX Sabra rats, 2) PTX rats receiving a physiological dose of 1,25(OH)-2-D(3), 3) PTX rats receiving 35 ng/h of PTH, and 4) rats receiving both PTH and 1,25(OH)(2)D(3), for 7 days via osmotic minipumps. Our results confirm that there is increased phosphaturia in the PTH+1,25(OH)(2)D(3)-infused animals despite blunting of urinary cAMP excretion, a reduced filtered load of phosphate, and lack of a phosphaturic effect by 1,25(OH)(2)D(3) alone. Both PTH and 1,25(OH)(2)D(3) significantly reduced expression of renal cortex NaPi-2 mRNA and NaPi-2 protein, and the administration of PTH together with 1,25(OH)(2)D(3) had additive effects in further decreasing NaPi-2 mRNA and NaPi-2 protein levels. Expression of two other epithelial transporters, type 1 Na-sulfate and type 1 Na-glucose cotransporters, were not different between the groups, suggesting specificity of the effects of PTH and 1,25(OH)(2)D(3) on phosphate transport. The effect of chronic administration of 1,25(OH)(2)D(3) has not been noted previously, and the cellular mechanisms and signaling processes that mediate the decrease in NaPi-2 remain to be determined.

Effect of calbindin D 28K on sodium transport by the luminal membrane of the rabbit nephron

We previously reported that in the rabbit, the vitamin D-dependent calcium binding protein 28K (CaBP 28K) increases calcium (Ca2+) transport in the distal tubule by opening a high affinity Ca2+ channel in the luminal membrane. Since Na+ and Ca2+ transports are interdependent in this membrane, we questioned whether the calbindin has any influence on Na+ transport. Luminal membranes from rabbit proximal and distal tubules were purified and 22Na uptake by the membrane vesicles was measured using the rapid filtration technique. The vesicles were loaded with 280 mM mannitol and 20 mM Tris-Hepes pH 7.4, with either 3 microM CaBP or the carrier. Incubation medium contained 1 mM 22NaCl, 278 mM mannitol, and 20 mM Tris-Hepes pH 7.4. The presence of 3 microM CaBP 28K in the distal luminal membrane vesicles increased the 0.5 mM Ca2+ uptake from 0.91 +/- 0.21 to 1.84 +/- 0.33 pmol/microg/10 s (P < 0.01) and decreased 1 mM Na+ uptake from 0.62 +/- 0.15 to 0.27 +/- 0.08 pmol/microg/10 s (P < 0.05). A similar decrease of Na+ uptake was observed in proximal luminal membrane experiments. The effect on Na+ uptake by the distal membrane was dose-dependent with a IC50 of 4.5 microM. Addition of 2 mM Ca2+ to the incubation medium decreased 1 mM Na + uptake from 0.62 +/- 0.15 to 0.49 +/- 0.12 pmol/microg/10 s (P < 0.05), but did not influence the effect of CaBP 28K on Na+ uptake. Experiments performed in the presence and absence of ethyl isopropyl amiloride (EIPA) suggest that the effect of calbindin involves the Na+/H+ exchanger activity.

22-Oxacalcitriol does not interfere with parathyroid hormone-induced phosphaturia or cyclic-AMP excretion

The vitamin D analogue, 22-oxacalcitriol [22-oxa-1,25(OH)2 vitamin D3], has pleiotropic effects similar to or greater than calcitriol but has markedly fewer calcemic and phosphatemic effects. To test the hypothesis that the lesser phosphatemic effect of 22-oxacalcitriol is due, at least in part, to a lack of interference with the phosphaturic effect of parathyroid hormone, acute clearance experiments were performed in parathyroidectomized rats receiving continuous 1-34 parathyroid hormone (PTH) infusion together with 22-oxacalcitriol (200 pmol.100 g body weight-1.min-1) or vehicle. In contrast to the previously reported inhibitory effect of calcitriol on PTH-induced phosphaturia, fractional excretion of phosphorus increased similarly in both groups, from 0.05 +/- 0.01 to 0.26 +/- 0.02 (p < 0.01) in the vehicle-infused animals and from 0.04 +/- 0.01 to 0.24 +/- 0.02 (p < 0.01) in the 22-oxacalcitriol-treated rats (p between groups not significant [n.s.]). Urinary cyclic AMP excretion also increased similarly, from 45.5 +/- 5.2 to 101.6 +/- 21.6 (p < 0.01) and from 45.4 +/- 5.6 to 102.6 +/- 16.7 pmol/min (p < 0.01), respectively (p between groups n.s.). In search for a nongenomic mechanism that might account for the disparate effects of 22-oxacalcitriol and calcitriol, OK cells, which are reminiscent of the mammalian proximal tubule cell, were stimulated with calcitriol and 22-oxacalcitriol and free intracellular calcium concentration was determined. At high concentrations, calcitriol caused a dose-dependent increase in [Ca2+]i; 22-oxacalcitriol had no effect on [Ca2+]i at any concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

The in vivo and in vitro effect of calmodulin antagonists on the renal actions of 25(OH) vitamin D3 in the rat

Previous work from this laboratory has demonstrated that 25(OH) vitamin D3 [25(OH)D3] acutely suppresses the phosphaturic action of parathyroid hormone (PTH) and interferes with the PTH-induced activation of adenylate cyclase (AC). Calmodulin inhibitors block vitamin D-induced Ca2+ transport in the gut and phosphorus uptake in renal BBMV's. We have examined whether calmodulin antagonists affect the renal action of 25(OH)D3. Acute clearance experiments were performed in PTH-infused parathyroidectomized rats receiving 25(OH)D3 after pretreatment with trifluoperazine (TFP) or promethazine (P). In vitro PTH-induced activation of renal AC was also studied in membrane preparations from pretreated rats in the presence of 25(OH)D3. 25(OH)D3 reduced the PTH-stimulated increase in fractional excretion of phosphorus (CP/CIn) from 0.292 +/- 0.024 to 0.195 +/- 0.018 (p less than 0.005) and urinary cAMP from 149.3 +/- 20.3 to 78.1 +/- 10.4 pmol/min (p less than 0.01) and also blunted AC activation in vitro. TFP but not P abolished the effects of 25(OH)D3 both in vivo and in vitro. R 24571 also abolished the in vitro effect of 25(OH)D3. Thus, (1) TFP abolishes both the antiphosphaturic and the AC/cAMP-related actions of 25(OH)D3, (2) P does not have these effects, and (3) R 24571 abolishes the in vitro effect of 25(OH)D3. These results suggest that the antiphosphaturic effect of 25(OH)D3 acting via the AC/cAMP system may be calmodulin dependent.

The metabolism and mechanism of action of 1,25-dihydroxyvitamin D3

Much has been learned about the formation of the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3. Information concerning its formation and catabolism has allowed a clear understanding of factors involved in the maintenance of plasma concentrations of the hormone. The effects of 1,25-dihydroxyvitamin D3 on calcium transporting cells in the intestine are marked and well defined. The tissue (intestinal tissue) is easily isolated and manipulated and hence, this is an ideal tissue in which to examine the mechanism of divalent cation transport. The mechanism by which 1,25-dihydroxyvitamin D3 brings about this effect should help in understanding sterol hormone action.

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)2D3 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 (M1,M2,M3) representing activation energies and transition temperatures (T1,T2). Physiologic 1,25(OH)2D3 repletion normalized these plots by stimulating phosphate transport at all temperatures, increasing T2 from 18 +/- 0.7 to 23.5 +/- 0.9 degrees C and decreasing M2 and M3 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)2D3 treatment resulted in a change in the Arrhenius plot of phosphate transport to a biphasic one with a transition temperature of 30 degrees 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)2D3 on phosphate transport.

Effects of vitamin D on renal handling of calcium, magnesium and phosphate in the hamster

The effects of 1,25(OH)2D3 on the renal handling of calcium (Ca), magnesium (Mg), and phosphate (Pi) in the thyroparathyroidectomized (TPTX) hamster were studied in the presence of exogenous PTH. Clearance experiments were performed in the following groups: acutely TPTX animals (group 1), acute TPTX plus continuous infusion of PTH in low or high doses sufficient to (1) reduce or (2) abolish the hypocalcemic effect of TPTX (groups 2 and 3, respectively), and acute TPTX plus Ca (group 4) or Pi (group 5) infusion. Each group was subdivided into control and experimental subgroups. In all animals an initial control phase was followed by a second phase in which experimental animals received an infusion of 1,25(OH)2D3 (1 U prime + 0.5 U/hr) while control animals received only the ethanol vehicle. GFR and urine flow rate, were not altered significantly in any of the groups. PCa and PMg increased significantly in group 3 only. Group 2 showed an increase in FECa (5.2 +/- 1.4 to 13.2 +/ 2.2%, P less than 0.001) and FEMg (7.3 +/- 1.3 to 17.3 +/- 2.2%, P less than 0.001) in response to 1,25(OH)2D3. Group 3 showed a significant increase in FEMg only (2.2 +/- 0.4 to 5.5 +/- 1.0%, P less than 0.01). The changes in the control groups were not significant. The administration of 1,25(OH)2D3 reduced phosphate excretion only in the presence of PTH. The FEPi decreased from 11.9 +/- 2.1 to 3.6 +/- 0.9% (P less than 0.001) in group 2 and 29.2 +/- 4.0 to 16.5 +/- 2.5% (P less than 0.02) in group 3.(ABSTRACT TRUNCATED AT 250 WORDS)

PTH independent sex difference in renal handling of inorganic phosphate in the rat: effect of oophorectomy

In order to evaluate the sex difference in the renal handling of inorganic phosphate (Pi) in the rat we performed clearance experiments using intact, thyroparathyroidectomized (TPTX), oophorectomized (OophX) and orchiectomized (OrchX) rats. During stepwise elevation of the Pi concentration in plasma (Pi-titration) to about 6 mmol/l the reabsorptive mechanism of Pi was saturated. The ratio Pi-reabs./GFR in intact males was higher than in females. A significant difference in this parameter was also observed in thyroparathyroidectomized rats: in females this value was 3.47 +/- 0.13, and in males it was 4.54 +/- 0.37 mumol/ml (P less than 0.001). Oophorectomy in the absence of parathyroid hormone (PTH) increased Pi-reabs./GFR from 3.18 +/- 0.36 to 4.12 +/- 0.24 mumol/ml (P less than 0.001); however, orchiectomy did not significantly change the reabsorption of Pi. In conclusion, the present results demonstrate a PTH independent sex difference in the renal handling of inorganic phosphate and are consistent with the hypothesis that estrogens may play a dominant role in this differentiation.

Evidence that sodium deprivation influences vitamin D dependent rat renal calcium binding protein

In order to provide some insight concerning the role of renal calcium binding protein (CaBP) in the functioning of the mammalian kidney, the response of renal CaBP to dietary alterations was examined. Three week old rats were fed diets deficient in calcium, phosphorus or sodium supplemented with vitamin D for a four week period. The specific activity of renal CaBP (as measured by the chelex resin assay; Ca2+ bound protein/Ca2+ bound resin per mg protein) in the 28,000 Mr region was found to increase four fold in rats fed the low phosphorus diet and two fold in rats fed the low calcium diet when compared to rats fed the control diet. Renal CaBP/mg protein from rats fed the low sodium diet decreased 50% from the control values. Changes in renal CaBP were confirmed by polyacrylamide gel analysis of the 28,000 Mr fraction by densitometric tracing using a purified CaBP marker. The greater response to dietary phosphorus restriction suggests that renal CaBP may be regulated by a mechanism different from that of intestinal CaBP. The decrease in renal CaBP in rats fed the low sodium diet suggests for the first time that sodium is required for vitamin D dependent distal tubular calcium transport processes.

Blockade of the renal tubular effects of vitamin D by cycloheximide in the rat

To further characterize the mechanisms by which 25(OH) vitamin D3 (25(OH)D3) and 1.25(OH)2 vitamin D3 (1,25(OH)2D3) suppress the phosphaturic action of parathyroid hormone (PTH) we have studied the effects of cycloheximide (cyclohex), a protein synthesis inhibitor, on the interaction between PTH and vitamin D metabolites in parathyroidectomized (PTX) rats, both in vivo and in vitro experiments. In clearance studies PTX PTH-infused rats were pretreated with cyclohex 2 h before the administration of vitamin D. In control, PTX PTH-infused rats not pretreated with cyclohex, the administration of 25(OH)D3 and 1,25(OH)2D3 was associated with a fall in fractional excretion of phosphate (CP/CIN) from 0.30 +/- 0.05 to 0.16 +/- 0.02 and from 0.31 +/- 0.05 to 0.13 +/- 0.01 (P less than 0.005) respectively. Cyclohex-pretreated PTX PTH-infused rats failed to respond to both 25(OH)D3 and 1,25(OH)2D3, and CP/CIN, which rose after PTH, remained 0.32 +/- 0.05 and 0.29 +/- 0.03 respectively. In vitro, both 25(OH)D3 and 1,25(OH)2D3 inhibited the PTH-induced activation of adenylate cyclase in the renal isolated membrane fractions. Pretreatment with cyclohex abolished this effect of vitamin D metabolites. These results show that cyclohex blocks the antiphosphaturic effects of both 25(OH)D3 and 1,25(OH)2D3 but does not alter the response to PTH. These findings are consistent with the possibility that the acute renal action of vitamin D depends on de novo synthesis of protein.

In vitro stimulation of phosphate uptake in isolated chick renal cells by 1,25-dihydroxycholecalciferol

Renal cells isolated from vitamin D-deficient chicks had an increased Na+-dependent phosphate uptake when preincubated with 1,25-dihydroxycholecalciferol [1,25-(OH)2D3]. Phosphate uptake in the absence of Na+ and methyl alpha-glucoside uptake dependent on Na+ were not affected. Phosphate uptake was stimulated 15% by 0.010 pM 1,25-(OH)2D3. Maximal enhancement of 30% was obtained with 100 pM. The uptake when fully stimulated by preincubation in vitro approximated the uptake of cells isolated from chicks that were previously repleted with 1,25-(OH)2D3 in vivo. Cells from repleted chicks were not stimulated additionally when preincubated with 1,25-(OH)2D3 in vitro. The increase in phosphate uptake could be measured after a 1-hr preincubation period; full response required at least 2 hr. Phosphate uptake induced by 1,25-(OH)2D3 was blocked by cycloheximide and actinomycin D. Enhancement of phosphate uptake was relatively specific for the 1,25-(OH)2D3 analog of vitamin D3. The potency order was 1,25-(OH)2D3 greater than 25-(OH)D3 = 1-(OH)D3 greater than 24,25-(OH)2D3 greater than D3. Kinetically, 1,25-(OH)2D3 increased the Vmax of the phosphate uptake system; the affinity for phosphate was unaffected. 3H-Labeled 1,25-(OH)2D3 was taken up by the isolated renal cells. It was estimated that the stimulation of phosphate uptake might be initiated by very few molecules of 1,25-(OH)2D3 per cell. It is proposed that 1,25-(OH)2D3 contributes importantly to the mechanisms by which phosphate transport is regulated in the kidney.

Effects of 1,25-(OH)2D3 administered in vivo on phosphate uptake by isolated chick renal cells

Renal cells from Vitamin D-deficient and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]-repleted chicks were isolated by a collagenase-hyaluronidase procedure. Exclusion of trypan blue and respiratory measurements indicate that the cells were functionally intact and metabolically active. The uptakes of phosphate and alpha-methylglucoside were stimulated markedly by Na+ in the extracellular medium. Phosphate uptake in the presence of Na+ was saturable with respect to phosphate concentration; half-maximal activity was obtained with approximately 0.2 mM. Three hours after 1,25-(OH)2D3 was injected into vitamin D-deficient chicks the Na+-dependent phosphate uptake by the isolated cells had increased about 40%, i.e., 2.00 compared with 1.44 nmol.min-1.mg protein-1. Phosphate uptake in the presence of K+ in the extracellular medium and alpha-methylglucoside uptake in the presence or absence of Na+ were unchanged. In a secondary response found 17 h after 1,25-(OH)2D3 injection, Na+-dependent phosphate uptake decreased. Serum concentrations of phosphorus and calcium were not measurably changed in the 3-h repleted bird, but both levels were increased 17 h after treatment. Administration of phosphate into vitamin D-deficient chicks, so that the serum concentration of phosphorus was raised to that of the 17-h 1,25-(OH)2D3 repleted animal, effected a comparable decrease in phosphate uptake. Serum calcium levels were not altered by this treatment. The actions of parathyroid hormone in stimulating adenylate cyclase and in inhibiting phosphate uptake were notably blunted in the vitamin D-deficient chick. Sensitivity to parathyroid hormone was not restored until several days after 1,25-(OH)2D3 repletion. These findings suggest that the initial response to 1,25-(OH)2D3, to increase renal phosphate uptake, and the secondary response, to decrease phosphate uptake, were by parathyroid hormone-independent processes. The results also indicate that the isolated renal cell represents an excellent model for studying the mechanism by which 1,25-(OH)2D3 regulates phosphate transport in the kidney.

Phosphate homeostasis and hypophosphatemia

The clinical importance of an understanding of phosphate metabolism is derived from the crucial role of this anion in the regulation of many cellular functions. In recent years, a greater appreciation of disorders of phosphate metabolism has been recognized because of more frequent monitoring of serum phosphate concentration as well as the increased utilization of therapeutic interventions that profoundly affect overall phosphate balance. The present review focuses on the factors tha regulate phosphate homeostasis, explores the pathophysiology and manifestations of the phosphate depletion syndrome and provides a framework for the diagnosis and rational treatment of patients with abnormalities in phosphate metabolism.

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.

Antiphosphaturic action of 25 (OH) vitamin D3 in experimental Fanconi syndrome

Renal handling of phosphorus was studied in the following groups of parathyroidectomized rats with maleate-induced Fanconi syndrome: 1) 6 rats receiving intravenous parathyroid hormone, 2) 6 rats receiving intravenous dibutyryl cyclic AMP (DBcAMP), 3) 6 rats undergoing volume expansion with saline, 4) 12 rats receiving intravenous 25 (OH)vitamin D3, 5) 12 rats with acute hypercalcemia induced by intravenous CaCl2, 6) 6 rats with phosphate deprivation, and 7) 6 rats receiving intravenous calcitonin. Parathyroid hormone and calcitonin failed to increase the urinary excretion of both cAMP and phosphorus. Likewise, DBcAMP failed to increase the urinary excretion of phosphorus. Extracellular volume expansion and hypercalcemia (serum calcium 12.9 +/- 0.7 mg/100 ml) did not alter the tubular reabsorption of phosphorus. In phosphate-deprived animals, the fractional excretion 0.16 +/- 0.05 (mean +/- SE) was lower than that in the control animals (maleate-treated without phosphate depletion), 0.46 +/- 0.04 (P less than 0.001). 25 (OH)vitamin D3 decreased the fractional excretion of phosphorus from 0.39 +/- 0.03 in the control (maleate-treated not receiving 25 (OH)vitamin D3) to 0.23 +/- 0.02 (P less than 0.001) in the experimental animals. The present study demonstrated an antiphosphaturic effect of 25(OH)vitamin D3 in experimental Fanconi syndrome; the mechanism of this action is not well understood.

Interactions between vitamin D deficiency and phosphorus depletion in the rat

To evaluate the role of vitamin D in the physiologic response to phosphorus depletion (P depleton) and the response to vitamin D administration in P depletion, we studied vitamin D-deficient (-D) rats, fed either a normal or low phosphorus diet and then injected intraperitoneally on alternate days with replacement vitamin D(3), 1.25 mug qod (D(3)); 1.25-dihydroxy-vitamin D(3)[1,25(OH)(2)D(3)] in physiologic, 54 ng qod (LD), and pharmacologic doses, 400 ng qod (HD); or vehicle alone (-D). The following results were obtained: (a) With P depletion, urinary excretion of inorganic phosphorus (Pi) fell to almost undetectable levels in -D rats, and two physiologic features of P depletion a calcemic effect and hypercalciuria, ensued. (b) With administration of vitamin D(3) or 1,25(OH)(2)D(3) in either doses to P-depleted rats, the renal retention of Pi was unaltered despite a significant elevation of serum Pi. (c) The calcemic response to P depletion was accentuated by vitamin D sterols, and the hypercalciuria of P depletion was reduced by 1,25(OH)(2)D(3), HD > LD > D(3). (d) In -D animals receiving normal Pi (+P), D(3), and 1,25(OH)(2)D(3), both LD and HD produced a significant calcemic and phosphatemic effect. (e) Urinary Pi excretion in +P animals was reduced slightly by vitamin D(3) whereas 1,25(OH)(2)D(3), both LD and HD, lowered urinary Pi markedly despite an increased serum Pi. (f) The serial values of serum Ca and Pi and urinary Ca in PD rats and the sequential values for urinary and serum Pi in +P rats indicated more rapid effects of 1,25(OH)(2)D(3), both HD and LD, compared with D(3). We conclude that: (a) The renal adaptation and physiologic response to PD does not require the presence of vitamin D. (b) 1,25(OH)(2)D(3) may directly enhance the renal tubular reabsorption of Pi even as serum Pi rises. (c) A hypocalciuric action of 1,25(OH)(2)D(3) in rats on low phosphorus diet could be direct or occur as a consequence of an increase in serum Pi produced by 1,25(OH)(2)D(3). The different sequential renal response to D(3) compared with 1,25-(OH)(2)D(3) raises the possibility that other natural forms of vitamin D(3) [i.e., 25(OH)D(3), 24,25(OH)(2)D(3), etc.] which may be present in vitamin D-fed rats but not those given only 1,25(OH)(2)D(3), could modify the actions of 1,25(OH)(2)D(3).

Has vitamin D a direct renal effect on the tubular reabsorption of phosphate? A study in parathyroidectomized (PTX) and non-PTX man

The effect of 1-alpha-hydroxycholecalciferol (1alpha-OH-D3) on the renal handling of phosphate and the immunoreactive parathyroid hormone in serum (i-PTH) has been studied in 10 patients with a wide range of glomerular filtration rate (GFR), maximal tubular reabsorption of phosphate (TmP) and i-PTH. The patients were treated with 2 microgram 1alpha-OH-D3 per day for approximately 80 days. Before and after this period of treatment, the TmP, i-PTH, 51Cr EDTA clearance, extracellular volume, standard bicarbonate, and serum calcium were measured in each patient. The TmP/GFR ratio was used as an index of the renal handling of phosphate. The index increased significantly (mean 26.5%, p less than 0.01) during the treatment, while i-PTH decreased significantly (mean 37.0%, p less than 0.01). A significant inverse correlation was demonstrated between the TmP/GFR index and i-PTH both before (r = -0.87; p less than 0.001) and after (r = -0.79; p less than 0.01) the administration of 1alpha-OH-D3, while none of the other factors investigated were correlated to the index. This may suggest that the stimulating effect of biologically active vitamin D on the tubular reabsorption of phosphate is mediated via the parallel suppression of PTH, but does not exclude that biologically active vitamin D exerts a direct effect on the human renal tubule. Therefore, the effect of 1alpha-OH-D3 was studied in 5 totally parathyroidectomized patients, in whom concomitant suppression of PTH would not occur. Estimation of TmP/GFR was performed 1) when the patients were vitamin D depleted and hypocalcemic, and 2) after 14-27 days of treatment with 1alpha-OH-D3 to obtain stable normocalcemia. In patients with absent parathyroid function, no increasing effect of 1alpha-OH-D3 on TmP/GFR could be demonstrated. It is therefore concluded 1) that 1alpha-OH-D3 exhibits no antiphosphaturic effect in the absence of PTH and 2) that the previously demonstrated antiphosphaturic effect of 1alpha-OH-D3 in man is mediated via a concomitant suppression of PTH.

Effect of 1,25-dihydroxyvitamin D3 on the renal handling of Pi in thyroparathyroidectomized rats

The kidney adapts its tubular capacity to transport inorganic phosphate (P(i)) according to the dietary supply of P(i) in both intact and thyropara-thyroidectomized (TPTX) rats. However, in TPTX rats the capability of the renal tubule to adapt to a high P(i) diet is diminished. In TPTX rats the production of the active vitamin D(3) metabolite, 1,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)], is also reduced. 1,25-(OH)(2)D(3) has been shown to have a marked effect on P(i) metabolism. Therefore the question arises whether the deficient production of 1,25-(OH)(2)D(3) contributes to the alteration of the tubular transport of P(i) observed in chronically TPTX rats. In the present investigation, vitamin D-replete rats were sham operated (SHAM) or thyroparathyroidectomized and then pair fed diets containing either 0.2 or 1.2 g/100 g P for 7 days. During this period, groups of SHAM and TPTX rats received i.p. 2 x 13 pmol/day of 1,25-(OH)(2)D(3), a dose which was shown to just normalize the decreased intestinal absorption of Ca and P(i) in TPTX rats. The capacity of tubular P(i) transport was then assessed by measuring the fractional excretion of P(i) (FEP(i)) at increasing plasma P(i) concentration ([P(i)](Pl)) obtained by acute infusion of P(i). The results show that in SHAM rats fed either P diet, 1,25-(OH)(2)D(3) has no effect on the renal handling of P(i). In TPTX rats fed 1.2 g/100 g P diet, 1,25-(OH)(2)D(3) increases FEP(i) over a wide range of [P(i)](Pl.) In TPTX rats fed a 0.2 g/100 g P diet, 1,25-(OH)(2)D(3) does not alter FEP(i) up to a [P(i)](Pl) of 3.0-3.5 mM, but does increase it at higher [P(i)](Pl.) In fact, on both diets TPTX rats supplemented with 1,25-(OH)(2)D(3) appear to have the same renal handling of P(i) as SHAM counterparts. The effect of 1,25-(OH)(2)D(3) was not associated with a change in urine pH or in urinary excretion of cyclic AMP and was maintained under marked extracellular volume expansion. It was associated with a rise in plasma calcium in the TPTX rats fed the high, but not the low, P diet. In TPTX rats fed 1.2 g/100 g P diet, 25-hydroxyvitamin D(3) in doses of 2 x 130 or 2 x 1,300 pmol/day i.p. did not increase FEP(i.)In conclusion, 1,25-(OH)(2)D(3) administered in physiological amounts to TPTX rats restores to normal the capability of the renal tubule to excrete P(i) and to adapt to large variation in dietary P(i). The results suggest that 1,25-(OH)(2)D(3) plays an important role in the regulation of the renal handling of P(i) and that the chronic change in the tubular capacity to transport P(i) after TPTX may be due to the decreased formation of 1,25-(OH)(2)D(3).

The role of phosphate in the action of vitamin D on the intestine

The response of chick intestine to vitamin D and its metabolites was studied in an organ culture preparation of chick ileum explants. Both 25-hydroxycholecalciferol (25-OHD(3)) at a concentration of 20 ng/ml or greater and 1,25-dihydroxycholecalciferol [1,25-(OH)(2)D(3)] at a concentration of 50 pg/ml or greater stimulated the rate of accumulation of [(32)P]phosphate and (45)Ca by the explants and the incorporation of [(3)H]thymidine into DNA. The accumulation of [(32)P]phosphate by the explants was against a concentration gradient and inhibited by ouabain and dinitrophenol. Two saturable mechanisms appeared to mediate the cellular accumulation of phosphate with K(a) of 0.0047 and 0.125 mM, respectively. The V(max) of the lower affinity transport mechanism was accelerated by 1,25-(OH)(2)D(3). Actinomycin D (5.0 mug/ml) did not block the intestinal response to 1,25-(OH)(2)D(3) stimulation of both [(32)p]phosphate and (45)Ca accumulation. Significant stimulation of [(32)P]phosphate accumulation was observed 30 min after the addition of 1,25-(OH)(2)D(3), preceding the sterol-induced increase in the rate of (45)Ca uptake by 30 min and the sterol-induced increase in [(3)H]thymidine incorporation into DNA by 150 min. Increasing extracellular phosphate concentration to 3.0 mM increased [(3)H]thymidine incorporation into DNA and the rate of (45)Ca uptake by the explants. Reducing extracellular phosphate concentration to 0.05 mM attenuated the response of the explants to 1,25-(OH)(2)D(3). From these observations it is postulated that the primary action of vitamin D sterols in the intestine is to enhance the ability of the mucosal cell to accumulate phosphate. The data suggest that restoration of intracellular phosphate levels may then permit expression of the cells' response to vitamin D sterols.

Metabolic acidosis in the vitamin D-deficient chick

In vitamin D-deficient chicks raised from age 1 day on a vitamin D-deficient diet, hyperchloremic metabolic acidosis accurred at 3 wk and persisted. Within 24 hr of administration of vitamin D, the acidosis and hypocalcemia were attentuated; during the subsequent 72 hr the severity of the metabolic acidosis but not that of the hypocalcemia was further attenuated. That further attenuation occurred despite hypocalcemia of unchanging severity and presumed continuing secondary hyperparathyroidism suggests the possibility that vitamin D deficiency may be a requirement for the expression of metabolic acidosis. Since in vitro and in vivo studies suggest that subphysiologic values of media and blood pH, respectively, are attended by reduced production of 1,25-(OH2D3, the most biologically active vitamin D metabolite known, the occurrence of acidosis in vitamin D deficiency may compound its metabolic consequences. The possible effects of acidosis must be considered in interpreting results of investigations of vitamin D metabolism in vitamin-D-deficient chicks.

Influence of vitamin D on bicarbonate reabsorption

The effects of acute administration of 25 hydroxycholecalciferol on bicarbonate reabsorption was studied in dogs. 25 hydroxycholecalciferol infused into the renal artery at a dose of 0.2 microgram/kg/hour led to a small but significant bilateral increase in bicarbonate reabsorption in intact dogs. 25 hydroxycholecalciferol had no effect on bicarbonate reabsorption in thyroparathyroidectomized dogs. The phosphaturia of bicarbonate administration was not altered by 25 hydroxycholecalciferol administration.