Biologic effects of 1,25-dihydroxycholecalciferol (a highly active vitamin D metabolite) in acutely uremic rats

Calcitriol, calcidiol, parathyroid hormone, and fibroblast growth factor-23 interactions in chronic kidney disease

Objective

To review the inter-relationships between calcium, phosphorus, parathyroid hormone (PTH), parent and activated vitamin D metabolites (vitamin D, 25(OH)-vitamin D, 1,25(OH)₂-vitamin D, 24,25(OH)₂-vitamin D), and fibroblast growth factor-23 (FGF-23) during chronic kidney disease (CKD) in dogs and cats.

Data Sources

Human and veterinary literature.

Human Data Synthesis

Beneficial effects of calcitriol treatment during CKD have traditionally been attributed to regulation of PTH but new perspectives emphasize direct renoprotective actions independent of PTH and calcium. It is now apparent that calcitriol exerts an important effect on renal tubular reclamation of filtered 25(OH)-vitamin D, which may be important in maintaining adequate circulating 25(OH)-vitamin D. This in turn may be vital for important pleiotropic actions in peripheral tissues through autocrine/paracrine mechanisms that impact the health of those local tissues.

Veterinary Data Synthesis

Limited information is available reporting the benefit of calcitriol treatment in dogs and cats with CKD.

Conclusions

A survival benefit has been shown for dogs with CKD treated with calcitriol compared to placebo. The concentrations of circulating 25(OH)-vitamin D have recently been shown to be low in people and dogs with CKD and are related to survival in people with CKD. Combination therapy for people with CKD using both parental and activated vitamin D compounds is common in human nephrology and there is a developing emphasis using combination treatment with activated vitamin D and renin-angiotensin-aldosterone-system (RAAS) inhibitors.

The biology and pathology of vitamin D control in bone

Vitamin D is a steroid pro-hormone, whose active metabolite binds the vitamin D receptor (VDR) which, in turn, binds to DNA sequences on target genes as a heterodimer with the retinoid-X receptor, resulting in regulation of gene expression. The vitamin D pro-hormone can be synthesized in the skin, in response to ultraviolet radiation; however, dietary sources have become increasingly important as a result of cultural changes over the past few centuries. Based on its initial discovery as an anti-rachitic factor, studies of the role of vitamin D and its receptor have largely focused on the skeleton. Investigations into the pathophysiologic basis and therapeutic responses of skeletal disorders associated with impaired vitamin D action have led to the identification of the molecular pathways involved in hormone activation and regulation of gene expression by the liganded VDR. These studies have also demonstrated that the skeletal actions of the VDR and its ligand are largely redundant if normal mineral ion homeostasis can be maintained by other means. However, investigations in animal models with tissue-specific ablation of the VDR or the enzyme required for hormone activation have demonstrated novel actions in skeletal tissues. The active vitamin D metabolite has been shown to have both paracrine and endocrine actions in other tissues as well.

25-hydroxycholecalciferol in poultry nutrition

Vitamin D is a complex of secosteroids that must undergo metabolic alterations to reach optimal biological activity. The parent compounds 1) ergocalciferol (D2) and 2) cholecalciferol (D3) can be synthesized in the leaves of many plants or in the skin of most animals, respectively. Transport of vitamin D steroids after absorption is associated with vitamin D binding proteins (DBP). In general, the relative binding affinities of the vitamin D steroids are: 25-hydroxy vitamin D3 [25-(OH)D3] = 24,25-dihydroxy vitamin D3 [24,25-(OH)2D3] = 25,26-dihydroxy vitamin D3 [25,26-(OH)2D3] > 25-hydroxy vitamin D2 (25-(OH)D2) > 1,25-dihydroxy vitamin D3 [1,25-(OH)2D3] > vitamin D3. The DBP in poultry does not bind D2 forms effectively, and therefore poultry can not use this form of vitamin D adequately. The concentration of 25-(OH)D3 in blood seems to be well correlated with dietary vitamin D intake or exposure to ultraviolet light. The 1 alpha hydroxylase enzyme in the kidney is subject to negative feedback regulation and is critical for formation of the active metabolite 1,25-(OH)2D3. The intracellular vitamin D receptor (VDR) specifically binds 1,25-(OH)2D3 and is necessary for cellular action. Increased levels of two to three orders of magnitude are required for 25-(OH)D3 to compete with 1,25-(OH)2D3 for binding on VDR. Feeding studies with 25-(OH)D3 suggest it has nearly twice the activity of vitamin D3. Hatchability studies have shown that 25-(OH)D3 supports good fertility and hatchability, whereas hens fed only 1,25-(OH)2D3 did not have normal hatchability. Likewise, 1,25-(OH)2D3 seems to reach toxic levels at dietary concentrations only two to three times optimal dietary levels whereas feeding 25-(OH)D3 for extended periods at levels 8 to 10 times requirement seems to have no adverse effects. It seems that 25-(OH)D3 is the most active metabolite of vitamin D3, ultimately capable of supporting both cellular functions and embryonic development in chickens and turkeys when fed as the sole source of vitamin D3.

Regulation of calcitriol receptor and its mRNA in normal and renal failure rats

Homologous up-regulation of calcitriol receptor (VDR) by calcitriol is believed to be a transcriptional event. In this experiment, we studied the effect of calcitriol on VDR in normal and renal failure rats. The time course of the effect of calcitriol on VDR mRNA showed a biphasic change in VDR mRNA in response to calcitriol. The concentration of intestinal VDR mRNA increased at six hours and reached peak levels approximately 15 hours after calcitriol injection. Thereafter, the mRNA began to decrease and by 48 hours the level had declined to below the control values. The VDR levels also increased, though they lagged behind the VDR mRNA, and nearly plateaued at 24 hours after calcitriol treatment. In renal failure, the concentrations of VDR were lower and the levels of VDR mRNA were higher than the respective values of normal rats, suggesting that VDR synthesis was inhibited at post-transcriptional sites. Chronic administration of calcitriol increased the VDR but lowered the VDR mRNA levels in both normal and renal failure rats. Infusion of uremic ultrafiltrate to normal rats resulted in lower VDR and higher VDR mRNA levels similar to those found in rats with renal failure. The results indicate that uremic toxins are responsible for the low VDR and high VDR mRNA in renal failure.

Production and metabolic clearance of calcitriol in acute renal failure

Metabolic clearance rate (MCR) and production rate (PR) of calcitriol were studied three and seven days after ischemic acute tubular necrosis (ATN). Creatinine clearance was decreased three days after clamping the renal arteries (0.42 +/- 0.03 ml/min/100 g, N = 6 in ATN vs. 0.68 +/- 0.09, N = 7 in sham controls; P less than 0.001). Plasma concentrations (24.1 +/- 1.9 pg/ml) and PR of calcitriol (9.8 +/- 0.91 ng/kg/day) were significantly lower in ATN rats three days after ischemic insult when compared to sham control rats, respectively (76.6 +/- 7.3 pg/ml, and 29.6 +/- 3.3 ng/kg/day; both P less than 0.01). The MCRs of calcitriol were not different between ATN (0.28 +/- 0.02 ml/min/kg) and sham control rats (0.27 +/- 0.01). By the seventh day after ischemic injury, when creatinine clearance of ATN rats returned to normal, both the PR and plasma concentrations of calcitriol also returned to normal values in these animals. In order to assess the effect of uremia on calcitriol metabolism, MCR and PR of calcitriol were measured in rats with reinfusion of their urines for 24 hours. The PR of calcitriol was significantly decreased (9.42 +/- 1.21; vs. controls, 20.5 +/- 2.9 ng/kg/day, P less than 0.001) in uremic animals. Since decreased PR of calcitriol was also accompanied by decreased MCR of calcitriol, plasma concentrations of calcitriol of the uremic rats with intact kidneys remained within normal values. We conclude that the PR of calcitriol is decreased early in ATN rats. Although the MCR was not decreased in mild ATN rats, it may decrease in severe acute renal failure.

gamma-Interferon stimulates production of 1,25-dihydroxyvitamin D3 by normal human macrophages

We show for the first time that normal human pulmonary alveolar macrophages (PAM) markedly enhance their basal rate of the production of [3H]-1,25(OH)2D3 when cultured in the presence of recombinant gamma-interferon (gamma-IFN). The rate of conversion of [3H]-25(OH)D3 to [3H]-1,25(OH)2D3 was dose-dependent in a linear fashion. A maximal production of 1,25(OH)2D3 by PAM occurred after exposure of PAM to gamma-IFN for one day. This maximum plateau-level was sustained for at least five days. The authenticity of the putative 1,25(OH)2D3 obtained from PAM was tested by demonstrating the exact comigration of [3H]-1,25(OH)2D3 with chemically synthesized 1,25(OH)2D3 in four different HPLC-systems.

Vitamin D and skeletal tissues

It is now accepted that vitamin D is an integral part of a complex endocrine system, one with far-reaching implications in mineral metabolism. Reviews of the sources, functions and metabolism of vitamin D, as currently understood, are presented as a prelude to discussions of the role of vitamin D in calcium and phosphorous homeostatis and possible specific roles for vitamin D in mineralized tissues. Data describing a possible regulatory function for vitamin D in bone and bone protein metabolism are presented. Some of the controversy which presently exists regarding the biochemical mechanism of the action of this vitamin is discussed. Finally, the possible relationship of vitamin D and disorders of skeletal tissues is described.

Naturally occurring 24,25-dihydroxyvitamin D3 is a mixture of both C-24R and C-24S epimers

Tritium-labeled 24,25-dihydroxyvitamin D3 was prepared both in vitro, by using chick kidney homogenates, and in vivo in rats from [26,27-methyl-3H]25-hydroxyvitamin D3. These compounds were mixed with synthetic 24(R),25- and 24(S),25-dihydroxyvitamin D3, converted to the corresponding trimethylsilyl ether derivatives, and analyzed by a high-pressure liquid chromatography procedure that separates the derivatized isomers. The tritium-labeled 24,25-dihydroxyvitamin D3 derivatives were found to be a mixture of both the 24(R) and 24(S) epimers; the ratio was found to be 96.4:3.6 in chick kidney homogenates and 96.8:3.2 in the serum of rats under physiological conditions. In addition, nonradioactive 24,25-dihydroxyvitamin D3 isolated from the serum of rats given large doses of vitamin D3 was shown to be an 89.5:10.5 mixture of the 24(R) and 24(S) isomers. When 25-hydroxy-24-oxo-vitamin D3 was utilized as a substrate, it was found to be more selectively reduced to 24(S),25-dihydroxyvitamin D3 than 24(R),25-dihydroxyvitamin D3 by the renal enzyme. The 24(S),25-dihydroxyvitamin D3 has been identified by ultraviolet absorption spectrophotometry, cochromatography with an authentic standard, and mass spectrometry. The reduced metabolites of 25-hydroxy-24-oxo-vitamin D3 were a 1:50 mixture of the 24(R) and 24(S) epimers. There are two known metabolic pathways leading to 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3; one is 24(R)-hydroxylation of 25-hydroxyvitamin D3 and the other is reduction of 25-hydroxy-24-oxo-vitamin D3. In contrast, 24(S),25-dihydroxyvitamin D3 is produced only by reduction of 25-hydroxy-24-oxo-vitamin D3 in the kidney. Therefore, naturally occurring 24,25-dihydroxyvitamin D3 is a mixture of the 24(R) and 24(S) isomers, and not just the 24(R) isomer as reported previously.

Vitamin D3 metabolites in rat epididymis: high 24,25-dihydroxy vitamin D3 levels in the cauda region

Normal male rats received six subcutaneous injections of 8.0 pmoles of tritiated 25-hydroxy vitamin D3 ([3H]25(OH)D3) or one intrajugular injection of 8.0 pmoles of high specific radioactivity [3H]-25(OH)D3. Lipid extracts of several tissues including the reproductive organs were subjected to sephadex LH-20 chromatography to determine the tissue distribution of the injected material and of the in vivo produced dihydroxylated cholecalciferol metabolites. The nature of the putative 25(OH)D3 and the 24,25-dihydroxy vitamin D3 (24,25(OH)2D3) from epididymis tissue was confirmed by high performance liquid chromatography (HPLC). The epididymis levels of 24,25(OH)2D3 were considerably higher in the cauda epididymis compared to kidney and caput epididymis levels. The other metabolites levels in this tissue were similar to those determined in the kidneys. The amounts of the three metabolites found in all other tissues were well below the cauda epididymis or kidney levels. The findings suggest a possible physiological role for 24,25(OH)2D3 in the epididymis, and are also consistent with data of others which indicated a possible action of 1,25-dihydroxy vitamin D3 (1,25(OH)2D3) in rat reproductive tissues.

23,25-Dihydroxy-24-oxovitamin D3: a metabolite of vitamin D3 made in the kidney

Kidney homogenates of rats produced a new metabolite of 25-hydroxyvitamin D3 which has been isolated in pure form after five column chromatographic steps. It was identified as 23,25-dihydroxy-24-oxovitamin D3 by means of ultraviolet and infrared absorption spectrophotometry, mass spectrometry, and proton nuclear magnetic resonance spectrometry. The stereochemistry at the C-23 position is as yet unknown. 25-Hydroxy-24-oxovitamin D3, which also has been isolated in pure form from this system, was found to be the precursor of the new metabolite in vitro. The production of the new metabolite was induced by two different methods: (a) perfusion of the kidneys with 1,25-dihydroxyvitamin D3 contained in the perfusate and (b) injection of 1,25-dihydroxyvitamin D3 in the intact animal. 23,25-Dihydroxy-24-oxovitamin D3 was not biologically active in an assay for intestinal calcium transport and bone calcium mobilization in the vitamin D deficient chick at a dose level of 5.3 nmol. A metabolic pathway is proposed to describe the results; it leads from 25-hydroxyvitamin D3 leads to 24(R),25-dihydroxyvitamin D3 leads to 25-hydroxy-24-oxovitamin D3 leads to 23,25-dihydroxy-24-oxovitamin D3.

1- but not 24-hydroxylation of vitamin D is required for growth and reproduction in rats

This study examines whether 1,25-dihydroxyvitamin D3 or 24,24-difluoro-25-hydroxyvitamin D3, an analogue of 25-hydroxyvitamin D3 blocked from undergoing 24-hydroxylation, can maintain normal growth and reproduction in the female rat. Vitamin D-deficient weanling rats were maintained from weaning through mating, pregnancy, and lactation with either 1,25-dihydroxyvitamin D3 (given by continuous subcutaneous infusion), 24,24-difluoro-25-hydroxyvitamin D3, 25-hydroxyvitamin D3, or vehicle. Body weight, plasma calcium levels, estrous cycling time, ability to give birth to live pups, litter weight, number of pups per litter, dam plasma calcium level during lactation, and pup growth to 9 wk of age were recorded. No striking differences were observed between the 25-hydroxyvitamin D3 groups and either the 1,25-dihydroxyvitamin D3 group or the 24,24-difluoro-25-hydroxyvitamin D3 group. However, significant differences in most parameters were observed between the vitamin D-deficient and metabolite- or analogue-dosed rats. The results demonstrate that 1,25-dihydroxyvitamin D3 and/or one of its metabolites is sufficient to maintain normal growth, development, and reproductive functions in the female rat. Because 24,24-difluoro-25-hydroxyvitamin D3 cannot be hydroxylated at C-24, the 24-hydroxylation of 25-hydroxyvitamin D3 is not essential for normal growth, development, and reproduction in the female rat.

Effects of anticonvulsants and methotrexate on calcium disposition

Administration of phenytoin for 5 days significantly reduced oral absorption and bone uptake of radiolabelled calcium in mice. Acute treatment with phenytoin had no effect on calcium disposition. Pretreatment (1--5 days) with phenobarbital also had no effect of calcium disposition. Methotrexate pretreatment for 5 days increased the bone uptake of radiolabelled calcium presumably by decreasing calcium renal clearance. It is concluded that phenytoin induced reduction in calcium metabolism is unrelated to a state of functional folate deficiency.

Vitamin D metabolism and calcium absorption

Vitamin D along with parathyroid hormone (PTH) and calcitonin (CT) are the three principal effectors of calcium and phosphorus homeostasis. The secosteroid, vitamin D3, is subject to metabolic conversion to its biologically active form(s) 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] prior to initiation of its physiologic responses in the intestine and skeletal system. The production of 1,25(OH)2D3 is stringently regulated by a variety of endocrine signals including PTH as well as the "calcium needs" of the organism. At the target intestine, 1,25-(OH)2D3 stimulates the intestinal absorption of calcium via a mechanism analogous to that of other steroid hormones. Definitive biochemical evidence exists supporting the existence in the intestine of a highly specific protein receptor for 1,25(OH)2D3. After formation of the steroid-receptor complex, it migrates to the nucleus of the cell and stimulates messenger-RNA synthesis for proteins (including a calcium-binding protein) which are necessary for the generation of the biologic response. Current efforts to biochemically characterize vitamin D-mediated intestinal calcium transport include efforts to understand the role of calcium-binding protein in this process, as well as to identify other protein components present either in the brush border or basal lateral membranes.

Regulation of the metabolism of 25-hydroxyvitamin D3 in primary cultures of chick kidney cells

A primary chick kidney cell culture is described, capable of forming 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], 24,25-dihydroxyvitamin D3 [24,25(OH)2D3], and 1,24,25-trihydroxyvitamin D3 [1,24,25(OH)3D3] over several days. The apparent Km values were 0.125 microM for the 1-hydroxylase and 2.1 microM for the 24-hydroxylase. Exogenous 1,25(OH)2D3 decreased 1-hydroxylase and increased 24-hydroxylase within 4 h. 24,25(OH)2D3 produced similar effects, but only in the absence of fetal calf serum. R and S isomers of 1,24,25(OH)3D3 were about fives times less active than 1,25(OH)2D3. Bovine parathyroid hormone stimulated the 1- and reduced the 24-hydroxylase in 6 h, but this only occurred in cultures either previously treated with 1,25(OH)2D3 and EGTA to lower Ca to 0.8 mM or in cultures grown in the presence of 25-hydroxyvitamin D3 (25(OH)D3). Under the latter condition, the sensitivity to bovine parathyroid hormone was enhanced, 0.04 U/ml producing a maximum response. Synthetic aminoterminal tetratriacontapeptide (1-34) human parathyroid hormone was equally effective. In the absence of D metabolites, estradiol for 6 h produced a dose-dependent inhibition of the 1-hydroxylase, but no change in the 24-hydroxylase. Progesterone, testosterone, and corticosterone had no significant effect. In cultures grown in the presence of 25(OH)D3 no reproducible effects were obtained with either 1 microM estradiol or 1 microM testosterone, alone or in combination, but 5 microM corticosterone decreased the 1- and increased the 24-hydroxylase. Changes in Ca and P concentrations of the medium as well as addition of ethane-l-hydroxy-1, 1-diphosphate for 48 h did not affect any of the hydroxylase activities. The modulation of the hydroxylase activities by vitamin D3 metabolites and parathyroid hormone suggests that these factors regulate the renal hydroxylase by direct actions, whereas it would appear that ethane-1-hydroxy-1,1-diphosphate, Ca, P, and steroid may exert their influence indirectly.

Morphometric analysis of skeletal response to vitamin D in uremic rats fed a low calcium diet

The response of tibial metaphyses to pharmacologic levels of vitamin D in uremic rats fed a low calcium diet was evaluated morphometrically. Uremic (5/6 nephrectomized) rats given vitamin D had increased percent metaphyseal hard tissue, trabecular surface perimeter and percent trabecular osteoid surface and reduced numbers of osteoblasts and osteoclasts per millimeter of trabecular perimeter compared to either uremic rats given placebo or sham-operated rats given vitamin D. It was concluded that the resistance of metaphyseal trabeculae in uremic rats to vitamin D was due in part to the increase in osteoid-covered surfaces which inhibited osteoclasis and subsequent remodeling. The pathogenesis of worsening osteomalacia as a consequence of vitamin D administration to uremic rats on a low calcium diet remains unclear.

Intestinal calcium absorption, serum phosphate, and parathyroid hormone in patients with chronic renal failure and osteodystrophy before and during hemodialysis

In 34 patients with chronic renal failure (CRF), fractional 47calcium absorption (Fa47Ca) was measured by an external counting method. A significant correlation was found with impairment of renal function, as expressed by the creatinine clearance. There was also a significant correlation of Fa47Ca with the serum phosphate (SeP) level and of immunoreactive parathyroid hormone (iPTH) with renal function. When the relationship of both SeP and Fa47Ca with creatinine clearance was excluded, no partial correlation between SeP and Fa47Ca appeared to exist. A significant increase of Fa47Ca and serum Ca and a significant decrease of SeP and iPTH were found in 12 patients 2 to 15 months after they were put on intermittent hemodialysis. The possible influence of SeP on intestinal calcium absorption is discussed, and it is suggested that impairment of intestinal absorption of calcium is not a main factor in development of renal osteodystrophy.

The interaction of 1,25-dihydroxyvitamin D3 with its intestinal mucosa receptor: kinetic parameters and structural requirements

Vitamin D3 and its metabolites comprise an endocrine system which plays a critical role in calcium homeostasis. The active form of vitamin D3 is 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. Chromatin localization of 1,25(OH)2D3 and sucrose density gradient centrifutation have demonstrated the presence of an intestinal mucosa cytosol receptor which specifically binds 1,25(OH)2D3. The kinetic parameters of 1,25(OH)2D3 binding to its receptor have been determined by hydroxylapatite and reconstituted chromatin cytosol assays. Utilization of these assays has also permitted a determination of the precise structural requirements of the vitamin D ligand for the intestinal receptor. Furthermore, it has been possible to propose two receptor-ligand models which are capable of accommodating the conformationaly modile A ring of the vitamin D seco-steroids.

The effect of 1alpha(OH)D3 and 1alpha,25(OH)2D3 on the bone in patients with renal osteodystrophy

Six patients with chronic renal disease and variable degrees of renal osteodystrophy were treated for three weeks with either 1alpha,25-dihydroxyvitamin D3 (1alpha25(OH)D3) or 1alpha,hydroxyvitamin D3 (1alpha(OH)D3) and both the biochemical and osseous responses measured. The most consistent changes seen were an increase in serum calcium concentration to normal, a decrease in immunoreactive parathyroid hormone toward normal, an increase in the extent of the calcification front and a decrease in the extent of fibrous dysplasia in the marrow cavity. Two important parameters which did not change significantly were serum alkaline phosphatase activity and the osteoid volume. These data, in conjunction with that from previous studies, indicate that therapy with 1alpha,25(OH)2D3 or 1alpha(OH)D3 does not heal the osteomalacia of renal osteodystrophy, but that it does suppress the secondary hyperparathyroidism, and ameliorate the osteitis fibrosa seen in patients with chronic renal disease. They raise the likelihood that additional factors, such as metabolites of vitamin D other than 1alpha,25(OH)2D3, play a role in regulating bone formation and/or mineralization.

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.

Intestinal 1,25-dihydroxyvitamin D3 binding protein: specificity of binding

The binding of metabolites of vitamin D and their analogs to the 3.7S chick intestinal cytosol receptor protein has been specifically studied by competitive binding techniques and polyethylene glycol precipitation of the complex. The structural requirements for the interaction between the vitamin D molecule and the receptor could be assessed without the nuclear chromatin binding step. These measurements have shown that 1,25-dihydroxyvitamin D3 and 1,25-dihydroxyvitamin D2 are equally competitive and are the most active. Of the structural features of the compounds, the 1 alpha-hydroxyl is most important followed by the 25-hydroxyl and the 3 beta-hydroxyl. The addition of a second hydroxyl near carbon 25 markedly reduces binding whether on the 26 carbon or the 24 carbon. A hydroxyl on C-24 could substitute to some degree for the 25-hydroxyl inasmuch as 24-hydroxyvitamin D3 was much more effective than vitamin D3 but less effective than 25-hydroxyvitamin D3. In general the patterns of binding affinities correlated well with the biological activity of the various analogs strongly supporting a physiological role for the 1,25-dihydroxyvitamin D3 binding protein. It also suggests that of the two-step receptor mechanism, the structural specificity is located in the initial interaction of the 1,25-dihydroxyvitamin D3 and the cytosol receptor.

Experimental diabetes reduces circulating 1,25-dihydroxyvitamin D in the rat

Duodenal calcium absorption and a vitamin D-dependent duodenal calcium-binding protein are depressed in rats with alloxan- or streptozotocin-induced diabetes. To test for possible abnormal vitamin D metabolism in diabetes we measured serum concentrations of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D in control, streptozotocin diabetic, and insulin-treated diabetic rats. The serum concentration of 1,25-dihydroxyvitamin D was depressed in untreated diabetic rats to one-eighth of the level in controls and was restored to control levels by insulin treatment. The serum concentration of 25-hydroxyvitamin D was the same in all three groups. Hence, effects of diabetes on duodenal calcium transport can be explained by reduced concentrations of 1,25-dihydroxyvitamin D resulting either from failure of renal 1alpha-hydroxylation of 25-hydroxyvitamin D or increased catabolism of 1,25-dihydroxyvitamin D.

The biological activity of synthetic 25,26-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol in vitamin D-deficient rats

The biological activity of synthetic 24,25 and 25,26 diOHD3 was studied in vitamin D-deficient rats. The purpose of this study was to investigate the influence of small doses of both metabolites (0.125-0.250 mug) upon intestinal calcium transport and bone calcium mobilization. Both metabolites were able to increase calcium absorption in rats maintained on a calcium-deficient diet, but failed to do it in rats on a normal calcium diet. Bilateral nephrectomy suppressed this effect. The "bone calcium mobilization" of both derivatives was measured in vitamin D and calcium- or phosphorus-deprived rats after one intravenous dose. When serum calcium was initially low, 24,25 and 25,26 diOHD3 increased serum calcium moderately, but the increment was only significant with 24,25 diOHD3. When serum calcium was normal before the injection, both metabolites decreased serum calcium significantly, and the decrease was greater with 24,25 diOHD3. Intraperitoneal administration of the metabolites for 5 consecutive days produced a significant increase of calcium in serum and bone ash.

[Immunoreactive parathyroid hormone, 25-hydroxycalciferol and bone histology in renal osteodystrophy (author's transl)]

Immunoreactive parathyroid hormone (iPTH) and 25-hydroxycalciferol (25(OH)D) serum levels were determined in 32 patients with renal osteopathy, they were correlated with the results of bone biopsy and other clinical parameters. iPTH was closely related to bone histology, it did not correspond to serum calcium and alkaline phosphatase, but the correlation to serum phosphate was statistically significant. 25(OH)D levels were not related to the histological findings of osteomalacia or increased bone resorption, while a correlation between the vitamin D metabolite and serum calcium could be observed. Since iPTH and 25(OH)D levels exhibited a significant correlation, an inhibitory effect of 25(OH)D on parathyroid gland function in renal failure was discussed.

Metabolism of 1,25-dihydroxyvitamin D3: evidence for side-chain oxidation

Approximately 7% of a 650-pmol dose of 25-hydroxyl[26,27-14C]vitamin D3 and 25% of a 325-pmol dose of 1,25-dihydroxyl[26,27-14C]vitamin D3 are metabolized to 14CO2 by vitamin D deficient rats. Nephrectomy prevents the metabolism of 25-hydroxy[26,27-14C]vitamin D3 to 14CO2 but not that of 1,25-dihydroxy[26,27-14C]vitamin D3. Less than 5% of the 14C from 24,25-dihydroxy[26,27-14C]vitamin D3 is metabolized to 14CO2. Feeding diets high in calcium and supplemented with vitamin D3 markedly diminishes the amount of 14CO2 formed from 25-hydroxy[26,27-14C]vitamin D3 but not that from 1,25-dihydroxyl[26,27-14C]vitamin D3. These results provide strong evidence that only 1-hydroxylated vitamin D compounds and especially 1,25-dihydroxyvitamin D3 undergo side-chain oxidation and cleavage to yield an unknown metabolite and CO2.

Jejunal and ileal absorption in patients with chronic renal disease. Effect of 1alpha-hydroxycholecalciferol

Calcium absorption in 30-cm segments of small intestine was measured by constant perfusion of test solutions containing different concentrations of calcium gluconate. In both the jejunum and ileum, calcium absorption rates increased progressively as luminal calcium concentration was increased stepwise between 1 and 20 mM. Although calcium transport was not saturable within these limits, unidirectional flux ratios of calcium (47Ca) suggest that calcium absorption is active in both the jejunum and ileum. Calcium absorption in patients with chronic renal disease was markedly depressed in both regions of the small intestine. This was due to decreased flux out of the lumen; flux in the reverse direction was normal. Flux ratios in the renal disease patients showed no evidence for active calcium transport. Treatment of these patients for 1 wk within 2 mug/day of 1alpha-hydroxycholecalciferol [1alpha-(OH)-D3] restored net calcium absorption and unidirectional calcium flux out of the lumen to normal values in the jejunum; in the ileum, 1alpha-(OH)-D3 increased calcium absorption 60-83% of normal at the various luminal calcium concentrations. 1alpha(OH)-D3 had no effect on unidirectional calcium flux into the lumen or on xylose and electrolyte absorption in either area of the small intestine.

Decrease in serum immunoreactive parathyroid hormone in rats and in parathyroid hormone secretion in vitro by 1,25-dihydroxycholecalciferol

The present study determined the effects of 1,25-dihydroxycholecalciferol on serum immunoactive parathyroid hormone and on parathyroid hormone secretion in vitro. Rats injected i.p. with 1,25-dihydroxycholecalciferol, 130 pmol (2 U)/140 g body wt, which is probably a physiologic dose, had a significant 43% decrease in serum immunoreactive parathyroid hormone at 4 h. In addition, this dose of 1,25-dihydroxycholecalciferol inhibited the serum immunoreactive parathyroid hormone response to hypocalcemia induced by phosphate injection. Because the increment in serum immunoreactive parathyroid hormone was less but the decrement in serum calcium more in phosphate plus 1,25-dihydroxycholecalciferol-treated than in phosphate plus vehicle-treated rats, the impaired serum immunoreactive parathyroid hormone response to 1,25-dihydroxycholecalciferol could not be attributed to the change in serum calcium. In studies of parathyroid hormone secretion from bovine parathyroid tissue in vitro, the concentration of 1,25-dihydroxycholecalciferol used for most experiments was 1nM, which is in the range found in rat serum. 1,25-Dihydroxycholecalciferol at 1 or 100 nM significantly inhibited parathyroid hormone secretion when medium calcium concentration was normal (1.5 mM), high (3.0 mM), and low (1.0 mM). Maximum inhibition ranged from 19 to 74%; inhibition was generally seen after 2 h of incubation; and inhibition was sustained or progressive thereafter. Vitamin A, 0.1 muM, caused a marked stimulation of parathyroid hormone secretion. 1,25-Dihydroxycholecalciferol at 1 nM markedly reduced (44%) the effect of vitamin A to stimulate parathyroid hormone secretion. This effect of 1,25-dihydroxycholecalciferol was maximal at 1 h and persisted thereafter. Another steroid, hydrocortisone, 10 muM, did not inhibit parathyroid hormone secretion, suggesting that the 1,25-dihydroxycholecalciferol effect was not a nonspecific inhibitory effect on parathyroid cells. Because other workers have shown that parathyroid hormone directly stimulates 1,25-dihydroxycholecalciferol secretion, our results are consistent with the concept that there is a feedback loop where parathyroid hormone directly stimulates secretion of 1,25-dihydroxycholecalciferol, which in turn directly inhibits secretion of parathyroid hormone.