Cold resistance of the brain during hibernation. Temperature sensitivity of the partial reactions of the Na+, K+-ATPase.

The regulatory effect of calcium added in vitro on 25-hydroxycholecalciferol metabolism was studied in kidney mitochondria and in renal tubules from vitamin D-deficient chicks. The addition of calcium (0.05 – 0.2 mm) to mitochondrial suspensions prepared with calcium-chelating agents caused a marked and dose-related stimulation of 1-hydroxylation. A sharp decline in the activity was induced by higher concentrations of calcium (0.3 – 0.7 mm). A similar but less striking biphasic effect of calcium on 1-hydroxylation was observed in mitochondria prepared in the absence of calcium chelating agents. The effect of calcium was not a consequence of accelerated mitochondrial translocation of either exogenous NADP or Mg²⁺ but was related to mitochondrial calcium content. The addition of inhibitors of the calcium uptake, i.e., LaCl₃ or ruthenium red, or a calcium ionophore (A 23187) significantly inhibited the calcium-induced stimulation of the 1-hydroxylation reaction. Similar calcium effects were also observed in renal tubules isolated from intact, but not from parathyroidectomized, vitamin D-deficient chicks. These data strongly suggest that mitochondrial calcium plays an important role in the regulation of 1-hydroxylase activity in kidney.     

Effects of dietary vitamin D and calcium on lysyl oxidase activity in chick bone metaphyses.

Activity of lysyl oxidase, an enzyme responsible for production of aldehydic precursors for lysine-derived collagen crosslinks, was measured in tibial metaphyses from chicks receiving different dietary levels of vitamin D and Ca for 2 weeks after hatching. Enzyme activities were increased twofold in D-deficient chicks compared to activities from chicks receiving control levels of vitamin D. Addition of Ca to the D-deficient diet had no effect on lysyl oxidase activity. It is suggested that vitamin D may play a role in the age-related decrease in lysyl oxidase activity that normally occurs in chick bone.     

Phosphate transport across the basolateral membrane of chick kidney proximal tubule cells

Characterization of the phosphate transport system across the basolateral membrane of renal proximal tubule has been attempted using isolated proximal tubule cells prepared from chicks. The Pi efflux system is independent of Na+ ions and is not influenced by the nature of the chief anion present in the bathing medium. Pi efflux is not sensitive to DIDS and it is concluded that a generalized anion transporter of band III type is not the chief agent for facilitating Pi exit from the cell across the basolateral membrane. Inhibition of efflux by vanadate is evidence for a specific carrier protein in the membrane. The carrier probably possesses thiol group(s) that are essential for activity. The carrier may effect electroneutral transport of Pi possibly in exchange for OH- ions. The activity of the transport process is not stimulated by depleting the cells of phosphate or inhibited by rearing the chicks on a vitamin D-deficient diet. The system is unlikely to be of great importance for the expression of various regulatory mechanisms that act on the kidney to control the excretion of Pi. The activity declines as the chicks mature however.     

Stimulatory effect of prostaglandin E2 on 1 alpha,25-dihydroxyvitamin D3 synthesis in rats.

The effect of prostaglandin E2 on accumulation in plasma of 1 alpha,25-dihydroxy[3H]vitamin D3 from 25-hydroxy[3H]vitamin D3 was studied in vivo using vitamin D-deficient thyroparathyroidectomized rats. Intra-arterial infusion of 10-50 micrograms of prostaglandin E2/h caused a significant stimulation of 1 alpha,25-dihydroxy[3H]vitamin D3 production. No significant changes in plasma Ca2+ and Pi concentrations or urinary cyclic AMP excretion were observed after prostaglandin E2 infusion. Theophylline did not enhance the effect of a submaximal dose of prostaglandin E2 on 1 alpha,25-dihydroxy[3H]vitamin D3 production. These data indicate that prostaglandin E2 stimulates plasma accumulation of 1 alpha,25-dihydroxy[3H]vitamin D3 independent of the adenylate cyclase/cyclic AMP system, and suggest that prostaglandin E2 has a modulatory role in the regulation of 25-hydroxyvitamin D3 1 alpha-hydroxylase in the kidney.     

The ionic control of 1,25-dihydroxyvitamin D-3 synthesis in isolated chick renal mitochondria. The role of potassium.

In previous studies it was found that change in the concentrations of Ca2+, H+, and HPO2-4 in the incubation medium altered the rates of synthesis of 1,25-dihydroxyvitamin D-3 (1,25(OH)2D-3) by isolated renal mitochondria obtained from D-deficient chicks. The present studies demonstrate that raising the medium concentration of K+ from 1 to 50 mM leads to a 6-fold increase in rate of 1,25(OH)2D-3 synthesis by isolated chick mitochondria; that the magnitnitude of this K+-dependent stimulation is enhaced by optimal concentrations of calcium (pCa = 5) and phosphate (pPi = 3) (3 mM) but not by pH (from 6.8 to 7.4); that the effect is not produced by similar changes in media Na+ concentration; and that the stimulatory effect of K+ is not blocked by ruthenium red, and inhibitor of calcium transport and of the calcium-dependent stimulation of mitochondrial 1,25(OH) 2D-3 synthesis. It was also found that valinomycin, a K+-specific ionophore, enhanced the sensitivity of the mitochondrial 1 alpha-hydroxylase activity to K+. In the presence of valinomycin, an increase of pK+ to 3 was sufficient to cause a significant stimulation of 1,25(OH)2D-3 synthesis. It was concluded that changes in the ion content of the mitochondrial matrix space regulated the activity of the 1 alpha-hydroxylase.     

Ca2+-dependent ATPases in the basolateral membrane of rat kidney cortex

The basolateral segment of the rat renal tubular plasma membrane possesses Ca2+-dependent ATPase activity which was independent of Mg2+. Two kinetic forms were found: one, was a high affinity (apparent Km for free Ca2+ of 172 nM) low capacity (Vmax of 144 nmol of Pi X min-1 mg-1 protein) type; the other, had low affinity (apparent Km of 25 microM) and high capacity (896 nmol of Pi X min-1 X mg-1 protein). Mg2+ inhibited both Ca2+-ATPases. The high affinity enzyme exhibited positive cooperativity with respect to ATP, with a n value of 1.6. Ca2+-ATPase activity was not affected by calmodulin and was not inhibited by vanadate. On the other hand, both high and low affinity Ca2+-ATPase activities were increased when 1,25-dihydroxycholecalciferol was given to vitamin D-deficient rats. Kinetically, the enhanced activities were due to an increase in the Vmax values; the apparent affinities for free Ca2+ were not changed. The physiological function of the vitamin D-sensitive, Mg+-independent, Ca2+-ATPase activities remains to be established.     

Effects of vitamin D-3 on phosphate and calcium transport across and composition of skeletal muscle plasma cell membranes

The effects of vitamin D-3 on calcium and phosphate transport in skeletal muscle plasma membranes were studied. Sarcolemma vesicles were isolated from vitamin D-deficient and vitamin D-treated (one week) chicks by sucrose density gradient centrifugation of a crude muscle plasma membrane fraction. Measurement of (Na+ + K+)-ATPase activity, cholesterol to phospholipid molar ratios and levels of intracellular marker enzymes showed a high degree of purification of the preparations. Administration of vitamin D-3 significantly increased active Ca2+ and phosphate uptake into the vesicles. The efflux of both ions from preloaded vesicles was only slightly altered by the sterol. Ca2+-ATPase activity was higher in sarcolemma from treated animals. This confirms that the effects of vitamin D-3 on calcium transport are related to the Ca2+ pump and not to the passive permeability properties of the membrane. No changes in the protein composition of vesicles from both experimental groups were observed. However, treatment with vitamin D-3 increased sphingomyelin and phosphatidylcholine concentrations. These changes in lipid structure may play a role in the effects of vitamin D-3 on transport characteristics of sarcolemma.     

(Ca2+ + Mg2+)-ATPase activity in plasma membrane of circulating mononuclear cells. Lack of a direct effect of vitamin D

The in vivo effect of vitamin D on (Ca2+ + Mg2+)-ATPase activity was examined in a plasma membrane fraction of rat circulating mononuclear cells (MPM). Although there was no significant difference in the ATPase activities in red blood cell ghosts, (Ca2+ + Mg2+)-ATPase activity in MPM was significantly higher (p less than 0.05) in long-term vitamin D3-replete rats (100 IU/day for 6 months) than that in vitamin D-deplete rats (for 6 months). In rats maintained on vitamin D-deficient diets for 5-7 weeks, in vivo administration of either vitamin D3, 2,000 IU orally, 5 days prior to killing or 1,25-dihydroxyvitamin D3, 2.4 nmol, intraperitoneally, 24 h prior to killing failed to show any significant effect on (Ca2+ + Mg2+)-ATPase activity in MPM. (Ca2+ + Mg2+)-ATPase activity in MPM from rats maintained on vitamin D-deficient diet with high calcium content (1.8%) was significantly higher (p less than 0.05) than that from rats maintained on vitamin D-deficient diet with low calcium content (0.3%). Moreover, in vitro addition of vitamin D3 metabolites did not show any effect on (Ca2+ + Mg2+)-ATPase activity in MPM. These data suggest that decreased (Ca2+ + Mg2+)-ATPase activity in MPM from long-term vitamin D-deplete rats resulted from an adaptation to low extracellular calcium rather than vitamin D depletion.     

Oxygen-dependent 1,25-dihydroxycholecalciferol-induced calcium ion transport in rat intestine.

O2-dependent CA2+ uptake by rat duodenal discs has been characterized and used in a revised assay for 1,25-dihydroxycholecalciferol-induced intestinal Ca2+ transport. Although both muscle and mucosal surfaces are exposed in this free-floating-disc assay, the Ca2+ influx across the muscle surface is small, not O2- or vitamin D-dependent, and can be subtracted out. Depriving the animals of food for 9-14 h before assay increases the O2-dependent uptake by about 75%. Half-saturation values for O2-dependent Ca2+ uptake as determined with this assay are: 0.8mM-Ca2+ (fed) and 0.5mM-Ca2+ (food-deprived) for vitamin D-deficient rats, and 0.9mM-Ca2+ (fed) and 1.5mM-Ca2+ (food-deprived) for rats dosed with 1,25-dihydroxycholecalciferol. The maximum velocity of uptake varies from 6.7nmol of Ca2+ per cm2/min (fed) to 7.0nmol of Ca2+ per cm2/min (food-deprived) for vitamin D-deficient rats and 16.7nmol of Ca2+ per cm2/min (fed) to 29 nmol of Ca2+ per cm2/min (food-deprived) for 1,25-dihydroxycholecalciferol-treated rats. By using a 5 min preincubation and 15 min incubation with 1.0mM-Ca2+, duodenal tissue taken from vitamin D-treated rats shows about a 3-fold increase in O2-dependent Ca2+ uptake when compared with tissue taken from vitamin D-deficient animals. The calcium ionophore A23187, depending on concentration, either has no significant effect on or inhibits the O2-dependent uptake, rather than increasing it. Actinomycin D, at a dose of 2 micrograms/g, inhibits the O2-dependent uptake in intestinal discs from both vitamin D-deficient and vitamin D-treated rats by 58 and 80% respectively, when administered in vivo 3 1/2 h before assay.     

Effect of oestrogen and 1,25-dihydroxycholecalciferol on 25-hydroxycholecalciferol metabolism in primary chick kidney-cell cultures.

Primary cultures of chick kidney cells convert 25-hydroxycholecalciferol into more-polar metabolites. Cells from vitamin D-deficient chicks have high 25-hydroxycholecalciferol 1 alpha-hydroxylase (1 alpha-hydroxylase) activity, but no 25-hydroxycholecalciferol 24-hydroxylase (24-hydroxylase) activity. Physiological concentrations of 1,25-dihydroxycholeclaciferol suppress 1 alpha-hydroxylase and induce 24-hydroxylase activity. The inhibition of 1 alpha-hydroxylase preceded the induction of 24-hydroxylase. In contrast, oestradiol-17 beta had no effect on the activity of either hydroxylase under a variety of experimental conditions. These results clearly demonstrate that 1,25-dihydroxycholecalciferol, but not oestrogen, acts directly on the kidney cells to regulate the metabolism of 25-hydroxycholecalciferol.     

Age- and sex-dependent stimulation of calcium uptake in duodenal cells by prolactin in vitamin D-deficient rats

Administration of ovine-prolactin (O-PRL) stimulated Ca2+ uptake in isolated duodenal cells prepared from vitamin D-deficient rats. The time course of this effect was biphasic: uptake activity reached a peak in 2.5 hrs followed by a decrease at 5 hrs to original levels. This stimulatory effect of O-PRL was observed in vitamin D-deficient male, but not in female rats. This stimulatory effect was observed in 16- and 26-week old, but not 9 week old, animals. Increase in Ca2+ uptake in duodenal cells was not due to a decrease in intracellular Ca2+ efflux. We measured serum Ca concentration in vitamin D-deficient female rats and found that serum Ca increased in D-deficient female rats between 16 and 52 weeks whereas a minimal increase was observed in D-deficient male rats. Although prolactin was shown to stimulate duodenal Ca2+ uptake, it appears that the source of the increase in levels of serum Ca in D-deficient female rats was not derived from an increase in Ca2+ uptake by prolactin in duodenum. The increase in serum calcium with time may explain why female D-deficient rats survive longer then male.     

Effect of alcohol on renal vitamin D metabolism in chickens.

Intraperitoneal administration of ethanol to young chickens (both vitamin D-replete and vitamin D-deficient) produced a significant impairment of renal 25 hydroxyvitamin D₃ 1α-hydroxylase (EC 1.14.13.13) activity with no significant change in serum calcium or phosphorus. In ethanol treated D-replete chicks the renal 25 hydroxyvitamin D₃ 24-hydroxylase activity was enhanced, and serum 25 hydroxyvitamin D₃ was significantly increased. The alkaline phosphatase levels in the D-deficient ethanol treated chicks were significantly less than the controls. Our data suggest that the impairment of the metabolic effects of vitamin D due to ethanol occurs chiefly via a renal, rather than a hepatic mechanism. Furthermore, 1α -hydroxylated metabolites of vitamin D would appear to be the logical treatment of choice for the bone disease of alcoholism.     

ATP in equilibrium with 32Pi exchange catalyzed by plasma membrane Ca(2+)-ATPase from kidney proximal tubules.

The Ca(2+)-stimulated adenosine 5'-triphosphate-orthophosphate (ATP in equilibrium with 32Pi) exchange reaction was studied using a vesicular preparation derived from plasma membrane of kidney proximal tubules. With native inside-out vesicles, ATP in equilibrium with 32Pi was stimulated by micromolar Ca2+ concentrations. Treatment of the vesicles with the Ca2+ ionophore A23187 that abolished Ca2+ accumulation, strongly inhibited ATP in equilibrium with 32Pi. When Ca(2+)-ATPase was solubilized with the nonionic detergent octaethylene glycol mono n-dodecyl ether, maximal activation of ATP in equilibrium with 32Pi required millimolar Ca2+ concentrations. These Ca2+ concentrations inhibited ATP hydrolysis. ATP in equilibrium with 32Pi exhibited a Michaelian dependence on Pi and Mg2+, was stimulated by ATP, and depended on the ATP/ADP ratio. ATP in equilibrium with 32Pi was modified by the osmolytes urea, trimethylamine-N-oxide, and sucrose, which are representative of the methylamines and polyols that normally accumulate in renal tissue. These compounds did not modify the apparent affinity for Pi; they affected the response to ADP in the same fashion as the overall rate of ATP in equilibrium 32Pi, and their effects depended on medium pH. These data show that the Ca(2+)-ATPase from plasma membrane kidney proximal tubules can operate simultaneously in forward and backward directions. They also show that ATP in equilibrium with 32Pi is modulated by the ligands Ca2+, ATP, ADP, Pi, Mg2+, and H+, and by organic solutes found in renal tissue.     

A rapid and sensitive in vitro assay of 25-hydroxyvitamin D3-1 alpha-hydroxylase and 24-hydroxylase using rat kidney homogenates

A sensitive and rapid in vitro assay of 25-hydroxyvitamin D3 [25-(OH)D3]-1 alpha- and 24-hydroxylase activities was developed using rat kidney homogenates. A potent inhibitor of the enzymes in rat plasma was removed by thoroughly perfusing rats with saline. Kidney homogenates prepared from vitamin D-deficient rats preferentially produced tritiated 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] from 25(OH) [3H]D3. Addition of 10 microliter or more of rat plasma to 3 ml of 10% kidney homogenates suppressed 1 alpha-hydroxylase activity dose-dependently. Thyroparathyroidectomy (TPTX) of vitamin D-deficient rats greatly abolished 1 alpha-hydroxylase activity. Administration of parathyroid hormone to the TPTX rats increased 1 alpha-hydroxylase activity and that of 1 alpha,25(OH)2D3 enhanced 24-hydroxylase markedly. Since this assay is technically simple, rapid and sensitive, it will be useful in studying the regulatory mechanism in the renal metabolism of 25(OH)D3 in mammals.     

1,25-Dihydroxyvitamin D3-mediated intestinal calcium transport. Biochemical identification of lysosomes containing calcium and calcium-binding protein (calbindin-D28K)

A variety of intestinal cell organelles and proteins have been proposed to mediate 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3)-stimulated calcium absorption. In the present study biochemical analyses were undertaken to determine the subcellular localization of 45Ca after calcium transport in vivo in ligated duodenal loops of vitamin D-deficient chicks injected with 1.3 nmol of 1,25-(OH)2D3 or vehicle 15 h prior to experimentation. Separation of Golgi, mitochondria, basal lateral membrane, and lysosome fractions in the epithelial homogenates was achieved by differential sedimentation followed by centrifugation in Percoll gradients and evaluation of appropriate marker enzyme activities. Both vitamin D-deficient and 1,25-(OH)2D3-treated chicks had the highest levels of 45Ca-specific activity in lysosomal fractions. The lysosomes were also the only organelles to exhibit a 1,25-(OH)2D3-mediated difference in calcium content, increasing to 138% of controls. Lysosomes prepared from 1,25-(OH)2D3-treated chicks also contained the greatest levels of immunoreactive calbindin-D28k (calcium-binding protein). Chloroquine, a drug known to interfere with lysosomal function, was tested and found to inhibit 1,25-(OH)2D3-stimulated intestinal calcium absorption. Neither 1,25-(OH)2D3 nor chloroquine affected [3H]2O transport. In additional experiments, microsomal membranes (105,000 X g pellets) were subjected to gradient centrifugation. The highest levels of 45Ca-specific activity and calcium-binding protein in material from 1,25-(OH)2D3-treated chicks were found in fractions denser than endoplasmic reticulum and may represent endocytic vesicles. In studies on intestinal mucosa of 1,25-(OH)2D3-treated birds fractionated after 30 min of exposure to lumenal Ca2+ or Ca2+ plus chloroquine, 45Ca was found to accumulate in lysosomes and putative endocytic vesicles, relative to controls. A mechanism involving vesicular flow is proposed for 1,25-(OH)2D3-mediated intestinal calcium transport. Endocytic internalization of Ca2+, fusion of the vesicles with lysosomes, and exocytosis at the basal lateral membrane complete the transport process.     

Effect of vitamin D status on cyclic AMP-dependent protein kinase activity and its heat-stable inhibitor in chick kidney

Cyclic AMP-dependent protein kinase activity in supernatants of homogenates of kidneys from vitamin D-deficient chicks is decreased to 70% of the level measured in kidneys from normal chicks. Activity was restored to normal by oral administration of vitamin D or 1,25-dihydroxyvitamin D3 for 1 or 2 weeks. Both isozymes of cAMP-dependent protein kinase were reduced to the same extent by vitamin D deficiency. The decreased enzyme activity could not be accounted for by a shift to the particulate fraction nor by an increased requirement for cyclic AMP. A heat stable, trichloroacetic acid-precipitable, trypsin-labile inhibitor of protein kinase activity was identified and quantitated in kidneys from vitamin D-deficient chicks (16 to 26 units/mg of protein) and from those given vitamin D (2 to 6 units/mg of protein). The measured difference in inhibitor levels could not be attributed to differential stability in kidney homogenates from vitamin D-deficient or -repleted chicks. The observed increase in inhibitor level with vitamin D deficiency is not sufficient to account for the decrease in cyclic AMP-dependent protein kinase activity, suggesting that the total amount of this enzyme activity is reduced in vitamin D deficiency.     

A guanine nucleotide-binding protein mediates 1,25-dihydroxy-vitamin D-3-dependent rapid stimulation of Ca2+ uptake in skeletal muscle.

1,25-Dihydroxyvitamin D-3 (1,25(OH)2D3) has been shown to increase Ca2+ uptake readily in skeletal muscle through a dihydropyridine-sensitive pathway, cAMP levels and adenylate cyclase activity. In the present study, fluoride (F-), a potent guanine nucleotide binding protein (G protein) stimulator, rapidly increases vitamin D-deficient skeletal muscle Ca2+ uptake in a dose-dependent manner and with a similar time-course as 1,25(OH)2D3. The increment is detected within 1 min (15%) and steadily increases up to 15 min (60%). The effects of 1,25(OH)2D3 and F- are also observed in muscle from normal, vitamin D-replete chicks. AlCl3, which is required for G protein stimulation by F-, potentiates the effects of F-, Ca2+ uptake in 1,25(OH)2D3-dependent muscle is potentiated by F- and, analogous to the hormone, the effects of F- can be suppressed by Ca(2+)-channel antagonists. Direct exposure of microsomal membranes to 1,25(OH)2D3 reduces the specific binding of [gamma-35S]GTP to the membranes 40%. Pretreatment of muscle with Bordetella pertussis toxin (PTX), known to inhibit Gi, or with cholera toxin (CTX), known to stimulate Gs, produces an acute elevation of muscle Ca2+ uptake. 1,25(OH)2D3 potentiates CTX, but has no additional effect on PTX-dependent Ca2+ uptake. These results indicate that an interaction with an inhibitory G protein coupled to adenylate cyclase may be part of the mechanism by which 1,25(OH)2D3 increase Ca2+ uptake through regulation of Ca(2+)-channel gating by a cAMP-dependent pathway in skeletal muscle.     

Stimulation of 1,25-dihydroxyvitamin D3 production by 1,25-dihydroxyvitamin D3 in the hypocalcaemic rat.

Serum 1,25-dihydroxyvitamin D3 concentration and renal 25-hydroxyvitamin D 1 alpha-hydroxylase activity were measured in rats fed various levels of calcium, phosphorus and vitamin D3. Both calcium deprivation and phosphorus deprivation greatly increased circulating levels of 1,25-dihydroxyvitamin D3. The circulating level of 1,25-dihydroxyvitamin D3 in rats on a low-calcium diet increased with increasing doses of vitamin D3, whereas it did not change in rats on a low-phosphorus diet given increasing doses of vitamin D3. In concert with these results, the 25-hydroxyvitamin D 1 alpha-hydroxylase activity was markedly increased by vitamin D3 administration to rats on a low-calcium diet, whereas the same treatment of rats on a low-phosphorus diet had no effect and actually suppressed the 1 alpha-hydroxylase in rats fed an adequate-calcium/adequate-phosphorus diet. The administration of 1,25-dihydroxyvitamin D3 to vitamin D-deficient rats on a low-calcium diet also increased the renal 25-hydroxy-vitamin D 1 alpha-hydroxylase activity. These results demonstrate that the regulatory action of 1,25-dihydroxyvitamin D3 on the renal 25-hydroxyvitamin D3 1 alpha-hydroxylase is complex and not simply a suppressant of this system.     

Vitamin D3 and 1,25-dihydroxyvitamin D3 stimulate the skeletal muscle-calcium mobilization in rachitic chicks

The Ca content in skeletal muscle relative to vitamin D3 intake was studied in chicks. It was found that the Ca content in rachitic chick muscle was significantly higher than normal and it decreased with vitamin D3 treatment. In 4-week-old chicks fed a vitamin D-deficient diet, the Ca content in leg muscle reached 9.86 +/- 1.07 mg/100 g wet wt, although in chicks receiving vitamin D3 in doses of 100 and 500 IU/kg diet, it was 7.80 +/- 0.72 and 6.08 +/- 0.61 mg/100 g wet wt, respectively. A single i.m. dose of 0.50 micrograms of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) or vitamin D3 caused a dramatic decrease in the muscle Ca content by 3 to 6 h after the injection. A simultaneous rise in the Ca level in blood serum was observed. However, at this time the Ca binding protein content in duodenal mucosa and the stimulation of Ca absorption were negligible. These findings allow the conclusion that the vitamin D deficiency in chicks leads to a surplus Ca accumulation in skeletal muscle. The administration of vitamin D3 or its metabolites causes rapid Ca release during the first 6 h. This may be the source of the Ca level increase in blood serum. In this respect 1,25(OH)2D3 was much more effective than vitamin D3.     

Effects of 1,25-dihydroxycholecalciferol administration on the rat renal vitamin K-dependent carboxylating system

We have shown previously that the in vitro activity of the renal vitamin K-dependent gamma-glutamyl carboxylase toward synthetic oligopeptide substrates is stimulated by administration of either parathyroid hormone (PTH) or 1,25-dihydroxycholecalciferol [1,25(OH)2D3] to rats [(1983) J. Biol. Chem. 258, 12783-12786]. Here we report that administration of 1,25(OH)2D3 to rats increases their levels of endogenous carboxylase substrate as well. Rats fed a vitamin D-deficient diet had highly elevated serum PTH levels while vitamin D-replete animals had undetectable levels. Furthermore, since PTH increases 1,25(OH)2D3 levels by stimulating renal 25-hydroxyvitamin D-1 alpha-hydroxylase, it is very likely that the stimulatory effects of PTH on the renal vitamin K-dependent carboxylating system are mediated by 1,25(OH)2D3.