Role of vitamin D metabolites in phosphate transport of rat intestine

Support of embryonic chick survival by vitamin D metabolites

The provision of 1,25-dihydroxyvitamin D3 as the only source of dietary vitamin D3 to laying hens failed to support normal embryonic development in their fertile eggs. Significant (P less than .001) improvement in embryonic survival to hatching in these eggs resulted from injections of 1,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, 25-hydroxyvitamin D3, or 24,24-difluoro-25-hydroxyvitamin D3 prior to incubation. Maximum embryonic survival with lowest embryonic mortality was observed when 0.20 micrograms/egg of 1,25-dihydroxyvitamin D3 or 0.60 micrograms/egg 25-hydroxyvitamin D3 was injected. These results indicate that several forms of vitamin D, two of which cannot be converted to 24,25-dihydroxyvitamin D3, can provide this activity; and of the vitamin D compounds tested, 1,25-dihydroxyvitamin D3 may be the most active in supporting embryonic survival in the chick when delivered directly by injection.

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.

A test of 1,25-dihydroxyvitamin D3 secretory capacity in normal subjects for application in metabolic bone diseases

The circulating concentration of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] varies in man in response to a variety of physiological stimuli. In pathological states, random plasma 1,25-(OH)2D3 levels reflect the sum of the underlying pathological process and the homeostatic corrections to those perturbations. Hence random 1,25-(OH)2D3 levels do not allow differentiation as to whether changes in 1,25-(OH)2D3 production or metabolism are primary or secondary to other changes. Similarly random levels do not provide much insight into the reserve of 1,25-(OH)2D3 secretory capacity. We have developed a single infusion of parathyroid extract (PTE) as a test of 1,25-(OH)2D3 secretory capacity. Normal responses in twelve males and six females consisted of an increase in plasma 1,25-(OH)2D3 levels of 78% over basal in the male and 77% over basal in the female subjects. The peak response occurred 18-22 h after the PTE was administered. This test of 1,25-(OH)2D3 secretory capacity should prove useful in the evaluation of patients with metabolic bone disease. As part of this study, the diurnal plasma 1,25-(OH)2D3 level was examined in six males and seven females. There was no significant diurnal rhythm between 8.00 and 22.00 h.

Changes in alkaline phosphatase activity in rabbit articular cartilage associated with ageing and joint contracture

Changes in alkaline phosphatase (ALPase) activity in rabbit articular cartilage induced by joint contracture were studied by light and electron microscopy. The knee joint was plaster-immobilized for 3--9 weeks in the flexion position. Three different age groups of rabbits were examined, at the age of 1 month (young), 1 year (adult), and over 2.5 years (old). ALPase activity was detected in the cartilage taken from the distal end of the femur, employing the lead citrate method. Positive activity was primarily localized in the plasma membrane of the chondrocytes and also in the pericellular matrix vesicles. Before immobilization the positive cells were distributed exclusively in the deep zone of the cartilage, whereas after immobilization activity was present in cells both in the middle and deep zones. This phenomenon was most evident in the old group.

1,25-Dihydroxyvitamin C3-stimulated active uptake of phosphate by rat jejunum

An assay system for measuring an active phosphate uptake in rat jejunal disks has been developed and used to study the rat's response to 1,25-dihydroxyvitamin D3. The active component of phosphate uptake is stimulated by added KCl, fructose, sodium, and CaCl2, but not MgCl2. Half-saturation values obtained with this assay are 0.08 mM (1,25-dihydroxyvitamin D3-treated) and 0.11 mM (D-deficient). The maximal velocities are 9.6 nmol . cm-2 . min-1 (D-deficient). Using a 5-min preincubation and a 15-min incubation, a dose-response curve for 1,25-dihydroxyvitamin D3 shows 6 pmol to be the lowest dose to give a significant response and 60 pmol to be the level at which the maximal response is reached. Measurement of phosphate uptake versus time after dosing with 1,25-dihydroxyvitamin D3 (300 pmol) revealed a biphasic response with peaks at 8 and 36 h and a trough at 20 h.

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.

Reduced plasma concentration of 1,25-dihydroxyvitamin D in children with moderate renal insufficiency

We measured the plasma concentration of 1,25-dihydroxyvitamin D (1,25(OH)2D) in 39 children comprising three groups; eight with moderate renal insufficiency (GFR of 25 to 50 ml/min/1.73 M2, seven of whom had tubulointerstitial disease), eight with severe renal insufficiency (on chronic hemodialysis), and 23 healthy control subjects. The mean plasma concentration of 1,25-(OH)2D was reduced by some 40% (P less than 0.002) in the children with moderate renal insufficiency, and by some 80% (P less than 0.001) in the children with severe renal insufficiency. In the children with moderate renal insufficiency, the reduced concentration of 1,25-(OH)2D was associated with increased serum concentrations of immunoreactive parathyroid hormone (iPTH) and reduced serum concentrations of 1,25-(OH)2D was associated with increased serum concentrations of immunoreactive parathyroid hormone (iPTH) and reduced serum concentrations of calcium and phosphorus. When analyzed over the range of renal function from normal through severely impaired, values of iPTH correlate inversely and significantly with those of 1,25-(OH)2D. Growth was impaired in four of the eight children with moderate renal insufficiency. The results of the current study suggest that in children with moderate renal insufficiency, a reduction in the renal synthesis and in the plasma concentration of 1,25-(OH)2D may be important pathogenetic events in disordered metabolism of calcium and phosphorus, including secondary hyperparathyroidism.

Receptors for 1,25-dihydroxyvitamin D3 in chick pancreas: a partial physical and functional characterization

1,25-Dihydroxyvitamin D3(1,25-(OH)2D3) receptors from the rachitic chick pancreas have been partially characterized. Analyses of these receptors by isokinetic gradient centrifugation and analytical gel filtration reveal a sedimentation coefficient (S) of 3.3-3.7, a molecular weight (Mr) of 58,500-68,000, and a calculated Stokes molecular radius (Rs) of 34-36 A. Polyethylenimine-ammonium sulfate precipitation of pancreatic cytosol partially purifies aporeceptor and reduces nonspecific binding (in part, 5.8S DBP), thus providing material more amenable to kinetic analyses, Binding studies incorporating this fractionated cytosol reveal an equilibrium dissociation constant (K4) of approximately 0.112 nM at 2 degrees C for the 1,25-(OH)2D3-receptor interaction. Competition studies further demonstrate a particular preference for 1,25-(OH)2D3 over 1,24(R),25-trihydroxyvitamin D3, 24(R),25-dihydroxyvitamin C3, and 25-hydroxyvitamin D3. The pancreatic receptor also binds to immobilized group-selective affinity ligands such as DNA, cibacron blue, and heparin, and can be eluted as a single macromolecular species during standard linear KCl gradients. Its interaction with these ligands supports the premise that the 1,25-(OH)2D3 receptors' fundamental mode of action is at the level of the cellular genome. Salt-dependent nuclear uptake and chromatin localization studies with this receptor in vitro also support this potential site of action. Significantly, a physiologic dose of 1,25-(OH)2[3H]D3 to rachitic chicks leads to the in vivo formation of a receptor-hormone complex as identified by DNA-cellulose chromatography. These observations provide further evidence that the pancreatic protein is a biologically relevant component of the chick pancreas which functions to accumulate hormone intracellularly under physiologic situations.

Relative effectiveness of vitamin D metabolites in increasing bone mineral solubility

Weanling rats were given a vitamin D-deficient diet containing 1.4% calcium and 1.0% phosphorus. After 4 weeks these deficient animals were injected for 7 days with selected doses of one of the following vitamin D metabolites: 25(OH)D3, 1,25(OH)2D3, 24,25(OH)2D3, 25,26(OH)2D3 or the ethanol vehicle. A vitamin D-replete group was placed on the same diet but injected with 50 IU of vitamin D3 once a week for the entire 5-week period. By the use of a modified Ussing chamber [1], the measurements of calcium fluxes into and from the rat calvaria were possible. These data enabled the apparent mineral solubilities to be derived. After 5 weeks on this diet the vitamin D-deficient rats had low levels of serum calcium (1.41 mM) and decreased mineral solubility when compared to the vitamin D-replete group. The apparent solubility of the bone mineral increased toward the vitamin D-replete level in calvaria from vitamin D metabolite-treated rats. However, these changes did not directly reflect the alterations in the level of serum calcium. At any given dose level, 1,25(OH)2D3 was the most effective metabolite in increasing serum calcium. In fact, the high dose (250 pmoles/day) was hypercalcemic. Next in effectiveness was 25(OH)D3. These two metabolites were equally effective in increasing mineral solubility. At a 10 times higher dose, the 24,25(OH)2D3 metabolite was able to normalize serum calcium and improve but not normalize mineral solubility. At the high dose (260 pmoles/day), the 25,26(OH)2D3 metabolite caused no effect on mineral solubility and minimal increases in serum calcium.

The control of calcium and phosphorus metabolism by the vitamin D endocrine system

Vitamin D, which is normally produced in the skin under ultraviolet irradiation, is the building block for a new endocrine system that involves hydroxylation on the 25-position in the liver followed by 1 alpha-hydroxylation in the kidney to produce the vitamin D hormone, 1 alpha, 25-(OH)2D3. This vitamin D hormone functions in the intestine, bone, and kidney to stimulate transport of calcium and phosphorus into the extracellular fluid compartment upon demand. The production of the vitamin D hormone is tightly feedback regulated directly or indirectly by calcium and phosphorus levels of the plasma. The vitamin D endocrine system is an important one in the regulation of calcium and phosphorus metabolism but is not solely responsible for the calcium and phosphorus transfer reactions occurring during reproduction. The vitamin D hormone functions in the target organs by a nuclear-mediated receptor-based mechanism probably involving the biogenesis of calcium and phosphorus transfer proteins. New target sites of 1,25-(OH)2D3 action in several tissues are suggested by this nuclear localization in those cells. Study of the vitamin D endocrine system has provided a new understanding of metabolic bone diseases and has provided new forms of vitamin D for their treatment. Thus a basic investigation of the regulation of calcium and phosphorus metabolism has rewarded medicine and science with new therapeutic approaches to disease problems.

Regulation of Na-dependent phosphate influx across the mucosal border of duodenum by 1,25-dihydroxycholecalciferol

Animals teated with disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP), at doses which decrease the renal production and/or the plasma levels of 1,25-dihydroxycholecalciferol [1,25(OH)2D3], display a reduced net absorption of phosphate. In this study we investigated whether EHDP-treatment and administration of 1,25(OH)2D3 to EHDP-treated animals affected the phosphate influx across the mucosal border of rabbit duodenum. The initial rate of phosphate influx into mucosal cells was measured in isolated intestine. In control, untreated rabbits, the phosphate influx shows a saturable, Na-dependent component and a diffusional, Na-independent uptake. In tissue from rabbits treated for 3 days with EHDP, the phosphate influx was found to be strongly reduced. EHDP-treatment decreased the Na-dependent, carrier mediated phosphate influx in duodenum. Administration of 1,25(OH)2D3 to EHDP-treated animals reversed the reduced phosphate influx. These effects were mainly apparent through changes in the J(mc)(max) of the phosphate influx, which was decreased from 211 +/- 38.7 nmol/cm2h in controls to 42.1 +/- 18.1 nmole/cm2 h in the EHDP-treated group and increased to 413 +/- 43.6 nmole/cm2 h by 1,25(OH)2D3. The treatment did not appear to affect the diffusional, Na-independent phosphate influx. EHDP-treatment did not affect the influx of alanine in this segment suggesting that EHDP-treatment affects only 1,25(OH)2D3-dependent transport mechanisms. The results suggest that 1,25(OH)2D3 modulates the number of carrier sites available at the mucosal membrane for Na-dependent phosphate entry.

Bone response to phosphate salts, ergocalciferol, and calcitriol in hypophosphatemic vitamin D-resistant rickets

We treated 11 children with vitamin D-resistant rickets with a phosphate mixture either alone (1.2 to 3.6 g per day) or combined with ergocalciferol (vitamin D2, to 50 x 103 IU per day) or with calcitriol (1,25-dihydroxyvitamin D3, 0.25 to 1 microgram per day). Serum calcitriol concentrations were normal in all patients. Calcitriol therapy circulating levels of the hormone to values above normal and increased intestinal phosphate absorption. In some patients this regimen decreased the need for phosphate supplements. None of the treatment regimens corrected the renal phosphate leak. Radiologic studies and bone histomorphometric analyses showed that phosphate (alone or with ergocalciferol) induced the mineralization of the growth plate but not of the endosteal bone surface. Combined calcitriol and phosphate therapy for a total of 2850 patient-days greatly improved the mineralization of trabecular bone. Short-term episodes of hypercalcemia were easily controlled by changes in calcitriol dosage. The data indicate that the combined calcitriol and phosphate regimen is useful in the treatment of vitamin D-resistent rickets.

In vitro synthesis of 1 alpha,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol by isolated calvarial cells

The question of whether the skeleton metabolizes 25-hydroxycholecalciferol [25(OH)D3] to more-polar products was studied. Calvarial cells were dispersed from 16-day old chicken embryos by using collagenase and then grown in culture in serum-free medium. Confluent cell cultures were incubated with 7 nM 25(OH)[3H]D3 for 2 hr, and the vitamin D metabolites were then extracted. At least four polar metabolites were produced. Based on separation by Sephadex LH-20 chromatography followed by high-pressure liquid chromatography, two of these metabolites were identified as 1,25-dihydroxycholecalciferol [1,25(OH)2D3] and 24,25-dihydroxycholecalciferol [24,25(OH)2D3]. These metabolites were also produced by cultured kidney cells but not by liver, heart muscle, or skin cells isolated from the same embryos. The specific activities of the calvarial 1- and 24-hydroxylases were similar in magnitude to those in isolated kidney cells. The specific activity of the calvarial 25(OH)D3:1-hydroxylase was inhibited by an 8-hr preincubation with 1,25(OH)2D3, whereas the 24-hydroxylase was enhanced. It is concluded that (i) vitamin D metabolism by isolated cells is organ-specific, (ii) calvarial cells produce active metabolites of vitamin D in significant amounts, (iii) vitamin D metabolism by calvarial cells is regulated by 1,25(OH)2D3, and (iv) locally produced, active metabolites could act locally, thereby adding a new dimension to the regulation of mineral metabolism by vitamin D metabolites.

The effect of dose rate of 1-alpha-hydroxycholecalciferol on calcium and phosphorus metabolism in sheep

1. A combination of a mineral balance and a radioisotope technique has been used to study the relationship between dose rate of 1-α-hydroxycholecalciferol (1α-OH-D3) and the magnitude and duration of its effect on the various processes of calcium and phosphorous metabolism in adult wether sheep.

2. The rate of absorption of Ca was markedly increased by treatment and maximum response occurred at the lowest dose rate.

3. Although sheep were already Ca-replete, the extra Ca absorbed was all retained and increased retention was brought about by a combination of an increase in bone accretion and a decrease in bone resorption. This finding conflicts with the generally-held belief that bone resorption is increased by cholecalciferol treatment.

4. The rates of absorption and retention of P were increased by 1α-OH-D3treatment and maximum response occurred at the lowest dose rate.

5. That P absorption could be increased by treatment suggests that not all the available dietary P was absorbed in the control period.

6. Although the loss of endogenous P in the faeces was unaltered by treatment, the secretion of P into the gut was increased, and the increase was directly related to increased serum inorganic P concentration.

7. Nearly all the extra P absorbed was retained and increased retention was achieved by a combination of an increased incorporation into and a decreased loss from the non-exchangeable pools of bone and soft tissues.

8. The time interval taken for absorption rates of Ca and P to return to normal after the end of each treatment was related to the dose rate of 1α-OH-D3. Although higher dose rates had little effect on the magnitude of response, they did prolong slightly the duration of response.

Calcium-dependent, parathyroid hormone-independent regulation of 1,25-dihydroxyvitamin D

The increase of plasma 1,25-dihydroxyvitamin D (1,25(OH)2D) in response to Ca restriction has been suggested to be essentially mediated by parathyroid hormone (PTH). In this study, we have assessed the influence of variations in calcium intake on plasma 1,25(OH)2D in pair-fed sham-operated (sham) and in hypocalcemic hypoparathyroid rats after thyroparathyroidectomy (TPTX). In sham rats, plasma 1,25(OH)2D increased from 189 +/- 16 to 486 +/- 41 pM when dietary calcium was inreased from 1.2% Ca to 0.2% Ca. This increase was associated with an increase in plasma PTH level. In TPTX rats, plasma 1,25(OH)2D increased from 112 +/- 9 to 332 +/- 36 pM when dietary calcium was decreased. In this case, the increase was not associated with a rise in plasma PTH level nor with an increase in urinary cyclic AMP. When TPTX rats were infused chronically with PTH (60 U/day), plasma 1,25(OH)2D was 62 +/- 9 pM when the 1.2% Ca diet was given and 281 +/- 45 pM with the 0.2% Ca diet. These reults confirm that the thyroparathyroid glands influence plasma 1,25(OH)2D but they also provide evidence for a PTH-independent response of plasma 1,25(OH)2D to Ca restriction.

Effect of 1,25-dihydroxyvitamin D-3 on phosphate uptake into chick intestinal brush border membrane vesicles

Brush border membrane vesicles prepared from the vitamin D-deficient chick duodenum take up phosphate and show an overshoot phenomenon in the presence of NaCl. Substitution of choline chloride for NaCl reduces phosphate uptake. Prior treatment of vitamin D-deficient chicks with 1,25-dihydroxy-vitamin D-3 increases the initial rate of Na+-dependent phosphate uptake into the brush border vesicles. This Na+-dependent phosphate uptake is a saturable process, exhibiting an apparent Km of 0.31 mM and a V of 385 pmol/mg per 15 s. Pretreatment of chicks with 1,25-dihydroxyvitamin D-3 leads to an increase in V (750 pmol/mg per 15 s) without significantly altering the apparent Km (0.33 mM). Addition of Ca2+, either in the presence or absence of the polyene antibiotic, filipin, or of calmodulin, has no effect on Na+-dependent phosphate uptake. Pretreatment of the vitamin D-deficient chick with a dose of cycloheximide sufficient to inhibit membrane protein synthesis blocks the 1,25-dihydroxyvitamin D-3-induced increase in alkaline phosphatase activity, but does not affect the stimulation of Na+-dependent phosphate uptake. From these data, it is concluded that 1,25-dihydroxyvitamin D-3 stimulates Na+-dependent phosphate transport at the brush border membrane of the enterocyte, that alkaline phosphatase is not directly involved in this process, and that this effect of 1,25-dihydroxyvitamin D-3 is independent of new protein synthesis.

Characterization of phosphate uptake in isolated chick intestinal cells

A preparation of isolated intestinal cells has been developed which permits the incubation of these cells in culture for 90 min. with preservation of viability and responsiveness to vitamin D analogues added in vitro. Characterization of phosphate transport suggested the presence of two transport processes which differed in their affinity for phosphate, calcium stimulation, and pH dependence. Both processes were facilitated by sodium transport. This cell preparation promises to be a valuable experimental model for the elucidation of the action of vitamin D on phosphate transport.

1,25-Dihydroxycholecalciferol in human serum and its relationship with other metabolites of vitamin D-3

A competitive protein binding assay for 1,25-dihydroxycholecalciferol has been developed using the hormone's nuclear receptor protein from chick intestinal mucosa. This nuclear receptor protein can be stored at -70 degrees C for several months and bound and free hormone can be separated easily with dextran coated charcoal. Results obtained using this assay agree well with those reported by other groups of workers. Serum levels of other vitamin D-3 metabolites, namely 25-hydroxycholecalciferol and 24,25-dihydroxycholecalciferol have also been measured and are shown in relation to 1,25-(OH)2D3 levels.

Stimulation of duodenal and ileal absorption of phosphate in the chick by low-calcium and low-phosphorus diets

The intestinal absorption of phosphate has been studied in vivo in the chick using ligated segments of duodenum and ileum. Feeding diets low in calcium (0.1%) and/or low in phosphorus (0.25%) caused an increase in the absorption of phosphate from both the duodenum and ileum. These changes are consistent with a putative increase in the renal production and mucosal uptake of 1,25-dihydroxycholecalciferol.

Is 24,25-dihydroxycholecalciferol a calcium-regulating hormone in man?

Small doses (1-10 microgram daily) of 24,25-dihydroxycholecalciferol (24,25-(OH)2D3), a renal metabolite of vitamin D of uncertain function, increased intestinal absorption of calcium in normal people and in patients with various disorders or mineral metabolism, including anephric subjects. In five of six patients studied, calcium balance increased, but, unlike 1,25-dihydroxycholecalciferol, 24,25-(OH)2D3 did not increase plasma or urinary calcium concentrations. These results suggest that 24,25-(OH)2D3 may be an important regulator of skeletal metabolism in man with potential value as a therapeutic agent.

The use of a Thiry-Vella loop of jejunum to study the intestinal absorption of calcium and inorganic phosphate in the conscious pig

1. Pigs, each surgically prepared with a Thiry-Vella jejunal loop were used to study the absorption of calcium and inorganic phosphate from the intestine.

2. The loops were perfused daily for 6–8 h with a nutrient solution and absorption from the perfusate was measured.

3. The technique employed minimized atrophy of the jejunal mucosa and enabled the measurement of hourly or daily changes in absorption rate of components of the luminal fluid.

4. No differences were observed when polyethylene glycol (PEG; molecular weight 4000), [14C]PEG or 51Cr-EDTA were used as markers of net water movement.

5. Increasing the concentration of Ca in the perfusate resulted in the demonstration of a two-component relationship between net absorption rate of Ca and intraluminal Ca concentration. An initial rapid absorption rate from 0 to 4 mm was found, then a slower rate from approximately 5 mm upwards which did not saturate at the highest concentration tested (25 mm).

6. Increasing the concentration of phosphate in the perfusion solution increased the net absorption of phosphate from that solution. Although a two-component system, similar to that for Ca, was not evident, net absorption of phosphate was not saturated at the highest concentration tested (50 mm).

7. The absorption of Ca was unaffected by the presence of phosphate in the solution but Ca (2.5 mm) enhanced the absorption of phosphate.

Physiological and pharmacological aspects of 24,25-dihydroxycholecalciferol in man

The present study describes the response to small oral doses (1--10 microgram/day) of 24,25-DHCC in man. Contrary to expectation, 24,25-DHCC was as potent as 1,25-DHCC in increasing intestinal absorption of calcium both in normal persons and in patients with a variety of disorders of calcium metabolism. Despite this increase in intestinal absorption, plasma and urine calcium did not increase after 24,25-DHCC as they did after 1,25-DHCC. Metabolic balance studies showed calcium balances to increase by 1.6 to 11.5 mmoles/day in 5 of the 6 patients studied. 24,25-DHCC increased intestinal absorption of calcium equally well in anephric patients, suggesting that conversion of 24,25-DHCC to 1,24,25-trihydroxycholecalciferol by the kidney cannot be the sole mechanism by which 24,25-DHCC expresses biological activity, even though in vitamin D deficient rats nephrectomy does abolish the ability of large doses of 24,25-DHCC to increase calcium absorption. It is concluded that 24,25-DHCC may be a calcium-regulating hormone in man. In view of the effects demonstrated here and its relatively high concentration in plasma and slow turnover rate, 24,25-DHCC has the properties that might be ideal for a long-acting stimulator of bone mineralisation. Further work is needed to explain why 24,25-DHCC has effects in man which are not readily seen in other species.

Intestinal Ca and phosphate transport: differential responses to vitamin D3 metabolites

The transport of Ca and inorganic phosphate (Pi) was studied in the absence of electrochemical gradients across rat intestine in vitro. 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) increased the active absorption of both Ca and Pi in all segments of the small intestine, with changes occurring only in absorptive fluxes, whereas secretory fluxes were unaffected. Active Ca absorption was greatest in the duodenum (greater than jejunum greater than ileum) and active Pi absorption was highest in jejunum (greater than duodenum greater than ileum), in agreement with earlier reports. 24R,25-dihydroxy-vitamin2D3 had similar effects on transport but was less potent. The ratios of Pi absorptive fluxes to Ca absorptive fluxes remained remarkably constant during 80-200% increases in absorption produced by 1,25(OH)2D3, suggesting coupled Ca-Pi transport or coordinate stimulation of Ca and Pi absorptive processes by hormonally active metabolites of vitamin D. The results seem most compatible with a differential distribution of vitamin D-responsive Ca and Pi absorptive cells with a predominance of cells with Ca absorptive sites occurring in duodenum, more Pi absorbing cells in jejunum, and a nearly equal ratio of each type in ileum.

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).

Vitamin D metabolism

During the past decade, an explosion of information has become available on the metabolism and function of vitamin D which is of great importance to clinicians in the treatment of metabolic bone disease. We have learned that vitamin D is the precursor of at least one hormone and that this hormone carries out functions in calcium and phosphorus metabolism bringing about mineralization of bone on one hand, and the prevention of hypocalcaemic tetany on the other. It may also function in the prevention of such degenerative bone diseases as osteoporosis. An important analogue of this hormone, 1alpha-hydroxyvitamin D3 has been prepared and is used successfully in the treatment of a variety of clinical conditions. This presentation will summarize these findings and their possible implications in these metabolic bone diseases.

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.

Effects of Solanum malacoxylon extract on rachitic chicks. Comparative study with vitamin D3

A comparative study of the effects of vitamin D3 and of a partially purified extract of Solanum malacoxylon has been carried out in rachitic chicks. Vitamin D3 and Solanum malacoxylon increased intestinal calcium absorption and serum calcium levels. They normalized the bone water and ash content. Vitamin D3 produced an increase of serum phosphate while Solanum malacoxylon further decreased the already low phosphate values. Vitamin D3 significantly increased the body weight increment of rachitic chicks, but Solanum malacoxylon did not. It appears that Solanum malacoxylon duplicates certain actions of vitamin D but lacks its phosphate-regulating and growth-promoting actions.

Role of 1,25-dihydroxyvitamin D3 on intestinal phosphate absorption in rats with a normal vitamin D supply

In vitamin D-deficient rats, impaired intestinal phosphorus (P) absorption can be corrected by 1,25-dihydroxyvitamin D(3)[1,25-(OH)(2)D(3)]. In the present study, it was investigated whether changes in 1,25-(OH)(2)D(3) production can influence intestinal P transport also in animals with a normal supply of vitamin D. The intestinal P absorption was evaluated in rats using both the in situ duodenal loop technique and the determination of the overall gastrointestinal absorption under three conditions known to influence the production of 1,25-(OH)(2)D(3): (a) variation in dietary P, (b) thyroparathyroidectomy (TPTX) with or without administration of parathyroid hormone (PTH), and (c) treatment with disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP). In all circumstances changes in duodenal absorption paralleled the changes in the overall fractional absorption. (a) Lowering dietary P stimulated P absorption. (b) TPTX decreased P absorption. This effect was corrected either by the administration of PTH or by the administration of 1,25-(OH)(2)D(3). (c) EHDP, when given at a dose known to inhibit 1,25-(OH)(2)D(3) formation, decreased the duodenal P absorption in both intact and TPTX animals. This effect was corrected by 1,25-(OH)(2)D(3). In the TPTX-EHDP-treated animals, the administration of PTH did not rectify the low duodenal P absorption. These results support the thesis that, in rats with normal vitamin D supply, variations in the endogenous production of 1,25-(OH)(2)D(3) change the rate of P absorption. However, these changes are in such magnitude that they are of relatively small importance when compared to the effect of variation in the dietary intake of P. These results also strongly suggest that the action of PTH on duodenal P transport is mediated by its effect on 1,25-(OH)(2)D(3) production, inasmuch as the effect of the hormone is abolished after blocking the renal 1-hydroxylation with EHDP.

1,25-dihydroxycholecalciferol and parathormone: effects on isolated osteoclast-like and osteoblast-like cells

The actions of 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] and parathormone, both effective bone-resorptive agents in vivo and in vitro, were tested on CT (osteoclast-like) and PT (osteoblast-like) bone cells maintained in culture. Both agents stimulated acid phosphatase activity and hyaluronate synthesis in the CT cells and decreased alkaline phosphatase, citrate decarboxylation, and collagen synthesis in the PT cells. Calcitonin inhibited the changes induced in the CT but not in the PT cells. The activity of 1,25-(OH)2D3 differed from that of parathormone in one key respect: it did not increase cellular cyclic adenosine monophosphate, whereas parathormone did. Prior incubation of the bone cells with 1,25-(OH)2D3 for 6 to 24 hours made the cells refractory to the effect of parathormone on cyclic adenosine monophosphate formation. These data suggest that 1,25-(OH)2D3 and parathormone induce bone resorption by affecting the same cell types (osteoblasts and osteoclasts) although at different cellular sites.

Phosphate control and 25-hydroxycholecalciferol administration in preventing experimental renal osteodystrophy in the dog

Previous studies from this laboratory demonstrated that secondary hyperparathyroidism in dogs with chronic renal disease may occur, at least in part, as a consequence of the need for progressive adaptation in renal phosphorus (P) excretion that occurs as glomerular filtration rate falls. However, the studies were of relatively short duration. Moreover, no information emerged regarding a potential role of calcium malabsorption in the pathogenesis of secondary hyperparathyroidism. The short duration of the protocol did not lend itself to the study of the effect of P control or the administration of vitamin D in the pathogenesis of renal osteodystrophy. In the present studies, 14 dogs with experimental chronic renal disease were studied serially for a period of 2 yr. Each animal was studied first with two normal kidneys on an intake of P of 1,200 mg/day. Then, renal insufficiency was produced by 5/6 nephrectomy. The dogs then were divided into three groups. In group I, 1,200 mg/day P intake was administered for the full 2 yr. In group II, P intake was reduced from the initial 1,200 mg/day, in proportion to the measured fall in glomerular filtration rate, in an effort to obviate the renal adaptation in P excretion. In group III, "proportional reduction" of P intake also was employed; but in addition, 20 mug of 25(OH)D(3) were administered orally three times a week. In group I, parathyroid hormone (PTH) levels rose throughout the 2-yr period reaching a final concentration of 557+/-70 U (normal 10-60). In group II, values for PTH remained normal throughout the 1st yr, increased modestly between the 12th and the 18th mo, but then did not rise after the 18th mo. In group III, no elevation of PTH levels was observed at any time; however, these animals were hypercalcemic. Histomorphologic analyses of the ribs of these dogs were performed serially throughout the 2-yr period. A linear relationship was obtained between the osteoclastic resorption surface and the concentration of circulating immunoreactive PTH. The osteoid volume was greater in group I animals when compared to those in group II. None of the morphologic abnormalities associated with renal osteodystrophy were observed in the animals in the third group.