Hormonal control of the renal conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol

Regulation of circulating parathyroid hormone levels: normal physiology and consequences in disorders of mineral metabolism

A review of the chemistry, the biosynthesis, the regulation of the secretion and the metabolism of parathyroid hormone (PTH) provides a background to problems related to the determination of immunoreactive PTH levels in the blood of normal subjects and of patients suffering from disorders of mineral metabolism. Immunoreactive PTH measurements have to be interpreted in the light of the immunoheterogeneity encountered and the occurrence of several circulating PTH species with different molecular weights and immunological properties.

25-hydroxycholecalciferol-enhanced bone maturation in the parathyroprivic state

In vitro evidence presently favors a direct osteolytic effect of biologically active vitamin D metabolites. Studies were designed to evaluate the effect of 25-hydroxycholecalciferol (25OHD3) on bone collagen and mineral maturation in vivo and its dependence on parathyroid hormone (PTH). After treatment of sham-operated control and parathyroidectomized (PTX) mature rats with either 25OHD3 or an oil vehicle for 2 wk, tibial bone mineral-collagen maturation was quantitated by bromoform-toluene density gradient fractionation techniques. Intestinal calcium absorption was measured by in vivo 45Ca transport procedures. In contrast to the control group, the response to 25OHD3 of PTX rats was dramatic. Bone mineral and matrix maturation were both accelerated by 25OHD3 treatment without concomitant reduction in total bone mineral or collagen content or changes in the intestinal calcium absorption. These observations support the premise that biologically active vitamin D metabolites stimulate bone tissue maturation, and that PTH is not required in this regard.

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.

Experimental renal osteodystrophy. The response to 25-hydroxycholecalciferol and dicholomethylene diphosphate therapy

Bone mineral and matrix maturation in chronically uremic, nonacidotic rats were investigated after 25-hydroxcholecalciferol (25OHD) and/or dichloromethylene diphosphonate (C12MDP) therapy utilizing bromoform-toluene density gradient fractionation and X-ray diffraction analyses. The bromoform-toluene density gradient analyses demonstrated that the progressive accumulation of less dense, more immature bone characteristic of progressive uremia was reversed by 25OHD and/or C12MDP therapy for a 2-wk period, and that after 4 wk of therapy the maturational profile of bones from chronically uremic animals treated with 250HD and/or C12MDP was comparable to that from nonuremic littermates. X-ray diffraction analysis revealed that by the 4th wk of therapy with 25OHD and C12MDP both the degree of crystallinity and the crystal size/perfection parameters in the uremic bones were comparable to those of nonuremic, pair-fed control littermates. Treatment for 4 wk with 25OHD resulted in enlarged and/or more perfect apatite crystallites, while C12MDP alone slightly inhibited crystal growth and/or perfection after 2 wk of treatment. Soft tissue calcification was diminished in uremic animals treated for 4 wk with C12MDP or a combined C2MDP/25OHD regimen, the latter being much more effective in this regard. The accumulated data in this study support the premise that the attendant accelerated bone resorption, soft tissue calcification, and abnormal mineralization and maturation of the skeletal tissue, well documented to characterize experimental ranal insufficiency, may be alleviated with therapeutic dosages of 25OHD and/or C12MDP.

True recurrence of hyperparathyroidism: proposed criteria of recurrence

Although recurrent hyperparathyroidism should not be rare on theoretical grounds, only a few cases of proved recurrence have been reported in the literature. In the present author's series only 4 patients (1 per cent) had true recurrence. Criteria for recurrence were: 1. Histological identification by biopsy and frozen section of all the parathyroid glands at the first operation. 2. Complete removal of the enlarged gland(s). 3. A normocalcaemic period of at least 1 year. 4. The finding of a tumour at the site of a previously normal-sized gland. The low incidence of recurrent hyperparathyroidism might be explained by the long period of time needed for the recurrence to develop. However, since diagnosis of hyperparathyroidism is now more easily made and patients are treated surgically earlier in the course of the disease, the incidence of true recurrence might be expected to rise. A meticulous exploration during operation and careful follow-up of the patients are therefore required.

Calcitonin: a general survey

Since the initial discovery of calcitonin, a considerable amount of information has been obtained concerning structure, function, secretion, and effects of the hormone from a number of species. The purpose of the present report is to review the studies of structure and function, the factors that influence secretion and synthesis, and the effects and mechanism of action of calcitonin. The clinical uses of the hormone and syndromes of calcitonin excess are discu,sed.

The ionic control of 1,25-dihydroxyvitamin D3 production in isolated chick renal tubules

Isolated renal tubules prepared from vitamin D-deficient chicks catalyze the 1 alpha-hydroxylation of 25-hydroxyvitamin D3 (250HD3) in vitro. The effect of calcium and phosphate on the rate of synthesis of the product, 1, 25-dihydroxyvitamin D3 (1,25(OH)2D3), was studied at two levels: the long-term effects of various dietary calcium and phosphate contents on the ability of the tubules to produce 1, 25 (OH)2D3, and the acute effects of different calcium and phosphate concentrations in the incubation medium on the rate of synthesis of 1,25(OH)2D3 by the tubules. Manipulation of dietary calcium and phosphate sufficient to produce marked changes in the concentration of calcium and phosphate in the serum led to altered rates of 1,25(OH)2D3 synthesis by the isolated renal tubules. The renal tubules isolated from chicks raised on a vitamin D-deficient diet containing 0.43% calcium and 0.3% P as inorganic phosphate showed the highest rate of synthesis of 1,25(OH)2D3. Diets containing more or less of either calcium or phosphate produced chicks whose renal tubules had a slower rate of 1,25(OH)2D3 production. The calcium, phosphate, and hydrogen ion content of the incubation medium were manipulated to determine the possible factors concerned with the immediate regulation of 1,25(OH)2D3 production. A calcium concentration of 0.5-1.0 mM was necessary for optimal enzymatic activity. Concentrations of calcium greater than this optimal concentration inhibited 1,25(OH)2D3 production if phosphate was also present, and this inhibition was more pronounced as the phosphate concentration was increased. The stimulation of 1,25(OH)2D3 production by calcium was less at pH 6.7 than at 7.4. Raising the phosphate concentration from 0 to 6 mM in the absence of calcium also stimulated the rate of synthesis of 1,25(OH)2D3. This stimulatory effect was blocked by 4 mM calcium. However, at 1-2 mM calciu, phosphate had a biphasic influence on 1,25(OH)2D3 production; extracellular concentrations of phosphate from 0.6 to 1.2 mM resulted in less 1,25(OH)2D3 production than higher or lower phosphate concentrations. This biphasic effect was seen both at pH 7.4 and 6.8.

Intranuclear localization and receptor proteins for 1,25-dihydroxycholecalciferol in chick intestine

1. The intranuclear distribution of cholecalciferol and its metabolites was studied in the intestine of rachitic chicks. 2. At high doses of cholecalciferol the nuclei contain the vitamin and its 25-hydroxy metabolite, but over 80% of this is localized on the nuclear membranes. The hormone, 1,25-dihydroxycholecalciferol, is found within the cell nuclei irrespective of the intake of cholecalciferol, but significant amounts could not be found with chromatin isolated free of nuclear membranes. 3. 1,25-Dihydroxycholecalciferol is associated in the nucleus with an acidic protein. Since one of the actions of 1,25-dihydroxycholecalciferol is to control the synthesis of mRNA for calcium-binding protein it was to be expected that the hormone would be bound to chromatin, as with the other steroid hormones. It is suggested that the hormone-receptor complex exists as part of an equilibrium mixture of the complex bound to the DNA and in a free form. 4. A protein extract of nuclei was obtained, which when incubated at 4 degrees C for 1h took up the 1,25-dihydroxycholecalciferol. The nature of this binding was studied. 5. There appear to be two nuclear proteins able to bind the hormone one of which is the intestinal nuclear receptor. The binding sites on this protein are saturable with the hormone, have an association constant of 2x10(9)m(-1) and show a high chemical specificity for the 1,25-dihydroxycholecalciferol. The number of nuclear binding sites for the hormone provided by this receptor is similar to the maximum intestinal hormone concentration so far observed. Its sedimentation coefficient is 3.5S, and is very close to that observed for the nuclear protein to which is attached the 1,25-dihydroxycholecalciferol formed in vivo from vitamin D. 6. The cytoplasmic protein has an association constant of 1x10(9)m(-1)and a sedimentation coefficient of 3.0S, but its relation with the nuclear receptor is not yet clear.

The effect of Prednisolone upon the metabolism and action of 25-hydroxy-and 1,25-dihydroxyvitamin D3

Treatment of vitamin D-deficient rats with Prednisolone(R) does not alter the rate of conversion of [(3)H]25-hydroxyvitamin D(3) to [(3)H]1,25-dihydroxyvitamin D(3), but the further conversion of [(3)H]1,25-dihydroxyvitamin D(3) to a more polar metabolite is more rapid in the Prednisolone(R)-treated animals. This more polar metabolite is biologically inactive, periodate-insensitive, and persists in the intestine as long as 1,25-dihydroxyvitamin D(3). Also, the time course of action of 1,25-dihydroxyvitamin D(3) upon intestinal calcium transport is altered by Prednisolone(R) treatment. Treatment with Prednisolone(R) did not change the magnitude of the initial response to 1,25-dihydroxyvitamin D(3) at 7 hr, but did decrease the response at 24 and 48 hr after a single dose of 1,25-dihydroxyvitamin D(3). The present results show that one of the means by which large doses of adrenal corticoids alter intestinal calcium transport is by stimulating the further metabolism of 1,25-dihydroxyvitamin D(3) to a more polar, biologically inactive intestinal metabolite.

Metabolism and action of the hormone vitamin D. Its relation to diseases of calcium homeostasis

Extensive experimental evidence has established a significant role of calciferol in the maintenance of normal calcium homeostasis. Present knowledge indicates that vitamin D(3) must first be converted to 25-OH-D(3) and then to 1,25(OH)(2)D(3), the most active known form of the steroid. Many of the factors regulating the rate of production of this last steroid from its precurser have been evaluated, and the concept that vitamin D functions as a steroid hormone seems to be well established. Deranged action of calciferol, caused by impaired metabolism of the steroid or through altered sensitivity of target tissues, may be involved in the pathophysiology of several disease states with abnormal calcium metabolism. It is noted that liver disease, osteomalacia due to anticonvulsant therapy, chronic renal failure, hypophosphatemic rickets, hypoparathyroidism, hyperparathyroidism, sarcoidosis and idiopathic hypercalciuria have possible relation to alterations in metabolism or action of vitamin D. The future clinical availability of 1,25(OH)(2)D(3) and other analogs of this steroid may offer potential therapeutic benefit in the treatment of certain of the disease entities discussed.

Effects of dietary calcium restriction and chronic thyroparathyroidectomy on the metabolism of (3H)25-hydroxyvitamin D3 and the active transport of calcium by rat intestine

Previous studies have shown that chronically thyroparathyroidectomized (TPTX) rats, fed a diet with restricted calcium but adequate phosphorus and vitamin D content, have higher levels of intestinal calcium absorption than controls. The results of recent acute experiments have suggested that parathyroid hormone (PTH) may be essential for regulating the renal conversion of 25-hydroxyvitamin D(3) (25-OH-D(3)) to 1,25-dihydroxyvitamin D(3) [1,25-(OH)(2)-D(3)] in response to dietary calcium deprivation. Since 1,25-(OH)(2)-D(3) is the form of the vitamin thought to be active in the intestine, increases in calcium transport mediated by this metabolite would not be expected to occur in the absence of the parathyroid glands if the preceding model is correct. The present study was undertaken to examine the chronic effects of both dietary calcium restriction and the absence of PTH on the metabolism of [(3)H]25-OH-D(3) and duodenal calcium-active transport in rats given thyroid replacement. These relatively long term studies confirm earlier observations which indicated that the adaptation of calcium absorption to a low calcium intake occurs in both sham-operated and TPTX animals. The present studies also demonstrated that despite reduced levels of 1,25-(OH)(2)-D(3) in the plasma of chronically TPTX animals fed a low calcium diet, the accumulation of this metabolite in at least one target tissue, intestinal mucosa, is identical in both the sham-operated and TPTX groups. A reduced, but continued level of 1,25-(OH)(2)-D(3) production, together with its selective accumulation by intestinal mucosa, probably explains the calcium adaptation which is observed inspite of the chronic absence of the parathyroid glands.

Radioreceptor assay for 1 alpha,25-dihydroxyvitamin D3

A competitive protein binding assay with a sensitivity of 80 picograms has been developed for 1alpha,25-dihydroxyvitamin D(3), the hormonal form of vitamin D(3). lalpha,25-Dihydroxyvitamin D(3) displaced tritiated hormone from a cytosol-chromatin receptor preparation isolated from chick small intestine, providing a simple assay for the hormone. The concentration of lalpha, 25-dihydroxyvitamin D(3) in human plasma, as determined by this assay, is approximately 6 nanograms per 100 milliliters; in patients with renal disease the concentration of this kidney-produced hormone is significantly lower.

Calcium absorption and calcium-binding protein synthesis: solanum malacoxylon reverses strontium inhibition

The ingestion of diets containing high concentrations of stable strontium inhibits calcium absorption and intestinal calcium-binding protein synthesis and, as shown by others, does so by inhibiting the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, the active form of vitamin D. The addition of the South American plant Solanum malacoxylon to strontium-containing diets counteracts the inhibitory action of dietary strontium, thereby indicating that the plant contains a factor which can mimic the action of 1,25-dihydroxycholecalciferol and representing the first such factor identified in a botanical source.

Stimulation of 24,25-dihydroxyvitamin D3 production by 1,25-dihydroxyvitamin D3

Vitamin D-deficient rats produce [(3)H]1,25-dihydroxyvitamin D(3) from [(3)H]25-hydroxyvitamin D(3) regardless of dietary content of calcium or phosphate. A daily dose of 130 picomoles of 1,25-dihydroxyvitamin D(3) for a period of 5 days reduces production of [(3)H]1,25-dihydroxyvitamin D(3) to essentially zero and stimulates production of [(3)H]24,25-dihydroxyvitamin D(3). A daily dose of 325 picomoles of 25-hydroxyvitamin D(3) has a similar but less dramatic effect. On the other hand, 650 picomoles daily of 24,25-dihydroxyvitamin D(3) given to vitamin D-deficient rats had no effect. Thus it appears that 1,25-dihydroxyvitamin D(3) is an important factor in the regulation of kidney metabolism of 25-hydroxyvitamin D(3).

Study of the renal tubular interactions of thyrocalcitonin, cyclic adenosine 3',5'-monophosphate, 25-hydroxycholecalciferol, and calcium ion

Acute clearance studies were performed in thyroparathyroidectomized animals to determine the actions and interactions of thyrocalcitonin (TCT), cyclic adenosine 3'5'-monophosphate (cAMP), 25-hydroxycholecalciferol (25HCC), and calcium ion on the reabsorption of phosphate, calcium, sodium, and potassium by the kidney. The infusion of 25HCC in a dosage of 60 U/h to moderately saline-expanded animals (2.5% body weight) induced a fall in the excretion of all of the ions under study after 90-120 min similar to that observed in previous experiments from this laboratory. Mean decrements in fractional excretion were: phosphate, 42.0% (P < 0.005); calcium, 25.0% (P < 0.005); sodium, 23.4% (P < 0.001); and potassium, 14.7% (P < 0.005). The superimposition of either porcine or salmon TCT (1-100 MRC U/h for 2 h) resulted in no further alterations in electrolyte excretion. However, the infusion of TCT during steady-state saline expansion, before the administration of 25HCC, obviated the renal transport effects of the vitamin D metabolite. Both in the latter studies, as well as those in which similar doses of TCT were given to hydropenic animals, the hormone itself failed to induce any consistent alteration in electrolyte excretion. Cyclic AMP (50 mg/h) caused an increase in the excretion of phosphate, sodium, and potassium and no change in calcium excretion. Like TCT, the nucleotide blocked the action of 25HCC on the kidney. Raising the mean level of serum ultrafilterable calcium to 3.02+/-0.25 mEq/liter from 1.62+/-0.17 mEq/liter likewise prevented enhanced ionic reabsorption due to 25HCC.

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

The acute effect of i.v. and direct intrarenal arterial infusion of 25-hydroxycholecalciferol (25HCC) and 1,25-dihydroxycholecalciferol (1,25-DHCC) on renal handling of phosphorus was evaluated in the following groups of rats: (a) intact animals, (b) parathyroidectomized (PTX) hypocalcemic rats, (c) PTX rats in which normocalcemia was maintained with calcium supplements and (d) PTX animals in which urinary phosphorus was augmented by (i) i.v. sodium phosphate, (ii) expansion of the extracellular fluid volume with normal saline, and (iii) i.v. parathyroid hormone (PTH). Clearances of inulin (C(In)), phosphorus (C(P)), and fractional clearances of phosphorus (C(P)/C(In)) of the experimental groups were compared with those of the corresponding control groups, and the clearances of the infused kidneys with those of the contralateral kidneys. In intact animals, i.v. 25HCC decreased C(P)/C(In) from 0.29+/-0.04 (mean +/-SE) to 0.19+/-0.04, and i.v. 1,25-DHCC decreased C(P)/C(In) from 0.25+/-0.04 to 0.15+/-0.02. The intrarenal infusion of both 25HCC and 1,25DHCC into intact animals failed to produce a unilateral change; however, it decreased C(P)/C(In) bilaterally. i.v. and intrarenal infusions of 25HCC or 1,25DHCC in PTX hypocalcemic and normocalcemic rats, and i.v. infusions of 25HCC in PTX rats receiving either sodium phosphate or normal saline, all failed to produce significant changes in C(P)/C(In). In contrast, 24HCC given i.v. to PTX animals receiving exogenous PTH was associated with a significant fall in C(P)/C(In), from 0.34+/-0.08 to 0.13+/-0.02. These results indicate that 25HCC enhances tubular reabsorption of phosphorus in rats, only in the presence of either endogenous or exogenous circulating PTH, but not in its absence and thus imply a PTH-dependent mechanism of 25HCC action on the kidney. This effect does not appear to be related to the conversion of 25HCC into 1,25DHCC, since the latter fails to affect tubular reabsorption of phosphorus in PTX rats.

Thyrocalcitonin and the jejunal absorption of calcium, water, and electrolytes in normal subjects

Jejunal absorption of calcium, water, and electrolytes was measured in 10 normal subjects by the triple-lumen perfusion method. During the control period, water and electrolyte movements were minimal when a bicarbonate-free test solution was infused. By contrast, bicarbonate-containing solutions were readily absorbed in the control period. Intravenous infusion of synthetic salmon calcitonin (SCT) (1 Medical Research Council U/kg wt/h) over 110-120 min resulted in a marked jejunal secretion of water, sodium, potassium, and chloride in 8 of the 10 subjects. This jejunal secretion occurred with both the bicarbonate-free and the bicarbonate-containing test solutions. Calcium absorption was not affected by SCT, and the serum calcium concentration did not fall during SCT infusion. These results suggest that diarrhea in patients with medullary carcinoma of the thyroid may be due to intestinal secretion secondary to high blood concentrations of thyrocalcitonin.

Calcium-mobilizing effect of large doses of 25-hydroxycholecalciferol in anephric rats

The effect of high doses of 25-hydroxycholecalciferol on plasma calcium concentration was studied in rats receiving a low-calcium normal vitamin D diet. In bilaterally nephrectomized animals, as in sham-operated controls, 62.5 nmol of 25-hyroxycholecalciferol did not produce a rise of plasma calcium concentration. In contrast, the administration of 125 or 625 nmol, doses 1,000-5,000 times the minimal active dose in D-deficient rats, was followed in both groups of animals by a significant increase of plasma calcium concentration. Removal of either parathyroids alone or parathyroid and thyroid glands did not suppress this effect. These data suggest that when large doses are used in vivo, the renal conversion of 25-hydroxycholecalciferol to more polar metabolites is not an obligatory step for its calcium-mobilizing action. The present study does not elucidate, however, the exact mechanism(s) of this effect.

Intestinal calcium transport: stimulation by low phosphorus diets

Rats maintained on a low phosphorus diet supplemented with 25-hydroxyvitamin D(3) show high intestinal calcium transport activity as compared to rats similarly treated but fed a diet containing adequate phosphorus. This increased transport activity is correlated with an increased biosynthesis of 1,25-dihydroxyvitamin D(3), the probable metabolically active form of the vitamin in the intestine.