THE RELATIONSHIP BETWEEN VITAMIN D AND PARATHYROID HORMONE

Vitamin D: Historical Overview

A history of vitamin D has been provided, dating from the earliest description of rickets, the disease resulting from vitamin D deficiency, to a current understanding of vitamin D metabolism and the mechanism of action of its hormonal form in regulating gene expression in target organs. Vitamin D is produced in skin by impact of 280-310 nm light on 7-dehydrocholesterol. The vitamin D is then converted in the liver to a circulating form, 25-hydroxyvitamin D that is converted largely, if not exclusively, in the kidney to the final hormone, 1α,25-dihydroxyvitamin D. This hormone functions through a nuclear receptor that regulates expression of key genes in target organs. Among its many resulting functions are increased intestinal calcium and phosphate absorption, bone calcium mobilization, and renal reabsorption of calcium. The resultant increase in serum calcium and phosphate supports bone mineralization, curing rickets, and osteomalacia. There are many other functions of vitamin D that remain to be described that contribute to its health supporting role.

Triennial Growth Symposium--Vitamin D: bones and beyond

The exploration of vitamin D metabolism and function has led to the discovery of active forms of vitamin D that find great usefulness in treating patients with bone disease or renal failure and also perhaps in topical application for the treatment of skin disorders, such as psoriasis. It may also be effective in some types of autoimmune disease. This warrants our attention to maintaining an adequate vitamin D level in our blood to assure that the expected functions of vitamin D take place. However, we must not get so overenthusiastic as to expect vitamin D to be effective in treating or preventing many diverse diseases and especially caution is urged in considering that vitamin D compounds might be used to suppress cancerous growth.

Actinomycin D Inhibition of Vitamin D Action

Injection of actinomycin D into rats completely prevents both the rise in serum calcium normally induced by vitamin D and the increased transport of calcium by everted intestinal sacs. Injection of excess parathyroid hormone did not alter this result; this eliminates the possibility that the inhibition of vitamin action was due to blocked hormone synthesis. As a result of these findings, a hypothesis concerning the mechanism of action of vitamin D is presented.

Limited availability of nutritional vitamin D causing inappropriate treatment of vitamin D deficiency rickets with a response resembling pseudohypoparathyroidism type II in a Japanese patient

Vitamin D deficiency rickets occasionally resembles pseudohypoparathyroidism type II (PHP type II) with respect to the response to exogenous PTH in the presence of hypocalcemia. We encountered a Japanese patient with stage 2 vitamin D deficiency rickets, who had increased urinary cAMP excretion and no response of urinary phosphate or N-acetyl-beta-D-glucosaminidase excretion to exogenous PTH under normocalcemic and normophosphatemic conditions, after treatment with 1,25(OH)2 vitamin D3. This case shows that it is possible for a response mimicking that of PHP type II to occur when the serum calcidiol level is low due to causes other than hypocalcemia and secondary hyperparathyroidism. When the serum calcidiol level is low, the appropriate treatment should be cholecalciferol or ergocalciferol. However, because neither is commercially available as a useful formulation in Japan, physicians are forced to inappropriately use calcitriol or analogs.

Hypercalcemia produced by parathyroid hormone suppresses experimental autoimmune encephalomyelitis in female but not male mice

Besides its role in regulating serum levels of calcium and phosphorus, 1alpha, 25-dihydroxyvitamin D3 (1,25-(OH)2D3) has potent effects on the immune system and suppresses disease in several animal models of autoimmune disorders including experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis. While the amount of 1,25-(OH)2D3 needed to prevent EAE is dependent on the gender of the mouse and amount of calcium available in the diet, the minimum levels of 1,25-(OH)2D3 sufficient to prevent disease cause hypercalcemia. To test if hypercalcemia independent of high levels of 1,25-(OH)2D3 can suppress EAE, we used a 25-hydroxyvitamin D3-1alpha-hydroxylase (1alpha-hydroxylase) knockout mouse strain. Because these 1alpha-hydroxylase knockout mice lack the parathyroid hormone (PTH)-regulated enzyme that synthesizes 1,25-(OH)2D3, hypercalcemia from increased bone turnover was created by continuous administration of PTH without changing the circulating levels of 1,25-(OH)2D3. This PTH-mediated hypercalcemia generated after EAE induction prevented disease in female mice but not male mice. When hypercalcemia was prevented by diet manipulation, PTH administration no longer prevented EAE. We conclude that hypercalcemia is able to prevent EAE after disease induction in female mice.

In vivo administration of 1,25-dihydroxyvitamin D3 suppresses the expression of RANKL mRNA in bone of thyroparathyroidectomized rats constantly infused with PTH

It is known that pharmacological or toxic doses of vitamin D induce bone resorption both in vivo and in vitro, whereas physiological doses of the vitamin have a protective effect on bone in vivo. To investigate the discrepancies of the dose-dependent effect of vitamin D on bone resorption, we examined the in vivo effect of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] on the expression of the receptor activator of nuclear factor-kappaB (NF-kappaB) ligand (RANKL) and osteoprotegerin (OPG) mRNAs in bone of thyroparathyroidectomized (TPTX) rats infused with or without parathyroid hormone (PTH). Continuous infusion of 50 ng/h of PTH greatly increased the expression of RANKL mRNA in bone of TPTX rats. Expression of OPG mRNA was not altered by PTH infusion. When graded doses of 1,25(OH)(2)D(3) was daily administered orally for 14 days to normocalcemic TPTX rats constantly infused with PTH, 0.01 and 0.1 microg/kg of 1,25(OH)(2)D(3) inhibited the PTH-induced RANKL mRNA expression, but 0.5 microg/kg of the vitamin did not inhibit it. Regulator of G protein signaling-2 (RGS-2) gene expression was suppressed by 1,25(OH)(2)D(3) dose-dependently, but PTH/PTHrP receptor mRNA expression was not altered. Bone morphometric analyses revealed that 1,25(OH)(2)D(3) suppressed PTH-induced osteoclast number in vivo. These results suggest that pharmacological or toxic doses of 1,25(OH)(2)D(3) stimulate bone resorption by inducing RANKL, but a certain range of physiological doses of the vitamin inhibit PTH-induced bone resorption, the latter mechanism appeared to be mediated, at least in part, by the suppression of the PTH/PTHrP receptor-mediated signaling.

Bone resorption activity of all-trans retinoic acid is independent of vitamin D in rats

The mechanism by which all-trans retinoic acid (ATRA) induces bone resorption is unknown. However, an interaction between vitamin A and vitamin D has been established. In fact, although the mechanism is still unclear, vitamin A has been shown to be a weak antagonist of the actions of vitamin D. Taking into account this interaction and the influence of vitamin D on other calcitropic hormones, such as parathyroid hormone, the effect of vitamin D on ATRA-induced bone resorption was investigated. Vitamin D-deficient rats were fed diets containing 0 or 150 micro g of ATRA/g of diet. The rats then were orally administered 0 or 625 ng of cholecalciferol (vitamin D(3)) daily. Various bone parameters were measured after 3-8 wk. Regardless of the presence or absence of vitamin D(3), ATRA was able to cause bone resorption. In addition to examining the effect of vitamin D on ATRA-induced bone resorption under normal conditions, this effect also was studied under conditions that inhibit bone mineralization or growth by altering dietary calcium (Ca) and phosphorus (P) levels. Changes in dietary levels of Ca and P did not affect the ability of ATRA to cause bone resorption. Interestingly, despite its ability to stimulate bone resorption, ATRA did not affect serum calcium or phosphorus levels. Overall, the ability of ATRA to cause bone resorption is not dependent on vitamin D(3), dietary Ca or dietary P.

Current understanding of the molecular actions of vitamin D

The important reactions that occur to the vitamin D molecule and the important reactions involved in the expression of the final active form of vitamin D are reviewed in a critical manner. After an overview of the metabolism of vitamin D to its active form and to its metabolic degradation products, the molecular understanding of the 1alpha-hydroxylation reaction and the 24-hydroxylation reaction of the vitamin D hormone is presented. Furthermore, the role of vitamin D in maintenance of serum calcium is reviewed at the physiological level and at the molecular level whenever possible. Of particular importance is the regulation of the parathyroid gland by the vitamin D hormone. A third section describes the known molecular events involved in the action of 1alpha,25-dihydroxyvitamin D3 on its target cells. This includes reviewing what is now known concerning the overall mechanism of transcriptional regulation by vitamin D. It describes the vitamin D receptors that have been cloned and identified and describes the coactivators and retinoid X receptors required for the function of vitamin D in its genomic actions. The presence of receptor in previously uncharted target organs of vitamin D action has led to a study of the possible function of vitamin D in these organs. A good example of a new function described for 1alpha,25-dihydroxyvitamin D3 is that found in the parathyroid gland. This is also true for the role of vitamin D hormone in skin, the immune system, a possible role in the pancreas, i.e., in the islet cells, and a possible role in female reproduction. This review also raises the intriguing question of whether vitamin D plays an important role in embryonic development, since vitamin D deficiency does not prohibit development, nor does vitamin D receptor knockout. The final section reviews some interesting analogs of the vitamin D hormone and their possible uses. The review ends with possible ideas with regard to future directions of vitamin D drug design.

Scurvy results in decreased collagen synthesis and bone density in the guinea pig animal model

The effect of severe ascorbic acid deficiency on bone remodeling and collagen synthesis was evaluated in a 21 day experiment, using the scorbutic guinea pig model. Animals (n = 6-7/group) were assigned to one of three groups: scorbutic, pair-fed ascorbic acid-replete, or ad libitum ascorbic acid-replete groups. After 2 weeks, scorbutic animals started voluntarily decreasing food intake and losing weight. By day 19-21, at which time bone and tissue samples were collected and analyzed, scorbutic animals decreased food intake to 46% of usual and lost 9% body weight. Serum 25OHD3, 1,25(OH)2D3, calcium, and albumin were significantly lower (p < 0.05) in the scorbutic animals than in the other groups. Bone mineral density and bone mineral content of the proximal and central femur were significantly lower in the scorbutic group than in the other groups (p < 0.05). Morphometric analysis of tibia indicated significantly lower bone volume, fewer and thinner trabeculae, and a thinner growth plate in the scorbutic group, compared to the pair-fed and ad libitum groups (p < 0.05). Osteoclast surface was about 60% higher in the scorbutic group than in the pair-fed and ad libitum control groups (0.05 < p < 0.10). Mechanical strength of the femur and lumbar vertebral body tended to be lower when bone mass was altered in the same group. Collagen synthesis of articular cartilage and tendons was lower in the scorbutic group than in the pair-fed or ad libitum groups (p < 0.05). In conclusion, scurvy but not food restriction, per se, results in alterations in bone mass and tissue collagen synthesis.

Pathophysiology of calcium metabolism in children with vitamin D-deficiency rickets

OBJECTIVE

To improve understanding of the biochemical events in vitamin D-deficiency rickets (VDR).

METHODS

We investigated 51 untreated patients, 2 to 36 months of age, during three stages of VDR. Nineteen of these patients were also studied during therapy with 5000 to 10,000 U vitamin D3 (cholecalciferol) and 0.5 to 1 gm calcium. Together with calcium and inorganic phosphate in serum and urine, we measured (1) parathyroid hormone (PTH) secretion (intact serum PTH) and action on the kidney (urinary adenosine 3',5'-cyclic monophosphate (cAMP)/creatinine ratio; (2) serum alkaline phosphatase level; (3) urinary hydroxyproline/creatinine ratio; and (4) serum 1,25-dihydroxyvitamin D (1,25(OH)2D) level.

RESULTS

The untreated patients had secondary hyperparathyroidism (high serum PTH and urinary cAMP/creatinine ratio), low calcium and phosphate concentrations in serum, and increased bone turnover (elevated serum alkaline phosphatase and OHP/creatinine ratio), whereas serum 1,25(OH)2D was low, normal, or even slightly elevated. Serum calcium level was positively correlated to serum 1,25(OH)2D and to OHP/creatinine ratio, indicating that normocalcemia in untreated rickets (stage 2) is at least partially maintained by 1,25(OH)2D-induced calcium mobilization from bone. There was no correlation between serum calcium and serum PTH, or between serum PTH and urinary cAMP/creatinine ratio or serum phosphate, indicating disturbed regulation and action of PTH. During vitamin D treatment, serum 1,25(OH)2D values increased to supranormal concentrations in association with the restoration of the physiologic relationship of PTH to serum calcium and phosphate concentrations and urinary cAMP/creatinine ratio.

CONCLUSION

Circulating 1,25(OH)2D has an important role in the pathophysiology of VDR before and during treatment, mainly by influencing the bone and kidney response to endogenous PTH.

Single-day therapy for nutritional vitamin D-deficiency rickets: a preferred method

A single-day large dose of vitamin D (stosstherapy) was given to 42 patients with nutritional vitamin D-deficiency rickets. Stosstherapy is safe and effective, obviates problems with compliance, and, by evoking a response in 4 to 7 days in nutritional rickets, becomes a valuable diagnostic aid for patients in whom initial findings do not clearly distinguish nutritional rickets from familial hypophosphatemic rickets.

Ultimobranchial body and parathyroid glands of the freshwater snake Natrix piscator in response to vitamin D3 administration

Vitamin D3 (20 I.U./100 g body wt) was administered to the freshwater snake Natrix piscator for 15 days. Elevation of serum calcium and inorganic phosphate levels was observed after the treatment. The ultimobranchial body became active whereas the parathyroid glands exhibited reduced activity.

Bone mineral loss during lactation occurs in absence of parathyroid tissue

The requirement of parathyroid tissue for bone mineral loss during lactation was investigated. Lactating rats parathyroidectomized (PTX) at day 2 of lactation and consuming a 2% calcium diet are hypercalcemic and hypophosphatemic at day 13 of lactation. The high-calcium diet supports normal growth of pups nursing PTX mothers. PTX lactating rats mobilize bone mineral to the same extent as euparathyroid lactating rats consuming the same diet. Non-lactating PTX rats lose no bone mineral over a similar time period, indicating lactation-specific bone mineral mobilization in the absence of parathyroid tissue. PTX rats were verified to have physiologically insignificant amounts of parathyroid tissue, as evidenced by severe hypocalcemia and/or death in each rat after a shift from a 2% calcium to a 0.02% calcium diet. These results conclusively demonstrate that lactation-associated bone mineral mobilization does not require parathyroid hormone or parathyroid tissue.

Effect of vitamin D status on the activity of carbonic anhydrase in chicken epiphysis and kidney

Chickens were raised for 6 weeks from the date of hatch under red light on a vitamin D-free diet; controls were given an oral vitamin D supplement. Vitamin D-deficient animals showed decreased total serum calcium concentration and decreased DNA content in epiphysis and kidney homogenates. In calcifying epiphysis, total carbonic anhydrase (CA) activity was decreased, but activity per microgram DNA was slightly increased and specific activity was double that of the controls. Polyacrylamide gel isoelectric focusing after preparation of the enzyme showed a picture similar to that seen after parathyroid hormone (PTH) administration in chicks; therefore, this could be considered a secondary hyperparathyroidism. The CA activation was not seen in the kidney which can be explained by induction of an endogenous inhibitor protein of the cyclic AMP-dependent protein kinase exclusively in the kidney in vitamin D deficiency. In an additional experiment, chickens were raised for 3 weeks from the date of hatch under red light on a vitamin D-free diet. Daily oral substitution by different vitamin D metabolites (1,25(OH)2D3, 25OHD3, 24,25(OH)2D3) over 7 days led to CA activation compared with controls probably by restoring protein kinase activity in the kidney. Our results show that CA activity is inversely correlated with serum calcium concentrations which is in agreement with a regulatory mechanism recently proposed by us.

Certain vitamin D metabolites potentiate the expression of parathyroid hormone bioactivity

With the development of a sensitive bioassay for the skeletal effects of parathyroid hormone (PTH), it has become possible to investigate the possible interaction between PTH and vitamin D3 metabolites. This assay is based on the stimulation of glucose-6-phosphate dehydrogenase (G6PD) activity in either the hypertrophic chondrocytes of the growth plate or the osteoblasts lining the metaphyseal trabeculae of rat metatarsals. The response to PTH is paralleled by the activity of dibutyryl cAMP. None of the vitamin D3 metabolites tested had any effect on enzyme activity when tested by themselves. However, both 1,25(OH)2D3 and 25(OH)D3 caused a dose-related potentiation of the response to PTH. Neither 1,24,25(OH)3D3 nor 1,25(OH)2D3 26,23-lactone potentiated the response to PTH. Because this potentiation of the response to PTH occurs after only 8 minutes, it is suggested that it represents a nongenomic response to the vitamin D3 metabolites.

The metabolism and functions of vitamin D

Vitamin D functions by stimulating intestinal calcium and phosphorus absorption, by stimulating bone calcium mobilization, and by increasing renal reabsorption of calcium in the distal tubule. These functions on bone and possibly kidney, but not intestine, require the parathyroid hormone. As a result of these functions, serum calcium and phosphorus concentrations are elevated to supersaturating levels required for the mineralization of bone to prevent rickets, osteomalacia, and hypocalcemic tetany. Recent experiments demonstrate that maintaining serum calcium and phosphorus levels in vitamin D-deficient rats in the normal range results in normal bone growth and mineralization. However, increased calcification results because bone resorption by osteoclasts is a vitamin D-dependent process. Thus, bone resorption, modeling and remodeling must be considered vitamin D-dependent processes. Vitamin D must be metabolized to 25-hydroxyvitamin D3 by the liver and subsequently by the kidney to 1,25-dihydroxyvitamin D3 before function. 1,25-Dihydroxyvitamin D3 is metabolized to a C-23 carboxylic acid (calcitroic acid) but the pathway is unknown. Although 25-hydroxyvitamin D3 is metabolized to 24R,25-dihydroxyvitamin D3, 25,26-dihydroxyvitamin D3 and 25-hydroxyvitamin D3-26,23-lactone, these pathways play no role in the function of vitamin D as shown by appropriate fluoro analogs of 25-hydroxyvitamin D3. 1,25-Dihydroxyvitamin D3 binds to a specific receptor in the intestinal nuclei to elicit a stimulation of calcium transport. 1,25-Dihydroxyvitamin D3 plus the receptor causes transcription of specific genes that code for calcium and phosphorus transport proteins. Only one protein, the calcium binding protein, has been identified as being vitamin D dependent. Two others have been described, but no clear description of the molecular mechanism of action of 1,25-dihydroxyvitamin D3 is yet available.

Effects of parathyroid hormone on puppies during development of Ca and vitamin D deficiency

The acute effects of parathyroid extract (PTE) were studied repeatedly in young dogs (prelabeled with 45Ca and [3H]tetracycline) during the development of calcium (Ca) and vitamin D deficiency. Blood Ca and radioactivity changes were monitored sequentially after subcutaneous PTE, injected seven times over 63 days. In control dogs, all sequential responses to acute PTE challenges were constant in both magnitude of increase and time at which maximum response occurred over the entire experiment. Under chronic Ca and D deficiency, plasma 25-hydroxyvitamin D in experimental dogs decreased continuously to very low levels at 63 days, whereas 1,25-dihydroxyvitamin D initially increased to a maximum at 32 days and thereafter decreased. In response to an acute challenge of PTE, dogs on the deficient diet for 3 and 10 days showed a greater response of blood Ca and 45Ca than the controls but subsequently showed a smaller response than controls after 49 and 63 days on the deficient diet. Compared with control dogs, the time of maximal response of blood Ca and 45Ca to PTE occurred much earlier in dogs that were on the deficient diet for 35-63 days. The blood [3H]tetracycline response (index of bone resorption) to exogenous PTE in the deficient dogs, however, was constant and similar to that of the control dogs during the entire period. The data suggest that the bone resorption response to PTE was normal in Ca- and D-deficient puppies with hypocalcemia.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of vitamin D deficiency on secretion of saliva by rat parotid gland in vivo

The role of vitamin D in parotid gland function was investigated by measuring the composition and rate of production of parotid saliva in response to pilocarpine injection in vitamin-D-deficient and replete rats in vivo. Rats fed a vitamin-D-free diet from weaning (G1) were studied after 8 weeks of diet at which time they had a decreased rate of growth, were hyperparathyroid and hypocalcaemic but still had detectable serum 1,25-dihydroxycholecalciferol (1,25(OH)2D3). Rats which were the offspring of vitamin-D-deficient mothers and which were maintained on a vitamin-D-free diet from weaning (G2) had a decreased rate of growth from birth, were hypocalcaemic and hyperparathyroid and at no time had any detectable serum 1,25(OH)2D3. In response to pilocarpine, the volume of parotid saliva produced by G1 animals was no different from the controls (G1 animals receiving supplemental vitamin D) whereas that produced by G2 animals was reduced more than 65%. The total amount of amylase secreted was unchanged in either group of experimental animals so that the concentration of amylase in the parotid saliva from G2 animals was increased. The concentration of calcium in parotid saliva changed in parallel with the changes in serum calcium in G1 and G2 animals. It is concluded that the primary source of parotid saliva calcium is the extracellular fluid and not zymogen granules and the transepithelial transport of this calcium is independent of vitamin D; the secretion of electrolytes and water, which in the parotid gland require extracellular calcium, is dependent on vitamin D. It is proposed that the vitamin is necessary for the synthesis of a protein(s) which is essential for the utilization of extracellular calcium in this secretion process.

25-Hydroxyvitamin D, 24, 25-dihydroxyvitamin D and 1,25-dihydroxyvitamin D in human cerebrospinal fluid

Samples of CSF and plasma were obtained simultaneously from 46 adult patients who had no endocrine disorders and were undergoing routine diagnostic lumbar puncture because of suspected or proved prolapse of a disc. Concentrations of 25-OHD, 24,25(OH)2D and 1,25(OH)2D were measured. The samples were purified by column chromatography and fractionated by HPLC. In the appropriate fractions the vitamin D metabolites were measured by PBA, and cytoreceptor assay. The results were as follows (median, range in brackets): 25-OHD in CSF 8.3 ng/ml (2.0-24.8), in plasma 14.5 ng/ml (7.0-36.0). 24,25(OH)2D in CSF 1.8 ng/ml (0.3-4.6) and 2.5 ng/ml (0.4-4.7) in plasma. 1.25(OH)2 D in CSF 25.0 pg/ml (2.2-39.0) and 31.0 pg/ml (10.1-55.0) in plasma. The correlations between plasma and CSF concentrations were as follows: 25-OHD r = 0.479 (P less than 0.001); 24,25(OH)2D r = 0.815 (P less than 0.001) and for 1.25(OH)2D r = 0.497 (P less than 0.001). Our findings showed vitamin D metabolites to be present in human CSF.

Effects of vitamin D and parathyroid hormone on cyclic AMP production by bone cells isolated from rat calvariae

Studies presented here were designed to investigate further the basis for an impaired cAMP response to parathyroid hormone (PTH) in osteoblastlike calvarial bone cells isolated from vitamin D-deficient rat pups. The goal was to perturb Ca, PTH, and vitamin D in vivo in order to see which factors might be responsible for the impaired in vitro bone cell cAMP response. Pups either were parathyroidectomized (PTX) 3-5 days, implanted with osmotic minipumps delivering high doses of PTH, given repeated, high doses of 1,25(OH)2D3, or were D-deficient (-D, i.e., born and suckled by D-deficient mothers). Osteoblastlike bone cells, isolated by sequential enzyme digestion and centrifugation, were exposed to PTH for 5 min in the presence of a phosphodiesterase inhibitor. In bone cells isolated from -D rat pups, both basal and PTH-induced cAMP accumulation were significantly lower than in +D bone cells. Earlier, we had shown that two daily injections of -D pups with 50 ng 1,25(OH)2D3 restores this reduced bone cAMP response of -D pups toward normal. In the present study, neither basal nor PTH-induced bone cell cAMP accumulation was affected by subjecting D-replete pups to PTX, PTH infusion, or repeated high doses of 1,25(OH)2D3 despite the fact that each treatment markedly changed serum Ca or serum immunoreactive PTH. The results indicate that the impaired bone cell cAMP response seen in -D pups is not a direct result of chronic hypocalcemia and that the "heterologous desensitization" seen in vitro with added 1,25(OH)2D3 could not be duplicated by in vivo treatment of +D pups with supraphysiologic doses of 1,25(OH)2D3.(ABSTRACT TRUNCATED AT 250 WORDS)

Responsiveness of vitamin D-deficient fetal rat limb bones to parathyroid hormone in culture

Radii and ulnae from 19-day fetal rats from normal or vitamin D-deficient mothers were treated with 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, or parathyroid hormone in vitro. Both sets of bones resorbed in response to all three agents. Statistical analysis indicated a purely additive model for the effects of vitamin D status and the bone-resorbing agents, with no evidence for interaction. The results suggest that the impaired calcemic response to parathyroid hormone seen in vitamin D-deficient animals in vivo is not the result of a specific unresponsiveness of vitamin D-deficient bone to parathyroid hormone.

Role of vitamin D in maternal skeletal changes during pregnancy and lactation: a histomorphometric study

The effect of vitamin D on bone changes during the reproductive cycle in female rats has been investigated. One group of female rats was maintained on a vitamin D-deficient diet and another group of a vitamin D-replete diet from weaning. Both groups were mated with normal males and changes in their bones were determined histomorphometrically during pregnancy, lactation, and after weaning. All vitamin D-deficient rats had bone changes typical of rickets. Pregnancy caused significant reductions in mineralized tissue of trabecular and cortical bone in the vitamin D-deficient rats. Lactation caused further significant reductions in mineralized tissues of cortical and trabecular bone in both the vitamin D-deficient and vitamin D-replete animals, with the greatest changes seen at weaning. Some restoration of mineralized tissues occurred following weaning. There was an increase in tetracycline-labeled bone surface in the vitamin D-replete animals during lactation, likely due to an increase in bone formation rates. In the vitamin D-deficient animals during lactation, there was a decrease in tetracycline-labeled bone surface, likely due to severely depressed bone mineralization. These results indicate that the mobilization of calcium from bone to maintain pregnancy and lactation occurs by a mechanism independent of vitamin D.

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.

Vitamin D and hyperparathyroidism: the Lumleian Lecture 1981

Vitamin D appears to influence parathyroid function indirectly through its effects on calcium metabolism rather than by a direct action of its metabolites on the parathyroid glands. In states of both secondary and primary hyperparathyroidism, the quantitative production of 1,25-(OH)2D may be determined by the prevailing concentration of serum 25-(OH)D but there appears to be some constraint that limits the formation of 1,25-0(OH)2D when the provision of its precursor exceeds the physiological. From the absence of this constraint in 'type 2 vitamin D dependency' it is inferred that it may operate through 'self-inhibition' of the renal production of 1,25-(OH)2D. It is shown that the level of serum 25-(OH)D may always exert some influence on the production of 1,25-(OH)2D and that this effect is facilitated by hyperparathyroidism. In developing vitamin D deficiency the reactive secondary hyperparathyroidism may thus function as an adaptive mechanism that sustains the level of serum 1,25-(OH)2D in the face of a diminishing serum 25-(OH)D. Failure of this adaptation and the development of a critical deficiency of 1,25-(OH)2D is regarded as the direct cause of defective mineralisation of bone. This concept would explain the absence of osteomalacia in some patients with very low levels of serum 25-(OH)D and the occurrence of defective osseous mineralisation in hypoparathyroidism.

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.

Effects and interactions of 24R,25(OH)2D3 and 1,25(OH)2D3 on bone

The effects of various combinations of therapy with 1 alpha,25-dihydroxycholecalciferol (1,25(OH)2D3) and 24R,25-dihydroxycholecalciferol (24R,25(OH)2D3) on structural and dynamic parameters of bone were evaluated in 40 chicks raised on a vitamin D-deficient diet from time of hatching and supplemented with the dihydroxylated metabolites. The results showed that: 1) the maintenance of volumetric density of bone is dependent on the presence of 1,25(OH)2D3, 2) lack of 1,25(OH)2D3 is associated with an increase in the number of osteocytes per unit volume of bone, most probably due to decreased amounts of bone formed by each osteoblast before becoming an osteocyte, 3) adequate quantities of either 24R,25(OH)2D3 or 1,25(OH)2D3 are needed to prevent accumulation of osteoid or the production of endosteal fibrosis, and 4) maintenance of normal tetracycline label width requires both hydroxylated compounds with one of them in sufficient amounts. The data of this study demonstrate that the integrity of certain parameters of bone structure could be maintained only with 1,25(OH)2D3, others with either dihydroxylated metabolites, and still others with a combination of both. These data underscore the biological activity of 24R,25(OH)2D3 by demonstrating its effectiveness on bone.

Effects of excess 1,25-dihydroxycholecalciferol in young rats

The oral administration for 5 days of excess 1 alpha,25-dihydroxycholecalciferol [1,25(OH)2D3] at doses of 1, 5, and 25 micrograms/kg to rats, beginning at the age of 2 or 10 days, produced dose-dependent reductions in weight development and additional calcification near the skeleton. Alizarin red S stained skeleton revealed calcific deposits near the bones of the head, near the neural arches, between the ribs, along the bones both of the fore limbs and, to a lesser extent, of the hind limbs. Histologically, the deposits appeared to be localized primarily in the sub-epithelial connective tissues. Starting treatment with 1,25(OH)2D3 (25 micrograms/kg for 5 days) at the age of 20 days produced additional calcification in 1 of 8 rats at only 1 location (lower jaw). Additional calcification as described above could no longer be induced by 1,25(OH)2D3 in 30-day-old rats using doses up to 25 micrograms/kg and 10 daily treatments. We conclude that the sensitivity of young rats to 1,25(OH)2D3-induced additional calcification, which differs in localization from that observed in adult rats, decreases with the maturation of the animals.

Resistance to the phosphaturic and calcemic actions of parathyroid hormone during phosphate depletion. Prevention by 1,25-dihydroxyvitamin D3

Recent observations indicate that in thyroparathyroidectomized (TPTX) rats fed a low (0.2 g/100 g) phosphorus diet, the tubular phosphaturic response to parathyroid hormone (PTH) remains markedly blunted even when it is assessed at normal or high plasma concentration and filtered load of inorganic phosphate (Pi). Because 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] decreases the tubular capacity to reabsorb Pi when chronically administered to TPTX rats, we have studied whether this vitamin D(3) metabolite could specifically increase the phosphaturic response to PTH in phosphate-deprived animals. The results show that in Vitamin D-replete TPTX rats fed a low (0.2 g/100 g) phosphorus diet, 1,25(OH)(2)D(3) (2 x 13 pmol/d i.p. for 7 d) markedly enhanced the acute tubular phosphaturic response to PTH (2.5 IU/h i.v.) without affecting the action of the peptide hormone on Ca reabsorption and cyclic-3',5'-AMP excretion. The influence of 1,25(OH)(2)D(3) on the phosphaturic response to PTH could not be ascribed to an increased plasma concentration and(or) filtered load of Pi during the administration of the peptide hormone. However, it could be, at least in part, related to the elevation in the basal level of plasma Pi which was observed in the 1,25(OH)(2)D(3)-treated animals. The results also indicate that 1,25(OH)(2)D(3) significantly enhanced the calcemic response to PTH, which was blunted in these conditions of phosphate deprivation. Unlike 1,25-(OH)(2)D(3), 25-hydroxyvitamin D(3) did not unmask the phosphaturic effect of PTH in phosphate-depleted animals, even when given in doses 100 times larger. Thus, 1,25(OH)(2)D(3) displays a selective and powerful activity in preventing the occurrence of tubular resistance to the phosphaturic action of PTH during Pi depletion. This finding suggests the existence of an important interaction between dietary Pi, 1,25(OH)(2)D(3), and PTH in the homeostasis of phosphate.

Resistance to parathyroid hormone in renal failure: role of vitamin D metabolites

Resistance to the calcemic action of parathyroid extract (PTE) was shown in thyroparathyroidectomized rats after 5 hr of renal failure that was induced by either bilateral nephrectomy (NPX) or ureter ligation (UL). Studies were carried out to investigate the relationship of parathyroid resistance to the vitamin D status of the animal. Concentrations of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) and 24,25-dihydroxycholecalciferol (24,25(OH)2D3) were similar in pooled sera samples from rats either UL or sham-operated and treated with PTE. Pretreatment with oral 25-hydroxycholecalciferol or with a combination of i.v. 24,25(OH)2D3 and 1,25(OH)2D3 prior to UL failed to alter the resistance. Resistance was also present in a group in vitamin-D-deficient rats. A similar group given 1 microgram of vitamin D2 showed more parathyroid resistance than did the group not given vitamin D2. In chronic renal failure of 28 day's duration, parathyroid resistance was again demonstrated, but, in contrast to the acute renal failure models, this was partly corrected by prior 1,25(OH)2D3 administration. These studies show that parathyroid resistance is not caused by an abnormality of vitamin D metabolism in the acute renal failure model, and we suggest that the phenomenon is due to the accumulation of one or more uremic factors.

Ultrastructural evaluation of the effects of 1,25-dihydroxyvitamin D3 on bone of thyroparathyroidectomized rats fed a low-calcium diet

Thyroparathyroidectomized rats fed a low-calcium-normal-phosphorus diet were administered 1 or 5 units of 1,25-dihydroxyvitamin D3-(1,25-[OH]2D3) or placebo daily for 7 days. 1,25-(OH)2D3 elevated serum and urine calcium and decreased serum phosphorus. Rats given 1 unit of 1,25-(OH)2D3 had increased numbers of osteoclasts in metaphyseal trabeculae, Ultrastructurally, osteoclasts, osteoblasts, and osteocytes in rats given 1 unit of 1,25-(OH)2D3, were similar to those in rats given placebo. In rats given 5 units of 1,25-(OH)2D3, osteoclasis was markedly increased. Osteoblasts were more numerous and interpreted to be active in matrix production and mineralization. Lamellated electron-dense bodies were observed adjacent to the plasma membranes of less active osteoblasts and were interpreted to be modified matrix. Most osteocytes in rats given 5 units of 1,25-(OH)2D3 were indistinguishable from osteocytes in rats given placebo. However,the pericellular space of some osteocytes in rats given 5 units of 1,25-(OH)2D3 contained electron-dense granular deposits that were interpreted to be calcium phosphate. It is concluded that 1,25-(OH)2D3 is able to significantly elevate serum calcium independent of dietary calcium, parathyroid hormone, and calcitonin primarily by increasing ostoeclasis with minimal dependence on osteocytic osteolysis.

Regulation of the receptor-mediated cyclic AMP response of kidney to parathyroid hormone in the vitamin D-deficient rat

Rats fed a diet deficient in vitamin D were found to exhibit a refractory cyclic AMP response of kidney slices to parathyroid hormone and a marked decrease in membrane parathyroid hormone-dependent adenylate cyclase activity. Both the characteristic calcium deficiency (hypocalcemia) and secondary elevation of circulating parathyroid hormone appeared before the first noticeable decrease in hormone-dependent enzyme activity. After repletion of D-deficient rats with vitamin D2, we found that serum calcium and parathyroid hormone were both restored to normal levels before the depressed enzyme response to the hormone was reversed. Moreover, infusion of parthyroid hormone into vitamin D-replete rats led to a marked reduction in parathyroid hormone-dependent adenylate cyclase activity, which was partly restored to control level 3 hours after discontinuing the hormone infusion. Taken as a whole, this study suggests that the elevated endogenous parathyroid hormone in the vitamin D-deficient rat is involved in the "down-regulation" of renal cyclic AMP responsiveness to the hormone. However, these experiments do not rule out the possibility that calcium deficiency and/or vitamin D per se participate in the regulation of the renal cyclic AMP response to parathyroid hormone.

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.

Selective deficiency of 1,25-dihydroxycholecalciferol. A cause of isolated skeletal resistance to parathyroid hormone

To investigate the role of vitamin D metabolites in the pathogenesis of pseudohypoparathyroidism, we studied an elderly man with a unique variant of the disease, which was characterized by hypocalcemia, elevated serum parathyroid hormone (513 +/- 13 pg per milliliter, mean +/- S.E.M., normal, less than 450) but normal renal responses (phosphate and cyclic AMP) to exogenous parathyroid extract. Treatment with parathyroid extract did not produce a calcemic effect, suggesting an isolated skeletal hyporesponsiveness to parathyroid hormone. Although 25-hydroxyvitamin D levels were not reduced, levels of 1,25-dihydroxycholecalciferol were extremely low (0.52 ng per deciliter; normal 3.3 +/- 0.06, S.D.). Treatment with 1,25-dihydroxycholecalciferol (1 microgram by mouth per day for four days) increased circulating levels to normal (4.60 ng per deciliter) and restored to normal the calcemic response to parathyroid (change in calcium 3.0 mg per deciliter). These data suggest that 1,25-dihydroxycholecalciferol deficiency may explain the skeletal resistance, but not the renal resistance, often present in classic pseudohypoparathyroidism.

Maintenance of a calcemic response to parathyroid hormone in D-deficient rats by the prevention of severe hyperparathyroidism

Parathyroid gland transplanted rats and hypophysectomized rats were raised from weaning on a diet without vitamin D and low in calcium (0.02%) for 4 weeks. At the enn of this period the animals of both experimental groups, when compared to their respective controls (i.e., sham-operated animals for parathyroid-transplanted ones, and hypophysectomized plus bovine growth hormone-supplemented ones for hypophysectomized rats) were characterized by (a) moderate or absent secondary hyperparathyroidism; (b) near normal bone calcium content; and (c) a maintained responsiveness to the calcemic effect of parathyroid extract (PTE). The PTE action is a bone effect that does not require the presence of the kidneys and is not related to changes in serum calcium and/or phosphorus concentrations. These results indicate that when severe hyperparathyroidism is prevented, the sensitivity of bone to the calcemic action of PTE can be maintained in D-deficient calcium-deprived rats. They also suggest that in these animals the main factor leading to resistance to PTH is the state of severe chronic hyperparathyroidism.