Control of 25-hydroxycholecalciferol metabolism by parathyroid glands

Measurement of plasma 1,25 dihydroxyvitamin D using a novel immunoextraction technique and immunoassay with iodine labelled vitamin D tracer

We evaluated a novel assay for the measurement of 1,25 dihydroxyvitamin D (1,25 (OH)2D). Immunoextraction of 1,25(OH)2D is performed using a mini column containing a solid-phase monoclonal antibody followed by radioimmunoassay (RIA) using an 125I-labelled 1,25(OH)2D derivative tracer and Sac-cell separation. The mean recovery of 1,25(OH)2D3 was 101%, linearity was excellent, inter- and intra-assay coefficients of variation were 9, 8 and 13% and 11, 10 and 14% at low, medium and high concentrations of 1,25 (OH)2D3, respectively. The cross-reactivity of vitamin D metabolites was < 0.0015% for 25-hydroxyvitamin D3, 24, 25 dihydroxyvitamin D3 and dihydrotachysterol and 0.54% for 1 alpha calcidol. 1,25 dihydroxyvitamin D2 cross-reactivity was 79%. The detection limit of the assay was 5 pmol/L. Comparison with a commercial radio receptor assay (RRA) and an in-house RIA gave regression equations of y = 0.94x + 11.8 (r = 0.98) and y = 0.91x-1.7 (r = 0.95), respectively, with no major discrepancies between the methods in all patient groups studied. Plasma concentrations of 1,25(OH)2D obtained with the assay were as follows: normal, unsupplemented subjects: mean 88, range 48-155 pmol/L, n = 68, patients with chronic renal failure: mean 11, range 3-36 pmol/L, n = 27, primary hyperparathyroidism: mean 198, range 130-299 pmol/L, n = 23, Paget's disease: mean 92, range 42-149 pmol/L, n = 24, osteomalacia: mean 43, range 27-61 pmol/L, n = 9. A minimum sample volume of 300 microL is required, the hands-on time is significantly less than other commercial assays and the measuring procedure is gamma counting rather than scintillation counting. The assay offers several advantages over previous methods and should allow more laboratories to offer measurement of 1,25(OH)2D as part of their repertoire.

Regulation of serum calcitriol by serum ionized calcium in rats during pregnancy and lactation

Serum calcitriol concentrations in rats follow a biphasic pattern during reproduction, with elevated levels during late pregnancy, a decline after parturition, and a rise to even higher levels during peak lactation. We have previously shown that serum calcitriol in rats at peak lactation correlates significantly with, and appears to be regulated by, serum ionized Ca (Ca2+), with parathyroid hormone (PTH) serving a permissive role. We have extended this study by determining if serum calcitriol also correlates with serum Ca2+ during late pregnancy, when calcitriol levels are clearly elevated, and during early lactation, when only modest increases in serum calcitriol are observed. Analyses of data combined from nonmated, 21-day pregnant (P), and 1-day lactating rats (L) revealed a significant regression (p < 0.001) of calcitriol on Ca2+, but a nonsignificant regression (p = 0.34) of calcitriol on serum PTH. An even stronger correlation (p < 0.001) between calcitriol and Ca2+ was found for the combined data for 5-, 8-, and 14-day L rats. The partial correlation coefficient for calcitriol versus Ca2+, with PTH as the independent variable, was highly significant (p < 0.01) for the data from both combined groups. However, the coefficient for calcitriol versus PTH, with Ca2+ as the independent variable, was not significant (p > 0.05). Fetal weights (uterus and contents) correlated significantly with both maternal calcitriol and Ca2+ concentrations (p < 0.01), but not with maternal PTH levels. Litter weights for 14-day-old pups likewise correlated significantly with maternal calcitriol and Ca2+ (p < 0.001). We conclude that hypocalcemia, induced by the demands for Ca for fetal calcification and milk production, appears to be a controlling factor in serum calcitriol elevation in late pregnancy and throughout lactation, whereas PTH may be important for calcitriol synthesis without playing a direct regulatory role.

Altered regulation of parathyroid hormone secretion by calcium in pregnant and lactating rats

We have previously shown that the serum parathyroid hormone (PTH) concentration in lactating (L) rats is not suppressed by high serum Ca2+ to the same extent as in nonmated (NM) rats. To investigate further Ca2+ regulation of PTH secretion, parathyroid cells from NM rats and rats in late pregnancy and at peak lactation were dispersed and incubated for 2 h in medium containing 0.52-2.05 mM Ca2+. Medium PTH was assayed with a homologous immunoradiometric assay (IRMA). At the two highest Ca2+ levels (1.81 and 2.05 mM), medium PTH was significantly higher (p = 0.031) for cells from L rats than for cells from NM rats. In contrast, significantly less (p < 0.001) PTH was secreted for the L group versus the NM group at medium Ca2+ values of 1.27 and 1.46 mM. Estimated set points for L and NM groups were 1.17 mM and 1.35 mM, respectively, corresponding closely to the prevailing serum Ca2+ for these two groups. Consistent with the present in vitro data, high serum PTH (> 40 pg/ml) in L rats occurred only at serum Ca2+ values below 1.27 mM. Elevated serum PTH at lower serum Ca2+ levels was also seen in pregnant rats. Dispersed parathyroid cells from 20- to 21-day pregnant rats secreted significantly more PTH (p = 0.028) than cells from NM rats at all Ca2+ levels tested (1.1-1.6 mM). In conclusion, the relationship between extracellular Ca2+ and PTH secretion is altered in rats during late pregnancy and at peak lactation, perhaps as part of the adaptation to the demands for calcium for pre- and postnatal growth.

Insulin markedly potentiates the capacity of parathyroid hormone to increase expression of 25-hydroxyvitamin D3-24-hydroxylase in rat osteoblastic cells in the presence of 1,25-dihydroxyvitamin D3

We have previously shown that insulin alters the renal metabolism of 25-hydroxyvitamin D. To examine the effect of insulin on vitamin D metabolism in bone, we have used UMR-106 osteoblast-like cells to study the regulation of 25(OH)D3-24-hydroxylase (24-hydroxylase) expression by insulin. The 24-hydroxylase is an important enzyme in degrading 1,25-dihydroxyvitamin D3 (1,25(OH)2D) in target tissues. Insulin alone had no effect on mRNA levels of the cytochrome P450 component (CYP24) of the 24-hydroxylase or on 24-hydroxylase activity itself in UMR cells. However, insulin increased the capacity of parathyroid hormone (PTH) to elevate CYP24 mRNA levels by 3-4-fold and to increase 24-hydroxylase activity by 2-fold in the presence of 1,25(OH)2D. Insulin increased the maximal responsiveness of UMR cells to PTH without altering their sensitivity. The action of insulin required the presence of 1,25(OH)2D and was partly dependent on new protein synthesis. Insulin-like growth factor 1 also potentiated the effects of PTH. This marked stimulation of the 24-hydroxylase by PTH and insulin may serve to regulate 1,25(OH)2D action and/or to produce 24,25-dihydroxyvitamin D in bone cells.

Tissue specificity and mechanism of vitamin D receptor up-regulation during dietary phosphorus restriction in the rat

Dietary phosphorus restriction up-regulates intestinal vitamin D receptor (VDR), but the tissue specificity of the up-regulation and the mechanism of receptor accumulation remain unknown. Therefore, the effects of low phosphorus diet (LPD) on VDR content in intestine, kidney, and splenic monocytes/macrophages were examined. Male Sprague-Dawley rats weighing 50-100 g were fed a normal diet (NPD; 0.6% Ca, 0.65% P) as controls followed by an LPD (0.6% Ca, 0.1% P) for 1-10 days (D1-D10). LPD rapidly decreased serum P levels by D1 from 11.11 +/- 0.19 mg/dl (mean +/- SE) to 4.98 +/- 0.37 mg/dl (n = 9). LPD increased total serum Ca from 10.54 +/- 0.09 mg/dl to 11.63 +/- 0.15, 12.17 +/- 0.15, and 12.39 +/- 0.18 mg/dl by D1, D2, and D3, respectively, and then remained stable. Serum 1,25-(OH)2D3 rapidly increased from 123 +/- 5.4 pg/ml to 304 +/- 35 pg/ml by D1, reached a plateau through D5, and then gradually increased to 464.9 +/- 27.7 pg/ml by D10. Intestinal VDR quantitated by ligand binding assay increased 3.5-fold from 169.6 +/- 13.7 fmol/mg of cytosol protein in rats fed NPD (n = 12) to a peak of 588.3 +/- 141.88 fmol/mg of protein by D3 (n = 6; p < 0.001) and then decreased to a plateau level of 2.5-fold greater than NPD (p < 0.05) during D5 to D10. In contrast, LPD did not up-regulate kidney or splenic monocyte/macrophage VDR. Northern blot analysis showed that intestinal VDR mRNA increased 2-fold by D2 (n = 3) of LPD and then gradually decreased to control levels after D5. In contrast, kidney VDR mRNA levels did not change during the first 5 days of P restriction and then subsequently decreased to 50% of NPD controls. The results of these studies indicate that VDR up-regulation during dietary phosphorus restriction is tissue-specific and that the mechanism of the up-regulation is time-dependent. Acutely (D1-D5), phosphorus restriction up-regulates intestinal VDR through increased VDR gene expression, whereas chronic (D5-D10) phosphorus restriction appears to alter VDR metabolism through nongenomic mechanisms that are consistent with prolongation of the half-life of the receptor. The nature of the tissue-specific regulation of VDR during phosphorus restriction remains to be determined.

Role of Vitamin D in the Pathogenesis and Treatment of Osteoporosis

Alterations in vitamin D metabolism normally occur after the menopause and with aging. Increases in skeletal remodeling as a consequence of estrogen deficiency lead to increased bone resorption, suppression of serum 1,25-dihydroxy-vitamin D and intestinal absorption of calcium and increases in urinary calcium. Age-related changes in vitamin D metabolism include diminished dermal production of 7-dehydrocholesterol, the precursor of previtamin D(3), vitamin D deficiency as a consequence of inadequate intake or exposure to sunshine (individuals may be institutionalized or housebound), a decline in intestinal vitamin D receptors and diminished intestinal absorption of calcium and secondary hyperparathyroidism which leads to further bone loss. These changes also have been implicated in the pathogenesis of senile osteoporosis. Treatment with vitamin D and its analogues such as 1,25-dihydrox-yvitamin D(3 )is sometimes of value in preventing bone loss and fractures in patients with postmenopausal osteoporosis and senile osteoporosis. However, the drugs have not been approved for treatment and prevention of osteoporosis.

Local action of phosphate depletion and insulin-like growth factor 1 on in vitro production of 1,25-dihydroxyvitamin D by cultured mammalian kidney cells

The hormonal form of vitamin D, 1,25(OH)2D, is synthesized mostly in proximal renal tubular cells. Experimental and clinical studies suggest that the growth hormone may be involved in growth-related fluctuations of plasma 1,25(OH)2D and in the increase of 1,25(OH)2D induced by in vivo phosphate deprivation, an action possibly mediated by insulin-like growth factor 1 (IGF 1). We tested the effects of phosphate depletion and IGF 1 addition on 1,25(OH)2D3 production in cultured kidney cells: opossum kidney (OK) cells, LLC-PK 1, and rabbit's proximal tubular cells. Confluent cell monolayers were preincubated in various phosphate concentrations, in the presence and absence of IGF 1. Then, 5 nM of [3H]25 (OH)D3 or 2 microM of 25 (OH)D3 were added to the medium and the cells were incubated for a further 120 min. The amount of biosynthesized 1,25(OH)2D3 in lipid extracts was determined after two different straight phase high performance liquid chromatographies. The experiment showed the following: (a) LLC-PK 1 and rabbit's cells expressed a detectable ability to synthesize 1,25(OH)2D3, while OK cells did not. (b) Partial or total phosphate deprivation increased the amount of 1,25(OH)2D3 produced, respectively in LLC-PK 1 and in rabbit's cells. (c) IGF 1 (25 ng/ml) increased 1,25(OH)2D3 production in rabbit's cells, particularly in phosphate-free medium (1.6-fold), and in LLC-PK 1 cells, in partial phosphate depletion (2.75-fold in 1 mM phosphate, P = 0.015, n = 5, and 3.2-fold in 0.5 mM phosphate, P = 0.043, n = 4). Our findings demonstrate a local action of phosphate depletion and of IGF 1 on 1,25-dihydroxyvitamin D3 production.

Sequential changes in vitamin D and calcium metabolism after successful renal transplantation

A prospective study was made of sequential changes in the metabolism of vitamin D and calcium in 19 allograft recipient during the first year after successful renal transplantation. All but one of the patients received cyclosporine A combined with corticosteroids and azathioprine as immunosuppressive therapy. Shortly after transplantation most patients showed transient hypocalcemia and hypophosphatemia. At the time of transplantation 17 of 19 patients had an elevated plasma intact parathyroid hormone (PTH) level, and at the close of follow-up one in four patients. In six other patients intact PTH was within the reference range, but high in relation to simultaneously measured serum ionized calcium. According, one year after transplantation less than half of the patients showed complete resolution of hyperparathyroidism. The change towards normal in the metabolism of vitamin D began within the first post-transplantation week irrespective of the onset of diuresis. One to two weeks after transplantation 1,25(OH)2D3 and 24,25(OH)2D3 reached the lower limit of normal range. In these renal allograft recipients who received cyclosporine A the long-term values of serum 1,25(OH)2D3 did not differ from those of normal subjects.

Hypoparathyroidism--a long-term follow-up experience with 1 alpha-vitamin D3 therapy

OBJECTIVE

Previous studies have suggested that alpha-D3 therapy can cause deterioration in renal function. We have, therefore, examined the long-term effect of 1 alpha-hydroxyvitamin D3 (alpha-D3) administration upon renal function in hypoparathyroid patients.

DESIGN

This is a prospective long-term follow-up study of alpha-D3 administration on hypoparathyroid patients at a mean daily dose of 1 microgram (range 0.5-2.5 micrograms) during a total of 2040 patient-months.

PATIENTS

Seventeen unselected patients (14 females and 3 males), two with primary and 15 with post-surgical hypoparathyroidism.

RESULTS

The significant effect of alpha-D3 on serum and urinary calcium was achieved during the first week of treatment and remained stable at the same range during the close follow-up of 2040 patient-months. No significant change was observed in the serum creatinine during the whole follow-up period. During follow-up, five women developed hypercalciuria and one patient developed hypercalcaemia that disappeared when the dose of the drug was reduced or discontinued.

CONCLUSIONS

From our study we concluded that alpha-D3 is a safe and effective drug in the long-term therapy of hypoparathyroid patients.

Loss of parathyroid hormone-stimulated 1,25-dihydroxyvitamin D3 production in aging does not involve protein kinase A or C pathways

Intestinal calcium absorption declines with aging as a result of decreased renal 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] biosynthesis. At least part of the decline in 1,25-(OH)2D3 may be due to acquired resistance to parathyroid hormone (PTH) stimulation of renal 25-hydroxyvitamin D1-hydroxylase (1-OHase) activity. To test whether aging rats can increase 1,25-(OH)2D3 production in response to PTH, male rats of the same litter were fed a normal Ca diet and were sacrificed at 175-225 g (young rats) or 3 months later at 350-425 g (aging rats). At sacrifice, basal serum 1,25-(OH)2D3 levels (88 +/- 16 versus 49 +/- 8 pg/ml, P < 0.05) and in vitro renal proximal tubule 1-OHase activity (178 +/- 15 versus 77 +/- 5 pmol/mg protein/5 minutes, n = 6, P < 0.001) were lower in aging animals. rPTH-(1-34) (10(-11) or 10(-7) M) increased in vitro 1,25-(OH)2D3 secretion by perifused renal proximal tubules from young but not aging rats. For young and aging rats, rPTH-(1-34) (10(-7) M) increased proximal tubule cAMP-dependent protein kinase (PKA) activity, and lower concentrations (10(-11) M) stimulated translocation of protein kinase C (PKC) activity from cytosolic to soluble membrane proximal tubule cell fractions. The results of this study show that PTH activation of 1,25-(OH)2D3 production may involve both signaling pathways, with the PKC pathway responsive to lower concentrations of the hormone. The acquired resistance to PTH stimulation of 1,25-(OH)2D3 production in aging appears not to involve the hormonal activation of PKA or PKC.

Long-term elevation of 1,25-dihydroxyvitamin D after short-term intravenous administration of pamidronate (aminohydroxypropylidene bisphosphonate, APD) in Paget's disease of bone

We report the prolonged biochemical changes that occurred in patients with Paget's disease when treated for 2-10 days with pamidronate disodium (3-amino-1-hydroxypropylidine-1,1-bisphosphonate, APD), by i.v. administration and observed for 6 months following therapy. In all 24 patients studied, bone resorption (measured by urinary hydroxyproline/creatinine ratio, OHP/Cr) fell sharply on treatment, from 0.12 +/- 0.02 (mean +/- SEM; above reference limits) to 0.04 +/- 0.008 (reference range 0.006-0.027 for females, 0.005-0.020 for males), remaining at this level for 6 months after therapy. A fall in serum ionized calcium (Ca2+) to just below the reference limits with treatment (1.11 +/- 0.02 mM; reference range 1.14-1.18 mM), followed by a rapid return to normal levels (1.14 +/- 0.02 mM, mean +/- SEM) within 8 days of treatment, was presumably due to the cessation of release of calcium from bone. This was followed by secondary hyperparathyroidism and a rise in serum 1,25-dihydroxyvitamin D [1,25(OH)2D]. The hormonal responses, however, were profound. Serum immunoreactive PTH (iPTH) rose to twice pretreatment values (86 +/- 11 pM, mean +/- SEM; reference range for iPTH, > 50 years, < 50 pM; < 50 years, < 40 pM), returning to normal 4-8 weeks after therapy. Serum 1,25-(OH)2D levels rose to three times pretreatment values (300 +/- 20 pM, mean +/- SEM; reference range 50-150 pM), remaining above reference limits 4-8 weeks after therapy (188 +/- 15 pM, mean +/- SEM) and returning to normal values only after 12 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased calcium intake does not suppress circulating 1,25-dihydroxyvitamin D in normocalcemic patients with sarcoidosis

Ca absorption is regulated by 1,25(OH)2D, and serum values vary inversely with Ca intake. In sarcoidosis, 1,25(OH)2D is produced by alveolar macrophages in response to gamma-interferon, and patients may develop hypercalcemia after prolonged exposure to sunlight and increased dermal production of vitamin D3. To determine if increased Ca intake suppresses serum 1,25(OH)2D in normocalcemic patients and to identify those at risk, 17 normal subjects and 11 patients were studied on a metabolic ward for two and one-half days while receiving first 400 and then 1,000 mg/d of Ca. On the low Ca intake, serum angiotensin-converting enzyme (ACE), an index of disease activity, was higher in only three of the patients than in the controls, mean serum 1,25(OH)2D was higher in the patients, and mean serum total Ca, serum Ca++, and urinary Ca were not different in the two groups. On the higher Ca intake, mean urinary Ca increased in both groups, but mean serum 1,25(OH)2D was suppressed only in the normal subjects. Thus, 1,25(OH)2D production is abnormally regulated, indicating that (a) normocalcemic patients with sarcoidosis are at risk for developing abnormal Ca metabolism, and (b) a better index of disease activity is provided by the oral Ca suppression test than by serum ACE.

Role of protein kinase C in parathyroid hormone stimulation of renal 1,25-dihydroxyvitamin D3 secretion

PTH is a major regulator of renal proximal tubule 1,25(OH)2D3 biosynthesis. However, the intracellular pathways involved in PTH activation of the mitochondrial 25-hydroxyvitamin D3-1 alpha-hydroxylase (1-OHase) remain unknown. PTH can activate both the adenylate cyclase/protein kinase A (PKA) and the plasma membrane phospholipase C/protein kinase C (PKC) pathways. The present study was undertaken to determine whether PKC may mediate PTH activation of renal 25-hydroxyvitamin D3-1 alpha-hydroxylase activity. Rat PTH 1-34 fragment in vitro translocated PKC activity from cytosolic to soluble membrane fraction from freshly prepared rat proximal tubules. Physiologic concentrations (10(-11)-10(-10) M) of rat PTH 1-34 fragment increased PKC translocation three- to fourfold while PKA activity ratio increased at PTH 10(-7) M. PTH stimulation of PKC and PKA was reduced in the presence of staurosporine (10 nM) by 41 and 29%, respectively. Sangivamycin (10 and 50 microM) also reduced PTH-stimulated PKC translocation, but did not alter PKA activity ratio. In vitro perifusion of renal proximal tubules with PTH (10(-11) M) increased 1,25(OH)2D3 steady-state secretion two- to fourfold. Sangivamycin at the same concentration that inhibited PKC translocation by 52% completely inhibited PTH-stimulated 1,25(OH)2D3 secretion. The present studies indicate that the phospholipase C/PKC pathway may mediate PTH stimulation of mammalian renal proximal tubule 1,25(OH)2D3 secretion.

Kinetics of 1,25-dihydroxyvitamin D metabolism in the aging rat

To determine whether postmaturational aging influences the kinetics of 1,25-dihydroxyvitamin D [1,25(OH)2D] metabolism in the rat, we measured the metabolic clearance (MCR) and production (PR) rates of 1,25(OH)2D in 6-, 12-, 18-, and 24-mo-old Fischer 344 rats using the constant infusion method. Plasma calcium, phosphorus, and parathyroid hormone (PTH), urinary calcium and phosphorus, and glomerular filtration rate (GFR) were also measured. MCR and PR increased 57 and 91%, respectively (when expressed per rat), and 32 and 39%, respectively (when expressed per kg body wt), between 6 and 24 mo of age, with the greatest increase occurring between 18 and 24 mo. The plasma concentration of 1,25(OH)2D remained unchanged. Plasma PTH, when compared with 6-mo-old animals, was significantly elevated at 18 mo (147%) and even higher at 24 mo (240%). GFR (51Cr-labeled EDTA clearance) remained unchanged through 18 mo but tended to be reduced at 24 mo. Plasma phosphorus tended to decrease with age, whereas plasma calcium, urinary calcium, and urinary phosphorus did not differ among the age groups. These data indicate that both clearance and production of 1,25(OH)2D increase with postmaturational aging in the rat. They also suggest that the early age-related rise in plasma PTH is neither a consequence of low plasma calcium nor of renal insufficiency.

Parathyroid hormone sensitizes long bones to the stimulation of bone resorption by 1,25-dihydroxyvitamin D3

In response to hypocalcemia the serum PTH level increases rapidly followed by a PTH-induced rise in 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] production. Therefore, bone is first exposed to increased PTH levels before increased 1,25-(OH)2D3 levels. In the present study the effect of pretreatment with PTH on 1,25-(OH)2D3-induced bone resorption was examined. Bone resorption was measured as release of prelabeled 45Ca during culture from 17-day-old fetal mice radii/ulnae and metatarsals. Radii/ulnae and metatarsals are characterized by differences in development. In radii/ulnae mature osteoclasts are present, whereas in metatarsals only different stages of preosteoclasts can be found. Preincubation for 24 h but not 4 h with PTH increases the stimulation of bone resorption by 1,25-(OH)2D3 in fetal radii/ulnae but not in metatarsals. Coincubation of PTH and 1,25-(OH)2D3 did not result in a significant change in bone resorption compared to 1,25-(OH)2D3 alone. The observed difference in the effect of pretreatment with PTH between radii/ulnae and metatarsals indicates that PTH does not stimulate the development of early osteoclast precursors but that a certain level of differentiation of the osteoclast precursor is required. Pretreatment with prostaglandin E2 resulted in an effect similar to that of PTH. Inhibition of prostaglandin synthesis by indomethacin prevented the potentiation of 1,25-(OH)2D3-induced bone resorption by pretreatment with PTH. Thus, the present study demonstrates that PTH sensitizes responses to 1,25-(OH)2D3. PTH must be present before 1,25-(OH)2D3 to observe a potentiation of 1,25-(OH)2D3-induced bone resorption.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypocalcemia, but not PTH or hypophosphatemia, induces a rapid increase in 1,25(OH)2D3 levels in rats

This study determined whether acute decreases in plasma ionized calcium (Ca2+) levels regulate plasma 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] levels independent of changes in parathyroid hormone (PTH) secretion and plasma phosphate levels. Chronically catheterized rats were subjected to a hypocalcemic clamp (mean decrement of Ca2+ levels 0.38 +/- 0.04 mM), a rat PTH-(1-34) infusion, and a PTH vehicle infusion for 2 h. Plasma NH2-terminal immunoreactive PTH levels were elevated 3.2- and 8.7-fold during hypocalcemia and PTH infusion, respectively. Plasma phosphate decreased by 23 +/- 4 and 42 +/- 3% during hypocalcemia and PTH infusion, respectively. In response to hypocalcemia, plasma 1,25(OH)2D3 levels increased promptly, were significantly elevated by 15 min (56 +/- 23% increase), and continued to increase until the end of the experiment at 5 h (350 +/- 30% increase). In contrast, no changes in plasma 1,25(OH)2D3 levels occurred during the PTH infusion, but levels were elevated by 5 h, i.e., 3 h after the end of the infusion (360 +/- 20% increase). No significant changes in 25(OH)D3 or 24,25(OH)2D3 levels occurred in any protocol. Thus hypocalcemia rapidly elevates 1,25(OH)2D3 levels in rats, but the increase is not caused by elevated PTH secretion, hypophosphatemia, or elevated 25(OH)D3 levels. Furthermore, the increase in 1,25(OH)2D3 levels by hypophosphatemia does not occur rapidly. These studies show that there is a calcium-dependent mechanism that is independent of changes in PTH secretion and that results in the rapid elevation of plasma 1,25(OH)2D3 levels to counteract hypocalcemia.

Calcemic response to parathyroid hormone in renal failure: role of phosphorus and its effect on calcitriol

The calcemic response to parathyroid hormone (PTH) is decreased in renal failure. The reduction of hyperphosphatemia improves the calcemic response to PTH in animals with advanced renal failure. However, since low calcitriol levels in renal failure may also contribute to the decreased calcemic response to PTH, the improved calcemic response observed during the reduction of serum phosphorus may be partially mediated by an increase in serum calcitriol levels. The present study evaluated the calcemic response to PTH in rats with moderate and advanced renal failure and how this response was modified by a high and a low phosphorus diet. In addition, the effect of a change in dietary phosphorus on calcitriol levels was also evaluated. A 48-hour continuous infusion of 1-34 rat PTH increased the serum calcium level to 18.2 +/- 0.4 mg/dl in normal rats, versus 13.7 +/- 0.9 and 12.1 +/- 0.2 mg/dl in rats with moderate and advanced renal failure, respectively. During the PTH infusion, a high phosphorus diet increased the serum phosphorus and resulted in a reduced calcemic response to PTH at each level of renal function; respective serum calcium levels were 13.8 +/- 0.6 mg/dl in normals, 11.2 +/- 0.2 mg/dl in moderate renal failure and 9.6 +/- 0.5 mg/dl in advanced renal failure. In normal rats and in rats with moderate renal failure, dietary phosphorus restriction during the PTH infusion increased serum calcitriol levels. In rats with advanced renal failure, serum calcitriol levels were lower than in the other two groups and were not affected by changes in dietary phosphorus.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid cell number and size in hypocalcemic young rats

Weanling rats were fed diets with normal (1%) or low (0.08% or 0.02%, respectively) Ca content for 28 days prior to sacrifice. The total volume of the parathyroids was estimated from serial sections. Volume density of secretory cells was calculated according to conventional stereological techniques, whereas cell number and cell size were estimated by the dissector method. Compared with controls the animals of the experimental groups developed moderate and severe hypocalcemia and their parathyroids were enlarged with a proportional growth of parenchyma and interstitium. Related to the body weight, secretory cell volume was highest in animals with severe hypocalcemia. In the enlarged glands the size of parathyroid secretory cells was increased by 30-40%, whereas total cell number was unaltered. Thus, the increased parathyroid size was due to cell hypertrophy rather than hyperplasia.

Calcium metabolism in elderly hypertensive patients: possible participation of exaggerated sodium, calcium and phosphate excretion

1. Renal handling of electrolytes, including calcium (Ca), in response to physiological saline infusion (20 mL/kg, i.v., for 2 h) as well as basal circulating levels of Ca-regulating hormones were compared in 27 hypertensive elderly females (mean age 80 +/- 9 years), in 44 normotensive elderly females (79 +/- 9 years) and in 19 young normotensive females (23 +/- 4 years). 2. The hypertensive elderly females showed excessive increase in urine volume and urinary excretions of sodium (Na), Ca and inorganic phosphate (P) in response to saline infusion, associated with slight but significant decrease in circulating levels of Na and ionized Ca compared with those in the other groups. These hypertensive elderly patients also showed characteristic features both in circulating blood pressure and Ca regulating factors; they showed significantly low levels of plasma renin activity and aldosterone concentration, significantly high plasma levels of atrial natriuretic peptide and noradrenalin, compared with those in young controls and normotensive elderly females. 3. Moreover they showed significant increase in basal serum levels of parathyroid hormone and 1,25-dihydroxyvitamin D, and significant decrease in basal serum levels of calcitonin, 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D, compared with those in the other groups. 4. These results suggest that the exaggerated natriuresis associated with excessive loss of Ca and P in urine may participate in the abnormality of Ca metabolism in low-renin hypertensive elderly patients.

Serum ionized calcium, as well as phosphorus and parathyroid hormone, is associated with the plasma 1,25-dihydroxyvitamin D3 concentration in normal postmenopausal women

Serum parathyroid hormone (PTH) and low-normal serum phosphorus (P) concentrations have well-known trophic effects on renal 1-hydroxylase. A role for serum ionized calcium (Ca2+) in the day-to-day regulation of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] has not been identified in normal humans. The associations between serum Ca2+, PTH, P, and plasma 1,25-(OH)2D3 were evaluated in a cross-sectional study of 275 healthy postmenopausal women. Partial correlations of Ca2+, PTH, and P (each controlled for the other two) with 1,25-(OH)2D3 were sought within quintiles of Ca2+. At low-normal concentrations (less than 1.26 mmol/liter, quintile 1) Ca2+ was inversely correlated with 1,25-(OH)2D3 (rp = -0.30, p = 0.028). At mid-normal concentrations Ca2+ was not significantly correlated with 1,25-(OH)2D3. At high-normal concentrations (greater than 1.32 mmol/liter, quintile 5) Ca2+ attenuated the positive associations of both PTH and low-normal P with 1,25-(OH)2D3. In quintile 5 Ca2+, PTH, and P together accounted for none of the variability in 1,25-(OH)2D3 (R2 = 0.03, p = 0.671). Women with Ca2+ below 1.32 mmol/liter were next examined by quintile of P. As expected, at low-normal concentrations (less than 1.03 mmol/liter, quintile 1) P was significantly correlated with 1,25-(OH)2D3 (rp = -0.32, p = 0.047). The association between PTH and 1,25-(OH)2D3 was statistically significant only at mid-normal concentrations of P (rp = 0.52, p = 0.001, quintile 3). We conclude that Ca2+, along with PTH and P, is associated with the plasma concentration of 1,25-(OH)2D3 in normal postmenopausal women.

Regulation of serum 1,25-dihydroxyvitamin D3 in lactating rats

To characterize further the mechanism(s) underlying the increased serum 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] concentration associated with lactation in the rat, we examined hormone biosynthesis [i.e., renal 25-hydroxyvitamin D-1 alpha-hydroxylase (1 alpha-hydroxylase) activity] and hormone disappearance in groups of lactating Holtzman rats and age- and sex-matched nonlactating controls. 1 alpha-Hydroxylase activity was significantly greater in kidneys from lactating rats (4.0 +/- 0.42 fmol.mg-1.min-1) on a basal diet than in those from nonmated females (1.4 +/- 0.08 fmol.mg-1.min-1), an increment sufficient to account for the observed fourfold elevation of 1,25(OH)2D3 in the dams. The increase occurs despite the lower serum 1,25(OH)2D3 levels in lactating than in nonlactating rats at 12 and 24 h after a bolus injection of 1,25(OH)2D3 (2 ng/g body wt). Elevation of serum 1,25(OH)2D3 is not a requisite consequence of lactation, however, because dams receiving supplemental calcium from food (1.6%) and water (0.3%) exhibited no increase of either serum 1,25(OH)2D3 or 1 alpha-hydroxylase activity compared with controls. In contrast, lactating rats that received a diet with only 0.1% calcium had 5-fold higher serum 1,25(OH)2D3 levels and 20-fold higher 1 alpha-hydroxylase activity than nonlactating rats on the same diet. We conclude that other factors in conjunction with lactation, but not the lactating state per se, promote the changes in 1,25(OH)2D3 metabolism observed.

Response of plasma 1,25-dihydroxyvitamin D in the human PTH(1-34) infusion test: an improved index for the diagnosis of idiopathic hypoparathyroidism and pseudohypoparathyroidism

Synthetic human parathyroid hormone (1-34) (hPTH(1-34] infusion test has been utilized in the differential diagnosis of hypoparathyroidism by examining the incremental response of urinary phosphate and cyclic adenosine monophosphate (AMP). The response of plasma levels of 1,25-dihydroxyvitamin D (1,25(OH)2D) in parathyroid hormone (PTH) infusion test was studied as a new criterion for the differential diagnosis of idiopathic hypoparathyroidism (IHP) and pseudohypoparathyroidism (PHP). Fourteen patients with IHP, 4 patients with PHP, and five control subjects were studied. All subjects received an intravenous infusion of 30 micrograms hPTH(1-34) over 5 minutes. The basal levels of plasma 1,25(OH)2D in patients with IHP and PHP were significantly lower than those in control subjects, but there was no significant difference between the levels in patients with IHP and in patients with PHP. The plasma levels of 1,25(OH)2D increased after the infusion of hPTH(1-34) and reached a peak 6 to 24 hours afterward. The 1,25(OH)2D increase at 24 hours afterward the infusion (delta 1,25(OH)2D) in control subjects and in patients with IHP were 18.1 +/- 3.91 (mean +/- SEM) and 24.1 +/- 2.80 pg/ml, respectively. There was no significant increase in patients with PHP (delta 1,25(OH)2D = 4.9 +/- 1.97 pg/ml). From these results, the measurement of delta 1,25(OH)2D in hPTH(1-34) infusion test is useful as a criterion for the differential diagnosis of hypoparathyroidism.

Regulation of 1,25-dihydroxyvitamin D3 by calcium in the parathyroidectomized, parathyroid hormone-replete rat

Parathyroid hormone (PTH) is a major stimulus for the renal production of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Elevated arterial blood ionized calcium ([Ca2+]) depresses serum 1,25-(OH)2D3 in nonparathyroidectomized rats even when serum PTH is maintained at high levels by infusion. However, suppression by [Ca2+] of endogenous PTH, causing the fall in 1,25-(OH)2D, cannot be excluded. To determine whether [Ca2+] regulates 1,25-(OH)2D3 in the absence of a variation in PTH, we parathyroidectomized (PTX) rats (post-PTX calcium levels less than 7.0 mg/dl), inserted arterial and venous catheters, and then replaced PTH using an osmotic pump. We varied [Ca2+] by infusing either 75 mM sodium chloride (control), 0.61 mumol/min of EGTA (EGTA), or calcium chloride at 0.61 mumol/min (low calcium) or 1.22 mumol/min (high calcium) for 24 h 5 days after surgery. Blood was then drawn from the rat through the arterial catheter. Compared with the control, [Ca2+] fell with EGTA, remained constant with the low-calcium infusion, and rose with the high-calcium infusion. 1,25-(OH)2D3 was correlated inversely with [Ca2+] in all four groups together (r = -0.635, n = 34, p less than 0.001), within the control group alone (r = -0.769, n = 11, p less than 0.002), and within the EGTA group alone (r = -0.774, n = 10, p less than 0.003). Serum phosphorus, PTH, and arterial blood pH were not different in any group, and none correlated with serum 1,25-(OH)2D3. We conclude that 1,25-(OH)2D3 levels are regulated by [Ca2+] independently of serum PTH, phosphorus, and acid-base status, all of which support the hypothesis that [Ca2+] is a principal regulator of serum 1,25-(OH)2D3 in the rat.

Serum vitamin D metabolite concentrations in primary hyperparathyroidism

Forty-nine patients with primary hyperparathyroidism were examined preoperatively and three months after parathyroid operation for their serum vitamin D metabolites and routine laboratory samples related to calcium metabolism. The preoperative serum 24,25-dihydroxyvitamin D level, mean (SE) was 1.86 (0.22) nmol/l and the postoperative level 5.35 (0.63) nmol/l, the difference being highly significant (P less than 0.001). Serum 1,25-dihydroxyvitamin D levels fell significantly (P less than 0.001) from a preoperative level of 175.5 (17.9) pmol/l to 102.8 (10.1) pmol/l postoperatively. The preoperative 25-hydroxyvitamin D level did not change significantly after surgery. The preoperative serum 24,25-dihydroxyvitamin D level was very low, especially in patients with bone disease while serum parathormone was significantly higher than in patients without bone disease.

Urinary excretion of cyclic AMP in cadmium-intoxicated rats

To obtain further information on the negative calcium balance caused by Cd, the factors associated with serum calcium and phosphorus homeostasis other than inhibition of intestinal calcium absorption were studied by using urinary cyclic 3',4'-adenosine monophosphate (cAMP). In rats exposed to Cd for 30 d, the levels of urinary excretion of cAMP after treatment with parathyroid hormone (PTH), parathyroidectomy (PTX), or 1 alpha-hydroxycholecalciferol (1 alpha-OH-D3) showed almost the same patterns as those of control rats: the response of urinary cAMP to treatment with PTH was not influenced by continuous oral administration of Cd. On the other hand, in rats exposed to Cd for 90 d without the other three treatments, the amount of urinary excretion of cAMP was markedly higher than in control rats. In PTX rats exposed to 90 d of Cd, urinary cAMP was unchanged, but it was markedly increased when the parathyroid was intact, with or without treatment with PTX. This phenomenon indicated hyperparathyroidemia in response to continuous oral administration of Cd for 90 d. The negative calcium balance with hyperparathyroidemia occurred after continuous oral administration of Cd and developed via increased urinary excretion of calcium. Urinary excretion of cAMP in Cd-exposed rats was unaffected by the administration of 1 alpha-OH-D3.

Control of DNA polymerase-alpha activity in regenerating rat liver by calcium and 1 alpha,25(OH)2D3

The late G1 surge of DNA polymerase-alpha activity and the initiation of DNA replication in the hepatocytes of partial hepatectomy-induced regenerating liver were severely reduced when the mitogenic partial hepatectomy was carried out in the hypocalcemic and 1,25(OH)2D3 (1 alpha,25-dihydroxycholecalciferol)-deficient environment of parathyroidectomized (PTX) or thyroparathyroidectomized (TPTX) rats. These inhibitions were prevented in TPTX rats by a postpartial hepatectomy injection of 1,25(OH)2D3, which also restored blood calcium to normocalcemic levels. Inhibition of active DNA polymerase-alpha accumulation and initiation of DNA synthesis in TPTX rats were also completely prevented by prefeeding the rats a low phosphorus diet, which stopped the lowering of the blood levels of calcium and 1,25(OH)2D3 following parathyroid removal. These studies indicate that the rise of DNA polymerase-alpha activity and the initiation of DNA replication in regenerating liver are controlled by cellular processes that rely on normal blood levels of calcium and 1,25(OH)2D3. Because DNA polymerase-alpha is the third DNA replication enzyme (the others are ribonucleotide reductase and thymidylate synthase) that has been shown to depend on parathyroid hormone and/or the circulating levels of calcium and 1,25(OH)2D3 that it controls, the authors concluded that the processes dependent on calcium and 1,25(OH)2D3 are parts of a mechanism that coordinately activates the DNA-replicating enzymes. The possibility that cyclic adenosine monophosphate (cAMP)-dependent protein kinases are involved in this replication mechanism is considered.

Physiologic regulation of the serum concentration of 1,25-dihydroxyvitamin D by phosphorus in normal men

We asked this question: in normal humans, is either a normal dietary intake or normal serum concentration of phosphorus a determinant of the serum concentration of 1,25(OH)2D? In seven normal men whose dietary phosphorus was decreased from 2,300 to 625 mg/d, each intake for 8-9 d, under strictly controlled, normal metabolic conditions, we measured serum concentrations of 1,25(OH)2D daily, and concentrations of phosphorus hourly throughout a 24-h period, before and after restriction. Decreasing dietary phosphorus induced: (a) a 58% increase in serum levels of 1,25(OH)2D; (b) a 35% decrease in serum levels of phosphorus measured in the afternoon; (c) a 12% decrease in the 24-h mean serum level of phosphorus; but, (d) no decrease in morning fasting levels of phosphorus. Serum concentrations of 1,25(OH)2D varied inversely and significantly with 24-h mean concentrations of phosphorus (r = -0.77, P less than 0.001). When these data are combined with those of our prior study in which dietary phosphorus was varied over an extreme range, the relationship between serum levels of 1,25(OH)2D and 24-h mean serum levels of phosphorus is even stronger (r = -0.90, P less than 0.001). In the aggregate, the results demonstrate that in normal men, dietary phosphorus throughout a normal range and beyond, can finely regulate the renal production and serum concentration of 1,25(OH)2D, and provide evidence that this regulation is mediated by fine modulation of the serum concentration of phosphorus.

Decreased receptor activation with age. Can it be explained by desensitization?

In the three endocrine/neuroendocrine systems discussed, there are demonstrable declines in post-maturational responsiveness. Parathyroid hormone stimulation of 1,25-dihydroxyvitamin D production declines with age in the kidney as does calcium absorption in the intestine. Chronotropic and inotropic responsiveness to beta-adrenergic agonists decreases with age in the myocardium, and performance on passive avoidance tasks related to memory dysfunction declines with age in rodents. In each case there is a corresponding decrease in receptor activation with age. Parathyroid hormone receptors are less able to activate adenylate cyclase in older rat kidneys; beta-adrenergic receptors have reduced density in some tissues, demonstrate reduced agonist affinity (are uncoupled), and are less able to activate adenylate cyclase in most tissues with age; and muscarinic receptors demonstrate mixed agonist affinity (are uncoupled) with age in rat hippocampal cells. This reduction in receptor activation can be attributed to desensitization to increased agonist concentrations. Parathyroid hormone receptor activation is restored by parathyroidectomy, beta-adrenergic agonists no longer desensitize in older animals, and muscarinic receptors from senescent rats pharmacologically mimic desensitized receptors. However, desensitization of receptor activation cannot fully account for the reduced hormonal responsiveness in any of these systems. Parathyroidectomy does not restore 1,25-dihydroxyvitamin D production or intestinal calcium absorption. There are age-related post receptor deficits in beta-adrenergic pathway that are not mediated by changes in serum catecholamines. In conclusion, there are significant changes in receptor and post-receptor biochemistry with age. The overall decreases in hormonal responsiveness are not due to a single biochemical defect in the system and are probably multiple in nature.

Evidence that calcitonin plays a role in the postnatal increase of serum 1 alpha,25-dihydroxyvitamin D

To investigate the changes in the 1 alpha,25-dihydroxyvitamin D [1,25(OH)2D] level and the role of parathyroid hormone (PTH) and calcitonin (CT) during the early neonatal periods, we measured 1,25(OH)2D, 25-hydroxyvitamin D [25(OH)D], PTH specific for mid-regions (mPTH) and urinary cAMP (UcAMP) to evaluate the renal tubular responsiveness to intrinsic PTH and CT, as well as serum Ca and P in 28 mothers at term delivery and in their babies at birth and 5 days of age. Cord serum 1,25(OH)2D levels were low (28.8 +/- 9.2 pg/ml, mean +/- SD), while maternal serum 1,25(OH)2D levels were high (62.2 +/- 22.6 pg/ml). The low 1,25(OH)2D value increased 2.5 times (62.2 +/- 22.6 pg/ml) in 5-day-old infants, reaching a high normal adult value, concomitant with the increases in mPTH and urinary cAMP/creatinine ratio (UcAMP/Cr). The correlations between 1,25(OH)2D and UcAMP/Cr, and 1,25(OH)2D and mPTH in all paired samples of babies at birth and at 5 days of age were r = 0.456, n = 50, P less than 0.01 and r = 0.341, n = 50, P less than 0.05, respectively. These data suggest that the parathyroid activation after birth is a major factor in the rapid 1,25(OH)2D increment at that time. CT levels were high in all paired samples and in 5-day-old infants. CT vs 1,25(OH)2D showed a significant correlation (r = 0.473, P less than 0.05, n = 24) as well as the relative increase of 1,25(OH)2D (delta pg/ml) after birth vs CT at age 5 days (r = 0.537, P less than 0.01, n = 24).(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary intake of phosphorus modulates the circadian rhythm in serum concentration of phosphorus. Implications for the renal production of 1,25-dihydroxyvitamin D

We recently reported that in healthy men, changes in the production rate (PR) of 1,25-dihydroxyvitamin D [1,25-(OH)2D] accounted for the 80% increase and the 30% decrease in its serum concentration that was induced by restriction and supplementation, respectively, of dietary phosphorus. These changes in PR and serum concentration of 1,25-(OH)2D could be mediated by changes in serum concentrations of phosphorus that occur after the morning fasting period. To examine this hypothesis, we measured serum concentrations of phosphorus in blood drawn at hourly intervals for 24 h in six healthy men in whom dietary phosphorus was initially maintained at 1,500 mg/70 kg body weight per day for 9 d, then restricted to 500 mg/d (coupled with orally administered aluminum hydroxide) for 10 d, and then supplemented to 3,000 mg/d for 10 d. When dietary phosphorus was normal, the serum concentration of phosphorus exhibited the normal circadian rhythm: a rapid decrease in early morning to a nadir at 1100, followed by an increase to plateau at 1600 h and a further increase to an acrophase (peak) at 0030 h. The variation in serum levels of phosphorus can be described as the sum of sinusoidal functions with periodicities of 24 and 12 h. Phosphorus restriction for 10 d induced a 40% reduction in the 24-h mean serum level of phosphorus, abolished the early afternoon rise in its serum level (i.e., the 12-h periodic component of the time series), and delayed the acrophase by 3 h to 0330 h. Phosphorus supplementation for 10 d induced a 14% increase in the 24-h mean serum level of phosphorus but no significant change in its morning fasting level, exaggerated the early afternoon rise in serum phosphorus, and advanced the acrophase by 9 h to 1530 h. The changes in the PR of 1,25-(OH)2D induced by restriction and supplementation of dietary phosphorus varied inversely and significantly with those induced in the 24-h mean serum level of phosphorus (R = -0.88, P less than 0.001). These data demonstrate that in healthy men, dietary phosphorus is an important determinant of the serum concentration of phosphorus throughout most of the day. The data suggest that diet-induced changes in serum levels of phosphorus mediate the changes in PR and serum concentration of 1,25(OH)2D.

Evaluation of a parathyroid hormone antagonist in an in vivo multiparameter bioassay

The antagonist properties of a bovine parathyroid hormone analogue ([Tyr34]bPTH-(7-34] amide were quantitatively assessed in vivo in a multiparameter assay to estimate the potency of the antagonist against the major actions of PTH. The analogue inhibited PTH-stimulated urinary excretion of phosphate and adenosine 3',5'-cyclic monophosphate in vitamin D-deficient thyroparathyroidectomized rats in a dose-dependent manner. At a molar dose ratio as low as 5:1 of antagonist to PTH, partial inhibition occurred. PTH stimulates the activity of 25-hydroxyvitamin D3-1 alpha-hydroxylase in renal proximal tubules. When coinfused with PTH, this analogue completely inhibited PTH-stimulated 1 alpha-hydroxylase activity at a molar dose ratio of 25:1 of antagonist to PTH and partially inhibited the activity at a molar dose ratio of 10:1. The analogue revealed no PTH-like agonist activity for stimulation of the 1 alpha-hydroxylase. Taken together, these studies indicate that [Tyr34]bPTH-(7-34) amide is a potent antagonist of several of the parameters of PTH action in vivo and demonstrate the feasibility of designing a PTH antagonist that can interact simultaneously with all the PTH receptors responsible for the hormone's major actions in vivo.

Effect of dietary acid and calcium on 25-hydroxyvitamin D metabolism by chick kidney

Studies of the effects of acidosis and dietary calcium on 25-hydroxyvitamin D3 (25OHD3) metabolism were conducted utilizing assays of mitochondrial 1-hydroxylase and 24-hydroxylase activity following 96 hours of acid loading. The results showed decreased 1-hydroxylase (206 +/- 6 vs. 132 +/- 22 fmol min-1 mg/mitochondrial protein-1, p less than 0.01), augmented 24-hydroxylase (48 +/- 14 vs. 180 +/- 30 fmol min-1 mg-1, p less than 0.001) and increments in plasma calcium and phosphate following acidosis. High calcium diet also increased 24-hydroxylase activity (48 +/- 14 vs. 160 +/- 24 fmol min-1 mg-1, p less than 0.001) and plasma calcium, but decreased plasma phosphate. The extramitochondrial concentrations of calcium, potassium, and phosphate which maximally stimulated the hydroxylases in vitro were not altered by the in vivo dietary perturbations. The increments in 1-hydroxylase and 24-hydroxylase resulting from the presence of calcium, potassium, or phosphate in optimal concentrations (10(-5) M, 100 mM, and 100 mM, respectively) were changed by both acid loading and high calcium diet, decreasing in the case of 1-hydroxylase and increasing in that of 24-hydroxylase. The data indicate that the suppression of 1-hydroxylase and augmentation of 24-hydroxylase following these dietary perturbations may depend on both changed total enzyme capacity and altered enzyme sensitivity to extramitochondrial ions and are consistent with mediation by calcium of the effects of acidosis.

Remission of hypoparathyroidism during lactation: evidence for a physiological role for prolactin in the regulation of vitamin D metabolism

We studied a young woman with surgical hypoparathyroidism who, on her usual maintenance dose of calcitriol, developed hypercalcaemia 9 d postpartum when lactation was established. Serum values of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) values were very high (127 pg/ml). The patient remained without exogenous calcitriol treatment for 40 d, during which time serum 1,25(OH)2D3 levels remained within the normal range and serum calcium fell with a half-time of 27 d. The requirements for calcitriol increased to antepartum levels when lactation had ceased. There was a close negative correlation between requirements for calcitriol and serum PRL values. After weaning, an episode of hypercalcaemia was induced by increasing the dose of calcitriol. On stopping calcitriol the serum 1,25(OH)2D3 fell to low values (4 pg/ml) within 2 d and serum calcium fell with a half-time of 3 d, necessitating the early reintroduction of calcitriol. We conclude that in hypoparathyroidism exogenous vitamin D requirements fall during lactation because of enhanced endogenous production of 1,25(OH)2D3. The lactation-associated increase in circulating 1,25(OH)2D3 concentrations thus results from a parathyroid hormone-independent mechanism, possibly by an effect of PRL on the 1 alpha-hydroxylase.

Impact of prepartum dietary phosphorus intake on calcium homeostasis at parturition

A study was conducted to determine the impact of dietary P intake on vitamin D metabolism and incidence of parturient paresis in aged dairy cows. Thirty dairy cows (10/group) were fed one of three experimental diets for approximately 28 d precalving. Phosphorus intake was .7, 1, or 3 times daily maintenance requirement and Ca intake was three times daily maintenance requirement for all cows. There was a 20% incidence of parturient paresis in each group. Prepartum dietary P intake had no effect on precalving or calving plasma Ca concentrations. Cows fed the low P (.7 times) diet had higher plasma Ca at 3 and 5 d postcalving than did cows fed P at 1 or 3 times maintenance. Plasma phosphorus concentrations reflected dietary P intake. Dietary P intake had no effect on plasma Mg, free hydroxyproline, 1,25-dihydroxyvitamin D, or 24,25-dihydroxyvitamin D concentrations. The range in dietary P from .7 to 3 times maintenance requirement had no effect on the incidence of parturient paresis. However, it did appear to influence Ca homeostasis during the postpartum period as cows fed the low P diet had higher plasma calcium concentrations postcalving. This may be a result of the low P diet enhancing intestinal C absorption by a vitamin D-mediated transport mechanism.

Effect of 1,25 dihydroxyvitamin D3 on the calcium and magnesium metabolism of lactating cows

Administration of 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) to lactating dairy cows resulted in increased dietary calcium absorption and elevated concentrations of plasma calcium. Dietary magnesium absorption was unaffected by 1,25(OH)2D3 however, plasma magnesium concentration was depressed. Injections of 1,25(OH)2D3 were effective in elevating plasma calcium concentrations in both normal and hypomagnesaemic cows. This indicates a potential use for 1,25(OH)2D3 to prevent and treat hypocalcaemic cows with or without concurrent hypomagnesaemia.

A new mechanism for induced vitamin D deficiency in calcium deprivation

Synthesis of vitamin D in the skin in response to ultraviolet light is the main determinant of vitamin D status in man and it is therefore surprising that rickets and osteomalacia, clinical signs of vitamin D deficiency, remain common in tropical and subtropical countries. Skin pigmentation can reduce vitamin D formation but this is a negligible limitation in people exposed to abundant ultraviolet light. Earlier studies in animals and man suggested that another environmental factor, the low calcium/high cereal diet typical of susceptible populations, might affect the efficiency of vitamin D utilization. We show here in rats that the rate of inactivation of vitamin D in the liver is increased by calcium deprivation. The effect is mediated by 1,25-dihydroxyvitamin D, produced in response to secondary hyperparathyroidism, which promotes hepatic conversion of vitamin D to polar inactivation products that are excreted in bile. This finding has widespread implications both for understanding the pathogenesis of endemic rickets and in that it provides a unifying mechanism for the development of vitamin D deficiency in many clinical disorders.

Evidence that calcitonin stimulates 1,25-dihydroxyvitamin D production and intestinal absorption of calcium in vivo

Although it is well established that parathyroid hormone and phosphate are important regulators of 1,25-dihydroxyvitamin D [1,25(OH)2D] production, it remains unclear whether calcitonin affects vitamin D metabolism in vivo. Experiments were performed in the rat to determine the effect of chronic calcitonin infusion (0.2 U X h-1) on plasma levels of vitamin D metabolites and on calcium metabolism. Thyroparathyroidectomized animals fed a calcium-replete or calcium-free diet were studied for as long as 2 wk before they were killed. In control rats, a calcium-free diet alone for 12 d resulted in an increase in 1,25(OH)2D levels from 24 +/- 5 to 139 +/- 37 pg . ml-1, P = 0.025. The infusion of calcitonin also stimulated 1,25(OH)2D levels compared with controls on a regular diet (80 +/- 17 vs. 38 +/- 6 pg . ml-1, P less than 0.05) and on a calcium-free diet (460 +/- 50 vs. 139 +/- 37 pg . ml-1, P less than 0.001). In addition, calcitonin increased plasma calcium levels in animals on a regular diet by 50%; this effect was most likely due to increased intestinal absorption of calcium, because removal of calcium from the diet markedly blunted this effect. In contrast, calcitonin administration did not significantly affect 25(OH)D plasma levels. Collectively, these data suggest that calcitonin and calcium are independent regulators of 1,25(OH)2D production and that calcitonin stimulates intestinal absorption of calcium, by increasing circulating levels of 1,25(OH)2D.

Synergism between cytosolic and mitochondrial oxidation: its possible relevance to the metabolism of vitamin D3 in the kidney

Using frozen liver sections and quantitative cytochemistry it has been established that when cells are allowed to oxidize a cytoplasmic and a mitochondrial substrate simultaneously the resulting oxidative activity is markedly higher than the sum of the oxidation of each substrate measured separately. In the present study this type of synergistic interaction has been confirmed in the kidney, particularly in cells of the pars recta. Our results support the evidence of the influence of cytoplasmic NADPH on the intramitochondrial oxidative process and it is suggested that, in cells of the pars recta, cytosolic NADPH may be involved in intramitochondrial mixed function oxidases such as 1 alpha-hydroxylase: these results could further elucidate the mechanism responsible for the production of the hormonal form of vitamin D3.