Regulation of parathyroid hormone gene expression by hypocalcemia, hypercalcemia, and vitamin D in the rat.

In vivo in the rat 1,25(OH)2D3 decreases and a low calcium increases PTH mRNA levels. We now report the effect of 3 and 8 wk of changes in dietary vitamin D and calcium on PTH mRNA levels. PTH mRNA levels were increased by 3 wk of calcium deficiency (five times), a vitamin D-deficient diet (two times), and combined deficiency (10 times), but not changed by high calcium. Vitamin D-deficient-diet rats' PTH mRNA did not decrease after a single large dose of 1,25(OH)2D3, but did decrease partially after repeated daily doses of 1,25(OH)2D3. Rats after a vitamin D-, calcium-deficient (-D-Ca) diet did not respond to changes in serum calcium at 1 h. Flow cytometry of isolated cells from parathyroid-thyroid tissue separated the smaller parathyroid from the larger thyroid cells and allowed an analysis of parathyroid cell number. In normal vitamin D/normal calcium (NDNCa) rats the parathyroid cells were 24.7 +/- 3.4% (n = 6) of the total cell number, whereas in -D-Ca rats they were 41.8 +/- 6.6% (n = 6) (P less than 0.05). That is, -D-Ca rats had 1.7 times the number of cells, whereas they had 10 times the amount of PTH mRNA, indicating the major contribution (6 times) of increased PTH gene expression per cell. Moreover, a calcium-deficient, more so than a vitamin D-deficient diet, amplifies the expression of the PTH gene, and vitamin D is necessary for an intact response of PTH mRNA to 1,25(OH)2D3 or calcium.     

Estrogen receptors and biologic response in rat parathyroid tissue and C cells.

The expression of the PTH and calcitonin genes is dramatically decreased by 1,25(OH)2D3 in vivo, and the PTH gene expression is increased by hypocalcemia. We have now studied the effect of estrogens on the expression of these genes in vivo. 17 beta-Estradiol, given to ovariectomized rats, led to a fourfold increase in PTH mRNA and calcitonin mRNA levels. These effects occurred 24 h after single injections of 37-145 nmol estradiol, or after constant infusions of 12 pmol/d for 1 or 2 wk, where there was no effect on serum calcium levels. The estrogen receptor mRNA was demonstrated in the thyroparathyroid tissue by polymerase chain reaction. The estrogen binding was localized to the parathyroid and C cells by immunohistochemistry. Uterus weight was increased by repeated larger doses (73 nmol/d x 7) of estradiol, but not by the small doses (12 pmol/d for 1 or 2 wk) which were effective on the PTH and calcitonin genes, suggesting a sensitive endocrine effect. These results confirm that the parathyroid and C cells are target organs for estrogen, leading to an increased expression of PTH and calcitonin, which by their combined anabolic effect on bone would help prevent osteoporosis.     

Parathyroid cell proliferation in normal and chronic renal failure rats. The effects of calcium, phosphate, and vitamin D.

Secondary hyperparathyroidism is characterized by an increase in parathyroid (PT) cell number, and parathyroid hormone (PTH) synthesis and secretion. It is still unknown as to what stimuli regulate PT cell proliferation and how they do this. We have studied rats with dietary-induced secondary hyper- and hypoparathyroidism, rats given 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and rats after 5/6 nephrectomy for the presence of PT cell proliferation and apoptosis. PT cell proliferation has been measured by staining for proliferating cell nuclear antigen (PCNA) and apoptosis by in situ detection of nuclear DNA fragmentation and correlated with serum biochemistry and PTH mRNA levels. A low calcium diet led to increased levels of PTH mRNA and a 10-fold increase in PT cell proliferation. A low phosphate diet led to decreased levels of PTH mRNA and the complete absence of PT cell proliferation. 1,25 (OH)2D3 (25 pmol/d x 3) led to a decrease in PTH mRNA levels and unlike the hypophosphatemic rats there was no decrease in cell proliferation. There were no cells undergoing apoptosis in any of the experimental conditions. The secondary hyperparathyroidism of 5/6 nephrectomized rats was characterized by an increase in PTH mRNA levels and PT cell proliferation which were both markedly decreased by a low phosphate diet. The number of PCNA positive cells was increased by a high phosphate diet. Therefore hypocalcemia, hyperphosphatemia and uremia lead to PT cell proliferation, and hypophosphatemia completely abolishes this effect. Injected 1,25 (OH)2D3 had no effect. These findings emphasize the importance of a normal phosphate and calcium in the prevention of PT cell hyperplasia.     

Marked suppression of secondary hyperparathyroidism by intravenous administration of 1,25-dihydroxy-cholecalciferol in uremic patients

Current evidence suggests that administration of 1,25(OH)2D3 to patients with chronic renal insufficiency results in suppression of secondary hyperparathyroidism only if hypercalcemia occurs. However, since the parathyroid glands possess specific receptors for 1,25(OH)2D3 and a calcium binding protein, there is considerable interest in a possible direct effect of 1,25(OH)2D3 on parathyroid hormone (PTH) secretion independent of changes in serum calcium. Recent findings indicate substantial degradation of 1,25(OH)2D3 in the intestine, therefore, it is possible that while oral administration of the vitamin D metabolite increases intestinal calcium absorption, the delivery of 1,25(OH)2D3 to peripheral target organs may be limited. We therefore compared the effects of orally or intravenously administered 1,25(OH)2D3 on the plasma levels of 1,25(OH)2D3 and the effects of these two modes of treatment on PTH secretion. Whereas oral administration of 1,25(OH)2D3 in doses adequate to maintain serum calcium at the upper limits of normal did not alter PTH levels, a marked suppression (70.1 +/- 3.2%) of PTH levels was seen in all 20 patients given intravenous 1,25(OH)2D3. Temporal studies suggested a 20.1 +/- 5.2% decrease in PTH without a significant change in serum calcium with intravenous 1,25(OH)2D3. In five patients the serum calcium was increased by the oral administration of calcium carbonate, the decrement in serum i-PTH was only 25 +/- 6.65% when compared with 73.5 +/- 5.08% (P less than 0.001) obtained by the administration of intravenous 1,25(OH)2D3. Thus, a similar serum calcium achieved by intravenous 1,25(OH)2D3 rather than calcium carbonate has a greater suppressive effect in the release of PTH. These studies indicate that 1,25(OH)2D3 administered intravenously rather than orally may result in a greater delivery of the vitamin D metabolite to peripheral target tissues other than the intestine and allow a greater expression of biological effects of 1,25(OH)2D3 in peripheral tissues. The use of intravenous 1,25(OH)2D3 thus provides a simple and extremely effective way to suppress secondary hyperparathyroidism in dialysis patients.     

Evidence that blood ionized calcium can regulate serum 1,25(OH)2D3 independently of parathyroid hormone and phosphorus in the rat.

This study asks whether arterial blood ionized calcium concentration (Ca++) can regulate the serum level of 1,25-dihydroxy-vitamin D3 [1,25(OH)2D3] independently of serum phosphorus and parathyroid hormone (PTH). We infused either PTH (bovine 1-34, 10 U/kg body wt/h) or saline into awake and unrestrained rats for 24 h, through a chronic indwelling catheter. PTH raised total serum calcium and arterial blood ionized calcium, yet serum 1,25(OH)2D3 fell from 35 +/- 6 (mean +/- SEM, n = 10) with saline to 12 +/- 3 pg/ml (n = 11, P less than 0.005 vs. saline). To determine if the decrease in serum 1,25(OH)2D3 was due to the elevated Ca++, we infused PTH into other rats for 24 h, along with varying amounts of EGTA. Infusion of PTH + 0.67 micron/min EGTA reduced Ca++, and 1,25(OH)2D3 rose to 90 +/- 33 (P less than 0.02 vs. PTH alone). PTH + 1.00 micron/min EGTA lowered Ca++ more, and 1,25(OH)2D3 increased to 148 +/- 29 (P less than 0.01 vs. saline or PTH alone). PTH + 1.33 micron/min EGTA lowered Ca++ below values seen with saline or PTH alone, and 1,25(OH)2D3 rose to 267 +/- 46 (P less than 0.003 vs. all other groups). Thus, during PTH infusion lowering Ca++ with EGTA raised 1,25(OH)2D3 progressively. There were no differences in serum phosphorus concentration or in arterial blood pH in any group infused with PTH. The log of serum 1,25(OH)2D3 was correlated inversely with Ca++ in all four groups infused with PTH (r = -0.737, n = 31, P less than 0.001), and also when the saline group was included (r = -0.677, n = 41, P less than 0.001). The results of this study indicate that serum 1,25(OH)2D3 may be regulated by Ca++ independent of PTH and serum phosphorus levels in the rat. Since 1,25(OH)2D3 regulates gastrointestinal calcium absorption, there may be direct feedback control of 1,25(OH)2D3, by its regulated ion, Ca++.     

Parathyroid hormone suppression by intravenous 1,25-dihydroxyvitamin D. A role for increased sensitivity to calcium.

Numerous in vitro studies in experimental animals have demonstrated a direct suppressive effect of 1,25-dihydroxyvitamin D (1,25(OH)2D) on parathyroid hormone (PTH) synthesis. We therefore sought to determine whether such an effect could be demonstrated in uremic patients undergoing maneuvers designed to avoid changes in serum calcium concentrations. In addition, the response of the parathyroid gland in patients undergoing hypercalcemic suppression (protocol I) and hypocalcemic stimulation (protocol II) before and after 2 wk of intravenous 1,25(OH)2D was evaluated. In those enlisted in protocol I, PTH values fell from 375 +/- 66 to 294 +/- 50 pg (P less than 0.01) after 1,25(OH)2D administration. During hypercalcemic suppression, the "set point" (PTH max + PTH min/2) for PTH suppression by calcium fell from 5.24 +/- 0.14 to 5.06 +/- 0.15 mg/dl (P less than 0.05) with 1,25(OH)2D. A similar decline in PTH levels after giving intravenous 1,25(OH)2D was noted in protocol II patients. During hypocalcemic stimulation, the parathyroid response was attenuated by 1,25(OH)2D. We conclude that intravenous 1,25(OH)2D directly suppresses PTH secretion in uremic patients. This suppression, in part, appears to be due to increased sensitivity of the gland to ambient calcium levels.     

The influence of calciotrophic hormones on lymphocyte transformation and interleukin-2 production in human mononuclear cells

Peripheral blood mononuclear cells (PBMC) display receptors for parathyroid hormone (PTH) and calcitonin (CT) and, when activated, express receptors for 1,25 dihydroxyvitamin D3 (1,25(OH)2D3). The role of these receptors in unclear. It is well established that 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) has a significant effect on lymphocyte transformation and interleukin-2 (IL-2) activity in PBMC's. We have proceeded to compare the effects of the calciotrophic hormones, 1,25(OH)2D3, 24,25 dihydroxyvitamin D3 (24,25(OH)2D3), 25 dihydroxyvitamin D3 (25(OH)D3), bovine (1-35)PTH (b(1-35)PTH) and salmon CT on both lymphocyte transformation and IL-2 activity in PBMC's from normal human volunteers. We have also sought an indication as to which subset of cells is responsible for the 1,25(OH)2D3 effect. Unlike the other calciotrophic hormones 1,25(OH)2D3 inhibited the proliferation of phytohemagglutinin (PHA), concanavalin A (ConA) and pokeweed mitogen (PWM) stimulated cells. 1,25(OH)2D3 had a significantly greater effect on PHA induced cell proliferation and only inhibited cells with the T-helper/inducer (TH, CD4 + ve) phenotype, this suggested that 1,25(OH)2D3 may act selectively on the cells with CD4 + ve phenotype. In addition, of the calciotrophic hormones, 1,25(OH)2D3 had a greater effect on IL-2 activity. These findings indicate that 1,25(OH)2D3 and not the other calciotrophic hormones has a significant immunomodulatory effect.     

Serum 1,25 dihydroxy vitamin D (1,25(OH)2D3), 25 hydroxy vitamin D (25(OH)D) and parathormone levels in diabetic retinopathy

OBJECTIVES: To investigate whether there is a relationship between serum 1,25 dihydroxy vitamin D3 [1,25(OH)2D3], which is an inhibitor of angiogenesis, concentrations and severity of diabetic retinopathy (DR). DESIGN AND METHODS: Serum 1,25(OH)2D3, 25 hydroxy vitamin D [25(OH)D] and parathormone (PTH) concentrations were measured in diabetic patients (n = 66) and nondiabetic healthy subjects (n = 20). RESULTS: The mean serum 1,25(OH)2D3 concentration in diabetic patients was lower than that in nondiabetics (57.3+/-21.44 vs. 89.4+/-18.01 pmol/L, p<0.001); mean 1,25(OH)2D3 concentrations fell with increasing severity of DR [being 63.4+/-17.26 pmol/L for background DR (BDR), 47.7+/-13.27 pmol/L for preproliferative DR (pre-PDR), and 43.1+/-19.45 pmol/L for proliferative DR (PDR)]. Compared with the control group, serum 25(OH)D concentrations were found to be decreased in diabetic patients (p<0.001).There were negative correlations between 1,25(OH)2D3 and age (r = -0.331, p<0.01) and duration of diabetes (r = -0.255, p<0.05). CONCLUSION: From these findings, it was found that there was an inverse relationship between the severity of the retinopathy, i.e., neovascularization, and serum 1,25(OH)2D3 concentrations, being the lowest in PDR and the highest in diabetic patients without retinopathy (NDR) patients. The measurement of serum 1,25(OH)2D3 concentrations might be helpful to predict severity of DR in patients with diabetes mellitus.     

Extrarenal synthesis of 1,25-dihydroxyvitamin D: sensitivity to glucocorticoid treatment

The response of circulating 1,25-dihydroxyvitamin D [1,25-(OH)2D] to challenge with vitamin D treatment both before and after 7-10 days of prednisone therapy (25 mg/day) was investigated in five anephric subjects, six patients with chronic renal failure (CRF), two patients with vitamin D intoxication and four patients with hypoparathyroidism. In anephric subjects serum 25-hydroxyvitamin D [25-(OH)D] rose from 58 +/- 48 (SD) to 377 +/- 221 (SD) nmol/l after administration of 150 micrograms of 25-(OH)D3 for 1 month. Serum 1,25-(OH)2D, which was barely detectable in only two out of five patients under basal conditions, rose to 30 +/- 21 pmol/l after 2 weeks of therapy with 25-(OH)D3, but fell to 10 +/- 5 pmol/l during prednisone treatment. In CRF patients circulating 1,25-(OH)2D rose from 37 +/- 24 to 58 +/- 24 pmol/l during 25-(OH)D3 therapy, but fell to 41 +/- 31 pmol/l during prednisone treatment. In two patients with rheumatoid arthritis, hypercalcaemia due to vitamin D intoxication was associated with raised levels of 1,25-(OH)2D (288 and 317 pmol/l). Administration of prednisone resulted in suppression of 1,25-(OH)2D levels (132 and 96 pmol/l respectively) and reduction of serum calcium to within the normal range. In the hypoparathyroid patients prednisone therapy did not affect circulating 25-(OH)D levels but serum 1,25-(OH)2D fell from 192 +/- 42 to 117 +/- 23 pmol/l and serum calcium from 2.41 +/- 0.21 to 2.20 +/- 0.05 mmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)     

维持性血透患者口服l，25（0H）2D3的治疗观察

目的观察维持性血透患者的继发性甲状旁腺功能亢进( SHPT)不同剂量 1, 25( OH) 2D3治疗效果.方法对维持性血透的 SHPT患者根据其血清免疫反应性甲状旁腺激素( iPTH)水平分 2组.小剂量组 [1, 25( OH) 2D3, 0.25ug口服, 1次 /d和大剂量组 [1, 25( OH) 2D3, 3.0ug口服, 2次 /周.分别观察两组治疗前后血清 iPTH、血钙、磷以及伴随的临床症状.结果小剂量组治疗后血清 iPTH水平较治疗前有明显下降 [(262.76± 197.81)pg/MLvs(191.81± 144.74)pg/ML,p<0.05],血钙上升、血磷下降 (p<0.01).大剂量组治疗后血清 iPTH水平明显下降 [(504.14± 387.01)pg/MLvs(182.56± 179.47)pg/ML,p<0.01],骨痛、皮肤瘙痒等症状明显减轻.结论口服 1, 25( OH) 2D3治疗维持性血透患者的 SHPT是有效的.     

Influence of parathyroid hormone, calcitonin, 1,25(OH)2 cholecalciferol, calcium, and the calcium ionophore A23187 on erythrocyte morphology and blood viscosity

Parathyroid hormone and calcitonin, both endocrine modulators of calcium homeostasis, may influence blood rheology. Parathyroid hormone is known to reduce erythrocyte survival, leading to anemia. Calcitonin has been found to have some vascular effects. We have analyzed the Influence of parathyroid hormone (10(-7) to 10(-10) mol/L), calcitonin (10(-6) to 10(-12) mol/L), 1,25(OH)2 cholecalciferol (10(-7) to 10(-10) mol/L), additional calcium in plasma (+1 and 2 mmol/L), and the calcium lonophore A23187 (50 micromol/L) on erythrocyte morphology and blood viscosity at high shear rate (94 s(-1)) and low shear rate (0.1 s(-1)) in vitro. The loading of erythrocytes with calcium by the ionophore A23187 produced a marked echinocytic shape transformation, an increased blood viscosity at high shear rate caused by decreased deformability of these cells, and a decreased viscosity at low shear rate caused by decreased aggregation of echinocytes. In contrast, increasing plasma calcium concentrations, parathyroid hormone, calcitonin, and 1,25(OH)2 vitamin D3 had no effect on erythrocyte morphology and blood viscosity. We conclude that an increase in intraerythrocytic calcium leads to severe echinocytosis and altered blood viscosity. The endocrine modulators of calcium homeostasis--namely, parathyroid hormone, calcitonin, and 1,25(OH)2 vitamin D3--apparently do not influence intraerythrocytic calcium to a significant degree and have, therefore, no influence on cell morphology and blood viscosity.     

Effects of dietary calcium restriction on 1,25-dihydroxyvitamin D3 net synthesis by rat proximal tubules

We measured in vitro 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) production by kidney proximal tubules prepared by Percoll density centrifugation from male and female rats. 1,25(OH)2D3 in tubule extracts was determined by a sensitive and specific radioreceptor assay. Ingestion of diets adequate in vitamin D3 and containing either normal calcium (1.2% Ca, NC), reduced calcium (0.6% Ca, RCD) or low calcium (0.002% Ca, LCD) increased 1,25(OH)2D3 net synthesis (for male rats, NC vs. RCD vs. LCD 1.8 +/- 0.1 SEM vs. 9 +/- 2 vs. 17 +/- 2 pmol/mg protein/20 min; P less than 0.05 for all comparisons). At either level of reduced calcium intake, tubules from male rats produced more 1,25(OH)2D3 than tubules from females. Serum 1,25(OH)2D3 and tubule cyclic adenosine monophosphate (cAMP) content rose in parallel with progressive dietary calcium restriction, and males had higher circulating 1,25(OH)2D3 and tubule cAMP content than females at each level of reduced calcium intake. L-Epinephrine (10(-4) mol/L), in vitro, increased tubule accumulation of 1,25(OH)2D3 and cAMP. Yohimbine, and alpha 2-receptor antagonist, blocked this response, whereas prazosin was without effect. Increased 1,25(OH)2D3 net synthesis by tubules from male vs. female rats partly explains the higher serum levels and enhanced mineral conservation demonstrated previously in male rats. Preparation of proximal tubules from vitamin D-replete rats permits studies in vitro of 1,25(OH)2D3 production and regulation under more physiologic conditions in which parathyroid hormone, inorganic phosphorus, and calcium may be varied independently.     

Size at birth, adult intestinal calcium absorption and 1,25(OH)(2) vitamin D

BACKGROUND: Adult bone mineral status is modified by early environmental influences, but the mechanism of this phenomenon is unknown. Intestinal calcium absorption and vitamin D metabolism are integrally involved in bone metabolism and may be programmed during early life. AIM: To examine the early-life influences on calcium absorption and its control in 322 post-menopausal female twins. METHODS: Intestinal calcium absorption was assessed by the stable strontium (Sr) method. Serum PTH, 25(OH) and 1,25(OH)(2) vitamin D were measured and recalled birth weight recorded. RESULTS: Fractional intestinal Sr absorption (alpha Sr) was correlated with serum 1,25(OH)(2) vitamin D (p<0.001), but not with 25(OH) vitamin D. Birth weight was inversely associated with serum 1,25(OH)(2) vitamin D (p=0.04), the association being independent of serum calcium, phosphate, creatinine and PTH. Birth weight was inversely correlated with alpha Sr (p=0.03), this association being independent of age, season, customary calcium intake and serum 25(OH) vitamin D; however, when serum 1,25(OH)(2) vitamin D was added into the model, the association became non-significant, suggesting that the association was partially mediated via serum 1,25(OH)(2) vitamin D. DISCUSSION: We found a significant inverse association between birth weight and intestinal calcium absorption that is partially explained by an association between serum 1,25(OH)(2) vitamin D and birth weight. This suggests a mechanism whereby the intra-uterine environment might affect adult skeletal status.     

老年男性骨质疏松与相关影响因素的关系

INTRODUCTIONOsteoporosisisacommondiseaseintheoldpeople.Theincidenceofosteoporosisinmenislaterthaninwomen,andthemorbidityislower,sothestudyonthemaleosteoporosisseemstobedefi-cient.Butsomeinvestigationdatashowthatalmostone-thirdofallhipfracturesworldwideoccurinmen.Themortalityinmenisevidenthigherthaninwomenofthesameage犤1-2犦.Itisveryimportanttostudymoreissuesofmaleosteoporosis.Inordertosurveyitspathogeny,wemeasurethebonemineraldensityin97oldmenbydualenergyX-rayabsorptiometry(DEXA).Atthe…     

Vitamin D metabolism and parathyroid function in man.

1. The metabolism of an intravenous pulse dose of double-isotope-labelled cholecalciferol has been studied in control subjects with widely differing states of vitamin D nutrition and in patients with primary disorders of parathyroid function. 2. The formation of labelled 1,25-dihydroxy-cholecalciferol [1,25-(OH)2D3] and labelled 24,25-dihydroxycholecalciferol [24,25-(OH)2D3] has been related to the prevailing concentrations in serum of 25-hydroxycholecalciferol [25-(OH)D3], immunoreactive parathyroid hormonel, calcium and orthophosphate (Pi). 3. In control subjects with relative vitamin D deficiency [serum 25-(OH)2D3 was related inversely to the serum 25-(OH)D3 and serum calcium, and directly to serum immunoreactive parathyroid hormone. No formation of 1,25-(OH)2D3 was detectable to form labelled 24,25(OH)2D3 preferentially. 4. No control subject produced significant amounts of both labelled 1,25-(OH)2D3 and labelled 24,25-(OH)2D3 simultaneously. 5. All subjects with primary hyperparathyroidism produced significant amounts of labelled 1,25-(OH)2D3 and labelled 24,25-(OH)2D3 simultaneously; the renal turnover of 25-(OH)D3 was apparently greater than in nutritionally matched controls. Serum labelled 1,25-(OH)2D3 in this disease was not correlated with serum 25-(OH)D3, immunoreactive parathyroid hormone, calcium or Pi. Production of labelled 24,25-(OH)2D3 was inappropriately high for the prevailing nutritional state. 6. The indirectly estimated their concentration of 1,25-(OH)2D3 showed only a fourfold variation in control subjects (45-180 pmol/l), compatible with its having a regulated hormonal function. 7. The data suggest that the production of 1,25-(OH)2D3 from a pulse dose of cholecalciferol is normally regulated, directly or indirectly, by the parathyroid hormone.     

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

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

Hypocalcemia may not be essential for the development of secondary hyperparathyroidism in chronic renal failure.

Hypocalcemia is the main factor responsible for the genesis of secondary hyperparathyroidism in chronic renal disease. Studies with parathyroid cells obtained from uremic patients indicate that there is a shift in the set point for calcium-regulated hormone (parathyroid hormone [PTH] secretion. Studies were performed in dogs to further clarify this new potential mechanism. Hypocalcemia was prevented in uremic dogs by the administration of a high calcium diet. Initially, ionized calcium was 4.79 +/- 0.09 mg/dl and gradually increased up to 5.30 +/- 0.05 mg/dl. Despite a moderate increase in ionized calcium, immunoreactive PTH (iPTH) increased from 64 +/- 7.7 to 118 +/- 21 pg/ml. Serum 1,25(OH)2D3 decreased from 25.4 +/- 3.8 to 12.2 +/- 3.6 pg/ml. Further studies were performed in two other groups of dogs. One group received 150-200 ng and the second group 75-100 ng of 1,25(OH)2D3 twice daily. The levels of 1,25(OH)2D3 increased from 32.8 +/- 3.5 to a maximum of 69.6 +/- 4.4 pg/ml. In the second group the levels of serum 1,25(OH)2D3 after nephrectomy remained normal during the study. Amino-terminal iPTH did not increase in either of the two groups treated with 1,25(OH)2D3. In summary, the dogs at no time developed hypocalcemia; however, there was an 84% increase in iPTH levels, suggesting that hypocalcemia, per se, may not be the only factor responsible for the genesis of secondary hyperparathyroidism.