Media calcium attenuates mitochondrial 1,25(OH)2D production in phosphorus or vitamin D-deprived rats

Vitamin D insufficiency is prevalent and vitamin D is inversely associated with parathyroid hormone and calcitriol in pregnant adolescents

Few large studies have assessed changes in calcitropic hormones and maternal 25-hydroxyvitamin D (25(OH)D) status across pregnancy, and how this may impact maternal bone turnover and neonatal hormone status. We aimed to identify determinants of 25(OH)D, parathyroid hormone (PTH), and calcitriol across pregnancy in a longitudinal study of 168 pregnant adolescents (≤18 years of age). Maternal 25(OH)D, PTH, and calcitriol were assessed at mid-gestation (∼26 weeks), delivery, and in cord blood. Data were related to measures of maternal anthropometrics, dietary intake, physical activity, and bone turnover markers. Approximately 50% of teens and their infants had serum 25(OH)D ≤ 20 ng/mL; 25(OH)D was lower in African Americans versus whites (p < 0.001). PTH increased across gestation (p < 0.001). Elevated PTH (≥60 pg/mL) was detected in 25% of adolescents at delivery, and was associated with increased concentrations of serum N-telopeptide (NTX) (p = 0.028). PTH and calcitriol did not significantly differ across the range of Ca intake consumed (257-3220 mg/d). In the group as a whole, PTH was inversely associated with 25(OH)D in maternal circulation at mid-gestation (p = 0.023) and at delivery (p = 0.019). However, when the cohort was partitioned by 25(OH)D status, this relationship was only present in those with 25(OH)D ≤ 20 ng/mL, suggestive of a threshold below which 25(OH)D impacts PTH during pregnancy. Mid-gestation 25(OH)D was inversely associated with calcitriol at delivery (p = 0.023), irrespective of Ca intake. Neonatal PTH and calcitriol were significantly lower than (p < 0.001), but unrelated to maternal concentrations. These findings indicate that maternal 25(OH)D status plays a role in calcitropic hormone regulation in pregnant adolescents.

Hormonal regulation of phosphate homeostasis in goats during transition to rumination

Regulatory processes in phosphorus (P) homeostasis in small ruminants are quite different compared to monogastric animals. Adaptive responses of modulating hormones [parathyroid hormone (PTH) and calcitriol] to feeding variable amounts of P are lacking. Therefore, the aim of this study was to examine the influence of high dietary P intake (control diet: 4 g kg(-1) dry matter; high-P diet: 8 g kg(-1) dry matter) on the expression levels of PTH receptor (PTHR), vitamin D receptor (VDR) and Na+-dependent Pi transporters (NaPi II) in kidney and jejunum of goats starting rumination. After 3 months of feeding, plasma phosphate (Pi) and PTH concentrations were increased in the high-P diet group, whereas calcium and calcitriol were not changed. The intestinal Na+-dependent Pi transport capacity was not influenced by a high-P diet and the expression of jejunal VDR, PTHR and NaPi IIb was not modified. Interestingly, renal Na+-dependent Pi transport capacity was significantly reduced and concomitantly the expression of PTHR and NaPi IIa was decreased. In conclusion, the adaptive response of renal Pi reabsorption in goats, which were in transition from non-ruminant to ruminant stage was comparable to that of monogastric animals. In contrast, the modulation of the intestinal Pi absorption was like in adult ruminants.

Extracellular calcium is a direct effecter of VDR levels in proximal tubule epithelial cells that counter-balances effects of PTH on renal Vitamin D metabolism

In renal proximal tubules, VDR is transiently decreased by parathyroid hormone (PTH) during times of hypocalcemia and returns to normal levels with the rise in serum calcium (Ca). In this study we tested the hypothesis that elevated extracellular Ca induces VDR in a human renal proximal cell line (HK-2G) stably expressing PTH receptor type I. Exposure of HK-2G cells to increasing Ca concentration, up to 3mM, induced the expression of VDR. The increase in VDR occurred within 1h and was sustained over 24h. The increase in VDR was also dose-dependently increased using 20-100 nM gadolinium, suggesting the induction of VDR is regulated via the extracellular Ca sensing receptor (CaSR) with is naturally expressed in HK-2G cells. In conclusion, an extracellular Ca concentration in the physiological range is capable of direct increase of renal proximal VDR expression, and the induction mechanism represents a strategy the body may use to counterbalance effects of PTH on renal Vitamin D metabolism.

High P diet induces acute secretion of parathyroidhormone without alteration of serum calcium levels in rats

To find whether a high phosphorus (P) diet stimulate the secretion of PTH, a high-P diet was fed to rats and an increase in serum P levels has occurred. All rats were fed a control diet (0.5% calcium (Ca), 0.5% P) for 7 days, while they were being adapted, for 1 hour at 8:00 AM and again at 8:00 PM. Four groups were switched to the high-P diet (0.5% Ca, 1.5% P) at the time of their morning meal for 1 hour. The other 4 groups continued to receive the control diet. Blood samples were collected from the rats in the remaining group, which served as a pre-feeding control. Every 30 minutes after the start of feeding (30, 60, 90, 120 min), blood samples were collected from the rats in the groups fed the control and high-P diets. Serum P concentrations increased upon intake of the high P diet, within 30 minutes after the start of feeding. Serum PTH levels also increased upon intake of the high P diet, within 30 minutes after the start of feeding, and the levels were significantly higher in the high-P group than in the control group. However, no significant difference was observed in serum Ca levels between the two groups. From these results, our findings suggest that an increase in serum P concentration might be a trigger of PTH secretion without any changes of serum calcium levels.

High phosphorus feeding decreases the expression of renal PTH/PTHRP-receptor mRNA in rats

Dietary intake of high phosphorus (P) is well-described to increase serum levels of PTH, however, how this increased serum PTH affects the PTH actions in major target tissues, particularly in kidney, remains uncovered. We therefore undertook to clarify this point in intact animals fed the high-P diet by examining various parameters of the PTH actions. Twelve weanling Wistar male rats were assigned randomly into the groups; a control group Ca: P = 1: 1 and a high-P group (Ca: P = 1: 3) fed the standard AIN 76 diet supplemented with P (0.5 and 1.5 g/100 g diet). After 3 week feeding, in the high-P diet group, we observed that serum Ca is lowered without difference in serum P when compared to those in the control group. Excretion of urine cAMP, an index of the renal PTH action, was also decreased with higher excretion of urine P by feeding the high P diet. In agreement with the decreased cAMP excretion, a clear reduction in the PTH/PTHrP receptor gene expression estimated by Northern blotting was observed in the kidney irrespective of increased levels of serum PTH. Thus, the present study indicated that high P dietary intake rather reduces the PTH actions in kidney though the serum PTH is increased.

Constitutive expression of 25-hydroxyvitamin D3-1alpha-hydroxylase in a transformed human proximal tubule cell line: evidence for direct regulation of vitamin D metabolism by calcium

Circulating levels of the active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) are dependent on activity of the renal mitochondrial cytochrome P450 enzyme, 25-hydroxyvitamin D3-1alpha-hydroxylase (1alpha-hydroxylase). Production of 1,25-(OH)2D3 occurs predominantly in the renal proximal tubule, with 1alpha-hydroxylase activity being impaired in renal insufficiency and renal disease. The expression and activity of 1alpha-hydroxylase are tightly regulated in response to serum levels of PTH, calcium, phosphate, and 1,25-(OH)2D3 itself. As a consequence of this, the characterization of 1alpha-hydroxylase in human renal tissue has proved difficult. In this study we have characterized constitutive 1alpha-hydroxylase expression in a simian virus 40-transformed human proximal tubule cell line, HKC-8. Initial analyses of [3H]25-hydroxyvitamin D3 (25OHD3) metabolism in these cells using straight and reverse phase HPLC revealed product peaks that coincided with authentic 1,25-(OH)2D3 as well as 24,25-dihydroxyvitamin D3 (24,25-(OH)2D3). Enzyme kinetic studies indicated that the Km for synthesis of 1,25-(OH)2D3 in HKC-8 cells was 120 nmol/liter 25OHD3, with a maximum velocity of 21 pmol/h/mg protein. This activity was inhibited by treatment with ketoconazole, but not diphenyl phenylenediamine. RT-PCR analysis of RNA from HKC-8 cells revealed a transcript similar in size to that observed in keratinocytes and primary cultures of human proximal tubule cells, and protein was detected by Western blot analysis. Synthesis of 1,25-(OH)2D3 was up regulated by treatment with forskolin (10 micromol/liter, 24 h) and was down-regulated by 1,25-(OH)2D3 (10 nmol/liter, 24 h). 1Alpha-hydroxylase activity in HKC-8 cells was also sensitive to the concentration of calcium. Cells grown in low calcium (0.5 mmol/liter) showed a 4.8-fold induction of 1alpha-hydroxylase, whereas treatment with medium containing high levels of calcium (2 mmol/liter) significantly inhibited 1,25-(OH)2D3 production. These data suggest that direct effects of calcium on proximal tubule cells may be an important feature of the regulation of renal 1,25-(OH)2D3 production.

Nonacidotic proximal tubulopathy transmitted as autosomal dominant trait

The family of a patient with a nonacidotic and hypercalciuric proximal tubulopathy was studied. The proband showed glycosuria, aminoaciduria, tubular proteinuria, renal hypophosphatemia, and urate tubular hyporeabsorption without bicarbonate loss. He also presented increased urine calcium excretion, plasma 1,25-dihydroxyvitamin D, and enteral calcium absorption. Clinical consequences of the tubulopathy were osteopenia and calcium kidney stones. Fifteen of the proband's relatives were studied; six of them had renal hypophosphatemia, 10 presented hypercalciuria, and three showed both hypercalciuria and hypophosphatemia. No other reabsorption defects were observed. High plasma levels of 1,25-dihydroxyvitamin D were found in 13 family members; their values correlated positively with calcium excretion and negatively with tubular phosphate reabsorption. None produced stones or had reduced mineral bone density. Hypophosphatemia and hypercalciuria occurred in the two generations studied; their transmission was independent of gender, and male-to-male transmission occurred for both defects. Our findings suggest that a genetic alteration of proximal tubular function could cause multiple reabsorption defects in the proband or renal phosphate leakage in the proband's relatives. The genotypic alteration causing the proximal dysfunctions may be monogenic, with an autosomal dominant pattern of inheritance and variable expressivity. Increased calcium excretion may be due to the proximal tubular alteration; alternatively, it may be the result of a genetic background predisposing to idiopathic hypercalciuria. Phosphate and calcium loss could stimulate 1,25-dihydroxyvitamin D synthesis in proximal tubular cells.