Media Calcium Attenuates Mitochondrial 1,25(OH)2D Production in Phosphorus or Vitamin D-Deprived Rats

Abstract

Although PTH and hypophosphatemia are the best known stimulators of 25-hydroxyvitamin D-la-hydroxylase, 1,25(OH),D3 production in rats and humans can be modulated by circulating calcium, independent of PTH. To test whether calcium modulates this function directly in mitochondria, we examined effects of calcium on la-hydroxylase in isolated mitochondrial preparations under basal and stimulated conditions. Rats were fed a low phosphorus (or matched control) diet for 4 or 7 d or a vitamin D-deficient (or matched control) diet for 2, 4, or 7 wk. Renal mitochondria were isolated and assayed for la-hydroxylase activity in the presence or absence of added calcium. Calcium did not alter la-hydroxylase in rats on control diets. After 4 d of low phosphorus diet, la-hydroxylase was increased 2-fold over basal activity; media calcium prevented this stimulatory response. By 7 d the calcium effect was not evident. After 4 wk of vitamin D deprivation, activity was -30-fold greater than controls; calcium reduced this response significantly (15-fold). A significant, but less marked inhibition of activity by calcium was present in rats subjected to 7 wk of vitamin D deprivation. Extramitochondrial calcium can directly modulate 1,25(OH),D3 production, but this effect appears to be secondary to the primary physiologic regulators of this function. The calcium effect can be overcome after longer term exposure to phosphorus deprivation, but is sustained in the presence of long term vitamin D deprivation. The best known active metabolite of vitamin D is 1,25(OH),D, which in humans and other mammals, effects numerous physiologic events. Production of 1,25(OH),D is tightly controlled at the final step of its biosynthetic pathway, performed in renal tubular mitochondria by a P450 enzyme system, the renal la-OHase (1). Although several factors have been shown to be trophic for this reaction (e.g. IGF-I, estrogen, thyroid hormone, calcitonin), the predominant physiologic regulators of this reaction in the intact organism are hypophosphatemia and PTH (2). The stimulatory effect of hypocalcemia on 1,25(OH),D production has generally been attributed to resultant secondary hyperparathyroidism (3). In contrast with the view that calcium regulation of la-OHase is entirely dependent on PTH effects, more recent work has shown that extracellular calcium is able to regulate this reaction in rats (4, 5) and humans (6) finding, we examined the ability of calcium to modulate I ) basal enzyme activity and 2) stimulated enzyme activity (using two standard dietary provocations) directly at the mitochondrial level. METHODS Animals.Male Holtzman rats were obtained from Harlan (Madison, WI) at weanling. In experiments examining effects of phosphorus deprivation, rats were housed in our animal facilities and placed on either a low phosphorus diet (Teklad, Madison, WI), containing 0.02% phosphorus, 0.6% calcium and 2200 U of vitamin D/kg, or a control diet containing 0.6% phosphorus, and otherwise matched to the experimental diet.Animals were killed after 4 or...

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

“…Prior work in rats fed a vitamin D deficient diet found vitamin D deficiency to result in a time-delayed 30-fold increase in the activity of the 1α-hydroxylase enzyme in renal mitochondria. 43 It has also been postulated that high serum 1,25(OH) 2 D during pregnancy may suppress 25(OH)D production via 1,25(OH) 2 D-mediated increased transcription of the 24-hydroxylase gene. 18 A recent meta-analyses designed to address this hypothesis during pregnancy found no relationship between 25(OH)D and 1,25(OH) 2 D, 18 but this study contained few longitudinal data, limiting the ability to detect relationships between 25(OH)D and 1,25(OH) 2 D that may be time-delayed.…”

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 vitro studies that have examined a role of [Ca 2+ ] e on either CYP27B1 or CYP24 systems have failed to demonstrate any direct effects at physiologically meaningful concentrations, while in vivo studies depict a general correlation between the rise in serum [Ca 2+ ] e with the reciprocal suppression and induction of CYP27B1 and CYP24, respectively. Recent reports clearly demonstrate CaSR detection in the proximal tubule suggesting that [Ca 2+ ] e plays an as yet undefined role in proximal kidney physiology [13][14][15][16][17]. Our laboratory recently reported the gene expression determinants of low and high calcium profiles in a kidney cortex model [18].…”

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

“…Parathyroidhormone (PTH) maintains serum calcium (Ca) concentration via increase of it's reabsorption in kidney and absorption in intestine by synthesized 1,25(OH) 2 D 3 [4,7]. Inversely, for phosphorus (P) homeostasis, PTH decreases serum P concentration by an increase in urinary excretion.…”