Metabolism of 25-hydroxyvitamin D3 in renal slices from the X-linked hypophosphatemic (Hyp) mouse: abnormal response to fall in serum calcium

The effect of the X-linked Hyp mutation on 25-hydroxyvitamin D3 (25-OH-D3) metabolism in mouse renal cortical slices was investigated. Vitamin D replete normal mice and Hyp littermates fed the control diet synthesized primarily 24,25-dihydroxyvitamin D3 (24,25-(OH)2D3); only minimal synthesis of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) was detected in both genotypes and 1,25-(OH)2D3 formation was not significantly greater in Hyp mice relative to normal littermates, despite hypophosphatemia and hypocalcemia in the mutants. Calcium-deficient diet fed to normal mice reduced serum calcium (p less than 0.01), increased renal 25-hydroxyvitamin D3-1-hydroxylase (1-OHase) activity (p less than 0.05), and decreased 25-hydroxyvitamin D3-24-hydroxylase (24-OHase) activity (p less than 0.05). In contrast, Hyp littermates on the calcium-deficient diet had decreased serum calcium (p less than 0.01), without significant changes in the renal metabolism of 25-OH-D3. Both normal and Hyp mice responded to the vitamin D-deficient diet with a fall in serum calcium (p less than 0.01), significantly increased renal 1-OHase, and significantly decreased renal 24-OHase activities. In Hyp mice, the fall in serum calcium on the vitamin D-deficient diet was significantly greater than that observed on the calcium-deficient diet. Therefore the ability of Hyp mice to increase renal 1-OHase activity when fed the vitamin D-deficient diet and their failure to do so on the calcium-deficient diet may be related to the resulting degree of hypocalcemia. The results suggest that although Hyp mice can respond to a disturbance of calcium homeostasis, the in vivo signal for the stimulation of renal 1-OHase activity may be set at a different threshold in the Hyp mouse; i.e. a lower serum calcium concentration is necessary for Hyp mice to initiate increased synthesis of 1,25(-OH)2D3.