Plasma 24,25-dihydroxyvitamin D3 concentrations in X-linked hypophosphatemic mice: studies using mass fragmentographic and radioreceptor assays.

Previous studies have suggested that both plasma 24,25-dihydroxyvitamin D [24,25-(OH)2D] concentrations and renal 25-hydroxyvitamin D-24-hydroxylase activity are increased in mice with X-linked hypophosphatemia (Hyp mice). However, because the plasma levels of 24,25-(OH)2D seemed surprisingly high, we repeated these assays using two different techniques. Mass fragmentographic and radioreceptor assays were employed to compare the plasma concentrations of 25-hydroxyvitamin D (25-OHD) and 24,25-(OH)2D in normal mice with those in Hyp mice. These assays yielded 24,25-(OH)2D concentrations much lower than previously reported in mice (both normal and Hyp). The concentrations of 25-OHD3 and 24,25-(OH)2D3, determined by mass fragmentography, were lower in Hyp mice than in controls [25-OHD3, 9.7 +/- 0.4 versus 14.6 +/- 0.6 ng/ml, p less than 0.01; 24,25-(OH)2D3, 7.1 +/- 0.3 versus 10.4 +/- 0.4 ng/ml, p less than 0.01]. Plasma 25-OHD concentration was the main determinant of plasma 24,25-(OH)2D, and the ratio of 25-OHD3 to 24,25-(OH)2D3 obtained from mass fragmentographic measurements did not differ between the two groups (1.40 +/- 0.05 versus 1.36 +/- 0.03 ng/ml, NS in normal and Hyp groups, respectively). Separate measurement of plasma 25-OHD, 24,25-(OH)2D, and 25-OHD3-26,23-lactone by radioreceptor assay showed no difference between either plasma 24,25-(OH)2D, or the ratio of 25-OHD concentration to 24,25-(OH)2D concentration among Hyp and control animals. In neither study was plasma phosphate concentration related to the 25-OHD3:24,25-(OH)2D3 ratio.(ABSTRACT TRUNCATED AT 250 WORDS)