Presystemic 24-hydroxylation of oral 25-hydroxyvitamin D3 in rats

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Abstract
The metabolism of 25‐hydroxyvitamin D3 (25‐OHD3) was compared following its intracardial or gastric administration. The rats were deprived of calcium and vitamin D. A mixture of radiolabeled (0.3 μCi) and stable (2 μg) 25‐OHD3 was given as a single dose. After 24 h the rats given the dose by gastric tube had significantly lower serum concentrations of 25‐OHD3 and 1,25‐dihydroxyvitamin D3 [1,25‐(OH)2D3] than those injected intracardially. In contrast, serum 24,25‐dihydroxyvitamin D3 [24,25‐(OH)2D3] was much higher in the rats given the 25‐OHD3 dose by gastric tube (6.2 nmol/liter ±1.3 SD, n = 7) compared to the intracardial group (0.9 nmol/liter ± 0.5, p < 0.001). The preceding results were based on specific radioactivity of metabolites. The same findings were obtained by reanalyzing the samples using conventional competitive binding assays for 25‐OHD3, 1,25‐(OH)2D3, and 24,25‐(OH)2D3. The results show that orally administered 25‐OHD3 is partly metabolized to 24,25‐(OH)2D3 presystemically.
Funding Information
  • Research Institute of the Queen Elizabeth Hospital