Effect of synthetic vitamin D derivatives on calcium and phosphate metabolism in intact vitamin O-replete rats

Effects of lα-hydroxylated metabolites of cholecalciferol on intestinal radiocalcium absorption in goats

1. Intestinal absorption of47Ca was measured by a double-isotope technique in goats treated with 1, 5 or 25 μg of 1,25-dihydroxycholecalciferol (1,25(OH)2D3). The effects of giving 1,25(OH)2D3by intravenous (iv) infusion for 30–36 h were compared at each dose level with the effects obtained by oral administration of 1,25(OH)2D3either in ethanol or protected against rumen degradation in fatty acid pellets.

2. Dose-dependent increments in absorption followed the treatments, with a doubling of absorption at the 1 μg dose and three- to fivefold increases with the 5 and 25 μg doses.47Ca absorption was equally stimulated 2 and 6 d after treatment but had returned to pretreatment levels 12–14 d after treatment.

3. Intravenous and protected oral administration of 1,25(OH)2D3stimulated47Ca absorption to the same extent, in spite of two- to fivefold higher plasma concentrations of 1,25(OH)2D3after iv treatment. Somewhat lower increments in47Ca absorption were seen using ethanol as the vehicle for oral administration.

4. The naturally occurring metabolites 1,24(R),25-trihydroxycholecalciferol and 1,25(S),26-trihydroxy-cholecalciferol had only one-tenth to one-fifteenth the potency of 1,25(OH)2D3in stimulating47Ca absorption, while synthetic lα-hydroxycholecalciferol appeared to be twice as effective as 1,25(OH)2D3when tested at a high (10μg) dose.