Effects of 1 alpha-hydroxylated metabolites of cholecalciferol on intestinal radiocalcium absorption in goats.

Intestinal absorption of 47Ca was measured by a double-isotope technique in goats treated with 1, 5 or 25 micrograms of 1,25-dihydroxycholecalciferol (1,25(OH)2D3). The effects of giving 1,25(OH)2D3 by intravenous (iv) infusion for 30-36 h were compared at each dose level with the effects obtained by oral administration of 1,25(OH)2D3 either in ethanol or protected against rumen degradation in fatty acid pellets. Dose-dependent increments in absorption followed the treatments, with a doubling of absorption at the 1 microgram dose and three- to fivefold increases with the 5 and 25 micrograms doses. 47Ca absorption was equally stimulated 2 and 6 d after treatment but had returned to pretreatment levels 12-14 d after treatment. Intravenous and protected oral administration of 1,25(OH)2D3 stimulated 47Ca absorption to the same extent, in spite of two- to fivefold higher plasma concentrations of 1,25(OH)2D3 after iv treatment. Somewhat lower increments in 47Ca absorption were seen using ethanol as the vehicle for oral administration. The naturally occurring metabolites 1,24(R),25-trihydroxycholecalciferol and 1,25(S),26-trihydroxycholecalciferol had only one-tenth to one-fifteenth the potency of 1,25(OH)2D3 in stimulating 47Ca absorption, while synthetic 1 alpha-hydroxycholecalciferol appeared to be twice as effective as 1,25(OH)2D3 when tested at a high (10 micrograms) dose.