Involvement of 1,25-dihydroxycholecalciferol in the short- and long-term increase of intestinal calcium absorption in laying hens: stimulation by gonadal hormones is partly independent of 1,25-dihydroxycholecalciferol.

The short- and long-term effects of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) and its interactions with sexual steroids on Ca absorption were studied in hens. A single injection of 375 ng 1,25(OH)2D3 did not increase intestinal calcium transport in D-repleted hens within 24 hr of administration. However, five daily injections of 1 alpha-(OH)D3(0.5 micrograms/kg), a low Ca intake, or oestrogen-testosterone (O.T) did increase calcium absorption in D-repleted immature pullets (measured by in vivo perfusion), mainly by increasing the diffusional component of Ca transport. O.T. further increased the stimulation by 1 alpha-(OH)D3 of calcium absorption and of duodenal CaBP concentration. At higher doses the stimulation by 1 alpha-(OH)D3 of calcium transport estimated by in situ ligated loop procedure was dose dependent and was not saturable at daily doses of less than 3 micrograms/kg. When Ca transport was stimulated by 2 micrograms/kg of 1 alpha-(OH)D3, an additional effect of O.T. was observed again without any increase in plasma levels of 1,25(OH)2D3. Both oestrogen and testosterone were necessary for this effect on Ca transport. This is evidence for an indirect effect on Ca transport, as both steroids are also necessary to induce medullary bone. It is concluded that O.T. increased intestinal absorption of calcium, first by stimulating 25(OH)D3 1 alpha-hydroxylase activity but also by another mechanism which is independent of 1,25(OH)2D3 metabolism.