Mediation of calcium adaptation by 1,25-dihydroxycholecalciferol.

Vitamin D-depleted chicks which were repleted with 25-hydroxycholecalciferol adapted to the feeding of a low or high calcium diet by respectively increasing or decreasing their rate of intestinal calcium absorption. The synthesis of 1,25-dihydroxycholecalciferol in chicks fed differing calcium diets correlated directly with intestinal calcium absorption activity, suggesting that the intestinal calcium adaptation response is mediated by the modulation of kidney 25-hydroxycholecalciferol-1-hydroxylase activity. This view was strongly supported by the demonstration that repletion of the chicks with 1,25-dihydroxycholecalciferol eliminated the ability of the chicks to adjust their rates of intestinal calcium absorption to dietary levels of calcium.