Vitamin d-stimulated intestinal calcium absorption may not involve calcium-binding protein directly.

Abstract

ONE of the earliest responses of chick intestine to cholecalciferol (vitamin D3) is the appearance of a soluble calcium-binding protein (CaBP) for which current evidence indicates a role in the intestinal absorption of calcium1,2. CaBP, which has never been detected in the intestine of cholecalciferol-deficient chickens, is synthesised de novo in response to cholecalciferol, apparently by the induction of the transcription of a specific mRNA for CaBP (ref. 3). In the chick CaBP biosynthesis is initiated after a lag of about 7 h after physiological doses of the vitamin. A correlation between the appearance of CaBP and the first detectable increase in calcium absorption in response to cholecalciferol was observed when CaBP was measured using immunological procedures4,5 but not using the less sensitive Chelex assay6. Consequently it has been generally accepted that CaBP plays a major role in chicks in cholecalciferol-stimulated calcium transport, but by a mechanism which has still to be described. These changes in intestinal CaBP levels and in calcium absorption are brought about by 1,25-dihydroxycholecalciferol (1,25-(OH)2D3), the hormonal metabolite of D3. We report here the sequence and time scale of changes in calcium absorption and CaBP levels in the intestine in response to 1,25-(OH)2D3. By comparison with events observed after cholecalciferol administration, the changes in response to the hormone turn out to be much more rapid and to have different rates of decay, with calcium transport being stimulated before CaBP can be detected immunologically.