The relationship between vitamin D-stimulated calcium transport and intestinal calcium-binding protein in the chicken.
1. The rapid stimulation of intestinal Ca(2+) transport observed in vitamin D-deficient chicks after receiving 1,25-dihydroxycholecalciferol has necessitated a re-evaluation of the correlation hitherto observed between this stimulation and the induction of calcium-binding protein synthesis. By 1h after a dose of 125ng of 1,25-dihydroxycholecalciferol, Ca(2+) transport is increased. This is at least 2h before calcium-binding protein can be detected immunologically and 1h before synthesis of the protein begins on polyribosomes, and thus the hormone stimulates Ca(2+) transport before calcium-binding-protein biosynthesis is induced. 2. The maximum increase in Ca(2+) transport observed after this dose of 1,25-dihydroxycholecalciferol (attained by 8h) is similar to that observed after 1.25-25mug of cholecalciferol, but the stimulation is only short-lived, in contrast with the effect observed after the vitamin. At later times after the hormone, however, when Ca(2+) transport has declined to its basal rate, the cellular content of calcium-binding protein remains elevated. 3. Calcium-binding protein is synthesized on free rather than membrane-bound polyribosomes, which implies that it is an intracellular protein. 4. Rachitic chicks require the presence of dietary calcium for maximum stimulation of calcium-binding protein production by cholecalciferol. 5. These results suggest that calcium-binding protein is an intracellular protein, and that its synthesis may be a consequence of the raised intracellular calcium content of the intestinal epithelial cells resulting from 1,25-dihydroxycholecalciferol-stimulated Ca(2+) transport. We propose that calcium-binding-protein synthesis is necessary for maintaining the stimulated rate of Ca(2+) transport, which is initiated by other factors.
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002645:Chickens; D002762:Cholecalciferol / Q000494:pharmacology; D004100:Dihydroxycholecalciferols / Q000494:pharmacology; D066298:In Vitro Techniques; D007408:Intestinal Absorption / Q000187:drug effects; D007413:Intestinal Mucosa / Q000187:drug effects / Q000378:metabolism*; D011132:Polyribosomes / Q000378:metabolism; D014807:Vitamin D / Q000494:pharmacology*; D014808:Vitamin D Deficiency / Q000378:metabolism
D002352:Carrier Proteins; D004100:Dihydroxycholecalciferols; D014807:Vitamin D; D002762:Cholecalciferol; D002118:Calcium