The relationship between vitamin D-stimulated calcium transport and intestinal calcium-binding protein in the chicken |
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| Authors: | Rosemary Spencer Marilyn Charman Peter W. Wilson D. Eric M. Lawson |
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| Affiliation: | Dunn Nutritional Laboratory, University of Cambridge and Medical Research Council, Milton Road, Cambridge CB4 1XJ, U.K. |
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| Abstract: | 1. The rapid stimulation of intestinal Ca2+ 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, Ca2+ 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 Ca2+ transport before calcium-binding-protein biosynthesis is induced. 2. The maximum increase in Ca2+ transport observed after this dose of 1,25-dihydroxycholecalciferol (attained by 8h) is similar to that observed after 1.25–25μg 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 Ca2+ 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 Ca2+ transport. We propose that calcium-binding-protein synthesis is necessary for maintaining the stimulated rate of Ca2+ transport, which is initiated by other factors. |
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