| Summary: | An in vivo assay was used to measure Ca-45 absorption from rat intestine in rats which had been adapted to diets which were either high or deficient in calcium. Results demonstrated that vitamin D was required for adaptation which was found to be strongest in the duodenum. The metabolites of vitamin D₃, i.e. 25-OH D₃, 24,25(OH)₂D₃ and 1,24,25(OH)₃D₃ all showed some adaptation response when supplied as dietary supplements; however, 1,25(OH)₂D₃ caused an increased Ca-45 absorption which was independent of dietary calcium intake and was not abolished by nephrectomy. Adaptation is defined as an altered efficiency of calcium absorption in response to a change in dietary calcium concentration. These data suggest that the controlled synthesis of 1,25(OH)₂D₃ is the mechanism which governs the intestinal adaptive response to calcium. Based on these data, an adaptive index was constructed to show each metabolite's effect on adaptation; with 1,25(OH)₂D₃ causing a 6% calcium uptake within 10 minutes from the duodenum into the blood. A low phosphorus diet was also shown to regulate calcium adaptation, presumably via synthesis of 1,25(OH)₂D₃, and this synthesis was shown to be independent of parathyroid hormone stimulation. === Medicine, Faculty of === Cellular and Physiological Sciences, Department of === Graduate
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