Calcium absorption in 30 cm segments of small intestine was measured by constant perfusion of test solutions containing different concentrations of calcium gluconate. In both the jejunum and ileum, calcium absorption rates increased progressively as luminal calcium concentration was increased stepwise between 1 and 20 mM. Although calcium transport was not saturable within these limits, unidirectional flux ratios of calcium (47Ca) suggest that calcium absorption is active in both the jejunum and ileum. Calcium absorption in patients with chronic renal disease was markedly depressed in both regions of the small intestine. This was due to decreased flux out of the lumen; flux in the reverse direction was normal. Flux ratios in the renal disease patients showed no evidence for active calcium transport. Treatment of these patients for 1 wk with 2 μg/day of 1α hydroxycholecalciferol [1α (OH) D3] restored net calcium absorption and unidirectional calcium flux out of the lumen to normal values in the jejunum; in the ileum, 1α (OH) D3 increased calcium absorption 60-83% of normal at the various luminal calcium concentrations. 1α (OH) D3 had no effect on unidirectional calcium flux into the lumen or on xylose and electrolyte absorption in either area of the small intestine.
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