Calcium absorption in the jejunum and ileum of patients with absorptive hypercalciuria was studied by in vivo intestinal perfusion. Net calcium absorption in the jejunum was markedly increased at four luminal calcium concentrations (1 to 20 mM) in the patients with absorptive hypercalciuria when compared to that in normal subjects. In the ileum, net calcium absorption was only slightly increased in the patients with absorptive hypercalciuria. Experiments with radioactive calcium (47Ca) revealed increased unidirectional flux out of the jejunal lumen in the patients but no difference in the unidirectional flux into the lumen, when compared to that in normal control subjects. Net magnesium absorption was normal in the patients. These results suggest that hyperabsorption of calcium in patients with absorptive hypercalciuria is mainly due to enhanced calcium transport in the jejunum and that the defect is specific for calcium since magnesium is absorbed normally.
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