A Characterization of Vitamin D-Independent Intestinal Calcium Absorption in the Osteopetrotic (op/op) Mouse

Previous work in our laboratory showed that the osteopetrotic (op/op) mouse possesses a vitamin D-independent mechanism of intestinal calcium absorption. This study was performed in an effort to further characterize the mechanism. The vitamin D-deficient op/op mouse absorbed calcium faster than either a vitamin D-deficient or 1,25-dihydroxyvitamin D3-supplemented wild-type mouse. This increased rate of absorption was not found at concentrations of calcium that result in diffusional calcium absorption. Thus, vitamin D-deficient op/op mice had intestinal calcium absorption similar to that of vitamin D-deficient wild-type littermates when increasing levels of calcium were administered. Also, mRNA and protein levels of calbindin-D9k were similar in vitamin D-deficient wild-type and op/op mice as well as in wild-type and op/op mice treated with 1,25-dihydroxyvitamin D3. Therefore, the mechanism of vitamin D-independent intestinal calcium absorption in the op/op mouse is distinct from vitamin D-dependent intestinal calcium absorption.