Intestinal transport of phosphate: action of vitamin D, calcium, and potassium

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Abstract
Everted loops of rat small intestine incubated in vitro produce a concentration difference of inorganic phosphate between the solutions bathing the serosal and mucosal surfaces as the result of transport out of the mucosal solution against a concentration difference. Cyanide or anaerobiosis inhibits this process. The concentrative transport of phosphate requires the presence of calcium and is enhanced by increase of potassium concentration in the medium. Loops from vitamin D-treated rats develop higher concentration ratios of phosphate between serosal and mucosal solutions than loops from vitamin D-deficient animals when the concentrations of calcium and potassium in the medium are the same. Complete removal of calcium from the system by EDTA inhibits transfer of phosphate against a concentration difference and eliminates the vitamin D effect. The findings suggest that the concentrative transport of phosphate across the intestinal wall is activated by calcium and potassium and that potassium can not satisfy the requirement for calcium. The effect of vitamin D can be to increase the availability of calcium to the system through an action on the permeability of cell surfaces to calcium.