Role of Vitamin D Metabolites in Phosphate Transport of Rat Intestine

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Abstract
Transport of phosphate by everted sacs of rat intestine was studied. Vitamin D stimulated this system and glucose as an oxidizable substrate was required in agreement with previous reports. Phosphate transport was highest in the upper duodenum in the presence of calcium in the medium. In this segment, calcium greatly increased phosphate transport. In the jejunum, however, calcium in the medium did not appreciably affect phosphate transport. In this segment vitamin D, 25-hydroxycholecalciferol and 1,25-dihydroxycholecalciferol administration, but not 24,25-dihydroxycholecalciferol, stimulated phosphate transport. Although the time course of response of jejunal phosphate transport was similar for 25-hydroxycholecalciferol and 1,25-dihydroxycholecalciferol, nephrectomy prevented the response to 25-hydroxycholecalciferol, but not to 1,25-dihydroxycholecalciferol. Thus it appears likely that 1,25-dihydroxycholecalciferol and not 25-hydroxycholecalciferol is the metabolically active form in this system.