Available information supports the dominance of the proximal intestine in inorganic phosphate (Pi) Pi. This approach allows the analysis of the mechanism and the regulation of Pi absorption under more authentic in vivo conditions. (J. Clin. Invest. 1993. 91:915-922.)

A B S T R A C T In the small intestine, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] stimulates both calcium (Ca) and inorganic phosphate (Pi) absorption. This is mediated through an increase in mucosal-toserosal flux (Jms) whereas the serosal-to-mucosal flux (Jsm) remains unchanged. We now report that in rat proximal colon, 1,25(OH)2D3 produces active Ca absorption without affecting Pi