Intestinal Inorganic Phosphate Transport

Abstract

Evidence indicating that the intestinal absorption of phosphate occurs, at least in part by regulated processes, has been available since the 1930’s when balance studies showed that vitamin D increased both calcium and phosphate absorption (1–3). However, it was not until the extensive studies of Harrison and Harrison in the early 1960’s that it became clear that vitamin D directly effected inorganic phosphate (Pi) absorption by mammalian small intestine (4). The transport-mediated changes in Pi concentration between the outer and inner compartments of everted gut sacs reported by the Harrisons are much larger than the changes that could result solely from effects of the transmural potential difference (PD) which is about 5 mV, so that these results (4) clearly indicate active Pi absorption. Nevertheless, in several studies in which bidirectional Pi fluxes were analyzed in relationship to the PD by means of the Ussing flux ratio test (5), the observed ratios were not different from those predicted as the result of the electrical gradients (6,7). This apparent discrepancy in results can most likely be explained by an increasing body of data which indicate that the activity of intestinal Pi absorptive processes tends to reflect the needs of the animal for this molecule.