Mechanism and regulation of intestinal phosphate absorption

Proper absorption of inorganic phosphate (Pi) from the lumen of the small intestine is of great importance for the achievement of Pi homeostasis. Although due to intralumenal H+ and Pi concentrations, Pi probably can be absorbed as H2PO4- by passive means in the duodenum, transepithelial transport of HPO4(2)- requires uptake from the lumen by an active transport system. The latter has been identified in many species as a Na(+)-Pi cotransport system at the brush-border membrane of the enterocyte. Although it is still a matter of debate whether the intestinal Na+ gradient-driven Pi transport system is electrogenic or electroneutral, there is agreement that the transporter accepts H2PO4- and HPO4(2)- alike. Recently, two laboratories independently isolated a Na(+)-Pi-binding protein which has been tentatively identified as part of the Na(+)-Pi cotransport system. Movement of Pi from the cytosol across the basolateral membrane into the interstitial space has only been preliminarily characterized as transfer by facilitated diffusion. Na(+)-Pi cotransport across the brush-border membrane is under control by the active vitamin D metabolite, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). The sterol increases the intrinsic activity, i.e. the number and/or mobility of Na(+)-Pi carriers through genomic and probably also nongenomic actions. In addition, the rate of Na(+)-gradient-driven Pi transport can be enhanced by the hormone also through reduction of transmembrane Na+ fluxes so that more energy for translocation becomes available from the transmembrane Na+ gradient. Evidence is accumulating that thyroid hormones as well as glucocorticoids, apart from stimulating vitamin D-independent Pi uptake, potentiate the effect of 1,25-(OH)2D3 on Na(+)-Pi cotransport across the brush-border membrane.