Phosphate transport in intestinal brush-border membrane.

In the small intestine of the rabbit the process of Na+-dependent uptake of phosphate occurs only at the brush-border of duodenal enterocytes. Li+ can replace Na+. The process is activated when either K+, Cs+, Rb+, or choline is present in the intravesicular space. The presence of membrane-permeable anions is essential for maximum rates of phosphate transport. We conclude that the mechanism of the phosphate carrier is electrogenic at pH 6-8, probably two Na+ moving with each H2PO4-. This will lead to the development of a positive charge within the vesicle. The variation of the Km for H2PO4- with pH is thought to be the consequence of the affinity of the carrier protein for H2PO4- increasing as the pH increases. Polyclonal antibodies against membrane vesicles isolated from rabbit duodenum, jejunum, and ileum were prepared. The antibodies raised against the ileum and jejunum both activated the phosphate transport process, while the anti-duodenum antibody preparation inhibited phosphate transport.