beta-Amino acid transport across the renal brush-border membrane is coupled to both Na and Cl.

Sodium-dependent beta-alanine uptake into dog renal brush-border membrane vesicles was studied. Kinetic analysis indicated a single transport system, highly specific for beta-amino acids, with Km = 35 microM at 100 mM NaCl. Sodium-dependent beta-alanine transport was markedly anion-dependent, being highest in the presence of chloride (Cl greater than Br greater than SCN greater than NO3 approximately I greater than F) and virtually nonexistent in the presence of gluconate and other nonphysiological chloride substitutes. In addition, it was observed that beta-alanine uptake could be driven against a concentration gradient by a chloride gradient. Similar results were found for sodium. Taken together, these observations provide strong evidence that beta-alanine transport across the renal brush-border membrane is coupled to both sodium and chloride. Studies of the dependence of beta-alanine flux on chloride and sodium concentrations indicated that one chloride ion and multiple sodium ions were involved in the beta-alanine transport event. beta-Alanine flux on chloride found to involve the net transfer of positive charge, consistent with these stoichiometric assignments. The hallucinogen harmaline inhibited beta-alanine uptake in a 1:1 fashion, presumably by acting at a single site on the transport molecule. The ability of harmaline to inhibit beta-alanine uptake was decreased when the chloride concentration was lowered but was unchanged when the sodium concentration was decreased. These results indicate that harmaline does not compete with sodium for a binding site on the carrier as has been suggested for other sodium-coupled transport systems, and that instead, chloride may be required for harmaline binding to the beta-alanine transporter.