Membrane mechanisms for electrogenic Na+-independent l-alanine transport in the lizard duodenal mucosa

Abstract

The active Na+-independent transport ofl-alanine across the duodenal mucosa of the lizard Gallotia galloti was studied in Ussing-type chambers using a computer-controlled voltage clamp. Addition of l-alanine to the Na+-free bathing solutions resulted in a significantl-alanine absorption ( J net) that was paralleled by an increase in transepithelial short-circuit current ( I sc) and potential difference (PD) without apparent changes in the tissue conductance. The concentration dependence of J net, PD, and I sc displayed Michaelis-Menten kinetics.l-alanine-induced electrical changes were completely inhibited by external alkaline pH or by the H+-ionophore carbonyl cyanide m-chlorophenyl-hydrazone in the bathing solution. The alanine-induced electrogenicity was dependent on the presence of extracellular K+ and could be blocked by serosal Ba2+ or mucosal orthovanadate. These results suggest the existence of an H+-coupledl-alanine cotransport at the apical membrane of enterocytes. The favorable H+ driving force is likely to be maintained by an apical vanadate-sensitive H+-K+-ATPase, allowing the extrusion of H+ in an exchange with K+. Potassium exit through a basolateral barium-sensitive conductance provides the key step for the electrogenicity of l-alanine absorption.