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

The active Na+-independent transport of L-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 significant L-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-chlorophenylhydrazone 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+-coupled L-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.