Electrogenic Na+-dependent L-alanine transport in the lizard duodenum. Involvement of systems A and ASC

L-Alanine transport across the isolated duodenal mucosa of the lizard Gallo tia galloti has been studied in Ussing chambers under short-circuit conditi ons. Net L-alanine fluxes, transepithelial potential difference (PD), and s hort-circuit current (Isc) showed concentration-dependent relationships. Na +-dependent L-alanine transport was substantially inhibited by the analog a lpha -methyl aminoisobutyric acid (MeAIB). Likewise, MeAIB fluxes were comp letely inhibited by L-alanine, indicating the presence of system A for neut ral amino acid transport. System A transport activity was electrogenic and exhibited hyperbolic relationships for net MeAIB fluxes, PD, and Isc, which displayed similar apparent K-m values. Na+-dependent L-alanine transport, but not MeAIB transport, was partially inhibited by L-serine and L-cysteine , indicating the participation of system ASC. This transport activity repre sents the major pathway for L-alanine absorption and seemed to operate in a n electroneutral mode with a negligible contribution to the L-alanine-induc ed electrogenicity. It is concluded from the present study that the active Na+-dependent L-alanine transport across the isolated duodenal mucosa of Ga llotia galloti results from the independent activity of systems A and ASC f or neutral amino acid transport.