Studies on isolated subcellular components of cat pancreas. III. Alanine-sodium cotransport in isolated plasma membrane vesicles.

Transport of alanine was studied in isolated plasma membrane vesicles from cat pancreas using a rapid filtration technique. The uptake is osmotically sensitive and the kinetics of L-alanine transport are biphasic showing a saturable and a nonsaturable component. The saturable component is seen only when a sodium gradient directed from the medium to the vesicular space is present. Under this condition an overshooting uptake of L-but not of D-alanine occurs. The Na+ gradient stimulated uptake of L-alanine is inhibited by L-serine and L-leucine and stimulated when the membrane vesicles had been preloaded with L-alanine, L-serine or L-leucine. The ionophore monensin inhibits stimulation of uptake caused by a sodium gradient. In the presence of valinomycin or carbonyl cyanide p-trifluoromethoxyphenylhydrazone (CFCCP), the sodium-dependent transport is augmented in vesicles preloaded with K2SO4 or H+ ions (intravesicular pH 5.5), respectively. In the presence of different anions, the Na+-dependent transport is stimulated according to increasing anionic penetration through membrane (lipid solubility). We conclude that a sodium dedpendent electrogenic amino acid transport system is present in pancreatic plasma membranes.