Effect of L-alanine and ouabain on membrane conductances and apical membrane potential in Aplysia intestine.

The proximal intestine of Aplysia californica was employed to assess the effect of alanine absorption on apical membrane K+ conductance (GKa) and basolateral membrane conductance (Gb) and the role of the electrogenic Na(+)-K(+)-adenosinetriphosphatase (Na+ pump) in the repolarization of apical membrane electrical potential difference (Va) after alanine-induced depolarization. Addition of 50 mM L-alanine (isosmotic substitution for mannitol) to the apical superfusate depolarized Va, reduced the ratio of apical to basolateral membrane resistances (Ra/Rb), and stimulated short-circuit current (Isc). Following these initial events, Va repolarized, Ra/Rb increased, and there was a slight decline in Isc. Apical high-K+ artificial seawater revealed an alanine-induced increase in GKa. Washout of alanine from the apical solution increased Ra/Rb above the prealanine control value. Thus alanine absorption is accompanied by an increase in Gb. Basolateral 0.1 mM ouabain abolished alanine-stimulated Isc but had little effect on Va ( < 3 mV depolarization) either before or after exposure to alanine. The repolarization of Va was not affected in tissues superfused with 0.1 mM basolateral ouabain for approximately 3 min even though the alanine-stimulated increase in Isc was abolished. Therefore, the electrogenic Na+ pump contributes minimally to the repolarization of Va in sea hare intestine. The origin of the hyperpolarization of Va resides therefore, at least in part, in the increase in GKa, which restores the driving force for Na(+)-alanine cotransport and prevents K+ accumulation in the enterocytes.