Conductive Choline Transport by Alveolar Epithelial Plasma Membrane Vesicles

Abstract

Choline is an important substrate in alveolar epithelia for both surfactant production and cellular maintenance. The underlying mechanisms of uptake and sites of membrane transport remain uncertain. To test the hypothesis that choline transport occurs at the basolateral side of alveolar epithelia by both Na+-independent and -dependent mechanisms, plasma membrane vesicles were prepared from the apical and basolateral membranes of mature porcine type II pneumocytes. Choline+transport was assayed by uptake of [3H]choline+by enriched apical or basolateral vesicles. In the presence of imposed, inside-negative charge gradients, basolateral vesicles exhibited early overshoot of [3H]choline+uptake unaffected by the presence or absence of external Na+(541 ± 53 vs 564 ± 79 pmol/mg protein (NS)). High sensitivity to hemicholinium-3 was observed in the presence or absence of Na+. In the absence of inside-negative charge gradients, uptake was reduced 12-fold in the presence or absence of Na+, and external choline+induced internal alkalization of acidified basolateral vesicles. Accumulative [3H]choline+uptakes by apical vesicles in the presence or absence of inside-negative charge gradients and Na+were insignificant. We conclude that predominant choline+uptake by type II pneumocytes occurs at the basolateral membrane by Na+-independent, electrogenic choline+conductance. The presence of electroneutral choline+/H+exchange is suggested.