Conductive choline transport by alveolar epithelial plasma membrane vesicles

Choline is an important substrate in alveolar epithelia for both surfactant production and cellular maintenance. The underlying mechanisms of uptake a nd sites of membrane transport remain uncertain. To test the hypothesis tha t choline transport occurs at the basolateral side of alveolar epithelia by both Na+-independent and -dependent mechanisms, plasma membrane vesicles w ere prepared from the apical and basolateral membranes of mature porcine ty pe II pneumocytes. Choline(+) transport was assayed by uptake of [H-3]choli ne(+) by enriched apical or basolateral vesicles. In the presence of impose d, inside-negative charge gradients, basolateral vesicles exhibited early o vershoot of [H-3]choline(+) uptake unaffected by the presence or absence of external Na+ (541 +/- 53 vs 564 +/- 79 pmol/mg protein (NS)). High sensiti vity 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 interna l alkalization of acidified basolateral vesicles. Accumulative [H-3]choline (+) uptakes by apical vesicles in the presence or absence of inside-negativ e charge gradients and Na+ were insignificant. We conclude that predominant choline(+) uptake by type II pneumocytes occurs at the basolateral membran e by Na+-independent, electrogenic choline(+) conductance. The presence of electroneutral choline(+)/H+ exchange is suggested. (C) 1998 Academic Press .