Two pathways for choline transport in eel erythrocytes: a saturable carrier and a volume-activated channel

Choline transport in eel (Anguilla anguilla) erythrocytes was investigated in cells suspended in isotonic and hypotonic media. In cells in isosmotic solution choline transport was mediated by a saturable system with a Michaelis constant (Km; 62 +/- 6 microM) similar to that of the choline carrier of human erythrocytes but a maximal transport rate (Vmax; 4.5 +/- 0.4 mmol.1 red blood cells-1.h-1) almost two orders of magnitude higher than that in human red blood cells. This pathway was inhibited by hemicholinium-3 and dodecyltrimethylammonium, but not by any of a range of anion transport inhibitors tested. Swelling the cells by suspending them in hyposmotic media activated a second choline transport component that was kinetically and pharmacologically distinct from the saturable system. The volume-activated component was nonsaturable (up to 50 mM choline). It was not inhibited by hemicholinium-3 or dodecyltrimethylammonium but was inhibited by anion transport inhibitors, the most potent of which was 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB; half-maximal inhibitory concentration = 14 microM). Dose-response curves for the effect of NPPB on swelling-activated choline transport and the swelling-activated transport of taurine, a sulfonic amino acid, were superimposable. It is postulated that the transport of choline and taurine (as well as that of other small organic solutes) in osmotically swollen fish erythrocytes is mediated by a volume-activated, anion-selective channel.