The Volume Changes Associated with the Operation of the Simple Transporter

The effects of hydrostatic pressure (0.1-50 MPa) on uridine transport mediated by the 'simple' facilitated nucleoside transporter of guinea-pig and human erythrocytes have been studied in an attempt to identify the volume changes which occur during transport. Pressure inhibited the zero-trans (influx or efflux) mode of uridine transport in guinea-pig cells significantly more (about 2.2-X) than equilibrium exchange. The equilibrium binding of H-3-nitrobenzylthioinosine, a potent specific inhibitor of nucleoside transport, to human red cells and ghosts, was not significantly altered by pressure suggesting that the permeation site was unperturbed. Thus pressure inhibited the transporter primarily by preventing the volume increase associated with the translocation step. Furthermore, the return of the 'empty' transporter was found to be rate-limiting because it required a larger increase in volume than when the transporter was loaded with substrate.