Effect of membrane potential on Na+-dependent sugar transport by ATP-depleted intestinal cells

The role of the membrane potential as a component of the thermodynamic driving force and as a determinant of kinetic parameters of Na+-dependent sugar transport was investigated using ATP-depleted isolated chicken intestinal cells. Inside-negative membrane potentials were established by incubating K+-loaded rotenone-inhibited cells with valinomycin in a low K+ medium. Overshoots of 3-O-methylglucose (3-OMG) accumulation as high as 10-fold were observed in the presence of valinomycin even in the absence of a Na+ chemical gradient. The magnitude of overshoot was diminished by decreasing the magnitude of the imposed K+ gradient and abolished altogether when nigericin was also included. An Eadie-Hofstee plot of initial flux data showed that the imposed membrane potential increases the Vmax of transport in the absence of a chemical gradient for Na+ from 3 to 12 nmol 3-OMG . mg protein-1 . min-1. The KT is not significantly altered. Similar kinetic results were obtained when a membrane potential as well as a Na+ gradient were imposed. These results suggest that the membrane potential is a more important contributor to alterations in the kinetics of transport than the Na+ chemical potential.