| Abstract: | The kinetics of Na+ efflux from Escherichia coli RA 11 membrane vesicles taking place along a favorable Na+ concentration gradient are strongly dependent on the generation of an electrochemical proton gradient. An energy-dependent acceleration of the Na+ efflux rate is observed at all external pHs between 5.5 and 7.5 and is prevented by uncoupling agents. The contributions of the electrical potential (delta psi) and chemical potential (delta pH) of H+ to the mechanism of Na+ efflux acceleration have been studied by determining the effects of (a) selective dissipation of delta psi and delta pH in respiring membrane vesicles with valinomycin or nigericin and (b) imposition of outwardly directed K+ diffusion gradients (imposed delta psi, interior negative) or acetate diffusion gradients (imposed delta pH, interior alkaline). The data indicate that, at pH 6.6 and 7.5, delta pH and delta psi individually and concurrently accelerate the downhill Na+ efflux rate. At pH 5.5, the Na+ efflux rate is enhanced by delta pH only when the imposed delta pH exceeds a threshold delta pH value; moreover, an imposed delta psi which per se does not enhance the Na+ efflux rate does contribute to the acceleration of Na+ efflux when imposed simultaneously with a delta pH higher than the threshold delta pH value. The results strongly suggest that the Na+-H+ antiport mechanism catalyzes the downhill Na+ efflux.(ABSTRACT TRUNCATED AT 250 WORDS) |
