RESUMEN
Our previous investigations have established that Na+ translocation across the Tetraselmis viridis plasma membrane (PM) mediated by the primary ATP-driven Na+-pump, Na+-ATPase, is accompanied by H+ counter-transport [Y.V. Balnokin et al. (1999) FEBS Lett 462:402-406]. The hypothesis that the Na+-ATPase of T. viridis operates as an Na+/H+ exchanger is tested in the present work. The study of Na+ and H+ transport in PM vesicles isolated from T. viridis demonstrated that the membrane-permeant anion NO3- caused (i) an increase in ATP-driven Na+ uptake by the vesicles, (ii) an increase in (Na(+)+ATP)-dependent vesicle lumen alkalization resulting from H+ efflux out of the vesicles and (iii) dissipation of electrical potential, deltapsi, generated across the vesicle membrane by the Na+-ATPase. The (Na(+)+ATP)-dependent lumen alkalization was not significantly affected by valinomycin, addition of which in the presence of K+ abolished deltapsi at the vesicle membrane. The fact that the Na+-ATPase-mediated alkalization of the vesicle lumen is sustained in the absence of the transmembrane deltapsi is consistent with a primary role of the Na+-ATPase in driving H+ outside the vesicles. The findings allowed us to conclude that the Na+-ATPase of T. viridis directly performs an exchange of Na+ for H+. Since the Na+-ATPase generates electric potential across the vesicle membrane, the transport stoichiometry is mNa+/nH+, where m> n.