Anion-dependent sodium ion conductance of platelet plasma membranes

External stimulate Na+ efflux from platelet plasma membrane vesicles. Efflux is apparently electrogenic since K+ diffusion potentials induced with valinomycin (interior positive) accelerate and potentials of the opposite polarity (interior negative) inhibit. In the presence of stimulatory anions, voltage-dependent Na+ efflux is much faster than Na+-Na+ exchange in the absence of an induced membrane potential. Anions stimulate voltage-dependent efflux in the following order: SCN- greater than I- greater than NO3- greater than Br- approximately acetate approximately Cl- greater than F- approximately SO42- greater than HPO42-, gluconate, and isethionate. Thiocyanate, the most stimulatory anion, increases Na+ efflux 20-fold in the presence of a membrane potential (interior positive). Stimulation of efflux by Cl- is a saturable phenomenon with a K0.5 of 41 mM and a maximal 2-3-fold stimulation over the basal level of efflux. Neither basal nor valinomycin-stimulated efflux was influenced by the presence of the platelet-aggregating agents thrombin, epinephrine, or ADP in the presence of fibrinogen.