Ionic selectivity of the Ca 2+/H + antiport in synaptic vesicles of sheep brain cortex

Abstract

As we previously reported, synaptic vesicles isolated from sheep brain cortex contain a Ca 2+/H + antiport that permits Ca 2+ accumulation inside the vesicles (∼5 nmol/mg protein) at expenses of the pH gradient generated by the H +-pumping ATPase. We observed that the system associates Ca 2+ influx to H + release and operates with low affinity for Ca 2+. In the present work, we found that Ca 2+/H + antiport mediates exchange of protons with other cations such as Zn 2+ and Cd 2+, suggesting that these cations and Ca 2+ share the same transporter molecules to enter the intravesicular space. Zn 2+ and Cd 2+ induce H + release in a concentration-dependent manner (fluorimetrically evaluated) and they inhibit the antiport-mediated Ca 2+ uptake by the vesicles (isotopically measured). In contrast, large cations such as Ba 2+ and Cs + do not alter Ca 2+ influx and they are unable to induce proton release from the vesicles. With respect to Sr 2+, which has an intermediary size relatively to the other groups of cations, we found that it does not induce H + liberation from the vesicles, but it has a concentration-dependent inhibitory effect on the Ca 2+-induced H + release and Ca 2+ uptake by the vesicles. These results indicate that the cation selectivity of the synaptic vesicles Ca 2+/H + antiport is essentially determined by the size of the cation transported into the vesicles.