Ca2+-H+ antiport activity in synaptic vesicles isolated from sheep brain cortex

Abstract

Synaptic vesicles isolated from sheep brain cortex exhibit an ATP-dependent Ca2+ accumulation that is inhibited by the protonophore uncoupler carbonyl cyanide m-chorophenylhydrazone (CCCP) and completely released by the Ca2+ ionophore ionomycin. This transport activity was sensitive to the V-type ATPase inhibitor, bafilomycin, but not to the P-type ATPase inhibitor, vanadate. We also observed that the proton gradient, established across the synaptic vesicle membranes in the presence of ATP, is partially dissipated by the addition of Ca2+ (100-860 [mu]M) in correlation to an increase of ATP hydrolysis by the H+-pumping ATPase. In contrast, the activity of the H+-ATPase, measured under uncoupling conditions (presence of CCCP), appears to be unaltered by the calcium ion. The Ca2+-induced H+ release visualized by fluorescence quenching of acridine orange correlates well with the Ca2+ uptake determined isotopically. These results indicate that synaptic vesicles accumulate Ca2+, via a low affinity Ca2+-H+ antiport system energized by the protonmotive force originated from the H+-pumping ATPase activity.