Extracellular Acidification Modifies Ca2+Fluxes in Rat Brain Synaptosomes☆

Abstract We examined the influence of external acidification on Ca 2+ fluxes ( 45 Ca 2+ influx and 45 Ca 2+ efflux) in rat brain synaptosomes. A change on external pH (pH e ) from 7.5 to 6.5 linearly decreased the 45 Ca 2+ uptake (5nmoles/mg protein/pHunit) and increased the 45 Ca 2+ efflux (1.5 fold/pH unit). These changes were both Na + dependent and amiloride sensitive suggesting that the Na + /Ca 2+ exchanger could be involved. The addition of the Ca 2+ channel blockers (diltiazem, verapamil, nifedipine) did not prevent the decrease of the 45 Ca 2+ uptake evoked by acid pH e and so the involvement of the voltage-sensitive Ca 2+ channels could be discarded. In order to determine whether the Na + /Ca 2+ exchanger was directly activated by H + or was indirectly activated by an internal mobilization of Ca 2+ from intrasynaptosomal stores we examined the effect of pH e variation on phophoinositide hydrolisis. An increase on phosphoinositide hydrolisis was observed at acid pH e values (7 and 6.5). The hydrolisis was amiloride insensitive. On the other hand 1mM neomycin did inhibit the effect of acidic pH e on Ca 2+ fluxes. Taken together, the results of our study provide evidence that external acidification stimulates phospholipase C leading to an increase in phosphoinositide hydrolisis and Ca 2+ mobilization. The increase in intracellular Ca 2+ would stimulate the Na + /Ca 2+ exchanger, increasing Ca 2+ efflux and reducing the global Ca 2+ influx.