Extracellular acidification modifies Ca2+ fluxes in rat brain synaptosomes

We examined the influence of external acidification on Ca2+ fluxes (45Ca2+ influx and 45Ca2+ efflux) in rat brain synaptosomes. A change on external pH (pHe) from 7.5 to 6.5 linearly decreased the 45Ca2+ uptake (5nmoles/mg protein/pHunit) and increased the 45Ca2+ efflux (1.5 fold/pH unit). These changes were both Na+ dependent and amiloride sensitive suggesting that the Na+/Ca2+ exchanger could be involved. The addition of the Ca2+ channel blockers (diltiazem, verapamil, nifedipine) did not prevent the decrease of the 45Ca2+ uptake evoked by acid pHe and so the involvement of the voltage-sensitive Ca2+ channels could be discarded. In order to determine whether the Na+/Ca2+ exchanger was directly activated by H+ or was indirectly activated by an internal mobilization of Ca2+ from intrasynaptosomal stores we examined the effect of pHe variation on phophoinositide hydrolisis. An increase on phosphoinositide hydrolisis was observed at acid pHe values (7 and 6.5). The hydrolisis was amiloride insensitive. On the other hand 1mM neomycin did inhibit the effect of acidic pHe on Ca2+ fluxes. Taken together, the results of our study provide evidence that external acidification stimulates phospholipase C leading to an increase in phosphoinositide hydrolisis and Ca2+ mobilization. The increase in intracellular Ca2+ would stimulate the Na+/Ca2+ exchanger, increasing Ca2+ efflux and reducing the global Ca2+ influx.