External alkalosis enhances the glutamate-induced destabilization of Ca2+ homeostasis in cultured cerebellar granule cells

The H+-dependent regulation of cytoplasmic Ca2+([Ca2+](i)) has been studied in experiments with fura-2 loaded cultured rat cerebellar granule cells. An increase in the external pH (pH(o)) produced a relatively small elevation of the basal [Ca2+](i) and a pronounced elevation of [Ca2+](i) during its partial recovery following 15-min glutamate (Glu, 100 mu M; glycin, 10 mu M; 0 Mg2+) application. In both cases, the removal of external Ca2+ prevented or abolished the effects of high pH(o) on [Ca2+](i). An increase in pH(o) against the background of the post-glutamate high-Ca2+ plateau caused no significant changes in the [Ca2+](i) level. The effect of pH(i) decrease on [Ca2+](i) was studied to reveal the existence of a reversed mode of operation of the Ca2+/2H(+) system. It was shown that preliminary intracellular acidification caused by protracted (15-min) superfusing of nerve cells with a low-pH (6.5) solution led to the appearance of a [Ca2+](i) response to a subsequent pH(o) returning down to 7.4 and, especially, to its increase up to 8.5. The analysis of the results obtained suggested that alkalinization of the external medium not only supresses the active extrusion of Ca2+ from cell but reverses the Ca+/H+ exchange resulting in an enhanced Ca2+ influx at the expense of the H+ efflux.