EXTERNAL ALKALOSIS ENHANCES THE GLUTAMATE -INDUCED DESTABILIZATION OFCA2+ HOMEOSTASIS IN CULTURED CEREBELLAR GRANULE CELLS

The H+-dependent regulation of cytoplasmic Ca2+([Ca2+](i)) has been st udied in experiments with fura-2 loaded cultured rat cerebellar granul e 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 rem oval 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 cau sed 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 subseq uent pH(o) returning down to 7.4 and, especially, to its increase up t o 8.5. The analysis of the results obtained suggested that alkalinizat ion of the external medium not only supresses the active extrusion of Ca2+ from cell but reverses the Ca+/H+ exchange resulting in an enhanc ed Ca2+ influx at the expense of the H+ efflux.