THE CALCIUM RESPONSE TO THE EXCITOTOXIN KAINATE IS AMPLIFIED BY SUBSEQUENT REDUCTION OF EXTRACELLULAR-SODIUM

The relation between intracellular and extracellular [Na+] and [Ca2+] and membrane potential during stimulation of non-N-methyl-D-aspartate glutamate receptors has been studied in cerebellar granule cells using the fluorescent indicators SBFI, fura-2 and the bisoxonol membrane po tential probe DiBaC(4)(3). Kainate increased both [Ca2+](i) (intracell ular [Ca2+]) and [Na+](i) (intracellular [Na+]) and depolarized the me mbrane. This elevation of [Ca2+](i) was only partially dependent on th e presence of extracellular Na+ at the time of kainate addition. Remov al of extracellular Na+ itself had a very minor effect on the [Ca2+](i ) or membrane potential of unstimulated cells. If extracellular Na+ wa s removed (in order to reverse the [Na+] gradient) or its concentratio n reduced during stimulation with kainate, the membrane depolarization recovered as expected. However, the intracellular level of sodium rec overed only very slowly and the [Ca2+](i) rose sharply, rather than re covering as might be expected on repolarization of depolarized cells p ossessing voltage sensitive calcium channels. This effect of extracell ular [Na+] reduction on [Ca2+](i) was mimicked by ouabain, another age nt that causes accumulation of [Na+] in cells. These results suggest t hat Na+/Ca2+ exchange may play a major role in calcium homeostasis in stimulated cells, and that the levels of Na+ inside and outside the ce ll are critical in determining the effect of receptor stimulation on t he intracellular [Ca2+].