ON THE ORIGIN OF A SUSTAINED INCREASE IN CYTOSOLIC CA2+ CONCENTRATION AFTER A TOXIC GLUTAMATE TREATMENT OF THE NERVE-CELL CULTURE

A sustained increase of cytosolic Ca2+ concentration, [Ca2+]i, (Ca2+ plateau) was induced by a 15-min treatment with 50 muM glutamate of cultured cerebellar granule cells and hippocampal neurons in a Mg2+-free solution. Plateau proved to be insensitive to inhibition of Na(o)+/Ca(i)2+ exchange caused by removal external Na+ in the post-glutamate period. A is similar to 10(5)-fold reduction of [Ca2+]o (from 1.5 mM to 20 nM) in the post-glutamate period caused in most cells only a slow and small decrease in [Ca2+]i, although the same low-Ca2+ trial before glutamate treatment caused in hippocampal cells very quick blockade of spontaneous [Ca2+]i oscillation and a decrease in the basal [Ca2+]i. The results suggest that the Ca2+ plateau is due to a suppression of the Ca2+ extrusion from the cell (in particular via Na+/Ca2+ exchange) rather than from a persistent increase in Ca2+ permeability of neuronal membrane.