THE INFLUENCE OF PH ON GLUTAMATE-INDUCED AND DEPOLARIZATION-INDUCED INCREASES OF INTRACELLULAR CALCIUM-CONCENTRATION IN CORTICAL-NEURONS INPRIMARY CULTURE

The present experiments, carried out on neocortical neurons in primary culture with measurements of cytosolic calcium concentrations ([Ca2+] (i)) by microspectrofluorometric techniques, were designed to study ho w changes in extra- and intracellular pH (pH(e) and pH(i), respectivel y) modulate the rise in [Ca2+](i) due to glutamate exposure or potassi um (K+)-induced depolarization. Although a reduction in pH(e)/pH(i) pe r se increased [Ca2+](i), the acidosis attenuated both the peak rise i n [Ca2+](i) following exposure to glutamate, and the plateau level obs erved during prolonged exposure. As a result, cells exposed to solutio ns with low pH consistently had lower [Ca2+](i) values upon glutamate exposure than cells studied at normal pH. Alkalosis, i.e., an increase in pH(e)/pH(i), had the opposite effect, accentuating the glutamate-i nduced [Ca2+](i) transients. Experiments designed to separate changes due to extra- and intracellular pH suggested that the decisive event w as the change in pH(e). These results are consistent with the known ef fect of pH(e) on calcium flux through NMDA-gated ion channels. However , lowering of pH(e) had an equivalent effect on the rise in [Ca2+](i) triggered by exposure of the cells to a K+ concentration of 50 mM. Thu s, acidosis reduces influx of calcium through both agonist-operated an d voltage-sensitive channels to such an extent that efflux/sequestrati on mechanisms suffice to maintain a lower [Ca2+](i).