THE INFLUENCE OF PH ON GLUTAMATE-INDUCED AND DEPOLARIZATION-INDUCED INCREASES OF INTRACELLULAR CALCIUM-CONCENTRATION IN CORTICAL-NEURONS IN PRIMARY 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 how changes in extra- and intracellular pH (pH(e) and pH(i), respectively) modulate the rise in [Ca2+](i) due to glutamate exposure or potassium (K+)-induced depolarization. Although a reduction in pH(e)/pH(i) per se increased [Ca2+](i), the acidosis attenuated both the peak rise in [Ca2+](i) following exposure to glutamate, and the plateau level observed during prolonged exposure. As a result, cells exposed to solutions 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-induced [Ca2+](i) transients. Experiments designed to separate changes due to extra- and intracellular pH suggested that the decisive event was the change in pH(e). These results are consistent with the known effect 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. Thus, acidosis reduces influx of calcium through both agonist-operated and voltage-sensitive channels to such an extent that efflux/sequestration mechanisms suffice to maintain a lower [Ca2+](i).