The influence of pH on glutamate- 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 (pHe and pHi, respectively) modulate the rise in [Ca2+]i due to glutamate exposure or potassium (K+)-induced depolarization. Although a reduction in pHe/pHi 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 pHe/pHi, 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 pHe. These results are consistent with the known effect of pHe on calcium flux through NMDA-gated ion channels. However, lowering of pHe 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.