Influence of Acid-base Changes on the Intracellular Calcium Concentration of Neurons in Primary Culture

The influence of changes in intra- and extracellular pH (pHi and pHe, respectively) on the cytosolic, free calcium concentration ([Ca2+]i) of neocortical neurons was studied by microspectrofluorometric techniques and the fluorophore fura-2. When, at constant pHe, pHi was lowered with the NH4Cl prepulse technique, or by a transient increase in CO2 tension, [Ca2+]i invariably increased, the magnitude of the rise being proportional to delta pHi. Since similar results were obtained in Ca(2+)-free solutions, the results suggest that the rise in [Ca2+]i was due to calcium release from intracellular stores. The initial alkaline transient during NH4Cl exposure was associated with a rise in [Ca2+]i. However, this rise seemed to reflect influx of Ca2+ from the external solution. Thus, in Ca(2+)-free solution NH4Cl exposure led to a decrease in [Ca2+]i. This result and others suggest that, at constant pHe, intracellular alkalosis reduces [Ca2+]i, probably by enhancing sequestration of calcium. When cells were exposed to a CO2 transient at reduced pHe, Ca2+ rose initially but then fell, often below basal values. Similar results were obtained when extracellular HCO3- concentration was reduced at constant CO2 tension. Unexpectedly, such results were obtained only in Ca(2+)-containing solutions. In Ca(2+)-free solutions, acidosis always raised [Ca2+]i. It is suggested that a lowering of pHe stimulates extrusion of Ca2+ by ATP-driven Ca2+/2H+ antiport.