INFLUENCE OF ACID-BASE CHANGES ON THE INTRACELLULAR CALCIUM-CONCENTRATION OF NEURONS IN PRIMARY CULTURE

The influence of changes in intra- and extracellular pH (pH(i) and pH(e), 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 pH(e), pH(i) 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 pH(i). Since similar results were obtained in Ca2+-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 Ca2+-free solution NH4Cl exposure led to a decrease in [Ca2+](i). This result and others suggest that, at constant pH(e), intracellular alkalosis reduces [Ca2+](i), probably by enhancing sequestration of calcium. When cells were exposed to a CO2 transient at reduced pH(e), Ca2+ rose initially but then fell, often below basal values. Similar results were obtained when extracellular HCO3- concentration was reduced at constant CO, tension. Unexpectedly, such results were obtained only in Ca2+-containing solutions. In Ca2+-free solutions, acidosis always raised [Ca2+](i). It is suggested that a lowering of pH(e) stimulates extrusion of Ca2+ by ATP-driven Ca2+/2H(+) antiport.