Influence of acid-base changes on the intracellular calcium concentration of neurons in primary culture

Abstract

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 ΔpH_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 HCO ^-₃ concentration was reduced at constant CO₂ 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.