During cerebral ischemia, accumulation of the glycolytic end product lactic acid may contribute to brain infarction. In vitro, lactic acid evokes a process of slowly evolving neuronal death characterized by a transient maintenance of cellular viability after initial injury. We examined effects of lactic acid on intracellular Ca2+ (Cai2+). Cultured neurons loaded with the fluorescent Ca2+ indicator fura 2 showed a marked increase in Cai2+ to as high as 600 nM. This increase occurred after lactic acid exposure when intracellular pH had normalized. Membrane potential was unaltered during this period, indicating that the Cai2+ increment was not a result of membrane depolarization. Increase in Ca2+ was prevented by incubating cultures in Ca(2+)-free solutions or exposing them to the L-type Ca2+ channel antagonist nimodipine. Cai2+ returned to resting levels within 20 min and remained normal during the remainder of the 4-h observation period. Neuronal Ca2+ homeostasis was disrupted after lethal exposure to lactic acid, in that subsequent exposure to 50 mM K+ failed to increase neuronal Cai2+. Cai2+ increment was integrated over a 20-min period to obtain a measure of neuronal Cai2+ load. This "calcium integral" was found to correlate directly with severity of neuronal damage observed 24 h later. Thus the Cai2+ increase integrated over time closely reflected the likelihood of lethal neuronal injury after lactic acid exposure.(ABSTRACT TRUNCATED AT 250 WORDS)