INTRACELLULAR CA2-ACID IN CULTURED MAMMALIAN NEURONS( TRANSIENTS EVOKED BY LACTIC)

During cerebral ischemia, accumulation of the glycolytic end product l actic acid may contribute to brain infarction. In vitro, lactic acid e vokes a process of slowly evolving neuronal death characterized by a t ransient maintenance of cellular viability after initial injury. We ex amined effects of lactic acid on intracellular Ca2+ (Ca-i(2+)). Cultur ed neurons loaded with the fluorescent Ca2+ indicator fura 2 showed a marked increase in Ca-i(2+) to as high as 600 nM. This increase occurr ed after lactic acid exposure when intracellular pH had normalized. Me mbrane potential was unaltered during this period, indicating that the Ca-i(2+) increment was not a result of membrane depolarization. Incre ase in Ca2+ was prevented by incubating cultures in Ca2+-free solution s or exposing them to the L-type Ca2+ channel antagonist nimodipine. C a-i(2+) returned to resting levels within 20 min and remained normal d uring the remainder of the 4-h observation period. Neuronal Ca2+ homeo stasis was disrupted after lethal exposure to lactic acid, in that sub sequent exposure to 50 mM K+ failed to increase neuronal Ca-i(2+). Ca- i(2+) increment was integrated over a 20-min period to obtain a measur e of neuronal Ca-i(2+) load. This ''calcium integral'' was found to co rrelate directly with severity of neuronal damage observed 24 h later. Thus the Ca-i(2+) increase integrated over time closely reflected the likelihood of lethal neuronal injury after lactic acid exposure. Howe ver, when lactic acid-induced Ca-i(2+) increments were prevented by po stincubating cultures in Ca2+-free solutions or by exposing them to ni modipine, neither time course nor extent of neuronal death was attenua ted: lactic acid-exposed neurons died according to the same time cours e, whether or not they had suffered large cytosolic Ca2+ loads during the first 4 h after acid exposure. These results suggest that lactic a cid induces a temporary opening of Ca2+-conducting channels, resulting in a large and sustained increase in neuronal Ca-i(2+). This increase in Ca2+ parallels lactic acid-induced damage but appears not to be ne cessary for progresssion of neuronal death. Thus neuronal death may, i n selected cases, proceed through processes independent of acute Ca2changes.