Intracellular Ca2+ transients evoked by lactic acid in cultured mammalian neurons
Nedergaard, M.
American Journal of Physiology 268(2 Pt 2): R506-R513
1995
ISSN/ISBN: 0002-9513 PMID: 7864248 Document Number: 451979
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 Ca-2+ (Ca-i-2+). Cultured neurons loaded with the fluorescent Ca-2+ indicator fura 2 showed a marked increase in Ca-i-2+ 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 Ca-i-2+ increment was not a result of membrane depolarization. Increase in Ca-2+ was prevented by incubating cultures in Ca-2+ -free solutions or exposing them to the L-type Ca-i-2+ channel antagonist nimodipine. Ca-i-2+ returned to resting levels within 20 min and remained normal during the remainder of the 4-h observation period. Neuronal Ca-2+ homeostasis was disrupted after lethal exposure to lactic acid, in that subsequent 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 measure of neuronal Ca-i-2+ load. This "calcium integral" was found to correlate 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. However, when lactic acid-induced Ca-i-2+ increments were prevented by postincubating cultures in Ca-2+-free solutions or by exposing them to nimodipine, neither time course nor extent of neuronal death was attenuated: lactic acid-exposed neurons died according to the same time course, whether or not they had suffered large cytosolic Ca-2+ loads during the first 4 h after acid exposure. These results suggest that lactic acid induces a temporary opening of Ca-2+-conducting channels, resulting in a large and sustained increase in neuronal Ca-i-2+. This increase in Ca-2+ parallels lactic acid-induced damage but appears not to be necessary for progression of neuronal death. Thus neuronal death may, in selected cases, proceed through processes independent of acute Ca-2+ changes.