M. Nedergaard, INTRACELLULAR CA2-ACID IN CULTURED MAMMALIAN NEURONS( TRANSIENTS EVOKED BY LACTIC), American journal of physiology. Regulatory, integrative and comparative physiology, 37(2), 1995, pp. 506-513
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.