Origin of intracellular Ca2+ elevation induced by in vitro ischemia-like condition in hippocampal slices

Microfluorometry was used to investigate the origin of intracellular Ca2+ ([Ca2+]i) elevation in field CA1 of gerbil hippocampal slices perfused with a glucose-free physiological medium equilibrated with a 95% N2/5% CO2 gas mixture (standard in vitro ischemia-like condition). Large [Ca2+]i elevation was detected about 4 min after the beginning of standard in vitro ischemia-like condition, which was accompanied with a negative shift of extracellular DC potential. When slices were perfused with Ca(2+)-free in vitro ischemia-like medium, large [Ca2+]i elevation was observed about 3.5 min after the beginning of Ca(2+)-free in vitro ischemia-like condition, however, the increase in [Ca2+]i was more gradual and of a lesser extent compared with that detected in the slices perfused with the standard in vitro ischemia-like medium that contained Ca2+. When slices were perfused with the Ca(2+)-free in vitro ischemia-like medium that contained dantrolene (50 microM) which is known to prevent Ca(2+)-induced Ca2+ release from intracellular Ca2+ stores, the increase in [Ca2+]i was more gradual and of a lesser extent compared with that detected in the slices perfused with the Ca(2+)-free in vitro ischemia-like medium that did not contain dantrolene. These results indicate that large [Ca2+]i elevation induced by in vitro ischemia-like condition in field CA1 of the hippocampus was caused by both Ca2+ influx from extracellular space and Ca2+ release from intracellular Ca2+ stores, and that a part of the Ca2+ release was due to Ca(2+)-induced Ca2+ release from intracellular Ca2+ stores.