THE EFFECTS OF 4 GENERAL-ANESTHETICS ON INTRACELLULAR [CA2+] IN CULTURED RAT HIPPOCAMPAL-NEURONS

It has been suggested that general anesthesia might arise as a consequence of increased cytoplasmic free ionized calcium concentration {[Ca2+](i)}. The effect of increased [Ca2+](i) might be to activate K+ channels or to modulate other ion channels important for the control of excitability, such as the GABA(A) receptor. A direct test of this hypothesis has not been reported. Microfluorimetry with the calcium-sensitive dye fura-2 was used to study the effects of four anesthetic agents on the regulation of intracellular free Ca2+ in hippocampal neurons cultured from the embryonic rat hippocampus. Basal intracellular free ionized calcium concentration [Ca2+](i) in the neurons was 50-100 nM. Depolarization of the neurons with 50 mM K+ resulted in the elevation of [Ca2+](i) to 200-800 nM, with subsequent recovery of [Ca2+](i) over several minutes. The volatile anesthetics halothane, enflurane and isoflurane did not alter basal [Ca2+](i), even above clinically relevant concentrations; however, they did inhibit elevation of [Ca2+](i) by high K+ stimulation. The intravenous anesthetic methohexital caused small increases in basal [Ca2+](i) at concentrations greater than or equal to 50 mu M; methohexital (5-50 mu M) also inhibited elevations of [Ca2+](i) induced by high K+. The evidence presented here suggests that the anesthetics studied do not produce their actions via sustained or transient increases in [Ca2+](i). However, all of the anesthetics studied appear to possess inhibitory effects on hippocampal voltage-dependent Ca2+ channels, in addition to their previously described effects at GABA(A) receptors.