Halothane and isoflurane alter calcium dynamics in rat cerebrocortical synaptosomes

An increase in synaptosomal Ca2+ triggers neurotransmitter release and volatile anesthetics have been shown to inhibit neurotransmitter release by inhibition of Ca2+ entry. We have examined the effect of isoflurane and halothane on the kinetics of increase and decrease of Ca2+ in rat cerebrocortical synaptosomes ([Ca2+](in)). We have also used specific Ca2+ antagonists to examine the role of L-, N-, and P-type Ca2+ channels. Synaptosomal [Ca2+](in) was measured spectrofluorometrically using fura-2 as a Ca2+ reporter; Ca2+ transients were initiated by depolarization with 40 mM KCI. We found that less than or equal to 1 minimum alveolar anesthetic concentration halothane and isoflurane decreased peak [Ca2+](in) by approximately 40%, that both anesthetics decreased the rate of [Ca2+](in) increase and decrease, that specific voltage-dependent calcium channel antagonists had little effect on peak or plateau [Ca2+](in), and that the volatile anesthetics increased the permeability of synaptosomal membranes to Ca2+. These results suggest that the volatile anesthetics, at clinically relevant concentrations, can alter Ca2+ homeostasis in the synapse. Implications: Clinically relevant concentrations of halothane and isoflurane markedly depress K+-evoked increases in rat cerebrocortical synaptosomal calcium (Ca2+) unrelated to L-, N-, and P-type voltage-dependent calcium channels and increase the Ca2+ permeability of the synaptosomal membrane. These changes in Ca2+ dynamics could have profound effects on Ca2+ signaling in the synapse