Halothane and isoflurane augment depolarization-induced cytosolic CA2+ transients and attenuate carbachol-stimulated CA2+ transients

BACKGROUND: Neuronal excitability is in part determined by Ca2+ availability that is controlled by regulatory mechanisms of cytosolic Ca2+ (Ca2+]cyt). Alteration of any of those mechanisms by volatile anesthetics (VAs) may lead to a change in presynaptic transmission and postsynaptic excitability. Using a human neuroblastoma cell line, the effects of halothane and isoflurane on cytosolic Ca2+ concentration (Ca2+]cyt) in response to K+ and carbachol stimulation were investigated. METHODS: Volatile anesthetic (0.05-1 mm) action on stimulated Ca2+]cyt transients were monitored in suspensions of SH-SY5Y cells loaded with fura-2. Potassium chloride (KCl; 100 mm) was used to depolarize and activate Ca2+ entry through voltage-dependent calcium channels; 1 mm carbachol was used to activate muscarinic receptor-mediated inositol triphosphate (IP3)-dependent intracellular Ca2+ release. Sequential stimulations, KCl followed by carbachol and vice versa, were used to investigate interactions between intracellular Ca2+ stores. RESULTS: Halothane and isoflurane in clinically relevant concentrations enhanced the K+-evoked Ca2+]cyt transient whether intracellular Ca2+ stores were full or partially depleted. In contrast, halothane and isoflurane reduced the carbachol-evoked Ca2+]cyt transient when the intracellular Ca2+ stores were full but had no effect when the Ca2+ stores were partially depleted by KCl stimulation. CONCLUSIONS: Volatile anesthetics acted on sites that differently affect the K+- and carbachol-evoked Ca2+]cyt transients. These data suggest the involvement of an intracellular Ca2+ translocation from the caffeine-sensitive Ca2+ store to the inositol triphosphate-sensitive Ca2+ store that was altered by halothane and isoflurane.