Differential inbibition of neuronal Na+-Ca2+ exchange versus store-operated Ca2+ channels by volatile anesthetics in pheocbromocytoma (PC12) cells

Background: Ca2+ influx is a key component of neuronal intracellular Ca2+ ([Ca2+](i)) regulation. The authors hypothesized that volatile anesthetic inhibition of neuronal activity is mediated by inhibition of Ca2+ influx via two major mechanisms: plasma membrane Na+-Ca2+ exchange (NCX) and the novel mechanism of Ca2+ influx triggered by endoplasmic reticulmn Ca2+ depletion (store-operated Ca2+ channels [SOCCs]). Methods: Differentiated rat pheochromocytoma cells loaded with the Ca2+ indicator fura-2 were Na+-loaded with 0 Ca2+, 145 m,m Na+ Tyrode's and 5 mu m cyclopiazonic acid plus 10 jam ryanodine (functionally isolating plasma membrane). influx-mode NCX was rapidly reactivated by 0 Na+ and 2.5 mM Ca2+. The protocol was repeated in the presence of volatile anesthetics (0.5-1.5 minimum alveolar concentration [MAC] halothane, isoflurane, or sevoflurane) or other drugs to characterize NCX. To examine SOCCs, endoplasmic reticulum Ca2+ was depleted by cyclopiazonic acid in 0 extracellular Ca2+, and Ca2+ influx was triggered by rapid reintroduction of extracellular Ca2+. The protocol was repeated in the presence of anesthetics or other drugs to characterize SOCCs. Results: influx via NCX was not inhibited by voltage-gated Ca2+ channel blockers but was sensitive to NCX inhibitors. Halothane and isoflurane (0.5-1.5 MAC) significantly inhibited NCX (P < 0.05; paired comparisons), whereas sevoflurane at less than 1.5 MAC did not inhibit NCX. SOCC-mediated Ca2+ influx was insensitive to a variety of Ca2+ channel blockers but was inhibited by Ni2+. Such influx was sensitive only to halothane at greater than 1 MAC but not isoflurane or sevoflurane. Conclusions: These data indicate that volatile anesthetics, especially halothane and isoflurane, interfere with neuronal [Ca2+](i) regulation by inhibiting NCX but not SOCC-mediated Ca2+ influx (except high concentrations of halothane).