Nitrous oxide enhances Na+/Ca++ exchange in the neuroblastoma cell line SK-N-SH

Changes in the concentration of cytosolic free calcium ([Ca++]i) play fundamental roles in the initiation and regulation of many neuronal processes. Altered regulation of [Ca++]i has been implicated in the action of some anesthetics. We investigated the effects of nitrous oxide (N2O) on Ca++ mobilization and membrane potential in the human neuroblastoma cell line SK-N-SH. [Ca++]i was monitored by fluorescence spectrophotometry of cells loaded with fura-2 or fluo-3. N2O reversibly suppressed carbachol-stimulated increases in [Ca++]i. N2O also inhibited increases in [Ca++]i induced by calcium ionophore or depolarization suggesting a mechanism involving enhanced efflux or sequestration of cytosolic Ca++. The inhibitory effect of N2O was attenuated when the transmembrane Na+ gradient was altered either by suspending cells in nominally Na(+)-free buffer or by pretreating cells with ouabain. The inhibitory effect of N2O was also attenuated by the Na+/Ca++ exchange inhibitor 3,4-dichlorobenzamil. The effects of N2O on membrane potential were measured fluorimetrically using bis(1,3-dibutylthiobarbituric acid)-trimethine oxonol. In the presence of N2O, resting membrane potential was hyperpolarized, a condition that would favor Ca++ efflux mediated by the electrogenic Na+/Ca++ exchanger. Taken together, these findings indicate that N2O suppresses carbachol-stimulated increases in [Ca++]i by enhancing Na+/Ca++ exchange activity. Enhancement of neuronal Na+/Ca++ exchange may contribute to the anesthetic action of N2O.