Abstract
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.
Footnotes
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Send reprint requests to: Professor R. B. Philp, Department of Pharmacology and Toxicology, Medical Sciences Building, The University of Western Ontario, London, Ontario, Canada N6A 5C1.
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↵1 This work was supported by a grant from the U.S. Office of Naval Research.
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↵2 Recipient of the Jefferson Davis Research Fellowship from the Undersea and Hyperbaric Medical Society. Current address: Department of Physiology, The University of Western Ontario, London, Ontario, Canada, N6A 5C1.
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↵3 Recipient of a Development Grant from the Medical Research Council of Canada
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↵4 In this series of experiments, basal Ca++ levels were also reduced by N2O (fig. 1B). However, in all subsequent experiments N2O did not significantly alter resting levels of [Ca++]i.
- Abbreviations:
- N2O
- nitrous oxide
- [Ca++]i
- cytosolic free calcium concentration
- DCB
- 3,4-diclorobenzamil
- MIA
- methylisobutylamiloride
- bisoxonol
- bis (1,3-dibutylthiobarbituric acid)-trimethine oxonol
- Caq
- aqueous concentration
- Received November 20, 1995.
- Accepted October 11, 1996.
- The American Society for Pharmacology and Experimental Therapeutics
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