Abstract
Volatile anesthetics inhibit mammalian voltage-gated Na+ channels, an action that contributes to their presynaptic inhibition of neurotransmitter release. We measured the effects of isoflurane, a prototypical halogenated ether volatile anesthetic, on the prokaryotic voltage-gated Na+ channel from Bacillus halodurans (NaChBac). Using whole-cell patch-clamp recording, human embryonic kidney 293 cells transfected with NaChBac displayed large inward currents (INa) that activated at potentials of –60 mV or higher with a peak voltage of activation of 0 mV (from a holding potential of –80 mV) or –10 mV (from a holding potential of –100 mV). Isoflurane inhibited INa in a concentration-dependent manner over a clinically relevant concentration range; inhibition was significantly more potent from a holding potential of –80 mV (IC50 = 0.35 mM) than from –100 mV (IC50 = 0.48 mM). Isoflurane positively shifted the voltage dependence of peak activation, and it negatively shifted the voltage dependence of end steady-state activation. The voltage dependence of inactivation was negatively shifted with no change in slope factor. Enhanced inactivation of INa was 8-fold more sensitive to isoflurane than reduction of channel opening. In addition to tonic block of closed and/or open channels, isoflurane enhanced use-dependent block by delaying recovery from inactivation. These results indicate that a prokaryotic voltage-gated Na+ channel, like mammalian voltage-gated Na+ channels, is inhibited by clinical concentrations of isoflurane involving multiple state-dependent mechanisms. NaChBac should provide a useful model for structure-function studies of volatile anesthetic actions on voltage-gated ion channels.
Footnotes
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This work was supported by National Institutes of Health Grants GM 58055 (to H.C.H.) and GM 68044 (to A.M.C.).
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.107.122929.
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ABBREVIATIONS: Nav, voltage-gated Na+ channel(s); NaChBac, prokaryotic voltage-gated Na+ channel from Bacillus halodurans; HEK, human embryonic kidney; INa, transient NaChBac Na+ current; V1/2a, voltage of half-maximal activation; V1/2in, voltage of half-maximal inactivation; Kv, voltage-gated K+ channel(s).
- Received March 16, 2007.
- Accepted June 13, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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