Isoflurane inhibits the tetrodotoxin-resistant voltage-gated sodium channel Nav1.8.
BACKGROUND
Voltage-gated sodium channels (Nav) mediate neuronal action potentials. Tetrodotoxin inhibits all Nav isoforms, but Nav1.8 and Nav1.9 are relatively tetrodotoxin-resistant (TTX-r) compared to other isoforms. Nav1.8 is highly expressed in dorsal root ganglion neurons and is functionally linked to nociception, but the sensitivity of TTX-r isoforms to inhaled anesthetics is unclear.


METHODS
The sensitivities of heterologously expressed rat TTX-r Nav1.8 and endogenous tetrodotoxin-sensitive (TTX-s) Nav to the prototypic inhaled anesthetic isoflurane were tested in mammalian ND7/23 cells using patch-clamp electrophysiology.


RESULTS
From a holding potential of -70 mV, isoflurane (0.53 +/- 0.06 mM, 1.8 minimum alveolar concentration at 24 degrees C) reduced normalized peak Na current (INa) of Nav1.8 to 0.55 +/- 0.03 and of endogenous TTX-s Nav to 0.56 +/- 0.06. Isoflurane minimally inhibited INa from a holding potential of -140 mV. Isoflurane did not affect voltage-dependence of activation, but it significantly shifted voltage-dependence of steady-state inactivation by -6 mV for Nav1.8 and by -7 mV for TTX-s Nav. IC50 values for inhibition of peak INa were 0.67 +/- 0.06 mM for Nav1.8 and 0.66 +/- 0.09 mM for TTX-s Nav; significant inhibition occurred at clinically relevant concentrations as low as 0.58 minimum alveolar concentration. Isoflurane produced use-dependent block of Nav1.8; at a stimulation frequency of 10 Hz, 0.56 +/- 0.08 mM isoflurane reduced INa to 0.64 +/- 0.01 versus 0.78 +/- 0.01 for control.


CONCLUSIONS
Isoflurane inhibited the tetrodotoxin-resistant isoform Nav1.8 with potency comparable to that for endogenous tetrodotoxin-sensitive Nav isoforms, indicating that sensitivity to inhaled anesthetics is conserved across diverse Nav family members. Block of Nav1.8 in dorsal root ganglion neurons could contribute to the effects of inhaled anesthetics on peripheral nociceptive mechanisms.
Herold
Karl F
KF
Department of Anesthesiology, Weill Cornell Medical College, New York, New York, USA.
Nau
Carla
C
Ouyang
Wei
W
Hemmings
Hugh C
HC
D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D002460:Cell Line; D045744:Cell Line, Tumor; D004351:Drug Resistance; D004558:Electric Stimulation; D004594:Electrophysiology; D005727:Ganglia, Spinal / Q000166:cytology / Q000187:drug effects; D007530:Isoflurane / Q000494:pharmacology*; D051379:Mice; D062559:NAV1.8 Voltage-Gated Sodium Channel; D009419:Nerve Tissue Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics; D009474:Neurons / Q000187:drug effects; D051381:Rats; D026941:Sodium Channel Blockers / Q000494:pharmacology*; D015222:Sodium Channels / Q000235:genetics; D013779:Tetrodotoxin / Q000494:pharmacology*; D014162:Transfection
D018685:Anesthetics, Inhalation; D062559:NAV1.8 Voltage-Gated Sodium Channel; D009419:Nerve Tissue Proteins; C568318:Scn10a protein, mouse; C568319:Scn10a protein, rat; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D013779:Tetrodotoxin; D007530:Isoflurane