Background: Voltage-gated Na channels modulate membrane excitability in excitable tissues. Inhibition of Na channels has been implicated in the effects of volatile anesthetics on both nervous and peripheral excitable tissues. The authors investigated isoform-selective effects of isoflurane on the major Na channel isoforms expressed in excitable tissues.
Methods: Rat Nav1.2, Nav1.4, or Nav1.5 alpha subunits heterologously expressed in Chinese hamster ovary cells were analyzed by whole cell voltage clamp recording. The effects of isoflurane on Na current activation, inactivation, and recovery from inactivation were analyzed.
Results: The cardiac isoform Nav1.5 activated at more negative potentials (peak INa at -30 mV) than the neuronal Nav1.2 (0 mV) or skeletal muscle Nav1.4 (-10 mV) isoforms. Isoflurane reversibly inhibited all three isoforms in a concentration- and voltage-dependent manner at clinical concentrations (IC50 = 0.70, 0.61, and 0.45 mm, respectively, for Nav1.2, Nav1.4, and Nav1.5 from a physiologic holding potential of -70 mV). Inhibition was greater from a holding potential of -70 mV than from -100 mV, especially for Nav1.4 and Nav1.5. Isoflurane enhanced inactivation of all three isoforms due to a hyperpolarizing shift in the voltage dependence of steady state fast inactivation. Inhibition of Nav1.4 and Nav1.5 by isoflurane was attributed primarily to enhanced inactivation, whereas inhibition of Nav1.2, which had a more positive V1/2 of inactivation, was due primarily to tonic block.
Conclusions: Two principal mechanisms contribute to Na channel inhibition by isoflurane: enhanced inactivation due to a hyperpolarizing shift in the voltage dependence of steady state fast inactivation (Nav1.5 approximately Nav1.4 > Nav1.2) and tonic block (Nav1.2 > Nav1.4 approximately Nav1.5). These novel mechanistic differences observed between isoforms suggest a potential pharmacologic basis for discrimination between Na channel isoforms to enhance anesthetic specificity.