Modification of motor nerve terminal excitability by alkanols and volatile anaesthetics

A method of local polarization-excitation was used to study changes in motor nerve terminal excitability produced by n-alkanols and volatile anaesthetics in mouse diaphragm preparations. Ethanol and propanol caused an exaggeration of 'accommodation', i.e., the increase in excitation threshold produced by a conditioning depolarization. Butanol, hexanol and octanol had mixed effects, producing a rise in the minimum threshold (threshold after removal of resting accommodation) in addition to an increase in accommodation. Volatile anaesthetics produced effects on excitability at concentrations comparable to minimum alveolar concentration. The action of enflurane was essentially only to increase accommodation while methoxyflurane produced an increase in threshold insensitive to conditioning polarization. Halothane and isoflurane produced intermediate effects. Accommodation curves were little affected by Ba2+ or 4-aminopyridine and were consistent with accommodation being a reflection of inactivation of the Na+ current system. We conclude that volatile anaesthetics, at concentrations comparable to those producing anaesthesia, may substantially modify Na+ channel gating and inactivation.