Use-dependent effects of amiodarone on Vmax in cardiac Purkinje and ventricular muscle fibers.

Superfusion with 5 micrograms/ml amiodarone for 3-4 h induced use-dependent Vmax block in dog Purkinje and guinea pig ventricular muscle fibers. The recovery from block was exponential with tau of 289 +/- 30 ms in Purkinje (n = 7) and 282 +/- 47 ms in muscle (n = 6) fibers. The onset of frequency-dependent Vmax block was rapid, i.e. reached steady state after 4.2 +/- 0.5 beats (n = 5). The combination of rapid interaction with sodium channel and the reported action potential lengthening make amiodarone unique among Class I antiarrhythmic drugs.