Frequency and voltage-dependent effects of mono-N-dealkyldisopyramide, the major metabolite of disopyramide, in canine ventricular tissue.

Mono-N-dealkyldisopyramide (MND), the major metabolite of disopyramide, reaches significant concentrations in patients; however, the contribution of MND to the antiarrhythmic or toxic effects of disopyramide is not known. We assessed the kinetics and magnitude of interaction of MND with the sodium channel in canine ventricular tissue superfused in vitro using Vmax as an index of sodium channel block. At a basic cycle length of 1000 msec, MND (4-32 micrograms/ml) produced a concentration-dependent depression of both Vmax and amplitude of the action potential and accelerated all phases of repolarization in Purkinje fibers. To assess rate-dependent block, Purkinje fibers were stimulated with pulse trains at interstimulus intervals of 400 to 2000 msec. MND produced a concentration- and rate-dependent increase in the magnitude of rate-dependent block. There was also a concentration-dependent increase in the kinetics of onset of block (decrease in rate constant). The rate constant increased with faster stimulation rates. Minimal tonic block occurred at clinically relevant concentrations. Recovery from rate-dependent block followed a single exponential time course with time constants of 5.23 +/- 0.90 and 4.88 +/- 0.94 sec for Vmax and activation time, respectively. There was no shift of the normalized Vmax-membrane potential relationship except at the highest concentration, 32 micrograms/ml. At cycle lengths of 250 to 1000 msec, MND (4 micrograms/ml) shortened all phases of repolarization in Purkinje fibers, the greatest shortening occurring at the longest cycle length. Prolongation of effective refractory period occurred only at rapid heart rates. Both action potential duration and effective refractory period were prolonged in ventricular muscle which was independent of rate.(ABSTRACT TRUNCATED AT 250 WORDS)