Lack of pharmacodynamic interactions between quinidine and digoxin in isolated atrial muscle of guinea pig heart

Quinidine has been reported to have no effect on the positive inotropic action of digoxin observed in isolated cardiac muscle preparations. This is surprising because quinidine has been shown to reduce Na⁺ influx in cardiac muscle. The conditions which increase Na⁺ influx stimulate the glycoside binding to Na⁺- and K⁺-activated Mg⁺⁺-dependent ATP phosphohydrolase (Na⁺,K⁺-ATPase), and therefore quinidine may be expected to have an opposite effect. Thus, the effects of quinidine on cardiac muscle and its possible interactions with digoxin were re-evaluated using electrically paced left atrial muscle preparations of guinea pig heart. Quinidine caused a frequency- and concentration-dependent decrease in maximal upstroke velocity and amplitude of the action potential without altering resting membrane potential. In addition, quinidine prolonged action potential duration markedly in a frequency-dependent manner. Despite action potential prolongation, the alkaloid reduced net Na⁺ influx as determined by a decrease in steady-state ouabain-sensitive ⁸⁶Rb⁺ uptake. Under these conditions, however, quinidine failed to reduce the rate of onset or the maximal positive inotropic effect of digoxin; or did it reduce digoxin binding to Na⁺,K⁺-ATPase in beating atrial muscle preparations. Benzocaine, which reduced net Na⁺ influx without increasing the action potential duration, also failed to affect the peak inotropic effect of digoxin or the glycoside binding. Quinidine had no direct effects on glycoside binding to isolated cardiac Na⁺,K⁺-ATPase. Moreover, [³H]ouabain binding to isolated enzyme was relatively insensitive to changes in Na⁺ concentrations between 1 and 8 mM although binding was stimulated clearly by Na⁺ above 8 mM. These results indicate that quinidine, at therapeutic concentrations, does not interact pharmacodynamically with digoxin in isolated cardiac muscle.