Paradoxical actions of quinidine in hyperpermeable heart cells

The effect of quinidine on the inotropic state was examined using small fragments of hyperpermeable ventricular muscle in vitro. In concentrations of 10(-5) and 10(-4) M, quinidine had no effect on the force of spontaneously contracting fibers. At a concentration of 10(-3) M, contractile force increased by at least twofold. We conclude that the negative inotropic action of quinidine is not by a direct action on intracellular organelles of the heart.