Effects of Cyclopropane and of Hypoxia on Transmembrane Potentials of Atrial, Ventricular and Purkinje Fibers

The effects of cyclopropane (6 to 8 vol% in Tyrode solution) and of hypoxia, on the transmembrane potentials of atrial, ventricular and Purkinje fibers of the dog heart, were studied. Action potentials were recorded during perfusion with solution equilibrated with 95% O 2 , and 5% CO 2 (control-Tyrode); during cyclopropane-Tyrode perfusion and during perfusion with solution in which nitrogen replaced cyclopropane (nitrogen-Tyrode). Changes in transmembrane potential observed during cyclopropane-Tyrode treatment were considered to be due to an action of cyclopropane only if they recovered or failed to appear during nitrogen-Tyrode perfusion. By interpretation on this basis it could not be determined that cyclopropane exerted an effect on atrial or ventricular fibers. In Purkinje fibers cyclopropane caused a significant increase in rate of repolarization during the plateau (phase 2) while the rate during the period of rapid repolarization (phase 3) decreased. The time required to repolarize to minus 60 millivolts was shortened significantly while the durations of the terminal phase of repolarization and of the total action potential were lengthened. A relative hypoxia averaging 300 mm Hg accelerated repolarization and reduced resting potential of atrial cells. Ventricular cells were affected similarly but only after two or more periods of hypoxia. Purkinje fibers were shown to be most resistant to this level of hypoxia.