Oscillatory afterpotentials in dog ventricular muscle fibers

Ventricular muscle fibers from dog hearts were superfused with K+-free, high Ca2+ solutions after exposure to a K+-free, Ca2+-free superfusate. Following a series of driven action potentials, oscillatory after potentials (OAPs) were elicited in all 40 preparations at a resting potential of greater than -60 mV. The OAPs appeared not to represent either an electrotonic influence from Purkinje fibers or reentry. They showed a non-monotonic dependence on the basic cycle length, as well as on the number of preceding action potentials. When the OAPs attained threshold, there was extra-excitation (triggered activity) and, occasionally, sustained rhythmic activity. Single premature impulses increased the amplitude of the OAPs, but single or multiple stimuli applied during sustained rhythmic activity failed to stop it. The amplitude of the OAPs increased with higher external [Ca2+] and with lower external [Na+]. The OAPs and triggered activity were suppressed in the presence of Mn2+, verapamil, and 2.7 mM K+. Our results indicate that OAPs and triggered activity can be evoked in the working myocardium at normal resting potentials under various conditions, and that they represent abnormal impulse formation rather than reentry.