Electrophysiological studies on the mechanism of electrically induced sustained rhythmic activity in the rabbit right atrium.

Abstract

SUMMARY Right atrial preparations with no spontaneous activity were isolated from rabbit hearts. Action potentials with specific oscillatory after potentials were recorded from some fibers in the upper pectinate muscle and in the pectinate muscle along the crista terminalis. The amplitude and rate of rise of the oscillatory potential increased as the stimulus interval decreased. In 12 of 35 preparations in Tyrode's solution, sustained rhythmic activity resulted from the depolarizing phase of the enhanced oscillatory potential. The action potential during sustained rhythmic activity was characterized by slow diastolic depolarization, as in the sinoatrial (SA) node. A single premature stimulus or a train of stimuli sometimes caused an acceleration of the rate of excitation rather than its suppression. Sustained rhythmic activity was maintained at a low resting potential and ceased spontaneously when the slope of the slow diastolic depolarization decreased and the maximum diastolic potential increased. Stimulation just after termination of sustained rhythmic activity neither increased the amplitude of the oscillatory potential nor initiated further sustained rhythmic activity. Initiation of new sustained rhythmic activity required a period of quiescence before electrical stimulation. These results suggest that the sustained rhythmic activity described in this paper results not from reentry but, rather, from spontaneous generation of action potentials by the atrial fibers having oscillatory afterpotentials. IT IS WELL KNOWN that premature impulses arising at certain times during the cardiac cycle may induce a burst of rapid repetitive activity (sustained rhythmic activity). Such activity has been assumed to result from either automatic ectopic pacemakers or reentry. 1 "" Sustained rhythmic activity which is initiated and terminated by single premature impulses generally has been thought to result from reentry. However, recent reports indicate that some atrial and ventricular specialized fibers can develop sustained ectopic rhythms that are initiated and sometimes terminated by premature impulses. 7 " 1 ' 2 So far, such sustained rhythmic activity has been observed mainly as the result of drugs such as catecholamines or digitalis and abnormal ionic environments such as a low sodium solution. The rhythms differ from those of automatic fibers, because they sometimes result from oscillatory afterpotentials that are enhanced by a series of extrinsic stimuli. Recently, in the rabbit right atrium with no spontaneous activity in Tyrode's solution, Tanaka et al. l:l found fibers that showed action potentials having oscillatory afterpotentials. In a preliminary report, they stated that such fibers respond to an increase in the rate of electrical stimulation with an increase in the amplitude of oscillatory afterpotentials and become spontaneously active in Tyrode's solution. 14