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

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Abstract
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. The sustained rhythmic activity apparently results not from reentry but, rather, from spontaneous generation of action potentials by the atrial fibers having oscillatory afterpotentials.