TRANSIENT OUTWARD CURRENT CONTRIBUTES TO WENCKEBACH-LIKE RHYTHMS IN ISOLATED RABBIT VENTRICULAR CELLS

Wenckebach-like rhythms in isolated rabbit ventricular cells are chara cterized by beat-to-beat increments in action potential duration (APD) and latency, giving rise to a beat-to-beat decrease in the recovery i nterval and culminating in a skipped beat. These systematic APD change s are associated with a beat-to-beat decrease in the slope of the earl y repolarizing phase (phase 1) of the action potential, which is parti ally controlled by the transient outward potassium current (I-to). Whe n I-to is blocked with 4-aminopyridine, periodic Wenckebach rhythms ar e replaced by aperiodic Wenckebach rhythms, in which the beat-to-beat changes in tl-le slope of phase I and in APD disappear but the beat-to -beat increase in latency remains. A beat-to-beat decrease in I-to, pa relleling the beat-to-beat changes in the slope of phase I and in APD, is seen in action-potential clamp experiments with Wenckebach rhythms previously recorded in the same cell. Simulations with an ionic model of I-to show cyclical changes in I-to consistent with the experimenta l data. These results demonstrate a key role for I-to in the generatio n of maintained periodic Wenckebach rhythms in isolated rabbit ventric ular cells.