Role of voltage oscillations in the automaticity of sheep cardiac Purkinje fibers.

The role of oscillatory potentials occurring near the threshold for the fast sodium current (ThVos) in the induction of spontaneous and repetitive activity was studied in sheep Purkinje fibers superfused in vitro. In low extracellular potassium concentration, the steepness and amplitude of diastolic depolarization increased and ThVos appeared during quiescence. ThVos amplitude increased progressively until its depolarizing phase reached the threshold potential for the initiation of the action potential. Drive increased the amplitude of diastolic depolarization and of ThVos, and longer drives induced faster and longer-lasting repetitive activity ("overdrive excitation"). In quiescent fibers, barium depolarized the resting membrane and initiated spontaneous discharge through ThVos. Acetylcholine had similar actions. Cesium hyperpolarized the membrane, thereby suppressing ThVos and related spontaneous activity. Tetrodotoxin and lidocaine also suppressed ThVos, but not the driven action potentials. In low extracellular potassium plus high extracellular calcium concentrations, drive induced ThVos as well as the oscillatory potentials related to calcium overload (Vos), but caused overdrive excitation through ThVos, even when caffeine was present. We conclude from our results that in Purkinje "dominant" pacemaker fibers (i) diastolic depolarization initiates spontaneous activity by attaining the threshold for the upstroke of the action potential through the depolarizing phase of a ThVos; (ii) the depolarizing phase of ThVos is caused by a tetrodotoxin-sensitive Na+ component; (iii) ThVos is voltage dependent in that a small depolarization of the resting membrane induces it and a small hyperpolarization suppresses it; (iv) ThVos can induce overdrive excitation; and (v) ThVos occurs in the absence of calcium overload and has distinguishing characteristics from the Vos induced by calcium overload.