Membrane currents, contractions, and aftercontractions in cardiac Purkinje fibers

We examined relationships between isometric tension and membrane currents in sheep Purkinje fibers voltage clamped by the two-microelectrode method. Oscillatory restitution of contractility was accompanied by a small oscillation in membrane current and by an aftercontraction. The membrane current oscillation resembled the transient inward current (TI) others have reported in the presence of strophanthidin. Twitches produced by voltage clamp depolarizations did not correlate with net outward current in normal solution, but when the early outward current was blocked by 0.5 mM 4-aminopyridine, the residual outward current did correlate with twitches elicited by strong depolarizing clamps, particularly in solutions containing higher than normal calcium concentrations. The results illustrate important similarities and differences between membrane current behavior in sheep Purkinje fibers and behavior others have reported in calf fibers. Correlations between restitution, aftercontractions, and TI's, and between twitch tension and a component of outward current, may arise because of calcium regulation of membrane conductance, electrogenic Na-Ca exchange, or a combination of these and other mechanisms.