Effects of acetylcholine on time-dependent currents in sheep cardiac Purkinje fibers.

Voltage clamp experiments were carried out on sheep Purkinje fibers to determine the effect of Ach on the time-dependent currents. On the pacemaker current (iK2) Ach 10(-6) mol . l(-1) had the following effects: shift of the activation curve by a few mV in the depolarizing direction, without change in the rectifier ratio. The potential dependence of the time constants for activation and deactivation was influenced in a similar way as the activation curve. Ach had no effect on the positive dynamic current (iqr) of the late plateau outward current (ix). The slow inward current (isi) as well as the transient inward current (T.I.) were reduced in amplitude and slowed in time course by Ach. The changes in pacemaker current are important in explaining the increased rate of diastolic depolarization in the presence of Ach. The decrease of slow inward current by Ach cannot be made responsible for the plateau shift or the prolongation of the action potential.