Changes by Acetylcholine of Membrane Currents in Rabbit Cardiac Purkinje Fibres

The ionic mechanisms underlying the effects of acetylcholine (ACh) on electrophysiological properties of rabbit cardiac Purkinje fibres have been analysed using the two-micro-electrode voltage-clamp technique on short preparations. In normal Tyrode solution, ACh shifted the membrane currents in the outward direction at potentials positive to the K+ equilibrium potential, EK, and in the inward direction at potentials negative to EK. When ACh effects were studied in various [K+]o the reversal potential for the ACh-induced current was changed in a way expected for a pure K+ electrode. The results indicate an increase of an inward-rectifying K+ conductance by ACh. ACh also decreased the magnitude of the late outward current (ix). The activation curve for this current was not modified, but the fully activated current was decreased. The effect of ACh on ix was more pronounced in the presence of catecholamines. The slow inward current (isi) was not changed by ACh in control conditions. However, the increase produced by beta-adrenergic stimulation in this current was suppressed by ACh in a concentration-dependent way.