The effects of acetylcholine on the electrical activity of canine cardiac Purkinje fibers.

We studied the effects of acetylcholine (ACh) on small bundles of canine cardiac Purkinje fibers exposed to normal, or low-chloride (isethionate) Tyrode's solution in a rapid superfusion system. In superfusate containing 4 mM K+, the resting potential of Purkinje fibers may be either "low," near -40 mV, or "high," near -90 mV. ACh, at 10(-6) to 10 (-5) M, increased the membrane potential from both the low and high resting levels and, in low-Cl solution, often induced a maintained shift in potential from the low to the high level. The increase in membrane potential caused by ACh was greater at the low than at the high level. ACh, at 10(-6) to 10(-5) M, reduced action potential duration in both normal and low-Cl Tyrode's solution, the effect being more marked in the latter. These effects of ACh were reversibly abolished by atropine (5 X 10(-5) M), indicating that they were mediated via muscarinic ACh receptors, and they probably result from an increase in membrane K+ conductance since 10(-5) M ACh reversibly reduced, by 13% on the average, the amplitudes of the steady changes in membrane potential evoked by applying small current pulses (-5 to -25 nA, 200 msec). ACh (10(-5) M) also diminished the rate of, or stopped, spontaneous activity arising from either level of membrane potential. The cessation of spontaneous slow response activity, arising from the low level, sometimes was accompanied by a maintained shift of the membrane potential to the high resting level. It is concluded that the action of ACh on Purkinje fibers is qualitatively similar to its action on sinoatrial nodal and atrial cells.