A Depressant Effect of Acetylcholine on the Idio-ventricular Pacemaker of the Isolated Perfused Rabbit Heart

In Langendorff preparations of the isolated rabbit heart, excision of the atria and severance of the atrio-ventricular bundle resulted in the onset of idio-ventricular rhythm at a rate much below the original sino-atrial rate. The mean decrease in rate in twelve experiments was 61%. Acetylcholine perfusion decreased the idio-ventricular rate still further. Physostigmine augmented this effect while atropine prevented or abolished it. Similar results were obtained with a rat heart. The results indicate that a cholinergic receptor mechanism is present at pacemaker sites in mammalian ventricles. However, when rabbit ventricles, beating under atrio-ventricular nodal rhythm or idio-ventricular rhythm, were cooled to the point of cardiac arrest, acetylcholine failed to cause reappearance of the cardiac beat.