Chronotropic response to acetylcholine injected into the sinus node artery of the isolated atrium of the dog.

Injection of a single bolus of acetylcholine into the sinus node artery of the isolated atrium of the dog produces a triphasic response similar to that previously observed after a single stimulus to the vagus nerves. There is an initial, brief but pronounced, cardiac deceleration followed by a brief, slight acceleration, which is then followed by a more prolonged deceleration which is of lesser magnitude than the initial deceleration. The most likely cause of the intermediate phase of cardiac acceleration is a direct effect of acetylcholine on the pacemaker cells, as it is also with the primary and tertiary phases of cardiac deceleration.