ABSTRACT
1. The effects of acetylcholine (ACh) on cardiac rate (electrocardiogram), contractile force and coronary flow recorded simultaneously were investigated in isolated guinea pig hearts perfused with Locke solution by the method of Langendorff. 2. Bolus injections of 0.5-550 nmol ACh induced oscillations of cardiac rate. These changes were not directly related to the doses of ACh injected (X**2 test, P > 0.05). 3. The presence of 10 micronM physostigmine in the Locke solution increased the number of heart rate oscillations elicited by ACh. 4. The electrocardiogram showed that the heart rate oscillations were due to wandering pacemakers, such as slow or fast junctional rhythm, and slow or fast indioventricular rhythmy, which were intermingled with sinus rhythym, A - V block or sinus bradycardia. 5. In most experiments, the increase in ventricular rate was associated with an increase in ventricular contractile force ("Bowditch Effect") and a simultaneous reduction of coronary flow. 6. The heart rate oscillations were not prevented bu reserpine or blockade of nicotinic receptors (hexamethonium plus gallamine) but were prevented blockade of muscarinic receptors with atropine. 7. We conclude that the heart rate oscillations induced by ACh are due to several electrophysiological mechanisms (automatism and/or conduction disturbances_ related to activation of muscarinic receptors