Catecholamines and cardiac rhythms in the unanesthetized dog with chronic AV block

The effects of epinephrine, norepinephrine, and isoproterenol were studied in the nonanesthetized dog with chronic atrioventricular block before and after blockade of cholinergic receptors. Epinephrine and norepinephrine reduced the atrial rate in a reflex manner, whereas isoproterenol increased it. In contrast, all three substances increased idioventricular rate. After atropine, atrial and ventricular rate increases were observed after administration of the three catecholamines. Moreover, blockade of cholinergic receptors led to an increase in ventricular acceleration induced by epinephrine and norepinephrine, but did not significantly modify the acceleration induced by isoproterenol. These results appear to indicate the possibility of the involvement (intensified by a reflex mechanism) of ventricular cholinergic innervation.