Direct effects of catecholamines and tyramine on sinoatrial conduction in isolated and blood-perfused dog atria.

We have investigated the effects of catecholamines (dopamine, norepinephrine, epinephrine and isoproterenol) and tyramine on sinoatrial conduction time (SACT), sinus cycle length (SCL) and developed tension (DT) in isolated atrial muscle, using an isolated and blood-perfused dog atrial preparation, perfused with heparinized blood from the carotid artery of the anesthetized donor dog. Each substance was continuously administered intraarterially into the cannulated sinus node artery. They had dose-dependent positive chronotropic and inotropic effects. Each produced not only a shortening but a prolongation of SACT. In experiments in which only a shortening occurred, the order of the potency for inducing the shortening of the SACT was isoproterenol greater than norepinephrine = epinephrine greater than dopamine greater than tyramine. The ratio of doses required to produce roughly a 15-25% shortening of the SACT was 1: 10: 10: 100: 300, respectively. The isoproterenol-induced shortening of the SACT was inhibited by treatment with propranolol. Tyramine-induced shortening was inhibited by imipramine. From these results, it is suggested that sympathomimetic amines induce a shortening of SACT through adrenergic beta-receptors and also readily induce a pacemaker shift in the SA nodal area.