alpha- and beta-Adrenergic effects of epinephrine on ventricular pacemakers in dogs.
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We studied the effects of epinephrine on idioventricular rhythm in 15 adult dogs with chronic complete heart block induced by the injection of formalin into the His bundle. Atropine (0.1 mg/kg) was given intravenously to attenuate any potential vagal effects, and epinephrine was infused in graded doses of 0.01-10.0 micrograms.kg-1.min-1. Two different responses were seen. In 12 dogs there was a concentration-dependent increase in ventricular rate following epinephrine infusion. These animals then were given the beta-blocker propranolol (0.5 mg/kg iv), and the epinephrine infusions were repeated. In this situation epinephrine concentrations less than 0.1 micrograms.kg-1.min-1 induced a decrease in ventricular rate with no associated change in arterial pressure. In four additional dogs this decrease in ventricular rate was prevented by alpha-blockade with phentolamine. In three dogs epinephrine less than 0.1 micrograms.kg-1.min-1 induced a decrease in ventricular rate without an associated change in arterial blood pressure. This decrease in rate was abolished by the alpha-blocker phentolamine. It therefore appears that an alpha-adrenergic effect on ventricular automaticity can occur in the intact animals. When this does not occur initially, it can be unmasked by propranolol and results in a slowing of ventricular rate unrelated to changes in blood pressure.