title: alpha- and beta-Adrenergic effects of epinephrine on ventricular pacemakers in dogs.
abstract: 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.
mesh_terms: D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D004285:Dogs; D004837:Epinephrine / Q000494:pharmacology*; D006321:Heart / Q000502:physiology* / Q000503:physiopathology; D006327:Heart Block / Q000503:physiopathology; D006339:Heart Rate / Q000187:drug effects; D006352:Heart Ventricles / Q000187:drug effects; D010646:Phentolamine / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D011941:Receptors, Adrenergic / Q000187:drug effects*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D011943:Receptors, Adrenergic, beta / Q000187:drug effects*
chemical_list: D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D011943:Receptors, Adrenergic, beta; D011433:Propranolol; D004837:Epinephrine; D010646:Phentolamine