AJP - Heart and Circulatory Physiology, Vol 242, Issue 4 677-H682, Copyright © 1982 by American Physiological Society
alpha- and beta-Adrenergic effects of epinephrine on ventricular pacemakers in dogs
A. J. Hordof, E. Rose, P. Danilo Jr and M. R. Rosen
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.
