Effects of catecholamines on cardiac chronotropic response to vagal stimulation in the dog

The influence of isoproterenol, norepinephrine, and dopamine on the cardiomoderator effects of moderate vagal stimulation was studied in anesthetized dogs. The drugs were administered at increasing doses in successive perfusions. Stimulation of the vagus nerve, the parameters of which remained constant throughout each experiment, was performed immediately before each sequence of perfusion and after 10-min perfusion. Isoproterenol at 0.025, 0.05, 0.1, and 0.2 microgram X kg-1 X min-1 raised heart rate dose relatedly but did not alter heart rate under vagal stimulation. Thus the amplitude of vagal bradycardic effects increased dose relatedly. Norepinephrine at 0.125, 0.25, 0.5, and 1 microgram X kg-1 X min-1 lowered heart rate through reflex hypertension. Heart rate under vagal stimulation remained constant. Thus the effects of vagal stimulation decreased as dose increased, finally becoming null. Dopamine at 0.5, 1, 2.5, and 5 micrograms X kg-1 X min-1 did not significantly alter heart rate, but at 10 and 20 micrograms X kg-1 X min-1, like norepinephrine, it raised blood pressure, causing a reflex fall in heart rate. At all doses, heart rate under vagal stimulation remained stable. Consequently, at the highest doses, the net effects of vagal stimulation were slight. These results suggest the simultaneous involvement of sympathetic-parasympathetic interactions both post- and prejunctionally. In the latter case, different mechanisms of regulation of neurotransmitter release are involved during vagal stimulation according to the sympathomimetic used. With isoproterenol, norepinephrine release seems more particularly affected, whereas with norepinephrine and dopamine, acetylcholine release is apparently inhibited.