Sympathetic-parasympathetic interaction and accentuated antagonism in conscious dogs

The heart rate response to vagal stimulation and the interaction with sympathetic activity was evaluated in conscious dogs at rest and during exercise; the latter was used as a tool to physiologically elevate sympathetic activity. In 20 dogs with a healed myocardial infarction and in 7 healthy dogs a bipolar electrode was chronically implanted around the right cervical vagus. Vagal stimulation (3 ms; 2.1 +/- 0.7 mA; 2, 4, 6, 8, 10, 12 Hz) was performed while dogs stood on the treadmill (heart rate 120 +/- 25 beats/min) and while they exercised (201 +/- 17 beats/min). Gradual increases of the frequency of vagal stimulation gradually enhanced the inhibitory effect on heart rate both before and during exercise. During exercise, heart rate reduction was significantly greater than that produced at rest at any frequency of stimulation (P less than 0.001). This difference widened as the frequency of stimulation increased and the interaction with or without the presence of exercise was significant (P less than 0.02). Vagal stimulation produced similar effects in the seven dogs without myocardial infarction. These data demonstrate that the vagal-sympathetic "accentuated antagonism" described in anesthetized animals is also present in conscious dogs.