Autonomic control of pacemaker activity in the atrioventricular junction of the dog

A stable atrioventricular (AV) junctional rhythm was induced in open-chest, anesthetized dogs by injecting pentobarbital into the sinus node artery. A factorial experimental design was used to quantify the changes in AV junctional rate as a function of the frequency of cardiac sympathetic and parasympathetic stimulation. The AV junctional pacemaker cells were more responsive to autonomic neural stimulation, but the vagal-sympathetic interactions were less pronounced than had previously been observed for the SA nodal pacemaker cells. In a group of seven animals, sympathetic stimulation at a frequency of 1.4 Hz increased the AV junctional rate by 102% from a control rate of 54 beats/min. In the same animals, vagal stimulation at a frequency of 8.4 Hz reduced the AV junctional rate by 56%. In three other animals, the AV junction was even more responsive; equivalent chronotropic effects were achieved with stimulation frequencies that were only about one-third of those cited above. There was a moderate, but significant, autonomic interaction: in the group of seven animals, the positive chronotropic effect of sympathetic stimulation at 1.4 Hz was 72% greater at the low level (0 Hz) than at the high level (8.4 Hz) of vagal activity.