Additive effects of acetylcholine released by vagal nerve stimulation on atrial rate

In reserpinized dogs, the decrease in atrial rate (AR) as a function of the frequency of stimulation (supramaximal strength) of a single vagal nerve can be described satisfactorily by a rectangular hyperbola (RH). If the RH of one vagal nerve has been determined and if one point of the RH of the other vagal nerve is known, then the latter RH can be determined. Both RHs being known and assuming that the acetylcholine released by the right and left vagal nerves is additive, the slowing of AR by simultaneous stimulation of both nerves can be predicted. A minor systematic deviation from the predicted AR during simultaneous stimulation is usually found, which could be accounted for by a time dependent decrease of the effect of vagal nerve stimulation. The results are qualitatively similar during norepinephrine infusion. We propose that, although the vagal innervation may vary from pacemaker cell to pacemaker cell, the ratio of the activities of the two vagal nerves is constant throughout the sinoatrial node.