Interaction of adrenaline and acetylcholine on cardiac pacemaker function. Functional inhomogeneity of the rabbit sinus node.

In the isolated rabbit sinus node, adrenaline induced a pacemaker shift toward the inferior part of the node (distance, 2.5 mm). Acetylcholine gave a pacemaker shift either toward the inferior part of the node (distance, 1.2 mm) or, at higher acetylcholine concentrations, toward cells near the upper part of the crista terminalis. After severing the superior from the inferior part of the preparation, we compared the chronotropic responses in the two parts. The node appeared to be functionally inhomogeneous: in the inferior part the adrenaline-induced acceleration was larger and the acetylcholine-induced deceleration was smaller than in the superior part. In the total preparation and the isolated inferior part, the acceleration due to adrenaline was suppressed by acetylcholine and the deceleration due to acetylcholine was enhanced by adrenaline. Such an interference could not be established in the isolated superior part. The acetylcholine-induced pacemaker also dominated during the simultaneous addition of the two neurotransmitters. We conclude therefore that the functional inhomogeneity of the sinus node explains the predominance of the effect of acetylcholine over that of adrenaline.