Cycle length dependence of the chronotropic effects of adrenaline and acetylcholine in the rabbit sinoatrial node.

Under the influence of adrenaline (Adr) and acetylcholine (Ach), pacemaker dominance in the sinoatrial node moves from one site to another. The Adr-center is located in the inferior part of the sinoatrial node. The Ach-center, an elongated cell group in the periphery of the sinoatrial node, is located nearer to the crista terminalis and is often more caudally located than the leading center under standard conditions. Under the combined influence of Ach and Adr, the Ach-center is dominant. The sinoatrial node preparation shows a cycle length-dependent chronotropic response to Adr. A greater shortening of cycle length is related to a longer basic cycle length. This agrees with previously reported findings concerning the positive chronotropic action of high calcium. The chronotropic response to Ach is not cycle length-dependent. In the presence of Ach, the effect of Adr remains cycle length dependent, although no shift of the pacemaker occurs. This demonstrates that cycle length dependence of chronotropic responses can be the property of one and the same cell group, and is not necessarily caused by different chronotropic responses of different cell groups.