Effect of neurotransmitters on the activation sequence of the isolated atrium.

Unipolar electrograms were recorded from 360 sites in 20 isolated, perfused canine right atria. In 52% of the preparations, spontaneous activation started in the sinus node region. In 32% of the preparations, spontaneous activation was initiated at the junction of the intercaval band and the crista terminalis. Simultaneous multicentric initiation occurred from both sites in 16% of the preparations. With infusion of incrementally increasing doses of norepinephrine, the superior sinus node site consistently assumed dominance. Infusion of incrementally increasing doses of acetylcholine produced no consistent pattern of pacemaker dominance. Propagation velocity during pacing was unaffected by either neurotransmitter. The results demonstrate the presence of two different pacemaker sites in the posterior right atrium that function spontaneously, devoid of any active or significant autonomic input. Exchange of dominance between the two sites occurs in the presence of norepinephrine and acetylcholine. The two sites appear to have a differential sensitivity to norepinephrine but are not differentially sensitive to acetylcholine. Changes in the pattern of activation seen with either neurotransmitters are mediated exclusively by the site of initiation and not by changes in conduction.