[The development of local nonexcitability in sinus node cells and other cells with diastolic depolarization in the right atrium of the rabbit under the action of acetylcholine].

The sinus node is heterogeneous structure. In the intact sinus node acetylcholine induced shift of pacemaker from the center of sinus node suggesting higher sensitivity of typical pacemaker cells to acetylcholine. The aim of this study was to compare the acetylcholine (10(-5), 10(-4) M) action on different cell types in the intercaval region as well as in the sinus node. The rabbit right atrium, with the sinus node region intact, was pinned in a tissue bath. One or two microelectrodes were impaled in the sinus node and atrial electrogram was obtained simultaneously. During superfusion with acetylcholine though the preparations retained regular rhythm some of the cells with diastolic depolarization became inexcitable, their amplitude fell below 30% of the control value while others did not. In pacemaker cells (upstroke velocity < 5 V/s; smooth transition from diastolic depolarization into upstroke) occurrence of inexcitability was the highest: 36 of 40 cells (90%). With increasing upstroke velocity, the occurrence of inexcitability decreased. Effects of acetylcholine were completely reversible and were blocked by pretreatment with atropine 10(-7) M. It may be suggested that particular sensitivity of typical pacemaker cells to acetylcholine could arise from two facts: 1. In typical pacemaker cells calcium current plays the main role in action potential upstroke; 2. The calcium current in the sinus node cells is particular sensitive to acetylcholine.