Selective stimulation of intracardiac pre-ganglionic vagal fibres of the dog atrium.

1. Direct electrical stimulation (0.05 ms, 5.0 V and 30 Hz) caused negative chronotropic and inotropic effects on the blood-perfused, isolated dog atrium. 2. These negative effects of electrical stimulation were significantly blocked not only by treatment with atropine or tetrodotoxin, but also by hexamethonium, although hexamethonium did not modify the effects of injected acetylcholine. 3. These results suggest that direct electrical stimulation on the isolated, blood-perfused dog atrium readily causes a release of acetylcholine by excitation of parasympathetic pre-ganglionic fibers.