Abstract
1. Effects of acetylcholine and adrenaline on the aconitine-inducedfibrillation were studied with microelectrodes on the right atrium isolated from young rabbits.
2. In the proper atrial muscle, acetylcholine (10-5-10-4) inhibiteddramatically the fibrillation and restored well synchronized and rhythmicaldischarges of equal magnitude.
3. In the sinoatrial node, acetylcholine also showed an inhibitory action on the irregular activity, producing an increase in resting potential, a decrease in slope of diastolic depolarization and a consequent decrease in pace-maker rhythm.
4. Contrarily, atropine (10-7) was found to facilitate the aconitine-induced atrial fibrillation. Thus, by repeating acetylcholine and atropine infusions alternately, the fibrillation was possible to start or stop several times.
5. Adrenaline (10-6) promoted the aconitine-induced fibrillation, inconcequence of an acceleration of pace-maker rhythm, frequent shift in pace-maker site and/or a production of two or more ectopic pace-makers. In the proper atrial muscle effect of adrenaline was not impressive, although it enhanced the rate of discharge and its irregularity.
6. Conflicting observations on effects of acetylcholine or adrenaline specially between the two kinds of fibrillation induced one by aconitine and the other by acetylcholine plus electrical stimulation were discussed, and dual sites of action of each drug were pointed out.
Finally, it was estimated that the cause of fibrillation in primary importance is a facilitation of sodium conductance which elicites an acceleration of the pace-maker potential.
