Abstract

The influence of hemicholinium No. 3 and/or vagus nerve stimulation on the acetylcholine content of rabbit atria was studied. It was established that stimulation of the vagus nerves up to complete disappearance of responses in animals injected with hemicholinium (1 mg/kg) results in a reduction of acetylcholine content to 33% of the control. Hemicholinium or vagal stimulation applied separately did not affect the atrial acetylcholine content. In the isolated atria hemicholinium (10 µg/ml) caused almost as marked decrease in acetylcholine content as was evoked by hemicholinium combined with vagal stimulation. The nerves were fully active at the time when the atrial acetylcholine content was depleted by hemicholinium. Their stimulation up to complete disappearance of response resulted in additional insignificant drop of atrial acetylcholine content. It is concluded that the content of acetylcholine of the intraatrial nerve fibres constitutes a small fraction of the total atrial content. The stores of extranervous acetylcholine, constituting up to 60% of the total content, may be released from the atrial muscle treated with hemicholinium under the influence of the nerve impulses. The same stores may be released from the active, isolated atria in which acetylcholine synthesis had been partially blocked by hemicholinium. No change was observed in the basic atrial activity after disappearance of the above fractions. The remaining, nonreleasable fraction is metabolically inactive or it is synthesized and stored at the sites not accessible to hemicholinium.