Studies on the action of acetylcholine on the sino‐auricular node of the dog

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Abstract
1 . The response of the sino-auricular node to direct injection of acetylcholine in the artery supplying the node has been studied in the dog. 2 . Acetylcholine in graded doses (2, 5, 10 μg) caused acceleration of the sinus rhythm. 3 . The time course of the positive chronotropic action of acetylcholine resembled that produced by adrenaline or noradrenaline. 4 . The positive chronotropic action produced by acetylcholine was blocked by dichloro-isopropylnoradrenaline and pronethalol and potentiated by cocaine. 5 . In reserpinized dogs, acetylcholine in similar doses produced sinus bradycardia. The positive chronotropic response could be temporarily restored by infusion of adrenaline or noradrenaline. 6 . Atropine blocked both the positive and negative chronotropic actions of acetylcholine.