Effects of acetylcholine, vagal stimulation and tyramine on the isolated atria of the tortoise

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Abstract
In the vagus-atrial preparation of the tortoise (Amyda japonica), a cold-blooded animal, acetylcholine and vagal stimulation, in the presence of atropine, produced positive inotropic effects which were inhibited by dichloroisoprenaline or pronethalol. Hexamethonium blocked the excitatory effect of acetylcholine but only slightly inhibited that of vagal stimulation. Tyramine exerted a positive inotropic action in this preparation, but had little effect on atria prepared from reserpine-treated animals. Tyramine was shown to release a substance from the tortoise atria which caused contraction of aortic strips taken from reserpine-treated rabbits, and which appeared to be a catecholamine.