Antagonism of Inotropic Responses of Frog Heart to Epinephrine and Acetylcholine.

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Abstract
In the isolated frog heart balanced responses to mixtures of epinephrine and acetylcholine were converted to positive inotropic responses by atropine, hexamethonium and tetraethylammonium. These drugs specifically and competitively antagonized acetylcholine depression when present with the latter. Prior perfusion with them as well as with diethylaminoethanol effectively blocked the positive inotropic action of epinephrine. This antagonism is believed to be non-specific and related to changes which raise the threshold to epinephrine.