Cardiostimulatory responses to vagal stimulation, nicotine, and tyramine

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Abstract
Isolated atria from control and sympathectomized cats were subjected to vagal stimulation and to administration of nicotine or tyramine. The catecholamine content of the atria, left ventricle, and right ventricle was determined using the trihydroxyindole method. Levels of catecholamines in the sympathectomized tissues were found to be less than 1/15 those of control tissues. Vagal stimula- tion resulted in a cardioinhibitory effect followed by a cardiostimulatory effect in both control and sympathectomized atria. No statistical difference in the cardioinhibitory or cardiostimulatory effects of vagal stimulation could be demonstrated between control atria and sympathectomized atria. In the presence of atropine, nicotine or tyramine exerted a cardiostimulatory effect on control atria which was greatly reduced in atria from sympathectomized animals. It is suggested that the vagus nerve innervating cat atria may not be a mixed nerve. The cardiostimulatory response to vagal stimulation may be due to catecholamines released from an extraneural store. The cardiostimulatory response to nicotine or tyramine may be due to catecholamine release from both extraneural and intraneural stores.

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