Selective activation of nicotinic and muscarinic transmission in cardiac sympathetic ganglion of the dog.

The myocardial contractile responses to electrical stimulation of different segments of the thoracic sympathetic trunk in the pentobarbital-anesthetized dog were studied. Electrical stimulation of the sympathetic trunk between the second and third thoracic ramus and caudal limb of the ansa subclavia elicited myocardial contractile responses which were not completely antagonized by the ganglionic blocking agent chlorisondamine. Atropine in addition to chlone had no effect. When the sympathetic trunk between the third and fourth ramus was electrically stimulated, the response was partially antagonized by atropine as well as by chlorisondamine alone. From these results, it is suggested that impulses travelling via the third thoracic ramus or above it mediate myocardial contractile responses in which the nicotinic type response predominates whereaas impulses travelling via the fourth thoracic ramus or below it mediate myocardial contractile responses in which the muscarinic type predominates. additionally, evidence presented indicates that a major proportion of both nicotinic and muscarinic receptors mediating myocardial contractile changes are located in the caudal cervical ganglion rather than the stellate ganglion of the dog.