Intrinsic innervation of the canine heart. Effects on conduction in the atrium, atrioventricular node, and proximal bundle branch.

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Abstract
The cardiac neural elements which survive surgical denervation of the heart were called the intrinsic innervation (ICN). These include postganglionic parasympathetic neurons and chromaffin cells. Specific activation of the ICN with nicotine (NIC) produces powerful negative inotropic and chronotropic effects on the heart. The influence of the ICN on dromotropic activity was assessed. Seven mongrel dogs successfully underwent surgical denervation of the heart. They were placed on cardiopulmonary bypass and conduction through the atrioventricular (AV) node and the right atrium was measured by an acrylic electrode plaque. Conduction through the right bundle branch (RBB) was measured using a 2nd recording electrode. Drugs administered were acetylcholine (ACh, 0.1-10.0 .mu.g); NIC (1-100 .mu.g); and tetrodotoxin (TTX, 30 .mu.g). All agents were injected by the intracoronary route. ACh and NIC caused significant increases in AV nodal conduction time, i.e., 5-49% and 11-49%, respectively. Intra-atrial, bundle branch and His-to-ventricular muscle conduction was affected inconsistently and the changes were nonsignificant. Responses to NIC, but not to ACh, were completely blocked by 30 .mu.g TTX. The effects of the ICN on ventricular conduction appear to parallel those of the parasympathetic innervation, i.e., profound influence on AV nodal conduction with little or no effect below this structure. The ICN does not appear to affect intra-atrial conduction, nor do there appear to be functionally important chromaffin cells that affect conduction time.