Responsiveness of intracardiac neurons to cholinergic stimulation in normal and thyroidectomized dogs

Hypothyroidism alters the responsiveness of sympathetically innervated structures. The present work was done to determine if the responsiveness of the intrinsic cardiac nerves (ICN) to nicotine is also affected by thyroidectomy (THX). Mongrel dogs were anesthetized and an electrode catheter was advanced into the right atrium for recording His bundle activity (HB). A second cannula was placed into the carotid artery with its tip near the coronary ostia, so that the responses to the drugs injected would be confined to the heart. Changes in the A-H interval of the HB were recorded in response to both acetylcholine (ACH) 2-50 ug, and nicotine HCl (NIC) 5-100 ug, before and after beta blockade with metoprolol (1-4 mg). There were no important differences between the two groups in the responses of A-V nodal conduction to ACH over the dose range used. However, the THX animals showed an apparent increase in responsiveness to NIC; ie., ca. 5% decrease in A-H at 10 ug (control) compared to ca. 30% increase after THX. The apparent catecholamine-releasing effect of NIC was also enhanced in the THX animals. In addition, the responses to ACH and NIC were compared in animals both 'on' and 'off' cardiac bypass. The data suggest that hypothyroidism increases the effectiveness of nicotinic stimulation of intracardiac neurons.