Selective stimulation of the parasympathetic preganglionic nerve fibres in the excised and blood-perfused SA node preparation of the dog

Summary

Experiments were conducted on the canine SA node preparation cross-circulated with the arterial blood from the donor dog through the cannulated sinus node artery. The sinus rate of 98±4 beats/min persisted with a regular rhythm over 10 h. Subthreshold electrical stimulation to the myocardium was applied to various portions of the right atrium. Stimulation of the caval margin caused a simple deceleration of the sinus rate, and a portion slightly anterior to the mid-point between the two caval ostia was most sensitive. Stimulation at the portion where the sinoatrial node was supposed to be caused far less deceleration, while that at the atrial free wall failed to affect sinus rhythm. The negative chronotropic response to subthreshold electrical stimulation was abolished either by injection of hexamethonium or pempidine, or by that of tetrodotoxin or atropine into the sinus node artery, while physostigmine enhanced it. These results indicate that the deceleration response induced by subthreshold electrical stimulation is due to excitation of preganglionic fibres of the parasympathetic nerve. On the other hand, the propranolol-sensitive acceleration did not occur until myocardial excitation was induced by a stronger intensity of stimulation. This suggests that the sympathetic postganglionic fibres have a higher electrical threshold than the parasympathetic preganglionic fibres. These results contradict previous observations in excised atria perfused with physiological salt solution in which hypoxia of the peripheral neurone cannot be avoided.