Cardiac Effects of Vagal Stimulation in the Anaesthetized Cat

Abstract

Continuous stimulation of the right vagus nerve in anaesthetized animals can produce cardiac asystole, followed by a reinitiation of the heart beat. The latter is commonly referred to as " vagal escape " (Bard, 1956a). Although the mechanism of the escape phenomenon is unresolved, evidence has accumulated which indicates that adrenergic factors may be involved, possibly through a local release of catecholamines by acetylcholine (Campos & Freidman, 1963;Moore, 1967).To evaluate the local release hypothesis, the effects of beta adrenergic and postganglionic parasympathetic blockade on the chronotropic action of the vagus were determined in this study. In the course of our experiments, interesting data were obtained on the effect of stimulation frequency on the duration of vagally induced cardiac asystole and the intracardiac origin of the ensuing vagal escape. The cat was used in all experiments because the vagus and cervical sympathetic nerves can be easily separated in this species (Liddell & Sherrington, 1929). METHODSCats of either sex, weighing between 2 and 5 kg, were anaesthetized intraperitoneally with a-chloralose, 85 mg/kg, and placed on a heated cat board. A tracheotomy was performed, and the exposed part of the cannula was covered with non-conducting electrical tape. Arterial blood pressure was recorded from the right femoral artery with a cannula leading to a Statham P 23 Db transducer and Sanborn Model 296 recorder. The right femoral vein was cannulated for injection of drug solutions. The Lead II electrocardiogram was recorded from subcutaneous needle electrodes with a Sanborn Model 100 Viso-Cardiette recorder. The left vagosympathetic trunk was isolated and cut. The corresponding right trunk was isolated, the cervical sympathetic section was isolated and severed, and the vagus was tied with suture and crushed central to the tie. Throughout the experiments, the right vagus was kept warm (-37' C) and moist with Locke-Ringer solution.The vagus was stimulated with bipolar platinum electrodes leading from American Electronics Laboratory stimulus isolation and stimulator units. Supramaximal voltage was determined for each preparation by selecting the voltage required to produce a maximal slowing of the heart rate at a stimulation frequency of 5 c/s.

Effects of isoprenaline and aminophylline on the chronotropic responses of the isolated guinea‐pig heart to vagal stimulation and acetylcholine

“…There is considerable evidence of an interaction between sympathetic and parasympathetic systems on heart rate (Lynch & Essex, 1956; Leaders & Long, 1962; Leaders, 1963;Miller, Pendleton & Richmond, 1968;Vassale, Mandel & Holder, 1970), though others have questioned this (Misu & Kirpekar, 1968;Warner & Russell, 1969).…”