Positive and Negative Inotropic Responses of the Atria and Ventricles to Vagosympathetic Stimulation in the Isovolumic Canine Heart

Abstract

The responses of the paced, isovolumic atria and ventricles to cervical vagosympathetic stimulation were examined in eight normal and seven sympathectomized dogs under chloralose anesthesia. Before atropine, stimulation usually caused decreases in both atrial and ventricular contractility but with significant quantitative and qualitative variation from animal to animal. The right atrium showed the greatest average decrease (>30%), followed by the left atrium (12%), right ventricle (4% to 12%), and the left ventricle (1% to 10%). After atropine, 0.5 mg/kg, all chambers responded positively to stimulation. The right ventricle was most responsive (about 30%), and the other chambers showed increases of 10% to 20%. These positive inotropic responses were eliminated by propranolol, 0.5 mg/kg, or by pentolinium tartrate, 0.5 mg/kg. In the sympathectomized animals, negative inotropic responses were usually greater and positive responses were reduced or eliminated. The data demonstrate that fibers exist in the cervical vagosympathetic trunks which have both negative and positive inotropic effects on all chambers of the heart. The net effect of vagosympathetic stimulation on a chamber's contractility reflects the relative numbers of the two fiber types which supply it. The fibers mediating the positive effects are probably preganglionic with significant postganglionic stations in the caudal cervical and stellate ganglia and perhaps in the heart itself.