Effects of Increased Reflex Sympathetic Activity on Contractile Force of the Heart

Reflexly induced changes in cardiac contractile force and arterial pressure in vagotomized dogs were compared with those produced by direct postganglionic cardiac sympathetic nerve stimulation, by i.v. injections of l-norepinephrine and by elevation of the intracranial pressure. Bilateral carotid occlusion and central vagal and central sciatic nerve stimulation produced a marked rise in arterial pressure but only a moderate increase in right ventricular contractile force. In contrast, direct stimulation of the cardiac sympathetic nerves and selected doses of l-norepinephrine caused a marked increase in right ventricular contractile force with only moderate elevation of blood pressure. Further, reflex stimulation resulted in a moderate dilation of the left ventricle in contrast to a reduction in size produced by l-norepinephrine. Elevation of the intracranial pressure provoked marked increases in both blood pressure and right ventricular contractile force. Experiments with vagotomized cats gave comparable results. The moderate positive inotropic effect of carotid occlusion in dogs was mediated both through the cardiac sympathetic nerves and the adrenal glands since removal of either structure alone reduced the response while combined adrenalectomy and cardiac sympathectomy abolished the effect. These results indicate that the predominant effect of reflexly augmented sympathetic activity through the carotid sinus, vagus and sciatic nerves is on vasomotor tone with only moderate influence on cardiac contractile force.