The vagus nerves exert a direct negative inotropic effect on the ventricular myocardium in a canine model.
In an innervated, paced, left heart preparation in which the left ventricle contracted against a constant volume of incompressible fluid (isovolumetric preparation), electrical stimulation of the distal end of either sectioned cervical vagus nerve decreased the peak tension generated by the teft ventricle. The vagi, therefore, exert a negative inotropic effect directly upon the ventricular myocardium.
Degeest et al. (Fri,) studied this question.