[Modification of inotropic vagus action by stretching of the heart atrium]

Isolated vagal innervated left rabbit atria were used to study the dependence of inotropic vagal action on the stretching of the auricle myocardium. Pressure-volume relationships, relationships between isovolumetric maxima, inotropic vagal action and filling of the auricle were made on the isolated, vagal-innervated auricles electrically driven by a constant frequency. Pressure-volume relationships and relationships of the isovolumetric maxima are described by means of a model, likewise the dependence of extra-pressure and extra-tension on the volume of the auricle, that is, on the filling pressure. The extra-pressure, derived from the auricle, is at its maximum when a volume of 152% of the volume at a filling pressure of 1 mm Hg is reached. When increasing the filling pressure, both the inotropic action of the vagal stimulation and the inotropic effect of an acetylcholine application decrease. Maximum vagal and acetylcholine action was obtained at filling pressures of 12-16 mm Hg. At a filling pressure of 38 mm Hg only 40% of the maximum vagal action was obtained.