Influence of vagal stimulation on ventricular contractility, O2 consumption, and coronary flow

Positive and negative inotropic responses of the atria and ventricles to vagosympathetic stimulation in the isovolumic canine heart

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.

Effects of frequency of contraction and ionic environment on the responses of heart muscle to acetylcholine

The effects of frequency of contraction and of external calcium and sodium concentration on the inotropic actions of acetylcholine (ACh) on isolated mammalian myocardium were examined. In stimulated left atria of cats, ACh reduced tension development, i.e. exerted a negative inotropic effect, which became more striking as the frequency of contraction was increased and was inhibited by atropine. In cat papillary muscles studied at a low frequency (<42/min), high concentrations of ACh augmented tension development, i.e. produced a positive inotropic effect, which was enhanced by atropine. At high frequencies (>72/min) this positive effect of ACh was abolished. Moreover, at high frequencies, ACh consistently further reduced the normal increment in tension associated with increasing frequency. The magnitude of the positive inotropic action of ACh on papillary muscle was abolished by lowering the extracellular [Na + ] and varied inversely with the extracellular [Ca 2+ ]. These findings were interpreted in terms of a dual action of ACh on the myocardium: (1) an action on the muscarinic receptor that reduces contractility and may be blocked with atropine, and (2) a direct effect on cellular membrane permeability to Ca 2+ that stimulates contractility and may be reduced by prior elevation of intracytoplasmic [Ca 2+ ].

Effect of decreasing the force of right atrial contraction on right ventricular stroke volume

The effect of reducing the force of right atrial contraction on right ventricular stroke volume was determined in anesthetized open-chest dogs. Atrial contractile force, measured directly with a strain-gauge arch, was decreased by electrical stimulation of the right vagus nerve; right ventricular stroke volume was measured with an electro-magnetic flowmeter. Heart rate was maintained constant by electrical stimulation of the right atrium.

Vagal stimulation decreased atrial contractile force by an average of 47±2.6 (SEM)% of the control, and stroke volume fell by 4.0±0.9%. During vagal stimulation, no change was observed in right ventricular contractile force. In addition, no consistent change was noted in mean right atrial or right ventricular pressures. The relation between the decrease in atrial force and stroke volume was not altered significantly by the rate of right atrial pacing. In 7 of 10 experiments, increasing the inhibition of atrial force produced a further decrease in stroke volume.

Since neural, mechanical, and humoral factors modify atrial contractile force in the intact animal, the present study provides more definitive evidence that the ability to alter atrial force represents another adaptive mechanism in the regulation of cardiac performance.