Excessive Sympathetic Nervous System Activity Decreases Myocardial Contractility

The objective of this study was to determine whether myocardial contractility is depressed by intense activation of the sympathetic nervous system. A massive sympathetic discharge was produced by injecting veratrine or sodium citrate into the cisterna magna of anesthetized rabbits (n = 10). Two and one-half hr later, the hearts were isolated and their left ventricular (LV) performance evaluated and compared with the LV performance of hearts isolated from control animals (n = 10). LV performance was evaluated from steady-state peak isovolumic systolic and end-diastolic pressures that were generated at various end-diastolic volumes (LV function curves). The relationship between peak LV systolic pressure (or the average peak developed LV wall stress) and LV end-diastolic volume was rotated downward (P less than 0.01) in the hearts removed from rabbits treated with veratrine or citrate. The LV end-diastolic pressure or LV end-diastolic wall stress of these hearts was not different from control at any end-diastolic volume. The diminished ability of the experimental hearts to develop systolic pressure or wall stress suggests that intense sympathetic activation depressed contractility. Severely damaged myofibers, located largely in the subendocardium, were found in these hearts. Furthermore, the depressed contractility was not related to pulmonary edema since only 2 of 10 rabbits developed edema.