Suppression of the Cardiac Performance during Recovery after Strenuous Sympathetic Nerve Activity

To examine whether myocardial contractility is suppressed during recovery from strenuous sympathetic nerve activity, the cardiac sympathetic nerve was stimulated for 1 min at 20 Hz in open-chest anesthetized dogs (n = 6). Myocardial contractility was compared between stimulation at 20 Hz (strenuous) and stimulation at 2 Hz (physiological range). Inotropic activity was assessed as the maximal value of the first derivative of left ventricular pressure. The inotropic activity was significantly below the prestimulation control level at 5 min after the end of the strenuous stimulation (68+/-7% of the control value; p < 0.05). The inotropic activity then increased toward the prestimulation level. In contrast, the inotropic activity with 2-Hz stimulation decreased toward the prestimulation level but never fell below that level during recovery. In another experiment (n = 10), 50% glucose solution was injected 5 min after the end of the strenuous stimulation (20 Hz), i.e. when the inotropic activity reached the nadir. The inotropic activity increased significantly (p < 0.01) above the prestimulation level between 7 and 20 min after stimulation. Maximal response (121+/-7% of the control value) was observed at approximately 10 min. These results suggest that transient myocardial suppression occurs after strenuous sympathetic nerve activity and that high-energy phosphate is substantially consumed during strenuous sympathetic activity, resulting eventually in myocardial suppression.