Disappearance of norepinephrine from the circulation following strenuous exercise.

Five subjects who had been training vigorously for 2 mo worked 5 min on the cycle ergometer at 1,480 +/- 100 kpm/min and at 1,920 kpm/min on separate days. Plasma norepinephrine (NE), blood lactate, and heart rate were determined during the postexercise periods. The same subjects were also tested at 1,480 +/- 100 kpm/min after a 2-mo detraining period, when subjects did not train. At 1,920 +/- 80 kpm/min, plasma NE was 4 ng/ml at the end of exercise. The NE concentration increased slightly in the first min postexercise, then decreased with a half-time of 2.8 min between 1 and 11 min postexercise. Detraining for 2 mo did not markedly affect the rate of disappearance of NE in the postexercise period in these five subjects. Between 11 and 30 min postexercise (1,480 +/- 100 kpm/min), heart rate was 25 beats/min higher after detraining, while NE was not significantly different. These data provide evidence that more rapid recovery of heart rate in trained individuals during this period is not likely due to a more rapid recovery from the sympathetic response to exercise.