EFFECTS OF EXERCISE INTENSITY AND DURATION ON NOREPINEPHRINE SPILLOVER AND CLEARANCE IN HUMANS

During dynamic exercise, blood flow to exercising muscle is closely ma tched to metabolic demands. This is made possible by metabolic vasodil ation, vasoconstriction in inactive vascular beds, and a rise in cardi ac output. The sympathetic nervous system plays an important role in r egulating this exercise response. In this study, we used steady-state infusions of tritiated norepinephrine ([H-3]NE) to determine the magni tude and time course of the arterial NE spillover response to sustaine d upright bicycle exercise at low (n = 11) and moderate-to-high (n = 1 4) exercise intensity (25 and 65% of maximum work load, respectively) in normal young subjects. In addition, we sought to examine whether ex ercise was associated with a change in NE clearance. During 30 min of low-level exercise, arterial NE spillover increased from 1.45 +/- 0.13 to 3.14 +/- 0.30 nmol . min-1 . m-2 (P < 0.01) and appeared to platea u at 20-30 min of exercise; NE clearance remained unchanged. During 20 min of moderate-to-high-intensity exercise, we found a substantial an d progressive rise of arterial NE spillover from 2.15 +/- 0.27 to 13.5 2 +/- 1.62 nmol . min-1 . m-2 (P < 0.01). NE clearance decreased from 0.91 +/- 0.05 to 0.80 +/- 0.051 . min-1 . m-2 (P < 0.05). These data s uggest that, during dynamic exercise, sympathetic nervous system activ ity is related to exercise intensity, and there appears to be an inter action between the effects of exercise intensity and duration on NE sp illover. In addition, at moderate-to-high exercise intensity, a small decrease of NE clearance contributes to the rise in plasma NE.