Plasma norepinephrine and muscle sympathetic discharge during rhythmic exercise in humans

The purpose of this study was to examine the relationship between plasma norepinephrine concentrations (PNE) and efferent muscle sympathetic nerve activity to noncontracting muscle (MSNA) during graded, rhythmic exercise in humans. In the initial study, six healthy men (ages 20-30 yr) performed 2-min bouts of two-arm cycling exercise at power outputs of 0, 10, 20, 40, 60 (n = 6), and 80 (n = 3) W. Heart rate (HR) was recorded and intraneural measurements of MSNA (right peroneal nerve) were made continuously for 2 min before (control) and during exercise at each work load. At least 2 wk later, subjects performed the same exercise bouts at which time HR was measured and a venous (forearm) blood sample was obtained for the subsequent determination of PNE by high-performance liquid chromatography. During exercise, HR increased progressively from 0 to 80 W. Neither MSNA nor PNE increased above control in response to arm cycling at 0, 10, and 20 W [0-16 +/- 1% (SE) of peak work load], but both variables increased progressively at the 40-, 60-, and 80-W (33 +/- 1 to 67 +/- 2% of peak work load) levels (all P less than 0.05). The individual MSNA and PNE responses (% change from control) over the six work loads were directly related (r = 0.80, P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)