Evidence for sympatholysis at the onset of forearm exercise.

The effect of augmented sympathetic outflow on forearm vascular conductance after single handgrip contractions of graded intensity was examined to determine whether sympatholysis occurs early in exercise (n = 7). While supine, subjects performed contractions that were 1 s in duration and 15, 30, and 60% of maximal voluntary contraction (MVC) in intensity. The contractions were repeated during control and lower body negative pressure (LBNP) (-40 mmHg) sessions. Forearm blood flow (FBF; Doppler ultrasound) and mean arterial pressure were measured continuously for 30 s before and 60 s after the single contractions. Vascular conductance (VC) was calculated. Total postcontraction blood flow increased in an exercise intensity-dependent manner. Compared with control, LBNP caused a reduction in baseline and postexercise FBF (P < 0.05), VC (P < 0.01), as well as total excess flow (P < 0.01). Specifically, during LBNP, baseline FBF and VC were reduced by 29 and 34% of control, respectively (P < 0.05). After the 15% MVC contraction, peak VC during LBNP was reduced by a magnitude similar to that during baseline (i.e., ~30%), but it was only reduced by 15% during both the 30 and 60% MVC trials (P < 0.01). It was concluded that the stimuli for exercise hyperemia during moderate and heavy, but not mild, handgrip exercise intensities, diminish the vasoconstrictor effects of LBNP. Furthermore, these data demonstrate that this sympatholysis occurs early in exercise.