Sympathetic modulation of blood flow and O₂ uptake in rhythmically contracting human forearm muscles

This study tested the effects of sympathetically mediated changes in blood flow to active muscles on muscle O₂ uptake (V̇O₂) in humans. Four minutes of graded (15-80% of maximum voluntary contraction, MVC) rhythmic handgrip exercise were performed. Forearm blood flow (FBF) (plethysmography) and deep vein O₂ saturation were measured each minute. Forearm O₂ uptake was calculated using the Fick principle. In protocol 1, exercise was performed while supine and again while upright to augment sympathetic outflow to the active muscles. Standing reduced FBF at rest from 3.6 to 2.2 ml · 100 ml^-1 · min^-1 (P < 0.05). During light exercise (15-40% MVC) FBF was unaffected by body position. Standing reduced FBF (P < 0.05) from 36.0 to 25.2 ml · 100 ml^-1 · min^-1 and forearm V̇O₂ from 38.2 to 28.1 ml · kg^-1 · min^{- 1} during the final work load. In protocol 2, exercise was performed while supine before and after local anesthetic block of the sympathetic nerves to the forearm. Sympathetic block increased FBF at rest from 3.1 to 8.9 ml · 100 ml^-1 · min^-1 (P < 0.05), and FBF was higher during all work loads. At 70-80% of MVC sympathetic block increased FBF from 35.4 to 50.7 ml · 100 ml^-1 · min^-1 (P < 0.05); and forearm V̇O₂ from 45.5 to 54.2 ml · kg^{- 1} · min^-1 (P < 0.05). These results suggest that in humans sympathetic nerves modulate blood flow to active muscles during light and heavy rhythmic exercise and that this restraint of flow can limit O₂ uptake in muscles performing heavy rhythmic exercise.