Sympathetic tone affects human limb vascular resistance during a maximal metabolic stimulus

To evaluate the relationship between heightened sympathetic tone and maximal metabolic vasodilation, peak forearm blood flow (ml.min-1.100 ml-1) was measured plethysmographically in 18 volunteers after the release of 10 min of arterial occlusion (the peak reactive hyperemic blood flow response, RHBF) both before and after a stimulus to induce heightened sympathetic tone. The stimulus was the application of ice to the forehead for 90 s just before and during RHBF measurements. Mean arterial cuff blood pressure (MAP; mmHg) was calculated, and corresponding resistance (R; mmHg.ml-1.min.100 ml) was derived from blood pressure divided by RHBF. During ice application, blood pressure rose (pre 92 vs. post 115 ml/mmHg; P less than 0.05), peak RHBF was unchanged (pre 38.8 vs. post 36.4 ml.min-1.100 ml-1; not significant), but R during the maximal metabolic stimulus rose (pre 2.5 vs. post 3.2 mmHg.ml-1.min.100 ml; P less than 0.05). To examine the effects of heightened sympathetic tone on conduit vessels, simultaneous measurements of maximal metabolic blood flow (RHBF) and brachial artery Doppler velocity (V, cm/s) were conducted (n = 5) with and without ice applied to the forehead. Velocity rose by 70% as flow remained constant. Thus brachial artery area (area = flow/velocity) and diameter decreased substantially (20% decrease in diameter). The increase in R noted with ice was due to an alpha-mediated response, since the increase in R was blocked by oral prazosin (n = 6) and was unaffected by maneuvers to alter myogenic tone (n = 5). We conclude that maximal metabolic vasodilation can be counteracted to some extent by the effects of heightened sympathetic tone. Moreover, some of the interaction between these two opposing influences takes place at the arterial level.