Ischemic muscle chemoreflex response elevates blood flow in nonischemic exercising human forearm muscle.

We tested the hypothesis that forearm blood flow (FBF) might be reduced during forearm exercise when a vasoconstrictor response was evoked by calf exercise during calf ischemia (CE + I). In nine healthy subjects, brachial artery FBF and finger-cuff mean arterial pressure (MAP) were measured beat by beat during rest and forearm exercise. CE + I initiated before 5 min of forearm exercise ( condition A) increased MAP by 24% and reduced resting forearm vascular conductance (FVC) by 24% such that FBF remained at the same level as without CE + I (control, condition C). With the onset of forearm exercise, the difference in FVC between condition A and condition C was abolished; consequently, the FBF adaptation to exercise was greater after 3 min of exercise in condition A (247.0 ± 14.8 ml/min) than in condition C(197.1 ± 9.4 ml/min, P< 0.05) because of the elevated MAP. Gradual stimulation of the chemoreflex by the addition of CE + I at 3 min of a 9-min bout of forearm exercise ( condition B) did not affect FVC such that progressive elevations in MAP resulted in proportional increases in FBF. We concluded that chemoreflex-mediated increases in systemic sympathetic nervous activity appear to affect resting FVC. Evidence from this study suggests that local factors responsible for initiating and maintaining vasodilation during moderate, small-muscle mass exercise can quickly override this vasoconstrictor influence such that FBF is elevated during exercise in direct proportion to the elevation in MAP.