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 dur ing forearm exercise when a vasoconstrictor response was evoked by calf exe rcise during calf ischemia (CE + I). In nine healthy subjects, brachial art ery 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 for earm exercise (condition A) increased MAP by 24% and reduced resting forear m vascular conductance (FVC) by 24% such that FBF remained at the same leve l as without CE + I (control, condition C). With the onset of forearm exerc ise, the difference in FVC between condition A and condition C was abolishe d; 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 fo rearm exercise (condition B) did not affect FVC such that progressive eleva tions in MAP resulted in proportional increases in FBF. We concluded that c hemoreflex-mediated increases in systemic sympathetic nervous activity appe ar to affect resting FVC. Evidence from this study suggests that local fact ors responsible for initiating and maintaining vasodilation during moderate , small-muscle mass exercise can quickly override this vasoconstrictor infl uence such that FBF is elevated during exercise in direct proportion to the elevation in MAP.