Large increases in systemic oxygen content cause substantial reductions in exercising forearm blood flow (FBF) due to increased vascular resistance. We hypothesized that 1) functional sympatholysis (blunting of sympathetic α-adrenergic vasoconstriction) would be attenuated during hyperoxic exercise and 2) α-adrenergic blockade would limit vasoconstriction during hyperoxia and increase FBF to levels observed under normoxic conditions. Nine male subjects (28 ± 1 years) performed forearm exercise (20% of maximum) under normoxic and hyperoxic conditions. Studies were performed in a hyperbaric chamber at 1 ATA (sea level) while breathing 21% O2 and at 2.82 ATA while breathing 100% O2 (estimated change in arterial O2 content ~6 ml O2/100ml). FBF (ml/min) was measured using Doppler ultrasound. Forearm vascular conductance (FVC) was calculated from FBF and blood pressure (arterial catheter). Vasoconstrictor responsiveness was determined using intra-arterial tyramine. FBF and FVC were substantially lower during hyperoxic compared to normoxic exercise (~20-25%; P < 0.01). At rest, vasoconstriction to tyramine (%decrease from pre-tyramine values) did not differ between normoxia and hyperoxia (P > 0.05). During exercise vasoconstrictor responsiveness was slightly greater during hyperoxia compared to normoxia (-22 ± 3 vs. -17 ± 2%; P < 0.05). However, during α-adrenergic blockade hyperoxic exercise FBF and FVC remained lower compared to normoxia (P < 0.01). Therefore, our data suggest that although the vasoconstrictor responsiveness during hyperoxic exercise was slightly greater, it likely does not explain the majority of the large reductions in FBF and FVC (~20-25%) during hyperbaric hyperoxic exercise.
- skeletal muscle blood flow
- Copyright © 2012, Journal of Applied Physiology