Vasoconstrictor responsiveness during hyperbaric hyperoxia in contracting human muscle

Casey DP, Joyner MJ, Claus PL, Curry TB. Vasoconstrictorresponsiveness during hyperbaric hyperoxia in contracting humanmuscle. J Appl Physiol 114: 217–224, 2013. First published Novem ber 8, 2012; doi:10.1152/japplphysiol.01197.2012.—Large increasesin systemic oxygen content cause substantial reductions in exercisingforearm blood flow (FBF) due to increased vascular resistance. Wehypothesized that 1) functional sympatholysis (blunting of sympa thetic -adrenergic vasoconstriction) would be attenuated duringhyperoxic exercise and 2) -adrenergic blockade would limit vaso constriction during hyperoxia and increase FBF to levels observedunder normoxic conditions. Nine male subjects (age 28 1 yr)performed forearm exercise (20% of maximum) under normoxic andhyperoxic conditions. Studies were performed in a hyperbaric cham ber at 1 atmosphere absolute (ATA; sea level) while breathing 21%O2 and at 2.82 ATA while breathing 100% O2 (estimated change inarterial O2 content 6 ml O2/100 ml). FBF (ml/min) was measuredusing Doppler ultrasound. Forearm vascular conductance (FVC) wascalculated from FBF and blood pressure (arterial catheter). Vasocon strictor responsiveness was determined using intra-arterial tyramine.FBF and FVC were substantially lower during hyperoxic exercisethan normoxic exercise ( 20 –25%; P 0.01). At rest, vasoconstric tion to tyramine (% decrease from pretyramine values) did not differ between normoxia and hyperoxia (P 0.05). During exercise, vaso constrictor responsiveness was slightly greater during hyperoxia than normoxia ( 22 3 vs. 17 2%; P 0.05). However, during -adrenergic blockade, hyperoxic exercise FBF and FVC remained lower than during 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.