Hypoxemia increases plasma catecholamine concentrations in exercising humans

To determine wheter plasma catecholamine concentrations (a measure of sympathetic nervous activity [SNA]) rise above normoxic levels during exercise with hypoxemia, we exercised seven men for 15 min at three loads that required from 40 to 88% of maximal O₂ uptake (V̇O(2 max)). Subjects breathed room air on one day and 11-12% O₂ on another with relative work loads corrected for the 24% fall in V̇O(2 max) during hypoxemia. Hypoxemia caused large increments in norepinephrine (NE) concentrations (radioenzyme technique) to 1.21 ± 0.20 ng/ml (mean ± SE), 2.79 ± 0.38, and up to 5.90 ± 0.75 (hypoxemia) compared with 0.89 ± 0.06, 1.66 ± 0.16, and 3.95 ± 0.39 in normoxia at the three loads, respectively (P<0.001). Epinephrine (E) concentration approximately doubled (P<0.001) in hypoxemia at each load when compared with normoxic levels (i.e., 0.10 ± 0.01 ng/ml, 0.23 ± 0.03, and 0.46 ± 0.06 in normoxia). However, hypoxemia did not significantly alter linear relationships between log plasma NE concentration and either heart rate (HR) or percent V̇O(2 max) utilized, or between HR and percent V̇O(2 max). Thus NE concentration, like HR, appeared to reflect relative severity of exercise and overall SNA in both hypoxemia and normoxia. Above 40% V̇O(2 max) during hypoxemia, circulating NE and E far exceeded levels known to have direct vasoconstrictor and metabolic effects on normoxic humans, but hypoxemia may blunt vasoconstriction in some regions.