Acute hypoxia decreases cardiac response to catecholamines in exercising humans.

Cardiac chronotropic response to adrenergic activity at rest and exercise has been studied in 8 sea-level natives on the first two days of exposure to high altitude hypoxia (3823 m, 473 mmHg). Maximal O2 uptake (VO2max) was determined at low altitude (day 0:D0) and high altitude (day 2:D2). Submaximal exercise tests were performed at low altitude (day 1:D1) and high altitude (days 3 and 4: D3, D4). Plasma venous norepinephrine (NE) and epinephrine (E) concentrations were determined at rest and at the end of submaximal exercise. From D0 to D2, maximal heart rate decreased by 7% (p less than 0.01), and VO2max decreased by 17% (p less than 0.01). During submaximal exercise, plasma NE did not vary significantly (D1: 1.36 +/- 0.57, D3: 1.48 +/- 0.51, D4: 1.31 +/- 0.54 ng.ml-1). In contrast, relative work load decreased at high altitude (% VO2max at D1, D3 and D4 were respectively: 90.2 +/- 6.1, 83.3 +/- 9.8, 76.9 +/- 8.2). Linear relationships were found, both at low and high altitudes, between NE and VO2, NE and % VO2max, and between the increases in NE and heart rate during exercise. Covariance analysis indicates that these relations shifted to the left at high altitude:for the same NE or increase in NE, VO2 or increase in heart rate was lower at high altitude. Variations in E were similar but not significant. We conclude that hypoxia induced a decrease in cardiac chronotropic response to adrenergic activation during submaximal exercise.