Acute Hypoxia Decreases Cardiac Response to Catecholamines in Exercising Humans

Abstract

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 : DO) 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 DO to D2, maximal heart rate decreased by 7% (p<0.01), and VO₂max decreased by 17% (p<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.

Key words

High altitude - adrenergic receptors - endurance - epinephrine - norepinephrine