Interleukin-6 response to exercise during acute and chronic hypoxia

Prolonged exercise is associated with increased plasma levels of the cytokine interleukin-6 (IL-6). Both circulating catecholamine levels and exercise intensity have been related to the exercise-derived IL-6. During hypoxia and acclimatization, changes in sympathetic activity is seen, and also a given workload becomes more intense in hypoxia. Therefore, hypoxia offers a unique opportunity to study the effect of catecholamines and intensity on exercise-derived IL-6. In the present study, eight Danish sea-level residents performed 60 min of cycle ergometer exercise at sea level (SL) (154 W, 45% maximal O₂ consumption, O₂max), in acute (AH) and chronic hypoxia (CH), at the same absolute (_abs) (AH_abs=154 W, 54% O₂max; CH_abs=154 W, 59% O₂max) and same relative (_rel) (AH_rel=130 W, 46% O₂max; CH_rel=120 W, 44% O₂max) workload. We hypothesized that the IL-6 response to exercise at the same absolute workload would be augmented during hypoxia compared with sea level, and that these changes would not correlate with changes in catecholamines. In AH_abs (2.35 pg·ml^–1) and CH_abs (3.34 pg·ml^–1) the IL-6 response to exercise was augmented (p<0.05) compared with that at sea level (0.78·ml^–1). In addition, after 60 min of bicycling at sea level, AH_rel (1.02 pg·ml^–1) and CH_rel (1.31 pg·ml^–1) resulted in similar IL-6 responses. The augmented IL-6 response during AH_abs and CH_abs did not match changes in circulating catecholamine levels when comparing all trials. We conclude that the plasma IL-6 concentration during exercise in hypoxia is intensity dependent, and that factors other than catecholamine levels are more important for its regulation.