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, V̇O2max), in acute (AH) and chronic hypoxia (CH), at the same absolute (abs) (AHabs=154 W, 54% V̇O2max; CHabs=154 W, 59% V̇O2max) and same relative (rel) (AHrel=130 W, 46% V̇O2max; CHrel=120 W, 44% V̇O2max) 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 AHabs (2.35 pg·ml−1) and CHabs (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, AHrel (1.02 pg·ml−1) and CHrel (1.31 pg·ml−1) resulted in similar IL-6 responses. The augmented IL-6 response during AHabs and CHabs 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.