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1 The Department of Infectious Diseases, Rigshospitalet, Copenhagen, Denmark; The Copenhagen Muscle Research Centre, Copenhagen, Denmark
2 The Copenhagen Muscle Research Centre, Copenhagen, Denmark; The August Krogh Institute, University of Copenhagen, Copenhagen, Denmark
3 The Copenhagen Muscle Research Centre, Copenhagen, Denmark
* To whom correspondence should be addressed. E-mail: cfischer{at}rh.dk.
Contracting skeletal muscle expresses large amounts of interleukin-6 (IL-6). Because (1) IL-6 mRNA expression in contracting skeletal muscle is enhanced by low muscle glycogen content, and (2) IL-6 seems to increase lipolysis and oxidation of fatty acids, we hypothesized that regular exercise training - associated with increased levels of resting muscle glycogen and enhanced capacity to oxidize fatty acids - would lead to a less pronounced increase in IL-6 mRNA in contracting muscle. Thus, before and after 10 weeks of knee-extensor endurance training, the skeletal muscle IL-6 mRNA expression was determined in young healthy men (n=7) in response to 3 hours of dynamic knee-extensor exercise using the same relative workload. Maximal power output, time to exhaustion during submaximal exercise, resting muscle-glycogen content, as well as citrate synthase (CS) and 3-hydroxyacyl-CoA dehydrogenase (HAD) enzyme activity were all significantly enhanced by training. Both before and after the training period, the skeletal muscle IL-6 mRNA expression increased markedly in response to the acute bout of exercise, with no difference between resting IL-6 mRNA expression pre- and post-training. However, although the absolute workload during the acute exercise was 44% higher (p<0.05) after the training period, the skeletal muscle IL-6 mRNA content increased 76-fold (95% confidence interval 20-210, p<0.05) in response to exercise before the training period, but only 8-fold (95% confidence interval 3-17, p<0.05) in response to exercise after the training. In addition, while plasma IL-6 increased in response to acute exercise both before and after the training (p<0.05 on both occasions), the plasma IL-6 levels were not higher after training despite the higher absolute work intensity. In conclusion, the magnitude of the exercise-induced IL-6 mRNA expression in contracting human skeletal muscle was markedly reduced by 10 weeks training.
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