Exercise and the prevention of chronic diseases: the role of cytokines and the anti-inflammatory effects of exercise
The main sources ofIL-6 in vivo are activated monocyres/macrophages, fibroblasts and endothelial cells (Akira et al. 1993); however, numerous other cellular sources of IL-6 have been identified, including T cells, B cells, neutrophils, eosinophils, osteoblasts, keratinocytes and myocytes (Akira et al. 1993). An early study indicated that monocytes were unlikely to be the source of the exercise-induced increase in the plasma IL-6 concentration (Ullum et al. 1994a) as one hour of strenuous exercise caused no changes in the amount of IL-6 mRNA detected in peripheral blood mononuclear cells despite an elevation in the plasma IL-6 level. This finding was later confirmed by Starkie et al. (2000), who demonstrated that monocyte intracellular IL-6 protein expression was unchanged following a bout of prolonged strenuous exercise; importantly, Starkie et al. (2001 a) also demonstrated that exercise had no significant effects on TNF-a or IL-1 ß production from monocytes. Several possible sites of origin were suggested for the exercise-induced increase in the circulating level ofIL-6, with contracting skeletal muscle receiving the most attention. Specifically, the intriguing finding that mRNA for IL-6 was elevated in the previously contracting skeletal muscle following prolonged exercise led to the hypothesis that strenuous exercise - marathon running in this case - caused the destruction of contracting myofibres, triggering an inflammatory response and the subsequent release ofIL-6 into the systemic circulation (Ostrowski et al. 1998b).