[Exercise induced release of cytokines from skeletal muscle: emphasis on IL-6]

ABSTRACT

Cytokines are a group of proteins that play a central role in mediating inflammatory responses to pathological stimuli such as infection and tissue damage. However, cytokine production is also modulated by a range of physiological stimuli such as exercise. Skeletal muscle has recently been identified as an endocrine organ. It has been suggested that cytokines or other peptides that are produced, expressed, and released by muscle fibers should be classified as "myokines". These myokines exert paracrine, endocrine and autocrine effects. IL-6 was discovered as a myokine because of the observation that it increases up to 100-fold in the circulation during physical exercise. Because of its metabolic roles, IL-6 production by skeletal muscle during physical activity created a paradox. As IL-6 is markedly produced and released in the postexercise period when insulin action is enhanced whereas, on the other hand, IL-6 has also been associated with obesity and reduced insulin action. Also, muscle mass and its function are influenced by different cytokines, in particular by IL-6, IL-15 and TNF-alpha more prominently during exercise. This review focuses on the myokines, their regulation by exercise and their roles in immune and metabolism, considering the effects of cytokines on muscle mass and function