Contraction-induced myokine production and release: is skeletal muscle an endocrine organ?

The concentration of plasma interleukin-6 (IL-6) increases during physical exercise, but until recently the cellular origin of this increase has been unknown. Recent work has identified that skeletal muscle is a major source of this increase and the release of IL-6 from muscle can mediate metabolic processes. IL-6 is, therefore, the first identified "myokine" released from muscle that can now be termed an endocrine organ.