Cytokine gene expression in human skeletal muscle during concentric contraction: evidence that IL-8, like IL-6, is influenced by glycogen availability

To determine the expression and induction of cytokines in human skeletal muscle during concentric contractions, eight males performed 60 min of bicycle exercise, with either a normal (Con) or reduced (Lo Gly) preexercise intramuscular glycogen content. Muscle biopsy samples were obtained before and after exercise and analyzed for glycogen and the mRNA expression of 13 cytokines. Resting muscle glycogen was higher (P<0.05) in Con compared with Lo Gly and was reduced (P<0.05) to 102+/-32 vs. 17+/-5 mmol U glycosyl/kg dry mass for Con and Lo Gly, respectively. We detected mRNA levels in human skeletal muscle for five cytokines, namely interleukin (IL)-1beta, IL-6, IL-8, IL-15, and tumor necrosis factor-alpha. However, muscle contraction increased (P<0.05) the mRNA expression of IL-6 and IL-8 alone. In addition, the fold change for both IL-8 and IL-6 was markedly higher (P<0.05) in Lo Gly compared with Con. Given these results, we analyzed venous blood samples, obtained before and during exercise, for IL-6 and IL-8. Plasma IL-6 was not different at rest, and although the circulating concentration of this cytokine increased (P<0.05) it increased to a greater extent (P<0.05) throughout exercise in Lo Gly. In contrast, plasma IL-8 was not affected by exercise or treatment. These data demonstrate that cytokines are not ubiquitously expressed in skeletal muscle and that only IL-6 and IL-8 mRNA are increased during contraction of this mode and duration. Furthermore, the mRNA abundance of IL-6 and IL-8 appears to be influenced by glycogen availability in the contracting muscle.