Skeletal myocytes are a source of interleukin-6 mRNA expression and protein release during contraction: evidence of fiber type specificity.

In this study, we aimed to determine whether skeletal muscle cells per se are a source of interleukin (IL)-6 during contraction and whether IL-6 production is fiber type specific. Muscle biopsy samples were collected from seven males before (PRE) and after (POST) completing 120 min of continuous bicycle ergometry. Biopsies were sectioned and analyzed for the following: IL-6 protein detected by immunohistochemistry (IHC), IL-6 mRNA content detected by in situ hybridization, fiber type measured by either IHC or myofibrillar ATPase activity stain, and glycogen content measured by periodic acid schiff (PAS) assay. Fibers were qualitatively categorized according to glycogen content to one of five groups (1-5), with 1 being very low (LOW) and 5 being very high (HIGH) glycogen. Total fluorescence (PRE vs. POST) and glycogen-dependent fluorescence (LOW vs. HIGH) of IL-6 protein were quantitated using Metamorph software. Total IL-6 protein was elevated from PRE to POST exercise (P<0.05). At PRE, IL-6 protein was evenly distributed across all fibers at low levels, consistent with glycogen distribution. At POST, IL-6 protein was greater (P<0.05) in HIGH compared with LOW glycogen fibers, which coincided with type 2 fibers. IL-6 mRNA was distributed peripherally in all fibers at PRE. At POST, however, IL-6 mRNA appeared predominantly in type 2 fibers, which also had higher glycogen content (P<0.05). These data demonstrate that myocytes per se are a source of IL-6 produced during contraction. Our data also suggest that type 2 fibers predominantly produce IL-6 during muscle contractile activity.