Recent studies suggest that Interleukin 6 (IL-6) is released from contracting skeletal muscles; however the cellular origin, the secretion kinetics, and signaling mechanisms regulating IL-6 secretion are unknown. To address these questions, we developed imaging methodology to study IL-6 in fixed mouse muscle fibers and in live animals in vivo. Using confocal imaging to visualize endogenous IL-6 protein in fixed muscle fibers, we found IL-6 in small vesicle structures distributed throughout the fibers under basal (resting) conditions. To determine the kinetics of IL-6 secretion, intact quadriceps muscles were transfected with EGFP-tagged IL-6 and five days later anesthetized mice were imaged before and after muscle contractions in situ. Contractions decreased IL-6-EGFP containing vesicles and protein by 62% (p<0.05), occurring rapidly and progressively over 25 minutes of contraction. However, contraction-mediated IL-6-EGFP reduction was normal in muscle-specific AMPKα2-inactive transgenic mice. In contrast, the AMPK activator AICAR decreased IL-6-EGFP vesicles, an effect that was inhibited in the transgenic mice. In conclusion, resting skeletal muscles contain IL-6-positive vesicles which are expressed throughout myofibers. Contractions stimulate the rapid reduction of IL-6 in myofibers, occurring through an AMPKα2-independent mechanism. This novel imaging methodology clearly establishes IL-6 as a contraction-stimulated myokine, and can be used to characterize the secretion kinetics of other putative myokines.
- Received September 12, 2012.
- Accepted June 8, 2013.
- © 2013 by the American Diabetes Association.
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