Autocrine Signaling may Regulate Exercise-Induced Interleukin-6 Release from Human Skeletal Muscle.

BACKGROUND: Contracting skeletal muscle releases interleukin-6 (IL-6) which is primarily cleared by the liver. The tissue-specific regulation of IL-6 and soluble IL-6 receptor (sIL-6R) kinetics remains, however, poorly understood. To characterize tissue-specific IL-6 and sIL-6R exchange during exercise, we combined simultaneous arteriovenous measurements across the contracting leg, splanchnic region, and the brain with, or without, pharmacological IL-6 receptor (IL-6R) blockade. METHODS: Thirty young, well-trained men were randomized in a double-blind, placebo-controlled design to receive either tocilizumab (IL-6R blockade) or placebo before performing moderate-intensity cycling exercise. Net tissue flux of IL-6 and sIL-6R was determined from arteriovenous concentration differences across the femoral, hepatic, and internal jugular circulation. RESULTS: During exercise, leg IL-6 release increased with both placebo and IL-6R blockade but was significantly augmented by IL-6R blockade, suggesting a potential role for autocrine regulation of local IL-6 release . Under control conditions, the splanchnic bed was the predominant site of IL-6 clearance; however, IL-6R blockade shifted splanchnic IL-6 net balance from uptake to release. In addition, the exercising leg was identified as the source of circulating sIL-6R during exercise, whereas no consistent sIL-6R exchange occurred across the splanchnic bed or brain. CONCLUSION: These findings suggest a potential role for IL-6 signaling in the autocrine regulation of IL-6 release from the contracting leg during exercise. Furthermore, exercise increases leg-derived sIL-6R, thereby contributing to increased circulating sIL-6R concentrations, while receptor-mediated splanchnic clearance appears to be a major determinant of systemic IL-6 availability during exercise in humans. CLINICAL TRIAL REGISTRATION NUMBER: Clinicaltrials.gov (NCT06334653).