Icing Following Muscle Injury Suppresses Muscle Pain-related Molecules But Exacerbates Fibrosis And Mitochondrial Content Recovery

PURPOSE: We and the others previously reported that icing treatment, commonly accepted as the gold standard of first aid for sports injuries, inhibits skeletal muscle regeneration. However, physiological effects of icing on damaged skeletal muscle are still incompletely understood. The aims of the present study were to investigate the impact of icing treatment on muscle pain-related molecules, fibrosis, and mitochondrial content during the recovery process after skeletal muscle injury. METHODS: Male Wistar rats (10 weeks of age) were randomly assigned to injured and injured with icing (ICE) groups. To induce muscle injury, bupivacaine (BPVC) was injected into fast plantaris muscles bilaterally in both groups. Rats in the ICE group received icing treatment (ice pack, 0 °C for 20 min) only once immediately after the BPVC injection. At 1, 3, 5, 7, 14, and 28 days after the injury, plantaris muscles were dissected, and qPCR, western blot, and histochemical analyses were performed. RESULTS: The mRNA expressions of muscle pain-related molecules (BKB2R, mPGES-1, and IL-1β) were significantly lower in ICE than in injured groups at 1 day after the injury (P < 0.05). In contrast, icing treatment significantly increased the fibrotic area at 28 days after the injury (P < 0.05). The expression levels of fibrosis-related mRNAs, TGF-β1 and α-SMA, were also higher in ICE than in injured groups at 7 and 28 days after the injury, respectively (P < 0.05 and P = 0.06, respectively). Although the changes in the expression of microRNA-494, a negative regulator of mitochondrial biogenesis, were not different between the two groups during the recovery period, the VDAC1 protein expression (as a marker of mitochondrial content) was significantly lower in ICE than in injured groups at 28 days after the BPVC injection (P < 0.05). CONCLUSIONS: These results indicated that icing treatment as a first aid for skeletal muscle injury would attenuate muscle pain early after the injury. However, although the precise mechanisms remain unclear, our data suggested that the icing treatment impairs late stage of skeletal muscle regeneration (promotion of fibrosis and inhibition of mitochondrial content recovery).