Skeletal muscle possesses a remarkable capacity for regeneration. Although the regulation of this process at the molecular level remains largely undefined, the plasminogen system appears to play a critical role. Specifically, mice deficient in either urokinase-type plasminogen activator (uPA-/- mice) or plasminogen demonstrate markedly impaired muscle regeneration after injury. In the present study, we tested the hypothesis that loss of the primary inhibitor of uPA, plasminogen activator inhibitor-1 (PAI-1), would improve muscle regeneration. Repair of the extensor digitorum longus muscle was assessed after cardiotoxin injury in wild-type, uPA-/-, and PAI-1-deficient (PAI-1-/-) mice. As expected, there was no uPA activity in the injured muscles of uPA-/- mice, and muscles from these transgenic animals demonstrated impaired regeneration. On the other hand, uPA activity was increased in injured muscle from PAI-1-/- mice to a greater extent than in wild-type controls. Furthermore, PAI-1-/- mice demonstrated increased expression of MyoD and developmental myosin after injury as well as accelerated recovery of muscle morphology, protein levels, and muscle force compared with wild-type animals. The injured muscles of PAI-1-null mice also demonstrated increased macrophage accumulation, contrasting with impaired macrophage accumulation in uPA-deficient mice. The extent of macrophage accumulation correlated with both the clearance of protein after injury and the efficiency of regeneration. Taken together, these results indicate that PAI-1 deficiency promotes muscle regeneration, and this protease inhibitor represents a therapeutic target for enhancing muscle regeneration.

This study investigated if vitamin C supplementation before and after eccentric exercise could reduce muscle soreness (MS), oxidative stress, and muscle function. Eighteen healthy men randomly assigned to either a placebo (P) or vitamin C (VC) (3 g/d) treatment group took pills for 2 wk prior and 4 d after performing 70 eccentric elbow extensions with their non-dominant arm. MS increased in both groups with significantly reduced MS for the first 24 h with VC. Range of motion was reduced equally in both groups after the exercise (P > or = 0.05). Muscle force declined equally and was unaffected by treatment. VC attenuated the creatine kinase (CK) increase at 48 h after exercise with similar CK after this time. Glutathione ratio (oxidized glutathione/total glutathione) was significantly increased at 4 and 24 h with P but VC prevented this change. These data suggest that vitamin C pretreatment can reduce MS, delay CK increase, and prevent blood glutathione oxidation with little influence on muscle function loss.

Although macrophages are thought to promote tissue repair, the molecular mechanisms involved remain to be elucidated. Mice deficient in urokinase‐type plasminogen activator (uPA−/−) demonstrate severely impaired skeletal muscle regeneration associated with decreased accumulation of macrophages. The hypothesis of this study was that macrophage‐derived uPA is required for macrophage accumulation in damaged muscle and for efficient skeletal muscle regeneration. In vitro experiments demonstrated that uPA was required for macrophage chemotaxis independent of its specific receptor (uPAR). In contrast, adhesion, phagocytosis and expression of selected growth factors and cytokines were not different between macrophages from wild‐type (WT) and uPA−/− mice. Intramuscular injection of exogenous uPA to injured muscle in uPA−/− mice restored macrophage accumulation and muscle regeneration. Specific depletion of macrophages using clodronate‐liposomes resulted in impaired muscle regeneration, mimicking the healing response in uPA−/− mice and confirming the requirement of macrophages for efficient muscle regeneration. Transfer of WT bone marrow cells to uPA−/− mice rescued macrophage accumulation and restored muscle regeneration. These results indicate that macrophage‐derived uPA is necessary for chemotaxis of macrophages and for efficient muscle regeneration, and as such, may represent a therapeutic target for enhancing macrophage chemotaxis and subsequent muscle regeneration.

Vitamin C supplementation (VC) (either 500 or 1000 mg/d for 2 wk) was compared to a placebo treatment (P) to ascertain if VC could influence oxidative stress. Twelve healthy males (25 +/- 1.4 y) were randomly assigned in a counter-balanced design with a 2-wk period between treatments. Data were analyzed using repeated measures ANOVA. Exercise intensity measures (VO(2), RER, RPE, HR, lactate) were similar across treatments. Resting blood oxidative-stress markers were unaffected by treatment. Exercise decreased total blood glutathione (TGSH) and reduced glutathione (GSH) and increased oxidized glutathione (GSSG) (P < 0.01) independent of treatment. Protein carbonyls (PC) increased 3.8 fold in the P (P < 0.01). VC attenuated the PC exercise response in a dose-dependent manner ( P < 0.01). Thiobarbituric acid reactive substances (TBARS) was not influenced by exercise (P = 0.68) or VC. These data suggest that VC supplementation can attenuate exercise-induced protein oxidation in a dose-dependent manner with no effect on lipid peroxidation and glutathione status.

Macrophages (Mp) and the plasminogen system play important roles in tissue repair following injury. We hypothesized that Mp-specific expression of urokinase-type plasminogen activator (uPA) is sufficient for Mp to migrate into damaged muscle and for efficient muscle regeneration. We generated transgenic mice expressing uPA only in Mp, and we assessed the ability of these mice to repair muscle injury. Mp-only uPA expression was sufficient to induce wild-type levels of Mp accumulation, angiogenesis, and new muscle fiber formation. In mice with wild-type uPA expression, Mp-specific overexpression further increased Mp accumulation and enhanced muscle fiber regeneration. Furthermore, Mp expression of uPA regulated the level of active hepatocyte growth factor (HGF), which is required for muscle fiber regeneration, in damaged muscle. In vitro studies demonstrated that uPA promotes Mp migration through proteolytic and non-proteolytic mechanisms, including proteolytic activation of HGF. In summary, Mp-derived uPA promotes muscle regeneration by inducing Mp migration, angiogenesis, and myogenesis.