Impaired regeneration of dystrophin-deficient muscle fibers is caused by exhaustion of myogenic cells

Duchenne muscular dystrophy is one of the most devastating myopathies. Muscle fibers undergo necrosis and lose their ability to regenerate, and this may be related to increased interstitial fibrosis or the exhaustion of satellite cells. In this study, we used mdx mice, an animal model of Duchenne muscular dystrophy, to assess whether muscle fibers lose their ability to regenerate after repeated cycles of degeneration-regeneration and to establish the role of interstitial fibrosis or exhaustion of satellite cells in this process. Repeated degenerative-regenerative cycles were induced by the injection of bupivacaine (33 mg/kg), a myotoxic agent. Bupivacaine was injected weekly into the right tibialis anterior muscle of male, 8-week-old mdx (N = 20) and C57Bl/10 (control, N = 10) mice for 20 and 50 weeks. Three weeks after the last injection, the mice were killed and the proportion of regenerated fibers was counted and reported as a fibrosis index. Twenty weekly bupivacaine injections did not change the ability of mdx muscle to regenerate. However, after 50 weekly bupivacaine injections, there was a significant decrease in the regenerative response. There was no correlation between the inability to regenerate and the increase in interstitial fibrosis. These results show that after prolonged repeated cycles of degeneration-regeneration, mdx muscle loses its ability to regenerate because of the exhaustion of satellite cells, rather than because of an increase in interstitial fibrosis. This finding may be relevant to cell and gene therapy in the treatment of Duchenne muscular dystrophy

Effects of methylene blue and indomethacin on sodium nitroprusside-induced relaxations of the guinea pig tracheal smooth muscle

The aim of the present investigation was to evaluate the potential modulatory effect of the guinea pig tracheal smooth muscle intrinsic tone on the relaxant responses to sodium nitroprusside and the effects of methylene blue on this response. Paired tracheal chains were mounted for isotonic contractions under 500 mg of tension in Krebs-Henseleit solution. The intrinsic modulatory tone was inhibited by indomethacin, in a concentration that did not have any effect over carbachol induced contractions. Sodium nitroprusside-induced relaxations were the same in the absence or presence of the modulatory tone. Methylene blue inhibited 50 per cent sodium nitroprusside-induced relaxations, in the presence or absence of the intrinsic system. This suggests that sodium nitroprusside-induced relaxations are mediated through guanylyl-cylase activation and that these are not under the modulation of the intrinsic prostaglandinergic tone.

Effects of methylene blue on the development of intrinsic contractile responses of the guinea pig tracheal smooth muscle

The aim of the present study was to investigate the effects of methylene bue, a guanylyl cyclase inhibitor, on the development of intrinsic contractile responses of the guinea pig tracheal smooth muscle. Paired tracheal chains were mounted for isotonic contractions under 500 mg of tension in Krebs-Henseleit solution. The intrinsic contractile tone modulated carbachol-induced contractions and was inhibited by indomethacin, suggesting the involvement of cyclooxygenase products on this tone. Methylene blue (5x10(-5)M) irreversibly inhibited the intrinsic contractile responses. considering that methylene blue prevents any endogenous production of cGMP, it would be expected to enhance the contractions. However, since methylene blue has effects over nitric oxide synthase and nitric oxide itself, we suggest that guanylyl cyclase activations is not important for the development of the intrinsic tone.