INCREASED SUSCEPTIBILITY OF EDL MUSCLES FROM MDX MICE TO DAMAGE-INDUCED BY CONTRACTIONS WITH STRETCH

Absence of dystrophin in mdx muscles may render the musde more suscept ible to damage when submitted to high stress levels. To test this, typ ically slow (soleus) and fast (EDL) limb muscles of dystrophic (mdx) a nd normal (C57BL/10) mice were submitted (in vitro) to a series of iso metric contractions, followed by a series of contractions with stretch es. Muscle injury was assessed by monitoring the force signal. Membran e damage was evaluated by bathing the muscle in Procion Red, a dye tha t does not penetrate intact fibres, and subsequent analysis by light m icroscopy. After isometric contractions, only a very small force drop (<3% of maximal isometric force) was observed which indicated that no injury had occurred in soleus and EDL muscles in either mdx or C57 str ains. After contractions with a stretch, a force drop of 10% was obser ved in soleus muscles from both strains and in EDL muscles from C57 mi ce. However, in mdx mice EDL muscles displayed an irreversible force d rop of 40-60%. Histological analysis of the muscles indicates that for ce drop is associated with membrane damage. These results show that ED L muscles from mdx mice are more vulnerable than their controls, suppo rting the structural role hypothesis for dystrophin. Furthermore, they suggest that contractions with stretches may contribute to the muscle damage and degeneration observed in DMD-patients.