Absence of dystrophin in mdx muscles may render the muscle more susceptible to damage when submitted to high stress levels. To test this, typically slow (soleus) and fast (EDL) limb muscles of dystrophic (mdx) and normal (C57BL/10) mice were submitted (in vitro) to a series of isometric contractions, followed by a series of contractions with stretches. Muscle injury was assessed by monitoring the force signal. Membrane damage was evaluated by bathing the muscle in Procion Red, a dye that does not penetrate intact fibres, and subsequent analysis by light microscopy. 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 strains. After contractions with a stretch, a force drop of 10% was observed in soleus muscles from both strains and in EDL muscles from C57 mice. However, in mdx mice EDL muscles displayed an irreversible force drop of 40-60%. Histological analysis of the muscles indicates that force drop is associated with membrane damage. These results show that EDL muscles from mdx mice are more vulnerable than their controls, supporting 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.

Frog sartorius muscles tetanized isometrically were released at a constant velocity from lengths lL to ls (A/ = l L -ls; ls >/0). The tension P~s redeveloped after the release was lower than the isometric tension Ps at Is, and higher than the isometric tension PL at lL. The tension deficit D is defined as the difference Ps-P*. The timing of the release during the tetanus did not influence D. D/Po was proportional to Al/lo. The proportionality constant k was equal to 1.35 -+ 0.19 (n = 8) when the velocity of release was 2.5 mm/s. When the muscles were released the same Al, D was found to be an exponential decreasing function of the velocity. The tension deficit was also found in experiments performed in the region ls < lo. The proportionality constant k was smaller, but the influence of the velocity of the release on D was not modified. When the velocity of the release was changed during the release, D changed accordingly, showing that the effects of Al and V are multiplicative. These facts suggest a working hypothesis based on the concept that the actin filaments which enter the overlap region during a release are strained by the tetanic stress and therefore unable to make normal cross-bridges.

A review of the literature suggests that the effects of nitric oxide (NO) on skeletal muscles fibers can be classified in two groups. In the first, the effects of NO are direct, due to nitrosation or metal nitrosylation of target proteins: depression of isometric force, shortening velocity of loaded or unloaded contractions, glycolysis and mitochondrial respiration. The effect on calcium release channels varies, being inhibitory at low and stimulatory at high NO concentrations. The general consequence of the direct effects of NO is to 'brake' the contraction and its associated metabolism. In the second group, the effects of NO are mediated by cGMP: increase of the shortening velocity of loaded or unloaded contractions, maximal mechanical power, initial rate of force development, frequency of tetanic fusion, glucose uptake, glycolysis and mitochondrial respiration; decreases of half relaxation time of tetanus and twitch, twitch time-to-peak, force maintained during unfused tetanus and of stimulus-associated calcium release. There is negligible effect on maximal force of isometric twitch and tetanus. The general consequence of cGMP-mediated effects of NO is to improve mechanical and metabolic muscle power, similar to a transformation of slow-twitch to fast-twitch muscle, an effect that we may summarize as a 'slow-to-fast' shift.

Fast skeletal muscles of mdx (X chromosome-linked muscular dystrophy) mice were injected after birth with a recombinant adenovirus containing a minidystrophin gene, a 6.3-kbp cDNA coding for the N-and Cterminal ends ofdystrophin. Adult muscles were challenged by forced lengthening during tetanic contractions. Stretchinduced mechanical and histological damages were much reduced in injected muscles, in (n = 7) or with the Ad-RSVI3gal (n = 7). When mice were 4 months old muscles were isolated under general anesthesia, which preserved circulation. The lateral part of the muscle was dissected out. The order of dissection of the Ad-RSVmDys limb vs. the other one was alternated from animal to animal.Isolated muscles were submitted to repetitive stimulations (125 Hz) for 300 msec, at 5-min intervals. During the first 160 msec, tension was developed isometrically and then a forced lengthening (1 mm; i.e., +7%, at 11.1 mm/sec) was imposed.After relaxation the muscle was returned to the resting length.Histology. At the end of the experiments, the muscles were soaked for 3 h in an oxygenated Krebs

The ky mouse mutant exhibits a primary degenerative myopathy preceding chronic thoraco-lumbar kyphoscoliosis. The histopathology of the ky mutant suggests that Ky protein activity is crucial for normal muscle growth and function as well as the maturation and stabilization of the neuromuscular junction. Muscle hypertrophy in response to increasing demand is deficient in the ky mutant, whereas adaptive fibre type shifts take place. The ky locus has previously been localized to a small region of mouse chromosome 9 and we have now identified the gene and the mutation underlying the kyphoscoliotic mouse. The ky transcript encodes a novel protein that is detected only in skeletal muscle and heart. The identification of the ky gene will allow detailed analysis of the impact of primary myopathy on idiopathic scoliosis in mice and man.