Soleus-specific myopathy induced by passive stretching under local tetanus

Force during stretches of rat skeletal muscles after hypertonia at short and long lengths

Following injection of tetanus toxin into rat gastrocnemius muscle to produce hypertonia, plantar flexor muscles were allowed to shorten (S, n=5) without restraint or held lengthened (L, n=3) by splinting. Saline injected rats served as control (n=5). One week after injection, peak forces during 3 stretches with passive muscles and acute isometric force deficits produced by 15 stretches of electrically stimulated muscles were examined under pentobarbital anesthesia. Isometric force and mass of plantar flexors were similar in S rats but 16% lower in L rats compared to control. Peak passive forces were highest in S rats but not different between L rats and control. At the end of the stretch protocol, isometric force deficits were 26% larger in S rats compared to L rats and 17% smaller in L rats compared to control. Acute isometric force deficits produced by stretches of active skeletal muscles were dependent on the muscle length maintained during hypertonia. Our animal model could be used to test rehabilitation interventions during hypertonia of skeletal muscles.

Recovery from 6 weeks of repeated strain injury to rat soleus muscles

Recovery from chronic strain injury (50 strains daily, five times weekly for 6 weeks to hyperactive soleus muscles) was followed for 3 months in female rats after cessation of chronic hyperactivity induced by pretreatment of the plantar flexor muscles with tetanus toxin. After 6 weeks of repeated strains, muscle mass decreased by 62%, myofiber areas were reduced by 87%, and noncontractile tissue expanded dramatically by 222%. Collagen content increased by almost ninefold (control 40 +/- 3 microg/mg, chronic injury 392 +/- 53 microg/mg), whereas the molar ratio of collagen (pyridinoline) crosslinks to collagen remained the same (control 0.20 +/- 0.01, chronic injury 0.16 +/- 0.01). After 3 months of ambulation, muscle mass returned to normal but myofiber areas remained smaller by 21%, noncontractile tissue was still markedly elevated by 18% with increased collagen content (107 +/- 15 microg/mg), and the molar ratio of crosslinks to collagen increased by 75% during recovery. Thus, rat soleus muscles recovered very slowly and incompletely from chronic strain injuries that produced muscle fibrosis, highlighting the necessity of devising preventative strategies for repeated strain injuries.

Fibrosis and intercellular collagen connections from four weeks of muscle strains

The effect of repeated cycles of muscle strain was studied in the soleus muscle of female rats. Muscle strains were repeated 3X/week for 1 month using two different strain protocols. Striking changes, including marked variability in fiber size, evidence of degradation and regeneration, and an expanded extracellular matrix were pronounced in the fast-stretched muscles but not in the slow-stretched muscles. However, the slow-stretched muscles did contain struts of connective tissue joining adjacent myofibers. Therefore, repeated muscle strains at high strain rates produced morphological changes similar to many myopathies, including fibrosis, whereas adaptation occurred in response to the same number of strains at slow strain rates. Such diverse tissue responses have relevance to the understanding of the mechanisms of skeletal muscle dysfunction in cumulative trauma disorders and in the design of preventive actions and treatments.

Prevention of myonecrosis in mdx mice: effect of immobilization by the local tetanus method

Sixteen mdx mice of three-week-old were injected with tetanus toxin into the right gastrocnemius muscle and then subjected to sustained dorsiflexion of the right ankle joint (local tetanus) for 2 to 56 days. Another 16 mdx mice of the same age were not injected with tetanus toxin, serving as controls. The incidence of centronucleated muscle fibers, which indicate preceding myodegeneration, was significantly reduced in the tetanus toxin injected soleus muscle (SOL) and extensor digitorum longus muscle (EDL), compared with that in the untreated control mdx mice. It seems that myonecrosis in the mdx mouse is facilitated by muscle movement itself and that immobilization of muscle prevents myonecrosis. This concept is helpful for understanding the mechanism underlying myonecrosis in patients with Duchenne muscular dystrophy, and for instruction concerning rehabilitation programs and activities in daily life.