Improved contractile function of the mdx dystrophic mouse diaphragm muscle after insulin-like growth factor-I administration.

Limited knowledge exists regarding the efficacy of insulin-like growth factor I (IGF-I) administration as a therapeutic intervention for muscular dystrophies, although findings from other muscle pathology models suggest clinical potential. The diaphragm muscles of mdx mice (a model for Duchenne muscular dystrophy) were examined after 8 weeks of IGF-I administration (1 mg/kg s.c.) to test the hypothesis that IGF-I would improve the functional properties of dystrophic skeletal muscles. Force per cross-sectional area was approximately 49% greater in the muscles of treated mdx mice (149.6 +/- 9.6 kN/m(2)) compared with untreated mice (100.1 +/- 4.6 kN/m(2), P < 0.05), and maintenance of force over repeated maximal contraction was enhanced approximately 30% in muscles of treated mice (P < 0.05). Diaphragm muscles from treated mice comprised fibers with approximately 36% elevated activity of the oxidative enzyme succinate dehydrogenase, and approximately 23% reduction in the proportion of fast IId/x muscle fibers with concomitant increase in the proportion of type IIa fibers compared with untreated mice (P < 0.05). The data demonstrate that IGF-I administration can enhance the fatigue resistance of respiratory muscles in an animal model of dystrophin deficiency, in conjunction with enhancing energenic enzyme activity. As respiratory function is a mortality predictor in Duchenne muscular dystrophy patients, further evaluation of IGF-I intervention is recommended.