Inhibition of myosatellite cell proliferation by gamma irradiation does not prevent the age-related increase of the number of dystrophin-positive fibers in soleus muscles of mdx female heterozygote mice.

In skeletal muscles of young mdx female heterozygote mice, there is a mosaic of dystrophin-positive and dystrophin-negative fiber segments. In older animals, there is a marked decline in the number of dystrophin-negative fiber segments. This phenomenon might be due to a fusion of dystrophin-competent satellite cells into the originally dystrophin-negative fiber segments during growth. To study this possibility, soleus muscles of 10-day-old mdx female heterozygotes were gamma irradiated (2000 rads) to inhibit subsequent myosatellite cell proliferation and fusion. In the irradiated soleus muscles of animals at 60 days, the relative amount of dystrophin measured by quantitative immunoblots was not significantly different from that of the contralateral nonirradiated muscles. The prevalence of dystrophin-negative fibers in the 60-day-old irradiated solei was not higher than in the nonirradiated contralateral muscles, implying that dystrophin-competent satellite cell fusion was not a significant factor in the observed conversion. A longitudinal expansion of the cytoplasmic domain of the original dystrophin-competent myonuclei during growth could explain the observed conversion phenomenon.