Variable involvement of rat skeletal muscles in paraoxon-induced necrotizing myopathy

We studied the incidence of paraoxon-induced myopathy in several rat skeletal muscles in relation to the morphometric properties and oxidative metabolism of their fibers. The necrosis was most pronounced in the predominantly oxidative-rich fiber-composed diaphragm. The purely oxidative-rich masseter and soleus muscles were also severely affected. All 7 mixed muscles with oxidative-poor fiber predominance were far less involved. A high correlation between oxidative capacity and the extent of the muscle fiber necrosis was evidenced in mixed muscles. No relation was found between the muscle fiber diameter and the susceptibility to necrosis. We conclude that muscles predominantly composed of highly oxidative fiber types are more susceptible to organophosphate-induced necrotizing myopathy. Oxidative capacity alone is not the only factor, however, as the mixed diaphragm was more involved than the purely oxidative-rich masseter and soleus. Several features of the distinct fiber types could be responsible for the variable vulnerability.