[Histochemical changes in skeletal muscles of racehorses susceptible to rhabdomyolysis after exertion. I. Early myopathological changes].

In this communication, the results of an enzyme histochemical study on m. gluteus medius of horses, sensitive to exertional myopathy, during attacks of rhabdomyolysis are presented. The activity and location of about 25 enzymes were examined. In the present report, the early metabolic changes are discussed. Within 6 min after an attack, some large rounded fibres (approximately 2%) were seen, which showed an intense red staining in the haematoxylin and eosin sections. These hypercontracted fibres showed an increase in activity of mitochondrial adenosine triphosphatase, indicating the presence of uncoupling and/or loose coupling of the mitochondria. This finding may point to a deficient production of ATP in the m. gluteus medius of horses sensitive for exertional myopathy and this deficiency may lead to pathological alterations in the skeletal muscles. The pathological fibres revealed a changed activity of other mitochondrial enzymes as well.