Expression of slow and fast myosin heavy chains in overload muscles of the developing rat.

The present study examines the developmental accumulation of slow myosin heavy chain in the extensor digitorum longus, soleus and plantaris muscles of rats after early post-natal imposition of mechanical overload by removal of synergistic muscles. The proportions of slow and fast myosin heavy chain were measured in each muscle by ELISA. Fibres expressing slow myosin were examined immunocytochemically using a monoclonal antibody specific for slow MHC. Between 30 and 60 days of age, MHC increases by 15% (p less than 0.001) in the soleus and by 27% (p less than 0.001) in the plantaris of normally developing, unoperated animals. The effect of overload on the soleus and plantaris is to accelerate the rate of increase in slow MHC accumulation so that levels are respectively 16 and 39% higher than controls by 30 days of age (p less than 0.001). By 60 days, the control soleus and plantaris attain levels of slow MHC roughly equivalent to their overloaded counterparts. In overloaded plantaris the increase in levels of slow myosin does not occur at the expense of fast myosin expression. In the EDL there is a normal developmentally regulated decrease in slow MHC accumulation, reflected by a 40% decrease in levels of slow MHC (p less than 0.0001) and a 50% decrease in the number of slow fibres (p less than 0.001), between 30 days and 20 weeks of age. This elimination of slow myosin accumulation in the EDL is unimpeded by chronic overload. Thus, muscles react to mechanical overload in a tissue specific manner.(ABSTRACT TRUNCATED AT 250 WORDS)