Adaptation of mouse skeletal muscle to a novel functional overload test: changes in myosin heavy chains and SERCA and physiological consequences

We have used a new approach to study the effects of overload on skeletal mu scle phenotype in mice. The method used avoids any traumatising contact wit h muscles and the inflammatory reaction that this may provoke. Blocks of le ad embedded in silicone were inserted under the skin of the lower part of t he back. After 1 month, a 17% hypertrophy was found to have occurred in the tonic soleus muscle, but no change was observed in the fast-twitch extenso r digitorum longus (EDL) muscle. The main effects on the contractile proper ties of the soleus muscle were a decrease in the tetanic relaxation rate an d a reduction in the maximal velocity of shortening. Immunohistological ana lysis of the soleus muscles revealed an increase in the proportion of fibre s that express myosin heavy chain (MHC) 1, from 54.2% to 73.9%, with a redu ction in the proportion of MHC2a-positive fibres, from 45.8% to 30.2%. Thes e changes were accompanied by an increase in the proportion of fibres that express the slow type of sarcoplasmic reticulum calcium pump (SERCA2a), fro m 61.8% to 84.7%. In EDL muscles, overload induced only minor changes. Thus , this method of overload affected the soleus muscle in particular. The obs erved changes in the control of muscle contraction were significantly large r than the changes in typical myofibrillar properties that were observed. T hese results indicate that there is a temporal dissociation between the rel ative expression of MHCs and SERCAs.