THE DEVELOPMENT OF COMPENSATORY HYPERTROPHY IN THE PLANTARIS MUSCLE OF THE RAT

The aim of this investigation was to study the time course of compensa tory hypertrophy (CH) over a seven week period after its surgical indu ction in the lower limb of the rat. CH of the left plantaris muscle of the rat was induced by denervation of the ipsilateral gastrocnemius a nd soleus muscles. Muscle fibres were classifed as type I, Ic, IIa and IIb. Hypertrophy of the muscle was first observed about ten days afte r induction of CH. All fibre types appeared to contribute to this hype rtrophy. During the period between four and twenty eight days there wa s a marked increase in the percentage of type I fibres, mainly at the expense of type IIa, as compared with control muscles. During this CH period so called 'intermediate' Ic fibres were found, indicating fibre type transition taking place. The isometric twitch time to peak tensi on (TPT) of the plantaris muscle was studied in situ. The TPT of CH mu scles remained the same during the experimental period of seven weeks. This might be explained by the effect of the increase in type I (slow ) fibres being masked by the far larger number of fast fibres, which s till accounted for approximately 79% of the total number of fibres aft er CH.