ADAPTATION OF RAT EXTENSOR DIGITORUM LONGUS MUSCLE TO GAMMA-IRRADIATION AND OVERLOAD

The right extensor digitorum longus (EDL) muscle of growing male rats was overloaded by ablation of its synergist tibialis anterior (TA) mus cle. Four weeks later, the overloaded muscle was heavier and contained larger type IIA, IIX and IIB fibres than either untreated contralater al muscle or control muscle from an untreated animal. The myonuclear-t o-myoplasmic volume ratio was maintained in the overloaded muscle. Ove rloaded EDL muscle, previously subjected to a dose of irradiation suff icient to sterilise satellite cells, and EDL muscle which had been onl y irradiated, were significantly lighter and contained significantly s maller fibres than controls, though a significant amount of normal EDL muscle growth did occur following either treatment. The myonuclear-to -myoplasmic volume ratio of the irradiated muscles was smaller than in controls. Overloaded muscle, with or without prior irradiation, posse ssed a smaller proportion of fibres containing IIB myosin heavy chain (MHC) and a larger proportion of fibres containing IIA and IIX MHC; a significant percentage of these fibres coexpressed either type IIA and IIX MHC or type IIX and IIB MHC. Thus in the absence of satellite cel l mitosis, muscles of young rats possess a limited capacity for normal growth but not for compensatory hypertrophy. Adaptations in MHC gene expression to chronic overload are completely independent of satellite cell activity.