The levels of c-myc and c-fos mRNA were investigated in rat skeletal muscle by Northern hybridization. During post-natal development in the rat, c-myc mRNA levels were similar at birth and at 7 and 21 days of age, but then declined at 90 days and were barely detectable at 1 year. c-fos mRNA levels followed this pattern of expression until 90 days, but showed a large increase at 1 year. Hypertrophy of soleus and plantaris muscles was induced either by severance of the tendon to the synergistic gastrocnemius (tenotomy) or by administration of the beta-adrenoceptor agonist clenbuterol. In both cases hypertrophy was associated with a rapid increase in c-myc mRNA levels. Following tenotomy the increase was both greater (8-fold) and more rapid (3 h) in soleus than in plantaris (2-3 fold, 12 h). Similar effects were observed during clenbuterol administration. Neither treatment caused any alteration in c-fos mRNA levels in the plantaris muscle. The results show that increased c-myc mRNA levels are an early event in the response of skeletal muscle to hypertrophic stimuli; it is argued that this occurs within the differentiated skeletal muscle fibres.

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