Effects of the inhibition of DNA synthesis on hypertrophying skeletal muscle

Hypertrophy of the soleus muscle was induced by tenotomy (cutting the tendon) of the synergistic gastrocnemius and plantaris muscles of 250- to 300-g male Sprague-Dawley rats. The effect of inhibition of connective tissue proliferation on skeletal muscle hypertrophy was examined with the use of inhibitors of DNA synthesis. Hydroxyurea and 1-β-D-arabinofuranosylcytosine (cytosine arabinoside), alone and in combination, were injected intraperitoneally. Normal saline was injected in control animals. Two days after tenotomy, muscle hypertrophy and DNA, RNA, and collagen turnover were studied. Compared to the muscles of control animals, muscles of animals injected with inhibitors of DNA synthesis differed markedly in many responses to tenotomy of their synergists. The increase in incorporation of labeled precursor into DNA was inhibited 93 to 98%. Rather than increasing, the concentration of DNA decreased 10 to 20%. The increase in total muscle DNA was inhibited by more than 90%. Histologic examination showed almost complete prevention of connective tissue cell proliferation. Increased incorporation of labeled precursor into RNA and the increase in RNA concentration were blocked to a large extent. Incorporation of labeled precursor into collagen hydroxyproline, total muscle collagen content, and collagen concentration were unaffected by drug treatment. Treatment with hydroxyurea resulted in an increase in hydroxyproline content in both sham-operated and hypertrophied muscle. A 2-fold increase in specific activity of the precursor proline pool with inhibitor treatment complicates interpretation of connective tissue changes. Despite these marked changes, prevention of connective tissue hyperplasia did not significantly alter hypertrophy of the soleus muscles, as measured by increase in wet weight, dry weight, or total muscle myofibrillar protein content. A proximal-distal gradient in RNA synthesis and concentration and in collagen synthesis was maintained in spite of almost total inhibition of DNA synthesis. The evidence suggests that connective tissue cell proliferation is not essential for skeletal muscle hypertrophy.