Effects of inhibiting DNA synthesis with hydroxyurea on stretch-induced skeletal muscle growth.

Growth of chicken patagialis muscle in response to passive stretch for 7 days was studied in the absence and presence of the DNA synthesis inhibitor hydroxyurea. In the absence of hydroxyurea, the DNA content and concentration of the stretched muscle increased 101% and 40%, respectively, relative to the unstretched contralateral control. Stretch-induced growth was characterized by large increases in muscle wet weight (56%), protein content (44%), RNA concentration (63%), cross-sectional area (67%), and mean fiber cross-sectional area (50%). In the presence of hydroxyurea, DNA content increased only 22% whereas DNA concentration decreased 11% in response to stretch. However, except for RNA concentration, which increased 84%, stretch-induced growth in the hydroxyurea-treated birds exhibited the same characteristics as those in the sham-treated animals. Therefore, the large increase in DNA content and concentration observed after 7 days of stretch-induced growth in the chicken patagialis muscle was not necessary for the growth to occur.

Effect of physical training on myocardial enzyme activities in aging rats.

Male Fischer rats of four age groups were subjected to a 3-mo exercise training program that consisted of a gradual increase to 1 h/day of forced swimming. Exercise was initiated at 1, 6, 12, or 17-22 mo of age. After the training period there was an increase in the heart weight relative to body weight in all groups, but heart weight was increased only in the two oldest groups. The specific activities of both actomyosin ATPase and creatine kinase isolated from cardiac muscle decreased with age. In animals that started exercise training at 6 mo of age the activities of both enzymes were higher than that of the age-matched sedentary controls, but the oldest animals (17-22 mo) responded negatively (i.e., a decreased enzymatic activity compared with sedentary individuals of the same age). These results suggest that, after a certain age, the initiation of endurance exercise may not result in the same adaptive response as occurs in younger animals. In the case of actomyosin ATPase this may be a consequence of a different distribution of myosin isozymes.