Cellular growth of skeletal muscle in weanling rats during dietary restrictions

The effects of feeding varying levels of a nutritionally balanced diet on the cellular growth of rat skeletal muscles were examined. Groups of male weanling rats were fed 50% (R-50), 40% (R-40), or 30% (R-30) of the intake of the ad libitum fed age-matched controls (C). An initial control group (IC) was killed at 25 days of age, and all other groups were sacrificed at 46 days. These chronic dietary restrictions slowed the rates of growth of each of the four skeletal muscles examined: the plantaris, soleus, extensor digitorum longus, and gastrocnemius. Longitudinal growth, as estimated from tibia length, was also significantly affected by the restrictions. DNA, protein, and RNA were examined in the plantaris. DNA content was the same in the IC and R-30 groups but increased significantly with each subsequent increase in food intake. The ratio of protein/DNA was the same for the C, R-50, and R-40 groups but was 15% lower for the R-30 group. Therefore, the restricted rats tended to maintain the normal ratio of protein/DNA even at the expense of halting DNA replication. While the transverse growth (cross-sectional area of muscle fibers) of the plantaris was significantly slowed by the chronic restrictions, muscle fiber number was the same for all groups at 46 days of age. These data indicate that in skeletal muscle, DNA content (nuclear number) does not equate with cell number, and the ratio of protein/DNA does not reflect cell size. Furthermore, nuclear number, not cell number, appears to be the major factor controlling postnatal skeletal muscle growth.