Abstract
1. Protein degradation in rat L6 myoblasts is inhibited by high concentrations of insulin as well as by foetal bovine serum and bovine colostrum, mixtures rich in growth-factor activity. 2. Growth factors achieve maximal effects within 2 h after addition to the cell cultures, but these diminish with time. Indeed, during incubations greater than 12 h, foetal calf serum actually stimulates protein breakdown. The changed response, however, is not due to the depletion of growth factors from serum. 3. Protein breakdown is stimulated by dexamethasone by a process that takes several hours to be expressed, but is more pronounced over a 4 h measurement period than over 18h. The glucocorticoid response is prevented by insulin or by cycloheximide. 4. Anabolic agents such as trenbolone, diethylstilboestrol and testosterone do not alter rates of intracellular protein breakdown and do not interfere with the glucocorticoid-induced catabolic response. 5. The results are consistent with anabolic steroids and related agents acting indirectly on muscle, perhaps via altering concentrations of growth factors of the somatomedin type.