INSULIN-LIKE GROWTH-FACTOR-I, ACTIN, AND MYOSIN HEAVY-CHAIN MESSENGER-RNAS IN SKELETAL-MUSCLE AFTER AN INJECTION OF GROWTH-HORMONE IN SUBJECTS OVER 60 YEARS OLD

Growth hormone (GH) increases the amount of insulinlike growth factor- I (IGF-I) mRNA in rat skeletal muscle, but this effect has not been de monstrated in human muscle. An autocrine effect of IGF-I produced in m uscle may be an important determinant of the increased muscle mass ass ociated with GH therapy. Thus, we examined IGF-I mRNA abundance in ske letal muscle biopsy samples taken 10 h after a subcutaneous injection of GH (0.03 mg/kg, n=6) or placebo (normal saline, n=5) in men and wom en over 60 years of age. Relative tissue concentrations of IGF-I mRNA were evaluated with a competitive reverse transcriptase-polymerase cha in reaction assay. Mean plasma IGF-I concentrations rose steadily afte r the GH injection, and were 74% higher in the GH group than in the co ntrol group at the time of the muscle biopsies. There was no consisten t difference between the GH and control groups in muscle IGF-I mRNA ab undance when expressed in relation to total RNA or polyadenylated RNA. However, one GH-treated subject had three times more IGF-I mRNA, rela tive to polyadenylated RNA, than the average control subject, There wa s no effect of GH on levels of mRNAs encoding the most abundant myofib rillar proteins, actin and myosin heavy chain. These data do not suppo rt the hypothesis that increased IGF-I mRNA abundance in skeletal musc le is required for the anabolic effect of GH in people over 60 years o f age.