Comparison of GH, IGF-I, and testosterone with mRNA of receptors and myostatin in skeletal muscle in older men

Growth hormone (GH), insulin-like growth factor I (IGF-I), and testosterone (T) are important mediators of muscle protein synthesis, and thus muscle m ass, all of which decline with age. We hypothesized that circulating hormon es would be related to the transcriptional levels of their respective recep tors and that this expression would be negatively related to expression of the myostatin gene. We therefore determined content of mRNA transcripts (by RT-PCR) for GH receptor (GHR), IGF-I, androgen receptor (AR), and myostati n in skeletal muscle biopsy samples from 27 healthy men >65 yr of age. Ther e were no significant relationships between age, lean body mass, or percent body fat and transcript levels of GHR, IGF-I, AR, or myostatin. Moreover, there were no significant correlations of serum GH, IGF-I, or T with their corresponding target mRNA levels (GHR, intramuscular IGF-I, or AR) in skele tal muscle. However, GHR was negatively correlated (r = -0.60, P = 0.001) w ith myostatin mRNA levels. The lack of apparent relationships of muscle tra nscripts with their respective ligands in healthy older adults suggests tha t age-related deficits in both GH and T may lead to an increase in myostati n expression and a disassociation in autocrine IGF-I effects on muscle prot ein synthesis, both of which could contribute to age-related sarcopenia.