title: Expression of IGF-I splice variants in young and old human skeletal muscle after high resistance exercise.
abstract: The mRNA expression of two splice variants of the insulin-like growth factor-I (IGF-I) gene, IGF-IEa and mechano growth factor (MGF), were studied in human skeletal muscle. Subjects (eight young, aged 25-36 years, and seven elderly, aged 70-82 years) completed 10 sets of six repetitions of single legged knee extensor exercise at 80 % of their one repetition maximum. Muscle biopsy samples were obtained from the quadriceps muscle of both the control and exercised legs 2.5 h after completion of the exercise bout. Expression levels of the IGF-I mRNA transcripts were determined using real-time quantitative RT-PCR with specific primers. The resting levels of MGF were significantly (approximately 100-fold) lower than those of the IGF-IEa isoform. No difference was observed between the resting levels of the two isoforms between the two subject groups. High resistance exercise resulted in a significant increase in MGF mRNA in the young, but not in the elderly subjects. No changes in IGF-IEa mRNA levels were observed as a result of exercise in either group. The mRNA levels of the transcription factor MyoD were greater at rest in the older subjects (P < 0.05), but there was no significant effect of the exercise bout. Electrophoretic separation of myosin heavy chain (MHC) isoforms showed the older subjects to have a lower (P < 0.05) percentage of MHC-II isoforms than the young subjects. However, no association was observed between the composition of the muscle and changes in the IGF-I isoforms with exercise. The data from this study show an attenuated MGF response to high resistance exercise in the older subjects, indicative of age-related desensitivity to mechanical loading. The data in young subjects indicate that the MGF and IGF-IEa isoforms are differentially regulated in human skeletal muscle.
authors.affiliation: Department of Physiology, Royal Free and University College Medical School, Rowland Hill Street, London NW3 2PF, UK.
authors.forename: M
authors.initials: M
authors.lastname: Hameed
authors.forename: R W
authors.initials: RW
authors.lastname: Orrell
authors.forename: M
authors.initials: M
authors.lastname: Cobbold
authors.forename: G
authors.initials: G
authors.lastname: Goldspink
authors.forename: S D R
authors.initials: SD
authors.lastname: Harridge
mesh_terms: D000328:Adult; D000368:Aged; D000375:Aging / Q000502:physiology*; D017398:Alternative Splicing / Q000502:physiology*; D015444:Exercise / Q000502:physiology*; D015870:Gene Expression / Q000502:physiology; D006801:Humans; D006984:Hypertrophy; D007334:Insulin-Like Growth Factor I / Q000235:genetics*; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D017570:MyoD Protein / Q000235:genetics; D018995:Myosin Heavy Chains / Q000235:genetics; D012333:RNA, Messenger / Q000032:analysis; D014891:Weight Lifting / Q000502:physiology
chemical_list: D017570:MyoD Protein; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I; D018995:Myosin Heavy Chains