M Hameed

Expression of IGF-I splice variants in young and old human skeletal muscle after high resistance exercise

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 (≈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.

Sarcopenia and hypertrophy: A role for insulin-like growth factor-1 in aged muscle?

HAMEED, M., S.D.R. HARRIDGE, and G. GOLDSPINK. Sarcopenia and hypertrophy: A role for insulin-like growth factor-1 in aged muscle? Exerc. Sport Sci. Rev., Vol. 30, No. 1, pp 15–19, 2002. Sarcopenia is the loss of muscle mass associated with the aging process. Although systemic or circulating growth hormone and insulin-like growth factor-1 levels fall as we age, this is likely to be of lesser importance in regard to muscle mass than the role of locally produced insulin-like growth factor-1s generated in the muscle in response to exercise.

Effects of eccentric cycling exercise on IGF-I splice variant expression in the muscles of young and elderly people.

Recovery from micro damage resulting from intensive exercise has been shown to take longer in older muscles. To investigate the factors that may contribute to muscle repair, we have studied the 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 response to 1 h of eccentric cycling exercise in young and old individuals. Subjects (nine young, aged 20–27 years and eight elderly, aged 67–75 years) completed an eccentric exercise protocol that consisted of 60 min of reverse pedal cycling. Workloads were chosen to give the same relative increases in oxygen uptake (VO2max) and heart rate in young and old subjects. Muscle biopsy samples were obtained from the quadriceps muscle before and 2¼ h after completion of the exercise bout and were analyzed for IGF‐IEa and MGF mRNA levels using real‐time quantitative PCR. No difference was observed between the baseline levels of the two splice variants between the two subject groups. Eccentric cycling exercise resulted in a significant increase in the mean MGF mRNA in both young and old subjects but did not alter IGF‐IEa mRNA levels in either age group. As reported previously ( Toft et al., 2002 ), the levels of serum creatine kinase and myoglobin, markers of muscle damage, were increased significantly from baseline and to 5 days after exercise in both young and old subjects. This supports previous research in suggesting that the MGF splice variant is sensitive to muscle damage‐inducing exercise and is differentially regulated compared with IGF‐IEa.

Clarification on IGF-I nomenclature

Journal of Physiology 547, 247–254 (2003)