NO EFFECT OF AGING ON SKELETAL-MUSCLE INSULIN-LIKE GROWTH-FACTOR MESSENGER-RNAS

This study examined the hypothesis that during aging insulin-like grow th factor (IGF) mRNAs are reduced in skeletal muscle. IGF-I, IGF-II, a nd IGF-binding protein-5 (IGFBP-5) mRNAs were measured with a ribonucl ease protection assay in the gastrocnemius of specific pathogen-free F ischer-344 rats. We hypothesized that IGF-I, IGF-II, and IGFBP-5 mRNA concentration (normalized to 18S RNA) in the gastrocnemius muscle of g rowing animals (3 mo) would be downregulated in a coordinated manner w ith muscle size during aging-associated atrophy. As indicated by muscl e wet weight and total protein content, the gastrocnemius muscle was g rowing in the 3-mo group (P < 0.01 smaller compared with 12 mo), fully developed at 12 mo, and was atrophied at 24 mo of age (P < 0.05 compa red with 12 mo). IGF-I mRNA concentration in the gastrocnemius of 12- and 24-mo-old rats was 39-49% less than in 3-mo-old rats (P < 0.05). C ontrary to our hypothesis, there was not a significant skeletal muscle IGF-I mRNA difference between middle age (12 mo) and senescence (24 m o). Thus IGF-I mRNA changed during maturation (3-12 mo) but not during aging (12-24 mo). Skeletal muscle IGF-II mRNA concentration was not d ifferent among 3-, 12-, and 24-mo-old animals. Furthermore, animal age did not have an effect on IGFBP-5 mRNA concentration. We conclude tha t the aging-associated atrophy of skeletal muscle is not caused by alt ered pretranslational regulation of IGF-I, IGF-II, or IGFBP-5 in skele tal muscle.