Desensitized morphological and cytokine response after stretch-shortening muscle contractions as a feature of aging in rats.

Recovery from contraction-induced injury is impaired with aging. At a young age, the secondary response several days following contraction-induced injury consists of edema, inflammatory cell infiltration, and segmental muscle fiber degeneration to aid in the clearance of damaged tissue and repair. This morphological response has not been wholly established at advanced age. Our aim was to characterize muscle fiber morphology 3 and 10 days following stretch-shortening contractions (SSCs) varying in repetition number (i.e. 0, 30, 80, and 150) for young and old rats. For muscles of young rats, muscle fiber degeneration was overt at 3 days exclusively after 80 or 150 SSCs and returned significantly closer to control values by 10 days. For muscles of old rats, no such responses were observed. Transcriptional microarray analysis at 3 days demonstrated that muscles of young rats differentially expressed up to 2144 genes while muscles of old rats differentially expressed 47 genes. Bioinformatic analysis indicated that cellular movement was a major biological process over-represented with genes that were significantly altered by SSCs especially for young rats. Protein levels in muscle for various cytokines and chemokines, key inflammatory factors for cell movement, increased 3- to 50-fold following high-repetition SSCs for young rats with no change for old rats. This age-related differential response was insightful given that for control (i.e. 0 SSCs) conditions, protein levels of circulatory cytokines/chemokines were increased with age. The results demonstrate ongoing systemic low-grade inflammatory signaling and subsequent desensitization of the cytokine/chemokine and morphological response to contraction-induced injury with aging - features which accompany age-related impairment in muscle recovery.