Abstract
Irisin is a myokine primarily recognized for its effect on browning adipose tissue, however emerging evidence suggests irisin also has a role in regulating skeletal muscle hypertrophy. PURPOSE: The purpose of this study was to determine if age or training status results in a differing systemic irisin concentrations following acute blood-flow restricted (BFR) resistance-exercise. METHODS: Fifteen untrained males (Younger: 24.7 ± 3.9 yrs, n = 8; Older: 68.3 ± 4.6 yrs, n = 7) performed a bout of full-body BFR resistance-exercise before and after twelve weeks of progressive full-body resistance exercise training. Blood samples were collected via venipuncture before, immediately after, and 3, 6, 24 and 48 hours post-BFR resistance-exercise. Samples were analyzed in duplicate to determine irisin concentration. RESULTS: A main effect for age (younger = 480.23 ± 34.2 pg/mL vs. older = 281.36 ± 36.57 pg/mL; p = 0.001) and interaction effect for training status x time (p = 0.02) were identified. No other main or interaction effects were noted (all p > 0.05). The concentration of irisin did not differ between any time points in the untrained condition, but differences were observed after completing twelve weeks of resistance training. In the trained condition, irisin decreased immediately post-BFR compared to baseline (Baseline = 413.13 ± 44.47 pg/mL vs. Post-BFR = 286.97 ± 47.76; p = 0.004). The concentration of irisin immediately post-BFR was also lower than 6 hours post-BFR (Post-BFR = 286.97 ± 47.76 pg/mL vs. 6-hours = 448.20 ± 47.54; p = 0.002). CONCLUSION: Older adults have a lower concentration of irisin before and after acute BFRE regardless of training status. These results suggest that the myokine irisin could contribute to differences in skeletal muscle hypertrophy response to resistance exercise between younger and older males. In the untrained condition, irisin does not respond to acute BFR, however in the trained condition irisin decreases immediately following BFR and then peaks by 6 hours of recovery. The training status of individuals alters the response of irisin to acute resistance training potentially manipulating the time course of downstream insulin-like growth factor-1 and myostatin which irisin is postulated to upregulate and downregulate, respectively.
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