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Background:Exercise promotes numerous phenotypic adaptations in skeletal muscle that contribute to improved function and metabolic capac-ity.An emerging body of evidence suggests that skeletal muscle also releases a myriad of factors during exercise,termedmyokines.The pur-pose of this study was to examine the effects of high-intensity interval training (HIIT) on the acute regulation of the mRNA expression of several myokines,including the prototypical myokine interleukin-6(IL-6),and recently identified myokines fibronectin type III domain-contain-ing protein 5(FNDC5)(irisin)and meteorin-like protein(METRNL). Methods:Both before and after a 20-day period of twice-daily high-volume HIIT,9 healthy males(20.5 ± 1.5 years performed a standardized bout of high-intensity interval exercise(HIIE;5×4 min at~80% pretraining peak power output)with skeletal muscle biopsy samples(vastus lat-eralis)obtained at rest,immediately following exercise,and at 3 h recovery. Results:Before training,a single bout of HIIE increased IL-6(p<0.05)and METRNL(p<0.05)mRNA expression measured at 3 h recovery when compared to rest.Following 20 days of HIIT,IL-6 and FNDC5 mRNA were increased at 3 h recovery from the standardized HIIE bout when compared to rest(both p<0.05).Resting METRNL and FNDC5 mRNA expression were higher following training(p<0.05),and there was an overall increase in FNDC5 mRNA post-training(main effect of training,p<0.05). Conclusion:In human skeletal muscle(1)an acute bout of HIIE can induce upregulation of skeletal muscle IL-6 mRNA both before and after a period of intensified HIIT;(2)Resting and overall FNDC5 mRNA expression is increased by 20 days of HIIT;and(3)METRNL mRNA expression is respon-sive to both acute HIIE and short-term intense HIIT.Future studies are needed to confirm these findings at the protein and secretion level in humans.