title: Exercise-induced irisin secretion is independent of age or fitness level and increased irisin may directly modulate muscle metabolism through AMPK activation.
abstract: BACKGROUND
  Irisin has been proposed to be a myokine mediating the effect of exercise on adipocyte browning. The physiology of irisin in humans is not completely understood.
  
  
  OBJECTIVE
  To study the physiology of irisin in healthy individuals with different age and fitness levels and to explore the direct effects of irisin on muscle metabolism.
  
  
  METHODS
  Treadmill exercise studies were conducted to measure circulating irisin at baseline and in response to exercise among old and young, physically active and sedentary individuals. Also, high- and moderate-intensity swimming was performed in adolescent men and women to study the effect of exercise intensity and the time course of irisin induction by acute bouts of exercise. Human myotubes were treated with recombinant irisin, and the effect on gene expression, cell signaling, and metabolism was examined.
  
  
  RESULTS
  Baseline circulating irisin was lower in old (vs young) and physically active (vs sedentary) subjects. Despite differences in basal levels, the percentage increase of irisin by acute bouts of exercise was not related to age or fitness level. The time course study revealed that circulating irisin increased immediately after high-intensity interval exercise and declined 1 hour thereafter. In vitro experiments showed that irisin facilitates glucose and lipid metabolism in human muscle through AMP kinase phosphorylation.
  
  
  CONCLUSIONS
  Despite the differences in basal irisin levels, exercise-induced irisin secretion is independent of age or fitness level. Increased irisin can directly modulate muscle metabolism through AMP kinase activation.
authors.affiliation: Division of Endocrinology, Diabetes, and Metabolism (J.Y.H., A.F., G.P., K.H.P., C.S.M.), Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215; School of Physical Education and Sports Science (V.M., A.K., A.S.), Aristotle University of Thessaloniki, Thessaloniki 54124, Greece; Department of Physical Education and Sport Sciences (I.F., I.I.D.), Democritus University of Thrace, Komotini 69100, Greece; Department of Family Medicine (K.H.P.), Hallym University Sacred Heart Hospital, Hallym University, Gyeonggi-do 431-796, Korea; Section of Endocrinology (C.S.M.), Boston VA Healthcare System, Harvard Medical School, Boston, Massachusetts 02130.
authors.forename: Joo Young
authors.initials: JY
authors.lastname: Huh
authors.forename: Vassilis
authors.initials: V
authors.lastname: Mougios
authors.forename: Athanasios
authors.initials: A
authors.lastname: Kabasakalis
authors.forename: Ioannis
authors.initials: I
authors.lastname: Fatouros
authors.forename: Aikaterina
authors.initials: A
authors.lastname: Siopi
authors.forename: Ioannis I
authors.initials: II
authors.lastname: Douroudos
authors.forename: Andreas
authors.initials: A
authors.lastname: Filippaios
authors.forename: Grigorios
authors.initials: G
authors.lastname: Panagiotou
authors.forename: Kyung Hee
authors.initials: KH
authors.lastname: Park
authors.forename: Christos S
authors.initials: CS
authors.lastname: Mantzoros
mesh_terms: D000263:Adenylate Kinase / Q000378:metabolism*; D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D015444:Exercise / Q000502:physiology*; D005260:Female; D005353:Fibronectins / Q000097:blood*; D006801:Humans; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000378:metabolism*; D010809:Physical Fitness / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology; D055815:Young Adult
chemical_list: C577593:FNDC5 protein, human; D005353:Fibronectins; D000263:Adenylate Kinase