title: Circulating bioactive and immunoreactive IGF-I remain stable in women, despite physical fitness improvements after 8 weeks of resistance, aerobic, and combined exercise training.
abstract: Insulin-like growth factor-I (IGF-I) is regulated by a number of IGF-binding proteins (IGFBPs) and proteases that influence IGF-I bioactivity. A specific IGF-I kinase receptor activation assay (KIRA) has been developed that determines the ability of IGF-I to activate the IGF-I receptor by quantification of intracellular receptor autophosphorylation on IGF-I binding. KIRA-assessed IGF-I bioactivity has not been utilized within the context of chronic exercise training paradigms. This study measured total and free immunoreactive IGF-I, bioactive IGF-I, and IGFBP-1, -2, and -3 before (Pre), during (Mid), and after (Post) 8 wk of exercise training in young, healthy women, who were randomized into one of four groups: control (n = 10), resistance (n = 18), aerobic (n = 13), and combined (n = 15) exercise training. The training programs were effective in improving physical fitness specific to the exercise mode engaged in: increases were observed for lean mass ( approximately 2%), aerobic fitness (6-7%), and upper (20-24%) and lower (15-48%) body strength (all P values < 0.05). By contrast, no time, group, or interaction effects were observed for the circulating IGF-I system, as immunoreactive total (Pre = 264 +/- 16 microg/l; Mid = 268 +/- 17 microg/l; Post = 271 +/- 17 microg/l), free (Pre = 0.70 +/- 0.1 microg/l; Mid = 0.63 +/- 0.1 microg/l; Post = 0.63 +/- 0.2 microg/l) and bioactive (Pre = 2.35 +/- 0.3 microg/l; Mid = 2.25 +/- 0.3 microg/l; Post = 2.33 +/- 0.3 microg/l) IGF-I were unchanged throughout the study. All IGFBP measures were also unchanged. We conclude that increased lean mass, aerobic fitness, and upper and lower body strength resulting from an 8-wk exercise training programs can occur without concomitant increases in either circulating bioactive or immunoreactive IGF-I, as well as associated IGFBPs. In terms of reflecting positive anabolic neuromuscular outcomes, these data do not support a role for endocrine-derived IGF-I.
authors.affiliation: Military Performance Division, The US Army Research Institute of Environmental Medicine, Natick, MA 01760, USA. Bradley.nindl@us.army.mil
authors.forename: Bradley C
authors.initials: BC
authors.lastname: Nindl
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Alemany
authors.forename: Alexander P
authors.initials: AP
authors.lastname: Tuckow
authors.forename: Kevin R
authors.initials: KR
authors.lastname: Rarick
authors.forename: Jeffery S
authors.initials: JS
authors.lastname: Staab
authors.forename: William J
authors.initials: WJ
authors.lastname: Kraemer
authors.forename: Carl M
authors.initials: CM
authors.lastname: Maresh
authors.forename: Barry A
authors.initials: BA
authors.lastname: Spiering
authors.forename: Disa L
authors.initials: DL
authors.lastname: Hatfield
authors.forename: Allan
authors.initials: A
authors.lastname: Flyvbjerg
authors.forename: Jan
authors.initials: J
authors.lastname: Frystyk
mesh_terms: D000293:Adolescent; D000328:Adult; D001823:Body Composition; D015444:Exercise; D005260:Female; D006801:Humans; D018969:Insulin-Like Growth Factor Binding Proteins / Q000097:blood*; D007334:Insulin-Like Growth Factor I / Q000032:analysis / Q000378:metabolism*; D053580:Muscle Strength; D010809:Physical Fitness; D017526:Receptor, IGF Type 1 / Q000378:metabolism; D055815:Young Adult
chemical_list: D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1