Circulating bioactive and immunoreactive IGF-I remain stable in women, despite physical fitness improvements after 8 weeks of resistance, aerobic, and combined exercise training.
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.
Nindl
Bradley C
BC
Military Performance Division, The US Army Research Institute of Environmental Medicine, Natick, MA 01760, USA. Bradley.nindl@us.army.mil
Alemany
Joseph A
JA
Tuckow
Alexander P
AP
Rarick
Kevin R
KR
Staab
Jeffery S
JS
Kraemer
William J
WJ
Maresh
Carl M
CM
Spiering
Barry A
BA
Hatfield
Disa L
DL
Flyvbjerg
Allan
A
Frystyk
Jan
J
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
D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1