Exercise and fasting activate growth hormone-dependent myocellular signal transducer and activator of transcription-5b phosphorylation and insulin-like growth factor-I messenger ribonucleic acid expression in humans.
BACKGROUND
GH secretion is increased by stress, such as fasting and exercise.


OBJECTIVE
Our study was designed to test the hypothesis that fasting and exercise increase GH levels, GH-dependent signal transducer and activator of transcription (STAT)-5b phosphorylation, and IGF-I mRNA levels in human skeletal muscle.


METHODS
Eight healthy men (25.5 +/- 4.3 yr) were studied for 6 h (a 4 h basal and a 2 h clamp period) in the basal state and after 72 h fasting and a 1-h ergometer cycling at 65% maximal oxygen uptake. Muscle biopsies were taken at t = 30 and 270 min.


METHODS
The study was conducted at a university clinical research unit.


RESULTS
During exercise, GH concentrations rapidly increased to greater than 10 ng/ml (P < 0.001). Thirty minutes after exercise, we observed a 4-fold increase in signal transducer and activator of transcription (STAT)-5 phosphorylation (P < 0.001) followed by an increase in IGF-I mRNA after 270 min (P = 0.026). During fasting, more sporadic GH bursts occurred, leading to an overall 3-fold increase in GH area under the curve(t=0-270) (P < 0.001). Similarly, STAT5 patterns were less consistent, with a tendency toward increased phosphorylation (P = 0.067, area under the curve(t=0-270)), whereas IGF-I mRNAs were persistently increased (P < 0.01).


CONCLUSIONS
Our data show that myocellular GH signaling is stimulated after exercise and fasting in terms of increased STAT5 phosphorylation and/or IGF-I gene expression. This suggests that exercise with brief, well-defined GH peaks leads to distinct STAT5 phosphorylation and subsequent IGF-I gene expression, whereas fasting induces more sporadic GH bursts and less distinct but more persistent activation of the GH signal.
Department of Medicine and Endocrinology, Aarhus University Hospital, 8000 Aarhus C, Denmark. mikkel.vendelbo@ki.au.dk
Mikkel H
MH
Vendelbo
Jens O
JO
Jørgensen
Steen B
SB
Pedersen
Lars C
LC
Gormsen
Sten
S
Lund
Ole
O
Schmitz
Niels
N
Jessen
Niels
N
Møller
D000328:Adult; D001706:Biopsy; D018592:Cross-Over Studies; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D005215:Fasting / Q000097:blood / Q000378:metabolism / Q000502:physiology*; D015870:Gene Expression / Q000187:drug effects; D013006:Growth Hormone / Q000494:pharmacology* / Q000502:physiology; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000235:genetics* / Q000378:metabolism; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism / Q000473:pathology; D010766:Phosphorylation / Q000187:drug effects; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D050799:STAT5 Transcription Factor / Q000378:metabolism*; D055815:Young Adult
D012333:RNA, Messenger; D050799:STAT5 Transcription Factor; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone