Use of a leucine clamp to demonstrate that IGF-I actively stimulates protein synthesis in normal humans.
Insulin-like growth factor I (IGF-I) is thought to mediate the anabolic action of growth hormone. A glucose and amino acid clamp technique was used to investigate the effects of a 3-h intravenous infusion of either 43.7 pmol.kg-1.min-1 (20 micrograms.kg-1.h-1) IGF-I or 3.4 pmol.kg-1.min-1 (0.5 mU.kg-1.min-1) insulin on whole body leucine turnover in five normal human volunteers. During the IGF-I infusion, IGF-I levels increased (P < 0.01; 26.6 +/- 2.8 to 88.9 +/- 14.2 nmol/l) and insulin levels fell (P < 0.05; 0.096 +/- 0.018 to 0.043 +/- 0.009 nmol/l). During the insulin infusion, insulin levels increased (P < 0.01; 0.057 +/- 0.013 to 0.340 +/- 0.099 nmol/l), and there was no change in IGF-I. There was no significant change in leucine production rate (Ra; a measure of protein degradation) during the IGF-I infusion (2.23 +/- 0.17 to 2.13 +/- 0.2 mumol.kg-1.min-1), but there was an increase (P < 0.03) in nonoxidative leucine disposal rate (Rd; a measure of protein synthesis; 1.83 +/- 0.15 to 2.05 +/- 0.21 mumol.kg-1.min-1). In contrast, insulin reduced (P < 0.02) leucine Ra (1.81 +/- 0.24 to 1.47 +/- 0.24 mumol.kg-1.min-1) and had no effect on nonoxidative leucine Rd (1.44 +/- 0.25 to 1.41 +/- 0.22 mumol.kg-1.min-1). We conclude that IGF-I, under conditions of adequate substrate supply, directly increases protein synthesis in contrast to insulin, which exerts its anabolic action by reducing proteolysis.
Russell-Jones
D L
DL
Department of Medicine, United Medical School, London, United Kingdom.
Umpleby
A M
AM
Hennessy
T R
TR
Bowes
S B
SB
Shojaee-Moradie
F
F
Hopkins
K D
KD
Jackson
N C
NC
Kelly
J M
JM
Jones
R H
RH
Sönksen
P H
PH
D000328:Adult; D006801:Humans; D007328:Insulin / Q000097:blood / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000378:metabolism / Q000494:pharmacology*; D007930:Leucine / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D014176:Protein Biosynthesis; D012016:Reference Values
D007328:Insulin; D007334:Insulin-Like Growth Factor I; D007930:Leucine