Changes in free rather than total insulin-like growth factor-I enhance insulin sensitivity and suppress endogenous peak growth hormone (GH) release following short-term low-dose GH administration in young healthy adults.
High-dose GH administration is commonly associated with impaired insulin sensitivity (S(I)) in humans. Paradoxically we have shown that low-dose GH (1.7 microg/kg.d) administration enhances beta-cell function in young healthy adults. In the present double-blind, placebo-controlled, cross-over study, we explored the physiological effects of this low GH dose on glucose metabolism in 12 young healthy adults (seven males, 19-29 yr). At pretreatment and after each 14-d treatment block, overnight metabolic profiles were assessed followed by a hyperinsulinemic euglycemic clamp, whereas fasting blood samples were collected weekly. In subjects treated with GH first (group A, n = 6), GH treatment increased total IGF-I (P < 0.05) and IGF binding protein-3 (P < 0.01) after 7 d, but these levels subsequently returned to pretreatment levels after 14 d. In contrast, free IGF-I increased (P < 0.05), and overnight GH pulse peak amplitude decreased (P < 0.01) after 14 d. In subjects treated with placebo first (group B, n = 6), all biochemical parameters were unchanged after placebo treatment, whereas the changes in free and total IGF-I were similar to those of group A after GH treatment. Combined clamp data from both groups A and B (n = 12) showed that 14-d GH treatment decreased overnight plasma insulin levels (P < 0.02) and hepatic glucose appearance (P < 0.05) and increased S(I) (P < 0.01). Of note, the GH-induced changes in S(I) positively correlated with the changes in free IGF-I (r = 0.72, P < 0.01). In conclusion, low-dose GH administration enhanced S(I) and suppressed endogenous peak GH release, and we hypothesize that these effects are the direct result of increased serum levels of free IGF-I.
Yuen
Kevin
K
Department of Paediatrics, University of Cambridge, Cambridge CB2 2QQ, United Kingdom.
Frystyk
Jan
J
Umpleby
Margot
M
Fryklund
Linda
L
Dunger
David
D
D000328:Adult; D018592:Cross-Over Studies; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D004334:Drug Administration Schedule; D005260:Female; D015309:Glucose Clamp Technique; D013006:Growth Hormone / Q000008:administration & dosage*; D019382:Human Growth Hormone / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000502:physiology*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008297:Male; D012016:Reference Values
D007328:Insulin; D019382:Human Growth Hormone; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone