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Effects of Recombinant Human Insulin-Like Growth Factor I Administration on the Growth Hormone (GH) Response to GH-Releasing Hormone in Obesity¹

Division of Endocrinology and Metabolism, Department of Internal Medicine, University of Turin, Turin; and Department of Pharmacology, University of Milan, Milan, Italy

Address all correspondence and requests for reprints to: E. Ghigo, M.D., Divisione di Endocrinologia, Ospedale Molinette, C.so Dogliotti 14, 10126 Torino, Italy. E-mail: ezio.ghigo{at}unito.it

Circulating GH levels are reduced in obesity due to true reduction of the 24-h GH production rate. GH insufficiency in obesity might reflect neuroendocrine abnormalities and/or alterations in peripheral hormones and metabolic factors. The somatotroph response to provocative stimuli including GHRH is markedly blunted in obese patients. However, the somatotroph responsiveness to GHRH in obesity shows also peculiar refractoriness to the inhibitory effect of glucose load. In this present study we aimed at verifying the effect of low dose rhIGF-I (20 µg/kg, sc, at 0 min) on the GH response to GHRH (1 µg/kg, iv, at 180 min) in obesity. With this goal in mind, six obese women with abdominal adiposity [OB; age (mean ± SEM), 32.3 ± 4.4 yr; body mass index, 32.8 ± 2.3 kg/m²] were studied. The effects of recombinant human insulin-like growth factor I (rhIGF-I) administration on circulating total IGF-I, insulin, and glucose levels were also evaluated. The results in OB were compared with those recorded in age-matched lean women (NW; age, 28.3 ± 1.2 yr; body mass index, 20.1 ± 0.5 kg/m²), in whom the inhibitory effect of rhIGF-I had already been shown. Basal IGF-I levels in OB were similar to those in NW (199.7 ± 33.3 vs. 274.4 ± 25.3 µg/L). The mean GH concentration over 3 h (from 0–180 min) in OB was lower than that in NW (0.9 ± 0.4 vs. 2.6 ± 0.8 µg/L; P = NS). Administration of GHRH induced a GH response in OB lower than that in NW (area under the curve from 180–270 min, 576.5 ± 137.5 vs. 1315.9 ± 189.9 µg/L·min; P < 0.02). Administration of rhIGF-I increased circulating IGF-I levels in both groups to the same percent extent (326.8 ± 28.3 and 420.3 ± 26.5 µg/L in OB and NW, respectively). rhIGF-I administration inhibited the GH response to GHRH in OB (240.1 ± 99.6 µg/L; P < 0.05) as well as in NW (730.2 ± 288.1 µg/L; P < 0.05), although it failed to lower the mean GH concentration over 3 h in either OB or NW. After rhIGF-I the GH response to GHRH in OB was slight and was still lower (P < 0.05) than that in NW; in fact, the percent decreases were similar in both groups (44.21 ± 14.06 and 48.21 ± 13.95 µg/L, in OB and NW, respectively). The mean insulin (107.1 ± 21.9 and 36.8 ± 7.2 pmol/L), but not glucose (4.0 ± 0.3 and 4.1 ± 0.1 mmol/L), levels calculated over 270 min, were higher (P = 0.005) in OB than in NW; rhIGF-I administration did not modify insulin and glucose levels in either group. Our study shows that the sc administration of a low rhIGF-I dose inhibits the somatotroph responsiveness to GHRH in obese as well as in normal subjects, indicating that somatotroph sensitivity to the inhibitory effect of rhIGF-I is preserved in obesity.

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