title: Suppression of the growth hormone (GH) response to clonidine and GH-releasing hormone by exogenous GH.
abstract: GH release in response to clonidine and human GH-releasing hormone-(1-44) (hGHRH-44) was assessed in 11 boys (aged 7-14 yr) with short stature, who had normal GH secretion. The response to these 2 provocative stimuli was repeated after, respectively, 2 and 3 days of treatment with human GH (0.1 U/kg, im). Exogenous GH significantly blunted the response to both clonidine [the mean 2-h integrated serum GH concentration falling from 1050 +/- 350 (+/- SEM) to 749 +/- 297 ng/ml X min; P = 0.03] and hGHRH-44, the 2-h integrated GH concentration falling from 1553 +/- 358 to 547 +/- 202 ng/ml X min; (P = 0.03). Plasma insulin-like growth factor (IGF-II) concentrations did not change after GH administration. In contrast, plasma IGF-I (somatomedin-C) concentrations increased from 97 +/- 16 ng/ml before administration of GH to 142 +/- 32 ng/ml (P = 0.05) after two days and 149 +/- 23 ng/ml (P less than 0.01) after the third treatment day. However, no correlation was found between the changes in response to clonidine or hGHRH-44 and changes in circulating levels of IGF-I. Our data confirm the existence of GH-dependent feedback inhibition of GH release during childhood and suggest that this inhibition operates, at least in part, at the level of the pituitary. While participation of the IGFs/somatomedins in this feedback loop cannot be excluded, the inhibitory effects of exogenous GH do not depend directly on circulating plasma IGF-I or IGF-II levels.
authors.forename: J M
authors.initials: JM
authors.lastname: Nakamoto
authors.forename: J M
authors.initials: JM
authors.lastname: Gertner
authors.forename: C M
authors.initials: CM
authors.lastname: Press
authors.forename: R L
authors.initials: RL
authors.lastname: Hintz
authors.forename: R G
authors.initials: RG
authors.lastname: Rosenfeld
authors.forename: M
authors.initials: M
authors.lastname: Genel
mesh_terms: D000293:Adolescent; D002648:Child; D003000:Clonidine / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D005246:Feedback; D013006:Growth Hormone / Q000097:blood / Q000494:pharmacology*; D013007:Growth Hormone-Releasing Hormone / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000097:blood; D007335:Insulin-Like Growth Factor II / Q000097:blood; D008297:Male; D013997:Time Factors
chemical_list: D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D003000:Clonidine