On the mechanism of growth hormone autofeedback regulation: possible role of somatostatin and growth hormone-releasing factor.
To determine the role of somatostatin (SRIF) and GH-releasing factor (GRF) in GH autofeedback, 20 micrograms rat GH in 2 microliter were injected into the third ventricle (IVT) 1 or 3 h before injection of the alpha 2-receptor stimulator clonidine (50 micrograms/kg, iv), which elevates plasma GH and TSH levels in normal rats. GH preinjected 1 or 3 h before clonidine significantly suppressed the clonidine-induced GH surge, whereas TSH release was not affected by GH. Preinjection of ovine LH IVT following the same protocol did not inhibit the clonidine-induced GH surge, suggesting a specific effect of IVT GH. Passive immunization with 400 microliters sheep antisomatostatin serum did not reverse the inhibition of the clonidine-induced GH surge by exogenous GH administered IVT either 1 or 3 h before clonidine. The TSH response was augmented by this procedure. Furthermore, IVT GH did not reduce the surges of GH and TSH elicited by GRF (250 ng/kg, iv) and TRH (150 ng/kg, iv) administered 1 or 3 h after IVT rat GH. These results suggest that GH autofeedback is mediated by reduced GRF secretion, rather than enhanced SRIF release.
Conway
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McCann
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Krulich
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D000818:Animals; D003000:Clonidine / Q000494:pharmacology; D013006:Growth Hormone / Q000008:administration & dosage / Q000378:metabolism* / Q000494:pharmacology; D013007:Growth Hormone-Releasing Hormone / Q000502:physiology*; D006706:Homeostasis; D007276:Injections, Intraventricular; D007986:Luteinizing Hormone / Q000008:administration & dosage / Q000494:pharmacology; D008297:Male; D051381:Rats; D012756:Sheep; D013004:Somatostatin / Q000502:physiology*; D013972:Thyrotropin / Q000097:blood; D013973:Thyrotropin-Releasing Hormone / Q000008:administration & dosage / Q000494:pharmacology; D013997:Time Factors
D013004:Somatostatin; D013973:Thyrotropin-Releasing Hormone; D007986:Luteinizing Hormone; D013972:Thyrotropin; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D003000:Clonidine