title: Low doses of estradiol partly inhibit release of GH in sheep without affecting basal levels.
abstract: Estradiol increases basal growth hormone (GH) concentrations in sheep and cattle. This study sought to determine the effects of estradiol on GH-releasing hormone (GRH)-stimulated GH release in sheep. Growth hormone secretory characteristics, the GH response to GRH, and steady-state GH mRNA concentrations were determined in castrated male lambs treated with 2 different doses of estradiol 17-beta for a 28-d experimental period. Although no differences between treatments in mean GH, basal GH, or GH pulse number were observed after 28 d of estradiol treatment, GH pulse amplitude was greater (P < 0.05) in the 2.00-cm implant-treated animals than in the control and 0.75-cm implant group. The effect of estradiol treatment on GRH-stimulated GH release revealed differences between the control and estradiol-treated animals (P < 0.05). The 15-min GH responses to 0.075 microg/kg hGRH in the control, 0.75-cm, and 2.00-cm implant groups, respectively, were 76 +/- 10, 22.6 +/- 2.1, and 43.6 +/- 15.0 ng/mL. Growth hormone mRNA content was determined for pituitary glands from the different treatment groups, and no differences in steady-state GH mRNA levels were observed. There were no differences in the mean plasma concentrations of IGF-I, cortisol, T(3), or T(4) from weekly samples. Growth hormone release from cultured ovine pituitary cells from control sheep was not affected by estradiol after 72 h or in a subsequent 3-h incubation with estradiol combined with GRH. These data suggest that estradiol has differing actions on basal and GRH-stimulated GH concentrations in plasma, but the increase in pulse amplitude does not represent an increased pituitary sensitivity to GRH.
authors.affiliation: Stark Neurosciences Research Institute, Indiana University School of Medicine, Indianapolis, IN 46202, USA.
authors.forename: A
authors.initials: A
authors.lastname: Hudmon
authors.forename: G
authors.initials: G
authors.lastname: Davenport
authors.forename: E S
authors.initials: ES
authors.lastname: Coleman
authors.forename: J L
authors.initials: JL
authors.lastname: Sartin
mesh_terms: D000818:Animals; D015152:Blotting, Northern / Q000662:veterinary; D004958:Estradiol / Q000494:pharmacology*; D013006:Growth Hormone / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D013007:Growth Hormone-Releasing Hormone / Q000502:physiology*; D006854:Hydrocortisone / Q000097:blood / Q000502:physiology; D015151:Immunoblotting / Q000662:veterinary; D007334:Insulin-Like Growth Factor I / Q000502:physiology; D016018:Least-Squares Analysis; D008297:Male; D010902:Pituitary Gland / Q000502:physiology; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D011897:Random Allocation; D012756:Sheep / Q000502:physiology*; D013974:Thyroxine / Q000097:blood / Q000502:physiology; D014284:Triiodothyronine / Q000097:blood / Q000502:physiology
chemical_list: D012333:RNA, Messenger; D014284:Triiodothyronine; D004958:Estradiol; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D013974:Thyroxine; D006854:Hydrocortisone