Regulation of growth hormone-releasing hormone and somatostatin from perifused, bovine hypothalamic slices. I. α2-adrenergic receptor regulation

• Published in: Domestic animal endocrinology 1997, Vol.14 (5), p.334-348
• Main Authors: West, C.R., Gaynor, P.J., Lookingland, K.J., Tucker, H.A.
• Format: Article
• Language: English
• ISSN: 0739-7240; 1879-0054
• DOI: 10.1016/S0739-7240(97)00030-1

An in vitro perifusion system was developed for bovine hypothalamic tissue to examine the role of α 2-adrenergic receptors in the regulation of growth hormone-releasing hormone (GHRH) and somatostatin (SRIF) release. Up to three sagittal slices (600 μm) of hypothalamus, immediately parallel to the midline, were cut in an oxygenated balanced salt solution at 4° C, placed in 5 cc syringes, and perifused at 37° C with oxygenated minimum essential medium-α at a flow rate of 0.15 ml/min. Three experiments were conducted, and medium effluent was collected every 20 min before (two samples), during (one or three samples), and after (six samples) treatment. Areas under GHRH and SRIF response curves (AUC), adjusted by covariance for pretreatment values, were calculated from samples collected during the treatment/post-treatment period. Location from which slices were cut, relative to the sagittal midline, had no effect on basal release of GHRH and SRIF, but variation in basal release of GHRH and SRIF differed among animals. Medium containing 60 mM KCl increased AUC for GHRH 39% and 161% for SRIF when compared with perifusion of medium alone, thereby verifying that tissue remained viable for at least 14 hr. Activation of α 2-adrenergic receptors with 10 −6 and 10 −4 M clonidine increased AUC for GHRH from 54.8 (control) to 79.1 and 108.7 ± 2.5 ng · ml −1 min for 10 −6 M and 10 −4 M clonidine, respectively. Guanabenz, another α 2-adrenergic receptor agonist, at 10 −8, 10 −6, and 10 −4 M also increased GHRH release from 45.5 (control) to 52.8, 66.2, and 86.7 ± 1.6 ng · ml −1 min, respectively. Clonidine and guanabenz did not affect release of SRIF. An α 2-adrenergic receptor antagonist, idazoxan, blocked clonidine-induced release of GHRH without affecting release of SRIF. We concluded that α 2-adrenergic receptor stimulation of in vivo growth hormone secretion in cattle is mediated via an increase in release of GHRH and not a change in release of SRIF.