ABSTRACT
Despite causing marked inhibition of somatic growth, glucocorticoids enhance both the response to GH-releasing hormone (GHRH) and the amplitude of naturally occurring GH secretory pulses in the male rat. The relative contribution of the two major hypothalamic regulatory factors for GH (somatostatin and GHRH) to these observed effects remains speculative. In the present studies, we have investigated endogenous and stimulated GH release in rats pretreated with glucocorticoid or vehicle, and the effects of passive immunoneutralization of somatostatin or GHRH.
In an initial study, four groups of eight rats were treated with either saline or various doses of a depot preparation of betamethasone: low dose, 0·85 mg; medium dose, 1·7 mg; high dose, 3·4 mg. All doses significantly suppressed body weight gain, total adrenal weight and concentrations of both plasma corticosterone and pituitary ACTH. Seven days after betamethasone treatment, GH responses to an i.v. injection of 1 μg human GHRH(1–29) were evaluated during pentobarbitone anaesthesia. Compared with saline-treated controls (peak GH concentration of 506·0±68·5 μg/l), peak GH levels were enhanced by the low dose (704·4±47·8 μg/l, P<0·05), unaltered by the medium dose (543±65·8 μg/l) and suppressed by the high dose (312·7±55·2 μg/l, P<0·05) of betamethasone. Similarly, the area under the secretory curves was increased by 46% following the low dose (P<0·01), unaltered by the medium dose and reduced by 33% after the high dose of betamethasone.
In a second study, rats were pretreated for 7 days before blood sampling with either the medium dose of betamethasone or saline. On day 5, 48 h before blood sampling, an indwelling venous catheter was fitted enabling sampling of conscious rats. On the day of study, blood samples were taken at 30-min intervals over an initial 2-h period (10.00–12.00 h). Following the sample at 12.00 h, rats were given the reconstituted and dialysed immunoglobulin fraction from either control sheep serum (NSIgG), sheep anti-rat GHRH serum (GHRHab) or sheep anti-somatostatin serum (SRIHab), and samples were taken for a further 90 min (12.30–14.00 h). Directly after the sample at 14.00 h, GH stimulation was effected in all rats using 1 μg human GHRH(1–29) with samples taken at 5, 10, 20 and 40 min following stimulation. During the initial sampling period, mean GH levels were significantly (P<0·005) higher in steroidpretreated animals than in saline-pretreated controls (29·3±5·8 vs 13·2±1·6 μg/l), with a higher amplitude secretory pulse occurring at 11.30 h (80·7±18·6 vs 26·4±4·1 μg/l, P < 0·01). Administration of GHRHab to saline-pretreated animals did not alter mean GH levels when compared with animals receiving control NSIgG (saline plus NSIgG, 9·3±1·1; saline plus GHRHab, 8±1·1 μg/l, P = NS). In contrast, the raised mean GH levels seen in betamethasone-pretreated rats receiving NSIgG (12·3 ±1·1 μg/l) were reduced by GHRHab administration (7·6±1·1 μg/l); these levels were not different from those of the saline-pretreated group suggesting that the observed permissive effect of glucocorticoids on GH secretion is mediated through enhanced GHRH activity. SRIHab increased mean basal GH levels to a similar extent in both saline- and betamethasone-pretreated groups (17·4±1·2 μg/l and 19·3 ±1·1 μg/l respectively, P<0·01 vs comparable NSIgG group). Administration of the various immunoglobulin fractions had no effect on GHRH-stimulated GH secretion except when SRIHab was given to betamethasone-pretreated animals, resulting in a significantly increased peak response (1467±93 μg GH/l, P<0·001) when compared with either saline- or betamethasone-pretreated rats given NSIgG (643±95 and 791±92 μg/l respectively). This enhancement following SRIHab administration was not seen in saline-pretreated animals (893±180 μg GH/l).
These results imply that glucocorticoid treatment increases basal GH levels through a GHRH-dependent mechanism and also increases pituitary sensitivity to exogenous GHRH when inhibitory somatostatin tone is blocked.
Journal of Endocrinology (1991) 131, 75–86
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