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The GH-releasing activity of the ₂-adrenergic agonist clonidine has been extensively studied in the rat, but the mechanism(s) by which clonidine stimulates GH release remains controversial. In the present study, we examined the effects of various doses of clonidine on spontaneous pulsatile GH secretion in conscious rats, and tested the hypothesis that the GH-releasing activity of clonidine is mediated primarily by an inhibition of hypothalamic somatostatin (SRIF) release. In the first experiment, free-moving adult male rats were given either saline or various doses of clonidine i.v. (30, 50 and 125 µg/kg) at times of spontaneous peaks (1100 h) and troughs (1300 h) in the GH rhythm. Clonidine, at all doses tested, failed to stimulate GH release when administered at the time of a spontaneous peak. In contrast, injection of clonidine at trough times (when SRIF tone is high) consistently augmented plasma GH levels (mean ± S.E.M. integrated GH release; 30 µg/kg, 1843·0±484·0; 50 µg/kg, 1469·0± 490·3; 125 µg/kg, 1675·6 ± 513·4 vs 201·3 ± 100·1 ng/ml per 45 min in saline-injected controls; P<0·05 or less). No significant regression was observed between increasing doses of clonidine and GH release. In the second experiment, i.v. administration of 30 µg clonidine/kg during a GH trough period, 30 min prior to GH-releasing factor (GRF) challenge, significantly potentiated the GH response to GRF compared with rats given saline (7218·7 ±806·6 vs 4206·9 ± 1068·1 ng/ml per 30 min; P<0·05). Clonidine treatment, at all doses tested, resulted in hyperglycaemia and behavioural effects. These results showed that: (1) clonidine is not a potent GH secretagogue in the rat; (2) when administered i.v., clonidine exerts a maximal GH-releasing activity already at the dose of 30 µg/kg; and (3) clonidine-induced GH release in the rat occurs mainly through an inhibition of hypothalamic SRIF release rather than by stimulating GRF secretion.

Journal of Endocrinology (1994) 141, 259–266

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