Effect of chronic clonidine administration on GH secretion in adult human subjects

It is well known that the acute administration of clonidine, an alpha 2-adrenergic agonist commonly used as an antihypertensive drug, stimulates GH secretion, likely via hypothalamic growth hormone releasing hormone (GHRH) release. Conversely, evidences of a hyperactivity of GHRH-GH-somatomedin C (SMC) axis during chronical administration of clonidine are controversial. In this study, GH and SMC levels have been evaluated in 16 hypertensive patients chronically treated with clonidine. The subjects were randomized to receive either TRH or saline with the aim of evaluating the non specific GH response to TRH as compared to spontaneous fluctuations during a control test. In basal conditions, GH and SMC concentrations in clonidine treated patients were similar to those observed in an age and sex matched group of normal untreated subjects. An abnormal increase in plasma GH occurred in 5 out of the 10 patients who received TRH, while plasma GH did not show significant variations during testing in the subjects who received saline. It is suggested that chronical administration of clonidine does not induce an hyperactivity of GHRH-GH-SMC axis as estimated by plasma GH and SMC concentrations, but may induce a disorder in hypothalamic control of GH secretion, possibly implicated in the abnormal GH responsivity to TRH.