Adrenergic regulation of growth hormone secretion in the ewe

This study examined the role of the adrenergic system in the regulation of growth hormone (GH) secretion in sheep. Intravenous infusion of noradrenaline (0.5 microgram/kg per min for 2 hr) totally suppressed plasma GH concentrations. Concomitant treatment of animals with the beta-adrenergic antagonist propranolol completely blocked the noradrenaline-induced suppression of GH. In contrast, intravenous injection of the centrally acting alpha 2-agonist clonidine (2 micrograms/kg) elicited a release of GH. To further investigate the central adrenergic regulation of GH secretion 10 micrograms of noradrenaline or adrenaline was microinjected (1 microliter) directly into the preoptic area of the hypothalamus of ovariectomized ewes. When the time of injection coincided with a GH trough period, both noradrenaline and adrenaline caused an increase in plasma GH concentrations, whereas if the injection coincided with an endogenous pulse of GH no additional GH response was obtained. In conclusion, these results provide evidence for the involvement of the adrenergic system in the regulation of GH secretion in sheep. Centrally, adrenergic pathways exert a stimulatory effect on GH release via an alpha 2-adrenergic system, whereas peripherally adrenergic pathways exert an inhibitory effect via beta-adrenergic mediated mechanisms. Furthermore, adrenergic stimulation of the preoptic area may inhibit somatostatin activity and directly facilitate a GH pulse. Alternatively, adrenergic innervation of the preoptic area may influence neurons (somatostatin or other) that project to the arcuate nucleus and stimulate the release of GH-releasing factor.