Evidence for a growth hormone releasing factor mediating alpha-adrenergic influence on growth hormone secretion in the rat.

The effects of adrenergic receptor agonists on GH secretion were studied in adult, male rats pretreated with reserpine and somatostatin antiserum. Frequent blood samples were obtained from intra-aortic cannulae. Plasma GH was determined by radioimmunoassay. Reserpine (10 mg/kg i.p.) caused a complete suppression of the normal, pulsatile secretion of GH in all animals. Administration of somatostatin antiserum resulted in rapid elevations of plasma GH in reserpine-pretreated rats with peak levels at 30 min. GH levels then fell but remained slightly elevated for the duration of the sampling period (8 h). Apomorphine (0.5 mg/kg i.p.) had no effect on plasma GH levels, whereas clonidine (0.5 mg/kg i.p.) induced release of GH in both antiserum treated and control rats. The results indicate that the alpha-adrenergic influence on the secretion of GH is mediated not by inhibition of somatostatin release but rather by effects on the release of a GHRF.