Stimulation by alpha-adrenergic mechanisms of the secretion of growth hormone-releasing factor (GRF) from perifused rat hypothalamus.

The possible involvement of adrenergic mechanisms in regulating the secretion of growth hormone (GH)-releasing factor (GRF) from the rat hypothalamus was examined in vitro with a perifusion system. A high potassium concentration (56 mM) stimulated GRF release from the hypothalamus. The infusion of clonidine (10(-4) M), an alpha 2-adrenergic stimulant, resulted in an increase in the spontaneous release of GRF. In the presence of propranolol (10(-5) M), a beta-adrenergic blocking agent, clonidine (10(-5) and 10(-4) M) stimulated GRF release more prominently in a dose-related manner, whereas propranolol (10(-5) and 10(-4) M) by itself did not affect the spontaneous GRF release. The stimulatory effect of clonidine (10(-4) M) on GRF release in the presence of propranolol was inhibited by yohimbine (10(-4) M), an alpha 2-adrenergic blocking agent. These findings suggest that alpha 2-adrenergic mechanisms play a role in stimulating GRF release from the hypothalamus in rats.