MECHANISM OF ACTION OF GROWTH HORMONE IN ALTERING ITS OWN SECRETION RATE: COMPARISON WITH THE ACTION OF DEXAMETHASONE

Abstract

ABSTRACT Administration of large doses of growth hormone (GH) (2 mg/100 g/day for 5 days) failed to modify growth hormone-releasing factor (GRF) activity in either 1) plasma and the hypothalamus of long-term hypophysectomized rats, 2) the hypothalamus of normal rats. An elevation of pituitary content of GH was observed in normal rats as a consequence of growth hormone treatment. In contrast to growth hormone, dexamethasone (50 μg/100 g/day for, 5 days) induced considerable decrease of GRF activity in both plasma and the hypothalamus of long-term hypophysectomized rats, as well as in the hypothalamus of normal rats. No alteration of pituitary GH content was observed in normal rats as a consequence of chronic dexamethasone treatment. GH (1 mg/100 g) given 30 minutes before administration of a hypothalamic extract blocked the GH-depleting activity of the latter, while dexamethasone (50 μg/100 g) given one hour before apparently was ineffective. It is suggested that growth hormone suppresses its own secretion rate acting directly on the pituitary gland, while the action of dexamethasone on GH secretion, takes place mainly at the level of the hypothalamus.