Growth hormone secretion and activation of cyclic AMP by growth hormone releasing hormone and gamma-aminobutyric acid in the neonatal rat pituitary.

The effect of factors influencing pituitary growth hormone secretion may be mediated by a combination of several intracellular mechanisms. The involvement of cyclic AMP (cAMP) in the GH stimulatory effect of tau-aminobutyric acid (GABA) and of growth hormone releasing hormone (GHRH) was studied in neonatal rat pituitaries. In the pituitaries of the newborn rats GH secretion was stimulated by forskolin and by isobutylmethylxantine (IBMX). GHRH but not GABA elevated pituitary cAMP concentration, whereas both drugs increased GH secretion from 2-day old pituitaries. IBMX did not augment the cAMP stimulating effect of GHRH in 2-day old, but potentiated it in older (7, 14 and 21-day old) pituitaries. The results indicate the presence of a functioning, but relatively immature intracellular signal transmission system in the 2-day old rat pituitary.