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 m ay be mediated by a combination of several intracellular mechanisms. T he involvement of cyclic AMP (cAMP) in the GH stimulatory effect of ta u-aminobutyric acid (GABA) and of growth hormone releasing hormone (GH RH) was studied in neonatal rat pituitaries. In the pituitaries of the newborn rats GH secretion was stimulated by forskolin and by isobutyl methylxantine (IBMX). GHRH but not GABA elevated pituitary cAMP concen tration, whereas both drugs increased GH secretion from 2-day old pitu itaries. 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) pituitari es. The results indicate the presence of a functioning, but relatively immature intracellular signal transmission system in the 2-day old ra t pituitary.