Effect of Synthetic Growth Hormone-Release Inhibiting Factor on the Secretion of Growth Hormone and Prolactin in Rats

The activities of the synthetic cyclized tetradecapeptide (GIF, Somatostatin) were investigated in isolated rat pituitaries in vitro and in rats treated with nembutal under ether anesthesia or with chlorpromazine (CPZ) under urethane anesthesia. It was found in in vitro study that 250nM or larger doses of the cyclized GIF partially but significantly inhibited GH release and that the incorporation of 14C-leucine into the pituitary GH was also significantly reduced by 25nM of the cyclized, whose dose neither affected the release of synthesized GH into the medium nor suppressed the release of immunoreactive GH. On the other hand, a single intravenous (iv) injection of 100μg of the cyclized GIF did not affect the basal levels of both serum GH and prolactin. The raised levels of serum GH induced by nembutal (2.5mg/100g body weight, iv) were completely suppressed by a concomitant iv administration of 100μg of GIF but not 10μg. CPZ (500μg/100g body weight, iv)-induced serum GH rise was also completely inhibited by 100μg of GIF, whose dose did not suppress CPZ-induced serum prolactin rise. In another study, 5μg of GIF was given by a single iv injection with nembutal and 10μg of GIF was immediately infused for 30min. No increase in serum GH levels was observed in any rat during the infusion, but the levels were abruptly elevated in 7 out of 9 rats 15min after the cessation of the infusion. The similar ‘rebound’was noticed in 4 out of 9 rats when the dose of GIF was reduced to one fifth. The levels of serum GH were also abruptly raised at the same interval in 3 out of 7 rats given iv both CPZ (50μg/100g body weight) and GIF, but not in any rat treated with GIF alone.