On thein Vitroandin VivoActivity of a New Synthetic Hexapeptide that Acts on the Pituitary to Specifically Release Growth Hormone*

His-DTrp-Ala-Trp-DPhe-LysNH2, His1,Lys6 GHRP, is a new synthetic hexapeptide which specifically elicits a dosage-related release of GH in vitro and in vivo without a concomitant release of LH, FSH, TSH, or PRL and, in limited in vivo studies, insulin or glucagon. Our results indicate that this small peptide has the attributes of a hypophysiotropic hormone. In vitro the minimum and maximum active dosages ranged from 1-10 ng/ml in the pituitary incubate assay. It was active in rats, monkeys, lambs, calves, and under special experimental conditions chicks, indicating its lack of species dependency. It was active when administered iv, sc, or ip to rats. After iv injection, GH levels rose within 2 min, peaked at +10-20 min, and by 2 h usually had returned to normal. It was not possible to directly compare the potencies of His1,Lys6GHRP, and the GH-releasing factors GHRF-44 and GHRF-40 after a single sc injection in rats because the time course of the GH response of these peptides was different. The GH response of His1,Lys6GHRP was longer in duration than either of these larger peptides. Both SRIF-14 and SRIF-28 inhibited the GH response of the hexapeptide; however, SRIF-28 was about four times more active than SRIF-14 in vitro and 7.5 times more active in vivo. When this small peptide was administered sc once or twice daily to immature rats for 9 or 25 days, the BW gain increased above the control. At the end of the weight gain studies the pituitary remained fully responsive to the peptide. Thus, His1,Lys6 GHRP may be a valuable peptide for investigating the function of the pituitary somatotrophs and, in addition, it has the potential for increasing BW gain of a variety of normal animals by inducing GH release via a direct pituitary site of action.