EFFECT OF GROWTH-HORMONE (GH)-RELEASING PEPTIDE (GHRP) ON THE RELEASEOF GH FROM CULTURED ANTERIOR-PITUITARY-CELLS IN CATTLE

The effect of His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP) on growth hormone (GH) release from cultured bovine anterior pituitary (AP) cells was s tudied in vitro with the interactive effects of GH-releasing factor (G RF: hpGRF (1-29)-NH2) and somatostatin (SRIF). The AP cells (5 X 10(4) cells per well) were incubated with media, and the media were changed 3 days after plating. After 3.5 days in culture, cells were incubated for 2 h with the peptides. GHRP stimulated GH release from cultured c ells in a dose-related manner. At doses from 10(-11) to 10(-7) M GHRP, the amount of GH released was significantly greater than the controls (P<0.05 to P<0.001). The amounts of GH released at lower doses of GHR P (10(-14) to 10(-12) M) were not significantly different from the con trols. GH concentrations after treatment with 10(-11) and 10(-7) M GHR P were 3.98 +/- 0.27 and 4.81 +/- 0.16 ng/ml, respectively. In experim ents performed similarly, the 10(-7) M GHRH, GHRP, and combined treatm ent with GHRP plus GHRH increased GH 126, 57, and 139%, respectively ( P<0.001). The GH releasing effects of either GHRH alone or GHRP plus G HRH were significantly more potent than that of GHRP alone (P<0.001). The additive effect was not significant when compared with GHRH alone. GH release induced by either GHRH or GHRP was significantly inhibited by SRIF (P<0.01) compared with the untreated control. The inhibitory effect of SRIF in combined treatment with GHRP plus GHRH was significa ntly less than that of SRIF with GHRH or with GHRP (P<0.01). The prese nt study suggests that GH-releasing peptide (GHRP) induced GH release in cattle via a direct action on anterior pituitary cells in vitro.