EFFECT OF GROWTH-HORMONE (GH)-RELEASING PEPTIDE (GHRP) ON THE RELEASE OF 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 studied in vitro with the interactive effects of GH-releasing factor (GRF: 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 cells 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 GHRP (10(-14) to 10(-12) M) were not significantly different from the controls. GH concentrations after treatment with 10(-11) and 10(-7) M GHRP were 3.98 +/- 0.27 and 4.81 +/- 0.16 ng/ml, respectively. In experiments performed similarly, the 10(-7) M GHRH, GHRP, and combined treatment 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 GHRH 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 significantly less than that of SRIF with GHRH or with GHRP (P<0.01). The present study suggests that GH-releasing peptide (GHRP) induced GH release in cattle via a direct action on anterior pituitary cells in vitro.