Effects of growth hormone (GH)-releasing hormone and its analogs on GH secretion from cultured adenohypophysial cells in cattle.

The effects of the human growth hormone (GH)-releasing hormone (hGRF(1-44)-NH2: hGRF-44), bovine GRF (bGRF(1-44)-NH2: bGRF-44), and their analogs (hGRF(1-29)-NH2, [D-Ala2]- hGRF(1-29)-NH2, bGRF(1-29)-NH2,[D-Ala2,Ala15]-bGRF(1-29)-NH2) as well as rat GRF(rGRF) on bovine GH release from anterior pituitary (AP) cells were studied in vitro in steers. The AP cells were incubated for 2 hr with the GRFs after preincubation for 3.5 d. Both of the hGRF-44 and hGRF(1-29)-NH2 (hGRF-29) significantly stimulated GH release from cultured cells at doses from 10(-14) to 10(-8)M (P < 0.01). The analog [D-Ala2]-hGRF-29 significantly induced GH release in media at doses from 10(-18) to 10(-8) M (P < 0.01). The bGRF-44, bGRF(1-29)-NH2 (bGRF-29), and [D-Ala2, Ala15]-bGRF-29 significantly induced GH release in media at doses as low as 10(-18), 10(-17), and 10(-16)M, respectively (P < 0.01). At doses from 10(-11) to 10(-8)M, there were no significant differences in GH-releasing potency between the hGRFs and bGRFs. The rGRF significantly stimulated GH release at doses ranging from 10(-14) to 10(-8) M (P < 0.01). The linear regression tests showed that the hGRFs, bGRFs, and rGRF, at doses from 10(-14) to 10(-8)M, induced GH release in a dose-related manner (P < 0.01). These results suggest that the hGRF, bGRF, and their analogs, as well as rGRF, are potent secretagogues of GH release from adenohypophysial cells in vitro in cattle.