EFFECTS OF GROWTH HORMONE-RELEASING HORMONE (GRF) ANALOGS, BOVINE ANDRAT GRF ON GROWTH-HORMONE SECRETION IN CATTLE IN-VIVO

Effects of bovine and human growth hormone-releasing hormone (GRF) ana logs (bGRF(1-29)NH2: bGRF-29, [D-Ala(2), Ala(15)]-bGRF-29, [D-Ala(2)]- hGRF(1-29)-NH2: [D-Ala(2)]-hGRF-29), bovine GRF (bGRF(1-44)-NH2: bGRF- 44), as well as rat GRF (rGRF) on GH release were studied in female ca lves. Intravenous (iv) bolus injections of 0.25 mu g/kg BW of bGRF-9, [D-Ala(2), Ala(15)]-bGRF-29, and [D-Ala(2)]hGRF-29 stimulated GH relea se. Plasma GH levels began to rise 10 min after the injection of each peptide, and significant increases in GH concentrations were obtained at 60, 180 and 150 min after the injection of bGRF-29, [D-Ala(2), Ala( 15)]-bGRF-29 and [D-Ala(2)]-hGRF-29, respectively. The concentrations of GH 80 min after the injection of [D-Ala(2), Ala(15)]-bGRF-29 were s ignificantly higher than those after the injection of [D-Ala(2)]-hGRF- 29 (except at 80 and 90 min) or bGRF-29. The iv bolus injections of 0. 25 mu g/kg BW of bGRF-44 and rGRF stimulated GH release, and the GH-re leasing potency of rGRF was approximately equal to that of bGRF-44. Th e plasma GH responses to the repeated iv injection of bGRF-29 or [D-Al a(2), Ala(15)]-bGRF-29 at 2-h intervals were examined. bGRF-29 acutely increased plasma GH levels after each injection, and the high GH leve ls decreased to the basal values within 2 h. In contrast, high GH leve ls induced by [D-Ala(2), Ala(15)]-bGRF-29 were gradually decreased but not lowered to basal values throughout the experiment. These results show that [D-Ala(2), Ala(15)]-bGRF-29 has longer-lasting and greater G H-releasing activity than the other GRF analogs in female calves, and the GH-releasing potency of rat GRF is approximately equal to that of bovine GRF in cattle in vivo.