Characteristics of growth hormone secretion responsiveness to growth hormone-releasing peptide-2 (GHRP-2 or KP102) in calves

This study was conducted to determine the effects of acute and chronic administration of GH-releasing peptide-2 (D-Ala-D-βNal-Ala-Trp-D-Phe-Lys-NH₂, GHRP-2 or KP102) on GH responsiveness in male Holstein calves. In the dose response study of acute administration, six calves were injected iv with saline or 6.25, 12.5 and 25.0 μg/kg body weight (BW) of KP102. The GH AUC (area under curve, ng/m/·min, mean ± SEM) for 60 min was significantly increased with 6.25 (6763 ± 125.6), 12.5 (1574.8 ± 318.0) and 25.0 (1578.7 ± 214.6) μg/kgBW of KP102 than with saline (78.6 ± 36.1) (P<0.01). GH responses were decreased by multiple injections of 12.5 μg/kgBW KP102 at every 2 h for 8 h. The GH AUC for 60 min was decreased from the first injection (1162.9 ± 313.3) to the second injection (604.7 ± 131.9), but the response was significantly higher for the first and second injections than the third (304.4 ± 173.1) and fourth injections (320.7 ± 144.2) (P<0.05). In the chronic administration, 8 calves were implanted subcutaneously with osmotic pumps (Alzet pump®). Each of the 4 calves was given with 12.5 μg/kgBW per hour KP102 and the other 4 calves served as the control. During the 14 day period, average daily gain was significantly increased (36.4%) over the control (P<0.05). Food efficiency was not significant, but numerically higher (29.4%) than the control. The plasma GH concentration was not increased by chronic administration of KP102, but IGF-I appeared to increase in KP102-treated calves more than the control. These results suggest that the synthetic KP102 can be used for enhancing the growth performance in domestic animals.