INFLUENCE OF PROTEIN NUTRITION ON THE RESPONSE OF GROWING LAMBS TO EXOGENOUS BOVINE GROWTH-HORMONE

Interactions between protein supply and the anabolic response to exogenous bovine (b) GH have been examined in two experiments using 28-35 kg lambs sustained entirely by intragastric infusion of volatile fatty acids (700 kJ/kg W0.75 per day) into the rumen and the casein (600 mg (low protein; LP) or 1200 mg (high protein; HP)/kg W0.75 per day) into the abomasum. Sheep received continuous i.v. infusions of bGH for 6 days in experiment 1 and for 18 days in experiment 2. Nitrogen balances were determined daily throughout both experiments and blood samples, from indwelling catheters, were assayed for GH, insulin-like growth factor-I (IGF-I), insulin and glucose. Infusion of bGH increased plasma GH concentration by five- to sixfold in all animals. There was an increase in N retention in both HP and LP animals over the first 2-3 days of GH administration. HP animals sustained higher N retentions (31%; P < 0.05) throughout the GH administration but LP animals did not. In contrast, plasma IGF-I concentrations increased progressively over the first 72 to 96 h of GH administration in all sheep and thereafter remained significantly (P < 0.05) elevated until termination of the GH infusion. In lambs which received both HP and LP infusions in experiment 1 the increase in IGF-I concentration by day 6 of GH administration was significantly (P < 0.05) greater when they received the higher protein intake. Plasma insulin concentrations increased rapidly (P < 0.05) with the onset of GH administration to levels which were 2.5 (LP)- and 4.8 (HP)-fold greater than those observed in the pre- and post-GH periods. Glucose concentration also increased during GH administration (P < 0.05), by 35% in LP animals and by 58% in HP animals. High protein availability appeared necessary to sustain a protein anabolic response where lambs received exogenous GH infusions, even though plasma IGF-I concentrations were elevated on both high and low protein treatments.