THE EFFECT OF IMMUNIZATION AGAINST SOMATOSTATIN AND BETA-AGONIST ADMINISTRATION ALONE AND IN COMBINATION ON GROWTH AND CARCASS COMPOSITION IN YOUNG STEERS

The effect of active immunization against somatostatin and beta-agonis t administration alone and in combination on growth, carcass compositi on and endocrine responses was investigated in young steers given a pe lleted diet of dried grass and barley (0.7:0.3) ad libitum far 16 week s. Immunization alone increased growth rate by proportionately 0.10 bu t had no effect on plasma growth hormone (GH) or insulin-like growth f actor-1 (IGF-I) concentrations (P > 0.10). Carcass protein content was unchanged but fat content tended to be increased. This tons possibly associated with increased (P < 0.10) plasma insulin concentrations obs erved in these animals. Cimaterol administration significantly increas ed growth rate (P < 0.05) and carcass weight (P < 0.01). Carcass prote in content was significantly (P < 0.01) increased and carcass fat teas significantly (P < 0.001) reduced. a Administration of cimaterol to i mmunized animals resulted in an enhancement of the repartitioning effe ct of the P agonist although growth rates were intermediate between th e separate treatments. The mechanism by which this was achieved is unc lear. Plasma IGF-1 concentrations tended (P < 0.10) to be lower in the se animals compared with a controls. Plasma insulin concentrations wer e also reduced (P < 0.05) in these animals. Both immunized animals and those given cimaterol alone showed an increased GH response to a sing le intravenous injection of growth hormone releasing factor (GRF) (P < 0.05). Animals given the combined treatment however showed a reduced response suggesting that the effects were not mediated through direct changes in GH secretion. The improved lean :fat ratio in the carcass w as observed to be mediated through an enhancement of the lipolytic res ponse to the P agonist with only small effects on protein accretion. I t is suggested that immunoneutralization of somatostatin may increase the sensitivity of adipose tissue to lipolytic stimuli. This could occ ur through changes in the secretion of gastrointestinal or pancreatic hormones as a result of neutralization of the inhibitory influences of somatostatin. Further work is required to clarify these effects.