PHYSIOLOGICAL CONSEQUENCES OF THE INDUCTION OF AUTOIMMUNITY TO ADRENOCORTICOTROPIN (ACTH)

Stress is a component of the productive life of grazing ruminants that results in the modification of metabolic efficiency due to the prefer ential re-direction of nutrients to essential tissues in order for the m to maintain their physiological integrity. This process is co-ordina ted by hormones of the hypothalamic-pituitary-adrenal (HPA) axis that regulate the secretion of the glucocorticoids that are responsible for chronic adaptation of the animals metabolism. This report summarizes the results of studies investigating the influence of the active immun ization of animals against the adrenal stimulatory hormone ACTH on car cass composition, endocrine function and energy metabolism in growing lambs. Decreases in carcass fatness in ACTH immune animals were associ ated with greater sensitivity in growth hormone (GH) and insulin secre tion to stress, however, no differences in feed conversion efficiency or nitrogen balance were found. ACTH immunization did increase energet ic efficiency by up to 20% during exercise as assessed by the consumpt ion of oxygen, particularly when the exercise was intensive which sugg ests that the animal resorted to anaerobic processes to generate suffi cient energy. This difference in oxygen consumption was partly attribu table to changes in both cortisol and endorphin status, since the repl acement of cortisol in immune animals and the blockade of mu and Sigma opioid receptors with naloxone decreased the difference in oxygen con sumption between the treatment groups. It would appear that immunologi cally induced changes in the stress hormone status of ruminants has a marked influence on metabolic efficiency and therefore productivity of sheep.