The influence of active immunization of animals against adrenocorticotropin on energy expenditure in the exercising sheep

Abstract

PI-oc. Amt. Sot. Anim. Prod. 1994 Vol. 20 THE INFLUENCE OF ACTIVE IMMUNIZATION OF ANIMALS AGAINST ADRENOCORTICOTROPIN ON ENERGY EXPENDITURE IN THE EXERCISING SHEEP A.Z. SHAHNEHA, J.M. GOODENA, R.D.G. RIGBYB and P.C. WYNN A ADcpt of Animal Science, University of Sydney, Cam den, N.S.W. 2570 *CSIRO Division of Animal Production, Locked Bag 1, Delivery Centre, Blacktown, N.S.W. 2148 Recent studies have shown that long term suppression of circulating levels of glucocorticoids by active immunization of prime lambs against the adrenal stimulatory hormone adrenocorticotropin (ACTH) decreases fat deposition (Shahneh et al. 1993). Since there was no discernible difference in feed intake between ACTH-immune and control animals, it was thought that the variation in fat deposition may be due to a difference in energetic efficiency in ACTH-immune animals. The present study investigates the influence of long term immunization against ACTH on the efficiency of energy utilization in the exercising lamb. The specific roles of l3-endorphin @Ep) and cortisol in the regulation of energy utilization were investigated. Border Leicester-Merino x Dorset Horn ewe lambs immunized against ACTH-ovalbumin (0.5 mg, l:l, w/w) in Freund' complete adjuvant (ACTH-immune) or adjuvant alone (control) were offered s 1ucerne:oats pellets (60:40) at 95% ad Zibitum. Animals were subjected to sequential periods of rest (1 hour), walking at 2.5 km/hour (30 minutes), rest (1 hour), walking at 5 km/hour (30 minutes) and rest (1 hour) on each of 3 days. On day 1, animals received a control saline infusion; on day 2 they were infused with the opioid receptor antagonist, naloxone (1 mg/kg liveweight.hour) and on day 3 cortisol (50 p&kg liveweighthour) was infused. Oxygen consumption was calculated from cardiac output using transittime ultrasound (Transonic Systems Inc) and blood oxygen concentrations using fibre-optic technology (Oximetrix, Abbott Laboratories). Exercise increased oxygen consumption, and plasma cortisol, /3Ep and lactate levels increased with the intensity. ACTH immunization decreased oxygen consumption at 5 km/hour only (Figure l), while at the same time suppressed the plasma levels of cortisol response to stress. (3Ep levels were stimulated 14 fold by this procedure for the duration of the experiment. Naloxone infusion stimulated ` plasma PEp levels and reduced cortisol level in ACTH-immune while increasing it in control animals. In contrast, the cortisol infusion had little effect on PEp levels in controls, but suppressed them 3 fold in ACTHimmune animals. Figure 1. Oxygen consumption in ACTH-immune (circles) and control (triangles) lambs In conclusion, the elevated anaerobic threshold in the ACTH-immune animals could not be explained by changes in either cortisol or PEp, since the naloxone and cortisol infusions did not modify the rate of oxygen consumption in either of the treatment groups. Therefore in exercising sheep the efficiency of energy utilization is independent of concentrations of these 2 hormones. This project is supported by the Meat Research Corporation. SHAHNEH, AZ., RIGBY, R.D.G., HOSKINSON, R.M. and WYNN, P.C. (1993). Proc. Endocr. Sot. Amt. 36: 157. 390