ADRENOCORTICOTROPIC HORMONE DOSE-RESPONSE AND SOME PHYSIOLOGICAL-EFFECTS OF TRANSPORTATION ON PREGNANT BRAHMAN CATTLE

The appropriate dose and the ability of exogenous ACTH to mimic the ph ysiological effects of a real stressor need to be determined. In Exp. 1, 25 pregnant Brahman heifers were injected i.v. with either 0 (salin e), .125, .25, .5, or 1 IU of ACTH/kg BW. Plasma cortisol was determin ed in blood samples collected during a 5-h period, and an integrated c ortisol response was calculated for each cow. The greater the dose of ACTH, the greater was the integrated cortisol response (P < .001). How ever, peak plasma cortisol in response to the four doses of ACTH did n ot differ (P > .6). The plasma cortisol concentrations returned to bas eline more slowly in those cows receiving the greater doses of ACTH, m aking their integrated areas of response greater. In Exp. 2, pregnant Brahman cows were either transported 48 km (n = 28), injected with 1 I U of ACTH/kg BW (n 21), or served as shams (n = 28). Each treatment wa s repeated at 60, 80, 100, 120, and 140 d of gestation. Shrink was gre ater for the transported cows than for either the ACTH-treated or sham cows, 14.3, 6.0, and 5.2 kg (P < .001). Shrink also decreased in resp onse to each subsequent application of treatment for all three treatme nt groups (P < .001). Transported cows had lower plasma cortisol conce ntrations after the first two applications of treatments (P < .006). T he range of doses of ACTH caused a similar peak cortisol release; howe ver, it took cortisol longer to return to baseline as ACTH dose increa sed. Repeated administration of exogenous ACTH did not cause the same amount of shrinkage as transportation, and the resultant cortisol conc entrations remained consistent for each administration. There was no a pparent carryover effect of repeated administration of ACTH at 20-d in tervals. Maximal plasma cortisol concentrations in Brahman cattle can be obtained with doses of ACTH much smaller than those traditionally i njected. However, larger doses of ACTH maintained plasma cortisol conc entrations for a longer duration. Repeated transportation caused a dec rease in cortisol release and shrinkage indicative of psychological ha bituation. Injections of ACTH did not cause the same physiological res ponse as transportation.