Pattern of Cortisol Release in Sheep following Administration of Synthetic ACTH or Imposition of Various Stressor Agents

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Abstract
This paper describes experiments with sheep, in which changes in plasma cortisol, after imposition of various stressor agents, were compared to the changes following administration of synthetic ACTH. The influence of stress associated with shearing, yarding, estrogen administration (30 .mu.g estradiol benzoate, i.v.), feeding and fasting on the plasma concentration of cortisol was monitored in 4 mature Merino ewes. They were placed in the experimental environment for 21 days before monitoring began. The ewes were treated in pairs, each pair being visually and acoustically isolated from the other. One pair of ewes was exposed to the particular stress and the remaining pair acted as their own controls. The treatments were reversed 2 days later. In the 2nd part of the experiment, 0, 0.01, 0.1, 1.0 or 10.0 IU synthetic ACTH were injected as an i.v. bolus, after endogenous secretion had been suppressed by administration of synthetic glucocorticoid. All blood samples were taken via an indwelling jugular catheter. A comparison of cortisol release, estimated from a plot of cortisol in plasma vs. time, following imposition of various stressor agents and administration of synthetic ACTH allowed stress to be defined in terms of synthetic ACTH equivalents. The most severe stress was associated with shearing (0.84 IU synthetic ACTH equivalents), less stress was imposed by yarding and handling (0.45 IU), and there appeared to be no effect attributable to feeding, fasting or estrogen administration. The similarity in the pattern of cortisol release following ACTH administration offers the possibility of defining acute, but not chronic, stress in terms of ACTH equivalents.