THE DISTRIBUTION OF CORTISOL AND ITS RATE OF TURNOVER IN NORMAL AND COLD-STRESSED SHORN SHEEP

Abstract

SUMMARY Cortisol pools were examined in six shorn sheep before (i.e. control conditions) and during their exposure to a cold, wet environment. Cortisol labelled with tritium was infused until equilibrium concentration [3H(eq)] was reached in the plasma; the infusion was then stopped and the instantaneous concentrations [3H(t)] determined at intervals during the next 20–40 min. A two-pool model was used to calculate results and the rate constants k10, k12 and k21 were used to describe catabolism of hormone from the central pool, movement from the central to the outer pool and in the reverse direction respectively. In the majority of cases when the fractional concentrations, [3H(t)]/-[3H(eq)], after infusion were plotted against time, curves representing control conditions indicated that the apparent rate of disappearance of cortisol from plasma was faster than from cold-stressed normothermic sheep. The apparent rate of disappearance of cortisol from the plasma of both control and cold-stressed normothermic sheep was faster than when the animals were hypothermic. These results were not obtained in two sheep when they were cold-stressed and normothermic and these are discussed. The rate constants k10, k12 and k21 obtained in the experiments bore the relation control < cold-stressed normothermic < hypothermic. The total amount of cortisol increased about 4 times, relative to control, when the sheep were cold-stressed and normothermic, and 40 times, relative to control, when they were hypothermic. Pools decreased in one cold-stressed normothermic sheep. Turnover rates were 7·7%, 7·0% and 3·7% of the total pool/min for sheep under control, cold-stressed normothermic and hypothermic conditions respectively. It is concluded that the greatly increased plasma cortisol concentrations detected in cold-stressed normothermic and hypothermic sheep were associated with great increases in total cortisol, distributed in relatively constant 'spaces' in the body under those conditions.