THE RATES OF PLASMA CORTISOL ENTRY AND CLEARANCE IN SHEEP BEFORE AND DURING THEIR EXPOSURE TO A COLD, WET ENVIRONMENT

Abstract

SUMMARY Plasma cortisol concentrations and entry rates increased greatly when nine shorn sheep were exposed to cold, wet conditions for periods up to 70 hr. The average entry rate in four of the cold-stressed animals before their rectal temperatures began to fall was 25 μg./min., approximately three to four times greater than in the same sheep before exposure (7·7 μg./min.). The metabolic clearance rates at this time remained unchanged. Plasma cortisol concentration began to increase about 2–3 hr. before the sheeps' rectal temperatures began to fall. The increase continued until concentrations of 100–200 μg./1. were reached after the sheeps' rectal temperature had fallen. Cortisol entry increased at this time to what may probably be maximal or near maximal rates in the sheep (about 150–200 μg./min.). Lowered clearance did not appear to contribute substantially to increased plasma cortisol concentrations at rectal temperatures above 34°. Since clearance rates did not begin to fall rapidly until rectal temperature fell below about 34°, cortisol entry during the terminal phase of hypothermia, approximately 76 μg./min., was very much less than the observed maximum and the high plasma cortisol concentrations measured during this period were residual and sustained by lowered clearance rates. The adrenal cortices and livers of the sheep after severe hypothermia were heavily infiltrated with fat. The effects of shearing alone, studied in a separate experiment, had a transitory effect since plasma cortisol concentrations and entry rates had returned to near their pre-shearing levels by about 27 hr.