Cortisol Turnover in Heat-Stressed Cows

Abstract

WHEN acutely exposed to a moderate heat stress (35 C) cows accustomed to the experimental procedure increased plasma cortisol significantly (P<.05) in the first 20 min. of exposure from 30 to 37 µg/liter. Plasma cortisol continued to rise for 2 hr. (P<.01) and plateaued around 43 µg/liter between 2 and 4 hr. after the onset of heat exposure. The metabolic clearance rate of cortisol remained unchanged over this period. Cortisol turnover rate plateaued between 2 and 4 hr. of heat stress at around 165 µg/min. after a pre-exposure rate of 117 µg/min. (P<.01). After 7 to 10 weeks of chronic, mild heat stress, (35 C) plasma cortisol was significantly (P<.01) depressed from 30 to 25 µg/liter. Metabolic clearance rate remained unchanged. Cortisol turnover rate also fell significantly (P<.05) from 117 to 97 µg/minute. There were no significant changes during the first 4 hr. of return to 18 C after 10 weeks of 35 C, but over the next 9 days the measured variables returned progressively to their pre-exposure values. There was poor agreement between cortisol turnover rate measured by isotope dilution and by continuous infusion; 430 vs. 117 µg/min. at 18 C, and 232 vs. 97 µg/min. at 35 C, respectively. It is concluded that the short-term response of cortisol to heat stress is part of an acute non-specific reaction which could be exhibited in response to a wide variety of stressors. Under conditions of prolonged heat exposure specific adjustments are made in the turnover rate and the plasma concentration of the calorigenic glucocorticoid hormone.