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Environmental Heat Exposure on Cattle Plasma Catecholamine and Glucocorticoids¹

Department of Dairy Husbandry, University of Missouri, Columbia 65201

ABSTRACT

Nonlactating Holstein cows were exposed for varying times to environmental heat to determine changes in plasma epinephrine, norepinephrine, and glucocorticoids (principally hydrocortisone). Short heat exposures of 40 to 43 C caused average increases of 45 and 42% in epinephrine and norepinephrine at 1 hour. Maximal increases (at 4.5 hours) of 127 and 84% were observed for epinephrine and norepinephrine. Glucocorticoids increased 38% at 1 hour and declined to the control at 4.5 hours. More moderate 3-day exposure to 35 C increased epinephrine 70% at 6 hours and norepinephrine 35% at 2 hours, which persisted throughout the 72-hour exposure. Glucocorticoids increased 62% by the second hour of exposure, reached a peak of 120% at 4 hours, then declined gradually to values not different from normal at 48 hours and remained at this level for duration of exposure.

Long heat exposure of 24 days showed epinephrine and norepinephrine means of 91 and 70% greater at 35 than at 18 C. At 35 C glucocorticoids were 13% higher than at 18 C on the third day of exposure, then declined and were lower by day 24.

These data show a high and sustained adrenosympathetic activity (indicated by increases in epinephrine and norepinephrine) during heat acclimation and a transient increase in plasma glucocorticoids, which then declined even though body temperatures remained elevated during long heat exposures.

¹ Contribution from the Missouri Agricultural Experiment Station. Journal Series 6229, approved by the director.

² Rockefeller Foundation Fellow. Present address: Instituto Colombiano Agropecurio, Apt. Aereo 7984, Bogota, Colombia, South America.