Abstract
Summary
Twelve heifers were exposed to 21° ambient temperature for 10 days, and then subjected to 4.5, 21, or 32° for 9 days in controlled environmental chambers. Serum prolactin (PRL) decreased linearly (P < 0.01; 0.6 ng/ml/°) as the temperature was reduced during the first day from 21 to 4.5°; serum PRL increased linearly (P < 0.05; 1.17 ng/ml/°) as the temperature was increased from 21 to 32°. Between Days 2 and 9 serum PRL averaged 2.6, 13.0, and 27.7 ng/ml (P < 0.05) at 4.5, 21, and 32°, respectively. Injection of thyrotropin-releasing hormone (TRH) caused serum PRL to increase within 5 min from 20.4 to 109.8 ng/ ml at 32°; at 21° serum PRL increased from 15.7 to 62.8 ng/ml, whereas at 4.5° serum PRL did not respond to TRH. Serum growth hormone (GH) averaged 4.0, 6.3, and 9.4 ng/ml at 4.5, 21, and 32°, respectively, but these means were not different (P > 0.10). TRH released GH at all temperatures tested, but the quantity released was unaffected by ambient temperature. Relative humidities of 50 and 90% did not significantly alter (P > 0.05) serum PRL or GH. We conclude that ambient temperature affects basal and TRH-stimulated concentrations of serum PRL but not GH in heifers.
The technical assistance of Eileen Bostwick, Larry Brock, and David Kreider is most appreciated.
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