Effects of thermal stress on plasma concentrations of luteinizing hormone, progesterone, prolactin and testosterone in the cycling ewe.

Naturally cycling white faced ewes were utilized to study the effects of continuously elevated environmental temperature and/or humidity on plasma concentrations of luteinizing hormone (LH), prolactin (PRL), progesterone (P4) and testosterone (TE) during the estrous cycle. Fourteen ewes were randomly allocated on the day of estrus (day 0) to either thermoneutral conditions (21.1 degrees C, 65% relative humidity) or elevated ambient temperature/humidity conditions (36.1 degrees C, 71% relative humidity) producing an average 1.4 degrees C hyperthermia. Animals remained in their respective environments and blood samples were collected daily until the next estrus or day 20, whichever occurred first. Starting at noon on day 14, blood was sampled every 2 hours. Concentrations of LH, PRL, P4 and TE were quantified using validated radioimmunoassays. Hyperthermic ewes exhibited 1) a significant decrease (P<0.05) in the incidence of behavioral estrus and a preovulatory LH surge at the expected time of the estrous cycle, 2) significantly lower (P<0.05) plasma P4 between days 7 and 13 of the cycle, 3) a six-fold increase of PRL levels (P<0.01). Plasma levels of TE were not significantly affected by hyperthermia. The only two experimental ewes which exhibited estrus and an LH surge also showed an unusual and significant peak in plasma P4 two days before estrus. These results confirm that elevated environmental temperatures that result in hyperthermia can induce endocrine imbalances in the ewe which may contribute to decreased reproductive efficiency in the heat-stressed female.

Influence of programmed circadian temperature changes upon levels of luteinizing hormone in the bovine.

PIP: A study was undertaken to determine the effects of controlled circadian patterns of elevated ambient temperatures (21-34 degrees C) upon the venous levels of luteinizing hormone (LH) in the bovine. Venous blood was collected from 8 16-month-old Angus heifers by indwelling cannulae to study LH levels in animals subjected to 2 ambient temperature regimes within 7 times 7.5 times 2.7 m psychrometric chambers. The control temperature regime was 17-21 degrees C diurnal variation, and the experimental temperature regime was 21-34 degrees C diurnal variation. The experimental regime significantly ( less than .05) decreased preovulatory and basal LH levels based upon 0800 and 1600 hour samples. Mean preovulatory LH levels were 6.4 ng/ml, where standard error of the mean (SEM) = 1.9 and number (n) = 16, in the control environment and 2.7 ng/ml (SEM = .9, N = 16) in the experimental regime. Comparable values for basal levels were .8 ng/ml (SEM = .02, n = 176). 7 of 8 animals exhibited an LH surge (greater than or equal to 4 ng/ml) while exposed to the control environment compared to only 3 of 8 at the experimental environment.^ieng