Plasma Concentrations of Luteinizing Hormone and Prolactin in the Ovariectomized Ewe during Induced Hyperthermia1

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Abstract
The effect of induced hyperthermia on the basal and estradiol-induced secretion of luteinizing hormone (LH) and on prolactin (PRL) secretion was studied in 12 ovariectomized ewes exposed every 14 days to 3 day treatment periods. Ambient conditions of dry bulb and dew point temperatures were adjusted to result in rectal temperatures that were either normal (control) or an average of 1.2.degree. C hyperthermic (experimental). Each ewe served as its own control in a switchback experimental design. On day 2 of the 3 day treatment period, blood plasma was sampled at 10 min intervals between 0900 and 1300 h to characterize episodic patterns of LH. At 1800 h on the same day, 50 .mu.g estradiol-17.beta. (E2.beta.) was injected i.m. to provoke a preovulatory-like surge of LH on day 3. Plasma concentrations of LH and PRL were determined in samples collected at 60 min intervals between 0800 and 1600 h on day 3. Induced hyperthermia caused a 4-fold increase in plasma PRL on day 3 (P < 0.01), while it caused a decrease (P < 0.01) in plasma LH on day 2. Mean concentrations of LH 14-22 h after E2.beta. were not significantly altered by hyperthermia. Maximum concentrations of estradiol-induced LH release during this time did not differ between treatment groups. Neither rhythmicity nor period of the major rhythm of episodic patterns of LH on day 2 was affected by treatment. The decrease in plasma LH on day 2 during hyperthermia was attributed to decreased LH secretion rate rather than to changes in blood volume or metabolic clearance of LH since neither seemed to be affected by experimental conditions. Neither LH nor PRL concentrations were significantly related to rectal temperature.