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Effects of heat stress on serum progesterone in cyclic ewes and on progesterone and cortisol response to ACTH in ovariectomized ewes

Summary. The daily mean of serum progesterone in cyclic ewes (N = 5) as well as the profile characteristics of progesterone and cortisol in response to an acute single dose (5 i.u./kg liveweight^0·75) of adrenocorticotrophic hormone (ACTH) into ovariectomized ewes (N = 4) was investigated during exposure to a constant thermoneutral temperature of 18 ± 1°C or to a daily cyclic heat stress temperature of 18°C–35°C–18°C, in an environmental chamber. Serum collected daily from the cyclic ewes was assayed for progesterone, while serum collected more frequently for 10 h, on the 14th day of exposure to the respective temperature, from the ovariectomized ewes was assayed for progesterone and cortisol by RIAs.

In cyclic ewes, heat stress increased the area under the daily progesterone curve (P < 0·09) but had no effect on progesterone concentration after the regression of the CL. In ovariectomized ewes, ACTH significantly elevated the response of both cortisol and progesterone (r = 0·75, P < 0·001) within 10–15 min of injection. In the ovariectomized ewes and during heat stress, the responses of progesterone and cortisol to ACTH were characterized by an initial acute rise, a transient drop, a steep elevation and a gradual but prolonged decline. During thermoneutral temperatures, this biphasic response pattern was not observed. Since progesterone concentration in the ovariectomized ewes in response to heat stress or the ACTH injection were within the range of basal progesterone observed during the preovulatory period in cyclic ewes, our results give no evidence of a direct role for adrenal progesterone in depressing pituitary function in ewes exposed to a daily cyclic heat stress.

Keywords: heat stress; ewes; ACTH; progesterone; cortisol