The effect of acclimation to elevated ambient temperature on progesterone levels in blood and corpora lutea of cycling and four-day pregnant rats

1. 1.|Progesterone concentration pattern in peripheral blood during the estrus cycle and fourth day of pregnancy were compared between heat acclimated [HA] rats kept at 35 ± 1°C and 25-40% relative humidity and controls [C] kept at 21 ± 1°C and 35-50% relative humidity. 2. 2.|Progesterone was determined by the use of protein binding assay. 3. 3.|The hormone concentrations for C and HA animals were, respectively, at proestrus 19.5 and 12.9 ng/ml (P < 0.01), estrus 14.5 and 17.4 ng/ml, metestrus 16.3 and 12.8 ng/ml and at diestrus 16.1 and 23.2 ng/ml (P < 0.05). 4. 4.|The pattern of progesterone levels observed in HA animals resembled that reported for 5-day cycle rats. 5. 5.|Progesterone concentrations during the first four days of pregnancy were consistently higher in HA animals than in C. 6. 6.|Progesterone levels of Corpora lutea on the fourth day of pregnancy were 147.1 and 49.5 ng/mg tissue (P < 0.001) for HA and C animals, respectively.