Characterization of Cytoplasmic Progesterone Receptors in the Bovine Endometrium during Proestrus and Diestrus2

Abstract

An exchange assay was developed to monitor concentrations of cytoplasmic progesterone receptors in the bovine endometrium in order to understand the regulation of uterine function by ovarian steroids. Thirteen 2-yr-old nulliparous Hereford and Angus crossbred heifers were assigned randomly to two groups. Group 1 heifers (n = 7) received a single im injection of 25 mg of PGF2 α-tham salt on d 9 of the estrous cycle and were sacrificed 72 h later. Only two of the seven heifers exhibited behavioral estrus on the morning of necropsy but all had one preovulatory follicle (17.1 ± 1.2 mm in diameter) and a regressed corpus luteum. Group 2 heifers (n = 6) were necropsied on d 12 of the cycle. The average weight of the corpus luteum in heifers of Group 2 was 3.49 ± .29 g. Jugular blood samples were obtained immediately prior to necropsy and assayed for progesterone and estradiol. Cytoplasmic progesterone receptor concentrations were determined by exchange assay. The Kd for the progesterone-receptor complex and the maximum concentration of receptors as determined by Scatchard analysis were 2.09 nM and 1,280 fmol/mg DNA, respectively, for heifers in Group 1. The concentration of cytoplasmic progesterone receptors in endometria of heifers during proestrus was greater (P<.05) than in heifers during diestrus (Group 1, 1,250 ± 249 vs Group 2, 597 ± 259 fmol/mg DNA). Serum estradiol levels during proestrus (6.46 ± 1.04 pg/ml) were greater (P<.01) than during diestrus (5.01 ± .37 pg/ml), while serum progesterone levels were greater (P<.01) during diestrus than during proestrus (Group 2, 2.41 ± .14 vs Group 1, <.25 ng/ml). Results from this study indicate that concentrations of endometrial cytoplasmic progesterone receptors are increased during proestrus when ovarian secretion of estrogen is generally maximal, whereas the increased secretion of progesterone during diestrus is associated with a significant reduction in cytoplasmic progesterone receptor concentration.