Effect of pregnancy and exogenous ovarian steroids on endometrial prolactin receptor ontogeny and uterine secretory response in pigs

Abstract

During gestation, pigs have constant circulating levels of prolactin (PRL), and lack decidual PRL and placental lactogens. Effects of PRL on uterine physiology in pigs may be due to changes in endometrial PRL receptors. In this study, effects of the conceptus and cyclic hormonal environment on endometrial PRL receptors were investigated. Endometrial PRL receptor numbers were similar between Days 8 and 15 for cyclic gilts. In contrast, endometrial PRL receptor numbers for pregnant gilts were similar between Days 8 and 11, then increased (p < 0.05) on Day 12 and remained elevated between Days 14 and 30. This day-by-status interaction approached significance (p < 0.06) and overall receptor numbers were higher (p < 0.01) for pregnant than for cyclic gilts. Pig conceptuses secrete estrogen between Days 11 and 12; therefore, regulation of endometrial PRL receptors by acute administration of estradiol was investigated. Uterine flushings and endometrium were collected from one uterine horn of cyclic gilts on Day 11; then, at 1, 6, 12, and 24 h following a single injection of estradiol valerate (EV; 5 mg, into adipose tissue), uterine flushings and endometrium were collected from the second uterine horn. Endometrial PRL receptor numbers were higher (p < 0.05) at both 1 h and 6 h after treatment with EV and then decreased (p < 0.02) by 12 h to below pretreatment values. In uterine flushings, total recoverable protein (p < 0.05), uteroferrin (p < 0.01), leucine aminopeptidase (p < 0.05), calcium (p < 0.03), sodium (p < 0.01), and potassium (p < 0.05) increased between 12 and 24 h following EV treatment. In another experiment, EV, progesterone (P4), EV plus P4, or corn oil (CO) was administered to ovariectomized gilts for 11 days to determine effects on endometrial PRL receptors and uterine secretions. Concentrations of PRL in serum were lower (p < 0.05) for gilts treated with P4 alone compared to pigs treated with EV, CO, or EV + P4. Endometrial PRL receptors were lower (p < 0.05) for gilts treated with EV alone or with EV + P4 than for gilts treated with CO or P4 alone; however, uterine secretory responses were greatest for gilts that received both steroids. Results of this study indicate that changes in receptor dynamics may account for PRL effects on uterine physiology in pigs, and that these receptor changes can be modulated by exogenous estrogens or estrogens of conceptus origin.