Changes of progesterone receptor (PR) protein and cellular localization in the endometrium were evaluated during the oestrous cycle and early pregnancy of the gilt. During the oestrous cycle, the concentration of total PR protein within the endometrium was highest on Days 0-5 and decreased on Day 10. The endometrial concentration of PR reached a nadir on Day 12 and this level was maintained throughout the remainder of the oestrous cycle (Day 18). In pregnant gilts, the concentration of endometrial PR protein from Day 10 to Day 18 was similar to that in cyclic gilts. Western blot analysis with antiserum specific for the A and B isoforms of PR indicated that porcine endometrium expresses both isoforms of PR. Immunostaining was detectable for both the A and B isoforms of PR from Day 0 to Day 12 of the oestrous cycle. However, no staining was observed on Day 15 and Day 18 of the oestrous cycle or pregnancy Immunocytochemical localization of PR in the endometrium of cyclic gilts and pregnant gilts indicated that there was intense staining for PR in surface epithelium and glandular epithelium during oestrus (Day 0) and on Day 5. However, the staining was less intense on Day 7 and Day 10 of the oestrous cycle and no epithelial staining was observed after Day 12. PRs were present in the stroma and myometrium throughout the oestrous cycle and early pregnancy. The presence of conceptuses during pregnancy did not affect the loss of PR from the uterine epithelium after Day 10 of gestation. Down-regulation of epithelial PR might be one factor involved in the timing of luteolysis during the oestrous cycle as well as conceptus growth and placentation during early pregnancy.

Conceptus secretion of oestrogen on Day 11 of gestation is involved with establishment of pregnancy in the pig. Changes in oestrogen receptor (ER) protein, mRNA and cellular localization in the endometrium were evaluated during the oestrous cycle and early pregnancy of the gilt. In nonpregnant gilts, concentration of nuclear ER in the endometrium increased from Days 0 to 12 followed by a decline on Day 15 of the oestrous cycle. In pregnant gilts, changes in endometrial nuclear ER during Days 10, 12, 15 and 18 were similar to that in cyclic pigs. Analysis of endometrial ER mRNA expression did not detect any difference between cyclic and pregnant pigs between Days 10 and 15 postoestrus. Expression of ER mRNA in endometrium of cyclic and pregnant gilts was greatest on Day 10 followed by a decline on Day 15. Endometrial ER mRNA increased on Day 18 of the oestrous cycle, but remained low during pregnancy. Immunocytochemical localization of ER in the endometria of cyclic and pregnant gilts indicated that there was intense staining for ER in stromal cells and moderate to strong staining in surface and glandular epithelial cells during oestrus (Day 0) and Day 18 of the oestrous cycle. However, stromal ER staining was absent from Days 5 to 15 of the oestrous cycle and continued to be suppressed on Day 18 of pregnancy. Immunocytochemical staining of ER in the surface and glandular epithelium was readily detectable from Days 0 to 12 of the oestrous cycle and during pregnancy. Intensity of staining for ER declined in surface epithelial cells on Day 15 in both cyclic and pregnant pigs whereas positive staining for ER in glandular epithelium was absent. Staining for ER on uterine surface epithelial cells increased during pro-estrus (Day 18) of cyclic gilts but remained similar to Day 15 in pregnant gilts. Changes in endometrial ER protein, mRNA and localization in surface epithelium are consistent with a physiological role for conceptus oestrogen secretion in uterine function and maternal recognition of pregnancy in the pig.

Mammalian conceptuses must provide a chemical signal to the maternal system to insure maintenance of corpus luteum (CL) function and of progesterone production and continuation of uterine endometrial secretory activity. These events insure that the developing conceptus is provided with appropriate nutrients, regulatory enzymes and endocrine state to allow successful establishment and maintenance of pregnancy. Pig blastocysts begin to produce estrogens by Day 11 of pregnancy, which prevents secretion of the uterine luteolytic factor (PGF2 alpha) in an endocrine direction, but allows secretion in an exocrine direction, i.e., into the uterine lumen. Therefore, CL are "protected." Blastocyst estrogens also trigger secretion of a group of proteins, including uteroferrin, an iron transport protein, and a family of protease inhibitors whose biosynthesis within the uterine glandular epithelium is under the control of progesterone. Estrogen also appears to promote accumulation of glucose and fructose within the uterine lumen. A complex in vivo "culture medium" is thereby established to promote conceptus development. Pig blastocysts do not undergo invasive implantation within the uterine lumen although they produce the protease, plasminogen activator. Invasion may be prevented by endometrial secretion of progesterone-induced protease inhibitors which are produced in large amounts. In addition to estrogens of conceptus origin, calcium and prostaglandins PGF2 alpha and E2 may affect the uterine vasculature, water and electrolyte transport, capillary permeability, conceptus steroid production, and related events during pregnancy. The blastocysts of the large domestic animals also secrete proteins which include a large glycoprotein (Mr approximately 600,000) and a small acidic protein (Mr approximately 17,000). The latter has been purified from sheep and named ovine trophoblast protein I. These proteins may play unique roles in early pregnancy with respect to establishment and maintenance of pregnancy in the ewe, sow, mare, and cow.

In Exp. I, 0.5 mg oestradiol or vehicle (0.5 ml absolute ethanol + 0.5 ml 0.9% NaCl) was injected i.v. at 08:00 h on Day 14 (onset of oestrus = Day 0). Blood samples were obtained via a jugular catheter at 30 and 1 min before oestradiol and every 30 min for 10 h afterwards. Plasma was obtained and assayed for 15-keto-13,14-dihydro-PGF-2 alpha (PGFM) by radioimmunoassay. Before oestradiol, PGFM basal values were higher (P less than 0.01) in pregnant (N = 10) than nonpregnant (N = 6) ewes (193 +/- 30 vs 67 +/- 8 pg/ml). However, at 4-10 h after oestradiol, pregnant ewes (N = 5) had less variable (P less than 0.01) PGFM values than did nonpregnant ewes (N = 5). In Exp II, conceptus secretory proteins (CSP) were obtained by pooling medium from cultures of Day-16 sheep conceptuses (N = 40). Ewes received 750 micrograms CSP + 750 micrograms plasma protein (N = 6) or 1500 micrograms plasma protein (N = 6) per uterine horn at 08:00 h and 18:00 h on Days 12-14. All ewes received 0.5 mg oestradiol at 08:00 h on Day 14 and blood samples were collected as in Exp. I and assayed for PGFM. On Day 15, 3 ewes in each group received 10 i.u. oxytocin and 3 received saline i.v. at 08:00 h and blood samples were taken continuously from 10 min before to 60 min after treatment. Mean PGFM response to oestradiol was suppressed (P = 0.05) in CSP- vs plasma protein-treated ewes (371 +/- 129 vs 1188 +/- 139 pg/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

Sheep conceptuses secrete a protein, ovine trophoblast protein-1 (oTP-1), between days 10 and 21 of gestation, that is responsible for establishment of pregnancy. oTP-1 inhibits uterine production of episodic release of prostaglandin (PG) F2 alpha (PGF) necessary for luteolysis and produced in response to oestradiol and oxytocin. oTP-1 does not compete with oxytocin for binding to its receptors or interfere with oxytocin stimulation of the inositol phospholipid second messenger system after oxytocin receptors are formed. However, oTP-1 may affect synthesis of oxytocin, oestrogen and/or progesterone receptors to alter the ability of the uterus to produce episodic pulses of PGF in response to episodic pulses of oxytocin. Porcine conceptuses secrete oestrogens between days 10 and 15 of pregnancy that are essential for establishment of pregnancy. Oestrogens, directly or indirectly, alter secretion of PGF from an endocrine direction (toward the uterine vasculature) to an exocrine direction (toward the uterine lumen). PGF sequestered in the uterine lumen is unavailable to exert a luteolytic effect on the corpus luteum (CL). Pig conceptus secretory proteins stimulate uterine production of PGF and PGE, but do not appear to be responsible for inhibition of luteolysis. Conceptus secretory proteins of sheep and pigs include proteins that have antiviral activity and are considered interferons. In sheep, oTP-1 has antiluteolytic, immunosuppressive, antiviral and possibly antiproliferative properties. The specific pig conceptus secretory protein(s) possessing antiviral activity has not been established. Unlike oTP-1, however, pig interferon-like protein(s) does not appear to possess antiluteolytic activity.