Ontogeny of oxytocin receptors and oxytocin-induced stimulation of prostaglandin synthesis in prepubertal heifers

Oxytocin receptor down-regulation is not necessary for reducing oxytocin-induced prostaglandin F(2alpha) accumulation by interferon-tau in a bovine endometrial epithelial cell line

Interferon-tau (IFNtau) is the embryonic signal responsible for pregnancy recognition in ruminants. The primary action of IFNtau is believed to be mediated through inhibition of prostaglandin F(2alpha) (PGF(2alpha)) released from the endometrial epithelial cells in response to oxytocin (OT). Our working hypothesis was that the antiluteolytic effect of IFNtau also involved modulation of PG production downstream of OT receptor (OTR) and/or cyclooxygenase 2 (COX2). There is currently no OT-sensitive endometrial cell line to study the molecular mechanisms underlying our hypotheses. Therefore, we established an immortalized bovine endometrial epithelial cell line (bEEL) exhibiting OT response. These cells were cytokeratin positive, expressed steroid receptors, and exhibited preferential accumulation of PGF(2alpha) over PGE(2). The bEEL cells were highly sensitive to OT, showing time- and concentration-dependent increase in COX2 transcript and protein and PGF(2alpha) accumulation. Interestingly, IFNtau (20 ng/ml) significantly reduced OT-induced PGF(2alpha) accumulation, but surprisingly, the effect was not mediated through down-regulation of either OTR or COX2. Rather, IFNtau up-regulated COX2 in a time- and concentration-dependent manner while decreasing OT-induced PG accumulation. This suggests that COX2 is not a primary target for the antiluteolytic effect of IFNtau. Because IFNtau reduced OT-stimulated PGF(2alpha) accumulation within 3 h, the mechanism likely involves a direct interference at the level of the OT signaling or transcription in addition to the down-regulation of OTR observed in vivo. In summary, bEEL cells offer a unique in vitro model for investigating the cellular and molecular mechanisms underlying OT and IFNtau response in relation with luteolysis and recognition of pregnancy in the bovine.

Mechanical stretch up-regulates the human oxytocin receptor in primary human uterine myocytes

Oxytocin receptor (OTR) expression is increased before the onset of labor in all models of parturition. However, the mechanisms responsible for the increase in OTR expression are uncertain. Animal data suggest that uterine stretch increases OTR mRNA expression. In primary cultures of human uterine smooth muscle cells obtained from nonpregnant (NP) women and pregnant women before (NL) and after (L) the onset of labor, we investigated the effect of stretch on the expression of OTR mRNA and DNA binding of activator protein-1 (AP-1), CCAAT/enhancer binding protein (C/EBP)beta, and nuclear factor-kappaB transcription factors. OTR expression was least in NL, intermediate in NP, and greatest in L cells. Stretch of NL cells resulted in up-regulation of OTR mRNA expression associated with increased OTR gene promoter activity. Stretch of NP and L cells did not affect OTR mRNA expression. The increased promoter activity was associated with increased DNA binding of C/EBP and AP-1 but not nuclear factor-kappaB transcription factors. Overexpression of C/EBP, but not AP-1, increased OTR promoter activity. We conclude that stretch of NL cells results in increased OTR mRNA expression probably through increased C/EBPbeta DNA binding. These data suggest that stretch contributes to the massive increase in OTR expression before the onset of human labor.

Steroid hormone modulation of prostaglandin secretion in the ruminant endometrium during the estrous cycle

Prostaglandins, produced from membrane phospholipids by the action of phospholipase A2, cyclooxygenase, and specific prostaglandin synthases, are important regulators of ovulation, luteolysis, implantation, and parturition in reproductive tissues. Destruction of the corpus luteum at the end of the estrous cycle in nonpregnant animals is brought about by the pulsatile secretion of prostaglandin F(2alpha) (PGF(2alpha)) from the endometrium. It has been known for many years that progesterone, estradiol, and oxytocin are the hormones responsible for luteolysis. To achieve luteolysis, two independent processes have to be coordinated; the first is an increase in the prostaglandin synthetic capability of the endometrium and the second is an increase in oxytocin receptor number. Although progesterone and estradiol can modulate the expression of the enzymes involved in prostaglandin synthesis, the primary reason for the initiation of luteolysis is the increase in oxytocin receptor on the endometrial epithelial cells. Results of many in vivo studies have shown that progesterone and estradiol are required for luteolysis, but it is still not fully understood exactly how these steroid hormones act. The purpose of this article is to review the recent data related to how progesterone and estradiol could regulate (initiate and then turn off) the uterine pulsatile secretion of PGF(2alpha) observed at luteolysis.

Mares with delayed uterine clearance have an intrinsic defect in myometrial function

Persistent, postmating endometritis affects approximately 15% of mares and results in reduced fertility and sizable economic losses to the horse-breeding industry. Mares that are susceptible to postmating endometritis have delayed uterine clearance associated with reduced uterine contractility. Unfortunately, the mechanism for reduced uterine contractility remains an enigma. The present study examined the hypothesis that mares with delayed uterine clearance have an intrinsic contractile defect of the myometrium. Myometrial contractility was evaluated in vitro by measuring isometric tension generated by longitudinal and circular uterine muscle strips in response to KCl, oxytocin, and prostaglandin F(2alpha) (PGF(2alpha)) for young nulliparous mares, older reproductively normal mares, and older mares with delayed uterine clearance. In addition, intracellular Ca(2+) regulation was evaluated using laser cytometry to measure oxytocin-stimulated intracellular Ca(2+) transients of myometrial cells loaded with a Ca(2+)-sensitive fluorescent dye, fluo-4. For all contractile agonists, myometrium from mares with delayed uterine clearance failed to generate as much tension as myometrium from older normal mares. Oxytocin-stimulated intracellular Ca(2+) transients were similar for myometrial cells from mares with delayed uterine clearance and from older normal mares, suggesting that the contractile defect did not result from altered regulation of intracellular Ca(2+) concentration. Furthermore, no apparent age-dependent decline was observed in myometrial contractility; KCl-depolarized and oxytocin-stimulated longitudinal myometrium from young normal mares and older normal mares generated similar responses. However, circular myometrium from young normal mares failed to generate as much tension as myometrium from older normal mares when stimulated with oxytocin or PGF(2alpha), suggesting possible age-related alterations in receptor-second messenger signaling mechanisms downstream of intracellular Ca(2+) release. In summary, for mares with delayed uterine clearance, an intrinsic contractile defect of the myometrium may contribute to reduced uterine contractility following breeding.