Prostaglandin F2 alpha-induced release of oxytocin from ovine corpora lutea in vitro

Maternal recognition of pregnancy in the horse: a mystery still to be solved

Maternal recognition of pregnancy in the horse is the sum of events leading to maintenance of pregnancy; in a narrow sense, maternal recognition of pregnancy refers to the physiological process by which the lifespan of the corpus luteum is prolonged. The horse is one of the few domestic species in which the conceptus-derived pregnancy recognition signal has not been identified. The presence of the conceptus reduces pulsatile prostaglandin F2α secretion by the endometrium during early gestation in the mare, partly attributed to the reduced expression of cyclooxygenase-2. Cyclooxygenase-2 has therefore been suggested as one of the regulators of endometrial prostaglandin F2α release modified by the antiluteolytic factor secreted by the conceptus. In addition, altered oxytocin responsiveness has been implicated in the adjustment of prostaglandin release in pregnant mares. While conceptus mobility has proven to be essential for establishment of pregnancy, conceptus-derived oestrogens and prostaglandins, principally prostaglandin E2, have not been confirmed as the critical antiluteolytic factor. Various ways to induce prolonged luteal function in the non-pregnant mare will be highlighted in the current review, specifically, how they may pertain to the process of maternal recognition of pregnancy. Furthermore, recently published microarray experiments comparing the transcriptome of pregnant and non-pregnant endometria and different stages of conceptus development will be reviewed. Findings include the prevention of conceptus adhesion, the provision of nutrients to the conceptus and the avoidance of immunological rejection, among others.

Sensitivity of bovine corpora lutea to prostaglandin F2alpha is dependent on progesterone, oxytocin, and prostaglandins

Prostaglandin (PG) F2alpha that is released from the uterus is essential for spontaneous luteolysis in cattle. Although PGF2alpha and its analogues are extensively used to synchronize the estrous cycle by inducing luteolysis, corpora lutea (CL) at the early stage of the estrous cycle are resistant to the luteolytic effect of PGF2alpha. We examined the sensitivity of bovine CL to PGF2alpha treatment in vitro and determined whether the changes in the response of CL to PGF2alpha are dependent on progesterone (P4), oxytocin (OT), and PGs produced locally. Bovine luteal cells from early (Days 4-5 of the estrous cycle) and mid-cycle CL (Days 8-12 of the estrous cycle) were preexposed for 12 h to a P4 antagonist (onapristone: OP; 10(-4) M), an OT antagonist (atosiban: AT; 10(-6) M), or indomethacin (INDO; 10(-4) M) before stimulation with PGF2alpha. Although OP reduced P4 secretion (p < 0.001) only in early CL, it reduced OT secretion in the cells of both phases examined (p < 0.001). OP also reduced PGF2alpha and PGE2 secretion (p < 0.01) from early CL. However, it stimulated PGF2alpha secretion in mid-cycle luteal cells (p < 0.001). AT reduced P4 secretion in early and mid-cycle CL (p < 0.05). Moreover, PGF2alpha secretion was inhibited (p < 0.05) by AT in early CL. The OT secretion and the intracellular level of free Ca2+ ([Ca2+]i) were measured as indicators of CL sensitivity to PGF2alpha. PGF2alpha had no influence on OT secretion, although [Ca2+]i increased (p < 0.05) in the early CL. However, the effect of PGF2alpha was augmented (p < 0.01) in cells after pretreatment with OP, AT, and INDO in comparison with the controls. In mid-cycle luteal cells, PGF2alpha induced 2-fold increases in OT secretion and [Ca2+]i. However, in contrast to results in early CL, these increases were magnified only by preexposure of the cells to AT (p < 0.05). These results indicate that luteal P4, OT, and PGs are components of an autocrine/paracrine positive feedback cascade in bovine early to mid-cycle CL and may be responsible for the resistance of the early bovine CL to the exogenous PGF2alpha action.