Localisation and temporal changes in prostaglandin G/H synthase-1 and -2 content in ovine intrauterine tissues in relation to glucocorticoid-induced and spontaneous labour

Abstract

Parturition in the ewe is preceded by an increase in the synthesis of prostaglandins (PGs) by gestational tissues. To establish the uterine source of these PGs, placental cotyledons, fetal membranes and maternal uterine tissues were collected from ewes (n=6) at spontaneous parturition. Solubilised tissue extracts were prepared and analysed by Western blots using polyclonal antibodies to PG G/H synthase-1 and -2 (PGHS-1 and PGHS-2). PGHS-1 was expressed by all intrauterine tissues at term labour. Densitometric analysis of Western blot autoradiographs showed that the fetal membranes and maternal cervix contained the largest amounts of PGHS-1. PGHS-1 enzyme content of ovine amnion was significantly greater than that of either chorion or allantois (P<0·05). PGHS-1 protein content of myometrial, endometrial and cotyledonary tissue extracts was minimal. Formation of the PGHS-2 isozyme was confined to placental tissue at term labour. PGHS-2 protein levels in sheep placenta were significantly higher than those of PGHS-1 in all intrauterine tissues examined. This result supports the hypothesis that PGHS-2 is a major contributor to PG formation at term labour.To elucidate the developmental changes in PGHS-1 and PGHS-2 relative to labour onset, an experimental paradigm of glucocorticoid-induced delivery was used. Previous characterisation and validation of this labour model demonstrated that direct, transabdominal, intrafetal injection of the synthetic glucocorticoid betamethasone (5·7 mg in 1 ml aqueous vehicle) on day 131 of gestation induced labour onset in 56·6 0·8 h (mean ...). As the latent period to induced-labour was known, the time course of enzyme formation could be ascertained. Sheep (n=20) were killed by barbiturate injection at various time intervals post-injection (0, 14, 28, 42 and 56 h). Tissue extracts collected at post-mortem examination were prepared and analysed by Western blots. PGHS-2 was induced in ovine cotyledon in a time-dependent fashion following glucocorticoid injection (P<0·05). There was a 12-fold increase in abundance between the time of betamethasone administration (0 h) and established labour (56 h). The PGHS-2 isozyme was not detected in any of the other tissues examined. In contrast, formation of the PGHS-1 isozyme did not change in relation to inducedlabour in any of the intrauterine tissues. This finding is consistent with constitutive formation of PGHS-1.Previous studies have demonstrated a rise in PG production in association with glucocorticoid-induced labour and spontaneous delivery. The results of the present study indicate that this rise in PG production is due to increased formation of the PGHS-2 isozyme in ovine cotyledon. PGHS-2 appears to be induced by exogenous glucocorticoid administration and/or the mechanisms controlling ovine parturition. The role of PG formation by the fetal membranes is yet to be elucidated.

Polyunsaturated fatty acids modulate prostaglandin synthesis by ovine amnion cells in vitro

“…In ovine foetal membranes, levels of PTGS1 were highest within the amnion (e.g. w25-fold higher than in the chorion) (McLaren et al 2000). PTGS1 was also found to be highly expressed within the guinea pig amnion, and its preferential inhibition was associated with prolonged gestation (Welsh et al 2005).…”

“…The increasing production of PGs by the feto-placental tissues during the 15-20 days prior to parturition is key to the successful initiation and progression of labour in the ewe (Olson et al 1985, Thorburn 1991, Gyomorey et al 2000. The increased production of PGs in intrauterine tissues towards the end of gestation could (at least in part) be due to increased expression/activity of PTGS1 and PTGS2 (McLaren et al 2000, Whittle et al 2000.…”

“…PG synthesis in the ovine amnion at term would appear to depend mainly on PTGS1 as it was found to be highly expressed in amniotic tissue from ewes in spontaneous labour, whereas PTGS2 was not detected (McLaren et al 1996(McLaren et al , 2000. In ovine foetal membranes, levels of PTGS1 were highest within the amnion (e.g.…”

The effect of a diet supplemented with the n-6 polyunsaturated fatty acid linoleic acid on prostaglandin production in early- and late-pregnant ewes

“…An increase in fetal cortisol production is a key element in the signalling pathway which leads to parturition in sheep via induction of placental enzymes that redirect steroidogenic pathways to favour the production of oestrogen at the expense of progesterone (Anderson et al 1975, Steele et al 1976). This change in progesterone/oestrogen ratio activates COX-2, ultimately leading to an increase in PG production (McClaren et al 1996(McClaren et al , 2000. However, Whittle et al (2000) showed that infusion of cortisol into the fetus beginning on day 125 of pregnancy increased both expression of COX-2 mRNA in placental trophoblast tissue and levels of fetal plasma PGE 2 , even in the prescence of the aromatase inhibitor 4-OHA.…”

“…Experiments using both gene-deletion and pharmacological treatments with specific COX-1 and COX-2 inhibitors have shown that parturition in mice is normally dependent on upregulation of COX-1 in the endometrium (Gross et al 1998, Reese et al 2000, Loftin et al 2002. In the ewe, however, COX-2 expression in the endometrium is increased during both spontaneous and dexamethasone (DEX)-induced labour, whereas there is no change in COX-1 expression at this time (Zhang et al 1996, Gibb et al 2000, McClaren et al 2000. In the uterus and placenta PG production and release can be modulated by a variety of physiological and pharmacological stimuli, for example oxytocin (OT; Lee and Silvia 1994), lipopolysaccharide (LPS;Flynn & Hoff 1995), calcium ionophore (CaI; Poyser 1987) and glucocorticoids (Whittle et al 2001).…”