Gestational- and labour-associated changes in the relative abundance of prostaglandin G/H synthase-1 and -2 mRNA in ovine placenta

The aim of this study was to establish the gestational- and labour-associated variation in the relative abundance of prostaglandin synthase-l (PGHS-1) and prostaglandin synthase-2 (PGHS-2) mRNA in ovine placenta (cotyledons). Cotyledons were collected from non-labouring ewes at 40-145 days of gestation (n = 25) and from ewes in active labour (145-147 days, n = 5). The relative abundance of PGHS-1 and PGHS-2 mRNA transcripts was determined by Northern blot analysis and laser densitometry, using a 2.3 kb sheep and a 1.2 kb mouse cDNA probe respectively. Data were expressed as a ratio of PGHS transcript hybridization/18S rRNA hybridization. During pregnancy, the relative abundance of PGHS-2 mRNA increased sevenfold, from 0.19 ± 0.04 at 40-85 days (n = 5) to 1.39 ± 0.05 at 140-145 days(n = 4) (P 0.05) during gestation. Neither PGHS-1 nor PGHS-2 mRNA relative abundance changed significantly in association with labour onset at term (n = 5) when compared with the relative abundance observed at 140-145 days (n = 4) (P > 0.05). The data obtained in this study are consistent with the hypothesis that PGHS-1 is constitutively expressed in ovine placenta during pregnancy and at the time of labour, and that PGHS-2 is induced during the second half of pregnancy. It remains to be established to what extent these two isozymes contribute to the net prostaglandin-forming capacity of the ovine placenta, particularly at the time of labour.