The isoform(s) of prostaglandin H synthase (PGHS) present in pregnant rat myometrium have been identified and the ontogeny of their expression studied during late gestation and parturition. Concentrations of PGHS have been related to changes in concentration of nuclear and cytosolic estrogen (ER) and progesterone receptors (PR) occurring at this time. Nuclear PR concentrations were maximal on days 16-18 of pregnancy, decreased from days 18 to 22 (delivery) and fell 24 hours postpartum. Nuclear ER concentrations increased significantly from days 20 to 22 of pregnancy and fell postpartum. Whereas the ratio of nuclear ER/PR was firmly in favour of progesterone action on days 16-20 it increased on day 22 corresponding to increased estrogen action. Western immunoblotting with specific antibodies revealed a single 72 kDa PGHS-1 isoform in myometrium at each timepoint. There was no evidence for the inducible PGHS-2 isoform in myometrium. Densitometric analysis showed the concentration of PGHS-1 increased significantly from day 16 to a maximum at the time of delivery on day 22 and decreased immediately afterwards. Expression of the constitutive PGHS-1 isoform is associated with the changing ratio of nuclear ER/PR and may therefore be hormonally regulated.
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