Changes in expression of contractile FP and relaxatory EP₂ receptors in pregnant rat myometrium during late gestation, at labor, and postpartum

Prostaglandins synthesized at parturition may act via specific myometrial receptors as mediators of uterine contractions. Several isoforms of eicosanoid (prostaglandin) receptors, identified by pharmacological means, are linked to contractile (FP, EP₁, EP₃) or relaxatory (EP₂, EP₄, IP, DP) intracellular pathways. Changes in mRNA expression of the contractile FP and the relaxatory EP₂ receptor were measured in myometrium throughout gestation, at parturition, and postpartum. Timed pregnant rats were killed at 0900 h on Day 16, 18, 20, 21, 21.5, or 22 (parturition) of pregnancy or one day postpartum (n = 5 animals/group). A longitudinal section of myometrium was removed, total RNA was extracted, and semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) was performed for glyceraldehyde phosphate dehydrogenase (GAPDH), calponin, EP₂, and FP receptor mRNA expression. Complementary DNA products were run on agarose gels and visualized, and the quantity of cDNA product was measured against a DNA mass ladder. RT-PCR product identity was confirmed by restriction enzyme cleavage. EP₂ receptor mRNA expression was highest at Day 16 and declined significantly to Day 21.5 and one day postpartum (p < 0.05, Student-Newman- Keuls procedure). Expression of FP receptor mRNA was low at Day 16 of gestation and increased significantly until delivery (p < 0.05, ANOVA) at Day 22, then fell to prepartum levels at one day postpartum. Myometrial activity at parturition may change from an active quiescent to an active contractile state in concert with a decline in expression of the relaxatory EP₂ receptors and up-regulation of contractile FP receptors.