Activin βA-subunit and activin receptors in human myometrium at term and during labour

Objective: To measure activin A content and to localise and semi-quantitate activin receptors in human myometrium at term and during labour. Design: Myometrium was collected from non-pregnant women (n = 6), pregnant women at term not in labour (n = 6) and at term in labour (n = 6). Setting: Monash Medical Centre, Melbourne, Australia. Main outcome measures: Tissue lysates of myometrium were analysed for activin A content using an enzyme-linked immunosorbent assay and activin receptor proteins IA, IIA and IIB using Western hybridisation. Activin β_A-subunit and activin receptors were localised in myometrium by immunohistochemistry. Results: Activin A was detected by ELISA in non-pregnant, pregnant and labouring myometrium. Levels were significantly higher in labouring myometrium. The three activin receptors IA, IIA and IIB were detected in all myometrial samples by Western hybridisation. Receptor IA was expressed in significantly higher levels in pregnant myometrium. Receptor IIA was very weakly expressed throughout. The expression of receptor IIB was similar in all three groups. Activin β_A-subunit and all three receptors were localised to the endothelial cells of myometrial blood vessels. Neither activin β_A-subunit nor any of the three activin receptors were immunolocalised to myometrial smooth muscle cells in the three groups. This result was confirmed by Western blotting for expression of activin receptors in isolated myometrial smooth muscle and microvascular endothelial cells. Conclusion: The myometrium is not a target for activin A during late pregnancy or labour. However, activin A may have a role in the regulation of microvascular endothelial cell function in the myometrium.