Hemoxygenase and nitric oxide synthase do not maintain human uterine quiescence during pregnancy

The nitric oxide (NO)-cGMP pathway has been proposed as a mechanism for rel axation of myometrium during pregnancy and as a modulator of labor. Carbon monoxide (CO), produced by hemeoxygenases (HO-1 and HO-2), also activates s oluble guanylate cyclase to increase cGMP, A recent study reported a large increase in HO-1 and HO-2 proteins during pregnancy, suggesting that the MO -CO pathway may be important in the maintenance of uterine quiescence durin g pregnancy. In this study we used Western blotting, reverse transcription- polymerase chain reaction, and immunohistochemistry to determine HO-1 and H O-2 expression in nonpregnant, pregnant, and laboring myometrium, Immunoloc alization of HO was also compared with endothelial and inducible nitric oxi de synthases (eNOS and iNOS). In contrast to HO-1 protein, which was not de tected in myometrium, HO-2 protein and mRNA were constitutively expressed, although there were no differences in expression between the groups. eNOS w as expressed in endothelial cells but not in myometrial. smooth muscle. iNO S protein was not detected in myometrium, These data do not support an up-r egulation of HO-1 and HO-2 during pregnancy and are not consistent with a r ole for NO of a major role for CO in human. myometrial quiescence. Our resu lts are also in keeping with HO-2 being an noninducible protein.