Functions of nitric oxide in the human female reproductive tract

Abstract

Nitric oxide (NO), synthesised enzymatically by nitric oxide synthases (NOS) from the amino acid L-arginine, is an important mediator of various physiological and pathophysiological processes in several organ systems. As a free radical gas with an unpaired electron, NO is extremely labile and it quickly reacts with its target proteins. The half-life of NO is only a few seconds during in vivo conditions. Thus, direct measurement of NO synthesis and its localisation in tissue have proved difficult. Therefore, indirect methods to identify and to localise NO production are usually employed. In this thesis, tissue samples from the human reproductive tract were obtained during operations due to legal sterilisation, benign gynaecological diseases, first trimester abortion, Caesarean section or directly after spontaneous vaginal delivery at term. The presence of an endogenous NO system within the Fallopian tube of cycling women as well as in the uterine body and cervix of pregnant women, was assessed by means of NADPH diaphorase staining, immunohistochemistry and immunoblotting. Further, to study effects of NO, tissue strips were mounted vertically in organ baths and substances known to regulate NO synthesis as well as NO donors were administered. NADPH diaphorase staining, a traditional but non-specific histochemical marker for NOS, indicated positive staining for NOS in all tissues examined. Two isoforms of NOS, endothelial NOS (eNOS) and inducible NOS (iNOS) were localised in the Fallopian tube and the uterine cervix at term. The expression of these two isoenzymes in the Fallopian tube of fertile women as well as in the uterine body and the cervix of term pregnant women was confirmed by immunoblotting. There was no difference in intensity of NADPH diaphorase staining or in NOS expression between samples from term pregnant non-labouring and labouring women. Contractility experiments further strengthened the notion of an endogenous NO system in the tissues examined. Addition of an NO donor resulted in concentration- dependent inhibition of contractile activity, thereby further indicating a physiological role of endogenously produced NO. Nitric oxide donors caused an effective inhibition of contractile activity in myometrial strips from both labouring and non-labouring women. Uterine cervical tissue specimens of first trimester as well as third trimester pregnant women were clearly responsive to NO in vitro. In the latter specimens, not only a dose-dependent inhibition of frequency and amplitude of contractions, but also a decrease in basal tone were registered. The present demonstration of a functional NO system within the female reproductive tract may have clinical implications.