title: The role of nitric oxide in human parturition
Creators: name: Thomson, Andrew John
Publisher: name: University of Glasgow
Year: 1998

description: Animal studies suggest that nitric oxide, a free radical derived from L-arginine by the action of nitric oxide synthase (NOS), is a fundamental mediator in the initiation of labour. The studies reported in this thesis have investigated the involvement of the L-arginine-nitric oxide system in the maintenance of myometrial quiescence and the initiation of parturition in human pregnancy. Further, the effects of nitric oxide donors on the pregnant human cervix have been determined. The studies reported in chapter 2, were performed to determine whether a decrease in NOS enzyme activity and expression are involved in the final activation of spontaneous labour at term. Using immunocytochemistry, each of the three isoforms of NOS was localised in myometrium, placenta and fetal membranes collected before and during labour at term. NOS enzyme activity in each of these tissues was determined by measuring the conversion of radiolabelled L-arginine to L-citrulline. There were no differences in either the expression or enzyme activity of NOS within each tissue following the onset of labour indicating that a decrease in NOS enzyme activity is not involved in the final activation of labour at term in the human. The enzymes superoxide dismutase (SOD) and xanthine oxidase (XO) are known to prolong and attenuate respectively, the biological activity of nitric oxide. Studies, presented in chapter 3, were therefore undertaken to determine whether changes in the uterine expression of these enzymes are associated with the onset of human parturition. Both SOD and XO were shown to be widely expressed within the pregnant uterus, although there was no difference in their expression in tissues collected before, compared with after the onset of labour. These data suggest that SOD and XO may be important in maintaining uterine quiescence in pregnancy, but that a change in the expression of these enzymes is not involved in the initiation of labour. In chapter 4, the expression of the constitutive isoforms of NOS, eNOS and bNOS, was investigated in the non-pregnant uterus, the preterm pregnant uterus and the term pregnant uterus to determine whether these isoforms are up-regulated in the uterus during pregnancy. Using immunocytochemistry and Western blotting, eNOS and bNOS protein concentrations were greater in preterm, pregnant myometrium than nonpregnant myometrium, (p&lt;0.01 and p&lt;0.05, respectively). eNOS, but not bNOS, protein concentration was lower (p&lt;0.05), in myometrial samples obtained at term compared with those obtained preterm. These data indicate that the constitutive isoforms of NOS are upregulated in human pregnancy and may play a role in maintenance of myometrial quiescence. Concurrent histological studies on tissues obtained before and during spontaneous labour, reported in chapter 5, revealed that inflammatory cells infiltrate the myometrium during labour. Analysis of individual cell types using immunocytochemistry showed significantly more inflammatory cells, predominantly neutrophils and macrophages, in labouring myometrium compared to non-labouring biopsies, (Scheffe's test, p = 0.0001). The infiltrate was present in both lower and upper uterine segment myometrium, with significantly more inflammatory cells present in the lower uterine segment myometrium, (Scheffe's test, p &lt; 0.02). These observations indicate that inflammatory cell infiltration is part of the physiological mechanisms that occur in the myometrium during labour. The cervical ripening effects of nitric oxide donors, isosorbide mononitrate (IMN, 40 mg) and glyceryl trinitrate (GTN, 500 mug), were determined in a prospective, randomised trial involving women scheduled for surgical termination of pregnancy in the first trimester. During the operative procedure, the cervical resistance was measured objectively using a force sensing apparatus. The data, presented in chapter 6, showed that both IMN and GTN significantly reduced the cervical resistance, compared with no treatment (p&lt;0.006 and p&lt;0.05, respectively). A further randomised trial involving 66 primigravid women was performed to compare the side effect profile of IMN in comparison with the prostaglandin analogue gemeprost. Significantly more women remained asymptomatic following IMN (64%) than following gemeprost (14%). Pretreatment with gemeprost resulted in abdominal pain in 73% of women and vaginal bleeding in 32%, compared with 3% and 0% respectively following IMN. These data, reported in chapter 7, suggest that nitric oxide donors may have advantages over the prostaglandins for cervical ripening. In summary, an up-regulation of NOS expression in myometrium during pregnancy suggests an involvement of the L-arginine-nitric oxide system in the maintenance of myometrial quiescence. A decrease in this system and in the expression of enzymes known to influence the bioavailability of nitric oxide, do not coincide with the onset of spontaneous labour at term, suggesting that a withdrawal of nitric oxide is not the final activating factor in the initiation of labour in women. This work has demonstrated that nitric oxide donors produce effective ripening of the human cervix in the first trimester of human pregnancy with fewer side effects than prostaglandins. Nitric oxide donors represent alternative cervical ripening agents, at least in the first trimester of human pregnancy.