Endogenous nitric oxide suppresses rat myometrial connexin 43 gap junction protein expression during pregnancy.

Nitric oxide (NO) synthase (NOS) is active in the gravid uterus, and its activity decreases prior to the onset of parturition. We tested the hypothesis that NO helps maintain uterine quiescence by suppressing the expression of genes necessary for parturition. Pregnant rats (18 days gestation) were treated with inducible NOS (iNOS) inhibitor N-iminoethyl-L-lysine (NIL) or endothelial NOS inhibitor nitro-L-arginine methyl ester (L-NAME); 24 h later, uteri were analyzed for myometrial connexin 43 (Cx43) protein by immunoblotting and mRNA by Northern analysis. Myometrial oxytocin receptors (OTR) were measured by radioligand binding, and decidual prostaglandin H synthase (PGHS) protein by immunoblotting. Uterine NOS blockade was verified by NOS activity assay. We found that NIL, but not L-NAME, significantly increased myometrial Cx43 protein to parturitional levels with treatment at 19 but not 17 days gestation. Steady state mRNA concentrations were not changed at 24 h. NOS inhibition did not increase the concentrations of OTR, or PGHS protein, nor did it decrease maternal serum progesterone. We conclude that endogenous uterine NO from iNOS suppresses myometrial Cx43 gap junction protein expression during rat pregnancy. Although the exact mechanism is unknown, an increase of uterine wall stretch due to inhibition of relaxation could account for increased Cx43 gene transcription.

Granulated metrial gland cells contain nitric oxide synthases during pregnancy in the rat.

Nitric oxide synthase (NOS) enzyme activity is present in rat decidua and metrial gland, and the activity decreases near the end of pregnancy. In this study, inducible and endothelial NOS isoforms were immunolocalized to rat granulated metrial gland (GMG) cells using anti-NOS antibodies proven to be isoform specific. These NOS-positive GMG cells are of the natural killer cell lineage as they stained positively for NKR-P1 cell surface receptor, and for perforin. The number of NOS-positive GMG cells corresponded with the level of decidual and metrial gland NOS enzyme activity. NOS activity declined when GMG cells containing NOS decreased in number. Uterine arteriolar vascular smooth muscle also stained positively for inducible NOS and the staining did not change with advancing gestation. Only a minority of uterine myocytes stained positively for inducible NOS and these were subjacent to the placental attachment site. Neuronal NOS immunostaining was not present in the decidua and the metrial gland.

Nitric oxide and pregnancy.

This review will consider whether nitric oxide (NO) contributes to maternal systemic vasodilation during pregnancy, regulates uterine and fetoplacental blood flow, and is involved in uterine quiescence prior to parturition. Also, whether a deficiency of NO contributes to the hypertensive disorder of pregnancy, preeclampsia, will be considered. The biosynthesis of NO increases in gravid rats and sheep, but the status in normal human pregnancy and preeclampsia is controversial. NO contributes to maternal systemic vasodilation and reduced vascular reactivity during normal pregnancy; however, the relative contribution of NO is variable depending on the animal species, vascular bed, and vessel size. Impaired relaxation responses to acetylcholine, but not bradykinin or NO donors, are observed in small arteries from women with preeclampsia, suggesting a receptor or signal transduction defect, although NO may play little, if any, role here. Uterine arteries have increased endothelial nitric oxide synthase (NOS) activity, protein expression, and guanosine 3',5'-cyclic monophosphate production during pregnancy; however, whether these mediate uterine vasodilation during pregnancy remains to be established. NOS is expressed in the human placental syncytiotrophoblast and in the fetoplacental and umbilical vascular endothelium where basal production of NO contributes to low fetoplacental vascular resistance. Controversy exists over the status of placental NOS in preeclampsia, although an abnormality of umbilical NOS activity is likely. Finally, the uterus has NOS activity, which decreases at the end of gestation, and exogenous NO relaxes the myometrium, but whether endogenous NO contributes to uterine quiescence during pregnancy has yet to be confirmed.

Nitric oxide synthase activity in the gravid rat uterus decreases a day before the onset of parturition.

OBJECTIVE: Our purpose was to determine the timing, tissue location, and isoform of the uterine nitric oxide synthase activity decrease at term in gravid rat uteri. STUDY DESIGN: Nitric oxide synthase specific activity was assayed in rat uteri 11 through 22 days' gestation by the difference in radiolabeled arginine to citrulline conversion with and without the cofactor reduced nicotinamide adenine dinucleotide phosphate. Nitric oxide synthase isoform was assessed by calcium sensitivity and subcellular location. RESULTS: Rat uterine nitric oxide synthase activity decreased between days 15 and 21 of gestation but did not decrease further at term (day 22), before and after the onset of labor. Decidual nitric oxide synthase activity exceeded the myometrial activity at 15 days' gestation, but then the two were equal at 18 through 22 days' gestation. The nitric oxide synthase activity was calcium insensitive except for half the decidual cytosolic activity on day 15. CONCLUSION: The decrease in pregnant rat uterine nitric oxide synthase activity coincides with the preparation of the uterus for parturition rather than the final activation of labor.

Nitric oxide synthase activity in pregnant rabbit uterus decreases on the last day of pregnancy.

OBJECTIVE: Our purpose was to test a potential role for the endogenous smooth muscle relaxant nitric oxide in the control of gestational uterine activity by quantifying and characterizing its synthetic enzyme, nitric oxide synthase, in uterine tissue at the end of pregnancy. STUDY DESIGN: We measured nitric oxide synthase activity through the conversion of tritiated L-arginine to tritiated L-citrulline in subcellular preparations of decidua and myometrium from pregnant rabbits at 27, 30, and 31 days' (term)gestation. Nitric oxide synthase was characterized by measuring its relative inhibition by arginine analogs and its calcium-calmodulin requirement. Nitric oxide synthase activities were compared by one-way analysis of variance with Fisher's post hoc test. RESULTS: Nitric oxide synthase activity in decidua was high at 27 days' gestation (6.32 +/- 1.10 pmol/mg protein per minute, n = 6), less with the approach of labor (30 days = 3.16 +/- 1.25 pmol/mg per minute, n = 4), and lowest at 31 days (1.07 +/- 0.29 pmol/mg per minute, n = 4, p < 0.05). Decidual nitric oxide synthase was calcium insensitive, and arginine analogs reduced activity with potencies consistent with their effect on the induced form of nitric oxide synthase. CONCLUSION: Decidual nitric oxide synthase activity, which has the characteristics of the inducible isoform of the enzyme, is significantly lower on the last day of gestation. This suggests a role for nitric oxide in the control of uterine contractility during pregnancy.