The effects of nitric oxide on the contractility of isolated uterine and aortic rings from pregnant rats

Activated NO pathway in uterine arteries during pregnancy in an IUGR rat model

Insufficient development of the uteroplacental circulation may contribute to the development of intrauterine growth restriction (IUGR). We developed a rat model of IUGR by administering a low-Na⁺ diet. This diet reduces maternal blood volume expansion and uteroplacental perfusion. We hypothesized that an impaired endothelial function in radial arteries decreases vasorelaxation and lowers placental perfusion in this IUGR model. The objective was to assess radial uterine artery responses to vasoactive agents in the IUGR model versus controls. The vasoactive agents included phenylephrine and carbachol, use of a pressurized artery myograph, in the absence or presence of inhibitors of nitric oxide (NO) synthase [ N-nitro-l-arginine methyl ester (l-NAME)], cyclooxygenase (Ibuprofen), and endothelium-dependent hyperpolarization {apamin/1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole}, allowing better characterization of the mechanism implicated in endothelium-dependent relaxation. The results show that 1) the diameter of uterine radial arteries was significantly decreased in the IUGR group; 2) sensitivity to phenylephrine was reduced in IUGR arteries, which could be returned to control group values by inhibition of NO production; 3) the relaxation response to carbachol was increased in IUGR rats, principally mediated by endothelium-dependent hyperpolarization in both groups; 4) NO synthase inhibition by l-NAME decreased the maximum relaxation to carbachol only in the IUGR group; and 5) relaxation response to NO donors is increased in IUGR compared with control radial arteries. Contrary to the hypothesis, results in the IUGR model indicate that the NO pathway is activated in radial uterine arteries, most likely in compensation for the reduction in blood uteroplacental perfusion. NEW & NOTEWORTHY In contrast to genetic or surgical models of intrauterine growth restriction, the diet-induced model of reduced maternal volume expansion shows the nitric oxide pathway to be activated in the uterine artery, possibly from increased shear stress and/or placental factors.

Oxytocin, its antagonist Atosiban, and preterm labor: a role for placental nitric oxide

OBJECTIVE

The aim of the present study was to understand the role played by Atosiban, an oxytocin receptor antagonist, on trophoblastic human cells, and the molecular bases of its efficacy and safety in the treatment of preterm labor. NO, peroxinitrite production and NOS expression have been evaluated on placenta obtained from term and preterm labors.

PATIENTS AND METHODS

We studied trophoblast cells isolated from selected placental tissue from 20 controls and 20 preterm patients after cesarean sections. Each sample was studied at basal state and after 2 hours incubation with oxytocin and Atosiban.

RESULTS

Significant variations of NO levels, peroxynitrite production and iNOS and eNOS expression both in the preterm, term samples and in each of the considered groups were observed. In the control group Atosiban re-established NO levels that were reduced after incubation with oxytocin, while in preterm samples NO levels were not only re-established but, after incubation with Atosiban, significantly increased compared to basal levels.

CONCLUSIONS

This confirms the beneficial role of Atosiban in prolonging the pregnancy of spontaneous labor at very early gestational periods. In conclusion, Atosiban might be an effective drug to prevent preterm labor, in the therapeutic approach to this pathology.

Effect of lipopolysaccharide on the production of prostaglandin E2 and inhibition of uterine contractions by nitric oxide in pregnant inducible nitric oxide synthase knockout mice

AIM

The aim of the present study was to evaluate the effect of lipopolysaccharide (LPS) on the production of prostaglandin E2 (PGE2) and inhibition by nitric oxide (NO) of spontaneous contractions of uterine rings from pregnant inducible isoform of nitric oxide synthase knockout (iNOS KO) mice.

MATERIAL AND METHODS

iNOS KO and wild-type mice were sacrificed 6 h after intraperitoneal (i.p.) administration of LPS on day 14 of gestation. Uterine rings were equilibrated in Krebs-Henseleit solution for isometric tension recording. In part of the uterine rings, placental tissues were left attached. The bathing solution was analyzed for PGE2 by radioimmunoassay. Changes in spontaneous contractions in response to cumulative concentrations of L-arginine, diethylamine/nitric oxide (DEA/NO), and 8-bromo-cyclic guanosine monophosphate (8-br-cGMP) were determined.

RESULTS

Treatment with LPS increased PGE2 production by uterine rings from wild-type and iNOS-KO mice. DEA/NO and 8-br-cGMP inhibited spontaneous contractions in uterine rings in the absence or presence of placenta, in both LPS-treated and LPS-untreated animals. LPS treatment attenuated maximal inhibition induced by the agents, both in the absence and presence of placental tissues in iNOS KO and wild-type mice.

CONCLUSION

LPS induces PG production in mice myometrium that is not dependent on the integrity of iNOS, while LPS could induce pathophysiological iNOS obstruct uterine quiescence by physiological iNOS. Infection affects uterine contractile activity through PG production, as well as through placental and genetic factors. NO may be a double-edged sword in pregnant mice myometrium.

In vitro study of the tocolytic effect of oroxylin A from Scutellaria baicalensis root

Scutellariae Radix is one of the well-known tocolytic Chinese herbs. Oroxylin A is isolated from the root of Scutellaria baicalensis. The main syndrome of preterm birth is caused by uterus contractions from excitatory factors. Administration of tocolytic agents is a strategy to prevent the occurrence of preterm births. The aim of this study was to investigate the effects of oroxylin A on contractions of uterine strips isolated from non-pregnant female Wistar rats (250~350 g). Contractions of the uterus were induced with acetylcholine (Ach) (1 μM), PGF_2α (0.1 μM), oxytocin (10^-3 U/ml), KCl (56.3 mM), tetraethylammonium (TEA; 1 and 10 mM), 4-aminopyridine (4-AP; 5 mM), glipizide (30 μM), a nitric oxide synthase (NOS) inhibitor (LNNA; 10^-3M), a β-receptor blocker (propranolol; 10 μM), and a cyclooxygenase inhibitor (indomethacin; 60 μM). The inhibitory effects of the amplitude and frequency of spontaneous contractions by oroxylin A were antagonized with Ach (IC₅₀ 22.85 μM), PGF_2α (IC₅₀27.28 μM), oxytocin (IC₅₀ 12.34 μM), TEA; 1 and 10 mM (IC₅₀ 52.73 and 76.43 μM), 4-AP (IC₅₀ 67.16 μM), and glipizide (IC₅₀27.53 μM), but oroxylin A was not influenced by Ca²⁺-free medium, LNNA, propranolol, or indomethacin. Otherwise, oroxylin A-mediated relaxation of the rat uterus might occur through opening of uterine calcium-dependent potassium channels or adenosine triphosphate potassium channel activation. This suggests that oroxylin A is the tocolytic principle constituent of Scutellariae Radix, and oroxylin A may provide a lead compound for new tocolytic drug development in the future.

Relationship between gene expression and function of uterotonic systems in the rat during gestation, uterine activation and both term and preterm labour

We have documented gestation- and labour- (preterm and term) dependent changes in expression of genes encoding contraction associated proteins in the rat uterus and correlated these changes with various parameters of uterine contractility. The data demonstrate increased expression of contractile agonist systems concurrent with decreased expression of relaxant systems after gestational day 20. Significant increases in expression of oxytocin (OT), its receptor (OTR), prostaglandin (PG) H synthase isoform 1 (PGHS-1) and PGF(2alpha) receptor (FP) occurred first, followed by increases in PGHS-2, connexin-43, endothelin-1 (ET-1) and the ET-1 receptor isoform ET(A). Expression of OTR and FP was significantly reduced during mid-gestation compared to non-pregnant animals. Expression of inducible nitric oxide synthase (iNOS) increased significantly during pregnancy then decreased concurrently with the increase in OTR and FP. Functional changes in uterine contractility accompany changes in gene expression. OT was the most potent contractile stimulant. Sensitivity of uterine strips to OT was reduced in early and mid-pregnancy then increased at uterine activation. Progesterone antagonist-induced preterm labour caused changes similar to those at normal term. Comparison of mRNA transcripts in separated endometrium and myometrium suggested that the endometrium is an important regulator of myometrial contractility, analogous to the relationship between endothelium and vascular smooth muscle. This novel combination of functional and genetic expression analyses provides new insight into the physiology of parturition.

Reduced Nitric Oxide in Amniotic Fluid of Patients with Chorioamnionitis

OBJECTIVE

Levels of nitric oxide (NO) and cytokines were assessed in amniotic fluid obtained from patients with severe chorioamnionitis (CAM) and appropriate controls.

METHODS

Amniotic fluid was obtained from 12 patients with CAM (17-24 weeks of gestation) and 89 patients undergoing diagnostic amniocentesis (16-18 weeks of gestation). The concentrations of NO, interleukin-6 (IL-6), and leukocyte elastase (LE) in amniotic fluid were then measured and compared.

RESULTS

The concentrations of NO, IL-6, and LE were all higher in CAM cases than in normal pregnant women. Furthermore, an inverse correlation between NO and LE was suggested in the CAM group.

CONCLUSIONS

These results indicate that in severe CAM, the action of NO might be reduced, not only due to blockage of action but also by degradation, despite increased production.

Expression of iNOS mRNA and inhibitory effect of NO on uterine contractile activity in rats are determined by local rather than systemic factors of pregnancy

Our purpose was to investigate whether the local or systemic factors of pregnancy are associated with inducible nitric oxide synthase (iNOS) mRNA expression and to determine the inhibitory effects of pharmacological agents that increase cGMP levels in rat myometrium. iNOS mRNA expression was determined in uterine tissues from nonpregnant rats and on day 17 of gestation in the pregnant and non-pregnant uterine horns by RT-PCR. In addition, uterine rings from the pregnant and non-pregnant uterine horns were placed in Krebs-Henseleit solution for isometric recordings of spontaneous contractions. Concentration-inhibition relationships to diethylamine/nitric oxide complex, 8-bromo-cGMP, and the selective phosphodiesterase V inhibitor were obtained. Compared to nonpregnant rats, expression of iNOS mRNA in myometrium increased during pregnancy, which was maximal on day 17, followed by a decrease on day 21 of gestation. Expression of iNOS mRNA at day 17 of gestation was greater in pregnant uterine horns than in nonpregnant ones. Maximal inhibition of phosphodiesterase V and increasing cGMP induced similar inhibition of spontaneous contractions in nonpregnant and pregnant uterine horns, while NO induced less inhibition in the former. The results suggest that the local pregnancy factor is needed for signal transduction from NO to soluble guanylate cyclase at a time when maximal expression of iNOS mRNA is evident.

Effect of nitric oxide on contractions of uterine and cervical tissues from pregnant rats

Our objective was to evaluate the role of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) pathway in rat uterine and cervical contractility at mid- and late gestation. Rings of uterus and cervix from Sprague Dawley rats on day 14 of pregnancy (mid-) and day 21 of pregnancy (late) were equilibrated at 2 g passive tension in organ chambers filled with Krebs-Henseleit solution and bubbled with 5% CO2 in air (37 degrees C, pH approximately 7.4). Rings were treated with an inhibitor of outward potassium current, tetraethylammonium, to activate phasic contractions, and the concentration-response relationships to diethylamine/NO and 8-bromo-cGMP (8-br-cGMP) were assessed. Baseline spontaneous activity was significantly higher at term gestation in both uterine and cervical rings compared with mid-gestation. Spontaneous contractile activity was not apparent in cervical rings from rats in mid-gestation, but was persistent after treatment with tetraethylammonium. Oxyhemoglobin did not affect NO-induced inhibition of activation by tetraethylammonium contractile activity in either cervical or uterine tissues in mid- or late gestation. The 8-br-cGMP concentration-dependently inhibited tetraethylammonium-activated contractions that were more pronounced in uterine tissues compared with cervical tissues in both mid- and late gestation. We concluded that activation of the NO-cGMP pathway inhibits both uterine and cervical smooth muscle contractility. Both tissues demonstrated refractoriness to NO at term.

Relaxant effects of parathyroid hormone and parathyroid hormone-related peptides on oviduct motility in birds and mammals: possible role of nitric oxide

Parathyroid hormone (PTH) and PTH-related peptides (PTHrP) have previously been shown to modulate the contractile state of numerous types of smooth muscle. The effects of N-terminal PTH and PTHrP on spontaneous in vitro contractility of oviducal smooth muscle using tissues from egg-laying Japanese quail (10-15 h post ovulation), 4 and 9 days pregnant mouse uterus were investigated. Myometrial tissues from both species contracted vigorously for several hours, when incubated in organ baths in De Jalon's solution gassed with 5%CO2/95%O2. Contractions were enhanced in high (1.2-2.5 mM) compared with low (0.1-0.5 mM) calcium (Ca) containing media. Bovine PTH(1-34) (bPTH(1-34)), human PTH(1-34 amide) (hPTHrP(1-34) amide), and hPTHrP(1-40) caused similar concentration-related inhibition of contractions in media containing 1.2mM Ca over a range of 10(-9) to 10(-7)M, whereas C-terminal hPTHrP(107-139) was devoid of such activity. Responses to bPTH(1-34) in 4 and 9-day pregnant mouse tissues were similar but hPTHrP(1-40) showed substantial loss of activity in 9-day, compared with 4-day pregnant mouse tissues. Repeated exposure of mouse uterine tissue to the peptides resulted in desensitisation of responses. The EC50 responses of mouse tissues were inhibited by the PTH/PTHrP receptor antagonist, hPTHrP(7-34) amide. Responses to bPTH(1-34) were also inhibited by both non-selective and selective neuronal nitric oxide synthase (NOS) inhibitors N(omega)-nitro-L-arginine methyl ester (0.01-1mM) and 7-nitroindazole (0.01-10 microM), respectively. Both NOS inhibitors were more effective in inhibiting bPTH(1-34)-induced relaxation in the absence of L-arginine compared with in the presence of 1mM L-arginine (a NOS substrate) in the incubation media. It is concluded that relaxant responses to N-terminal PTH and PTHrP peptides are well conserved in oviducal and uterine tissues from avian and mammalian species. The results also suggest that NO may be responsible for mediating relaxant activities of these peptides in pregnant mouse uterine tissue.

EFFECTS OF NITRIC OXIDE DONORS ON NON-PREGNANT AND PREGNANT RAT UTERINE AND AORTIC SMOOTH MUSCLE

OBJECTIVE

To compare the effects of nitric oxide (NO) donors, diethylamine/nitric oxide (DEA/NO) and nitroglycerin (NTG), on isolated uterine and aortic tissues from non-pregnant, mid and late pregnant rats.

METHODS

The uterus and thoracic aorta were obtained from non-pregnant (estrous cycle) and pregnant Sprague-Dawley rats on day 14 and day 21. The uterine and aortic rings were incubated in organ chambers filled with Krebs-Henseleit solution bubbled with 5% CO2 in air for isometric tension recordings. Cumulative concentration-response relationships to DEA/NO and NTG were obtained in the aortic rings contracted with phenylephrine and in spontaneously contracting uterine rings.

RESULTS

The sensitivity and the maximal inhibitory effects of DEA/NO did not differ in aortic tissues of any group. DEA/NO-induced Maximal inhibition of spontaneous contractions of uterine tissues from mid-pregnant rats was greater (although not significantly) than in the tissues from non-pregnant animals (with similar sensitivity), but it was significantly depressed in tissues from late pregnant rats. The sensitivity to and maximal inhibitory effects of NTG were less in aortic tissues from late pregnant versus mid-pregnant and non-pregnant rats. In uterine tissues from late pregnant rats the effect of NTG was negligible. The inhibitory action of both NO donors was much more pronounced in aortic versus uterine tissues.

CONCLUSIONS

Uterine smooth muscle is less sensitive than vascular smooth muscle to NO. Uterine smooth muscle from late pregnant animals demonstrates refractoriness to both DEA/NO and NTG, while vascular smooth muscle from late pregnant animals demonstrates refractoriness to NTG, but not to DEA/NO.

New developments in the management of preterm labor

Current management of preterm labor has not changed the incidence of preterm delivery; therefore, significant research effort has been concentrated on the search for new methods of management. New tocolytics like inhibitors of cyclooxygenase 2 and nitric oxide donors have been tested in animal models and in preliminary clinical trials with promising results. Inhibition of cervical ripening may be one alternative to tocolysis. This new approach has a potential to be a valuable method of management of preterm labor if human studies confirm the promising results reported in animals. Growing evidence suggests that premature delivery may be associated with infection or fetal growth abnormalities, with dire consequences to the fetus. If these associations are to be included in risk and benefit assessment, then inhibition of preterm labor may prove to be detrimental to the fetus.

Changes in intrafollicular pressure in the rat ovary by nitric oxide and by alteration of systemic blood pressure

BACKGROUND

ovulation is associated with degradation of the follicular apex vasodilatation and increased permeability of ovarian vessels. These changes may maintain or increase intrafollicular pressure (IFP) at ovulation to cause rupture of the follicular wall.

OBJECTIVE

to investigate the possible regulation of IFP during the ovulatory process.

STUDY DESIGN

immature Sprague-Dawley rats were primed with pregnant mare serum gonadotrophin (PMSG; 10IU) and given hCG (10IU) 48h later. The ovary was exposed 48-60h after PMSG, micropipette inserted into the Graafian follicle and the IFP measured at three time periods: preovulatory (PO) 48h after PMSG; midovulatory (MO) 4-7h after hCG; late ovulatory (LO) 9-12h after hCG. The offset of the nitric oxide synthase (NOS) inhibitor L-arginine methyl ester (L-NAME), the alpha(1)-adrenoceptor agonist phenylephrine and the beta-adrenoceptor agonist isoprenaline were tested.

RESULTS

phenylephrine given i.v. increased the systemic blood pressure, and significantly decreased the IFP in the LO phase (78% of pre-treatment value). Local administration of phenylephrine or isoprenaline (1ml of 1.5-15 microM) by superfusion over the ovary did not change the IFP. Local administration of L-NAME (1ml of 2 microM) significantly lowered (P<0.05) the IFP in the MO and LO phases, but was without effect in the PO phase.

CONCLUSION

this study reveals that IFP regulation may be related to changes of the systemic blood pressure and that NO may be one local ovarian mediator in IFP regulation.

Effect of placental tissue on inhibition of uterine contraction by nitric oxide donors

OBJECTIVE

Our purpose was to test the hypothesis that placental tissue modulates the effect of nitric oxide on spontaneous uterine contractility in pregnant rats.

STUDY DESIGN

Rings (approximately 4 mm) of uterus taken from rats on day 14 (midpregnancy, n = 6), day 18 (late pregnancy, n = 4), and day 22 (term, n = 4) of gestation were placed in organ chambers filled with Krebs-bicarbonate buffer bubbled with 5% carbon dioxide in air (37 degrees C, pH approximately 7.4) for isometric tension recording. In some rings a piece of placenta was left attached to the uterine wall. In the other rings the fetuses, placentas, and membranes were removed completely. Change of spontaneous contractions of the rings (percentage change of basal integral activity for 10 minutes) in response to cumulative concentrations of the nitric oxide donors diethylamine-nitric oxide and nitroglycerin (10(-6) mol/L to 10(-4) mol/L) were compared between rings with and without placenta.

RESULTS

Diethylamine-nitric oxide and nitroglycerin inhibited spontaneous uterine contractions in rings from midpregnancy, in both the absence and the presence of placenta. In rings from midpregnancy, the maximal inhibition of contractions by diethylamine-nitric oxide but not by nitroglycerin was significantly (P <.05) higher in the presence (26.7% +/- 3.5% of basal activity) than in the absence (39. 6% +/- 3.3%) of placenta. Inhibition of contraction by nitric oxide donors in rings from late and term pregnancy was less than in midpregnancy, and the presence of placental tissue did not influence the responses.

CONCLUSIONS

The presence of placental tissue enhances inhibition of uterine contractility by agents that spontaneously release nitric oxide, such as diethylamine-nitric oxide, but not by nitroglycerin, which requires metabolic transformation for nitric oxide to be released. Refractoriness to nitric oxide near or at term does not depend on the presence or absence of placental tissue.