Nitric oxide donor-induced inhibition of pregnant rat uterine spontaneous contractile activity and release of nitric oxide from uterus measured by microdialysis.

Our aim was to study whether nitric oxide (NO) donor-induced inhibition of pregnant rat myometrium contractility correlates with the release of NO. Uterine rings from mid-pregnant and late pregnant Sprague-Dawley rats were used for isometric tension recording. Concentration-response relationships to sodium nitroprusside (SNP), nitroglycerine (NTG) and diethylamine (DEA)/NO were assessed. The time course of NO release after addition to the organ chambers of the 3 NO-donors was assessed by the detection of NO products NOx (NO3+NO2) using the microdialysis probe by a HPLC-NO detector system. DEA/ NO induced greater inhibition of the spontaneous contractile activity of uterine rings from mid-pregnant rats than SNP or NTG. In uterine rings from late pregnant rats, however, the maximal inhibition of the contractility by all 3 NO-donors were significantly less. The NOx levels measured in the uterine ring walls from either mid-pregnant or late pregnant rats significantly increased after DEA/ NO as compared to the basal levels or the levels after NTG or SNP. The decrease of NO-donor-induced inhibition of rat myometrium contractility, with unchanged formation of NOx, at term, suggests that the changes in NO signaling are responsible for gestational age-dependent attenuation of the inhibitory effect.

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.

Expression of inducible nitric oxide synthase messenger RNA, but not guanylate cyclase messenger RNA, depends on gestational age in rat myometrium.

We studied the expression of inducible nitric oxide (NO) synthase (iNOS) and soluble guanylate cyclase (sGC) mRNAs in pregnant rat myometrium. Expression of iNOS and sGC alpha1, beta1 and beta2 mRNA was analyzed in non-pregnant and pregnant (days 10, 14, 17 and 21) Wistar rats by reverse transcription-polymerase chain reaction. Expression of iNOS mRNA increased during pregnancy but decreased on day 21 of gestation. Expression of GC alpha1 mRNA was greater than GC beta1 mRNA at all time points. Expression of uterine GC alpha1 and GC beta1 mRNA did not change significantly during pregnancy and did not differ significantly from non-pregnant levels. The values of sGC beta2 mRNA were below the limit of detection. In conclusion, the expression of iNOS mRNA increased during pregnancy in the myometrium and decreased at term, while the expression of sGC mRNA was not affected by pregnancy. Thus, it is the changes in NO production, rather than changes in its target, that are responsible for uterine quiescence during pregnancy and initiation of labor.

Effects of L-type Ca(2+)-channel blockade, K(+)(ATP)-channel opening and nitric oxide on human uterine contractility in relation to gestational age and labour.

Relative uterine inactivity during pregnancy changes to vigorous rhythmic contractility during labour. We hypothesized that mechanisms involved in the regulation of uterine quiescence and contractility differ between term and preterm myometrium and in labour and non-labour states. Myometrial strips, prepared from biopsies taken at Caesarean section from labouring and non-labouring women preterm and at term, were mounted in organ chambers for isometric tension recording. Oxytocin (10(-9) mol/l) was added to maintain stable contractions, and effects of various inhibitors of uterine contractility were studied. The inhibitory effects of L-type Ca(2+)-channel blocker nifedipine and ATP-sensitive K(+)-channel opener pinacidil were greater in myometrium from the non-labour versus the labour group, both preterm and at term. In addition, pinacidil's effect was greater at term compared with preterm in the non-labour group. Mg(2+) and the nitric oxide donor sodium nitroprusside significantly inhibited contractility in all groups without significant differences with regard to labour or gestational age. Decreased inhibition of human uterine contractility by L-type Ca(2+)-channel blockers and K(+)(ATP)-channel openers in preterm and term labour may reflect changes in expression and activity of these channels. Effects of nitric oxide and Mg(2+) are not affected by gestational age or labour.

Calcitonin gene-related peptide dilates the pregnant rat uterine vascular bed via guanylate cyclase, ATP- and Ca-sensitive potassium channels and gap junctions.

We studied the mechanism of calcitonin gene-related peptide (CGRP)-induced vasorelaxation in isolated uterine vascular beds of pregnant rats. The vascular beds were perfused in situ with Krebs buffer containing dextran and indomethacin, an inhibitor of cyclooxygenase. Baseline perfusion pressure was maintained with norepinephrine. When applied as a bolus, CGRP caused a decreased perfusion pressure in uterine vascular beds that was dose-dependent and equal in both mid-pregnant and late-pregnant rats. The non-selective inhibitor of nitric oxide synthase (NOS), Nomega-nitro-L-arginine methyl ester (L-NAME), did not significantly affect CGRP-induced vasodilatation in vascular beds of either group. CGRP-induced vasodilatation was not influenced by preincubation with the inhibitors of adenylate cyclase (SQ 22536 or MDL 12330A), but was significantly attenuated by the selective inhibitor of soluble guanylate cyclase (ODQ). The vasorelaxant effect of CGRP was not significantly influenced by the inhibitor of voltage-gated potassium (KV) channels (4-aminopyridin), but was significantly attenuated by an inhibitor of calcium-regulated potassium (KCa) channels (tetraethylammonium) and by an inhibitor of adenosine triphosphate-sensitive potassium (KATP) channels (glibenclamide). The gap junction uncoupling agent (carbenoxolone) also significantly attenuated the CGRP-induced decrease in perfusion pressure. We conclude that vasorelaxation induced by CGRP in the pregnant rat uterine vascular bed is not dependent on endothelial nitric oxide. In the uterine circulation of late-pregnant rats, the CGRP effect involves activation of soluble guanylate cyclase, but not adenylate cyclase, and does involve KCa and KATP channels and gap junctions.

The role of endothelium-derived hyperpolarizing factor in the regulation of the uterine circulation in pregnant rats.

OBJECTIVE: The purpose of this study was to determine whether endothelium-derived hyperpolarizing factor regulates rat uterine circulation in pregnant rats. STUDY DESIGN: Intact isolated uterine vascular beds from late pregnant rats were perfused in situ with Krebs buffer that contained dextran, indomethacin, N-nitro-L-arginine methyl ester, and phenylephrine. Endothelium-derived hyperpolarizing factor-induced decreases in perfusion pressure in response to acetylcholine were analyzed. RESULTS: The decrease in perfusion pressure induced by endothelium-derived hyperpolarizing factor was significantly attenuated by 4-aminopyridine and was abolished by a combination of 4-aminopyridine and tetraethylammonium. Endothelium-derived hyperpolarizing factor-induced decrease in perfusion pressure was abolished by potassium chloride and attenuated by miconazole, but not linoleyl hydroxamic acid. Endothelium-derived hyperpolarizing factor-induced decrease in perfusion pressure persisted after perfusion with solutions that contained 2 inhibitors of nitric oxide synthase and a scavenger of nitric oxide. Nitric oxide exerted negative feedback on the endothelium-derived hyperpolarizing factor effects. CONCLUSION: In the pregnant rat uterine vascular beds, endothelium-derived hyperpolarizing factor release is activated by a delayed rectifier type of voltage-sensitive potassium channel. Endothelium-derived hyperpolarizing factor does not seem to be related to nitric oxide or to products of lipoxygenase or cytochrome p450 mono-oxygenase pathways of arachidonic acid metabolism.

Inhibition of cyclooxygenase but not nitric oxide synthase influences effects on the human omental artery of the thromboxane A2 mimetic U46619 and 17beta-estradiol.

OBJECTIVE: These experiments were performed to study the influence of endothelial factors on the contractile effect of the thromboxane A(2) analog U46619 and on the relaxant action of 17beta-estradiol on isolated human omental arteries from nonpregnant women, women with normal pregnancies, and women with preeclampsia. STUDY DESIGN: Arterial rings (3 mm) with or without endothelium were suspended in organ chambers filled with Krebs buffer, 37 degrees C, aerated with 5% carbon dioxide in air, pH approximately 7.4, for isometric tension recording. Rings were incubated with indomethacin, N(omega)-nitro-L -arginine, or 17beta-estradiol, alone or in combination. The concentration that produced 50% of maximal effect, the area under the curve, and the maximal effect of U46619, normalized with respect to a reference contraction in response to potassium chloride, were compared. RESULTS: Neither indomethacin nor N(omega)-nitro-L -arginine changed the basal tone of omental artery rings. Neither N(omega)-nitro-L -arginine nor removal of the endothelium affected either the contractile action of U46619 or the relaxant action of 17beta-estradiol. In contrast, indomethacin potentiated the contractile effect of U46619 and abolished the inhibitory effect of 17beta-estradiol in rings from all three groups. The effects of U46619 and 17beta-estradiol were significantly less in rings from women with normal pregnancy than in those from women with preeclampsia. Tissues from women with preeclampsia demonstrated the highest contractile response to U46619. CONCLUSION: The inhibitory effect of 17beta-estradiol is not due to increased production of endothelial nitric oxide but rather involves inhibitory products of the cyclooxygenase pathway. The effect of indomethacin may result from inhibited production or release of eicosanoids or from sensitization of thromboxane A(2) receptors.

Endothelium dependence and gestational regulation of inhibition of vascular tone by magnesium sulfate in rat aorta.

OBJECTIVE: The aim of this study was to investigate the role of nitric oxide in the vasorelaxant effect of magnesium sulfate during pregnancy. STUDY DESIGN: Segments of 3 mm of the aorta, with or without intact endothelium, from 16- or 22-day-pregnant rats were mounted in organ chambers with standard Krebs solution or low-magnesium Krebs solution for measurement of isometric tension. The rings were contracted with phenylephrine, and cumulative concentration-response curves for magnesium were determined after incubation with various inhibitors. RESULTS: Magnesium relaxed the aortic rings from pregnant rats in a concentration-dependent manner. The relaxation was significantly lower on day 22 of gestation than on day 16 of gestation. Removal of the endothelium or incubation with 10(-4)-mol/L N omega-nitro-L -arginine methyl ester (a nitric oxide synthase inhibitor), 10(-5)-mol/L 6-anilino-5,8-quinolinedione (a guanylate cyclase inhibitor), or 10(-5)-mol/L indomethacin (a cyclooxygenase inhibitor) significantly decreased the relaxant effect of magnesium on aortic rings from 16-day-pregnant but not 22-day-pregnant rats. Treatment with minimally effective concentrations of a nitric oxide donor (3 x 10(-10)-mol/L sodium nitroprusside) or a cyclic guanosine monophosphate analog (10(-6)-mol/L 8-bromo-cyclic guanosine monophosphate) restored the response to magnesium. CONCLUSIONS: The relaxant effect of magnesium on rat aortic rings was dependent on both endothelium and gestational age and was lower at term than during late pregnancy. The endothelium appears to potentiate the vasorelaxant effects of magnesium through the nitric oxide-cyclic guanosine monophosphate and cyclooxygenase systems.

The effect of 17beta-estradiol on isolated omental arteries from preeclamptic women.

OBJECTIVE: To study the effect of 17beta-estradiol on isolated omental arteries from preeclamptic women. STUDY DESIGN: Rings of omental artery with intact endothelium were mounted in organ chambers for isometric tension recording. We studied the effect of pharmacological concentrations of 17beta-estradiol on potassium chloride-induced tension and the concentration-contraction relationships for norepinephrine and calcium. RESULTS: Cumulative application of 17beta-estradiol, in a concentration-dependent manner, relaxed potassium chloride contracted rings. Sixty minutes of incubation with 17beta-estradiol (10(-5)mol/l) attenuated the tension developed in response to potassium chloride, norepinephrine and calcium. Tamoxifen (10(-6)mol/l) did not antagonize the inhibitory actions of 17beta-estradiol. CONCLUSIONS: Pharmacological concentrations of 17beta-estradiol retain the capability for relaxing omental artery rings from preeclamptic women. The loss of refractoriness to norepinephrine, increased responsiveness to calcium ions and decreased ability of 17beta-estradiol to inhibit calcium-induced tension may be responsible for increased vascular reactivity in preeclampsia.

Modulation of rat uterine contractility by mast cells and their mediators.

OBJECTIVE: This study was designed to test the possibility that mast cells play a role in the regulation of uterine contractility. STUDY DESIGN: Histamine and rat mast cell protease II levels were determined by radioenzymatic assay and standard radial immunodiffusion techniques, respectively, in uterine tissues from Wistar rats with timed pregnancies. Isolated uterine strips from nonsensitized and ovalbumin-sensitized nonpregnant and pregnant Wistar rats were used for isometric tension recording. Contractile responses to compound 48/80, carbachol, ovalbumin, normal rabbit serum, antirat immunoglobulin E, and 5-hydroxytryptamine were obtained. Antagonists methysergide, ketanserin, 5,8,11, 14-eicosatetraynoic acid, diphenhydramine, and sodium meclofenamate were also used. RESULTS: Tissue levels of rat mast cell protease II and histamine were decreased during delivery compared with prepartum and postpartum levels. Carbachol and compound 48/80 stimulated uterine contractility, and responses were highest during late gestation (day 16 to term). Responses to ovalbumin of uterine tissues in rats sensitized to the antigen were highest at midpregnancy and decreased during the last 10 days of gestation. Ovalbumin challenge in vitro increased the frequency and magnitude of contractions in tissues from ovalbumin-sensitized rats. Compound 48/80 and antirat immunoglobulin E stimulated contractility in both control and sensitized rats. None of the antagonists prevented the contractile responses. CONCLUSIONS: Activation of mast cells is an effective mechanism for stimulation of uterine contractility and may play an important role in the control of term and preterm parturition.

The effects of pregnancy and smooth muscle contractility on cervical distensibility in the rat.

OBJECTIVE: We sought to compare distensibility of the isolated rat cervix from nonpregnant rats (n = 6), rats at midgestation (n = 5), and rats at term gestation (n = 4). STUDY DESIGN: The cervix was excised, and one cervical channel was cannulated from both ends and positioned in the organ chamber for perfusion-superfusion by a peristaltic pump at an intraluminal pressure of 30 mm Hg for 30 minutes. After the equilibration period, perfusion was stopped, the outlet was closed, and the cervix was inflated with a syringe pump. The volume was increased at a rate of 3.33 microL/s until intraluminal pressure reached approximately 120 mm Hg. The outlet was then opened, and the cervix was perfused at 30 mm Hg of intraluminal pressure for another 30 minutes. The volume-pressure relationships were obtained 3 times without any agent present and in the presence of 60-mmol/L potassium chloride or 10(-4)-mol/L 3-morpholinosydnonimine hydrochloride (also known as SIN-1). RESULTS: The volume-pressure relationship was shifted to the right during progression of pregnancy, demonstrating increased compliance of the cervix. The nonspecific depolarizing agent potassium chloride or the nitric oxide donor 3-morpholinosydnonimine did not affect volume-pressure relationships in cervices from nonpregnant rats, rats at midgestation, or rats at term gestation. CONCLUSION: The volume-pressure relationship in the isolated cannulated rat cervix reflects the resistance of the organ to increased intraluminal pressure. The compliance of the cervix is increased as pregnancy progresses, demonstrating decreased resistance to stretch. Activation or inhibition of cervical smooth muscle does not contribute to the physical properties of the cervix, which controls compliance-resistance.

Role of nucleotide cyclases in the inhibition of pregnant rat uterine contractions by the openers of potassium channels.

OBJECTIVE: Our objective was to study the involvement of adenylate and guanylate cylases in spontaneous uterine contractions and inhibition induced by the opening of potassium channels. STUDY DESIGN: Uterine rings from rats at mid and term gestation and from rats at term gestation in labor were suspended in organ chambers for isometric tension recording. Concentration-response relationships to an opener of adenosine triphosphate-dependent potassium channels, levcromakalim, or to an opener of calcium-dependent potassium channels, NS 1619, were studied in the absence and presence of inhibitors of adenylate cyclase (MDL 12330 A, 2 x 10(-5) mol/L; SQ 22536, 10(-4) mol/L) or guanylate cyclase (LY 83583,3 x 10(-6) mol/L). RESULTS: MDL 12330 A and SQ 22536 accentuated contractions in rings from rats at mid gestation but not at term gestation or at term gestation in labor. LY 83583 inhibited contractions in the rings from all 3 groups. Levcromakalim was equally effective in inhibiting contractions of rings from all 3 groups. MDL 12330 A, but not SQ 22536, decreased sensitivity and maximal inhibition induced by levcromakalim (term gestation greater than mid gestation greater than term gestation in labor). LY 83583 decreased the sensitivity to and maximal inhibition induced by levcromakalim in rings from pregnant rats at mid gestation. NS 1619 attenuated contraction of rings from rats at mid gestation and, to a lesser extent, at term gestation but accentuated contractions in rings from animals at term gestation in labor. MDL 12330 A, but not SQ 22536 or LY 83583, attenuated the changes induced by NS 1619 in rings from all 3 groups. CONCLUSIONS: (1) The influence of nucleotide cyclases on basal uterine contractility depends on gestational age. (2) The inhibition of uterine contractions that results from the opening of calcium-dependent potassium channels depends on adenylate cyclase, whereas that of adenosine triphosphate-dependent potassium channels depends on both adenylate and guanylate cyclases. 3. Activation of adenosine triphosphate-dependent potassium channels is more efficient than activation of calcium-dependent potassium channels. 4. The inhibition induced by calcium-dependent potassium channel openers, but not adenosine triphosphate-dependent potassium channel openers, decreases as pregnancy progresses, and at delivery the activation of spontaneous contractile activity is evident.

The role of cyclic nucleotides in the spontaneous contractility and responsiveness to nitric oxide of the rat uterus at midgestation and term.

OBJECTIVES: The aim of this investigation was to study the effects of pharmacologic manipulation of cyclic nucleotide levels on the uterine spontaneous contractile activity and responsiveness to nitric oxide in pregnant rats at midgestation and term. STUDY DESIGN: Uterine rings from pregnant rats at midgestation and term were placed in organ chambers for isometric tension recording. Concentration-response curves were obtained for phosphodiesterase and guanylate cyclase inhibitors, membrane-permeable cyclic nucleotide analogs, and forskolin. In addition, the effects of minimal inhibitory concentrations of these agents on the concentration-response relationships for diethylamine nitric oxide (a nitric oxide donor) were studied. RESULTS: Nonselective phosphodiesterase inhibitors induced more inhibition of contractions of uterine rings from pregnant rats at term than at midgestation and zaprinast induced less. Inhibitors of guanylate cyclase and membrane-permeable analogs of cyclic guanosine monophosphate were equally effective in tissues from pregnant rats at midgestation and term. All agents attenuated the inhibitory effect of diethylamine nitric oxide at midgestation; however, dibutyryl cyclic adenosine monophosphate and papaverine increased the inhibitory effect of diethylamine nitric oxide in tissues from pregnant animals at term. CONCLUSIONS: Cyclic nucleotides modulate both spontaneous and nitric oxide-induced changes in uterine contraction during pregnancy. Application of nonselective inhibitors of phosphodiesterase, as well as membrane-permeable analogs of cyclic adenosine monophosphate, may counteract refractoriness to nitric oxide at term.

Adenylate cyclase and potassium channels are involved in forskolin- and 1,9-dideoxyforskolin-induced inhibition of pregnant rat uterus contractility.

OBJECTIVE: We sought to study the contribution of potassium channels in the effect of forskolin and 1,9-dideoxyforskolin on uterine contractility in the pregnant rat. STUDY DESIGN: Rings taken from the middle portions of uterine horns from rats at 16 days of gestation were positioned in organ chambers containing physiologic salt solution bubbled with 5% carbon dioxide in air (37 degrees C, pH approximately 7.4) for isometric tension recording under 2 g passive tension. The effects of cumulative concentrations of forskolin and 1,9-dideoxyforskolin in the absence or presence of an adenylate cyclase inhibitor (MDL-12,330A, 10(-5) mol/L), a nonselective potassium channel blocker (tetrabutylammonium, 10(-4) mol/L), or an adenosine triphosphate-dependent potassium channel blocker (glibenclamide 10(-5) mol/L) were studied. RESULTS: Both forskolin and, to a lesser extent, 1,9-dideoxyforskolin inhibit uterine contractions. Tetrabutylammonium, glibenclamide, and MDL-12, 330A attenuated the effects of forskolin, whereas glibenclamide was less effective against 1,9-dideoxyforskolin. CONCLUSION: Activation of adenylate cyclases, as well as adenosine triphosphate-dependent potassium channels and, to a greater extent, calcium-dependent potassium channels, is involved in the inhibitory effect of forskolin in uterine rings from rats at 16 days of gestation. Inhibition of uterine contractions by 1,9-dideoxyforskolin is less than that by forskolin and involves activation of adenylate cyclase and calcium-dependent potassium channels. Whether activation of guanylate cyclase is involved in the effect of the agents on calcium-dependent potassium channels needs further investigation. 1, 9-Dideoxyforskolin is not an inactive isomer of forskolin in rat uterine rings.

Responses of isolated perfused uterine vascular beds of nonpregnant and pregnant rats to endogenous and exogenous nitric oxide.

The responses to endothelial vasodilators and exogenous nitric oxide (NO) were characterized in intact isolated uterine vascular beds of nonpregnant, midpregnant and late-pregnant rats perfused with Kreb's buffer (37 degrees C, 5% CO(2) in air, pH approximately 7.4) containing 2% dextran and indomethacin. Phenylephrine increased perfusion pressure in the vascular beds equally in all three groups. In the presence of phenylephrine, N(omega)-nitro-L-arginine methyl ester (L-NAME) significantly augmented perfusion pressure in the order: nonpregnant

Effect of nitric oxide and carbon monoxide on uterine contractility during human and rat pregnancy.

OBJECTIVE: We sought to study the effects of authentic nitric oxide and carbon monoxide on the contractile activity of pregnant human and rat myometrium. STUDY DESIGN: Strips were prepared from uterine biopsy specimens of 10 pregnant, nonlaboring women at term gestation undergoing cesarean delivery. In addition, rings were prepared from the uteri of pregnant rats at midterm (day 14) and at term (day 22) gestation (n = 10-12). The tissues were mounted in organ chambers filled with Krebs-Henseleit solution continuously aerated with 5% carbon dioxide in air (37 degrees C, pH approximately 7.4) for isometric tension recording. The effects of nitric oxide and carbon monoxide gases on spontaneous contractile activity were studied. Responses to hemin (hemoxygenase substrate), which produces endogenous carbon monoxide, were also examined. Responses to nitric oxide and carbon monoxide were also studied in aortic and tail artery rings from pregnant rats after contraction with phenylephrine. RESULTS: Nitric oxide significantly inhibited contractility of human myometrium at term (area under the concentration-response curve, 145.36 +/- 30.02 vs 40.56 +/- 22.81 in controls; P <.05) and rat myometrium at midterm gestation (264.23 +/- 47.86 vs 121.82 +/- 23.50; P <.05) but not at term. No statistically significant inhibition was induced in human or rat myometrium by carbon monoxide, whereas hemin significantly attenuated contractility in human myometrium at term and in rat myometrium at midterm gestation (P <. 05). Nitric oxide, carbon monoxide, and hemin relaxed aortic and tail artery rings. CONCLUSIONS: Authentic nitric oxide inhibits rat uterine contractile activity at midterm gestation but not at term. However, nitric oxide inhibits human myometrium activity at term. Authentic carbon monoxide does not appear to modulate uterine contractility, whereas hemin may have some inhibitory properties.

Roles of potassium channels and nitric oxide in modulation of uterine contractions in rat pregnancy.

OBJECTIVE: We sought to study the involvement of potassium channels in the inhibition by nitric oxide of spontaneous contractions in isolated uterine rings from midterm and term pregnant rats. STUDY DESIGN: Uterine rings from Sprague-Dawley rats at midterm and term gestation were used for isometric tension recording. The inhibition of spontaneous contractile activity by potassium channel openers and nitric oxide was studied in the absence and presence of potassium channel inhibitors. RESULTS: The adenosine triphosphate-dependent potassium channel opener levcromakalim inhibited spontaneous contractions in rings from both midterm and term pregnant rats in a concentration-dependent manner, and the effects were significantly attenuated by pretreatment with selective inhibitor of the adenosine triphosphate-dependent potassium channel inhibitor glibenclamide. The opener of calcium-dependent potassium channel NS 1619 inhibited spontaneous contractions in rings from midterm but significantly less so in rings from term pregnant rats in a concentration-dependent manner, and the effect was significantly attenuated by pretreatment with potassium channel inhibitors tetraethylammonium and tetrabutylammonium but not with glibenclamide. Rings from midterm and term pregnant rats were more sensitive to the inhibitory effect of levcromakalim compared with NS 1619. Nitric oxide donor diethylamine-nitric oxide inhibited spontaneous contractions in rings from midterm but significantly less in rings from term pregnant rats in a concentration-dependent manner, and the effect was attenuated by tetraethylammonium and tetrabutylammonium but not by glibenclamide. CONCLUSIONS: There is gestational age-dependent refractoriness to calcium-dependent potassium but not adenosine triphosphate-dependent potassium channel opener-induced inhibition of spontaneous contractile activity of isolated rat uterine rings. Nitric oxide inhibits uterine contractions by opening of calcium-dependent potassium channels in pregnant rat myometrium. Refractoriness to nitric oxide toward term may result from decreased probability to open or number of calcium-dependent potassium channels.

Relaxation kinetics of the aorta in N omega-nitro-L-arginine methyl ester-treated pregnant rats.

OBJECTIVE: To test the hypothesis that vascular relaxation kinetics are prolonged in pregnant rats treated chronically with N omega-nitro-L-arginine methyl ester (L-NAME). METHODS: Timed pregnant rats (on day 15 of a 22-day gestation) were implanted with infusion pumps containing vehicle (controls) or L-NAME (50 mg/d). L-NAME pumps were retained until day 22 (group 1), or removed on day 20 (group 2). All rats were killed at term. Aortic rings were mounted in organ chambers containing physiologic salt solution (PSS) for isometric tension recording, contracted with high-K+ PSS (60 mM), and allowed to relax in normal-K+ PSS. Relaxation kinetics were quantified as time for 50% and 80% relaxation. After contraction with phenylephrine, responses to cumulative concentrations of methacholine were studied in the absence and presence of L-arginine (L-Arg) (10(-3) M). RESULTS: Responses to methacholine were inhibited completely in group 1 and partially in group 2 (P < .05). The inhibition in both groups was reversed by L-Arg. The rate of relaxation was significantly slower in groups 1 and 2 (P < .05) as compared with controls. Mechanical removal of the endothelium caused prolongation of relaxation in controls and group 2 (P < .05), but not in group 1. Preincubation of aortic rings from untreated controls with L-NAME (in vitro, 10(-4) M) did not affect relaxation. CONCLUSION: The endothelium modulates the rate of vascular relaxation by a factor other than nitric oxide. N omega-nitro-L-arginine methyl ester (L-NAME) prolongs vasorelaxation by endothelium-dependent and -independent mechanisms. Prolongation of vascular relaxation kinetics may be a mechanism to elevate blood pressure and peripheral vascular resistance in preeclampsia.

The effect of amniotic fluid on the human omental artery in vitro.

OBJECTIVE: The aim of the study was to determine the effect of amniotic fluid on the in vitro contractility of the human omental artery. STUDY DESIGN: Amniotic fluid and a segment of omentum were obtained from each of 5 patients at the time of planned cesarean delivery at normal term gestation for the indication of previous cesarean delivery. The omental artery was cleaned and cut into 3-mm rings, which were placed in 10-mL organ chambers for isometric tension recording. The chambers were filled with Krebs-Henseleit solution bubbled with 5% carbon dioxide in air and maintained at 37 C, pH 7.4. The rings were then equilibrated at 1 g passive tension for 90 minutes. The amniotic fluid was centrifuged for 10 minutes at 3000 rpm to remove all debris. Increasing volumes of supernatant (10-2000 microL) were added to the omental artery rings at baseline tone or after contraction with U46619 (10(-7) mol/L) or potassium chloride (60 mmol/L) to detect contractile and relaxant effects, respectively. Time-solvent control preparations were also run in parallel. RESULTS: Amniotic fluid had no effect on the basal tone of omental artery rings. Amniotic fluid had no effect on the tension in rings previously contracted with either U46619 or potassium chloride. CONCLUSIONS: Amniotic fluid has no direct effect on isolated human omental artery. The catastrophic hemodynamic changes associated with the syndrome of amniotic fluid embolism are not due to a direct effect of circulating amniotic fluid on vascular tone but rather may be due to secondary responses

Endothelium-dependent and -independent mechanisms of vasorelaxation by corticotropin-releasing factor in pregnant rat uterine artery.

Corticotropin-releasing factor (CRF), a potent vasorelaxant, is increased tremendously during human pregnancy. Placenta is the main source for this increase. CRF is thought to be important in modulating vascular resistance and uteroplacental blood flow during pregnancy. Here we investigated pathways mediating a vasorelaxant effect of CRF in the uterine artery. Two-millimeter segments of uterine artery (o.d. 300-400 microm) from day 18 pregnant rats were mounted in a small vessel myograph and precontracted with norepinephrine, and relaxation responses to CRF were studied. CRF relaxed the uterine artery in a concentration-dependent manner. Relaxation of uterine artery by CRF was abolished completely by alpha-helical CRF 9-41 (CRF antagonist, 1 micromol) and partially by removal of endothelium, Nomega-nitro-L-arginine methyl ester (nitric oxide synthase inhibitor, 0.1 mmol), 6-anilino-5,8-quinolinedione (guanylate cyclase inhibitor, 10 micromol), or thiopental/miconazole (cytochrome P-450 inhibitors, 0.3 mmol/30 micromol), but remained unaffected by indomethacin (cyclo-oxygenase inhibitor, 10 micromol). Relaxation was also inhibited when depolarizing solution (K+, 120 mmol) was used for precontraction. In deendothelized preparations, relaxation was not inhibited by 9-tetrahydro-2-furanyl-9H-purin-6-amine (adenylate cyclase inhibitor, 0.2 mmol), glibenclamide (adenosine triphosphate-dependent K+ channel blocker, 10 micromol), tetrabutyl ammonium (nonspecific K+ channel blocker, 1 mmol), nitrendipine (voltage-gated Ca++ channel blocker, 1 micromol), or when vessels were precontracted with depolarizing solution. CRF causes vasorelaxation by receptor-operated, endothelium-dependent and -independent pathways. The endothelium-dependent relaxation is mediated by nitric oxide-cyclic guanosine monophosphate pathway and endothelium-derived hyperpolarizing factor but not prostacyclin. However, cyclic adenosine monophosphate, K+ channels, or Ca++ channels are not involved in endothelium-independent vasorelaxation by CRF.