Uterine Vascular Tone Research Articles

Gestational Hypoxia Inhibits Pregnancy-Induced Upregulation of Ca2+ Sparks and Spontaneous Transient Outward Currents in Uterine Arteries Via Heightened Endoplasmic Reticulum/Oxidative Stress.

Hypoxia during pregnancy profoundly affects uterine vascular adaptation and increases the risk of pregnancy complications, including preeclampsia and fetal intrauterine growth restriction. We recently demonstrated that increases in Ca2+ sparks and spontaneous transient outward currents (STOCs) played an essential role in pregnancy-induced uterine vascular adaptation. In the present study, we hypothesize that gestational hypoxia suppresses Ca2+ sparks/STOCs coupling leading to increased uterine vascular tone via enhanced endoplasmic reticulum (ER)/oxidative stress. Uterine arteries were obtained from nonpregnant and near-term pregnant sheep residing in low altitude or acclimatizing to high-altitude (3801 m) hypoxia for ≈110 days. High-altitude hypoxia suppressed pregnancy-induced upregulation of RyR1 and RyR2 (ryanodine receptor 1 and 2) protein abundance, Ca2+ sparks, and STOCs in uterine arteries. Inhibition of Ca2+ sparks/STOCs with the RyR inhibitor ryanodine significantly increased pressure-dependent myogenic tone in uterine arteries from low-altitude normoxic pregnant animals but not those from high-altitude hypoxic pregnant animals. Gestational hypoxia significantly increased ER/oxidative stress in uterine arteries. Of importance, the hypoxia-mediated suppression of Ca2+ sparks/STOCs and increase in myogenic tone in uterine arteries of pregnant animals were reversed by inhibiting ER/oxidative stress. Of great interest, the impaired sex hormonal regulation of STOCs in high-altitude animals was annulled by scavenging reactive oxygen species but not by inhibiting ER stress. Together, the findings reveal the differential mechanisms of ER and oxidative stresses in suppressing Ca2+ sparks/STOCs and increasing myogenic tone of uterine arteries in hypoxia during gestation, providing new insights into the understanding of pregnancy complications associated with hypoxia.

Ryanodine receptor subtypes regulate Ca2+ sparks/spontaneous transient outward currents and myogenic tone of uterine arteries in pregnancy.

Our recent study demonstrated that increased Ca2+ sparks and spontaneous transient outward currents (STOCs) played an important role in uterine vascular tone and haemodynamic adaptation to pregnancy. The present study examined the role of ryanodine receptor (RyR) subtypes in regulating Ca2+ sparks/STOCs and myogenic tone in uterine arterial adaptation to pregnancy. Uterine arteries isolated from non-pregnant and near-term pregnant sheep were used in the present study. Pregnancy increased the association of α and β1 subunits of large-conductance Ca2+-activated K+ (BKCa) channels and enhanced the co-localization of RyR1 and RyR2 with the β1 subunit in the uterine artery. In contrast, RyR3 was not co-localized with BKCa β1 subunit. Knockdown of RyR1 or RyR2 in uterine arteries of pregnant sheep downregulated the β1 but not α subunit of the BKCa channel and decreased the association of α and β1 subunits. Unlike RyR1 and RyR2, knockdown of RyR3 had no significant effect on either expression or association of BKCa subunits. In addition, knockdown of RyR1 or RyR2 significantly decreased Ca2+ spark frequency, suppressed STOCs frequency and amplitude, and increased pressure-dependent myogenic tone in uterine arteries of pregnant animals. RyR3 knockdown did not affect Ca2+ sparks/STOCs and myogenic tone in the uterine artery. Together, the present study demonstrates a novel mechanistic paradigm of RyR subtypes in the regulation of Ca2+ sparks/STOCs and uterine vascular tone, providing new insights into the mechanisms underlying uterine vascular adaptation to pregnancy.

Pregnancy Increases Ca2+ Sparks/Spontaneous Transient Outward Currents and Reduces Uterine Arterial Myogenic Tone.

Spontaneous transient outward currents (STOCs) at physiological membrane potentials of vascular smooth muscle cells fundamentally regulate vascular myogenic tone and blood flow in an organ. We hypothesize that heightened STOCs play a key role in uterine vascular adaptation to pregnancy. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep. Ca2+ sparks were measured by confocal microscopy, and STOCs were determined by electrophysiological recording in smooth muscle cells. Percentage of Ca2+ spark firing myocytes increased dramatically at the resting condition in uterine arterial smooth muscle of pregnant animals, as compared with nonpregnant animals. Pregnancy upregulated the expression of RyRs (ryanodine receptors) and significantly boosted Ca2+ spark frequency. Ex vivo treatment of uterine arteries of nonpregnant sheep with estrogen and progesterone imitated pregnancy-induced RyR upregulation. STOCs occurred at much more negative membrane potentials in uterine arterial myocytes of pregnant animals. STOCs in uterine arterial myocytes were diminished by inhibiting large-conductance Ca2+-activated K+ (BKCa) channels and RyRs, thus functionally linking Ca2+ sparks and BKCa channel activity to STOCs. Pregnancy and steroid hormone treatment significantly increased STOCs frequency and amplitude in uterine arteries. Of importance, inhibition of STOCs with RyR inhibitor ryanodine eliminated pregnancy- and steroid hormone-induced attenuation of uterine arterial myogenic tone. Thus, the present study demonstrates a novel role of Ca2+ sparks and STOCs in the regulation of uterine vascular tone and provides new insights into the mechanisms underlying uterine vascular adaptation to pregnancy.

Pregnancy Enhances Calcium Spark Activity Independent of Altitude in Ovine Uterine Arterial Myocytes

During pregnancy, uterine artery blood flow increases in order to accommodate the growing fetus. However, the molecular mechanisms by which this change in uterine blood flow occurs remains to be elucidated. The BKCa channel, which is activated in response to ryanodine receptor (RyR) generated increases in intracellular Ca2+ known as Ca2+ sparks, helps regulate uterine artery blood flow. BKCa channel activation leads to vascular smooth muscle relaxation, thus decreasing uterine artery vascular tone and increasing uterine blood flow during pregnancy. Further to this, long term hypoxia due to high altitude exposure diminishes pregnancy – related increases in uterine blood flow. Based on these relationships we hypothesized that both pregnancy and membrane depolarization would increase Ca2+ spark activity, whereas ryanodine and long term hypoxia would inhibit Ca2+ spark activity. Uterine arteries were isolated from near‐term pregnant (n=6) and non‐pregnant sheep (n=6) in normoxia, as well as near‐term pregnant (n=6) and non‐pregnant sheep (n=6) exposed to hypoxic conditions at White Mountain (elevation 3,801 m) for 100 plus days. Isolated uterine arteries were subjected to three distinct ex vivo treatments: (1) control (saline), (2) 30mM potassium (30K), which causes membrane depolarization, and (3) 30K with ryanodine (10 μM), an RyR antagonist. Indeed, Ca2+ spark activity was increased in pregnant uterine arteries as compared to non‐pregnant uterine arteries, whereas hypoxia reduced spark frequency in all the aforementioned groups. Treatment with 30K increased Ca2+ spark activity in all groups. In comparison, 30K + ryanodine treatment decreased Ca2+ spark activity. Pregnancy increased the full width of sparks. However, pregnancy or hypoxia caused only minor changes in the amplitude and temporal aspects of sparks. These findings support the claim that membrane depolarization, due to 30K treatment, causes an increase in Ca2+ spark activity whereas ryanodine inhibits release of Ca2+ from the sarcoplasmic reticulum and thus decreases Ca2+ sparks. Taken together, these findings support the notion that the ryanodine receptors and BKCa channels work in conjunction with each other. This study provides further insight into the mechanisms that regulate uterine artery vascular tone and blood flow, and thus may prove useful in designing treatments for pregnancy complications that arise from increased uterine vascular tone, such as intrauterine growth restriction.Support or Funding InformationThis work is supported by The National Institutes of Health, Eunice Kennedy Shriver National Institute of Child Health and Human Development grant number HD083132, by the National Science Foundation under Grant No. MRI 0923559, and the Loma Linda University School of Medicine. VW was a Walter E Macpherson Medical Student Summer Research Fellow.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Pregnancy Augments G Protein Estrogen Receptor (GPER) Induced Vasodilation in Rat Uterine Arteries via the Nitric Oxide - cGMP Signaling Pathway.

BackgroundThe regulation of vascular tone in the uterine circulation is a key determinant of appropriate uteroplacental blood perfusion and successful pregnancy outcome. Estrogens, which increase in the maternal circulation throughout pregnancy, can exert acute vasodilatory actions. Recently a third estrogen receptor named GPER (G protein-coupled estrogen receptor) was identified and, although several studies have shown vasodilatory effects in several vascular beds, nothing is known about its role in the uterine vasculature.AimThe aim of this study was to determine the function of GPER in uterine arteries mainly during pregnancy. Uterine arteries were isolated from nonpregnant and pregnant rats.MethodsVessels were contracted with phenylephrine and then incubated with incremental doses (10−12–10−5 M) of the selective GPER agonist G1.ResultsG1 induced a dose-dependent vasodilation which was: 1) significantly increased in pregnancy, 2) endothelium-dependent, 3) primarily mediated by NO/cGMP pathway and 4) unaffected by BKca channel inhibition.ConclusionThis is the first study to show the potential importance of GPER signaling in reducing uterine vascular tone during pregnancy. GPER may therefore play a previously unrecognized role in the regulation of uteroplacental blood flow and normal fetus growth.

Hypoxia Represses ER-α Expression and Inhibits Estrogen-Induced Regulation of Ca2+-Activated K+ Channel Activity and Myogenic Tone in Ovine Uterine Arteries: Causal Role of DNA Methylation.

Previous in vivo study demonstrated that chronic hypoxia during gestation was associated with estrogen receptor-α (ER-α) gene repression in ovine uterine arteries. Yet, it remains undetermined whether hypoxia had a direct effect and if DNA methylation played a causal role in hypoxia-mediated ER-α gene repression. Thus, this study tested the hypothesis that prolonged hypoxia has a direct effect and increases promoter methylation resulting in ER-α gene repression and inhibition of estrogen-mediated adaptation of uterine vascular tone. Uterine arteries isolated from nonpregnant and pregnant sheep were treated ex vivo with 21.0% O2 and 10.5% O2 for 48 hours. Hypoxia significantly increased ER-α promoter methylation at both specificity protein-1 and upstream stimulatory factor binding sites, decreased specificity protein-1 and upstream stimulatory factor binding to the promoter, and suppressed ER-α expression in uterine arteries of pregnant animals. Of importance, the effects of hypoxia were blocked by a methylation inhibitor 5-aza-2'-deoxycytidine. In addition, hypoxia abrogated steroid hormone-mediated increase in ER-α expression and inhibited the hormone-induced increase in large-conductance Ca(2+)-activated K(+) channel activity and decrease in myogenic tone in uterine arteries of nonpregnant animals, which were reversed by 5-aza-2'-deoxycytidine. The results provide novel evidence of a direct effect of hypoxia on heightened promoter methylation that plays a causal role in ER-α gene repression and ablation of steroid hormone-mediated adaptation of uterine arterial large conductance Ca(2+)-activated K(+) channel activity and myogenic tone in pregnancy.

793: Expression of cytochrome P450 2J2 (CYP2J2) and soluble epoxide hydrolase (sEH) in labored versus non-labored human myometrium

Epoxyeicosatrienoic acids (EETs) have been shown to induce relaxation of uterine vascular tone and diminish myometrial contractility in vitro. Epoxyeicosanoids are thought to facilitate uterine quiescence in vivo. The synthesis and degradation of EETs is principally regulated by CYP 2J2, a cytochrome P450 epoxygenase, and soluble epoxide hydrolase (sEH) respectively. We hypothesized that human pregnant myometrium exposed to labor would exhibit diminished expression of CYP 2J2 and increased expression of sEH compared to non-labored controls.

Potassium Channels and Uterine Vascular Adaptation to Pregnancy and Chronic Hypoxia

During a normal course of pregnancy, uterine vascular tone is significantly decreased resulting in a striking increase in uterine blood flow, which is essential for fetal development and fetal growth. Chronic hypoxia during gestation may adversely affect the normal adaptation of uterine vascular tone and increase the risk of preeclampsia and fetal intrauterine growth restriction. In this review, we present evidence that the regulation of K+ channels is an important mechanism in the adaptation of uterine vascular tone to pregnancy and hypoxia. There are four types of K+ channels identified in arterial smooth muscle cells: 1) voltage-dependent K(+) (Kv) channels, 2) Ca2+-activated K(+) (KCa) channels, 3) inward rectifier K(+) (KIR) channels, and 4) ATP-sensitive K(+) (KATP) channels. Pregnancy differentially augments the expression and activity of K(+) channels via downregulation of protein kinase C signaling in uterine and other vascular beds, leading to decreased uterine vascular tone and increased uterine blood flow. Sex steroid hormones play an important role in the pregnancy- mediated alteration of K+ channels in the uterine vasculature. In addition, chronic hypoxia alters uterine vascular K(+) channels expression and activities via modulation of steroid hormones/receptors-mediated signaling, resulting in increased uterine vascular tone during pregnancy.

Novel insights into the regulation of uterine vascular tone by sphingosine 1-phosphate

s / Placenta 34 (2013) A1–A99 A3 stem cells in action. Vasculogenesis and angiogenesis of the fetal vasculature in the human placenta from mesenchymal stem cells and the sequential expression of endothelial adhesion molecules during vasculogenesis is a classic example of direct incorporation of stem cells in a growing vascular system. Modulation of the endothelial barrier and junctional phenotype with gestational age and altered maternal and placental milieu reflect paracrine repair and regeneration by vascular stem cells. Indeed, the phenotypic plasticity of the placental endothelial cells suggests their close proximity to the stem cell in the differentiation time line. This phenotypic plasticity has been successfully used by cell culture experimentalists for barrier studies. Moreover, de-differentiation of placental/umbilical cord derived endothelial cells in culture to a more mesenchymal/pericytic phenotype suggests that differentiation processes are not unidirectional. Recent studies of gene expression profiles of placental fetal endothelium combined with functional studies reporting an immature phenotype in the diabetic milieu further strengthen the stem cell but one nature and provide examples of endothelial de-differentiation in vivo. The phenotypic plasticity of fetal endothelial cells is advantageous to the fetus. It allows increased angiogenesis and permeability during gestation to sustain fetal growth; VEGF studies demonstrate this vividly. However, this very plasticity may then leave the fetus vulnerable to maternal diseases; the induced immature barrier phenotype or the accelerated aging upon sustained oxidative stress will lead to placental inefficiency and fetal morbidity. Fetal endothelial cells are not alone, other cell types in the placenta also show examples of plasticity and indeed trans-differentiation evidenced by differentiation of trophoblast cells during modulating of spiral arteries and expression of endothelial markers such as eNOS and caveolin-1. http://dx.doi.org/10.1016/j.placenta.2013.06.012 PL2.3. NOVEL INSIGHTS INTO THE REGULATION OF UTERINE VASCULAR TONE BY SPHINGOSINE 1-PHOSPHATE

Chronic Hypoxia Differentially Up-Regulates Protein Kinase C-Mediated Ovine Uterine Arterial Contraction via Actin Polymerization Signaling in Pregnancy1

Chronic hypoxia (CH) during pregnancy is associated with increased uterine vascular tone. The present study tested the hypothesis that CH up-regulates protein kinase C (PKC)-mediated actin polymerization, resulting in enhanced uterine vascular contraction in pregnancy. Uterine arteries were isolated from nonpregnant (NPUA) and near-term (∼140 days of gestation) pregnant (PUA) sheep that had been maintained at sea level (∼300 m) or exposed to high altitude (3801 m) hypoxia for 110 days. In normoxic animals, the induced contractions by the PKC activator phorbol 12,13-dibutyrate (PDBu) were greater in NPUA than in PUA, which was abrogated by an actin polymerization inhibitor cytochalasin B (Cyto B). In hypoxic animals, PDBu-induced contractions were significantly increased in PUA but not in NPUA, which was inhibited by Cyto B. In contrast, neither pregnancy nor hypoxia affected Cyto B-mediated inhibition of norepinephrine (NE)-induced contractions. Prolonged ex vivo treatment of NPUA with 17beta-estradiol and progesterone significantly attenuated PDBu-induced actin polymerization and contractions, and the hormonal treatment did not alter the inhibitory effect of Cyto B on PDBu- or NE-induced contractions in either normoxic or hypoxic animals. 2-(2-Amino-3-methoxyphenyl)-4H-1-benzopyran-4-one potentiated PDBu-mediated actin polymerization and enhanced PDBu-induced contractions of PUA in normoxic but not hypoxic animals, which was abrogated by Cyto B. The results suggest that chronic hypoxia during pregnancy causes attenuation of steroid hormone-mediated ERK1/2 signaling and results in increased actin polymerization and uterine vascular tone, linking gestational hypoxia to aberrant uteroplacental circulation.

Role of BKCa channels in regulating uterine vascular contractility: effect of high altitude hypoxia

BKCa channels play a key role in the increased uterine artery blood flow in pregnancy. The present study tested whether chronic hypoxia attenuates pregnancy‐mediated enhanced BKCa channel activities and increases PKC‐mediated uterine vascular tone. Uterine arteries were isolated from nonpregnant (NPUA) and nearterm pregnant (PUA) sheep maintained at sea level (~300 m) or high altitude (3801 m) for 110 days. In contrast to previous studies in normoxic sheep, PKC activator PDBu‐induced contractions were increased in PUA, which was not significantly different from that of NPUA in hypoxic sheep. BKCa channel blocker TEA increased PDBu‐induced contractions in NPUA but not PUA. However, TEA increased norpinephrine (NE)‐induced contractions in both NPUA and PUA. Both BKCa channel opener, NS1619 and SKCa channel opener, NS309 caused minimal relaxations (~20%) of NPUA and PUA pre‐contracted by NE in the present or absence of endothelial cells. In addition, Western blot showed a decrease in the β1, but not α, subunit of BKCa channels in hypoxic PUA as compared with normaxic PUA. The results suggest that chronic hypoxia selectively attenuates basal uterine vascular BKCa channel activity, but not BKCa channel activation in pregnant animals, which may contribute to enhanced uterine vascular tone observed in sheep during pregnancy at high altitude hypoxia. (Supported in part by NIH grant HD31226)

Pregnancy Upregulates Large-Conductance Ca 2+ -Activated K + Channel Activity and Attenuates Myogenic Tone in Uterine Arteries

Uterine vascular tone significantly decreases whereas uterine blood flow dramatically increases during pregnancy. However, the complete molecular mechanisms remain elusive. We hypothesized that increased Ca(2+)-activated K(+) (BK(Ca)) channel activity contributes to the decreased myogenic tone of uterine arteries in pregnancy. Resistance-sized uterine arteries were isolated from nonpregnant and near-term pregnant sheep. Electrophysiological studies revealed a greater whole-cell K(+) current density in pregnant compared with nonpregnant uterine arteries. Tetraethylammonium and iberiotoxin inhibited K(+) currents to the same extent in uterine arterial myocytes. The BK(Ca) channel current density was significantly increased in pregnant uterine arteries. In accordance, tetraethylammonium significantly increased pressure-induced myogenic tone in pregnant uterine arteries and abolished the difference in myogenic responses between pregnant and nonpregnant uterine arteries. Activation of protein kinase C produced a similar effect to tetraethylammonium by inhibiting BK(Ca) channel activity and increasing myogenic tone in pregnant uterine arteries. Chronic treatment of nonpregnant uterine arteries with physiologically relevant concentrations of 17β-estradiol and progesterone caused a significant increase in the BK(Ca) channel current density. Western blot analyses demonstrated a significant increase of the β1, but not α, subunit of BK(Ca) channels in pregnant uterine arteries. In accordance, steroid treatment of nonpregnant uterine arteries resulted in an upregulation of the β1, but not α, subunit expression. The results indicate that the steroid hormone-mediated upregulation of the β1 subunit and BK(Ca) channel activity may play a key role in attenuating myogenic tone of the uterine artery in pregnancy.

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Pregnancy Downregulates Actin Polymerization and Pressure-Dependent Myogenic Tone in Ovine Uterine Arteries

Pregnancy is associated with significantly decreased uterine vascular tone and increased uterine blood flow. The present study tested the hypothesis that the downregulation of actin polymerization plays a key role in reduced vascular tone of uterine arteries in the pregnant state. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep. Activation of protein kinase C significantly increased the filamentous:globular actin ratio and contractions in the uterine arteries, which were inhibited by an actin polymerization inhibitor cytochalasin B. The basal levels of filamentous:globular actin were significantly higher in nonpregnant uterine arteries than those in near-term pregnant sheep. Prolonged treatment (48 hours) of nonpregnant sheep with 17β-estradiol (0.3 nmol/L) and progesterone (100.0 nmol/L) caused a significant decrease in the filamentous:globular actin. In accordance, the treatment of near-term pregnant sheep for 48 hours with an estrogen antagonist ICI 182 780 (10.0 μmol/L) and progesterone antagonist RU 486 (1.0 μmol/L) significantly increased the levels of filamentous:globular actin. Increased intraluminal pressure from 20 to 100 mm Hg resulted in an initial increase in uterine arterial diameter and vascular wall Ca(2+) concentrations, followed by a decrease in the diameter at a constant steady-state level of Ca(2+). Cytochalasin B blocked pressure-induced myogenic constrictions without effect on vascular wall Ca(2+) levels and eliminated the differences in pressure-dependent myogenic tone between nonpregnant sheep and near-term pregnant sheep. The results indicate a key role of actin polymerization in protein kinase C-induced myogenic contractions and suggest a novel mechanism of sex steroid hormone-mediated downregulation of actin polymerization underlying the decreased myogenic tone of uterine arteries in pregnancy.

Role of K ATP and L-type Ca 2+ channel activities in regulation of ovine uterine vascular contractility: effect of pregnancy and chronic hypoxia

Our objective was to determine whether the pregnancy and high altitude long-term hypoxia-mediated changes in uterine artery contractility were regulated by K(ATP) and L-type Ca(2+) channel activities. Uterine arteries were isolated from nonpregnant and near-term pregnant ewes that had been maintained at sea level (∼300 m) or exposed to high altitude (3801 m) for 110 days. Isometric tension was measured in a tissue bath. Pregnancy increased diazoxide, but not verapamil-induced relaxations. Long-term hypoxia attenuated diazoxide-induced relaxations in near-term pregnant uterine arteries, but enhanced verapamil-induced relaxations in nonpregnant uterine arteries. Diazoxide decreased the maximal response (E(max)) of phenylephrine-induced contractions in near-term pregnant uterin arteries but not nonpregnant uterine arteries in normoxic sheep. In contrast, diazoxide had no effect on phenylephrine-induced E(max) in near-term pregnant uterine arteries but decreased it in nonpregnant uterine arteries in long-term hypoxia animals. Verapamil decreased the E(max) and pD(2) (-logEC(50)) of phenylephrine-induced contractions in both nonpregnant uterine arteries and near-term pregnant uterine arteries in normoxic and long-term hypoxia animals, except nonpregnant uterine arteries of normoxic animals in which verapamil showed no effect on the pD(2). The results suggest that pregnancy selectively increases K(ATP), but not L-type Ca(2+) channel activity. Long-term hypoxia decreases the K(ATP) channel activity, which may contribute to the enhanced uterine vascular myogenic tone observed in pregnant sheep at high altitude hypoxia.

Direct Chronic Effect of Steroid Hormones in Attenuating Uterine Arterial Myogenic Tone

Pregnancy is associated with a significant decrease in uterine vascular tone and an increase in uterine blood flow. The present study tested the hypothesis that estrogen and progesterone differentially regulate the extracellular signal–regulated kinase (ERK)1/2 and protein kinase C (PKC) signaling pathways in vascular smooth muscle, resulting in a decrease in uterine vascular myogenic tone in pregnancy. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep. Chronic treatment (48 hours) of nonpregnant uterine arteries with 17β-estradiol and progesterone caused a significant decrease in PKC-mediated contractions and pressure-induced myogenic tone. In accordance, treatment of near-term pregnant uterine arteries for 48 hours with ICI 182780 and RU 486 significantly increased PKC-induced contractions and myogenic tone. In contrast, acute treatment for 30 minutes had no effect on uterine artery contractility. An ERK1/2 inhibitor, PD098059, restored the chronic effect of steroids on PKC-mediated contractions in nonpregnant sheep. ERK1/2 protein and mRNA levels were greater in near-term pregnant as compared with nonpregnant uterine arteries. 17β-Estradiol and progesterone increased ERK1/2 protein in nonpregnant sheep. In agreement, ICI 182780 and RU 486 caused significant decreases in ERK1/2 protein in near-term pregnant sheep. Western blot showed 6 PKC isozymes, α, β I , β II , δ, ε, and ζ, in the uterine arteries. 17β-Estradiol and progesterone decreased the particulate:cytosolic ratios of PKCα, ε, and ζ, respectively, in nonpregnant sheep. ICI 182780 and RU 486 increased the ratios in near-term pregnant sheep. The results indicate a direct chronic effect of the steroid hormones in the upregulation of ERK1/2 expression and downregulation of the PKC signaling pathway, resulting in attenuated myogenic tone of the uterine artery in pregnancy.

Review article: steroid hormones and uterine vascular adaptation to pregnancy.

Pregnancy is a physiological state that involves a significant decrease in uterine vascular tone and an increase in uterine blood flow, which is mediated in part by steroid hormones, including estrogen, progesterone, and cortisol. Previous studies have demonstrated the involvement of these hormones in the regulation of uterine artery contractility through signaling pathways specific to the endothelium and the vascular smooth muscle. Alterations in endothelial nitric oxide synthase expression and activity, nitric oxide production, and expression of enzymes involved in PGI(2) production contribute to the uterine artery endothelium-specific responses. Steroid hormones also have an effect on calcium-activated potassium channel activity, PKC signaling pathway and myogenic tone, and alterations in pharmacomechanical coupling in the uterine artery smooth muscle. This review addresses current understanding of the molecular mechanisms by which steroid hormones including estrogen, progesterone, and cortisol modulate uterine artery contractility to alter uterine blood flow during pregnancy with an emphasis on the pregnant ewe model.

Pregnancy attenuates uterine artery pressure-dependent vascular tone: role of PKC/ERK pathway

The mechanisms of adaptation of uterine artery vascular tone to pregnancy are not fully understood. The present study tested the hypothesis that pregnancy decreases the PKC-mediated Ca(2+) sensitivity of the contractile process and attenuates myogenic tone in resistance-sized uterine arteries. In pressurized uterine arteries from nonpregnant (NPUA) and near-term pregnant (PUA) sheep, we measured, simultaneously in the same tissue, vascular diameter and vessel wall intracellular Ca(2+) concentration ([Ca(2+)](i)) as a function of intraluminal pressure. In both NPUA and PUA, membrane depolarization with KCl caused a rapid increase in [Ca(2+)](i) and a decrease in diameter. A pressure increase from 20 to 100 mmHg resulted in a transient increase in diameter that was associated with an increase in [Ca(2+)](i), followed by myogenic contractions in the absence of further changes in [Ca(2+)](i). In addition, activation of PKC by phorbol 12,13-dibutyrate induced a decrease in diameter in the absence of changes in [Ca(2+)](i). Pressure-dependent myogenic responses were significantly decreased in PUA compared with NPUA. However, pressure-induced increases in [Ca(2+)](i) were not significantly different between PUA and NPUA. The ratio of changes in diameter to changes in [Ca(2+)](i) was significantly greater for pressure-induced contraction of NPUA than that of PUA. Inhibition of PKC by calphostin C significantly attenuated the pressure-induced vascular tone and eliminated the difference of myogenic responses between NPUA and PUA. In contrast, the MAPKK (MEK) inhibitor PD-098059 had no effect on NPUA but significantly enhanced myogenic responses of PUA. In the presence of PD-098059, there was no difference in pressure-induced myogenic responses between NPUA and PUA. The results suggest that pregnancy downregulates pressure-dependent myogenic tone of the uterine artery, which is partly due to increased MEK/ERK activity and decreased PKC signal pathway leading to a decrease in Ca(2+) sensitivity of myogenic mechanism in the uterine artery during pregnancy.

Cortisol-mediated regulation of uterine artery contractility: effect of chronic hypoxia

We previously demonstrated that cortisol regulated alpha(1)-adrenoceptor-mediated contractions differentially in nonpregnant and pregnant uterine arteries. Given that chronic hypoxia during pregnancy has profound effects on maternal uterine artery reactivity, the present study investigated the effects of chronic hypoxia on cortisol-mediated regulation of uterine artery contractions. Pregnant (day 30) and nonpregnant ewes were divided between normoxic control and chronically hypoxic [maintained at high altitude (3,820 m), arterial Po(2): 60 mmHg for 110 days] groups. Uterine arteries were isolated and contractions measured. In hypoxic animals, cortisol (10 ng/ml for 24 h) increased norepinephrine-induced contractions in pregnant, but not in nonpregnant, uterine arteries. The 11beta-hydroxysteroid dehydrogenase inhibitor carbenoxolone did not change cortisol effects in nonpregnant uterine arteries, but abolished it in pregnant uterine arteries by increasing norepinephrine pD(2) (-log EC(50)) in control tissues. The dissociation constant of norepinephrine-alpha(1)-adrenoceptors was not changed by cortisol in nonpregnant, but decreased in pregnant uterine arteries. There were no differences in the density of glucocorticoid receptors between normoxic and hypoxic tissues. Cortisol inhibited the norepinephrine-induced increase in Ca(2+) concentrations in nonpregnant arteries, but potentiated it in pregnant arteries. In addition, cortisol attenuated phorbol 12,13-dibutyrate-induced contractions in normoxic nonpregnant and pregnant uterine arteries, but had no effect on the contractions in hypoxic arteries. The results suggest that cortisol differentially regulates alpha(1)-adrenoceptor- and PKC-mediated contractions in uterine arteries. Chronic hypoxia suppresses uterine artery sensitivity to cortisol, which may play an important role in the adaptation of uterine vascular tone and blood flow in response to chronic stress of hypoxia during pregnancy.

Effect of Ro 61-1790, a selective endothelin-A receptor antagonist, on systemic and uterine hemodynamics and fetal oxygenation in sheep

Objective: Endothelin-1 is reportedly elevated in preeclampsia, and studies in our laboratory have shown that infusion of endothelin-1 produces increased mean arterial pressure, hemoconcentration, and proteinuria, while decreasing uterine blood flow. If a role in preeclampsia is confirmed for endothelin-1, therapeutic intervention may involve selective endothelin-A receptor blockers. Thus, this study was designed to determine the effects of Ro 61-1790, a selective endothelin-A receptor inhibitor, on mean arterial pressure, heart rate, and uteroplacental blood flow in pregnant and nonpregnant sheep. Study Design: Seven pregnant and seven nonpregnant sheep were instrumented with indwelling femoral artery and vein catheters and bilateral uterine artery flow probes, allowing determination of mean arterial pressure, heart rate, and uteroplacental blood flow. After baseline administration, animals received intravenous Ro 61-1790 either 1 mg/kg or 3 mg/kg, and hemodynamic responses were monitored for 120 minutes. Results: Intravenous administration of Ro 61-1790 at 1 mg/kg had no effect on the parameters measured in either group studied. However, at 3 mg/kg, Ro 61-1790 caused an increase in total uterine blood flow in nonpregnant sheep from 22 ± 6 mL/min to 51 ± 15 mL/min. This occurred in the absence of significant changes in mean arterial pressure. In pregnant animals, administration of 3 mg/kg Ro 61-1790 decreased mean (± SEM) uteroplacental blood flow by 20% (from 771 ± 130 mL/min to 621 ± 110 mL/min) and increased uterine vascular resistance transiently. Administration of Ro 61-1790 had no significant effect on fetal mean arterial pressure, heart rate, or oxygenation. Conclusions: These data suggest that endogenous endothelin-1 may play an important role in regulating uterine vascular tone in pregnant and nonpregnant sheep and that inhibition of endogenous endothelin-1 may lead to reductions in uteroplacental perfusion in pregnant animals. (Am J Obstet Gynecol 2002;186:55-60.)