Pregnancy modifies the large conductance Ca2+-activated K+ channel and cGMP-dependent signaling pathway in uterine vascular smooth muscle

Genome-Wide Scan for Selective Sweeps Reveals Novel Loci Associated with Prolificacy in Iranian Sheep Breeds in Comparison with Highly Prolific Exotic Breed

Domestication and selection significantly changed phenotypic traits in modern domestic animals. To identify the genomic regions associated with prolificacy in this study, 837 ewes from three Iranian indigenous sheep breeds, consisting of Baluchi, Lori-Bakhtiari, and Zandi uniparous breeds, and one Greek highly prolific dairy sheep, namely Chios, were genotyped using OvineSNP50K arrays. Statistical tests were then performed using different and complementary methods based on either site frequency (FST) and haplotype (hapFLK) between populations, followed by a pathway analysis of the genes contained in the selected regions. The results revealed that for the top 0.01 percentile of the obtained FST values, 16 genomic regions on chromosomes 2, 3, 4, 7, 8, 9, 13, 14, 16, 18, 19, and 20, and for hapFLK values, 3 regions located on chromosomes 3, 7, and 13, were under selection. A bioinformatic analysis of these genomic regions showed that these loci overlapped with potential candidate genes associated with prolificacy in sheep including GNAQ, COL5A2, COL3A1, HECW1, FBN1, COMMD3, RYR1, CCL28, SERPINA14, and HSPA2. These regions also overlapped with some quantitative trait loci (QTLs) linked to prolificacy traits, milk yield, and body weight. These findings suggest that future research could further link these genomic regions to prolificacy traits in sheep.

Ca2+-Activated K+ Channels and the Regulation of the Uteroplacental Circulation

Adequate uteroplacental blood supply is essential for the development and growth of the placenta and fetus during pregnancy. Aberrant uteroplacental perfusion is associated with pregnancy complications such as preeclampsia, fetal growth restriction (FGR), and gestational diabetes. The regulation of uteroplacental blood flow is thus vital to the well-being of the mother and fetus. Ca²⁺-activated K⁺ (K_Ca) channels of small, intermediate, and large conductance participate in setting and regulating the resting membrane potential of vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) and play a critical role in controlling vascular tone and blood pressure. K_Ca channels are important mediators of estrogen/pregnancy-induced adaptive changes in the uteroplacental circulation. Activation of the channels hyperpolarizes uteroplacental VSMCs/ECs, leading to attenuated vascular tone, blunted vasopressor responses, and increased uteroplacental blood flow. However, the regulation of uteroplacental vascular function by K_Ca channels is compromised in pregnancy complications. This review intends to provide a comprehensive overview of roles of K_Ca channels in the regulation of the uteroplacental circulation under physiological and pathophysiological conditions.

High altitude differentially modulates potassium channel-evoked vasodilatation in pregnant human myometrial arteries

High-altitude (>2500 m or 8200 ft) residence reduces uterine artery blood flow during pregnancy, contributing to an increased incidence of preeclampsia and intrauterine growth restriction. However, not all pregnancies are affected by the chronic hypoxic conditions of high-altitude residence. K+ channels play important roles in the uterine vascular adaptation to pregnancy, promoting a reduction in myogenic tone and an increase in blood flow. We hypothesized that, in pregnancies with normal fetal growth at high altitude, K+ channel-dependent vasodilatation of myometrial arteries is increased compared to those from healthy pregnant women at a lower altitude (∼1700 m). Using pharmacological modulation of two K+ channels, ATP-sensitive (KATP ) and large-conductance Ca2+ -activated (BKCa ) K+ channels, we assessed the vasodilatation of myometrial arteries from appropriate for gestational age (AGA) pregnancies in women living at high or low altitudes. In addition, we evaluated the localization of these channels in the myometrial arteries using immunofluorescence. Our results showed an endothelium-dependent increase in KATP -dependent vasodilatation in myometrial arteries from high versus low altitude, whereas vasodilatation induced by BKCa activation was reduced in these vessels. Additionally, KATP channel co-localization with endothelial markers was reduced in the high-altitude myometrial arteries, which suggested that the functional increase in KATP activity may be by mechanisms other than regulation of channel localization. These observations highlight an important contribution of K+ channels to the human uterine vascular adaptation to pregnancy at high altitude serving to maintain normal fetal growth under conditions of chronic hypoxia. KEY POINTS: High-altitude (>2500 m or 8200 ft) residence reduces uterine blood flow during pregnancy and fetal growth. Animal models of high altitude/chronic hypoxia suggest that these reductions are partially due to reduced vascular K+. channel responses, such as those elicited by large conductance Ca2+ -activated (BKCa ) and ATP-sensitive (KATP ) K+ channel activation. We found that women residing at high versus low altitude during pregnancy showed diminished myometrial artery vasodilatory responses to endothelium-independent BKCa channel activation but greater responses to endothelium-dependent KATP channel activation. Our observations indicate that KATP channels play an adaptive role in maintaining myometrial artery vasodilator sensitivity under chronic hypoxic conditions during pregnancy. Thus, KATP channels represent potential therapeutic targets for augmenting uteroplacental blood flow and, in turn, preserving fetal growth in cases of uteroplacental hypoperfusion.

Potassium Channels in the Uterine Vasculature: Role in Healthy and Complicated Pregnancies

A progressive increase in maternal uterine and placental blood flow must occur during pregnancy to sustain the development of the fetus. Changes in maternal vasculature enable an increased uterine blood flow, placental nutrient and oxygen exchange, and subsequent fetal development. K⁺ channels are important modulators of vascular function, promoting vasodilation, inducing cell proliferation, and regulating cell signaling. Different types of K⁺ channels, such as Ca²⁺-activated, ATP-sensitive, and voltage-gated, have been implicated in the adaptation of maternal vasculature during pregnancy. Conversely, K⁺ channel dysfunction has been associated with vascular-related complications of pregnancy, including intrauterine growth restriction and pre-eclampsia. In this article, we provide an updated and comprehensive literature review that highlights the relevance of K⁺ channels as regulators of uterine vascular reactivity and their potential as therapeutic targets.

Uteroplacental Circulation in Normal Pregnancy and Preeclampsia: Functional Adaptation and Maladaptation

Uteroplacental blood flow increases as pregnancy advances. Adequate supply of nutrients and oxygen carried by uteroplacental blood flow is essential for the well-being of the mother and growth/development of the fetus. The uteroplacental hemodynamic change is accomplished primarily through uterine vascular adaptation, involving hormonal regulation of myogenic tone, vasoreactivity, release of vasoactive factors and others, in addition to the remodeling of spiral arteries. In preeclampsia, hormonal and angiogenic imbalance, proinflammatory cytokines and autoantibodies cause dysfunction of both endothelium and vascular smooth muscle cells of the uteroplacental vasculature. Consequently, the vascular dysfunction leads to increased vascular resistance and reduced blood flow in the uteroplacental circulation. In this article, the (mal)adaptation of uteroplacental vascular function in normal pregnancy and preeclampsia and underlying mechanisms are reviewed.

Hypoxia and Mitochondrial Dysfunction in Pregnancy Complications

Hypoxia is a common and severe stress to an organism's homeostatic mechanisms, and hypoxia during gestation is associated with significantly increased incidence of maternal complications of preeclampsia, adversely impacting on the fetal development and subsequent risk for cardiovascular and metabolic disease. Human and animal studies have revealed a causative role of increased uterine vascular resistance and placental hypoxia in preeclampsia and fetal/intrauterine growth restriction (FGR/IUGR) associated with gestational hypoxia. Gestational hypoxia has a major effect on mitochondria of uteroplacental cells to overproduce reactive oxygen species (ROS), leading to oxidative stress. Excess mitochondrial ROS in turn cause uteroplacental dysfunction by damaging cellular macromolecules, which underlies the pathogenesis of preeclampsia and FGR. In this article, we review the current understanding of hypoxia-induced mitochondrial ROS and their role in placental dysfunction and the pathogenesis of pregnancy complications. In addition, therapeutic approaches selectively targeting mitochondrial ROS in the placental cells are discussed.

Hydrogen Sulfide Relaxes Human Uterine Artery via Activating Smooth Muscle BKCa Channels

Opening of large conductance calcium-activated and voltage-dependent potassium (BK_Ca) channels hyperpolarizes plasma membranes of smooth muscle (SM) to cause vasodilation, underling a key mechanism for mediating uterine artery (UA) dilation in pregnancy. Hydrogen sulfide (H₂S) has been recently identified as a new UA vasodilator, yet the mechanism underlying H₂S-induced UA dilation is unknown. Here, we tested whether H₂S activated BK_Ca channels in human UA smooth muscle cells (hUASMC) to mediate UA relaxation. Multiple BK_Ca subunits were found in human UA in vitro and hUASMC in vitro, and high β1 and γ1 proteins were localized in SM cells in human UA. Baseline outward currents, recorded by whole-cell and single-channel patch clamps, were significantly inhibited by specific BK_Ca blockers iberiotoxin (IBTX) or tetraethylammonium, showing specific BK_Ca activity in hUASMC. H₂S dose (NaHS, 1–1000 µM)-dependently potentiated BK_Ca currents and open probability. Co-incubation with a Ca²⁺ blocker nifedipine (5 µM) or a chelator (ethylene glycol-bis (β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA), 5 mM) did not alter H₂S-potentiated BK_Ca currents and open probability. NaHS also dose-dependently relaxed phenylephrine pre-constricted freshly prepared human UA rings, which was inhibited by IBTX. Thus, H₂S stimulated human UA relaxation at least partially via activating SM BK_Ca channels independent of extracellular Ca²⁺.

Using High-Density SNP Array to Reveal Selection Signatures Related to Prolificacy in Chinese and Kazakhstan Sheep Breeds

Selection signature provides an efficient tool to explore genes related to traits of interest. In this study, 176 ewes from one Chinese uniparous breed and three Kazakhstan multiparous breeds are genotyped using Affymetrix 600K HD single nucleotide polymorphism (SNP) arrays, F-statistics (Fst), and a Cross Population Extend Haplotype Homozygosity Test (XPEHH). These are conducted to identify genomic regions that might be under selection in three population pairs comprised the one multiparous breed and the uniparous breed. A total of 177 and 3072 common selective signatures were identified by Fst and XPEHH test, respectively. Nearly half of the common signatures detected by Fst were also captured by XPEHH test. In addition, 1337 positive and 1735 common negative signatures were observed by XPEHH in three Kazakhstan multiparous breeds. In total, 242 and 798 genes were identified in selective regions and positive selective regions identified by Fst and XPEHH, respectively. These genes were further clustered in 50 gene ontology (GO) functional terms and 66 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways in enrichment analysis. The GO terms and pathways were relevant with reproductive processes, e.g., oxytocin signaling pathway, thyroid hormone synthesis and GnRH signaling pathway, vascular smooth muscle contraction and lipid metabolism (alpha-Linolenic acid metabolism and Linoleic acid metabolism), etc. Based on the findings, six potential candidate genes ESR1, OXTR, MAPK1, RYR1, PDIA4, and CYP19A1, under positive selection related to characteristics of multiparous sheep breeds were revealed. Our results improve our understanding of the mechanisms of selection that underlies the prolificacy trait in sheep, and provide essential references for future sheep breeding.

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

AIMS

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.

METHODS AND RESULTS

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.

CONCLUSION

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⁺ (BK_Ca) channels and RyRs, thus functionally linking Ca²⁺ sparks and BK_Ca 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.

Involvement of sex hormonal regulation of K+ channels in electrophysiological and contractile functions of muscle tissues

Sex hormones, such as testosterone, progesterone, and 17β-estradiol, control various physiological functions. This review focuses on the sex hormonal regulation of K⁺ channels and the effects of such regulation on electrophysiological and contractile functions of muscles. In the cardiac tissue, testosterone and progesterone shorten action potential, and estrogen lengthens QT interval, a marker of increased risk of ventricular tachyarrhythmias. We have shown that testosterone and progesterone in physiological concentration activate KCNQ1 channels via membrane-delimited sex hormone receptor/eNOS pathways to shorten the action potential duration. Mitochondrial K⁺ channels are also involved in the protection of cardiac muscle. Testosterone and 17β-estradiol directly activate mitochondrial inner membrane K⁺ channels (Ca²⁺ activated K⁺ channel (K_Ca channel) and ATP-sensitive K⁺ channel (K_ATP channel)) that are involved in ischemic preconditioning and cardiac protection. During pregnancy, uterine blood flow increases to support fetal growth and development. It has been reported that 17β-estradiol directly activates large-conductance Ca²⁺-activated K⁺ channel (BK_Ca channel) attenuating arterial contraction. Furthermore, 17β-estradiol increases expression of BK_Ca channel β1 subunit which enhances BK_Ca channel activity by DNA demethylation. These findings are useful for understanding the mechanisms of sex or generation-dependent differences in the physiological and pathological functions of muscles, and the mechanisms of drug actions.

Effect of Oxidative Stress on the Estrogen-NOS-NO-KCa Channel Pathway in Uteroplacental Dysfunction: Its Implication in Pregnancy Complications

During pregnancy, the adaptive changes in uterine circulation and the formation of the placenta are essential for the growth of the fetus and the well-being of the mother. The steroid hormone estrogen plays a pivotal role in this adaptive process. An insufficient blood supply to the placenta due to uteroplacental dysfunction has been associated with pregnancy complications including preeclampsia and intrauterine fetal growth restriction (IUGR). Oxidative stress is caused by an imbalance between free radical formation and antioxidant defense. Pregnancy itself presents a mild oxidative stress, which is exaggerated in pregnancy complications. Increasing evidence indicates that oxidative stress plays an important role in the maladaptation of uteroplacental circulation partly by impairing estrogen signaling pathways. This review is aimed at providing both an overview of our current understanding of regulation of the estrogen-NOS-NO-K_Ca pathway by reactive oxygen species (ROS) in uteroplacental tissues and a link between oxidative stress and uteroplacental dysfunction in pregnancy complications. A better understanding of the mechanisms will facilitate the development of novel and effective therapeutic interventions.

Gestational Hypoxia and Developmental Plasticity

Hypoxia is one of the most common and severe challenges to the maintenance of homeostasis. Oxygen sensing is a property of all tissues, and the response to hypoxia is multidimensional involving complicated intracellular networks concerned with the transduction of hypoxia-induced responses. Of all the stresses to which the fetus and newborn infant are subjected, perhaps the most important and clinically relevant is that of hypoxia. Hypoxia during gestation impacts both the mother and fetal development through interactions with an individual's genetic traits acquired over multiple generations by natural selection and changes in gene expression patterns by altering the epigenetic code. Changes in the epigenome determine "genomic plasticity," i.e., the ability of genes to be differentially expressed according to environmental cues. The genomic plasticity defined by epigenomic mechanisms including DNA methylation, histone modifications, and noncoding RNAs during development is the mechanistic substrate for phenotypic programming that determines physiological response and risk for healthy or deleterious outcomes. This review explores the impact of gestational hypoxia on maternal health and fetal development, and epigenetic mechanisms of developmental plasticity with emphasis on the uteroplacental circulation, heart development, cerebral circulation, pulmonary development, and the hypothalamic-pituitary-adrenal axis and adipose tissue. The complex molecular and epigenetic interactions that may impact an individual's physiology and developmental programming of health and disease later in life are discussed.

Chronic hypoxia upregulates DNA methyltransferase and represses large conductance Ca2+-activated K+ channel function in ovine uterine arteries

Chronic hypoxia during gestation suppresses large-conductance Ca2+-activated K+ (BKCa) channel function and impedes uterine arterial adaptation to pregnancy. This study tested the hypothesis that chronic hypoxia has a direct effect in upregulating DNA methyltransferase (DNMT) and epigenetically repressing BKCa channel beta-1 subunit (KCNMB1) expression in uterine arteries. Resistance-sized uterine arteries were isolated from near-term pregnant sheep maintained at ∼300 m above sea level or animals acclimatized to high-altitude (3,801 m) hypoxia for 110 days during gestation. For ex vivo hypoxia treatment, uterine arteries from normoxic animals were treated with 21.0% O2 or 10.5% O2 for 48 h. High-altitude hypoxia significantly upregulated DNMT3b expression and enzyme activity in uterine arteries. Similarly, ex vivo hypoxia treatment upregulated DNMT3b expression and enzyme activity that was blocked by a DNMT inhibitor 5-aza-2'-deoxycytidine (5-Aza). Of importance, 5-Aza inhibited hypoxia-induced hypermethylation of specificity protein (SP) 1 binding site at the KCNMB1 promoter and restored transcription factor binding to the KCNMB1 promoter, resulting in the recovery of KCNMB1 gene expression in uterine arteries. Furthermore, 5-Aza blocked the effect of hypoxia and rescued BKCa channel activity and reversed hypoxia-induced decrease in BKCa channel-mediated relaxations and increase in myogenic tone of uterine arteries. Collectively, these results suggest that chronic hypoxia during gestation upregulates DNMT expression and activity, resulting in hypermethylation and repression of KCNMB1 gene and BKCa channel function, impeding uterine arterial adaptation to pregnancy.

Calcium- and voltage-gated BK channels in vascular smooth muscle

Ion channels in vascular smooth muscle regulate myogenic tone and vessel contractility. In particular, activation of calcium- and voltage-gated potassium channels of large conductance (BK channels) results in outward current that shifts the membrane potential toward more negative values, triggering a negative feed-back loop on depolarization-induced calcium influx and SM contraction. In this short review, we first present the molecular basis of vascular smooth muscle BK channels and the role of subunit composition and trafficking in the regulation of myogenic tone and vascular contractility. BK channel modulation by endogenous signaling molecules, and paracrine and endocrine mediators follows. Lastly, we describe the functional changes in smooth muscle BK channels that contribute to, or are triggered by, common physiological conditions and pathologies, including obesity, diabetes, and systemic hypertension.

The large-conductance voltage- and Ca2+ -activated K+ channel and its γ1-subunit modulate mouse uterine artery function during pregnancy

KEY POINTS

The uterine artery (UA) markedly vasodilates during pregnancy to direct blood flow to the developing fetus. Inadequate UA vasodilatation leads to intrauterine growth restriction and fetal death. The large-conductance voltage- and Ca2+ -activated K+ (BKCa ) channel promotes UA vasodilatation during pregnancy. We report that BKCa channel activation increases the UA diameter at late pregnancy stages in mice. Additionally, a BKCa channel auxiliary subunit, γ1, participates in this process by increasing channel activation and inducing UA vasodilatation at late pregnancy stages. Our results highlight the importance of the BKCa channel and its γ1-subunit for UA functional changes during pregnancy.

ABSTRACT

Insufficient vasodilatation of the uterine artery (UA) during pregnancy leads to poor utero-placental perfusion, contributing to intrauterine growth restriction and fetal loss. Activity of the large-conductance Ca2+ -activated K+ (BKCa ) channel increases in the UA during pregnancy, and its inhibition reduces uterine blood flow, highlighting a role of this channel in UA adaptation to pregnancy. The auxiliary γ1-subunit increases BKCa activation in vascular smooth muscle, but its role in pregnancy-associated UA remodelling is unknown. We explored whether the BKCa and its γ1-subunit contribute to UA remodelling during pregnancy. Doppler imaging revealed that, compared to UAs from wild-type (WT) mice, UAs from BKCa knockout (BKCa-/- ) mice had lower resistance at pregnancy day 14 (P14) but not at P18. Lumen diameters were twofold larger in pressurized UAs from P18 WT mice than in those from non-pregnant mice, but this difference was not seen in UAs from BKCa-/- mice. UAs from pregnant WT mice constricted 20-50% in response to the BKCa blocker iberiotoxin (IbTX), whereas UAs from non-pregnant WT mice only constricted 15%. Patch-clamp analysis of WT UA smooth muscle cells confirmed that BKCa activity increased over pregnancy, showing three distinct voltage sensitivities. The γ1-subunit transcript increased 7- to 10-fold during pregnancy. Furthermore, γ1-subunit knockdown reduced IbTX sensitivity in UAs from pregnant mice, whereas γ1-subunit overexpression increased IbTX sensitivity in UAs from non-pregnant mice. Finally, at P18, γ1-knockout (γ1-/- ) mice had smaller UA diameters than WT mice, and IbTX-mediated vasoconstriction was prevented in UAs from γ1-/- mice. Our results suggest that the γ1-subunit increases BKCa activation in UAs during pregnancy.

Augmented H2S production via cystathionine-beta-synthase upregulation plays a role in pregnancy-associated uterine vasodilation

Endogenous hydrogen sulfide (H2S) synthesized via metabolizing L-cysteine by cystathionine-beta-synthase (CBS) and cystathionine-gamma-lyase (CSE) is a potent vasodilator and angiogenic factor. The objectives of this study were to determine if human uterine artery (UA) H2S production increases with augmented expression and/or activity of CBS and/or CSE during the menstrual cycle and pregnancy and whether exogenous H2S dilates UA. Uterine arteries from nonpregnant (NP) premenopausal proliferative (pPRM) and secretory (sPRM) phases of the menstrual cycle and pregnant (P) women were studied. H2S production was measured by the methylene blue assay. CBS and CSE mRNAs were assessed by quantitative real-time PCR, and proteins were assessed by immunoblotting and semiquantitative immunofluorescence microscopy. Effects of H2S on rat UA relaxation were determined by wire myography ex vivo. H2S production was greater in NP pPRM and P than NP sPRM UAs and inhibited by the specific CBS but not CSE inhibitor. CBS but not CSE mRNA and protein were greater in NP pPRM and P than NP sPRM UAs. CBS protein was localized to endothelium and smooth muscle and its levels were in a quantitative order of P >NP UAs of pPRM>sPRM. CSE protein was localized in UA endothelium and smooth muscle with no difference among groups. A H2S donor relaxed P > NP UAs but not mesentery artery. Thus, human UA H2S production is augmented with endothelium and smooth muscle CBS upregulation, contributing to UA vasodilation in the estrogen-dominant physiological states in the proliferative phase of the menstrual cycle and pregnancy.

MicroRNA-210 Targets Ten-Eleven Translocation Methylcytosine Dioxygenase 1 and Suppresses Pregnancy-Mediated Adaptation of Large Conductance Ca2+-Activated K+ Channel Expression and Function in Ovine Uterine Arteries

Gestational hypoxia inhibits large conductance Ca2+-activated K+ (BKCa) channel expression and function in uterine arterial adaptation to pregnancy. Given the findings that microRNA-210 (miR-210) is increased in hypoxia during gestation and preeclampsia, the present study sought to investigate the role of miR-210 in the regulation of BKCa channel adaptation in the uterine artery. Gestational hypoxia significantly increased uterine vascular resistance and blood pressure in pregnant sheep and upregulated miR-210 in uterine arteries. MiR-210 bound to ovine ten-eleven translocation methylcytosine dioxygenase 1 mRNA 3' untranslated region and decreased ten-eleven translocation methylcytosine dioxygenase 1 mRNA and protein abundance in uterine arteries of pregnant sheep, as well as abrogated steroid hormone-induced upregulation of ten-eleven translocation methylcytosine dioxygenase 1 expression in uterine arteries of nonpregnant animals. In accordance, miR-210 blocked pregnancy- and steroid hormone-induced upregulation of BKCa channel β1 subunit expression in uterine arteries. Functionally, miR-210 suppressed BKCa channel current density in uterine arterial myocytes of pregnant sheep and inhibited steroid hormone-induced increases in BKCa channel currents in uterine arteries of nonpregnant animals. Blockade of endogenous miR-210 inhibited hypoxia-induced suppression of BKCa channel activity. In addition, miR-210 decreased BKCa channel-mediated relaxations and increased pressure-dependent myogenic tone of uterine arteries. Together, the results demonstrate that miR-210 plays an important role in the downregulation of ten-eleven translocation methylcytosine dioxygenase 1 and repression of BKCa channel function in uterine arteries, revealing a novel mechanism of epigenetic regulation in the maladaptation of uterine hemodynamics in gestational hypoxia and preeclampsia.

Pregnancy Reprograms Large-Conductance Ca2+-Activated K+ Channel in Uterine Arteries: Roles of Ten-Eleven Translocation Methylcytosine Dioxygenase 1-Mediated Active Demethylation

The large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel is of critical importance in pregnancy-mediated increase in uterine artery vasodilation and blood flow. The present study tested the hypothesis that active DNA demethylation plays a key role in pregnancy-induced reprogramming and upregulation of BK_Ca channel β1 subunit (BKβ1) in uterine arteries. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep. Pregnancy significantly increased the expression of ten-eleven translocation methylcytosine dioxygenase 1 (TET1) in uterine arteries. A half-palindromic estrogen response element was identified at the TET1 promoter, and estrogen treatment increased TET1 promoter activity and TET1 expression in uterine arteries. In accordance, pregnancy and steroid hormone treatment resulted in demethylation of BKβ1 promoter by increasing 5-hydroxymethylcytosine and decreasing 5-methylcytosine at the CpG in the Sp1_-380 binding site that is of critical importance in the regulation of the promoter activity and BKβ1 expression. Inhibition of TET1 with fumarate significantly decreased BKβ1 expression in uterine arteries of pregnant animals and blocked steroid hormone-induced upregulation of BKβ1. Functionally, fumarate treatment inhibited pregnancy and steroid hormone-induced increases in BK_Ca channel current density and BK_Ca channel-mediated relaxations. In addition, fumarate blocked pregnancy and steroid hormone-induced decrease in pressure-dependent myogenic tone of the uterine artery. The results demonstrate a novel mechanism of estrogen-mediated active DNA demethylation in reprogramming of BK_Ca channel expression and function in the adaption of uterine circulation during pregnancy.

Survival and growth of C57BL/6J mice lacking the BK channel, Kcnma1: lower adult body weight occurs together with higher body fat

Big conductance potassium (BK) channels contribute to K⁺ flow and electrical behavior in many cell types. Mice made null for the gene (Kcnma1) producing the BK channel (BK^KO) exhibit numerous deficits in physiological functions. Breeding mice lacking a single allele of Kcnma1 (C57BL/6J background) had litter sizes of approximately eight pups. For the period of maternal care (P0–P21), pup deaths peaked at P1 with a second less severe interval of death peaking near P13. Early deaths were twice as likely during a 20‐month period of building construction compared with the quiescent period after cessation of construction. Births during construction were not consistent with Mendelian predictions indicating the likelihood of a specific disadvantage induced by this environmental stressor. Later BK^KO pup deaths (~P13) also were more numerous than Mendelian expectations. After weaning, weight gain was slower for BK^KO mice compared with wild‐type littermates: 5 g less for male BK^KO mice and 4 g less for female BK^KO mice. Body composition determined by quantitative magnetic resonance indicated a higher fat proportion for wild‐type female mice compared with males, as well as a higher hydration ratio. Both male and female BK^KO mice showed higher fat proportions than wild‐type, with female BK^KO mice exhibiting greater variation. Together, these results indicate that BK^KO mice suffered disadvantages that lead to prenatal and perinatal death. A metabolic difference likely related to glucose handling led to the smaller body size and distinct composition for BK^KO mice, suggesting a diversion of energy supplies from growth to fat storage.

Direct effect of chronic hypoxia in suppressing large conductance Ca(2+)-activated K(+) channel activity in ovine uterine arteries via increasing oxidative stress

KEY POINTS

Chronic hypoxia has a direct effect in down-regulating the BKCa channel β1 subunit and inhibiting the BKCa channel activity in uterine arteries of pregnant sheep. Oxidative stress plays a causal role in hypoxia-mediated suppression of BKCa channel function. The steroid hormone-induced effect on BKCa channels is a target of hypoxia-mediated oxidative stress. Inhibition of oxidative stress ameliorates the adverse effect of hypoxia both ex vivo and in vivo in pregnant sheep exposed to long-term high-altitude hypoxia. Our findings provide novel evidence of a causative role of oxidative stress in hypoxia-mediated inhibition of the BKCa channel activity in uterine arteries and new insights in understanding and alleviating pregnancy complications associated with gestational hypoxia such as pre-eclampsia and fetal growth restriction.

ABSTRACT

Uterine arteries of pregnant sheep acclimatized to long-term high-altitude hypoxia were associated with a decrease in large-conductance Ca(2+)-activated K(+) (BKCa) channel activity. The present study tested the hypothesis that prolonged hypoxia has a direct effect in suppressing BKCa channel activity by increasing oxidative stress. Uterine arteries were isolated from non-pregnant and near-term (∼142 days) pregnant sheep, and were treated ex vivo with 21.0 or 10.5% O2 for 48 h. The hypoxia treatment significantly increased the production of reactive oxygen species in uterine arteries, which was blocked by N-acetylcysteine. In uterine arteries of pregnant sheep, hypoxia significantly inhibited BKCa channel current density, decreased NS1619-induced relaxations and increased pressure-dependent tone, which were annulled by N-acetylcysteine. In accordance, hypoxia resulted in down-regulation of BKCa channel β1 subunit, which was restored in the presence of N-acetylcysteine. In addition, the N-acetylcysteine treatment significantly increased BKCa channel β1 subunit abundance and BKCa channel current density in uterine arteries from pregnant sheep exposed to high-altitude hypoxia (3801 m, PaO2: 60 mmHg) for 110 days. In uterine arteries of non-pregnant animals, hypoxia inhibited steroid hormone-induced up-regulation of BKCa channel current density and NS1619-mediated relaxations, which were reversed by N-acetylcysteine. Furthermore, the synthetic superoxide dismutase and catalase mimetic EUK-134 also ablated the effects of hypoxia on BKCa channel currents in uterine arteries. The results demonstrate a direct effect of hypoxia in inhibiting the BKCa channel activity in uterine arteries via increased oxidative stress.

Smooth muscle relaxation and activation of the large conductance Ca(++)-activated K+ (BK(Ca)) channel by novel oestrogens

BACKGROUND AND PURPOSE

Oestrogens can interact directly with membrane receptors and channels and can activate vascular BK(Ca) channels. We hypothesized that novel oestrogen derivatives could relax smooth muscle by an extracllular effect on the α and β1 subunits of the BK(Ca) channel, rather than at an intracellular site.

EXPERIMENTAL APPROACH

We studied the effects of novel oestrogens on the tension of pre-contracted isolated rat aortic rings, and on the electrophysiological properties of HEK 293 cells expressing the hSloα or hSloα+β1 subunits. Two of the derivatives incorporated a quaternary ammonium side-chain making them membrane impermeable.

KEY RESULTS

Oestrone, oestrone oxime and Quat DME-oestradiol relaxed pre-contracted rat aorta, but only Quat DME-oestradiol-induced relaxation was iberiotoxin sensitive. However, only potassium currents recorded in HEK 293 cells over-expressing both hSloα and hSloβ1 were activated by oestrone, oestrone oxime and Quat DME-oestradiol.

CONCLUSION AND IMPLICATIONS

The novel oestrogens were able to relax smooth muscle, but through different mechanisms. In particular, oestrone oxime required the presence of the endothelium to exert much of its effect, whilst Quat DME-oestradiol depended both on NO and BK(Ca) channel activation. The activation of BK(Ca) currents in HEK 293 cells expressing hSloα+β1 by Quat DME-oestradiol is consistent with an extracellular binding site between the two subunits. The binding site resides between the extracellular N terminal of the α subunit and the extracellular loop between TM1 and 2 of the β1 subunit. Membrane-impermeant Quat DME-oestradiol lacks an exchangeable hydrogen on the A ring obviating antioxidant activity.

Prolonged uterine artery nitric oxide synthase inhibition modestly alters basal uteroplacental vasodilation in the last third of ovine pregnancy

Mechanisms regulating uteroplacental blood flow (UPBF) in pregnancy remain unclear, but they likely involve several integrated signaling systems. Endothelium-derived nitric oxide (NO) is considered an important contributor, but the extent of its involvement is unclear. Bolus intra-arterial infusions of nitro-l-arginine methyl ester (l-NAME) modestly decrease ovine basal UPBF; however, the doses and duration of infusion may have been insufficient. We, therefore, examined prolonged uterine artery (UA) NO synthase inhibition with l-NAME throughout the last third of ovine pregnancy by performing either continuous 30-min UA infusion dose responses (n = 4) or 72-h UA infusions (0.01 mg/ml) at 104-108, 118-125, and 131-137 days of gestation (n = 7) while monitoring mean arterial pressure (MAP), heart rate (HR), and UPBF. Uteroplacental vascular resistance (UPVR) was calculated, and uterine cGMP synthesis was measured. Thirty-minute UA l-NAME infusions did not dose dependently decrease UPBF, increase UPVR, or decrease uterine cGMP synthesis (P > 0.1); however, MAP rose and HR fell modestly. Prolonged continuous 72-h UA l-NAME infusions decreased UPBF ∼32%, increased UPVR ∼68% (P ≤ 0.001), and decreased uterine cGMP synthesis 70% at 54-72 h (P ≤ 0.004); the noninfused uterine horn was unaffected. These findings were associated with ∼10% increases in MAP and decreases in HR that were greater at 104-108 than 118-125 and 131-137 days of gestation (P = 0.006). Although uterine and UA NO and cGMP synthesis increase severalfold during ovine pregnancy, they contribute modestly to the maintenance and rise in UPBF in the last third of gestation. Thus, local UA NO may primarily modulate vasoconstrictor responses. Notably, the systemic vasculature appears more sensitive than the uterine vasculature to NO synthase inhibition.

Epigenetic upregulation of large-conductance Ca2+-activated K+ channel expression in uterine vascular adaptation to pregnancy

Our previous study demonstrated that pregnancy increased large-conductance Ca(2+)-activated potassium channel β1 subunit (BKβ1) expression and large-conductance Ca(2+)-activated potassium channel activity in uterine arteries, which were abrogated by chronic hypoxia. The present study tested the hypothesis that promoter methylation/demethylation is a key mechanism in epigenetic reprogramming of BKβ1 expression patterns in uterine arteries. Ovine BKβ1 promoter of 2315 bp spanning from -2211 to +104 of the transcription start site was cloned, and an Sp1-380 binding site that contains CpG dinucleotide in its core binding sequences was identified. Site-directed deletion of the Sp1 site significantly decreased the BKβ1 promoter activity. Estrogen receptor-α bound to the Sp1 site through tethering to Sp1 and upregulated the expression of BKβ1. The Sp1 binding site at BKβ1 promoter was highly methylated in uterine arteries of nonpregnant sheep, and methylation inhibited transcription factor binding and BKβ1 promoter activity. Pregnancy caused a significant decrease in CpG methylation at the Sp1 binding site and increased Sp1 binding to the BKβ1 promoter and BKβ1 mRNA abundance. Chronic hypoxia during gestation abrogated this pregnancy-induced demethylation and upregulation of BKβ1 expression. The results provide evidence of a novel mechanism of promoter demethylation in pregnancy-induced reprogramming of large-conductance Ca(2+)-activated potassium channel expression and function in uterine arteries and suggest new insights of epigenetic mechanisms linking gestational hypoxia to aberrant uteroplacental circulation and increased risk of preeclampsia.

The renal circulation in normal pregnancy and preeclampsia: is there a place for relaxin?

During the first trimester of human pregnancy, the maternal systemic circulation undergoes remarkable vasodilation. The kidneys participate in this vasodilatory response resulting in marked increases in renal plasma flow (RPF) and glomerular filtration rate (GFR). Comparable circulatory adaptations are observed in conscious gravid rats. Administration of the corpus luteal hormone relaxin (RLN) to nonpregnant rats and humans elicits vasodilatory changes like those of pregnancy. Systemic and renal vasodilation are compromised in midterm pregnant rats by neutralization or elimination of circulating RLN and in women conceiving with donor eggs who lack a corpus luteum and circulating RLN. Although RLN exerts both rapid (minutes) and sustained (hours to days) vasodilatory actions through different molecular mechanisms, a final common pathway is endothelial nitric oxide. In preeclampsia (PE), maternal systemic and renal vasoconstriction leads to hypertension and modest reduction in GFR exceeding that of RPF. Elevated level of circulating soluble vascular endothelial growth factor receptor-1 arising from the placenta is implicated in the hypertension and disruption of glomerular fenestrae and barrier function, the former causing reduced Kf and the latter proteinuria. Additional pathogenic factors are discussed. Last, potential clinical ramifications include RLN replacement in women conceiving with donor eggs and its therapeutic use in PE. Another goal has been to apply knowledge gained from investigating circulatory adaptations in pregnancy toward identifying and developing novel therapeutic strategies for renal and cardiovascular disease in the nonpregnant population. So far, one candidate to emerge is RLN and its potential therapeutic use in heart failure.

Chronic hypoxia during gestation enhances uterine arterial myogenic tone via heightened oxidative stress

Chronic hypoxia during gestation has profound adverse effects on the adaptation of uteroplacental circulation in pregnancy. Yet, the underlying mechanisms are not fully understood. The present study tested the hypothesis that enhanced production of reactive oxygen species (ROS) in uterine arteries plays a critical role in the maladaptation of uterine circulation associated with chronic hypoxia. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep maintained at sea level (~300 m) or exposed to high-altitude (3801 m) hypoxia for 110 days. Hypoxia significantly increased ROS production in uterine arteries of pregnant, but not nonpregnant, sheep. This was associated with a significant increase in NADPH oxidase (Nox) 2, but not Nox1 or Nox4, protein abundance and total Nox activity in uterine arteries of pregnant animals. Chronic hypoxia significantly increased pressure-dependent uterine arterial myogenic tone in pregnant sheep, which was abrogated by a Nox inhibitor apocynin. Additionally, the hypoxia-induced increase in myogenic reactivity of uterine arteries to phorbol 12,13-dibutyrate in pregnant sheep was blocked by apocynin and tempol. In consistence with the myogenic responses, the hypoxia-mediated down-regulation of BKCa channel activity in uterine arteries of pregnant animals was reversed by apocynin. The findings suggest that heightened oxidative stress in uterine arteries plays a key role in suppressing the BKCa channel activity, resulting in increased myogenic reactivity and maladaptation of uteroplacental circulation caused by chronic hypoxia during gestation.

Large conductance Ca2+-activated K+ channels modulate uterine α1-adrenergic sensitivity in ovine pregnancy

The uteroplacental vasculature is refractory to α-adrenergic stimulation, and large conductance Ca(2+)-activated K(+) channels (BK(Ca)) may contribute. We examined the effects of uterine artery (UA) BK(Ca) inhibition with tetraethylammonium (TEA) on hemodynamic responses to phenylephrine (PE) at 101 to 117 days and 135 to 147 days of ovine gestation, obtaining dose responses for mean arterial pressure (MAP), heart rate (HR), and uteroplacental blood flow (UPBF) and vascular resistance (UPVR) before and during UA TEA infusions. The UA α(1)-adrenergic receptors (α1-ARs) were assessed. The PE increased MAP and UPVR and decreased HR and UPBF dose dependently at both gestations (P < .001, analysis of variance). The %▵MAP was less at 135 to 147 days before and during TEA infusions (P ≤ .008); however, responses during TEA were greater (P ≤ .002). The PE increased %▵UPVR>>%▵MAP, thus %▵UPBF fell. The TEA enhanced PE-mediated increases in %▵UPVR at 135 to 147 days (P ≤ .03). The UA α(1)-AR expression was unchanged in pregnancy. Uterine vascular responses to PE exceed systemic vascular responses throughout pregnancy and are attenuated by BK(Ca) activation, suggesting BK(Ca) protect UPBF.

Chronic hypoxia inhibits pregnancy-induced upregulation of SKCa channel expression and function in uterine arteries

Small-conductance Ca(2+)-activated K(+) (SKCa) channels are crucial in regulating vascular tone and blood pressure. The present study tested the hypothesis that SKCa channels play an important role in uterine vascular adaptation in pregnancy, which is inhibited by chronic hypoxia during gestation. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep maintained at sea level (≈300 m) or exposed to high-altitude (3801 m) hypoxia for 110 days. Immunohistochemistry revealed the presence of SKCa channels type 2 (SK2) and type 3 (SK3) in both smooth muscles and endothelium of uterine arteries. The expression of SK2 and SK3 channels was significantly increased during pregnancy, which was inhibited by chronic hypoxia. In normoxic animals, both SKCa channel opener NS309 and a large-conductance (BKCa) channel opener NS1619 relaxed norepinephrine-contracted uterine arteries in pregnant but not nonpregnant sheep. These relaxations were inhibited by selective SKCa and BKCa channel blockers, respectively. NS309-induced relaxation was largely endothelium-independent. In high-altitude hypoxic animals, neither NS1691 nor NS309 produced significant relaxation of uterine arteries in either nonpregnant or pregnant sheep. Similarly, the role of SKCa channels in regulating the myogenic reactivity of uterine arteries in pregnant animals was abrogated by chronic hypoxia. Accordingly, the enhanced SKCa channel activity in uterine arterial myocytes of pregnant animals was ablated by chronic hypoxia. The findings suggest a novel mechanism of SKCa channels in regulating myogenic adaptation of uterine arteries in pregnancy and in the maladaptation of uteroplacental circulation caused by chronic hypoxia during gestation.

Large conductance Ca2+-activated and voltage-activated K+ channels contribute to the rise and maintenance of estrogen-induced uterine vasodilation and maintenance of blood pressure

Uterine blood flow (UBF) increases greater than 4-fold 90 min after systemic estradiol-17β (E2β) in nonpregnant sheep and remains elevated longer than 6-8 h; mean arterial pressure (MAP) is unchanged. Large-conductance Ca(+2)-activated (BK(Ca)) and voltage-activated (K(V)) K(+) channels contribute to the acute rise in UBF; their role in maintaining UBF and MAP longer than 90 min is unknown. We examined this in five nonpregnant, ovariectomized ewes with uterine artery (UA) flow probes and catheters in a UA for infusion of K(+) channel inhibitors and uterine vein to sample venous effluent. Animals received systemic E2β (1.0 μg/kg; control), E2β+UA tetraethylammonium (TEA; 0.4-0.8 mm, n = 4), and E2β+UA 4-aminopyridine (4-AP; 0.01-0.08 mm, n = 4) to block BK(Ca) and K(V), respectively, while monitoring MAP, heart rate, and UBF. Uterine cGMP synthesis was measured. Ninety minutes after E2β, UBF rose 4.5-fold, uterine vascular resistance (UVR) fell greater than 5-fold and MAP was unchanged [78 ± 0.8 (sem) vs. 77 ± 1.5 mm Hg] in control studies and before UA inhibition with TEA and 4-AP. Between 90 and 120min, UBF, UVR, and MAP were unchanged after E2β alone. E2β+TEA dose dependently decreased ipsilateral UBF and increased UVR (24 ± 8.9 and 38 ± 16%, respectively, at 0.8 mm; P < 0.03); MAP was unchanged. Contralateral UBF/UVR were unaffected. E2β+4-AP also dose dependently decreased ipsilateral UBF and increased UVR (27 ± 5.3 and 76 ± 18%, respectively, at 0.08 mm; P < 0.001); however, MAP rose 27 ± 6.9% (P ≤ 0.006). E2β increased uterine cGMP synthesis greater than 3.5-fold and was unaffected by local K(+) channel inhibition. BK(Ca) and K(V) contribute to the rise and maintenance of E2β-induced uterine vasodilation, which is partially cGMP dependent. Systemic vascular K(V) also contributes to maintaining MAP after systemic E2β.

Function and regulation of large conductance Ca(2+)-activated K+ channel in vascular smooth muscle cells

Large conductance Ca(2+)-activated K(+) (BK(Ca)) channels are abundantly expressed in vascular smooth muscle cells. Activation of BK(Ca) channels leads to hyperpolarization of cell membrane, which in turn counteracts vasoconstriction. Therefore, BK(Ca) channels have an important role in regulation of vascular tone and blood pressure. The activity of BK(Ca) channels is subject to modulation by various factors. Furthermore, the function of BK(Ca) channels are altered in both physiological and pathophysiological conditions, such as pregnancy, hypertension and diabetes, which has dramatic impacts on vascular tone and hemodynamics. Consequently, compounds and genetic manipulation that alter activity and expression of the channel might be of therapeutic interest.

Pregnancy increases myometrial artery myogenic tone via NOS- or COX-independent mechanisms

Myogenic tone (MT) is a primary modulator of blood flow in the resistance vasculature of the brain, kidney, skeletal muscle, and perhaps in other high-flow organs such as the pregnant uterus. MT is known to be regulated by endothelium-derived factors, including products of the nitric oxide synthase (NOS) and/or the cyclooxygenase (COX) pathways. We asked whether pregnancy influenced MT in myometrial arteries (MA), and if so, whether such an effect could be attributed to alterations in NOS and/or COX. MA (200-300 μm internal diameter, 2-3 mm length) were isolated from 10 nonpregnant and 12 pregnant women undergoing elective hysterectomy or cesarean section, respectively. In the absence of NOS and/or COX inhibition, pregnancy was associated with increased MT in endothelium-intact MA compared with MA from nonpregnant women (P < 0.01). The increase in MT was not due to increased Ca(2+) entry via voltage-dependent channels since both groups of MA exhibited similar levels of constriction when exposed to 50 mM KCl. NOS inhibition (N(ω)-nitro-L-arginine methyl ester, L-NAME) or combined NOS/COX inhibition (L-NAME/indomethacin) increased MT in MA from pregnant women (P = 0.001 and P = 0.042, respectively) but was without effect in arteries from nonpregnant women. Indomethacin alone was without effect on MT in MA from either nonpregnant or pregnant women. We concluded that MT increases in MA during human pregnancy and that this effect was partially opposed by enhanced NOS activity.

Chronic hypoxia suppresses pregnancy-induced upregulation of large-conductance Ca2+-activated K+ channel activity in uterine arteries

Our previous study demonstrated that increased Ca(2+)-activated K(+) (BK(Ca)) channel activity played a key role in the normal adaptation of reduced myogenic tone of uterine arteries in pregnancy. The present study tested the hypothesis that chronic hypoxia during gestation inhibits pregnancy-induced upregulation of BK(Ca) channel function in uterine arteries. Resistance-sized uterine arteries were isolated from nonpregnant and near-term pregnant sheep maintained at sea level (≈ 300 m) or exposed to high-altitude (3801 m) hypoxia for 110 days. Hypoxia during gestation significantly inhibited pregnancy-induced upregulation of BK(Ca) channel activity and suppressed BK(Ca) channel current density in pregnant uterine arteries. This was mediated by a selective downregulation of BK(Ca) channel β1 subunit in the uterine arteries. In accordance, hypoxia abrogated the role of the BK(Ca) channel in regulating pressure-induced myogenic tone of uterine arteries that was significantly elevated in pregnant animals acclimatized to chronic hypoxia. In addition, hypoxia abolished the steroid hormone-mediated increase in the β1 subunit and BK(Ca) channel current density observed in nonpregnant uterine arteries. Although the activation of protein kinase C inhibited BK(Ca) channel current density in pregnant uterine arteries of normoxic sheep, this effect was ablated in the hypoxic animals. The results demonstrate that selectively targeting BK(Ca) channel β1 subunit plays a critical role in the maladaption of uteroplacental circulation caused by chronic hypoxia, which contributes to the increased incidence of preeclampsia and fetal intrauterine growth restriction associated with gestational hypoxia.

Mineralocorticoids participate in the reduced vascular reactivity of pregnant rats

The renin-angiotensin-aldosterone (RAA) system is markedly activated in pregnancy. We evaluated if mineralocorticoid receptors (MR), a major component of the RAA system, are involved in the reduced vascular reactivity associated with pregnancy. Canrenoate (MR antagonist; 20 mg·kg(-1)·day(-1)) was administered to nonpregnant (NP) rats for 7 days and to pregnant rats from day 15 to 22 of gestation. These were killed on day 17, 19, or 22 of gestation and, for NP rats, after 7 days treatment. Constrictor responses to phenylephrine (PhE) and KCl were measured in endothelium-denuded thoracic aortic rings under the influence of modulators of potassium (activators) and calcium (blocker) channels. Responses to the constrictors were blunted from days 17 to 22 of gestation. Although canrenoate increased responses to PhE and KCl, it did not reverse their blunted responses in gestation. NS-1619 and cromakalim (respectively, high-conductance calcium-activated potassium channels and ATP-sensitive potassium channel activators) diminished responses to both PhE and KCl. Inhibition by NS-1619 on responses to both agonists was decreased under canrenoate treatment in NP, but the reduced influence of NS-1619 during gestation was reversed by the mineralocorticoid antagonist. Cromakalim reduced the response to PhE significantly in the pregnant groups; this effect was enhanced by canrenoate. Finally, nifedipine (calcium channel blocker) markedly reduced KCl responses but to a lesser extent at the end of pregnancy, an inhibiting effect that was increased with canrenoate treatment. These data demonstrate that treating rats with a MR antagonist increased vascular reactivity but that it differentially affected potassium and calcium channel activity in aortas of NP and pregnant animals. This suggests that aldosterone is one of the components involved in vascular adaptations to pregnancy.

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.

Differential effects on nitric oxide-mediated vasodilation in mesenteric and uterine arteries from cytomegalovirus-infected mice

Chronic cytomegalovirus (CMV) infections are implicated in vascular diseases. Recently, we showed that an active mouse CMV (mCMV) infection in nonpregnant mice increased endothelial-dependent vasodilation in isolated mesenteric and uterine arteries. In late pregnancy, while increased vasodilation was found in mesenteric arteries from infected mice, there was a dramatic decrease in uterine arteries. Understanding the mechanisms for these vascular changes during CMV infections is important for pregnancy outcomes and long-term consequences of this chronic infection. Increased nitric oxide (NO) is implicated in CMV-associated atherosclerosis, and CMV replication is dependent on prostaglandin H synthase (PGHS) activity. Alternatively, CMV infections decrease NO under inflammatory conditions. We therefore hypothesized that changes in the contribution by NO or PGHS-induced vasodilators would explain the increased or decreased endothelial-dependent vasodilation in arteries from nonpregnant and late pregnant mice, respectively. We found that the contribution by NO to methacholine-induced vasodilation was significantly increased in mesenteric, but not uterine, arteries isolated from nonpregnant and pregnant mCMV-infected mice. Prostaglandin inhibition did not affect endothelial-dependent vasodilation in any group. Vasodilation responses to sodium nitroprusside, an NO donor, were increased in mesenteric and uterine arteries isolated only from mCMV-infected nonpregnant mice. These results explain the increased vasodilation responses observed in mesenteric arteries from mCMV-infected mice; however, the decreased vasodilation in uterine arteries from pregnant mice could not be explained by these mechanisms. Thus CMV infection affects the contribution of NO differently in endothelial-dependent vasodilation in pregnant compared with nonpregnant mice and also in the mesenteric compared with the uterine vascular bed.

Regulation of the cGMP-cPKG pathway and large-conductance Ca2+-activated K+ channels in uterine arteries during the ovine ovarian cycle

The follicular phase of the ovine ovarian cycle demonstrates parallel increases in ovarian estrogens and uterine blood flow (UBF). Although estrogen and nitric oxide contribute to the rise in UBF, the signaling pathway remains unclear. We examined the relationship between the rise in UBF during the ovarian cycle of nonpregnant sheep and changes in the uterine vascular cGMP-dependent pathway and large-conductance Ca(2+)-activated K(+) channels (BK(Ca)). Nonpregnant ewes (n = 19) were synchronized to either follicular or luteal phase using a vaginal progesterone-releasing device (CIDR), followed by intramuscular PGF(2alpha), CIDR removal, and treatment with pregnant mare serum gonadotropin. UBF was measured with flow probes before tissue collection, and second-generation uterine artery segments were collected from nine follicular and seven luteal phase ewes. The pore-forming alpha- and regulatory beta-subunits that constitute the BK(Ca), soluble guanylyl cyclase (sGC), and cGMP-dependent protein kinase G (cPKG) isoforms (cPKG(1alpha) and cPKG(1beta)) were measured by Western analysis and cGMP levels by RIA. BK(Ca) subunits were localized by immunohistochemistry. UBF rose >3-fold (P < 0.04) in follicular phase ewes, paralleling a 2.3-fold rise in smooth muscle cGMP and 32% increase in cPKG(1alpha) (P < 0.05). sGC, cPKG(1beta), and the BK(Ca) alpha-subunit were unchanged. Notably, expression of beta(1)- and beta(2)-regulatory subunits rose 51 and 79% (P <or= 0.05), respectively. Increases in endogenous ovarian estrogens in follicular-phase ewes result in increases in UBF associated with upregulation of the cGMP- and cPKG-dependent pathway and increased vascular BK(Ca) beta/alpha-subunit stoichiometry, suggesting enhanced BK(Ca) activation contributes to the follicular phase rise in UBF.