Upregulation of eNOS in pregnant ovine uterine arteries by chronic hypoxia

In vivo mitochondria-targeted protection against uterine artery vascular dysfunction and remodelling in rodent hypoxic pregnancy

Gestational hypoxia adversely affects uterine artery function, increasing complications. However, an effective therapy remains unidentified. Here, we show in rodent uterine arteries that hypoxic pregnancy promotes hypertrophic remodelling, increases constrictor reactivity via protein kinase C signalling, and triggers compensatory dilatation via nitric oxide-dependent mechanisms and stimulation of large conductance Ca2+ -activated K+ -channels. Maternal in vivo oral treatment with the mitochondria-targeted antioxidant MitoQ in hypoxic pregnancy normalises uterine artery reactivity and prevents vascular remodelling. From days 6-20 of gestation (term ∼22 days), female Wistar rats were randomly assigned to normoxic or hypoxic (13-14% O2 ) pregnancy ± daily maternal MitoQ treatment (500 µm in drinking water). At 20 days of gestation, maternal, placental and fetal tissue was frozen to determine MitoQ uptake. The uterine arteries were harvested and, in one segment, constrictor and dilator reactivity was determined by wire myography. Another segment was fixed for unbiased stereological analysis of vessel morphology. Maternal administration of MitoQ in both normoxic and hypoxic pregnancy crossed the placenta and was present in all tissues analysed. Hypoxia increased uterine artery constrictor responses to norepinephrine, angiotensin II and the protein kinase C activator, phorbol 12,13-dibutyrate. Hypoxia enhanced dilator reactivity to sodium nitroprusside, the large conductance Ca2+ -activated K+ -channel activator NS1619 and ACh via increased nitric oxide-dependent mechanisms. Uterine arteries from hypoxic pregnancy showed increased wall thickness and MitoQ treatment in hypoxic pregnancy prevented all effects on uterine artery reactivity and remodelling. The data support mitochondria-targeted therapy against adverse changes in uterine artery structure and function in high-risk pregnancy. KEY POINTS: Dysfunction and remodelling of the uterine artery are strongly implicated in many pregnancy complications, including advanced maternal age, maternal hypertension of pregnancy, maternal obesity, gestational diabetes and pregnancy at high altitude. Such complications not only have immediate adverse effects on the growth of the fetus, but also they can also increase the risk of cardiovascular disease in the mother and offspring. Despite this, there is a significant unmet clinical need for therapeutics that treat uterine artery vascular dysfunction in adverse pregnancy. Here, we show in a rodent model of gestational hypoxia that in vivo oral treatment of the mitochondria-targeted antioxidant MitoQ protects against uterine artery vascular dysfunction and remodelling, supporting the use of mitochondria-targeted therapy against adverse changes in uterine artery structure and function in high-risk pregnancy.

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.

Endothelial Dysfunction Driven by Hypoxia-The Influence of Oxygen Deficiency on NO Bioavailability

Cardiovascular diseases (CVDs) are the leading cause of death worldwide. The initial stage of CVDs is characterized by endothelial dysfunction, defined as the limited bioavailability of nitric oxide (NO). Thus, any factors that interfere with the synthesis or metabolism of NO in endothelial cells are involved in CVD pathogenesis. It is well established that hypoxia is both the triggering factor as well as the accompanying factor in cardiovascular disease, and diminished tissue oxygen levels have been reported to influence endothelial NO bioavailability. In endothelial cells, NO is produced by endothelial nitric oxide synthase (eNOS) from L-Arg, with tetrahydrobiopterin (BH₄) as an essential cofactor. Here, we discuss the mechanisms by which hypoxia affects NO bioavailability, including regulation of eNOS expression and activity. What is particularly important is the fact that hypoxia contributes to the depletion of cofactor BH₄ and deficiency of substrate L-Arg, and thus elicits eNOS uncoupling-a state in which the enzyme produces superoxide instead of NO. eNOS uncoupling and the resulting oxidative stress is the major driver of endothelial dysfunction and atherogenesis. Moreover, hypoxia induces impairment in mitochondrial respiration and endothelial cell activation; thus, oxidative stress and inflammation, along with the hypoxic response, contribute to the development of endothelial dysfunction.

High Altitude Reduces NO-Dependent Myometrial Artery Vasodilator Response During Pregnancy

The chronic hypoxia of high-altitude (HA) residence reduces uterine artery blood flow during pregnancy, likely contributing to an increased frequency of preeclampsia and intrauterine growth restriction. We hypothesized that this lesser pregnancy blood flow rise was due, in part, to reduced vasodilation of myometrial arteries (MAs). Here, we assessed MA vasoreactivity in healthy residents of high (2902±39 m) or low altitude (LA; 1669±10 m). MA contractile responses to potassium chloride, phenylephrine, or the thromboxane A2 agonist U46619 did not differ between LA and HA women. Acetylcholine vasodilated phenylephrine or U466119 preconstricted MAs at LA, yet had no effect on HA MAs. In contrast, another vasodilator, bradykinin, relaxed MAs from both altitudes similarly. At LA, the NO synthase inhibitor L-N^G-nitroarginine methyl ester decreased both acetylcholine and bradykinin vasodilation by 56% and 33%, respectively. L-N^G-nitroarginine methyl ester plus the COX (cyclooxygenase) inhibitor indomethacin had similar effects on acetylcholine and bradykinin vasodilation (68% and 42% reduction, respectively) as did removing the endothelium (78% and 50% decrease, respectively), suggesting a predominantly NO-dependent vasodilation at LA. However, at HA, L-N^G-nitroarginine methyl ester did not change bradykinin vasodilation, whereas indomethacin or endothelium removal decreased it by 28% and 72%, respectively, indicating impaired NO signaling at HA. Suggesting that the impairment was downstream of eNOS (endothelial NO synthase), HA attenuated the vasodilation elicited by the NO donor sodium nitroprusside. We concluded that reduced NO-dependent MA vasodilation likely contributes to diminished uteroplacental perfusion in HA pregnancies.

MicroRNAs in Uteroplacental Vascular Dysfunction

Pregnancy complications of preeclampsia and intrauterine growth restriction (IUGR) are major causes of maternal and perinatal/neonatal morbidity and mortality. Although their etiologies remain elusive, it is generally accepted that they are secondary to placental insufficiency conferred by both failure in spiral artery remodeling and uteroplacental vascular malfunction. MicroRNAs (miRNAs) are small no-coding RNA molecules that regulate gene expression at the post-transcriptional level. Increasing evidence suggests that miRNAs participate in virtually all biological processes and are involved in numerous human diseases. Differentially expressed miRNAs in the placenta are typical features of both preeclampsia and IUGR. Dysregulated miRNAs target genes of various signaling pathways in uteroplacental tissues, contributing to the development of both complications. In this review, we provide an overview of how aberrant miRNA expression in preeclampsia and IUGR impacts the expression of genes involved in trophoblast invasion and uteroplacental vascular adaptation.

Mild Hypobaric Hypoxia Enhances Post-exercise Vascular Responses in Young Male Runners

It has been reported that sustained post-exercise vasodilation may be linked to exercise-induced angiogenesis. The present study aimed to evaluate whether mild hypobaric hypoxia enhances the post-exercise reduction in systemic vascular resistance in young male runners. Seven male intercollegiate runners (aged 19–21 years) performed maximal incremental treadmill running under conditions of hypobaric hypoxia (corresponding to 2,200 m above sea level, hereinafter referred to as HH) and normobaric normoxia (corresponding to sea level, hereinafter referred to as NN). A third exercise test was performed under NN conditions, consisting of submaximal exercise with the same absolute exercise volume as was achieved during HH (submaximal exercise under NN conditions, hereinafter referred to as NNsubmax). Blood pressure and cardiac output (CO) were measured before and at 15, 30, and 60 (p60) minutes after exercise. Compared with NN, exercise time was shorter in HH and NNsubmax conditions (p < 0.05). Systolic blood pressure and mean blood pressure (MBP) were lower after exercise in HH conditions (p < 0.05). No condition-related differences were found in CO. Total peripheral resistance (TPR, defined as the ratio of MBP to CO) was significantly lower after exercise compared to baseline for all conditions (p < 0.05). However, the decrease in TPR was maintained longer after exercise in HH compared with NN and NNsubmax conditions (p < 0.05). At p60, TPR was lower than baseline for HH conditions (p < 0.05), whereas after exercise in NN, and NNsubmax conditions, TPR recovered to baseline by p60. Decreases in systemic vascular resistance after exercise were maintained longer under mild HH conditions compared with NN despite the lower exercise volume of the former.

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.

Maternal supraphysiological hypercholesterolemia associates with endothelial dysfunction of the placental microvasculature

Maternal physiological or supraphysiological hypercholesterolemia (MPH, MSPH) occurs during pregnancy. MSPH is associated with foetal endothelial dysfunction and atherosclerosis. However, the potential effects of MSPH on placental microvasculature are unknown. The aim of this study was to determine whether MSPH alters endothelial function in the placental microvasculature both ex vivo in venules and arterioles from the placental villi and in vitro in primary cultures of placental microvascular endothelial cells (hPMEC). Total cholesterol < 280 mg/dL indicated MPH, and total cholesterol ≥280 mg/dL indicated MSPH. The maximal relaxation to histamine, calcitonin gene-related peptide and adenosine was reduced in MSPH venule and arteriole rings. In hPMEC from MSPH placentas, nitric oxide synthase (NOS) activity and L-arginine transport were reduced without changes in arginase activity or the protein levels of endothelial NOS (eNOS), human cationic amino acid 1 (hCAT-1), hCAT-2A/B or arginase II compared with hPMEC from MPH placentas. In addition, it was shown that adenosine acts as a vasodilator of the placental microvasculature and that NOS is active in hPMEC. We conclude that MSPH alters placental microvascular endothelial function via a NOS/L-arginine imbalance. This work also reinforces the concept that placental endothelial cells from the macro- and microvasculature respond differentially to the same pathological condition.

Oxidative Stress during Ovarian Torsion in Pediatric and Adolescent Patients: Changing The Perspective of The Disease

Among the different causes of gynecological acute pelvic pain, ovarian torsion represents a surgical emergency. It is a rare case in the pediatric/adolescent aged group that must be included in the differential diagnosis of any girl with abdominal pain or pelvic/abdominal mass. Current recommendations suggest that laparoscopic detorsion should be performed in order to preserve the integrity of the ovaries and fertility, although oophoropexy may be considered in case of severe necrosis. Nevertheless, maintaining the circulation of the ovary after detorsion deteriorates the tissue injury and leads to a pathologic process called ischaemia/reperfusion (I/R) injury, which is characterized by oxidative stress. During the detorsion process, an excess amount of molecular oxygen is supplied to the tissues, and reactive species of oxygen (ROS) such as superoxide radical (O₂^-), hydrogen peroxide (H₂O₂), hydroxyl radical (OH•), as well as reactive nitrogen species (RNS) are produced in excess. ROS, RNS and their toxic products cause DNA damage and lipid peroxidation in the cellular and mitochondrial membranes, leading to cell death. In spite of attention on this topic, currently there is no shared and clear evidence about the use of anti-inflammatory and antioxidant agents to prevent I/R damage after laparoscopic ovarian detorsion. Considering this element, future research should aim to develop shared protocols for the clinical use (route of application, dosage and time of application) of antioxidants after laparoscopic management of this condition.

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.

Crosstalk between oxygen- and nitric oxide-dependent signaling pathways in angiogenesis

With every heart beat blood rushes through a complex network of tubes to deliver essential ingredients of life, oxygen and nutrients. Consequently, this network of blood vessels is an indispensable part of vertebrate physiology. Its organization and architecture is highly dynamic in its form and function. Understanding how blood vessels develop, a process referred to as angiogenesis, is equally important as to know how they function considering that failure or misalignment of this process results in disorder and disease, in many cases of which death is inevitable. Much has been learned about the angiogenic process and the critical contributors of blood vessel function. A central determinant is oxygen, an evident contributor given the fact that oxygen delivery is a primary feature of blood vessel function. Not only is oxygen however essential for mitochondrial energy production, it also serves as a key molecule in various biochemical reactions, such as the formation of nitric oxide (NO), on its part a critical regulator of vascular tone and vessel homeostasis. Hence, oxygen abundance relates to the production of NO, and NO in turn regulates oxygen delivery and consumption. Given the importance of the intrinsic link these two molecules exert on angiogenesis and vessel function; this review shall highlight our current understanding on how these two molecules cooperate to form blood vessels.

The effects of oxidative stress on female reproduction: a review

Oxidative stress (OS), a state characterized by an imbalance between pro-oxidant molecules including reactive oxygen and nitrogen species, and antioxidant defenses, has been identified to play a key role in the pathogenesis of subfertility in both males and females. The adverse effects of OS on sperm quality and functions have been well documented. In females, on the other hand, the impact of OS on oocytes and reproductive functions remains unclear. This imbalance between pro-oxidants and antioxidants can lead to a number of reproductive diseases such as endometriosis, polycystic ovary syndrome (PCOS), and unexplained infertility. Pregnancy complications such as spontaneous abortion, recurrent pregnancy loss, and preeclampsia, can also develop in response to OS. Studies have shown that extremes of body weight and lifestyle factors such as cigarette smoking, alcohol use, and recreational drug use can promote excess free radical production, which could affect fertility. Exposures to environmental pollutants are of increasing concern, as they too have been found to trigger oxidative states, possibly contributing to female infertility. This article will review the currently available literature on the roles of reactive species and OS in both normal and abnormal reproductive physiological processes. Antioxidant supplementation may be effective in controlling the production of ROS and continues to be explored as a potential strategy to overcome reproductive disorders associated with infertility. However, investigations conducted to date have been through animal or in vitro studies, which have produced largely conflicting results. The impact of OS on assisted reproductive techniques (ART) will be addressed, in addition to the possible benefits of antioxidant supplementation of ART culture media to increase the likelihood for ART success. Future randomized controlled clinical trials on humans are necessary to elucidate the precise mechanisms through which OS affects female reproductive abilities, and will facilitate further explorations of the possible benefits of antioxidants to treat infertility.

eNOS activation and NO function: differential control of steroidogenesis by nitric oxide and its adaptation with hypoxia

Nitric oxide (NO) plays a role in a wide range of physiological processes. Aside from its widely studied function in the regulation of vascular function, NO has been shown to impact steroidogenesis in a number of different tissues. The goal of this review is to explore the effects of NO on steroid production and further, to discern its source(s) and mechanism of action. Attention will be given to the regulation of NO synthases in specific endocrine tissues including ovaries, testes, and adrenal glands. The effects of hypoxia on generation of NO and subsequent effects on steroid biosynthesis will also be examined. Finally, a potential model for the interaction of hypoxia on NO synthesis and steroid production is proposed.

Remodeling of fetoplacental arteries in rats due to chronic hypoxia

The increased fetoplacental vascular resistance due to chronic hypoxia cannot be explained by simple hypoxic vasoconstriction, as it sustains to some degree after recovery in normobaric environment. To verify a hypothesis that fetoplacental arteries undergo remodeling of their walls similar to remodeling of pulmonary arteries in hypoxic pulmonary hypertension, we used a model of the chronically hypoxic rat placenta. Han Wistar pregnant rats were exposed to 14-day hypoxia (10% of oxygen) during the 6th to 19th day of pregnancy. Chronic hypoxia elicited in both intraplacental (prelabyrinthine) and chorionic plate (insertion) arteries significant narrowing of their lumina. Irregular thickening of their adventitia due to an increase in collagen fibers as well as ground substance was observed; reticular fibers were fragmented. Because of remodeling of fetoplacental arteries, a model of chronically hypoxic rat placenta could simulate human preplacental hypoxia and consequent effects.

Role of KATP and L-type Ca2+ channel activities in regulation of ovine uterine vascular contractility: effect of pregnancy and chronic hypoxia

OBJECTIVE

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.

STUDY DESIGN

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.

RESULTS

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).

CONCLUSION

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.

Nitric oxide inhibits ACTH-induced cortisol production in near-term, long-term hypoxic ovine fetal adrenocortical cells

We previously reported that in the sheep fetus, long-term hypoxia (LTH) resulted in elevated basal plasma adrenocorticotropic hormone (ACTH(1- 39)) whereas the cortisol levels were not different from normoxic controls. We also showed that LTH enhances endothelial nitric oxide synthase (eNOS) expression in the fetal adrenal. This study was designed to determine the effect of NO on cortisol production in adrenocortical cells from LTH fetal sheep. Ewes were maintained at high altitude (3820 m) from ∼40 days' gestation (dG) to near term. Between 138 and 141 dG, fetal adrenal glands were collected from LTH and age-matched normoxic control fetuses. Adrenal cortical cells were pretreated with sodium nitroprusside (SNP), nitro-L-arginine methyl ester (L-NAME), L-arginine, or diethyleneamine NO (DETA-NO) and then challenged with 10 nmol/L ACTH. Cortisol responses were compared after 1 hour. Adrenocorticotropic hormone -induced cortisol secretion was significantly higher in LTH versus control (P < .01). Enhancement of NO with L-arginine resulted in a significant reduction of ACTH-mediated cortisol production in the LTH group. DETA-NO also caused a significant decrease in ACTH-mediated cortisol production (P < .05). Inhibition of NOS with L-NAME significantly increased cortisol production in the LTH group (P < .05 compared to ACTH alone), whereas the effect on the control group was not significant. Nitric oxide synthase activity was significantly higher in the LTH group compared to control, but this difference was eliminated following ACTH treatment. These data indicate that LTH enhances adrenal cortical sensitivity to the inhibitory effects of NO on cortisol production. Nitric oxide may, therefore, play an important role in regulating ACTH-induced cortisol production in the LTH fetal adrenal.

Changes of renal AT1/AT2 receptors and structures in ovine fetuses following exposure to long-term hypoxia

BACKGROUND/AIMS

The present study tested the hypothesis that chronic hypoxia adversely affects renal development in the ovine fetus.

METHODS

Kidneys were collected from near-term fetuses of pregnant ewes maintained at sea level or high altitude (3,801 m, PaO(2): approx. 60 mm Hg) for 110 days (n = 6 for each group).

RESULTS

Long-term high altitude hypoxia reduced the fetal kidney/body weight ratio. Histological analysis showed a significant enlargement in the Bowman's space and swelling of tubule epithelial cells in the kidney of the hypoxic fetus. The histological alterations were limited to the cortical, but not medullary, zone. These alterations were associated with an increase in serum creatinine and a decrease in the BUN-to-creatinine ratio in hypoxic fetuses. Angiotensin II receptors (AT(1)R and AT(2)R) were detected in the glomerular and tubular regions of the kidney. Chronic hypoxia caused a significant increase in AT(1)R and a decrease in AT(2)R protein and mRNA abundance, resulting in a large increase in the AT(1)R/AT(2)R ratio in the fetal kidney.

CONCLUSION

The results demonstrate an adverse effect of chronic hypoxia on renal AT(1)R and AT(2)R expression and functions in the fetus, suggesting a possible role of fetal hypoxia in the programming of renal diseases in fetal origins.

Direct chronic effect of steroid hormones in attenuating uterine arterial myogenic tone: role of protein kinase c/extracellular signal-regulated kinase 1/2

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 17beta-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. 17beta-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, alpha, beta(I), beta(II), delta, epsilon, and zeta, in the uterine arteries. 17beta-Estradiol and progesterone decreased the particulate:cytosolic ratios of PKCalpha, epsilon, and zeta, 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.

Long-term hypoxia increases endothelial nitric oxide synthase expression in the ovine fetal adrenal

This study was designed to test the hypothesis that fetal adrenal nitric oxide synthase (NOS) is elevated in response to long-term hypoxia (LTH). Pregnant ewes were maintained at high altitude (3820 m) for approximately the last 100 days of gestation. Between days 138 and 141 of gestation, adrenal glands were collected from LTH fetuses and age-matched normoxic controls. Quantitative real-time polymerase chain reaction (qRT-PCR) and Western analysis were used to quantify NOS expression, and NOS distribution was examined by immunohistochemistry and double-staining immunofluorescence for endothelial NOS (eNOS) and 17alpha-hydroxylase (CYP17). Neuronal NOS (nNOS) was expressed at very low levels and with no differences between groups. Expression of eNOS was significantly greater in the LTH group compared with control. Neuronal NOS was distributed throughout the cortex while the greatest density of eNOS was observed in the zona fasciculata/reticularis area and eNOS co-localized with CYP17. We conclude that LTH enhances eNOS expression in the inner adrenal cortex which may play a role in regulation of cortisol biosynthesis in the LTH fetus.

No evidence for an enhanced role of endothelial nitric oxide in the maintenance of arterial blood pressure in the IUGR sheep fetus

The fetus makes a number of physiological adaptations to a restriction of placental substrate supply, including a decrease in body growth and an increase in peripheral vasoconstriction which maintains mean arterial pressure (MAP) and supports a redistribution of cardiac output to key fetal organs. It is not known, however, whether chronic restriction of placental substrate supply results in an enhanced or diminished role for vasodilators such as endothelial nitric oxide in the regulation of MAP. We hypothesised that there is an increased contribution of NO to blood pressure regulation in growth restricted fetuses and that a 2h infusion of a nitric oxide synthase inhibitor, N(omega)-nitro-l-arginine methyl ester (l-NAME) would result in an augmented rise in MAP in chronically hypoxemic, placentally restricted (PR, n=8) fetuses compared to controls (n=6) in late gestation. There was no difference in the increase in fetal MAP and decrease in HR during l-NAME infusion between Control and PR fetuses. In the PR group, fetuses with lower mean gestational PaO(2) had a lower increase in MAP during l-NAME infusion. Thus we have found no evidence for an enhanced role of NO in the maintenance of MAP in the chronically hypoxemic IUGR fetus.

Effect of long-term high-altitude hypoxia on fetal pulmonary vascular contractility

Hypoxia in the fetus and/or newborn is associated with an increased risk of pulmonary hypertension. The present study tested the hypothesis that long-term high-altitude hypoxemia differentially regulates contractility of fetal pulmonary arteries (PA) and veins (PV) mediated by differences in endothelial NO synthase (eNOS). PA and PV were isolated from near-term fetuses of pregnant ewes maintained at sea level (300 m) or high altitude of 3,801 m for 110 days (arterial Po(2) of 60 Torr). Hypoxia had no effect on the medial wall thickness of pulmonary vessels and did not alter KCl-induced contractions. In PA, hypoxia significantly increased norepinephrine (NE)-induced contractions, which were not affected by eNOS inhibitor N(G)-nitro-l-arginine (l-NNA). In PV, hypoxia had no effect on NE-induced contractions in the absence of l-NNA. l-NNA significantly increased NE-induced contractions in both control and hypoxic PV. In the presence of l-NNA, NE-induced contractions of PV were significantly decreased in hypoxic lambs compared with normoxic animals. Acetylcholine caused relaxations of PV but not PA, and hypoxia significantly decreased both pD(2) and the maximal response of acetylcholine-induced relaxation in PV. Additionally, hypoxia significantly decreased the maximal response of sodium nitroprusside-induced relaxations of both PA and PV. eNOS was detected in the endothelium of both PA and PV, and eNOS protein levels were significantly higher in PV than in PA in normoxic lambs. Hypoxia had no significant effect on eNOS levels in either PA or PV. The results demonstrate heterogeneity of fetal pulmonary arteries and veins in response to long-term high-altitude hypoxia and suggest a likely common mechanism downstream of NO in fetal pulmonary vessel response to chronic hypoxia in utero.

Diminished NO release in chronic hypoxic human endothelial cells

The present study addressed whether chronic hypoxia is associated with reduced nitric oxide (NO) release due to decreased activation of endothelial NO synthase (eNOS). Primary cultures of endothelial cells from human umbilical veins (HUVECs) were used and exposed to different oxygen levels for 24 h, after which NO release, intracellular calcium, and eNOS activity and phosphorylation were measured after 24 h. Direct measurements using a NO microsensor showed that in contrast to 1-h exposure to 5% and 1% oxygen (acute hypoxia), histamine-evoked (10 μM) NO release from endothelial cells exposed to 5% and 1% oxygen for 24 h (chronic hypoxia) was reduced by, respectively, 58% and 40%. Furthermore, chronic hypoxia also lowered the amount and activity of eNOS enzyme. The decrease in activity could be accounted for by reduced intracellular calcium and altered eNOS phosphorylation. eNOS Ser1177 and eNOS Thr495 phosphorylations were reduced and increased, respectively, consistent with lowered enzyme activity. Akt kinase, which can phosphorylate eNOS Ser1177, was also decreased by hypoxia, regarding both total protein content and the phosphorylated (active) form. Moreover, the protein content of β- actin, which is known to influence the activity of eNOS, was almost halved by hypoxia, further supporting the fall in eNOS activity. In conclusion, chronic hypoxia in HUVECs reduces histamine-induced NO release as well as eNOS expression and activity. The decreased activity is most likely due to changed eNOS phosphorylation, which is supported by decreases in Akt expression and phosphorylation. By reducing NO, chronic hypoxia may accentuate endothelial dysfunction in cardiovascular disease.

Altered Penile Vascular Reactivity and Erection in the Zucker Obese-Diabetic Rat

INTRODUCTION

The combination of the independent risk factors for erectile dysfunction, obesity, hypertension, and diabetes are manifested collectively in a condition known as metabolic syndrome X. However, the exact mechanism(s) by which the combination of these factors contributes to erectile dysfunction have yet to be elucidated.

AIM

We hypothesized that protein kinase C (PKC) and Rho-kinase enhanced vascular tone and thus contributed to erectile dysfunction in this condition.

METHODS

Erectile function was evaluated by recording voltage-dependent increases in intracavernosal pressure following stimulation of the cavernosal nerve in 16- to 20-week-old lean and obese-diabetic Zucker rats. Cavernosal tissue contractile and relaxation responses were evaluated in vitro when contracted with phenylephrine, endothelin-1 and relaxed by Rho-kinase, PKC inhibitors or sodium nitroprusside. Additionally, cavernosal tissue Rho-kinase, protein kinase, and nitric oxide synthase isoform expression were evaluated by Western blot.

RESULTS

The voltage-dependent erectile responses were suppressed by >30% in the obese-diabetic Zucker rat. The maximal stress generated by cavernosal tissue from the obese-diabetic was significantly greater than the lean response by greater than 0.8 mN/mm(2) for both phenylephrine and endothelin stimulation. The PKC inhibitor, chelerythrine, inhibited more than 30% of the phenylephrine-induced and 70% of the endothelin-1-induced contractions. Rho-kinase inhibition, with either Y-27632 or HA-1077, revealed impaired relaxations of nearly 30% in tissue from obese-diabetic animals. Western blot analysis revealed increased protein expression of PKC alpha and delta and Rho-kinase alpha and beta but no loss for endothelial or neuronal nitric oxide synthase.

CONCLUSIONS

In this rodent model both PKC and Rho-kinase signaling elements may contribute to an enhanced vasoconstriction state of the penile smooth muscle that was differentially dependent on the agonist used. The enhanced vasoconstrictive state of the tissue could contribute to the reduced voltage-dependent erectile response in the obese-diabetic Zucker rat.

Chronic hypoxia in vivo reduces placental oxidative stress

Decreased placental oxygenation and increased oxidative stress are implicated in the development of preeclampsia. Oxidative stress arises from imbalance between pro-versus anti-oxidants and can lead to biological oxidation and apoptosis. Because pregnant women living at high altitude (3100 m, HA) have lowered arterial PO2 and an increased incidence of preeclampsia, we hypothesized that HA placentas would have decreased anti-oxidant enzyme activity, increased oxidative stress (lipid peroxidation, protein oxidation and nitration) and greater trophoblast apoptosis than low-altitude (LA) placentas. We measured enzymatic activities, lipid and protein oxidation and co-factor concentrations by spectrophotometric techniques and ELISA in 12 LA and 18 HA placentas. Immunohistochemistry (IHC) was used to evaluate nitrated proteins and specific markers of apoptosis (activated caspase 3 and M30). Superoxide dismutase activity was marginally lower (p=0.05), while glutathione peroxidase activity (p<0.05), thioredoxin concentrations (p<0.005) and thioredoxin reductase activity p<0.01 were all reduced in HA placentas. Decreased anti-oxidant activity was not associated with increased oxidative stress: lipid peroxide content and protein carbonyl formation were lower at HA (p<0.01). We found greater nitrotyrosine residues in the syncytiotrophoblast at 3100 m (p<0.05), but apoptosis did not differ between altitudes. Our data suggest that hypoxia does not increase placental oxidative stress in vivo. Nitrative stress may be a consequence of hypoxia but does not appear to contribute to increased apoptosis. Lowered placental concentrations of anti-oxidants may contribute to the susceptibility of women living at HA to the development of preeclampsia, but are unlikely to be etiological.

Hypoxia-induced endothelial NO synthase gene transcriptional activation is mediated through the tax-responsive element in endothelial cells

Although hypoxia is known to induce upregulation of endothelial NO synthase (eNOS) gene expression, the underlying mechanism is largely unclear. In this study, we show that hypoxia increases eNOS gene expression through the binding of phosphorylated cAMP-responsive element binding (CREB) protein (pCREB) to the eNOS gene promoter. Hypoxia (1% O2) increased both eNOS expression and NO production, peaking at 24 hours, in bovine aortic endothelial cells, and these increases were accompanied by increases in pCREB. Treatment with the protein kinase A inhibitor H-89 or transfection with dominant-negative inhibitor of CREB reversed the hypoxia-induced increases in eNOS expression and NO production, with concomitant inhibition of the phosphorylation of CREB induced by hypoxia, suggesting an involvement of protein kinase A/pCREB-mediated pathway. To map the regulatory elements of the eNOS gene responsible for pCREB binding under hypoxia, we constructed an eNOS gene promoter (-1600 to +22 nucleotides) fused with a luciferase reporter gene [pGL2-eNOS(-1600)]. Hypoxia (for 24-hour incubation) increased the promoter activity by 2.36+/-0.18-fold in the bovine aortic endothelial cells transfected with pGL2-eNOS(-1600). However, progressive 5'-deletion from -1600 to -873 completely attenuated the hypoxia-induced increase in promoter activity. Electrophoretic mobility shift, anti-pCREB antibody supershift, and site-specific mutation analyses showed that pCREB is bound to the Tax-responsive element (TRE) site, a cAMP-responsive element-like site, located at -924 to -921 of the eNOS promoter. Our data demonstrate that the interaction between pCREB and the Tax-responsive element site within the eNOS promoter may represent a novel mechanism for the mediation of hypoxia-stimulated eNOS gene expression.

Preserved placental oxygenation and development during severe systemic hypoxia

Local tissue oxygenation profoundly influences placental development. To elucidate the impact of hypoxia on cellular and molecular adaptation in vivo, pregnant mice at embryonic days 7.5–11.5 were exposed to reduced environmental oxygen (6–7% O2) for various periods of time. Hypoxia-inducible factor (HIF)-1α mRNA was highly expressed in the placenta, whereas HIF-2α was predominantly found in the decidua, indicating that HIF-1 is a relevant oxygen-dependent factor involved in placental development. During severe hypoxia, HIF-1α protein was strongly induced in the periphery but, however, not in the labyrinth layer of the placenta. Accordingly, no indication for tissue hypoxia in this central area was detected with 2-(2-nitro-1 H-imidazol-1-yl)- N-(2,2,3,3,3-pentafluoropropyl)acetamide staining and VEGF expression as hypoxic markers. The absence of significant tissue hypoxia was reflected by preserved placental architecture and trophoblast differentiation. In the search for mechanisms preventing local hypoxia, we found upregulation of endothelial nitric oxide synthase (NOS) expression in the labyrinth layer. Inhibition of NOS activity by Nω-nitro-l-arginine methyl ester application resulted in ubiquitous placental tissue hypoxia. Our results show that placental oxygenation is preserved even during severe systemic hypoxia and imply that NOS-mediated mechanisms are involved to protect the placenta from maternal hypoxia.

Molecular cloning of ovine endothelial nitric oxide synthase and expression in COS-7 cells

While studies of human and bovine endothelial nitric oxide synthase (eNOS) demonstrate activation by Ca(2+)/calmodulin, recent progress demonstrates that eNOS phosphorylation can alter sensitivity to intracellular free calcium ([Ca(2+)](i)). The sheep, however, is widely used as a model for cardiovascular adaptation to pregnancy and ovine uterine artery endothelial cell (UAEC) eNOS undergoes pregnancy-specific (P) enhancement of activity associated with increased Ca(2+) and protein kinase signaling in response to a number of agonists, including adenosine triphosphate (ATP). The degree of homology between the ovine and human full-length cDNAs was not previously known and yet is necessary to determine the validity in using an ovine model to study human physiology. The objectives of this study were to isolate and validate the clone of ovine eNOS cDNA and investigate ovine eNOS activation when expressed in COS-7 cells. The ovine eNOS cDNA has high homology to published human and bovine sequences and shares identity with the bovine amino acid sequence. When ovine eNOS was transiently expressed in COS-7 cells (COS-7/oeNOS), A23187 increased specific catalytic activity in a dose- and time-dependent manner. A23187-stimulated activation of eNOS was, however, also accompanied by phosphorylation of eNOS S1179 and dephosphorylation of T497, demonstrating that an increase in [Ca(2+)](i) may not be the sole mechanism of activation. The physiologic relevance of this was further underscored by the finding that ATP dose-dependently increased peak [Ca(2+)](i) and eNOS activity in COS-7/oeNOS, but also increased eNOS p-S1179 and decreased p-T497. This finding was similar to those in ovine P-UAEC treated with the Ca(2+)-mobilizing agonist ATP, wherein activation of eNOS was again concomitant with a rise p-S1179 as well as a slight decrease in p-T497. In conclusion, we describe the full-length ovine eNOS cDNA sequence and show that both physiologic and nonphysiologic calcium-mobilizing agents, which activate ovine eNOS in COS-7 and P-UAEC, do so in association with changes in eNOS phosphorylation. Given this information we can now begin to dissect the relationship between Ca(2+) elevation and specific phosphorylation events in eNOS activation in the ovine model, and thereby gain insight into the possible basis for pregnancy-related dysfunction.

Heart mitochondrial nitric oxide synthase is upregulated in male rats exposed to high altitude (4,340 m)

Male rats exposed for 21 days to high altitude (4,340 m) responded with arrest of weight gain and increased hematocrit and testosterone levels. High altitude significantly (58%) increased heart mitochondrial nitric oxide (NO) synthase (mtNOS) activity, whereas heart cytosolic endothelial NOS (eNOS) and liver mtNOS were not affected. Western blot analysis found heart mitochondria reacting only with anti-inducible NOS (iNOS) antibodies, whereas the postmitochondrial fraction reacted with anti-iNOS and anti-eNOS antibodies. In vitro-measured NOS activities allowed the estimation of cardiomyocyte capacity for NO production, a value that increased from 57% (sea level) to 79 nmol NO.min(-1).g heart(-1) (4,340 m). The contribution of mtNOS to total cell NO production increased from 62% (sea level) to 71% (4340 m). Heart mtNOS activity showed a linear relationship with hematocrit and a biphasic quadratic association with estradiol and testosterone. Multivariate analysis showed that exposure to high altitude linearly associates with hematocrit and heart mtNOS activity, and that testosterone-to-estradiol ratio and heart weight were not linearly associated with mtNOS activity. We conclude that high altitude triggers a physiological adaptive response that upregulates heart mtNOS activity and is associated in an opposed manner with the serum levels of testosterone and estradiol.

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

We previously demonstrated that cortisol regulated α1-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 Po2: 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 11β-hydroxysteroid dehydrogenase inhibitor carbenoxolone did not change cortisol effects in nonpregnant uterine arteries, but abolished it in pregnant uterine arteries by increasing norepinephrine pD2(–log EC50) in control tissues. The dissociation constant of norepinephrine-α1-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 Ca2+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 α1-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.

Calcium homeostasis and contraction of the uterine artery: effect of pregnancy and chronic hypoxia

The present study tested the hypothesis that chronic hypoxia alters pregnancy-mediated adaptation of Ca2+ homeostasis and contractility in the uterine artery. Uterine arteries were isolated from nonpregnant and near-term pregnant ewes of normoxic control or high-altitude (3820 m) hypoxic (oxygen pressure in the blood [PaO2], 60 mm Hg) treatment for 110 days. Contractions and intracellular-free Ca2+ concentration ([Ca2+]i) were measured simultaneously in the same tissue. In normoxic animals, pregnancy increased norepinephrine (NE), but not 5-hydroxy-thymide (5-HT) or KCl, contractile sensitivity in the uterine artery. Chronic hypoxia significantly attenuated NE-induced contractions in the pregnant, but not nonpregnant, uterine arteries. Similarly, 5-HT-mediated contractions of nonpregnant arteries were not changed. In the pregnant uterine artery, chronic hypoxia significantly increased NE-mediated Ca2+ mobilization, but decreased the Ca2+ sensitivity. In addition, hypoxia increased the calcium ionophore A23187-induced relaxation in pregnant, but not nonpregnant, uterine arteries. However, the A23187-mediated reduction of [Ca2+]i was significantly impaired in hypoxic arteries. In contrast, hypoxia significantly increased the slope of the [Ca2+]i-tension relationship of A23187-induced reductions in [Ca2+]i and tension in the pregnant uterine artery. The results suggest that the contractility of nonpregnant uterine artery is insensitive to moderate chronic hypoxia, but the adaptation of sympathetic tone that normally occurs in the uterine artery during pregnancy is inhibited by chronic hypoxia. In addition, changes in Ca2+ sensitivity of myofilaments play a predominant role in the adaptation of uterine artery contractility to pregnancy and chronic hypoxia.

Effects of chronic hypoxia on maternal vasodilation and vascular reactivity in guinea pig and ovine pregnancy

During pregnancy, exposure to chronic hypoxia is thought to be associated with an increased risk of preeclampsia and fetal intrauterine growth restriction (IUGR). While some studies suggest that this process may be mediated through effects of chronic hypoxia on uterine artery vasodilation and growth, these observations are likely to be species specific and may represent genetic variability in maternal adaptation to hypoxia. This review is a comparative analysis of the effects of chronic hypoxia on vascular reactivity in pregnant and nonpregnant guinea pig and sheep. Data suggest that exposure to chronic hypoxia is associated with enhanced uterine artery blood flow in the sheep, whereas, in the guinea pig, blood flow is decreased.

Increased nitric oxide production following chronic hypoxia contributes to attenuated systemic vasoconstriction

Attenuated vasoconstrictor reactivity following chronic hypoxia (CH) is associated with endothelium-dependent vascular smooth muscle (VSM) cell hyperpolarization and diminished intracellular [Ca(2+)]. We tested the hypothesis that increased production of nitric oxide (NO) after CH contributes to blunted vasoconstrictor responsiveness. We found that basal NO production of mesenteric arteries from CH rats (barometric pressure = 380 Torr; 48 h) was greater than that of controls (barometric pressure = 630 Torr). In addition, studies employing pressurized mesenteric arteries (100-200 microM ID) abluminally loaded with the Ca(2+) indicator fura 2-AM demonstrated that although NO synthase (NOS) inhibition normalized agonist-induced vasoconstrictor responses between groups, VSM cell [Ca(2+)] in vessels from CH rats remained diminished compared with controls. To determine whether elevated NO production following CH results from increased NOS protein levels, we performed Western blots for NOS isoforms by using mesenteric arteries from control and CH rats. Endothelial NOS levels did not differ between groups, and other NOS isoforms were not detected in these samples. Selective endothelial loading of fura 2-AM was employed to test the hypothesis that elevated endothelial cell [Ca(2+)] following CH accounts for enhanced NOS activity. These experiments demonstrated greater endothelial cell [Ca(2+)] in mesenteric arteries isolated from CH rats compared with controls. We conclude that enhanced production of NO resulting from elevated endothelial cell [Ca(2+)] contributes to attenuated reactivity following CH by decreasing VSM cell Ca(2+) sensitivity.

Chronic hypoxia opposes pregnancy-induced increase in uterine artery vasodilator response to flow

We tested the hypotheses that pregnancy increases the uterine artery (UA) vasodilator response to flow and that this increase is impaired under conditions of chronic hypoxia (30 days, simulated elevation 3,960 m). UA were isolated from 24 normoxic or chronically hypoxic midpregnant guinea pigs and studied with the use of pressure myography. Normoxic pregnancy increased UA flow vasodilator response and protected against a rise in wall shear stress (WSS). Chronic hypoxia opposed these effects, prompting vasoconstriction at high flow and increasing WSS above levels seen in normoxic pregnant UA. The nitric oxide synthase inhibitor N(G)-nitro-l-arginine (l-NNA) eliminated the pregnancy-associated increase in flow vasodilation in normoxic UA, suggesting that increased nitric oxide production was responsible. The considerable residual vasodilation after nitric oxide synthase and cyclooxygenase inhibition implicated endothelial-derived hyperpolarizing factor (EDHF) as an additional contributor to flow vasodilation. l-NNA increased flow vasodilation in UA from chronically hypoxic animals, suggesting that chronic hypoxia may have lowered EDHF or elevated peroxynitrite production. In conclusion, flow is an important physiological vasodilator for the acute and more chronic UA dimensional changes required to increase uteroplacental blood flow during normal pregnancy. Chronic hypoxia may be a mechanism that opposes the pregnancy-associated rise in UA flow vasodilation, thereby increasing the incidence of preeclampsia and intrauterine growth restriction at a high altitude.

Possible mechanisms underlying pregnancy-induced changes in uterine artery endothelial function

The last 10 years has seen a dramatic increase in our understanding of the mechanisms underlying the pregnancy-specific adaptation in cardiovascular function in general and the dramatic changes that occur in uterine artery endothelium in particular to support the growing fetus. The importance of these changes is clear from a number of studies linking restriction of uterine blood flow (UBF) and/or endothelial dysfunction and clinical conditions such as intrauterine growth retardation (IUGR) and/or preeclampsia in both humans and animal models; these topics are covered only briefly here. The recent developments that prompts this review are twofold. The first is advances in an understanding of the cell signaling processes that regulate endothelial nitric oxide synthase (eNOS) in particular (Govers R and Rabelink TJ. Am J Physiol Renal Physiol 280: F193-F206, 2001). The second is the emerging picture that uterine artery (UA) endothelial cell production of nitric oxide (NO) as well as prostacyclin (PGI2) may be as much a consequence of cellular reprogramming at the level of cell signaling as due to tonic stimuli inducing changes in the level of expression of eNOS or the enzymes of the PGI2 biosynthetic pathway (cPLA2, COX-1, PGIS). In reviewing just how we came to this conclusion and outlining the implications of such a finding, we draw mostly on data from ovine or human studies, with reference to other species only where directly relevant.

Flow-mediated dilatation of the brachial artery in pregnancy at high altitude

OBJECTIVE

Pregnancy at high altitude has been associated with increased prevalence of pre-eclampsia and reduced maternal oestrogen levels, factors that have been associated with endothelial dysfunction. The aim of this study was to examine the effect of high altitude (4370 m above sea level) on endothelial function during pregnancy as assessed by a non-invasive method.

DESIGN

Cross-sectional study.

SETTING

Two maternity units providing routine antenatal care: one at high altitude (District General Hospital--IPSS in Cerro de Pasco, Peru) and one at sea level (Instituto Materno-Perinatal in Lima, Peru).

POPULATION

Sixty pregnant women at 6-42 weeks of gestation resident at high altitude (Cerro de Pasco, Peru, 4370 m above sea level) and 54 at sea level (Lima, Peru). Comparisons were performed also in 11 and 14 non-pregnant women at each altitude, respectively.

METHODS

Endothelial function was assessed by flow-mediated dilatation of the brachial artery using high-resolution ultrasound.

MAIN OUTCOME MEASURES

Differences in flow mediated dilatation of the brachial artery in two groups of pregnant women, one at high altitude and one at sea level.

RESULTS

Both at high altitude and sea level flow-mediated dilatation of the brachial artery increased in the first two trimesters to levels 32% higher than non-pregnant controls. However, in the third trimester, flow-mediated dilatation of the brachial artery was lower than non-pregnant levels. Resting vessel size increased during pregnancy by 15% compared with non-pregnant controls at term, with no difference between the two populations at high and low altitude. Pregnancy at high altitude, compared with sea level, was associated with 59% lower baseline blood flow and 76% higher reactive hyperaemia. Similarly, non-pregnant controls at high altitude compared with sea level demonstrated similar flow-mediated dilatation of the brachial artery and 40% lower resting blood flow of the brachial artery. However, the difference in reactive hyperaemia did not reach statistical significance.

CONCLUSION

These data suggest that, during pregnancy at high altitude, endothelial function, as assessed by flow-mediated dilatation of the brachial artery, is not impaired.

Cortisol-mediated potentiation of uterine artery contractility: effect of pregnancy

During pregnancy, maternal plasma cortisol concentrations approximately double. Given that cortisol plays an important role in the regulation of vascular reactivity, the present study investigated the potential role of cortisol in potentiation of uterine artery (UA) contractility and tested the hypothesis that pregnancy downregulated the cortisol-mediated potentiation. In vitro cortisol treatment (3, 10, or 30 ng/ml for 24 h) produced a dose-dependent increase in norepinephrine (NE)-induced contractions in both nonpregnant and pregnant (138-143 days gestation) sheep UA. However, this cortisol-mediated response was significantly attenuated by approximately 50% in pregnant UA. The 11 beta-hydroxysteroid dehydrogenase (11-beta HSD) inhibitor carbenoxolone did not change the effect of cortisol in nonpregnant UA but abolished its effect in pregnant UA by increasing the NE pD(2) in control tissues from 6.20 +/- 0.05 to 6.59 +/- 0.11. The apparent dissociation constant value of NE alpha(1)-adrenoceptors was not changed by cortisol in pregnant UA but was decreased in nonpregnant UA. There was no difference in glucocorticoid receptor density between nonpregnant and pregnant UA. Cortisol significantly decreased endothelial nitric oxide (NO) synthase protein levels and NO release in both nonpregnant and pregnant UA, but the effect of cortisol was attenuated in pregnant UA by approximately 50%. Carbenoxolone alone had no effects on NO release in nonpregnant UA but was decreased in pregnant UA. These results suggest that cortisol potentiates NE-mediated contractions by decreasing NO release and increasing NE-binding affinity to alpha(1)-adrenoceptors in nonpregnant UA. Pregnancy attenuates UA sensitivity to cortisol, which may be mediated by increasing type-2 11-beta HSD activity in UA.

Effect of uterine blood flow occlusion on shear stress-mediated nitric oxide production and endothelial nitric oxide synthase expression during ovine pregnancy

During normal pregnancy, uterine blood flow (UBF) is increased in association with elevations of endothelial nitric oxide (NO) production and endothelial nitric oxide synthase (eNOS) expression. Shear stress increases endothelial-derived NO production to reduce vasomotor tone. We hypothesized that decreasing in vivo UBF, and thus shear stress, will decrease NO and/or eNOS levels. In this experiment, one of the main uterine arteries of chronically instrumented late pregnant sheep (125 +/- 1 days' gestation [mean +/- SEM]; n = 15) was occluded for 24 h. Cardiovascular parameters (systemic and uterine arterial pressure, heart rate [HR], and ipsilateral and contralateral UBF) and NO(2)/NO(3) (NO(x)) levels were evaluated. Although UBF measured using Transonic flow probes was reduced unilaterally 41.5% +/- 2.1%, uterine perfusion pressure only fell 12.2% +/- 4.5%. Systemic arterial blood pressure and HR were unaltered. Using radioactive microspheres, ipsilateral UBF was reduced approximately 28% during occlusion. The redistribution of UBF to other reproductive tissues suggests that collateral circulation develops in response to occlusion. Systemic arterial and uterine venous NO(x) levels were reduced 22.1% +/- 6.7% and 22.6% +/- 7.6%, respectively, during occlusion. Treatment with microspheres produced an unexpected initial ( approximately 2.5 h) increase in systemic arterial and uterine venous NO(x) levels by 116% +/- 30% and 97% +/- 49%, respectively. Despite a decline in NO(x) levels after 6 h, no significant differences versus preocclusion NO(x) levels were detected by 24 h of occlusion in this experimental group. In contrast, NO(x), UBF, and uterine perfusion pressure levels unexpectedly failed to return to baseline values following release of occlusion. No differences in uterine artery eNOS expression were demonstrated by Western analysis from occlusion. Thus, our data suggest that shear stress may mediate in vivo vasomotor tone via production of NO(x).

Endothelial vasodilator production by uterine and systemic arteries. VI. Ovarian and pregnancy effects on eNOS and NO(x)

Normal pregnancy and the follicular phase of the ovarian cycle are both estrogen-dominated physiological states that are characterized by elevations in uterine blood flow and endothelial nitric oxide synthase (eNOS) protein expression in the uterine artery (UA) endothelium. It is unknown if elevations in mRNA level account for the changes in protein or eNOS activity. We tested the hypothesis that pregnancy and the follicular phase are associated with increases in eNOS mRNA and the consequent elevated expression of eNOS protein results in increased circulating nitric oxide (NO) levels. UA were obtained from pregnant (PREG; n = 8; 110-130 days gestation; term = 145 +/- 3 days), nonpregnant luteal (LUT; n = 6), nonpregnant follicular (FOL; n = 6), and nonpregnant ovariectomized (OVEX; n = 6) sheep. Circulating NO levels were analyzed as total NO(2)-NO(3) (NO(x)). Western analysis performed on UA endothelial-isolated proteins demonstrated that eNOS protein levels were OVEX = LUT < or = FOL < PREG (P < 0.05), whereas eNOS mRNA expression (RT-PCR) in UA endothelial cells obtained by limited collagenase digestion was OVEX < LUT < FOL < PREG (P < 0.05). Pregnancy dramatically elevated eNOS protein (4.1- to 6.9-fold) and mRNA (2.4- to 6.9-fold) over LUT controls (P < 0.01). Circulating NO(x) levels were not altered by ovariectomy or the ovarian cycle but were elevated from 4.4 +/- 1.1 microM in LUT to 12 +/- 4, 22 +/- 3, and 41 +/- 3 microM at 110, 120, and 130 days gestation (P < 0.01). Systemic NO(x) levels in singleton (12.5 +/- 1.6 microM) were less (P < 0.01) than in multiple (twin 27.6 +/- 6.5 microM; triplet = 46 +/- 10 microM) pregnancies. Therefore, the follicular phase and, to a much greater extent, pregnancy are associated with elevations in UA endothelium-derived eNOS expression, although significant increases in systemic NO(x) levels were only observed in the PREG group (multiple > singleton). Thus, although UA endothelial increases in eNOS protein and mRNA levels are associated with high estrogen states, increases in local UA NO production may require additional eNOS protein activation to play its important role in the maintenance of uterine blood flow in pregnancy.

Upregulation of nitric oxide synthase in mice with severe hypoxia-induced pulmonary hypertension

BACKGROUND

The importance of nitric oxide (NO) in hypoxic pulmonary hypertension has been demonstrated using nitric oxide synthase (NOS) knockout mice. In that model NO from endothelial NOS (eNOS) plays a central role in modulating pulmonary vascular tone and attenuating hypoxic pulmonary hypertension. However, the normal regulation of NOS expression in mice following hypoxia is uncertain. Because genetically engineered mice are often utilized in studies of NO, we conducted the present study to determine how hypoxia alters NOS expression in wild-type mice.

METHOD

Mice were exposed to sea level, ambient conditions (5280 feet) or severe altitude (17,000 feet) for 6 weeks from birth, and hemodynamics and lung NOS expression were assessed.

RESULTS

Hypoxic mice developed severe pulmonary hypertension (right ventricular systolic pressure [RVsP] 60 mmHg) as compared with normoxic mice (27 mmHg). Using quantitative reverse-transcription PCR, it was found that expressions of eNOS and inducible NOS (iNOS) increased 1.5-fold and 3.5-fold, respectively, in the lung. In addition, the level of lung eNOS protein was increased, neuronal NOS (nNOS) protein was unchanged, and iNOS was below the limit of detection. Immunohistochemistry demonstrated no change in lung iNOS or nNOS staining in either central or peripheral areas, but suggested increased eNOS in the periphery following hypoxia.

CONCLUSION

In mice, hypoxia is associated with increases in lung eNOS, possibly in iNOS, but not in nNOS; this suggests that the pattern of lung NOS expression following hypoxia must be considered in studies using genetically engineered mice.