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

Surviving birth at high altitude

This Symposium Review examines challenges to surviving birth and infancy at high altitudes. Chronic exposure to the environmental hypoxia of high altitudes increases the incidence of maternal vascular disorders of pregnancy characterized by placental insufficiency, restricted fetal growth and preterm delivery, and impairs pulmonary vascular health during infancy. While each condition independently contributes to excess morbidity and mortality in early life, evidence indicates vascular disorders of pregnancy and infantile pulmonary vascular dysfunction are intertwined. By integrating our recent scientific and clinical observations in Bolivia with existing literature, we propose potential avenues to reduce the infant mortality burden at high altitudes and reduce pulmonary vascular disease in highland neonates, and emphasize the need for further research to address unresolved questions.

Altered placental ion channel gene expression in preeclamptic high-altitude pregnancies

High-altitude (>2,500 m) residence increases the risk of pregnancy vascular disorders such as fetal growth restriction and preeclampsia, each characterized by impaired placental function. Genetic attributes of highland ancestry confer relative protection against vascular disorders of pregnancy at high altitudes. Although ion channels have been implicated in placental function regulation, neither their expression in high-altitude placentas nor their relationship to high-altitude preeclampsia has been determined. Here, we measured the expression of 26 ion-channel genes in placentas from preeclampsia cases and normotensive controls in La Paz, Bolivia (3,850 m). In addition, we correlated gene transcription to maternal and infant ancestry proportions. Gene expression was assessed by PCR, genetic ancestry evaluated by ADMIXTURE, and ion channel proteins localized by immunofluorescence. In preeclamptic placentas, 11 genes were downregulated (ABCC9, ATP2A2, CACNA1C, KCNE1, KCNJ8, KCNK3, KCNMA1, KCNQ1, KCNQ4, PKD2, and TRPV6) and two were upregulated (KCNQ3 and SCNN1G). KCNE1 expression was positively correlated with high-altitude Amerindian ancestry and negatively correlated with non-high altitude. SCNN1G was negatively correlated with African ancestry, despite minimal African admixture. Most ion channels were localized in syncytiotrophoblasts (Cav1.2, TRPP2, TRPV6, and Kv7.1), whereas expression of Kv7.4 was primarily in microvillous membranes, Kir6.1 in chorionic plate and fetal vessels, and MinK in stromal cells. Our findings suggest a role for differential placental ion channel expression in the development of preeclampsia. Functional studies are needed to determine processes affected by these ion channels in the placenta and whether therapies directed at modulating their activity could influence the onset or severity of preeclampsia.

Can Maternal Exercise Prevent High-Altitude Pulmonary Hypertension in Children?

Leslie, Eric, Ann L. Gibson, Laura V. Gonzalez Bosc, Christine Mermier, Sean M. Wilson, and Michael R. Deyhle. Review: can maternal exercise prevent high-altitude pulmonary hypertension in children? High Alt Med Biol. 24:1-6, 2023.-Chronic high-altitude exposure reduces oxygen delivery to the fetus during pregnancy and causes pathologic pulmonary artery remodeling, This increases the risk of high-altitude pulmonary hypertension (PH), which is a particularly fatal disease that is difficult to treat. Therefore, finding ways to prevent high-altitude PH, including during the neonatal period, is preferable. Cardiorespiratory exercise can improve functional capacity and quality of life in patients with high-altitude PH. However, similar to other treatments and surgical procedures, the benefits are not enough to cure the disease after a diagnosis. Cardiorespiratory exercise by mothers during pregnancy (i.e., maternal exercise) has not been previously evaluated to prevent the development of high-altitude PH in children born and living at high altitude. This focused review describes the pathophysiology of high-altitude PH and the potential benefit of maternal exercise for preventing the disease caused by high-altitude pregnancies.

Evidence of Nitric Oxide Impairment During Hypertensive Pregnancies

Hypertensive disorders of pregnancy complicate up to 10% of pregnancies worldwide, and they can be classified into (1) gestational hypertension, (2) preeclampsia, (3) chronic hypertension and (4) chronic hypertension with preeclampsia. Nitric oxide (NO) plays an essential role in the haemodynamic adaptations observed during pregnancy. It has been shown that the nitric oxide pathway's dysfunction during pregnancy is associated with placental- and vascular-related diseases such as hypertensive disorders of pregnancy. This review aims to present a brief definition of hypertensive disorders of pregnancy and physiological maternal cardiovascular adaptations during pregnancy. We also detail how NO signalling is altered in the (a) systemic vasculature, (b) uterine artery/spiral arteries, (c) implantation and (d) placenta of hypertensive disorders during pregnancy. We conclude by summarizing the anti-hypertensive therapy of hypertensive disorders of pregnancy as a specific management strategy.

Acute hypoxia induced dysregulation of clock-controlled ovary functions

High altitudes or exposure to hypoxia leads to female reproductive disorders. Circadian clocks are intrinsic time-tracking systems that enable organisms to adapt to the Earth's 24-h light/dark cycle, which can be entrained by other environmental stimuli to regulate physiological and pathological responses. In this study, we focused on whether ovarian circadian clock proteins were involved in regulating female reproductive dysfunction under hypoxic conditions. Hypobaric hypoxia was found to induce a significantly prolonged estrous cycle in female mice, accompanied by follicular atresia, pituitary/ovarian hormone synthesis disorder, and decreased LHCGR expression in the ovaries. Under the same conditions, the levels of the ovarian circadian clock proteins, CLOCK and BMAL1, were suppressed, whereas E4BP4 levels were upregulated. Results from granulosa cells (GCs) further demonstrated that CLOCK: BMAL1 and E4BP4 function as transcriptional activators and repressors of LHCGR in ovarian GCs, respectively, whose responses were mediated by HIF1ɑ-dependent (E4BP4 upregulation) and ɑ-independent (CLOCK and BMAL1 downregulation) manners. The LHCGR agonist was shown to efficiently recover the impairment of ovulation-related gene (EREG and PGR) expression in GCs induced by hypoxia. We conclude that hypoxia exposure causes dysregulation of ovarian circadian clock protein (CLOCK, BMAL1, and E4BP4) expression, which mediates female reproductive dysfunction by impairing LHCGR-dependent signaling events. Adjusting the timing system or recovering the LHCGR level in the ovaries may be helpful in overcoming female reproductive disorders occurring in the highlands.

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.

Pathological features of early pregnancy disorders in women living at high altitude in KSA

Objectives

Pregnancy at high altitudes is relatively challenging because of hypobaric hypoxia, which is associated with a smaller uterine artery diameter and diminished blood flow. Here, we investigated the histopathological characteristics of early pregnancy disorders among pregnant women living in a high-altitude region (approximately 2200 m above sea level).

Methods

This cross-sectional study used retrospective data collection from a single tertiary hospital in a high-altitude region in KSA. Age and histopathology were analyzed in 495 women diagnosed with early pregnancy disorders (mainly miscarriage) in 2018-2020.

Results

Approximately one-fifth of pregnancies in this high-altitude region were lost before 24 weeks' gestation, whereas 1150/6044 experienced miscarriage; 495 samples from those participants were sent for histopathological analysis. A total of 269 (54.34%) patients were younger than 35 years. Missed miscarriages accounted for 49.3% of miscarriages, followed by inevitable miscarriages (34.2%), which had a relatively higher frequency among mothers older than 35 years. The correlation between age and inevitable miscarriage was significant; ectopic pregnancy was diagnosed in 6.8%, molar pregnancy was detected in 6.26%, and blighted ovum was observed in 3.4%.

Conclusion

The miscarriage rate in this high-altitude region was 19% of all pregnancies. Approximately half of the affected women were in their 30s. Missed miscarriage cases were relatively high in this region. The percentage of molar pregnancy was higher than those reported in prior studies, thus suggesting a need for monitoring and genetic workup practices. Furthermore, studies involving a larger population at high altitudes will be crucial for further risk assessment in addition to national studies on women living at sea level.

How hypoxia slows fetal growth: insights from high altitude

A continuous oxygen supply is vital for fueling fetal growth, yet we know surprisingly little as to the precise mechanisms by which hypoxia reduces fetal growth. Animal models, while important, are limited by the enormous variability in mammalian physiologic responses to pregnancy. One of the earliest models for understanding hypoxia-induced reductions in fetal growth came from studies at high altitude that showed high altitude to have one of the strongest depressant effects on fetal growth. But all populations are not equally affected. Dolma et al., have added valuable information now showing that that Ladakhis, like Tibetans and Andeans, have a lower-than-expected frequency of small-for-gestational age (SGA) infants. Consistent with previous reports, these authors also found that uterine artery diameters at mid pregnancy (week 26–28) were larger in women giving birth to appropriate-for-gestational-age (AGA) than SGA infants at high but, interestingly, not at low altitude. Much remains to be learned about the physiologic pathways by which hypoxia impairs fetal growth. The variability among high-altitude populations and increasing sophistication of tools for investigating causal mechanisms have the potential to expand our presently limited means for identifying new treatments for hypoxia-related complications of pregnancy and fetal life.

High Altitude Pregnancies and Vascular Dysfunction: Observations From Latin American Studies

An estimated human population of 170 million inhabit at high-altitude (HA, above 2,500 m). The potential pathological effects of HA hypobaric hypoxia during gestation have been the focus of several researchers around the world. The studies based on the Himalayan and Central/South American mountains are particularly interesting as these areas account for nearly 70% of the HA world population. At present, studies in human and animal models revealed important alterations in fetal development and growth at HA. Moreover, vascular responses to chronic hypobaria in the pregnant mother and her fetus may induce marked cardiovascular impairments during pregnancy or in the neonatal period. In addition, recent studies have shown potential long-lasting postnatal effects that may increase cardiovascular risk in individuals gestated under chronic hypobaria. Hence, the maternal and fetal adaptive responses to hypoxia, influenced by HA ancestry, are vital for a better developmental and cardiovascular outcome of the offspring. This mini-review exposes and discusses the main determinants of vascular dysfunction due to developmental hypoxia at HA, such as the Andean Mountains, at the maternal and fetal/neonatal levels. Although significant advances have been made from Latin American studies, this area still needs further investigations to reveal the mechanisms involved in vascular dysfunction, to estimate complications of pregnancy and postnatal life adequately, and most importantly, to determine potential treatments to prevent or treat the pathological effects of being developed under chronic hypobaric hypoxia.

Influence of geographic conditions on body length of male newborns in Kyrgyzstan

Newborn length has been reported by many researchers to be reduced at high altitudes. However, many of these studies lacked adequate control of the ethnic group which may be confounding the altitude differences. In addition, few studies have examined the sources of variation in birth weight at high altitudes that may be related to ethnic group adaptation to the stresses of this hypoxic environment. In our study, we tested the hypotheses that the effect of altitude differences in newborn length depends on ethnic variation. Samples of 3359 healthy male newborns from different areas in Kyrgyzstan between the years 2003 and 2011 were analyzed for altitude and ethnic variation on male newborn length. Our results indicate significant decrease in male newborn length as a latitude increase. It is concluded that ethnic group difference in pregnancy outcome reflects a better state of adaptation to high altitude in this healthy indigenous population and that long-term genetic selection may be the most plausible explanation for these ethnic differences.

Placental Ischemia Says "NO" to Proper NOS-Mediated Control of Vascular Tone and Blood Pressure in Preeclampsia

In this review, we first provide a brief overview of the nitric oxide synthase (NOS) isoforms and biochemistry. This is followed by describing what is known about NOS-mediated blood pressure control during normal pregnancy. Circulating nitric oxide (NO) bioavailability has been assessed by measuring its metabolites, nitrite (NO₂) and/or nitrate (NO₃), and shown to rise throughout normal pregnancy in humans and rats and decline postpartum. In contrast, placental malperfusion/ischemia leads to systemic reductions in NO bioavailability leading to maternal endothelial and vascular dysfunction with subsequent development of hypertension in PE. We end this article by describing emergent risk factors for placental malperfusion and ischemic disease and discussing strategies to target the NOS system therapeutically to increase NO bioavailability in preeclamptic patients. Throughout this discussion, we highlight the critical importance that experimental animal studies have played in our current understanding of NOS biology in normal pregnancy and their use in finding novel ways to preserve this signaling pathway to prevent the development, treat symptoms, or reduce the severity of PE.

Blood pressure and hypertensive disorders of pregnancy at high altitude: a systematic review and meta-analysis

OBJECTIVE

Exposure to high altitude (≥2500 m) is associated with increased arterial blood pressure. During pregnancy, even a mild elevation of maternal blood pressure is associated with reduced birthweight and increased prevalence of pregnancy complications. This study aimed to systematically assess the impact of altitude on maternal blood pressure at term and on the prevalence of hypertensive disorders of pregnancy.

DATA SOURCES

PubMed, Ovid Embase, Cochrane Library, Medline, Web of Science, and ClinicalTrials.gov were searched (inception to November 11, 2020).

STUDY APPRAISAL AND SYNTHESIS METHODS

Observational, cohort, or case-control studies were included if they reported a high-altitude and appropriate control pregnant population. Studies published >50 years ago were excluded; 2 reviewers independently assessed articles for eligibility and risk of bias.

RESULTS

At high altitude, maternal systolic and diastolic blood pressure at term was higher than at low altitude (4.8±1.6 mm Hg; P<.001; 4.0±0.8 mm Hg; P<.001, respectively). Hypertensive disorders of pregnancy were more common at high altitude (odds ratio, 1.31 [1.03-1.65]; P<.05). The prevalence of gestational hypertension was nearly twice as high at high altitude (odds ratio, 1.92 [1.15-3.22]; P<.05) but the prevalence of preeclampsia was half as high (odds ratio, 0.57 [0.46-0.70]; P<.001). The likelihood of stillbirth was increased by 63% in pregnancies at high altitude compared with low altitude (odds ratio, 1.63 [1.12-2.35]; P<.01).

CONCLUSION

Maternal blood pressure is higher at term in pregnancies at high altitude than low altitude, accompanied with an increased risk of gestational hypertension but not preeclampsia. Risk of stillbirth at high altitude is also increased. With a growing population residing at high altitude worldwide, it is essential to clearly define the associated risk of adverse pregnancy outcomes.

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.

Fetal growth, high altitude, and evolutionary adaptation: a new perspective

Residence at high altitude is consistently associated with low birthweight among placental mammals. This reduction in birthweight influences long-term health trajectories for both the offspring and mother. However, the physiological processes that contribute to fetal growth restriction at altitude are still poorly understood, and thus our ability to safely intervene remains limited. One approach to identify the factors that mitigate altitude-dependent fetal growth restriction is to study populations that are protected from fetal growth restriction through evolutionary adaptations (e.g., high altitude-adapted populations). Here, we examine human gestational physiology at high altitude from a novel evolutionary perspective that focuses on patterns of physiological plasticity, allowing us to identify 1) the contribution of specific physiological systems to fetal growth restriction and 2) the mechanisms that confer protection in highland-adapted populations. Using this perspective, our review highlights two general findings: first, that the beneficial value of plasticity in maternal physiology is often dependent on factors more proximate to the fetus; and second, that our ability to understand the contributions of these proximate factors is currently limited by thin data from altitude-adapted populations. Expanding the comparative scope of studies on gestational physiology at high altitude and integrating studies of both maternal and fetal physiology are needed to clarify the mechanisms by which physiological responses to altitude contribute to fetal growth outcomes. The relevance of these questions to clinical, agricultural, and basic research combined with the breadth of the unknown highlight gestational physiology at high altitude as an exciting niche for continued work.

Enhanced Placental Mitochondrial Respiration in Tibetan Women at High Altitude

Living at high altitudes is extremely challenging as it entails exposure to hypoxia, low temperatures, and high levels of UV radiation. However, the Tibetan population has adapted to such conditions on both a physiological and genetic level over 30,000–40,000 years. It has long been speculated that fetal growth restriction is caused by abnormal placental development. We previously demonstrated that placentas from high-altitude Tibetans were protected from oxidative stress induced by labor compared to those of European descent. However, little is known about how placental mitochondria change during high-altitude adaptation. In this study, we aimed to uncover the mechanism of such adaptation by studying the respiratory function of the placental mitochondria of high-altitude Tibetans, lower-altitude Tibetans, and lower-altitude Chinese Han. We discovered that mitochondrial respiration was greater in high-altitude than in lower-altitude Tibetans in terms of OXPHOS via complexes I and I+II, ETS_max capacity, and non-phosphorylating respiration, whereas non-ETS respiration, LEAK/ETS, and OXPHOS via complex IV did not differ. Respiration in lower-altitude Tibetans and Han was similar for all tested respiratory states. Placentas from high-altitude Tibetan women were protected from acute ischemic/hypoxic insult induced by labor, and increased mitochondrial respiration may represent an acute response that induces mitochondrial adaptations.

High altitude regulates the expression of AMPK pathways in human placenta

INTRODUCTION

High-altitude (>2500 m) residence augments the risk of intrauterine growth restriction (IUGR) and preeclampsia likely due, in part, to uteroplacental hypoperfusion. Previous genomic and transcriptomic studies in humans and functional studies in mice and humans suggest a role for AMP-activated protein kinase (AMPK) pathway in protecting against hypoxia-associated IUGR. AMPK is a metabolic sensor activated by hypoxia that is ubiquitously expressed in vascular beds and placenta.

METHODS

We measured gene expression and protein levels of AMPK and its upstream regulators and downstream targets in human placentas from high (>2500 m) vs. moderate (~1700 m) and low (~100 m) altitude.

RESULTS

We found that phosphorylated AMPK protein levels and its downstream target TSC2 were increased in placentas from high and moderate vs. low altitude, whereas the phosphorylated form of the downstream target translation repressor protein 4E-BP1 was increased in high compared to moderate as well as low altitude placentas. Mean birth weights progressively fell with increasing altitude but no infants, by study design, were clinically growth-restricted. Gene expression analysis showed moderate increases in PRKAG2, encoding the AMPK γ2 subunit, and mechanistic target of rapamycin, MTOR, expression.

DISCUSSION

These results highlight a differential regulation of placental AMPK pathway activation in women residing at low, moderate or high altitude during pregnancy, suggesting AMPK may be serving as a metabolic regulator for integrating hypoxic stimuli with placental function.

Introduction to Environmental Harmful Factors

In this Chapter, we systematically and comprehensively described various environmental harmful factors. They were classified into four aspects: physical factors, chemical factors, biological factors, and physiological and psychological stress factors. Their classification, modes of presence, toxicity and carcinogenicity, routes of exposure to human and toxic effects on the female reproductive health were introduced. It is expected that the exposure routes could be controlled and eliminated, and the pathogenic mechanism of environmental harmful factors should be investigated and explained to protect female reproductive health.

Developmental and reproductive physiology of small mammals at high altitude: challenges and evolutionary innovations

High-altitude environments, characterized by low oxygen levels and low ambient temperatures, have been repeatedly colonized by small altricial mammals. These species inhabit mountainous regions year-round, enduring chronic cold and hypoxia. The adaptations that allow small mammals to thrive at altitude have been well studied in non-reproducing adults; however, our knowledge of adaptations specific to earlier life stages and reproductive females is extremely limited. In lowland natives, chronic hypoxia during gestation affects maternal physiology and placental function, ultimately limiting fetal growth. During post-natal development, hypoxia and cold further limit growth both directly by acting on neonatal physiology and indirectly via impacts on maternal milk production and care. Although lowland natives can survive brief sojourns to even extreme high altitude as adults, reproductive success in these environments is very low, and lowland young rarely survive to sexual maturity in chronic cold and hypoxia. Here, we review the limits to maternal and offspring physiology - both pre-natal and post-natal - that highland-adapted species have overcome, with a focus on recent studies on high-altitude populations of the North American deer mouse (Peromyscus maniculatus). We conclude that a combination of maternal and developmental adaptations were likely to have been critical steps in the evolutionary history of high-altitude native mammals.

The Influence of Bisphenol a on the Nitrergic Nervous Structures in the Domestic Porcine Uterus

Bisphenol A (BPA) is one of the most common environmental pollutants among endocrine disruptors. Due to its similarity to estrogen, BPA may affect estrogen receptors and show adverse effects on many internal organs. The reproductive system is particularly vulnerable to the impact of BPA, but knowledge about BPA-induced changes in the innervation of the uterus is relatively scarce. Therefore, this study aimed to investigate the influence of various doses of BPA on nitrergic nerves supplying the uterus with the double immunofluorescence method. It has been shown that even low doses of BPA caused an increase in the number of nitrergic nerves in the uterine wall and changed their neurochemical characterization. During the present study, changes in the number of nitrergic nerves simultaneously immunoreactive to substance P, vasoactive intestinal polypeptide, pituitary adenylate cyclase-activating peptide, and/or cocaine- and amphetamine-regulated transcript were found under the influence of BPA. The obtained results strongly suggest that nitrergic nerves in the uterine wall participate in adaptive and/or protective processes aimed at homeostasis maintenance in the uterine activity under the impact of BPA.

AMPK activation in pregnant human myometrial arteries from high-altitude and intrauterine growth-restricted pregnancies

High-altitude (>2,500 m) residence increases the incidence of intrauterine growth restriction (IUGR) due, in part, to reduced uterine artery blood flow and impaired myometrial artery (MA) vasodilator response. A role for the AMP-activated protein kinase (AMPK) pathway in protecting against hypoxia-associated IUGR is suggested by genomic and transcriptomic studies in humans and functional studies in mice. AMPK is a hypoxia-sensitive metabolic sensor with vasodilatory properties. Here we hypothesized that AMPK-dependent vasodilation was increased in MAs from high versus low-altitude (<1,700 m) Colorado women with appropriate for gestational age (AGA) pregnancies and reduced in IUGR pregnancies regardless of altitude. Vasoreactivity studies showed that, in AGA pregnancies, MAs from high-altitude women were more sensitive to vasodilation by activation of AMPK with A769662 due chiefly to increased endothelial nitric oxide production, whereas MA responses to AMPK activation in the low-altitude women were endothelium independent. MAs from IUGR compared with AGA pregnancies had blunted vasodilator responses to acetylcholine at high altitude. We concluded that 1) blunted vasodilator responses in IUGR pregnancies confirm the importance of MA vasodilation for normal fetal growth and 2) the increased sensitivity to AMPK activation in AGA pregnancies at high altitude suggests that AMPK activation helped maintain MA vasodilation and fetal growth. These results highlight a novel mechanism for vasodilation of MAs under conditions of chronic hypoxia and suggest that AMPK activation could provide a therapy for increasing uteroplacental blood flow and improving fetal growth in IUGR pregnancies.NEW & NOTEWORTHY Intrauterine growth restriction (IUGR) impairs infant well- being and increases susceptibility to later-in-life diseases for mother and child. Our study reveals a novel role for AMPK in vasodilating the myometrial artery (MA) from women residing at high altitude (>2,500 m) with appropriate for gestational age pregnancies but not in IUGR pregnancies at any altitude.

Nitric oxide metabolism in the human placenta during aberrant maternal inflammation

KEY POINTS

Nitric oxide (NO) is a gasotransmitter with important physiological and pathophysiological roles in pregnancy. There is limited information available about the sources and metabolism of NO and its bioactive metabolites (NOx) in both normal and complicated pregnancies. The present study characterized and quantified endogenous NOx in human and mouse placenta following determination of the stability of exogenous NOx in placental homogenates. NOx have differential stability in placental homogenates. NO and iron nitrosyl species (FeNOs), are relatively unstable in placental homogenates from normal placentas. Exogenous NO, nitrite and nitrosothiols react with placental homogenates to form iron nitrosyl complexes. FeNOs were also detected endogenously in mouse and human placenta. NOx levels in placental villous tissue are increased in fetal growth restriction vs. placentas from women with normal pregnancies, particularly in fetal growth restriction associated with pre-eclampsia. Villitis was not associated, however, with an increase in NOx levels in either normotensive or pre-eclamptic placentas. The results call for further investigation of FeNOs in normal and complicated pregnancies.

ABSTRACT

Nitric oxide (NO) is a gasotransmitter with important roles in pregnancy under both physiological and pathophysiological conditions. Although products of NO metabolism (NOx) also have significant bioactivity, little is known about the role of NO and NOx under conditions of aberrant placental inflammation during pregnancy. An ozone-based chemiluminescence approach was used to investigate the stability and metabolic fate of NOx in human placental homogenates from uncomplicated pregnancies in healthy mothers compared to that in placental tissue from normotensive and pre-eclamptic pregnancies complicated with fetal growth restriction (FGR) with and without villitis of unknown aetiology. We hypothesized that placental NOx would be increased in FGR vs. normal tissue, and be further increased in villitis vs. non-villitis placentas. Findings indicate that nitrate, nitrite and nitrosothiols, but not NO or iron nitrosyl species (FeNOs), are relatively stable in placental homogenates from normal placentas, and that NO, nitrite and nitrosothiols react with placental homogenates to form iron nitrosyl complexes. Furthermore, NOx levels in placental villous tissue are increased in FGR vs. placentas from women with normal pregnancies, particularly in FGR associated with pre-eclampsia. However, in contrast to our hypothesis, villitis was not associated with an increase in NOx levels in either normotensive or pre-eclamptic placentas. Our results also strongly support the involvement of FeNOs in both mouse and human placenta, and call for their further study as a critical mechanistic link between pre-eclampsia and fetal growth restriction.

Increased uterine artery blood flow in hypoxic murine pregnancy is not sufficient to prevent fetal growth restriction†

Incomplete maternal vascular responses to pregnancy contribute to pregnancy complications including intrauterine growth restriction (IUGR) and preeclampsia. We aimed to characterize maternal vascular dysfunction in a murine model of fetal growth restriction as an approach toward identifying targetable pathways for improving pregnancy outcomes. We utilized a murine model of late-gestation hypoxia-induced IUGR that reduced E18.5 fetal weight by 34%. Contrary to our hypothesis, uterine artery blood flow as measured in vivo by Doppler ultrasound was increased in mice housed under hypobaric hypoxia (385 mmHg; 5500 m) vs normoxia (760 mmHg; 0 m). Using wire myography, uterine arteries isolated from hypoxic mice had similar vasodilator responses to the two activators A769662 and acetylcholine as those from normoxic mice, although the contribution of an increase in nitric oxide production to uterine artery vasodilation was reduced in the hypoxic vs normoxic groups. Vasoconstrictor responses to phenylephrine and potassium chloride were unaltered by hypoxia. The levels of activated adenosine monophosphate-activated protein kinase (AMPK) were reduced with hypoxia in both the uterine artery and placenta as measured by western blot and immunohistochemistry. We concluded that the rise in uterine artery blood flow may be compensatory to hypoxia but was not sufficient to prevent fetal growth restriction. Although AMPK signaling was reduced by hypoxia, AMPK was still receptive to pharmacologic activation in the uterine arteries in which it was a potent vasodilator. Thus, AMPK activation may represent a new therapy for pregnancy complications involving reduced uteroplacental perfusion.

Peroxisome proliferator-activated receptor gamma blunts endothelin-1-mediated contraction of the uterine artery in a murine model of high-altitude pregnancy

The environmental hypoxia of high altitude (HA) increases the incidence of intrauterine growth restriction (IUGR) approximately threefold. The peroxisome proliferator-activated receptor γ (PPAR-γ), a ligand-activated nuclear receptor that promotes vasorelaxation by increasing nitric oxide and downregulating endothelin-1 (ET-1) production, has been implicated in IUGR. Based on our prior work indicating that pharmacologic activation of the PPARγ pathway protects against hypoxia-associated IUGR, we used an experimental murine model to determine whether such effects may be attributed to vasodilatory effects in the uteroplacental circulation. Using wire myography, ex vivo vasoreactivity studies were conducted in uterine arteries (UtA) isolated from pregnant mice exposed to hypoxia or normoxia from gestational day 14.5 to 18.5. Exposure to troglitazone, a high-affinity PPARγ agonist-induced vasorelaxation in UtA preconstricted with phenylephrine, with HA-UtA showing increased sensitivity. Troglitazone blunted ET-1-induced contraction of UtA in hypoxic and normoxic dams equivalently. Immunohistological analysis revealed enhanced staining for ET-1 receptors in the placental labyrinthine zone in hypoxic compared to normoxic dams. Our results suggest that pharmacologic PPAR-γ activation, via its vasoactive properties, may protect the fetal growth under hypoxic conditions by improving uteroplacental perfusion and thereby justify further investigation into PPARγ as a therapeutic target for IUGR in pregnancies complicated by hypoxia.

Effect of high altitude on human placental amino acid transport

Women residing at high altitudes deliver infants of lower birth weight than at sea level. Birth weight correlates with placental system A-mediated amino acid transport capacity, and severe environmental hypoxia reduces system A activity in isolated trophoblast and the mouse placenta. However, the effect of high altitude on human placental amino acid transport remains unknown. We hypothesized that microvillous membrane (MVM) system A and system L amino acid transporter activity is lower in placentas of women living at high altitude compared with low-altitude controls. Placentas were collected at term from healthy pregnant women residing at high altitude (HA; >2,500 m; n = 14) or low altitude (LA; <1,700 m; n = 14) following planned, unlabored cesarean section. Birth weight, but not placenta weight, was 13% lower in HA pregnancies (2.88 ± 0.11 kg) compared with LA (3.30 ± 0.07 kg, P < 0.01). MVM erythropoietin receptor abundance, determined by immunoblot, was greater in HA than in LA placentas, consistent with lower placental oxygen levels at HA. However, there was no effect of altitude on MVM system A or L activity, determined by Na⁺-dependent [¹⁴C]methylaminoisobutyric acid uptake and [³H]leucine uptake, respectively. MVM abundance of glucose transporters (GLUTs) 1 and 4 and basal membrane GLUT4 were also similar in LA and HA placentas. Low birth weights in the neonates of women residing at high altitude are not a consequence of reduced placental amino acid transport capacity. These observations are in general agreement with studies of IUGR babies at low altitude, in which MVM system A activity is downregulated only in growth-restricted babies with significant compromise.NEW & NOTEWORTHY Babies born at high altitude are smaller than at sea level. Birth weight is dependent on growth in utero and, in turn, placental nutrient transport. We determined amino acid transport capacity in placentas collected from women resident at low and high altitude. Altitude did not affect system A amino acid transport across the syncytiotrophoblast microvillous membrane, suggesting that impaired placental amino acid transport does not contribute to reduced birth weight in this high-altitude population.

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.