Chronic Hypoxia Inhibits Contraction of Fetal Arteries by Increased Endothelium-Derived Nitric Oxide and Prostaglandin Synthesis

Near to One's Heart: The Intimate Relationship Between the Placenta and Fetal Heart

The development of the fetal heart is exquisitely controlled by a multitude of factors, ranging from humoral to mechanical forces. The gatekeeper regulating many of these factors is the placenta, an external fetal organ. As such, resistance within the placental vascular bed has a direct influence on the fetal circulation and therefore, the developing heart. In addition, the placenta serves as the interface between the mother and fetus, controlling substrate exchange and release of hormones into both circulations. The intricate relationship between the placenta and fetal heart is appreciated in instances of clinical placental pathology. Abnormal umbilical cord insertion is associated with congenital heart defects. Likewise, twin-to-twin transfusion syndrome, where monochorionic twins have unequal sharing of their placenta due to inter-twin vascular anastomoses, can result in cardiac remodeling and dysfunction in both fetuses. Moreover, epidemiological studies have suggested a link between placental phenotypic traits and increased risk of cardiovascular disease in adult life. To date, the mechanistic basis of the relationships between the placenta, fetal heart development and later risk of cardiac dysfunction have not been fully elucidated. However, studies using environmental exposures and gene manipulations in experimental animals are providing insights into the pathways involved. Likewise, surgical instrumentation of the maternal and fetal circulations in large animal species has enabled the manipulation of specific humoral and mechanical factors to investigate their roles in fetal cardiac development. This review will focus on such studies and what is known to date about the link between the placenta and heart development.

Intermittent maternal hypoxia has an influence on regional expression of endothelial nitric oxide synthase in fetal arteries of rabbits

BACKGROUND

Maternal hypoxia induces sustained fetal adaptations associated with changes in gene expression. We hypothesized that intermittent maternal hypoxia has an influence on regional expression of endothelial nitric oxide synthase (eNOS) in fetal arteries of New Zealand White rabbits.

METHODS

Timed-pregnant New Zealand White rabbits (term = 30 ± 1 d) were randomly assigned to a normoxic control group (n = 5) or a hypoxia group (12% O2, n = 5) during days 10-29 of pregnancy. At the end of pregnancy (29 d gestation), blood samples were collected from mothers and fetuses. Carotid and femoral arteries of fetuses were extracted for eNOS mRNA and protein concentration and analysis of total NOS activities.

RESULTS

Our data demonstrate that chronic intermittent maternal hypoxia significantly increased eNOS mRNA and protein concentrations and total NOS activities in carotid artery segments but decreased eNOS mRNA and protein concentrations and total NOS activities in femoral artery segments in the same fetuses. Vascular endothelial cells, but not smooth muscle cells, of fetal rabbits exhibited positive immunostaining for the eNOS protein.

CONCLUSION

These observations suggest that chronic hypoxia can regulate regional expression of eNOS as an adaptive response to hypoxic stress in fetal arteries.

Protective effect of N-acetylcysteine on liver damage during chronic intrauterine hypoxia in fetal guinea pig

Chronic exposure to hypoxia during pregnancy generates a stressed intrauterine environment that may lead to fetal organ damage. The objectives of the study are (1) to quantify the effect of chronic hypoxia in the generation of oxidative stress in fetal guinea pig liver and (2) to test the protective effect of antioxidant treatment in hypoxic fetal liver injury. Pregnant guinea pigs were exposed to either normoxia (NMX) or 10.5% O(2) (HPX, 14 days) prior to term (65 days) and orally administered N-acetylcysteine ([NAC] 10 days). Near-term anesthetized fetuses were excised and livers examined by histology and assayed for malondialdehyde (MDA) and DNA fragmentation. Chronic HPX increased erythroid precursors, MDA (NMX vs HPX; 1.26 ± 0.07 vs 1.78 ± 0.07 nmol/mg protein; P < .001, mean ± standard error of the mean [SEM]) and DNA fragmentation levels in fetal livers (0.069 ± 0.01 vs 0.11 ± 0.005 OD/mg protein; P < .01). N-acetylcysteine inhibited erythroid aggregation and reduced (P < .05) both MDA and DNA fragmentation of fetal HPX livers. Thus, chronic intrauterine hypoxia generates cell and nuclear damage in the fetal guinea pig liver. Maternal NAC inhibited the adverse effects of fetal liver damage suggestive of oxidative stress. The suppressive effect of maternal NAC may implicate the protective role of antioxidants in the prevention of liver injury in the hypoxic fetus.

Chronic fetal hypoxia produces selective brain injury associated with altered nitric oxide synthases

OBJECTIVE

The purpose of this study was to investigate the impact of chronic hypoxia on the nitric oxide synthase isoenzymes in specific brain structures.

STUDY DESIGN

Time-mated pregnant guinea pigs were exposed to 10.5% molecular oxygen for 14 days (animals with chronic fetal hypoxia; HPX) or room air (control animals; NMX); L-N6-(1-iminoethyl)-lysine (L-NIL; an inducible nitric oxide synthase inhibitor, 1 mg/kg/d) was administered to HPX group for 14 days (L-NIL + HPX). Fetal brains were harvested at term. Multilabeled immunofluorescence was used to generate a brain injury map. Laser capture microdissection and quantitative polymerase chain reaction were applied; cell injury markers, apoptosis activation, neuron loss, total nitric oxide, and the levels of individual nitric oxide synthase isoenzymes were quantified.

RESULTS

Chronic hypoxia causes selective fetal brain injury rather than global. Injury is associated with differentially affected nitric oxide synthases in both neurons and glial cells, with inducible macrophage-type nitric oxide synthase up-regulated at all injury sites. L-NIL attenuated the injury, despite continued hypoxia.

CONCLUSION

These studies demonstrate that chronic hypoxia selectively injures the fetal brain in part by the differential regulation of nitric oxide synthase isoenzymes in an anatomic- and cell-specific manner.

Partial contributions of developmental hypoxia and undernutrition to prenatal alterations in somatic growth and cardiovascular structure and function

OBJECTIVE

The objective of the study was to compare and contrast the effects of developmental hypoxia vs undernutrition on fetal growth, cardiovascular morphology, and function.

STUDY DESIGN

On day 15 of gestation, Wistar dams were divided into control, hypoxic (10% O(2)), or undernourished (35% reduction in food intake) pregnancy. On day 20, fetal thoraces were fixed, and the fetal heart and aorta underwent quantitative histological analysis. In a separate group, fetal aortic vascular reactivity was determined via wire myography.

RESULTS

Both hypoxic and undernourished pregnancy was associated with asymmetric fetal growth restriction. Pregnancy complicated by hypoxia promoted fetal aortic thickening without changes in cardiac volumes when expressed as a percentage of total heart volume. In contrast, maternal undernutrition affected fetal cardiac morphology without changes in aortic structure. Fetal aortic vascular reactivity was also differentially affected by hypoxia or undernutrition.

CONCLUSION

Developmental hypoxia or undernutrition in late gestation has differential effects on fetal cardiovascular morphology and function.

Brain injury caused by chronic fetal hypoxemia is mediated by inflammatory cascade activation

The prevalence of cerebral palsy (CP) shows little temporal or geographic variation and is associated with preterm birth, maternal/fetal infection/inflammation, and fetal growth restriction (IUGR), a potential surrogate for chronic fetal hypoxemia (CHX). We previously demonstrated CHX causes a fetal inflammatory response syndrome (FIRS). Herein, we test the hypothesis that CHX may cause fetal brain injury by upregulating inflammatory cytokine cascades, culminating in apoptosis pathway activation. Time-mated guinea pigs were housed in 12% or 10.5% O(2) for the last 21% of gestation. Chronic fetal hypoxemia increased the lactate/pyruvate and decreased the glutathione (GSH)/oxidized glutathione (GSSH) ratios, confirming a shift to a prooxidant state. The end result was a >30% decrease in hippocampal neuron density. Based on a microarray spotted with 113 cytokines and receptors, 22 genes were upregulated by CHX in proportion to the degree of hypoxia; the findings were confirmed by quantitative polymerase chain reaction (PCR). Thus, CHX triggers fetal brain inflammation inversely proportional to its severity characterized by increased apoptosis and neuronal loss. We suggest CHX fetal brain injury is not directly caused by oxygen deprivation but rather is an adaptive response that becomes maladaptive.

Intrauterine hypoxia upregulates proinflammatory cytokines and matrix metalloproteinases in fetal guinea pig hearts

OBJECTIVE

Intrauterine infection increases proinflammatory cytokines in the fetus. We hypothesize that proinflammatory cytokines and matrix metalloproteinases (MMPs) are upregulated in fetal hearts in response to hypoxic stress.

STUDY DESIGN

Timed-pregnant guinea pigs were exposed to either hypoxia (10.5% O(2), 14 day) or normoxia (room air). Left ventricles of fetal hearts were excised from anesthetized age-matched fetuses and frozen until ready for study. Messenger RNA of pro- (TNF-alpha, IL-6, IL-1beta) and anti- (IL-4, TGF, IFN-gamma) inflammatory cytokines and MMP2 and 9 was quantified by real-time PCR, MMP proteins by Western analysis, and MMP activity by gel zymography.

RESULTS

Chronic hypoxia increased (P < .05) TNF-alpha, IL-6, MMP2, and MMP9 mRNA levels but not IL-4, TGF, or IFN-gamma. Hypoxia increased protein levels of MMP9 but not MMP2, despite a hypoxia-induced increase in MMP2 activity.

CONCLUSION

Intrauterine hypoxia may be an important stimulus in local generation of selected proinflammatory cytokines and MMPs in fetal hearts.

Chronic hypoxia differentially increases glutathione content and gamma-glutamyl cysteine synthetase expression in fetal guinea pig organs

OBJECTIVE

Glutathione is a natural antioxidant in the fetus and adult. We sought to determine whether maternal hypoxia alters glutathione levels in fetal organs as an adaptive response to the reduced oxygenation.

STUDY DESIGN

Timed pregnant guinea pigs were housed in either a Plexiglas chamber containing 10.5% O(2) from 46 to 60 days gestation (HPX, n=6) or in room air, as the normoxic control (NMX, n=5). Pregnant guinea pigs were anesthetized at near term ( approximately 60 days, term=65 days) and liver, lungand kidney were excised from anesthetized fetuses and stored frozen (-80 degrees C) prior to sample processing. Using the hypoxia marker, pimonidazole, we measured a hypoxia-induced increase in stained cells of fetal liver compared to no change in either the lung or kidney. To measure the effect of hypoxia among different organs, total glutathione (GSH) content and protein levels of gamma-glutamyl cysteine synthetase (gamma-GCS) were measured from the same organs.

RESULTS

Maternal hypoxia increased (P<0.05) total glutathione levels by 121% in the fetal liver but had no effect in either fetal lung or kidney. Chronic hypoxia increased (P<0.05) gamma-GCS protein levels in all three fetal organs studied.

CONCLUSION

These results demonstrate that the fetal response to maternal hypoxia may be organ specific. The increase in fetal liver glutathione via upregulation of gamma-GCS may be an important adaptive response to prolonged hypoxic stress.

Chronic hypoxia decreases endothelial nitric oxide synthase protein expression in fetal guinea pig hearts

OBJECTIVES

The underlying cellular mechanisms mediating hypoxia-induced adaptations in the fetus are poorly understood. We tested the hypothesis that hypoxia up-regulates endothelial nitric oxide synthase (NOS3, type III) protein expression in fetal hearts similar to that observed in adult hearts as a cardioprotective adaptation. This study investigates the effect of chronic hypoxia on NOS3 protein expression in hearts and carotid arteries of fetal guinea pigs exposed to normoxia or intrauterine hypoxia.

METHODS

Time-mated pregnant guinea pigs (term = 65 days) were housed in either normoxic room air (NMX) or exposed to 12% O(2) (hypoxia; HPX) for 14 or 28 days of duration. At near term ( approximately 60 days of gestation), pregnant mothers were anesthetized and fetal guinea pig hearts and carotid arteries were excised from NMX and HPX animals and frozen until ready for study. In addition, hearts were also excised from anesthetized adult nonpregnant female guinea pigs exposed to either NMX or HPX for 14 days. NOS3 protein was extracted from all tissues and quantified using Western blot analysis. Fetal heart samples were also prepared for localization of NOS3 protein using immunohistochemistry.

RESULTS

Chronic hypoxia increased both maternal and fetal hematocrit after 28 days of duration. HPX decreased NOS3 protein levels in fetal guinea pig hearts by 29% after 28 days compared to NMX controls. In contrast, HPX increased both NOS3 protein levels in adult hearts by 62% and fetal carotid arteries by fourfold after 14 days of exposure compared to their respective NMX controls. Positive immunostaining of NOS3 protein of fetal hearts was localized in both cardiomyocytes and endothelial cells.

CONCLUSION

Contrary to our hypothesis, the hypoxia-induced decrease in fetal guinea pig heart NOS3 protein contrasts to the protein levels measured in either adult hearts or fetal carotid arteries. These results suggest that the NOS protein expression is altered differently by hypoxia in fetal and adult hearts and in a peripheral fetal artery exposed to the same level of hypoxia. Thus, the functional role of NO in the fetal heart during hypoxia may differ from that of the adult.

Increased myogenic tone in 7-month-old adult male but not female offspring from rat dams exposed to hypoxia during pregnancy

Intrauterine growth restriction (IUGR) increases the risk of cardiovascular disease later in life. Vascular dysfunction occurs in adult offspring from animal models of IUGR including maternal undernutrition, but the influence of reduced fetal oxygen supply on adult vascular function is unclear. Myogenic responses, essential for vascular tone regulation, have not been evaluated in these offspring. We hypothesized that 7-mo-old offspring from hypoxic (12% O(2); H) or nutrient-restricted (40% of control; NR) rat dams would show greater myogenic responses than their 4-mo-old littermates or control (C) offspring through impaired modulation by vasodilators. Growth restriction occurred in male H (P < 0.01), male NR (P < 0.01), and female NR (P < 0.02), but not female H, offspring. Myogenic responses in mesenteric arteries from males but not females were increased at 7 mo in H (P < 0.01) and NR (P < 0.05) vs. C offspring. There was less modulation of myogenic responses after inhibition of nitric oxide synthase (P < 0.05), prostaglandin H synthase (P < 0.005), or both enzymes (P < 0.001) in arteries from 7-mo male H vs. C offspring. Thus reduced vasodilator modulation may explain elevated myogenic responses in 7-mo male H offspring. In contrast, there was increased modulation of myogenic responses in arteries from 7-mo female H vs. C or NR offspring after inhibition of both enzymes (P < 0.05). Thus increased vasodilator modulation may maintain myogenic responses in female H offspring at control levels. In summary, vascular responses in adult offspring from adverse intrauterine environments are impaired in a gender-specific, age-dependent, and maternal insult-dependent manner, with males more profoundly affected.