Age-dependent modulation of endothelium-dependent vasodilatation by chronic hypoxia in ovine cranial arteries

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

Postnatal development alters functional compartmentalization of myosin light chain kinase in ovine carotid arteries

The rate-limiting enzyme for vascular contraction, myosin light chain kinase (MLCK), phosphorylates regulatory myosin light chain (MLC₂₀) at rates that appear faster despite lower MLCK abundance in fetal compared with adult arteries. This study explores the hypothesis that greater apparent tissue activity of MLCK in fetal arteries is due to age-dependent differences in intracellular distribution of MLCK in relation to MLC₂₀. Under optimal conditions, common carotid artery homogenates from nonpregnant adult female sheep and near-term fetuses exhibited similar values of V_max and K_m for MLCK. A custom-designed, computer-controlled apparatus enabled electrical stimulation and high-speed freezing of arterial segments at exactly 0, 1, 2, and 3 s, calculation of in situ rates of MLC₂₀ phosphorylation, and measurement of time-dependent colocalization between MLCK and MLC₂₀. The in situ rate of MLC₂₀ phosphorylation divided by total MLCK abundance averaged to values 147% greater in fetal (1.06 ± 0.28) than adult (0.43 ± 0.08) arteries, which corresponded, respectively, to 43 ± 10% and 31 ± 3% of the V_max values measured in homogenates. Confocal colocalization analysis revealed in fetal and adult arteries that 33 ± 6% and 20 ± 5% of total MLCK colocalized with pMLC₂₀, and that MLCK activation was greater in periluminal than periadventitial regions over the time course of electrical stimulation in both age groups. Together, these results demonstrate that the catalytic activity of MLCK is similar in fetal and adult arteries, but that the fraction of total MLCK in the functional compartment involved in contraction is significantly greater in fetal than adult arteries.

Cardiovascular and Cerebrovascular Implications of Growth Restriction: Mechanisms and Potential Treatments

Fetal growth restriction (FGR) is a common complication of pregnancy, resulting in a fetus that fails to reach its genetically determined growth potential. Whilst the fetal cardiovascular response to acute hypoxia is well established, the fetal defence to chronic hypoxia is not well understood due to experiment constraints. Growth restriction results primarily from reduced oxygen and nutrient supply to the developing fetus, resulting in chronic hypoxia. The fetus adapts to chronic hypoxia by redistributing cardiac output via brain sparing in an attempt to preserve function in the developing brain. This review highlights the impact of brain sparing on the developing fetal cardiovascular and cerebrovascular systems, as well as emerging long-term effects in offspring that were growth restricted at birth. Here, we explore the pathogenesis associated with brain sparing within the cerebrovascular system. An increased understanding of the mechanistic pathways will be critical to preventing neuropathological outcomes, including motor dysfunction such as cerebral palsy, or behaviour dysfunctions including autism and attention-deficit/hyperactivity disorder (ADHD).

Hypoxic modulation of fetal vascular MLCK abundance, localization, and function

Changes in vascular contractility are among the most important physiological effects of acute and chronic fetal hypoxia. Given the essential role of myosin light-chain kinase (MLCK) in smooth muscle contractility and its heterogeneous distribution, this study explores the hypothesis that subcellular changes in MLCK distribution contribute to hypoxic modulation of fetal carotid artery contractility. Relative to common carotid arteries from normoxic term fetal lambs (FN), carotids from fetal lambs gestated at high altitude (3,802 m) (FH) exhibited depressed contractility without changes in MLCK mRNA or protein abundance. Patterns of confocal colocalization of MLCK with α-actin and 20-kDa regulatory myosin light chain (MLC20) enabled calculation of subcellular MLCK fractions: 1) colocalized with the contractile apparatus, 2) colocalized with α-actin distant from the contractile apparatus, and 3) not colocalized with α-actin. Chronic hypoxia did not affect MLCK abundance in the contractile fraction, despite a concurrent decrease in contractility. Organ culture for 72 h under 1% O2 decreased total MLCK abundance in FN and FH carotid arteries, but decreased the contractile MLCK abundance only in FH carotid arteries. Correspondingly, culture under 1% O2 depressed contractility more in FH than FN carotid arteries. In addition, hypoxia appeared to attenuate ubiquitin-independent proteasomal degradation of MLCK, as reported for other proteins. In aggregate, these results demonstrate that the combination of chronic hypoxia followed by hypoxic culture can induce MLCK translocation among at least three subcellular fractions with possible influences on contractility, indicating that changes in MLCK distribution are a significant component of fetal vascular responses to hypoxia.

The Neurovascular Unit: Effects of Brain Insults During the Perinatal Period

The neurovascular unit (NVU) is a relatively recent concept in neuroscience that broadly describes the relationship between brain cells and their blood vessels. The NVU incorporates cellular and extracellular components involved in regulating cerebral blood flow and blood-brain barrier function. The NVU within the adult brain has attracted strong research interest and its structure and function is well described, however, the NVU in the developing brain over the fetal and neonatal period remains much less well known. One area of particular interest in perinatal brain development is the impact of known neuropathological insults on the NVU. The aim of this review is to synthesize existing literature to describe structure and function of the NVU in the developing brain, with a particular emphasis on exploring the effects of perinatal insults. Accordingly, a brief overview of NVU components and function is provided, before discussion of NVU development and how this may be affected by perinatal pathologies. We have focused this discussion around three common perinatal insults: prematurity, acute hypoxia, and chronic hypoxia. A greater understanding of processes affecting the NVU in the perinatal period may enable application of targeted therapies, as well as providing a useful basis for research as it expands further into this area.

Changes in Endothelial Nitric Oxide Production in Systemic Vessels during Early Ontogenesis-A Key Mechanism for the Perinatal Adaptation of the Circulatory System

Nitric oxide (NO) produced in the wall of blood vessels is necessary for the regulation of vascular tone to ensure an adequate blood supply of organs and tissues. In this review, we present evidence that the functioning of endothelial NO-synthase (eNOS) changes considerably during postnatal maturation. Alterations in NO-ergic vasoregulation in early ontogeny vary between vascular beds and correlate with the functional reorganization of a particular organ. Importantly, the anticontractile effect of NO can be an important mechanism responsible for the protectively low blood pressure in the immature circulatory system. The activity of eNOS is regulated by a number of hormones, including thyroid hormones which are key regulators of the perinatal developmental processes. Maternal thyroid hormone deficiency suppresses the anticontractile effect of NO at perinatal age. Such alterations disturb perinatal cardiovascular homeostasis and lead to delayed occurring cardiovascular pathologies in adulthood. The newly discovered role of thyroid hormones may have broad implications in cardiovascular medicine, considering the extremely high prevalence of maternal hypothyroidism in human society.

Fetal Cerebrovascular Maturation: Effects of Hypoxia

The human cerebral vasculature originates in the fourth week of gestation and continues to expand and diversify well into the first few years of postnatal life. A key feature of this growth is smooth muscle differentiation, whereby smooth muscle cells within cerebral arteries transform from migratory to proliferative to synthetic and finally to contractile phenotypes. These phenotypic transformations can be reversed by pathophysiological perturbations such as hypoxia, which causes loss of contractile capacity in immature cerebral arteries. In turn, loss of contractility affects all whole-brain cerebrovascular responses, including those involved in flow-metabolism coupling, vasodilatory responses to acute hypoxia and hypercapnia, cerebral autoregulation, and reactivity to activation of perivascular nerves. Future strategies to minimize cerebral injury following hypoxia-ischemic insults in the immature brain might benefit by targeting treatments to preserve and promote contractile differentiation in the fetal cerebrovasculature. This could potentially be achieved through inhibition of receptor tyrosine kinase-mediated growth factors, such as vascular endothelial growth factor and platelet-derived growth factor, which are mobilized by hypoxic and ischemic injury and which facilitate contractile dedifferentiation. Interruption of the effects of other vascular mitogens, such as endothelin and angiotensin-II, and even some miRNA species, also could be beneficial. Future experimental work that addresses these possibilities offers promise to improve current clinical management of neonates who have suffered and survived hypoxic, ischemic, asphyxic, or inflammatory cerebrovascular insults.

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.

Long-term hypoxia uncouples Ca2+ and eNOS in bradykinin-mediated pulmonary arterial relaxation

Bradykinin-induced activation of the pulmonary endothelium triggers a rise in intracellular Ca²⁺ that activates nitric oxide (NO)-dependent vasorelaxation. Chronic hypoxia is commonly associated with increased pulmonary vascular tone, which can cause pulmonary hypertension in responsive individuals. In the present study, we tested the hypothesis that long-term high-altitude hypoxia (LTH) diminishes bradykinin-induced Ca²⁺ signals and inhibits endothelial nitric oxide synthase (eNOS), prostacyclin (PGI₂), and large-conductance K⁺ (BK_Ca) channels in sheep, which are moderately responsive to LTH, resulting in decreased pulmonary arterial vasorelaxation. Pulmonary arteries were isolated from ewes kept near sea level (720 m) or at high altitude (3,801 m) for >100 days. Vessel force was measured with wire myography and endothelial intracellular Ca²⁺ with confocal microscopy. eNOS was inhibited with 100 μM N^G-nitro-l-arginine methyl ester (l-NAME), PGI₂ production was inhibited with 10 µM indomethacin that inhibits cyclooxygenase, and BK_Ca channels were blocked with 1 mM tetraethylammonium. Bradykinin-induced endothelial Ca²⁺ signals increased following LTH, but bradykinin relaxation decreased. Furthermore, some vessels contracted in response to bradykinin after LTH. l-NAME sensitivity decreased, suggesting that eNOS dysfunction played a role in uncoupling Ca²⁺ signals and bradykinin relaxation. The Ca²⁺ ionophore A-23187 (10 µM) elicited an enhanced Ca²⁺ response following LTH while relaxation was unchanged although l-NAME sensitivity increased. Additionally, BK_Ca function decreased during bradykinin relaxation following LTH. Western analysis showed that BK_Ca α-subunit expression was increased by LTH while that for the β₁ subunit was unchanged. Overall, these results suggest that those even moderately responsive to LTH can have impaired endothelial function.

Chronic hypoxia alters fetal cerebrovascular responses to endothelin-1

In utero hypoxia influences the structure and function of most fetal arteries, including those of the developing cerebral circulation. Whereas the signals that initiate this hypoxic remodeling remain uncertain, these appear to be distinct from the mechanisms that maintain the remodeled vascular state. The present study explores the hypothesis that chronic hypoxia elicits sustained changes in fetal cerebrovascular reactivity to endothelin-1 (ET-1), a potent vascular contractant and mitogen. In fetal lambs, chronic hypoxia (3,820-m altitude for the last 110 days of gestation) had no significant effect on plasma ET-1 levels or ETA receptor density in cerebral arteries but enhanced contractile responses to ET-1 in an ETA-dependent manner. In organ culture (24 h), 10 nM ET-1 increased medial thicknesses less in hypoxic than in normoxic arteries, and these increases were ablated by inhibition of PKC (chelerythrine) in both normoxic and hypoxic arteries but were attenuated by inhibition of CaMKII (KN93) and p38 (SB203580) in normoxic but not hypoxic arteries. As indicated by Ki-67 immunostaining, ET-1 increased medial thicknesses via hypertrophy. Measurements of colocalization between MLCK and SMαA revealed that organ culture with ET-1 also promoted contractile dedifferentiation in normoxic, but not hypoxic, arteries through mechanisms attenuated by inhibitors of PKC, CaMKII, and p38. These results support the hypothesis that chronic hypoxia elicits sustained changes in fetal cerebrovascular reactivity to ET-1 through pathways dependent upon PKC, CaMKII, and p38 that cause increased ET-1-mediated contractility, decreased ET-1-mediated smooth muscle hypertrophy, and a depressed ability of ET-1 to promote contractile dedifferentiation.

Metabolic Profiles in Ovine Carotid Arteries with Developmental Maturation and Long-Term Hypoxia

Background

Long-term hypoxia (LTH) is an important stressor related to health and disease during development. At different time points from fetus to adult, we are exposed to hypoxic stress because of placental insufficiency, high-altitude residence, smoking, chronic anemia, pulmonary, and heart disorders, as well as cancers. Intrauterine hypoxia can lead to fetal growth restriction and long-term sequelae such as cognitive impairments, hypertension, cardiovascular disorders, diabetes, and schizophrenia. Similarly, prolonged hypoxic exposure during adult life can lead to acute mountain sickness, chronic fatigue, chronic headache, cognitive impairment, acute cerebral and/or pulmonary edema, and death.

Aim

LTH also can lead to alteration in metabolites such as fumarate, 2-oxoglutarate, malate, and lactate, which are linked to epigenetic regulation of gene expression. Importantly, during the intrauterine life, a fetus is under a relative hypoxic environment, as compared to newborn or adult. Thus, the changes in gene expression with development from fetus to newborn to adult may be as a consequence of underlying changes in the metabolic profile because of the hypoxic environment along with developmental maturation. To examine this possibility, we examined the metabolic profile in carotid arteries from near-term fetus, newborn, and adult sheep in both normoxic and long-term hypoxic acclimatized groups.

Results

Our results demonstrate that LTH differentially regulated glucose metabolism, mitochondrial metabolism, nicotinamide cofactor metabolism, oxidative stress and antioxidants, membrane lipid hydrolysis, and free fatty acid metabolism, each of which may play a role in genetic-epigenetic regulation.

Role of the sympathetic autonomic nervous system in hypoxic remodeling of the fetal cerebral vasculature

Fetal hypoxia triggers compensatory angiogenesis and remodeling through mechanisms not fully elucidated. In response to hypoxia, hypoxia-inducible factor drives expression of cytokines that exert multiple effects on cerebral structures. Among these, the artery wall is composed of a heterogeneous cell mix and exhibits distinct patterns of cellular differentiation and reactivity. Governing these patterns are the vascular endothelium, smooth muscle (SM), adventitia, sympathetic perivascular nerves (SPN), and the parenchyma. Although an extensive literature details effects of nonneuronal factors on cerebral arteries, the trophic role of perivascular nerves remains unclear. Hypoxia increases sympathetic innervation with subsequent release of norepinephrine (NE), neuropeptide-Y (NPY), and adenosine triphosphate, which exert motor and trophic effects on cerebral arteries and influence dynamic transitions among SM phenotypes. Our data also suggest that the cerebrovasculature reacts very differently to hypoxia in fetuses and adults, and we hypothesize that these differences arise from age-related differences in arterial SM phenotype reactivity and proximity to trophic factors, particularly of neural origin. We provide an integration of recent literature focused on mechanisms by which SPN mediate hypoxic remodeling. Our recent findings suggest that trophic effects of SPN on cerebral arteries accelerate functional maturation through shifts in SM phenotype in an age-dependent manner.

Cerebrovascular adaptations to chronic hypoxia in the growth restricted lamb

Chronic moderate hypoxia induces angiogenic adaptation in the brain, reflecting a modulatory role for oxygen in determining cerebrovascular development. Chronic intrauterine fetal hypoxia, such as occurs in intrauterine growth restriction (IUGR) is likely to lead to a reduction in oxygen delivery to the brain and long-term neurological abnormalities. Thus we investigated whether vascular remodeling and vascular abnormalities were evident in the brain of IUGR newborn lambs that were chronically hypoxic in utero. Single uterine artery ligation (SUAL) surgery was performed in fetuses at ∼ 105 days gestation (term ∼ 145 days) to induce placental insufficiency and IUGR. Ewes delivered naturally at term and lambs were euthanased 24h later. IUGR brains (n = 9) demonstrated a significant reduction in positive staining for the number of blood vessels (laminin immunohistochemistry) compared with control (n = 8): from 1650 ± 284 to 416 ± 47 cells/mm(2) in subcortical white matter (SCWM) 1793 ± 298 to 385 ± 20 cells/mm(2) in periventricular white matter (PVWM), and 1717 ± 161 to 405 ± 84 cells/mm(2) in the subventricular zone (SVZ). The decrease in vascular density was associated with a significant decrease in VEGF immunoreactivity. The percentage of blood vessels exhibiting endothelial cell proliferation (Ki67 positive) varied regionally between 14 to 22% in white matter of control lambs, while only 1-3% of blood vessels in IUGR brains showed proliferation. A 66% reduction in pericyte coverage (α-SMA and desmin) of blood vessels was observed in SCWM, 71% in PVWM, and 73% in SVZ of IUGR lambs, compared to controls. A reduction in peri-vascular astrocytes (GFAP and laminin) was also observed throughout the white matter of IUGR lambs, and extravasation of albumin into the brain parenchyma was present, indicative of increased permeability of the blood brain barrier. Chronic hypoxia associated with IUGR results in a reduction in vascular density in the white matter of IUGR newborn brains. Vascular pericyte coverage and peri-vascular astrocytes, both of which are essential for stabilisation of blood vessels and the maintenance of vascular permeability, were also decreased in the white matter of IUGR lambs. In turn, these vascular changes could lead to inadequate oxygen supply and contribute to under-perfusion and increased vulnerability of white matter in IUGR infants.

The fetal cerebral circulation: three decades of exploration by the LLU Center for Perinatal Biology

For more than three decades, research programs in the Center of Perinatal Biology have focused on the vascular biology of the fetal cerebral circulation. In the 1980s, research in the Center demonstrated that cerebral autoregulation operated over a narrower pressure range, and was more vulnerable to insults, in fetuses than in adults. Other studies were among the first to establish that compared to adult cerebral arteries, fetal cerebral arteries were more hydrated, contained smaller smooth muscle cells and less connective tissue, and had endothelium less capable of producing NO. Work in the 1990s revealed that pregnancy depressed reactivity to NO in extra-cerebral arteries, but elevated it in cerebral arteries through effects involving changes in cGMP metabolism. Comparative studies verified that fetal lamb cerebral arteries were an excellent model for cerebral arteries from human infants. Biochemical studies demonstrated that cGMP metabolism was dramatically upregulated, but that contraction was far more dependent on calcium influx, in fetal compared to adult cerebral arteries. Further studies established that chronic hypoxia accelerates functional maturation of fetal cerebral arteries, as indicated by increased contractile responses to adrenergic agonists and perivascular adrenergic nerves. In the 2000s, studies of signal transduction established age-dependent roles for PKG, PKC, PKA, ERK, ODC, IP3, myofilament calcium sensitivity, and many other mechanisms. These diverse studies clearly demonstrated that fetal cerebral arteries were functionally quite distinct compared to adult cerebral arteries. In the current decade, research in the Center has expanded to a more molecular focus on epigenetic mechanisms and their role in fetal vascular adaptation to chronic hypoxia, maternal drug abuse, and nutrient deprivation. Overall, the past three decades have transformed thinking about, and understanding of, the fetal cerebral circulation due in no small part to the sustained research efforts by faculty and staff in the Center for Perinatal Biology.

Melatonin improves cerebrovascular function and decreases oxidative stress in chronically hypoxic lambs

Chronic hypoxia during gestation and delivery results in oxidative stress and cerebrovascular dysfunction in the neonate. We assessed whether melatonin, a potent antioxidant and potential vasodilator, improves the cerebral vascular function in chronically hypoxic neonatal lambs gestated and born in the highlands (3600 m). Six lambs received melatonin (1 mg/kg per day oral) and six received vehicle, once a day for 8 days. During treatment, biometry and hemodynamic variables were recorded. After treatment, lambs were submitted to a graded FiO2 protocol to assess cardiovascular responses to oxygenation changes. At 12 days old, middle cerebral arteries (MCA) were collected for vascular reactivity, morphostructural, and immunostaining evaluation. Melatonin increased fractional growth at the beginning and improved carotid blood flow at all arterial PO2 levels by the end of the treatment (P < 0.05). Further, melatonin treatment improved vascular responses to potassium, serotonin, methacholine, and melatonin itself (P < 0.05). In addition, melatonin enhanced the endothelial response via nitric oxide-independent mechanisms in isolated arteries (162 ± 26 versus 266 ± 34 AUC, P < 0.05). Finally, nitrotyrosine staining as an oxidative stress marker decreased in the MCA media layer of melatonin-treated animals (0.01357 ± 0.00089 versus 0.00837 ± 0.00164 pixels/μm2 , P < 0.05). All the melatonin-induced changes were associated with no systemic cardiovascular alterations in vivo. In conclusion, oral treatment with melatonin modulates cerebral vascular function, resulting in a better cerebral perfusion and reduced oxidative stress in the neonatal period in chronically hypoxic lambs. Melatonin is a potential therapeutic agent for treating cerebrovascular dysfunction associated with oxidative stress and developmental hypoxia in neonates.

Age-related remodeling of small arteries is accompanied by increased sphingomyelinase activity and accumulation of long-chain ceramides

The structure and function of large arteries alters with age leading to increased risk of cardiovascular disease. Age‐related large artery remodeling and arteriosclerosis is associated with increased collagen deposition, inflammation, and endothelial dysfunction. Bioactive sphingolipids are known to regulate these processes, and are also involved in aging and cellular senescence. However, less is known about age‐associated alterations in small artery morphology and function or whether changes in arterial sphingolipids occur in aging. We show that mesenteric small arteries from old sheep have increased lumen diameter and media thickness without a change in media to lumen ratio, indicative of outward hypertrophic remodeling. This remodeling occurred without overt changes in blood pressure or pulse pressure indicating it was a consequence of aging per se. There was no age‐associated change in mechanical properties of the arteries despite an increase in total collagen content and deposition of collagen in a thickened intima layer in arteries from old animals. Analysis of the sphingolipid profile showed an increase in long‐chain ceramide (C14–C20), but no change in the levels of sphingosine or sphingosine‐1‐phosphate in arteries from old compared to young animals. This was accompanied by a parallel increase in acid and neutral sphingomyelinase activity in old arteries compared to young. This study demonstrates remodeling of small arteries during aging that is accompanied by accumulation of long‐chain ceramides. This suggests that sphingolipids may be important mediators of vascular aging.

In this study, we have investigated remodeling of small arteries in a large animal model of aging, the sheep. We show that there is age‐related formation of neointima and increased collagen deposition that is accompanied by changes in sphingolipid metabolism resulting in ceramide accumulation in the tissues. These are the first data implicating sphingolipids as important mediators of vascular aging in small arteries. Given that aging is a major risk factor for cardiovascular disease, our study opens a new area for further research into the mechanisms that underlie vascular remodeling in aging.

Vasotrophic regulation of age-dependent hypoxic cerebrovascular remodeling

Hypoxia can induce functional and structural vascular remodeling by changing the expression of trophic factors to promote homeostasis. While most experimental approaches have been focused on functional remodeling, structural remodeling can reflect changes in the abundance and organization of vascular proteins that determine functional remodeling. Better understanding of age-dependent hypoxic macrovascular remodeling processes of the cerebral vasculature and its clinical implications require knowledge of the vasotrophic factors that influence arterial structure and function. Hypoxia can affect the expression of transcription factors, classical receptor tyrosine kinase factors, non-classical G-protein coupled factors, catecholamines, and purines. Hypoxia's remodeling effects can be mediated by Hypoxia Inducible Factor (HIF) upregulation in most vascular beds, but alterations in the expression of growth factors can also be independent of HIF. PPARγ is another transcription factor involved in hypoxic remodeling. Expression of classical receptor tyrosine kinase ligands, including vascular endothelial growth factor, platelet derived growth factor, fibroblast growth factor and angiopoietins, can be altered by hypoxia which can act simultaneously to affect remodeling. Tyrosine kinase-independent factors, such as transforming growth factor, nitric oxide, endothelin, angiotensin II, catecholamines, and purines also participate in the remodeling process. This adaptation to hypoxic stress can fundamentally change with age, resulting in different responses between fetuses and adults. Overall, these mechanisms integrate to assure that blood flow and metabolic demand are closely matched in all vascular beds and emphasize the view that the vascular wall is a highly dynamic and heterogeneous tissue with multiple cell types undergoing regular phenotypic transformation.

Maturation and differentiation of the fetal vasculature

Rapid postnatal growth and differentiation of fetal arterial smooth muscle is coordinated by a cacophony of growth factors, one of the most important of which is vascular endothelial growth factor (VEGF). In fetal arterial smooth muscle, VEGF influences both the expression and intracellular organization of contractile proteins and helps mediate hypoxic vascular remodeling. Numerous factors influence the expression of VEGF and its receptors, including chronic hypoxia, maternal food restriction, glucocorticoids, and miRNA. Continued study of the coupling between VEGF and transcription factors such as myocardin that govern smooth muscle differentiation, offers great promise for better clinical management of neonates at risk for cardiovascular dysregulation.

Hypoxic depression of PKG-mediated inhibition of serotonergic contraction in ovine carotid arteries

Chronic hypoxia attenuates soluble guanylate cyclase-induced vasorelaxation in serotonin (5-HT)-contracted ovine carotid arteries. Because protein kinase G (PKG) mediates many effects of soluble guanylate cyclase activation through phosphorylation of multiple kinase targets in vascular smooth muscle, we tested the hypothesis that chronic hypoxia reduces the ability of PKG to phosphorylate its target proteins, which attenuates the ability of PKG to induce vasorelaxation. We also tested the hypothesis that hypoxia attenuates PKG expression and/or activity. Arteries from normoxic and chronically hypoxic (altitude of 3,820 m for 110 days) fetal and adult sheep were denuded of endothelium and equilibrated with 95% O2-5% CO2 in the presence of nitro-l-arginine methyl ester (l-NAME) and N(G)-nitro-l-arginine (l-NNA) to inhibit residual endothelial nitric oxide synthase. Concentration-response relations for 5-HT were determined in the presence of prazosin to minimize activation of α-adrenergic receptors. The PKG activator 8-(p-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate (8-pCTP-cGMP) reduced agonist binding affinity of the 5-HT receptor in a concentration-dependent manner that was attenuated by hypoxia. Expression and activity of PKG-I was not significantly affected by chronic hypoxia in either fetal or adult arteries, although PKG-I abundance was greater in fetal arteries. Pretreatment with the large conductance calcium-sensitive potassium channel (BK) inhibitor iberiotoxin attenuated the vasorelaxation induced by 8-pCPT-cGMP in normoxic but not chronically hypoxic arteries. These results support the hypothesis that hypoxia attenuates the vasorelaxant effects of PKG through suppression of the ability of PKG to activate large conductance calcium-sensitive potassium channels in arterial smooth muscle. The results also reveal that this hypoxic effect is greater in fetal than adult arteries and that chronic maternal hypoxia can profoundly affect fetal vascular function.

Chronic hypoxia and VEGF differentially modulate abundance and organization of myosin heavy chain isoforms in fetal and adult ovine arteries

Chronic hypoxia increases vascular endothelial growth factor (VEGF) and thereby promotes angiogenesis. The present study explores the hypothesis that hypoxic increases in VEGF also remodel artery wall structure and contractility through phenotypic transformation of smooth muscle. Pregnant and nonpregnant ewes were maintained at sea level (normoxia) or 3,820 m (hypoxia) for the final 110 days of gestation. Common carotid arteries harvested from term fetal lambs and nonpregnant adults were denuded of endothelium and studied in vitro. Stretch-dependent contractile stresses were 32 and 77% of normoxic values in hypoxic fetal and adult arteries. Hypoxic hypocontractility was coupled with increased abundance of nonmuscle myosin heavy chain (NM-MHC) in fetal (+37%) and adult (+119%) arteries. Conversely, hypoxia decreased smooth muscle MHC (SM-MHC) abundance by 40% in fetal arteries but increased it 123% in adult arteries. Hypoxia decreased colocalization of NM-MHC with smooth muscle α-actin (SM-αA) in fetal arteries and decreased colocalization of SM-MHC with SM-αA in adult arteries. Organ culture with physiological concentrations (3 ng/ml) of VEGF-A(165) similarly depressed stretch-dependent stresses to 37 and 49% of control fetal and adult values. The VEGF receptor antagonist vatalanib ablated VEGF's effects in adult but not fetal arteries, suggesting age-dependent VEGF receptor signaling. VEGF replicated hypoxic decreases in colocalization of NM-MHC with SM-αA in fetal arteries and decreases in colocalization of SM-MHC with SM-αA in adult arteries. These results suggest that hypoxic increases in VEGF not only promote angiogenesis but may also help mediate hypoxic arterial remodeling through age-dependent changes in smooth muscle phenotype and contractility.

Fetal cerebral oxygenation: the homeostatic role of vascular adaptations to hypoxic stress

The mammalian fetus is highly adapted for growth in a low-O(2) environment in which arterial O(2) tensions average near 30 mm Hg. Acute decreases in O(2) tension below this value elicit vasodilatation, but the responses are blunted compared to those observed in adults. Chronic hypoxia in the fetus stimulates a pattern of cerebrovascular remodeling that results in an increased wall thickness and decreased overall contractility and also depresses the capacity for cerebral vasodilatation through decreases in NO release, soluble guanylate cyclase activity, and expression of PKG substrates. Many of these hypoxic effects appear to be homeostatic and may be mediated by VEGFs, which increase in direct response to hypoxia and, in turn, can dramatically alter the expression and function of multiple contractile proteins in cerebrovascular smooth muscle through both endothelium-dependent and endothelium-independent effects on large artery smooth muscle.

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.

Endothelium-dependent control of vascular tone during early postnatal and juvenile growth

Endothelial dysfunction can develop at an early age in children with risk factors for cardiovascular disease. A clear understanding of the nature of this dysfunction and how it can worsen over time requires detailed information on the normal growth-related changes in endothelial function on which the pathological changes are superimposed. This review summarizes our current understanding of these normal changes, as derived from studies in four different mammalian species. Although the endothelium plays an important role in controlling vascular tone from birth onward, the vasoactive molecules that mediate this control often change during postnatal or juvenile growth. The specifics of this transition to an adult endothelial cell phenotype can vary depending on the vascular bed. During growth, the contribution of nitric oxide to endothelium-dependent dilation generally increases in the lung, cerebral cortex, and skeletal muscle, but decreases in the intestine. Endothelial capacity for release of other vasoactive factors (e.g., cyclooxygenase products, hydrogen peroxide, carbon monoxide) can also increase or decrease during growth. Although these changes have been well documented, there is less information on their underlying cellular or molecular events. Further research is required to clarify these mechanisms, and to evaluate the functional significance of such shifts in endothelial phenotype.

Effects of chronic hypoxia on soluble guanylate cyclase activity in fetal and adult ovine cerebral arteries

A broad variety of evidence obtained largely in pulmonary vasculature suggests that chronic hypoxia modulates vasoreactivity to nitric oxide (NO). The present study explores the general hypothesis that chronic hypoxia also modulates cerebrovascular reactivity to NO, and does so by modulating the activity of soluble guanylate cyclase (sGC), the primary target for NO in vascular smooth muscle. Pregnant and nonpregnant ewes were maintained at either sea level or at 3,820 m for the final 110 days of gestation, at which time middle cerebral arteries from term fetal lambs and nonpregnant adults were harvested. In both fetal and adult arteries, NO-induced vasodilatation was attenuated by chronic hypoxia and completely inhibited by 10 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a selective inhibitor of sGC. sGC abundance (in ng sGC/mg protein) measured via Western immunoblots was approximately 10-fold greater in fetal (17.6 +/- 1.6) than adult (1.7 +/- 0.3) arteries but was not affected by chronic hypoxia. The specific activity of sGC (in pmol cGMP.microg sGC(-1).min(-1)) was similar in fetal (255 +/- 64) and adult (280 +/- 75) arteries and was inhibited by chronic hypoxia in both fetal (120 +/- 10) and adult (132 +/- 26) arteries. Rates of cGMP degradation (in pmol cGMP.mg protein(-1).min(-1)) were similar in fetal (159 +/- 59) and adult (134 +/- 36) arteries but were not significantly depressed by chronic hypoxia in either fetal (115 +/- 25) or adult (108 +/- 25) arteries. The cGMP analog 8-(p-chlorophenylthio)-cGMP was a more potent vasorelaxant in fetal (pD(2) = 4.7 +/- 0.1) than adult (pD(2) = 4.3 +/- 0.1) arteries, but its ability to promote vasodilatation was not affected by chronic hypoxia in either age group. Together, these results reveal that hypoxic inhibition of NO-induced vasodilatation is attributable largely to attenuation of the specific activity of sGC and does not involve significant changes in sGC abundance, cGMP-phosphodiesterase activity, or the vasorelaxant activity of protein kinase G.

Effects of alpha-lipoic acid on endothelial function in aged diabetic and high-fat fed rats

BACKGROUND AND PURPOSE

This study was conducted to investigate the effects of alpha-lipoic acid (alpha-LA) on endothelial function in diabetic and high-fat fed animal models and elucidate the potential mechanism underlying the benefits of alpha-LA.

EXPERIMENTAL APPROACH

Plasma metabolites reflecting glucose and lipid metabolism, endothelial function, urinary albumin excretion (UAE), plasma and aortic malondialdehyde (MDA) and urinary 8-hydroxydeoxyguanosine (8-OHdG) were assessed in non-diabetic controls (Wistar rats), untreated Goto-Kakizaki (GK) diabetic and high-fat fed GK rats (fed with atherogenic diet only, treated with alpha-LA and treated with vehicle, for 3 months). Vascular eNOS, nitrotyrosine, carbonyl groups and superoxide anion were also assessed in the different groups.

KEY RESULTS

alpha-LA and soybean oil significantly reduced both total and non-HDL serum cholesterol and triglycerides induced by atherogenic diet. MDA, carbonyl groups, vascular superoxide and 8-OHdG levels were higher in GK and high-fat fed GK groups and fully reversed with alpha-LA treatment. High-fat fed GK diabetic rats showed significantly reduced endothelial function and increased UAE, effects ameliorated with alpha-LA. This endothelial dysfunction was associated with decreased NO production, decreased expression of eNOS and increased vascular superoxide production and nitrotyrosine expression.

CONCLUSIONS AND IMPLICATIONS

alpha-LA restores endothelial function and significantly improves systemic and local oxidative stress in high-fat fed GK diabetic rats. Improved endothelial function due to alpha-LA was at least partially attributed to recoupling of eNOS and increased NO bioavailability and represents a pharmacological approach to prevent major complications associated with type 2 diabetes.

Executive summary of the workshop on oxygen in neonatal therapies: controversies and opportunities for research

One of the most complex areas in perinatal/neonatal medicine is the use of oxygen in neonatal therapies. To address the knowledge gaps that preclude optimal, evidence-based care in this critical field of perinatal medicine, the National Institute of Child Health and Human Development organized a workshop, Oxygen in Neonatal Therapies: Controversies and Opportunities for Research, in August 2005. The information presented at the workshop included basic and translational oxygen research; a review of completed, ongoing, and planned clinical trials; oxygen administration for neonatal resuscitation; and a review of the collaborative home infant monitoring evaluation study. This article provides a summary of the discussions, focusing on major knowledge gaps, with prioritized suggestions for studies in this area.

Hypoxic adaptation during development: relation to pattern of neurological presentation and cognitive disability

Children with acute hypoxic-ischaemic events (e.g. stroke) and chronic neurological conditions associated with hypoxia frequently present to paediatric neurologists. Failure to adapt to hypoxia may be a common pathophysiological pathway linking a number of other conditions of childhood with cognitive deficit. There is evidence that congenital cardiac disease, asthma and sleep disordered breathing, for example, are associated with cognitive deficit, but little is known about the mechanism and whether there is any structural change. This review describes what is known about how the brain reacts and adapts to hypoxia, focusing on epilepsy and sickle cell disease (SCD). We prospectively recorded overnight oxyhaemoglobin saturation (SpO2) in 18 children with intractable epilepsy, six of whom were currently or recently in minor status (MS). Children with MS were more likely to have an abnormal sleep study defined as either mean baseline SpO2 <94% or >4 dips of >4% in SpO2/hour (p = .04). In our series of prospectively followed patients with SCD who subsequently developed acute neurological symptoms and signs, mean overnight SpO2 was lower in those with cerebrovascular disease on magnetic resonance angiography (Mann-Whitney, p = .01). Acute, intermittent and chronic hypoxia may have detrimental effects on the brain, the clinical manifestations perhaps depending on rapidity of presentation and prior exposure.

Chronic hypoxic decreases in soluble guanylate cyclase protein and enzyme activity are age dependent in fetal and adult ovine carotid arteries

The present study tests the hypothesis that chronic hypoxia enhances reactivity to nitric oxide (NO) through age-dependent increases in soluble guanylate cyclase (sGC) and protein kinase G (PKG) activity. In term fetal and adult ovine carotids, chronic hypoxia had no significant effect on mRNA levels for the beta1-subunit of sGC, but depressed sGC abundance by 16% in fetal and 50% in adult arteries, through possible depression of rates of mRNA translation (15% in fetal and 50% in adult) and/or increased protein turnover. Chronic hypoxia also depressed the catalytic activity of sGC, but only in fetal arteries (63%). Total sGC activity was reduced by chronic hypoxia in both fetal (69%) and adult (37%) carotid homogenates, but this effect was not observed in intact arteries when sGC activity was measured by timed accumulation of cGMP. In intact arteries treated with 300 microM 3-isobutyl-1-methylxanthine (IBMX), chronic hypoxia dramatically enhanced sGC activity in fetal (186%) but not adult (89%) arteries. This latter observation suggests that homogenization either removed an sGC activator, released an sGC inhibitor, or altered the phosphorylation state of the enzyme, resulting in reduced activity. In the absence of IBMX, chronic hypoxia had no significant effect on rates of cGMP accumulation. Chronic hypoxia also depressed the ability of the cGMP analog, 8-(p-chlorophenylthio)-cGMP, to promote vasorelaxation in both fetal (8%) and adult (12%) arteries. Together, these results emphasize the fact that intact and homogenized artery studies of sGC activity do not always yield equivalent results. The results further suggest that enhancement of reactivity to NO by chronic hypoxia must occur upstream of PKG and can only be possible if changes in cGMP occurred in functional compartments that afforded either temporal or chemical protection to the actions of phosphodiesterase. The range and age dependence of hypoxic effects observed also suggest that some responses to hypoxia must be compensatory and homeostatic, with reactivity to NO as the primary regulated variable.