Developmental changes in plasma catecholamine concentrations during normoxia and acute hypoxia in the chick embryo

Cardiovascular decline in offspring during the perinatal period in an ovine model of fetal growth restriction

Fetal growth restriction (FGR) increases the risk cardiovascular disease (CVD) in adulthood. Placental insufficiency and subsequent chronic fetal hypoxemia are causal factors for FGR, leading to a redistribution of blood flow that prioritizes vital organs. Subclinical signs of cardiovascular dysfunction are evident in growth-restricted neonates; however, the mechanisms programming for CVD in adulthood remain unknown. This study aimed to determine the potential mechanisms underlying structural and functional changes within the heart and essential (carotid) and nonessential (femoral) vascular beds in growth-restricted lambs. Placental insufficiency was surgically induced in ewes at 89 days gestational age (dGA, term = 148dGA). Three age groups were investigated: fetal (126dGA), newborn (24 h after preterm birth), and 4-wk-old lambs. In vivo and histological assessments of cardiovascular indices were undertaken. Resistance femoral artery function was assessed via in vitro wire myography and blockade of key vasoactive pathways including nitric oxide, prostanoids, and endothelium-dependent hyperpolarization. All lambs were normotensive throughout the first 4 wk of life. Overall, the FGR cohort had more globular hearts compared with controls (P = 0.0374). A progressive decline in endothelium-dependent vasodilation was demonstrated in FGR lambs compared with controls. Further investigation revealed that impairment of the prostanoid pathway may drive this reduction in vasodilatory capacity. Clinical indicators of CVD were not observed in our FGR lambs. However, subclinical signs of cardiovascular dysfunction were present in our FGR offspring. This study provides insight into potential mechanisms, such as the prostanoid pathway, that may warrant therapeutic interventions to improve cardiovascular development in growth-restricted newborns.NEW & NOTEWORTHY Our findings provide novel insight into the potential mechanisms that program for cardiovascular dysfunction in growth-restricted neonates as our growth-restricted lambs exhibited a progressive decline in endothelium-dependent vasodilation in the femoral artery between birth and 4 wk of age. Subsequent analyses indicated that this reduction in vasodilatory capacity is likely to be mediated by the prostanoid pathway and prostanoids could be a potential target for therapeutic interventions for fetal growth restriction (FGR).

Effects of Developmental Hypoxia on the Vertebrate Cardiovascular System

Developmental hypoxia has profound and persistent effects on the vertebrate cardiovascular system, but the nature, magnitude, and long-term outcome of the hypoxic consequences are species specific. Here we aim to identify common and novel cardiovascular responses among vertebrates that encounter developmental hypoxia, and we discuss the possible medical and ecological implications.

β3-Adrenoceptor, a novel player in the round-trip from neonatal diseases to cancer: Suggestive clues from embryo

The role of the β-adrenoceptors (β-ARs) in hypoxia-driven diseases has gained visibility after the demonstration that propranolol promotes the regression of infantile hemangiomas and ameliorates the signs of retinopathy of prematurity (ROP). Besides the role of β2-ARs, preclinical studies in ROP have also revealed that β3-ARs are upregulated by hypoxia and that they are possibly involved in retinal angiogenesis. In a sort of figurative round trip, peculiarities typical of ROP, where hypoxia drives retinal neovascularization, have been then translated to cancer, a disease equally characterized by hypoxia-driven angiogenesis. In this step, investigating the role of β3-ARs has taken advantage of the assumption that cancer growth uses a set of strategies in common with embryo development. The possibility that hypoxic induction of β3-ARs may represent one of the mechanisms through which primarily embryo (and then cancer, as an astute imitator) adapts to grow in an otherwise hostile environment, has grown evidence. In both cancer and embryo, β3-ARs exert similar functions by exploiting a metabolic shift known as the Warburg effect, by acquiring resistance against xenobiotics, and by inducing a local immune tolerance. An additional potential role of β3-AR as a marker of stemness has been suggested by the finding that its antagonism induces cancer cell differentiation evoking that β3-ARs may help cancer to grow in a nonhospital environment, a strategy also exploited by embryos. From cancer, the round trip goes back to neonatal diseases for which new possible interpretative keys and potential pharmacological perspectives have been suggested.

Critical developmental windows for morphology and hematology revealed by intermittent and continuous hypoxic incubation in embryos of quail (Coturnix coturnix)

Hypoxia during embryonic growth in embryos is frequently a powerful determinant of development, but at least in avian embryos the effects appear to show considerable intra- and inter-specific variation. We hypothesized that some of this variation may arise from different protocols that may or may not result in exposure during the embryo’s critical window for hypoxic effects. To test this hypothesis, quail embryos (Coturnix coturnix) in the intact egg were exposed to hypoxia (~15% O₂) during “early” (Day 0 through Day 5, abbreviated as D0-D5), “middle” (D6-D10) or “late” (D11-D15) incubation or for their entire 16–18 day incubation (“continuous hypoxia”) to determine critical windows for viability and growth. Viability, body mass, beak and toe length, heart mass, and hematology (hematocrit and hemoglobin concentration) were measured on D5, D10, D15 and at hatching typically between D16 and D18 Viability rate was ~50–70% immediately following the exposure period in the early, middle and late hypoxic groups, but viability improved in the early and late groups once normoxia was restored. Middle hypoxia groups showed continuing low viability, suggesting a critical period from D6-D10 for embryo viability. The continuous hypoxia group experienced viability reaching <10% after D15. Hypoxia, especially during late and continuous hypoxia, also inhibited growth of body, beak and toe when measured at D15. Full recovery to normal body mass upon hatching occurred in all other groups except for continuous hypoxia. Contrary to previous avian studies, heart mass, hematocrit and hemoglobin concentration were not altered by any hypoxic incubation pattern. Although hypoxia can inhibit embryo viability and organ growth during most incubation periods, the greatest effects result from continuous or middle incubation hypoxic exposure. Hypoxic inhibition of growth can subsequently be “repaired” by catch-up growth if a final period of normoxic development is available. Collectively, these data indicate a critical developmental window for hypoxia susceptibility during the mid-embryonic period of development.

Sildenafil therapy for fetal cardiovascular dysfunction during hypoxic development: studies in the chick embryo

KEY POINTS

Common complications of pregnancy, such as chronic fetal hypoxia, trigger a fetal origin of cardiovascular dysfunction and programme cardiovascular disease in later life. Sildenafil treatment protects placental perfusion and fetal growth, but whether the effects of sildenafil transcend the placenta to affect the fetus is unknown. Using the chick embryo model, here we show that sildenafil treatment directly protects the fetal cardiovascular system in hypoxic development, and that the mechanisms of sildenafil protection include reduced oxidative stress and increased nitric oxide bioavailability; Sildenafil does not protect against fetal growth restriction in the chick embryo, supporting the idea that the protective effect of sildenafil on fetal growth reported in mammalian studies, including humans, is secondary to improved placental perfusion. Therefore, sildenafil may be a good candidate for human translational antioxidant therapy to protect the chronically hypoxic fetus in adverse pregnancy.

ABSTRACT

There is a need for developing clinically translatable therapy for preventing fetal origins of cardiovascular disease in pregnancy complicated by chronic fetal hypoxia. Evidence shows that sildenafil protects placental perfusion and fetal growth. However, whether beneficial effects of sildenafil transcend onto the fetal heart and circulation in complicated development is unknown. We isolated the direct effects of sildenafil on the fetus using the chick embryo and hypothesised that sildenafil also protects fetal cardiovascular function in hypoxic development. Chick embryos (n = 11 per group) were incubated in normoxia or hypoxia (14% O₂ ) from day 1 and treated with sildenafil (4 mg kg^-1  day^-1 ) from day 13 of the 21-day incubation. Hypoxic incubation increased oxidative stress (4-hydroxynonenal, 141.1 ± 17.6% of normoxic control), reduced superoxide dismutase (60.7 ± 6.3%), increased phosphodiesterase type 5 expression (167 ± 13.7%) and decreased nitric oxide bioavailability (54.7 ± 6.1%) in the fetal heart, and promoted peripheral endothelial dysfunction (70.9 ± 5.6% AUC of normoxic control; all P < 0.05). Sildenafil treatment after onset of chronic hypoxia prevented the increase in phosphodiesterase expression (72.5 ± 22.4%), protected against oxidative stress (94.7 ± 6.2%) and normalised nitric oxide bioavailability (115.6 ± 22.3%) in the fetal heart, and restored endothelial function in the peripheral circulation (89.8 ± 2.9%). Sildenafil protects the fetal heart and circulation directly in hypoxic development via mechanisms including decreased oxidative stress and enhanced nitric oxide bioavailability. Sildenafil may be a good translational candidate for human antioxidant therapy to prevent fetal origins of cardiovascular dysfunction in adverse pregnancy.

Hypoxia-induced contraction of chicken embryo mesenteric arteries: mechanisms and developmental changes

The fetal cardiovascular responses to acute hypoxia include a redistribution of the cardiac output toward the heart and the brain at the expense of other organs, such as the intestine. We hypothesized that hypoxia exerts a direct effect on the mesenteric artery (MA) that may contribute to this response. Using wire myography, we investigated the response to hypoxia (Po2 ~2.5 kPa for 20 min) of isolated MAs from 15- to 21-day chicken embryos (E15, E19, E21), and 1- to 45-day-old chickens (P1, P3, P14, P45). Agonist-induced pretone or an intact endothelium were not required to obtain a consistent and reproducible response to hypoxia, which showed a pattern of initial rapid phasic contraction followed by a sustained tonic contraction. Phasic contraction was reduced by elimination of extracellular Ca2+ or by presence of the neurotoxin tetrodotoxin, the α1-adrenoceptor antagonist prazosin, or inhibitors of L-type voltage-gated Ca2+ channels (nifedipine), mitochondrial electron transport chain (rotenone and antimycin A), and NADPH oxidase (VAS2870). The Rho-kinase inhibitor Y27632 impaired both phasic and tonic contraction and, when combined with elimination of extracellular Ca2+, hypoxia-induced contraction was virtually abolished. Hypoxic MA contraction was absent at E15 but present from E19 and increased toward the first days posthatching. It then decreased during the first weeks of life and P45 MAs were unable to sustain hypoxia-induced contraction over time. In conclusion, the results of the present study demonstrate that hypoxic vasoconstriction is an intrinsic feature of chicken MA vascular smooth muscle cells during late embryogenesis and the perinatal period.

A review of fundamental principles for animal models of DOHaD research: an Australian perspective

Epidemiology formed the basis of ‘the Barker hypothesis’, the concept of ‘developmental programming’ and today’s discipline of the Developmental Origins of Health and Disease (DOHaD). Animal experimentation provided proof of the underlying concepts, and continues to generate knowledge of underlying mechanisms. Interventions in humans, based on DOHaD principles, will be informed by experiments in animals. As knowledge in this discipline has accumulated, from studies of humans and other animals, the complexity of interactions between genome, environment and epigenetics, has been revealed. The vast nature of programming stimuli and breadth of effects is becoming known. As a result of our accumulating knowledge we now appreciate the impact of many variables that contribute to programmed outcomes. To guide further animal research in this field, the Australia and New Zealand DOHaD society (ANZ DOHaD) Animals Models of DOHaD Research Working Group convened at the 2nd Annual ANZ DOHaD Congress in Melbourne, Australia in April 2015. This review summarizes the contributions of animal research to the understanding of DOHaD, and makes recommendations for the design and conduct of animal experiments to maximize relevance, reproducibility and translation of knowledge into improving health and well-being.

Sympathetic Innervation Promotes Arterial Fate by Enhancing Endothelial ERK Activity

RATIONALE

Arterial endothelial cells are morphologically, functionally, and molecularly distinct from those found in veins and lymphatic vessels. How arterial fate is acquired during development and maintained in adult vessels is incompletely understood.

OBJECTIVE

We set out to identify factors that promote arterial endothelial cell fate in vivo.

METHODS AND RESULTS

We developed a functional assay, allowing us to monitor and manipulate arterial fate in vivo, using arteries isolated from quails that are grafted into the coelom of chick embryos. Endothelial cells migrate out from the grafted artery, and their colonization of host arteries and veins is quantified. Here we show that sympathetic innervation promotes arterial endothelial cell fate in vivo. Removal of sympathetic nerves decreases arterial fate and leads to colonization of veins, whereas exposure to sympathetic nerves or norepinephrine imposes arterial fate. Mechanistically, sympathetic nerves increase endothelial ERK (extracellular signal-regulated kinase) activity via adrenergic α1 and α2 receptors.

CONCLUSIONS

These findings show that sympathetic innervation promotes arterial endothelial fate and may lead to novel approaches to improve arterialization in human disease.

Effect of Bosentan on Claudication Distance and Endothelium-Dependent Vasodilation in Hispanic Patients With Peripheral Arterial Disease

Endothelin (ET) is involved in the etiopathogenesis of peripheral arterial disease (PAD). We hypothesized that ET antagonism might improve the endothelial function, inflammatory status, and symptoms in PAD. This pilot randomized clinical trial was designed to determine the clinical efficacy, pleiotropic effects, and safety of dual ET-receptor antagonist bosentan in Hispanic patients with PAD presenting intermittent claudication. The Bosentan Population-Based Randomized Trial for Clinical and Endothelial Function Assessment on Endothelin Antagonism Therapy was a 12-month, randomized, controlled, parallel-group, double-blind, proof-of-concept pilot study evaluating the effect of bosentan on absolute claudication distance (primary efficacy end point), flow-mediated arterial dilation, and C-reactive protein levels (primary pleiotropic end points) in patients with PAD with Rutherford category 1 to 2 of recent diagnosis. Secondary end points included ankle-brachial index, subjective claudication distance, and safety. Of the 629 screened subjects, 56 patients were randomized 1:1 to receive bosentan for 12 weeks (n = 27) or placebo (n = 29). Six months after the initiation, a significant treatment effect in flow-mediated arterial dilation of 2.43 ± 0.3% (95% CI 1.75 to 3.12; p = 0.001), absolute claudication distance of 283 ± 23 m (95% CI 202 to 366; p = 0.01), ankle-brachial index of 0.16 ± 0.03 (95% CI 0.09 to 0.23; p = 0.001), and a decrease in C-reactive protein levels of -2.0 ± 0.5 mg/L (95% CI -2.8 to -1.1; p = 0.02) were observed in the bosentan-treated group compared to the control group. No severe adverse effects were found in the bosentan group. In conclusion, in Hispanic patients with intermittent claudication, bosentan was well tolerated and improved endothelial function and claudication distance as well as inflammatory and hemodynamic states.

Functional differences between the arteries perfusing gas exchange and nutritional membranes in the late chicken embryo

The chicken extraembryonic arterial system comprises the allantoic arteries, which irrigate the gas exchange organ (the chorioallantoic membrane, CAM) and the yolk sac (YS) artery, which irrigates the nutritional organ (the YS membrane). We compared, using wire myography, the reactivity of allantoic and YS arteries from 19-day chicken embryos (total incubation 21 days). The contractions induced by KCl, the adrenergic agonists norepinephrine (NE, nonselective), phenylephrine (α1), and oxymetazoline (α2), electric field stimulation (EFS), serotonin, U46619 (TP receptor agonist), and endothelin (ET)-1 and the relaxations induced by acetylcholine (ACh), sodium nitroprusside (SNP, NO donor), forskolin (adenylate cyclase activator), and isoproterenol (β-adrenergic agonist) were investigated. Extraembryonic allantoic arteries did not show α-adrenergic-mediated contraction (either elicited by exogenous agonists or EFS) or ACh-induced (endothelium-dependent) relaxation, whereas these responses were present in YS arteries. Interestingly, the intraembryonic segment of the allantoic artery showed EFS- and α-adrenergic-induced contraction and ACh-mediated relaxation. Moreover, glyoxylic acid staining showed the presence of catecholamine-containing nerves in the YS and the intraembryonic allantoic artery, but not in the extraembryonic allantoic artery. Isoproterenol- and forskolin-induced relaxation and ET-1-induced contraction were higher in YS than in allantoic arteries, whereas serotonin- and U46619-induced contraction and SNP-induced relaxation did not significantly differ between the two arteries. In conclusion, our study demonstrates a different pattern of reactivity in the arteries perfusing the gas exchange and the nutritional membranes of the chicken embryo.

Adenylyl cyclase signaling in the developing chick heart: the deranging effect of antiarrhythmic drugs

The adenylyl cyclase (AC) signaling system plays a crucial role in the regulation of cardiac contractility. Here we analyzed the key components of myocardial AC signaling in the developing chick embryo and assessed the impact of selected β-blocking agents on this system. Application of metoprolol and carvedilol, two commonly used β-blockers, at embryonic day (ED) 8 significantly downregulated (by about 40%) expression levels of AC5, the dominant cardiac AC isoform, and the amount of Gsα protein at ED9. Activity of AC stimulated by forskolin was also significantly reduced under these conditions. Interestingly, when administered at ED4, these drugs did not produce such profound changes in the myocardial AC signaling system, except for markedly increased expression of Giα protein. These data indicate that β-blocking agents can strongly derange AC signaling during the first half of embryonic heart development.

The role of nitric oxide in the cardiovascular response to chronic and acute hypoxia in White Leghorn chicken (Gallus domesticus)

AIM

Prenatal hypoxia due to placental insufficiency results in deleterious phenotypes and compensatory mechanisms including increased sympathetic tone. Utilizing the embryonic chicken model, we investigated (i) changes in nitric oxide (NO)-mediated tone in response to chronic hypoxic development and (ii) the in vivo role of NO-mediated tone during acute hypoxic exposure, which has not been previously studied. We hypothesized that NO tone on the cardiovascular system would be unaffected by chronic hypoxic incubation in White Leghorn chicken (Gallus domesticus) embryos.

METHODS

We measured arterial pressure, heart rate and femoral blood flow (via a Doppler flow probe) in response to acute hypoxia (10% O2 ) and pharmacological manipulations in normoxic- and hypoxic (15% O2 )-incubated embryos. This was performed at 70 and 90% of total incubation time (21 days). At 70% of incubation (day 15), blood volume and chorioallantoic membrane development are maximal; 90% of incubation (day 19) is 1 day prior to lung ventilation.

RESULTS

Acute hypoxic exposure decreased femoral flow in both 90% groups, but increased femoral artery resistance in the hypoxic group. NO tone increased during development, but was not affected by hypoxic incubation. Inhibition of NO production by L-NAME (100 mg kg(-1) ) revealed that NO plays a significant role in the flow response to hypoxia.

CONCLUSION

Chronic hypoxic incubation has no effect on cardiovascular NO tone during White Leghorn chicken development. In the intact animal, NO function during acute hypoxic stress is suppressed by hypoxic incubation, indicating that chronic hypoxic stress dampens the NO contribution.

Opposing effects of hypoxia on catecholaminergic locus coeruleus and hypocretin/orexin neurons in chick embryos

Terrestrial vertebrate embryos face a risk of low oxygen availability (hypoxia) that is especially great during their transition to air-breathing. To better understand how fetal brains respond to hypoxia, we examined the effects of low oxygen availability on brain activity in late-stage chick embryos (day 18 out of a 21-day incubation period). Using cFos protein expression as a marker for neuronal activity, we focused on two specific, immunohistochemically identified cell groups known to play an important role in regulating adult brain states (sleep and waking): the noradrenergic neurons of the Locus Coeruleus (NA-LC), and the Hypocretin/Orexin (H/O) neurons of the hypothalamus. cFos expression was also examined in the Pallium (the avian analog of the cerebral cortex). In adult mammalian brains, cFos expression changes in a coordinated way in these areas. In chick embryos, oxygen deprivation simultaneously activated NA-LC while deactivating H/O-producing neurons; it also increased cFos expression in the Pallium. Activity in one pallial primary sensory area was significantly related to NA-LC activity. These data reveal that at least some of the same neural systems involved in brain-state control in adults may play a central role in orchestrating prenatal hypoxic responses, and that these circuits may show different patterns of coordination than seen in adults.

Temperature-dependent sex determination modulates cardiovascular maturation in embryonic snapping turtles Chelydra serpentina

We investigated sex differences in cardiovascular maturation in embryos of the snapping turtle Chelydra serpentina, a species with temperature-dependent sex determination. One group of eggs was incubated at 26.5°C to produce males. Another group of eggs was incubated at 26.5°C until embryos reached stage 17; eggs were then shifted to 31°C for 6 days to produce females, and returned to 26.5°C for the rest of embryogenesis. Thus, males and females were at the same temperature when autonomic tone was determined and for most of development. Cholinergic blockade increased resting blood pressure (P(m)) and heart rate (f(H)) in both sexes at 75% and 90% of incubation. However, the magnitude of the f(H) response was enhanced in males compared with females at 90% of incubation. β-adrenergic blockade increased P(m) at 75% of incubation in both sexes but had no effect at 90% of incubation. β-adrenergic blockade reduced f(H) at both time points but produced a stronger response at 90% versus 75% of incubation. We found that α-adrenergic blockade decreased P(m) in both sexes at 75% and 90% of incubation and decreased f(H) at 75% of incubation in both sexes. At 90% of incubation, f(H) decreased in females but not males. Although these data clearly demonstrate sexual dimorphism in the autonomic regulation of cardiovascular physiology in embryos, further studies are needed to test whether differences are caused by endocrine signals from gonads or by a hormone-independent temperature effect.

Developmental changes in mesenteric artery reactivity in embryonic and newly hatched chicks

At birth, the intestine becomes the sole site for nutrient absorption requiring a dramatic increase in blood flow. The vascular changes accompanying this transition have been partly characterized in mammals. We investigated, using wire myography, the developmental changes in chick mesenteric artery (MA) reactivity. Rings of the MA from 15-day (E15) and 19-day (E19) chicken embryos (total incubation 21 days) as well as non-fed 0–3-h-old (NH3h) and first-fed 1-day-old (NH1d) newly hatched chicks contracted in response to KCl, norepinephrine (NE), U46619, and endothelin (ET)-1 and relaxed in response to acetylcholine (ACh), sodium nitroprusside (SNP), and forskolin indicating the presence of electro- and pharmaco-mechanical coupling as well as cGMP- and cAMP-mediated relaxation. In ovo development and transition to ex ovo life was accompanied by alterations in the response of the MAs, but a different developmental trajectory was observed for each reactivity pathway tested. Thus, the contractile efficacy of KCl underwent a linear increase (E15 < E19 < NH3h < NH1d). The efficacy of NE and U46619 increased in ovo, but not ex ovo (E15 < E19 = NH3h = NH1d) and the efficacy of ET-1 peaked at E19 (E15 < E19 > NH3h = NH1d). The relaxations elicited by ACh (endothelium-dependent), SNP, and forskolin did not undergo significant developmental changes. In conclusion, the ability of chick MAs to constrict in response to pharmacological stimuli increases during the embryonic period, but no dramatic changes are induced by hatching or the first feeding. Maturation of vasodilator mechanisms precedes that of vasoconstrictor mechanisms. Alterations of the delicate balance between vasoconstrictors and vasodilators may play an important role in perinatal intestinal diseases.

Modulation of dopaminergic system and neurobehavioral functions in delayed neuropathy induced by organophosphates

Acute exposure to organophosphate pesticides (OPs) is associated with the development of a syndrome called organophosphate-induced delayed neuropathy (OPIDN) which is not mediated through hyper-cholinergic crisis. The present study has been designed to examine the role of alterations in dopaminergic system and neurobehavioral deficits in OPIDN. Rats were administered an acute dose of monocrotophos (MCP, 20 mg/kg body weight, orally) or dichlorvos (DDVP, 200 mg/kg body weight, subcutaneously), 15-20 min after treatment with antidotes (atropine (20 mg/kg body weight) and 2-pralidoxime (100 mg/kg body weight) intraperitoneally) to induce OPIDN. At biochemical level, an increase in dopamine, norepinephrine, and homovanillic acid levels were observed in brain of MCP- or DDVP-treated animals compared to controls. This was accompanied by increased intracellular calcium levels and lipid peroxidation in the cerebral cortex of OP-exposed animals. In addition, deficits in locomotor activity and spatial memory were observed in animals exposed to either MCP or DDVP. These results clearly suggest the role of dopaminergic system in memory and motor deficits observed in delayed neuropathy induced by OPs.

Maturation of the angiotensin II cardiovascular response in the embryonic White Leghorn chicken (Gallus gallus)

Angiotensin II (Ang II) is an important regulator of cardiovascular function in adult vertebrates. Although its role in regulating the adult system has been extensively investigated, the cardiovascular response to Ang II in embryonic vertebrates is relatively unknown. We investigated the potential of Ang II as a regulator of cardiovascular function in embryonic chickens, which lack central nervous system control of cardiovascular function throughout the majority of incubation. The cardiovascular response to Ang II in embryonic chickens was investigated over the final 50% of their development. Ang II produced a dose-dependent increase in arterial pressure on each day of development studied, and the response increased in intensity as development progressed. The Ang II type-1 receptor nonspecific competitive peptide antagonist [Sar(1) ile(8)] Ang II blocked the cardiovascular response to subsequent injections of Ang II on day 21 only. The embryonic pressure response to Ang II (hypertension only) differed from that of adult chickens, in which initial hypotension is followed by hypertension. The constant level of gene expression for the Ang II receptor, in conjunction with an increasing pressure response to the peptide, suggests that two Ang II receptor subtypes are present during chicken development. Collectively, the data indicate that Ang II plays an important role in the cardiovascular development of chickens; however, its role in maintaining basal function requires further study.

Ultrastructural evidence of a vesicle-mediated mode of cell degranulation in chicken chromaffin cells during the late phase of embryonic development

In the present investigation, we attempted to determine whether ultrastructural features indicative of a vesicle-mediated mode of cell secretion were detectable in chick chromaffin cells during embryo development. The adrenal anlagen of domestic fowls were examined at embryonic days (E) 12, 15, 19 and 21 by electron microscopy quantitative analysis. Morphometric evaluation revealed a series of granule and cytoplasmic changes highly specific for piecemeal degranulation (PMD), a secretory process based on vesicular transport of cargoes from within granules for extracellular release. At E19 and E21 we found a significant peak in the percentage of granules exhibiting changes indicative of progressive release of secretory materials, i.e. granules with lucent areas in their cores, reduced electron density, disassembled matrices, residual cores and membrane empty containers. A dramatic raise in the density of 30-80-nm-diameter, membrane-bound, electron-dense and electron-lucent vesicles--which were located either next to granules or close to the plasma membrane--was recognizable at E19, that is, during the prehatching phase. The cytoplasmic burst of dense and clear vesicles was paralleled by the appearance of chromaffin granules showing outpouches or protrusions of their profiles ('budding features'). These ultrastructural data are indicative of an augmented vesicle-mediated transport of chromaffin granule products for extracellular release in chick embryo chromaffin cells during the prehatching stage. In conclusion, this study provides new data on the fine structure of chromaffin cell organelles during organ development and suggests that PMD may be part of an adrenomedullary secretory response that occurs towards the end of chicken embryogenesis. From an evolutionary point of view, this study lends support to the concept that PMD is a secretory mechanism highly conserved throughout vertebrate classes.

Chronic prenatal hypoxia sensitizes beta-adrenoceptors in the embryonic heart but causes postnatal desensitization

Prenatal hypoxia in mammals causes fetal growth restriction and catecholaminergic overstimulation that, in turn, alter signaling pathways associated with adrenergic receptors. Beta-adrenoceptors (beta-ARs) are essential for fetal cardiac development and regulation of cardiac contractility. We studied the effects of chronic prenatal hypoxia on cardiac beta-AR signaling and the incidence of alterations in the juvenile beta-AR system due to the embryonic treatment. We measured functional beta-AR density (B(max)) through binding with [(3)H]CGP-12177 and the effect of agonists on beta-AR-dependent contractility (pEC(50)) through concentration-response curves to epinephrine. Eggs from broiler chickens were incubated in normoxia (N, 21% O(2)) or chronic hypoxia (H, 14% O(2)). Cardiac tissue from embryos and juveniles was used (15 and 19 day of embryonic development and 14 and 35 days posthatching, E19, E15, P14, and P35, respectively). Relative cardiac enlargement was found in the hypoxic groups at E15, E19, and P14, but not P35. B(max) significantly decreased in E19H. B(max) more than doubled posthatching but decreased from P14 to P35. The sensitivity to epinephrine was lower in E19N compared with E15N, but hypoxia increased the sensitivity to agonist in both E15H and E19H. Despite maintained receptor density, the P35H juvenile displayed a decreased sensitivity to beta-AR agonist, something that was not seen in P14H. The postnatal decrease in beta-AR sensitivity as an effect of chronic prenatal hypoxia, without a concomitant change in beta-AR density, leads us to conclude that the embryonic hypoxic challenge alters the future progression of beta-AR signaling and may have important implications for cardiovascular function in the adult.

Importance of adrenergic receptors in prenatally induced cognitive impairment in the domestic chick

In the domestic chick, mild hypoxia (24h of 14% oxygen) at two stages of embryonic development results in post-hatch memory deficiencies tested using a discriminated bead avoidance task. The nature of the memory loss depends on the gestational age at which the hypoxia occurs. Hypoxia on embryonic day 10 (E10) of a 21 day incubation results in chicks with no short-term memory 10 min after training, whereas hypoxia on day 14 (E14) results in chicks with good labile memory 30 min after training but no consolidation of memory into permanent storage (120 min). Hypoxia at E14 is associated with increased plasma levels of noradrenaline and therefore we suggest that altered catecholamine exposure within the brain contributes to cognitive problems by modifying the responsiveness of brain beta-adrenoceptors. In ovo administration of noradrenaline, or the beta(2)-adrenoceptor agonist formoterol, at E14 had the same effect on memory consolidation as hypoxia. Following hypoxia at E14, memory could be rescued after training by central injection of a beta(3)-adrenoceptor agonist, but not by a beta(2)-adrenoceptor agonist. The differences in the responsiveness of memory processing to beta(2)-adrenoceptor agonists suggests alterations to the receptors or downstream of the receptor activation. However, both types of beta-adrenoceptor agonists rescued memory in E10 treated chicks implying that at this age hypoxia does not affect the receptors. In summary, hypoxia or increased levels of stress hormones during incubation alters beta-adrenoceptor responsiveness; the outcome of the insult depends upon the cellular developmental processes at a given embryonic stage.

Developmental changes in the effects of prostaglandin E2 in the chicken ductus arteriosus

Prostaglandin E(2) (PGE(2)) is the major vasodilator prostanoid of the mammalian ductus arteriosus (DA). In the present study we analyzed the response of isolated DA rings from 15-, 19- and 21-day-old chicken embryos to PGE(2) and other vascular smooth muscle relaxing agents acting through the cyclic AMP signaling pathway. PGE(2) exhibited a relaxant response in the 15-day DA, but not in the 19- and 21-day DA. Moreover, high concentrations of PGE(2) (>or= 3 microM in 15-day and >or= 1 microM in 19-day and 21-day DA) induced contraction of the chicken DA. The presence of the TP receptor antagonist SQ29,548, unmasked a relaxant effect of PGE(2) in the 19- and 21-day DA and increased the relaxation induced by PGE(2) in the 15-day DA. The presence of the EP receptor antagonist AH6809 abolished PGE(2)-mediated relaxation. The relaxant responses induced by PGE(2) and the beta-adrenoceptor agonist isoproterenol, but not those elicited by the adenylate cyclase activator forskolin or the phosphodiesterase 3 inhibitor milrinone, decreased with maturation. High oxygen concentrations (95%) decreased the relaxation to PGE(2). The relaxing potency and efficacy of isoproterenol and milrinone were higher in the pulmonary than in the aortic side of the DA, whereas no regional differences were found in the response to PGE(2). We conclude that, in contrast to the mammalian situation, PGE(2) is a weak relaxant agent of the chicken DA and, with advancing incubation, it even stimulates TP vasoconstrictive receptors.

High oxygen prevents fetal lethality due to lack of catecholamines

The catecholamine norepinephrine is required for fetal survival, but its essential function is unknown. When catecholamine-deficient [tyrosine hydroxylase (Th) null] mouse fetuses die at embryonic day (E)13.5-14.5, they resemble wild-type (wt) fetuses exposed to hypoxia. They exhibit bradycardia (28% reduction in heart rate), thin ventricular myocardium (20% reduction in tissue), epicardial detachment, and death with vascular congestion, hemorrhage, and edema. At E12.5, before the appearance of morphological deficits, catecholamine-deficient fetuses are preferentially killed by experimentally induced hypoxia and have lower tissue Po(2) levels than wt siblings. By microarray analysis (http://www.ncbi.nlm.nih.gov/geo; accession no. GSE10341), hypoxia-inducible factor-1 target genes are induced to a greater extent in null fetuses than in wt siblings, supporting the notion that mutants experience lower oxygen tension or have an enhanced response to hypoxia. Hypoxia induces a 13-fold increase in plasma norepinephrine levels, which would be expected to increase heart rate, thereby improving oxygen delivery in wt mice. Surprisingly, increasing maternal oxygen (inspired O(2) 33 or 63%) prevents the effects of catecholamine deficiency, restoring heart rate, myocardial tissue, and survival of Th null fetuses to wt levels. We suggest that norepinephrine mediates fetal survival by maintaining oxygen homeostasis.

Early fetal hypoxia leads to growth restriction and myocardial thinning

Hypoxia is necessary for fetal development; however, excess hypoxia is detrimental. Hypoxia has been extensively studied in the near-term fetus, but less is known about earlier fetal effects. The purpose of this study was to determine the window of vulnerability to severe hypoxia, what organ system(s) is most sensitive, and why hypoxic fetuses die. We induced hypoxia by reducing maternal-inspired O2 from 21% to 8%, which decreased fetal tissue oxygenation assessed by pimonidazole binding. The mouse fetus was most vulnerable in midgestation: 24 h of hypoxia killed 89% of embryonic day 13.5 (E13.5) fetuses, but only 5% of E11.5 and 51% of E17.5 fetuses. Sublethal hypoxia at E12.5 caused growth restriction, reducing fetal weight by 26% and protein by 45%. Hypoxia induced HIF-1 target genes, including vascular endothelial growth factor (Vegf), erythropoietin, glucose transporter-1 and insulin-like growth factor binding protein-1 (Igfbp-1), which has been implicated in human intrauterine growth restriction (IUGR). Hypoxia severely compromised the cardiovascular system. Signs of heart failure, including loss of yolk sac circulation, hemorrhage, and edema, were caused by 18-24 h of hypoxia. Hypoxia induced ventricular dilation and myocardial hypoplasia, decreasing ventricular tissue by 50% and proliferation by 21% in vivo and by 40% in isolated cultured hearts. Epicardial detachment was the first sign of hypoxic damage in the heart, although expression of epicardially derived mitogens, such as FGF2, FGF9, and Wnt9b was not reduced. We propose that hypoxia compromises the fetus through myocardial hypoplasia and reduced heart rate.

Greater hypoxia-induced cell death in prenatal brain after bacterial-endotoxin pretreatment is not because of enhanced cerebral energy depletion: a chicken embryo model of the intrapartum response to hypoxia and infection

Infection is a risk factor for adult stroke and neonatal encephalopathy. We investigated whether exposure to bacterial endotoxin increases hypoxia-induced brain cell death and impairs cerebral metabolic compensatory responses to hypoxia. Prehatching chicken embryos (incubation day 19) were exposed to bacterial lipopolysaccharide (LPS) (3 mg Salmonella typhimurium LPS per egg) or hypoxia (4% ambient O(2) for 1 h), alone or in combination with LPS, followed 4 h later by hypoxia. Cerebral cell death and glial activation were assessed histologically. Further, chicken embryo brains were studied by magnetic resonance imaging (MRI) and spectroscopy (MRS) to assess haemodynamic and metabolic responses. In most brain areas, combined LPS/hypoxia resulted in a 30- to 100-fold increase in terminal deoxynucleotidyl transferase dUTP nick end labelling -positive cells, compared to control and single-insult groups. Glial activation correlated with the severity of cell death and was significantly greater in the combined-insult group (P<0.05). Hypoxia was associated with a 10-fold increase in lactate/N-acetyl-aspartate (NAA), an approximately 20% increase in total creatine/NAA, rapid decreases in T2 and T2(*), and a reduction in direction-averaged brain-water diffusion (D(av)) by approximately 15%. Liposaccharide pretreatment did not alter the magnitude or timing of these responses, but engendered baseline shifts (increased Cho/NAA, Cr/NAA, and Dav, and reduced T2(*)). In conclusion, LPS greatly increased hypoxia-induced brain damage in this model and induced changes in baseline haemodynamics and metabolism but did not affect the magnitude of the glycolytic response to hypoxia. The damage-enhancing effects of LPS are not because of additional energy depletion but because of a synergistic toxic component.

β2- and β3-Adrenoceptors activate glucose uptake in chick astrocytes by distinct mechanisms: a mechanism for memory enhancement?

Isoprenaline, acting at beta-adrenoceptors (ARs), enhances memory formation in single trial discriminated avoidance learning in day-old chicks by mechanisms involving alterations in glucose and glycogen metabolism. Earlier studies of memory consolidation in chicks indicated that beta3-ARs enhanced memory by increasing glucose uptake, whereas beta2-ARs enhance memory by increasing glycogenolysis. This study examines the ability of beta-ARs to increase glucose uptake in chick forebrain astrocytes. The beta-AR agonist isoprenaline increased glucose uptake in a concentration-dependent manner, as did insulin. Glucose uptake was increased by the beta2-AR agonist zinterol and the beta3-AR agonist CL316243, but not by the beta1-AR agonist RO363. In chick astrocytes, reverse transcription-polymerase chain reaction studies showed that beta1-, beta2-, and beta3-AR mRNA were present, whereas radioligand-binding studies showed the presence of only beta2- and beta3-ARs. beta-AR or insulin-mediated glucose uptake was inhibited by phosphatidylinositol-3 kinase and protein kinase C inhibitors, suggesting a possible interaction between the beta-AR and insulin pathways. However beta2- and beta3-ARs increase glucose uptake by two different mechanisms: beta2-ARs via a Gs-cAMP-protein kinase A-dependent pathway, while beta3-ARs via interactions with Gi. These results indicate that activation of beta2- and beta3-ARs causes glucose uptake in chick astrocytes by distinct mechanisms, which may be relevant for memory enhancement.

The effect of hypoxia at different embryonic ages on impairment of memory ability in chicks

Hypoxia during the prenatal period is a principal antecedent to cognitive impairment after birth. In this study we have investigated the duration, severity and timing of acute hypoxia during chick embryonic development to elucidate the relative importance of these factors. Our results show that 24h of hypoxia (exposure to 14% oxygen) at embryonic day 10 (E10) results in significant impairment of intermediate and long-term memory in the post-hatch chick, which is the same as we observed with 4 days of hypoxia. At E14, 24h of hypoxia, 5min of anoxia, but not 1h of hypoxia, resulted only in impaired long-term memory; the same as 4 days of hypoxia from E14. Corticosterone levels, measured post-hatch as an indicator of a stress response, were significantly elevated in response to E10 hypoxia, and E14 hypoxia (both 1 and 24h) and anoxia. In a separate experiment we exposed embryos to 24h of hypoxia from E6 to E16, and found that memory deficits resulted from hypoxia at E9 and E10, and E13-E15, while corticosterone concentrations at hatch were significantly raised following E10-E16 hypoxia. These results demonstrate that the developmental age when the insult occurs determines the nature of the cognitive deficit and, if the severity of the insult is sufficient, then the outcome, or deficits in memory ability, are consistent whether the insult is acute or chronic. Importantly, there are two critical stages in development, which in the chick are around E10 and E14, when acute hypoxia results in significant adverse cognitive effects after hatch. These time-points correspond to two different stages in growth and development.

Hemodynamic vulnerability to acute hypoxia in day 10.5-16.5 murine embryos

AIM

We tested the hypothesis that murine embryonic cardiovascular (CV) function is vulnerable to transient changes in maternal transplacental oxygen support during the critical period of CV morphogenesis.

METHODS

We measured maternal heart rate (MHR), maternal blood pressure (MBP), and embryonic heart rate (EHR) during mechanical ventilatory support, then induced transient maternal hypoxia daily from gestation day (ED) 10.5 to ED16.5 in pregnant ICR mice. Hypoxia was induced by suspending mechanical ventilation for 30 s or by the replacement of inspired oxygen with nitrogen (75% or 100%) for 30 s while maintaining ventilation.

RESULTS

We noted a rapid onset of maternal hypotension in response to hypoxia that quickly recovered following reoxygenation. Following a brief lag time that was not gestation specific, EHR decreased in response to hypoxia. The magnitude of embryo bradycardia and the rate of EHR decline and recovery displayed gestation specific patterns. The magnitude of embryo bradycardia was similar from ED10.5 to ED13.5 and then increased with gestation. Before ED13.5, only 40% of embryos recovered to the baseline EHR following transient maternal hypoxia (vs 80% of embryos after ED 13.5). EHR following recovery exceeded baseline EHR after ED15.5. Nitrogen inhalation (75% or 100%) produced changes in maternal and embryonic hemodynamics similar to suspended ventilation induced hypoxia.

CONCLUSIONS

The mammalian embryo is vulnerable to transient decreases in maternal oxygenation during the critical period of organogenesis and the gestational specific EHR response to hypoxia may reflect both increased embryonic oxygen demand and the maturation of neurohumoral heart rate regulation.

Birth and hatching: Key events in the onset of awareness in the lamb and chick

The neuroanatomical and neurophysiological development of the embryo and fetus and unique features of the physiological environment of the fetal brain, features which are lost at birth, support recent conclusions that under normal circumstances awareness (or consciousness) is probably not exhibited by the ovine embryo-fetus before birth and that it appears for the first time only after birth. However, there has apparently been no evaluation of whether or not similar mechanisms modulate awareness-related functions in domestic chicks before and after hatching. This comparative review, in seeking to rectify this, arrived at the following conclusions. First, the neural apparatus of both lambs and chicks appears to be too immature to support any states resembling awareness during at least the first half of pregnancy or incubation. Second, electroencephalographic (EEG) activity, which evolves subsequently, shows that states of sleep-like unconsciousness are likely to be continuously present in lambs until after birth, and that such states at least predominate in chicks until after hatching. Third, as in fetal lambs, epochs of so-called 'wakefulness' previously reported in chick embryos do not seem likely to represent short periods of awareness in ovo. Fourth, several neurosuppressive mechanisms, with some unique features, also operate or have the potential to operate in chicks before hatching, but a dearth of published information currently hinders a full comparison with those demonstrated to operate in fetal lambs. Fifth, contradicting the intuitive perception that vocalisation pre-hatching by the chick indicates the presence of awareness, published evidence suggests that vocalisation before and during hatching occurs mostly during EEG states indicating sleep-like unconsciousness. Sixth, as seems to be the case for newborn lambs after birth, it is possible that demonstrable awareness may appear for the first time only after hatching in chicks, presumably through waning neurosuppression and burgeoning neuroactivation, but such awareness seems to take longer to manifest itself. However, additional research in chicks is recommended to further assess this suggestion. Particular attention should be given to the status of vocal interactions between hen and chick which begin several days before hatching, and to the operation of neurosuppressive and neuroactivating mechanisms throughout the last 40% of incubation and during and after hatching.

Development of the ovine fetal cardiovascular defense to hypoxemia towards full term

We tested the hypothesis that fetal cardiovascular responses to hypoxemia change close to full term in relation to the prepartum increase in fetal basal cortisol and investigated, in vivo, the neural and endocrine mechanisms underlying these changes. Fetal heart rate and peripheral hemodynamic responses to 1 h of hypoxemia were studied in 25 chronically instrumented sheep within three narrow gestational age ranges: 125–130 ( n = 13), 135–140 ( n = 6), and >140 ( n = 6) days (full term ∼145 days). Chemoreflex function and plasma concentrations of vasoconstrictor hormones were measured. Reductions in fetal arterial Po2 during hypoxemia were similar at all ages. At 125–130 days, hypoxemia elicited transient bradycardia, femoral vasoconstriction, and increases in plasma concentrations of catecholamines, neuropeptide Y (NPY), AVP, ACTH, and cortisol. Close to full term, in association with the prepartum increase in fetal basal cortisol, there was a developmental increase in the magnitude and persistence of fetal bradycardia and in the magnitude of the femoral constrictor response to hypoxemia. The mechanisms mediating these changes close to full term included increases in the gain of chemoreflex function and in the magnitudes of the fetal NPY and AVP responses to hypoxemia. Data combined irrespective of gestational age revealed significant correlations between fetal basal cortisol and fetal bradycardia, femoral resistance, chemoreflex function, and plasma AVP concentrations. The data show that the fetal cardiovascular defense to hypoxemia changes in pattern and magnitude just before full term because of alterations in the gain of the neural and endocrine mechanisms mediating them, in parallel with the prepartum increase in fetal basal cortisol.

Ontogeny of chicken ductus arteriosus response to oxygen and vasoconstrictors

The present study aimed to characterize the contractile reactivity of the chicken ductus arteriosus (DA) from the last stage of prenatal development and throughout the perinatal period. Isolated DA rings from 15-day, noninternally-pipped 19-day, and externally-pipped 21-day embryos were studied using myograph techniques. On embryonic day 15, the chicken DA did not respond to O(2) (0 to 21%), norepinephrine (NE), or phenylephrine (Phe) but contracted in response to high-K(+) solution, the inhibitor of voltage-gated channels 4-aminopyridine, U-46619, and endothelin (ET)-1. These responses increased with advancing incubation age. Contractile responses to O(2), NE, and Phe were present in the 19- and 21-day embryo. Oxygen-induced contraction was restricted to the pulmonary side of the DA and was augmented by the nitric oxide synthase inhibitor N(omega)-nitro-l-arginine methyl ester and the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one and reduced by the peptidic ET(A) and ET(B)-receptor antagonist PD-142,893. Transmural electrical stimulation of nerves, the nonselective cyclooxygenase (COX) inhibitor indomethacin, the COX-1 inhibitor valeryl salicylate, the COX-2 inhibitor nimesulide, the inhibitor of ATP-sensitive K(+) channels glibenclamide, and the inhibitor of Ca(2+)-activated K(+) channels tetraethylammonium did not cause contraction of the DA rings at any age. We conclude that transition to ex ovo life is accompanied by dramatic changes in chicken DA reactivity. At 0.7 incubation, excitation-contraction and pharmacomechanical coupling for several contractile agonists are already present, whereas the constrictor effects of O(2) and cathecolamines appear later in development and are located in the pulmonary side of the DA.

Impact of hypoxia on early chick embryo growth and cardiovascular function

Oxygen tension is a critical factor for appropriate embryonic and fetal development. Chronic hypoxia exposure alters cardiovascular (CV) function and structure in the late fetus and newborn, yet the immature myocardium is considered to be less sensitive to hypoxia than the mature heart. We tested the hypothesis that hypoxia during the period of primary CV morphogenesis impairs immature embryonic CV function and embryo growth. We incubated fertile white Leghorn chick embryos in 15% oxygen (hypoxia) or 21% oxygen (control) until Hamburger-Hamilton stage 21 (3.5 d). We assessed in ovo viability and dysmorphic features and then measured ventricular pressure and dimensions and dorsal aortic arterial impedance at stage 21. Chronic hypoxia decreased viability and embryonic wet weight. Chronic hypoxia did not alter heart rate or the ventricular diastolic indices of end-diastolic pressure, maximum ventricular -dP/dt, or tau. Chronic hypoxia decreased maximum ventricular +dP/dt and peak pressure, increased ventricular end-systolic volume, and decreased ventricular ejection fraction, consistent with depressed systolic function. Arterial afterload (peripheral resistance) increased and both dorsal aortic SV and steady-state hydraulic power decreased in response to hypoxia. Thus, reduced oxygen tension during early cardiac development depresses ventricular function, increases ventricular impedance (afterload), delays growth, and decreases embryo survival, suggesting that a critical threshold of oxygen tension is required to support morphogenesis and cardiovascular function in the early embryo.

Prenatal hypoxia impairs memory function but does not result in overt structural alterations in the postnatal chick brain

We showed previously that hypoxia in ovo impairs memory consolidation in the chick tested 2 days after hatching. Our present aim was to investigate whether we could detect any morphological effects of the same prenatal hypoxia. Hypoxia was induced by half-wrapping the egg with an impermeable membrane from either days 10-18 (W10-18 chicks) or days 14-18 (W14-18 chicks) of incubation (hatching approximately 21 days). Measurement of blood gases showed that reducing the surface area of the egg for gas exchange resulted in reduced pO2 and increased pCO2 2 days after wrapping. Although this hypoxia was sufficient to impair cognitive processing in the postnatal chick, our data suggest that it did not produce overt structural alterations or changes in the number of neurons, glutamine synthetase-immunoreactive cells or immunoreactivity to synaptophysin in the presynaptic vesicles in the multimodal integration (cortical) area compared to controls. Hence, we found no differences in the astrocyte to neuron ratio, synaptic density and/or vesicle number. Analysis of the ontogeny of astrocytes during the prenatal period of hypoxia showed them to be present at embryonic day 12, but not at the earlier ages examined. Although we found cognitive deficits in chicks from embryos made hypoxic during incubation, our regimen of prenatal hypoxia did not alter any of the parameters measured in the brains. This does not preclude the possibility that changes have occurred at the cellular or molecular levels or in specific neurotransmitter systems.

Cardiovascular development in embryos of the American alligator Alligator mississippiensis: effects of chronic and acute hypoxia

Chronic hypoxic incubation is a common tool used to address the plasticity of morphological and physiological characteristics during vertebrate development. In this study chronic hypoxic incubation of embryonic American alligators resulted in both morphological (mass) and physiological changes. During normoxic incubation embryonic mass, liver mass and heart mass increased throughout the period of study, while yolk mass fell. Chronic hypoxia(10%O2) resulted in a reduced embryonic mass at 80% and 90% of incubation. This reduction in embryonic mass was accompanied by a relative enlargement of the heart at 80% and 90% of incubation, while relative embryonic liver mass was similar to the normoxic group. Normoxic incubated alligators maintained a constant heart rate during the period of study, while mean arterial pressure rose continuously. Both levels of hypoxic incubation(15% and 10%O2) resulted in a lower mean arterial pressure at 90%of incubation, while heart rate was lower in the 10%O2 group only. Acute (5 min) exposure to 10%O2 in the normoxic group resulted in a biphasic response, with a normotensive bradycardia occurring during the period of exposure and a hypertensive tachycardic response occurring during recovery. The embryos incubated under hypoxia also showed a blunted response to acute hypoxic stress. In conclusion, the main responses elicited by chronic hypoxic incubation, namely, cardiac enlargement, blunted hypoxic response and systemic vasodilation, may provide chronically hypoxic embryos with a new physiological repertoire for responding to hypoxia.

The role of catecholamines in memory impairment in chicks following reduced gas exchange in ovo

We have shown previously that reducing gas exchange to chick embryos by half wrapping eggs with an impermeable membrane from either days 14-18 (W14-18) or days 10-18 (W10-18) of the 21 day incubation results in post-hatch memory deficits. In the W10-18 chicks, short-term memory following training is impaired, whereas in the W14-18 chicks, memory is intact for 30 min but does not consolidate into long-term storage. The reduction in gas exchange caused by half wrapping eggs resulted in alterations in hematocrit, O2 and CO2 tensions suggesting that the embryos are hypoxic and hypercapnic. Our aim was to test the hypothesis that increases in circulating levels of catecholamines in ovo, as a result of hypoxia, lead to a disturbance of the central noradrenergic pathways resulting in cognitive impairment. Noradrenaline is critical for memory consolidation and a disturbance during development could compromise cognitive ability. In the present study, plasma noradrenaline levels were significantly elevated compared with control levels 2 days after hatch in W14-18 chicks. There was also a decrease in tissue noradrenaline concentration in the anterior forebrain in both W14-18 and W10-18 chicks. The differential ability of centrally administered beta2- and beta3-adrenoceptor agonists to overcome the memory deficit post-training, suggests altered responsiveness of central beta2-adrenoceptors to noradrenaline in W14-18 chicks. By comparing the W10-18 and W14-18 chicks with those from eggs wrapped from W10-14 we show that it is the timing of the prenatal hypoxia, rather than its duration, that determines the nature of cognitive dysfunction. We conclude that prenatal hypoxia induced by restriction of gas exchange can disrupt or alter central noradrenergic transmission causing cognitive impairment.

Maturation of cardiovascular control mechanisms in the embryonic emu (Dromiceius novaehollandiae)

Our understanding of avian embryonic cardiovascular regulation has been based on studies in chickens. The present study was undertaken to determine if the patterns established in chickens are generally applicable to the emu, a ratite bird species. We studied cardiovascular physiology over the interval from 60% to 90% of the emu's 50-day incubation period. During this period, embryonic emus exhibit a slight fall in resting heart rate (from 171 beats min(-1) to 154 beats min(-1)) and a doubling of mean arterial pressure (from 1.2 kPa to 2.6 kPa). Exposures to 15% or 10% O(2) initially decreased heart rate during the first period of emu incubation studied [60% of incubation (60%I)] but increased heart rate in the 90%I group. Arterial pressure responded to hypoxia with an initial depression (-1.6 kPa) at 60%I and 70%I but showed no response during the later periods of incubation (80%I and 90%I). In addition, tonic stimulation of both cholinergic and adrenergic (alpha and beta) receptors was present on heart rate at 70%I, with the cholinergic and beta-adrenergic tone increasing in strength by 90%I. Arterial pressure was dependent on a constant beta-adrenergic and constant alpha-adrenergic tone from 60%I to 90%I. A comparison with embryonic white leghorn chickens over a similar window of incubation revealed that emus and white leghorn chickens both possess an adrenergic tone on heart rate and pressure but that only emus possess a cholinergic tone on heart rate. Collectively, these data indicate that the maturation of cardiovascular control systems differs between white leghorn chickens and emus, inviting investigation of additional avian species to determine other patterns.

Cardiovascular and endocrine responses to acute hypoxaemia during and following dexamethasone infusion in the ovine fetus

This study investigated the effects of fetal treatment with dexamethasone on ovine fetal cardiovascular defence responses to acute hypoxaemia, occurring either during or 48 h following the period of glucocorticoid exposure. To address the mechanisms underlying these responses, chemoreflex function and plasma concentrations of catecholamines, neuropeptide Y (NPY) and vasopressin were measured. Under general halothane anaesthesia, 26 Welsh Mountain sheep fetuses were surgically prepared for long-term recording at between 117 and 120 days of gestation (dGA; term is approximately 145 days) with vascular catheters and a Transonic flow probe around a femoral artery. Following at least 5 days of recovery, fetuses were randomly assigned to one of two experimental groups. After 48 h of baseline recording, at 125 +/- 1 dGA, half of the fetuses (n = 13) were continuously infused I.V. with dexamethasone for 48 h at a rate of 2.06 +/- 0.13 microg kg-1 h-1. The remaining 13 fetuses were infused with heparinized saline at the same rate (controls). At 127 +/- 1 dGA, 2 days from the onset of infusions, seven fetuses from each group were subjected to 1 h of acute hypoxaemia. At 129 +/- 1 dGA, 2 days after the end of infusions, six fetuses from each group were subjected to 1 h of acute hypoxaemia. Similar reductions in fetal partial pressure of arterial oxygen occurred in control and dexamethasone-treated fetuses during the acute hypoxaemia protocols. In control fetuses, acute hypoxaemia led to transient bradycardia, femoral vasoconstriction and significant increases in plasma concentrations of catecholamines, vasopressin and NPY. In fetuses subjected to acute hypoxaemia during dexamethasone treatment, the increase in plasma NPY was enhanced, the bradycardic response was prolonged, and the plasma catecholamine and vasopressin responses were diminished. In fetuses subjected to acute hypoxaemia 48 h following dexamethasone treatment, femoral vasoconstriction and plasma catecholamine and vasopressin responses were enhanced, whilst the prolonged bradycardia and augmented plasma NPY responses persisted. These data show that fetal treatment with dexamethasone modifies the pattern and magnitude of fetal cardiovascular responses to acute oxygen deprivation. Modifications to different mechanisms mediating the fetal defence responses to acute hypoxaemia that occur during dexamethasone treatment may reverse, persist or even become enhanced by 48 h following the treatment period.

Chronic moderate hypoxia and protein malnutrition both induce growth retardation, but have distinct effects on arterial endothelium-dependent reactivity in the chicken embryo

Deviations in the rate of intrauterine growth may change organ system development, resulting in cardiovascular disease in adult life. Arterial endothelial dysfunction often plays an important role in these diseases. The effects of two interventions that reduce fetal growth, chronic hypoxia and protein malnutrition, on arterial endothelial function were investigated. Eggs of White Leghorn chickens were incubated either in room air or in 15% O2 from d 6 until d 19 of the 21-d incubation. Protein malnutrition was induced by removal of 10% of the total albumen content at d 0. In vitro reactivity of the femoral artery in response to vasodilators was measured at d 19. Both chronic hypoxia and protein malnutrition reduced embryonic body weight at d 19 by 14% without affecting relative brain weight. Chronic hypoxia or protein malnutrition did not change sensitivity to the exogenous nitric oxide donor, sodium nitroprusside (5.74 +/- 0.15 versus 5.85 +/- 0.23 and 6.05 +/- 0.18 versus 6.01 +/- 0.34, respectively). Whereas protein malnutrition did not modify arterial sensitivity to acetylcholine (7.00 +/- 0.10 versus 7.12 +/- 0.05), chronic hypoxia reduced sensitivity to this endothelium-dependent vasodilator (6.57 +/- 0.07 versus 7.02 +/- 0.06). In the presence of Nomega-nitro-l-arginine methyl ester, this difference in sensitivity to acetylcholine was no longer apparent (6.31 +/- 0.13 versus 6.27 +/- 0.06), indicating that chronic exposure to hypoxia reduced sensitivity to acetylcholine by lowering nitric oxide release. In additional experiments, a decrease in basal nitric oxide release in arteries of 3- to 4-wk-old chickens that had been exposed to in ovo chronic hypoxia was observed (increase in K+ contraction: -0.16 +/- 0.33 N/m versus 0.68 +/- 020 N/m). Protein malnutrition and chronic hypoxia both induce disproportionate growth retardation, but only the latter impairs arterial endothelial function. Intrauterine exposure to chronic hypoxia induces changes in arterial endothelial properties that may play a role in the development of cardiovascular disease in adult life.

Cardiovascular regulation during hypoxia in embryos of the domestic chicken Gallus gallus

Renewed interest in the use of the embryonic chicken as a model of perinatal cardiovascular regulation has inspired new questions about the control mechanisms that respond to acute perturbations, such as hypoxia. The objectives of this study were to determine the cardiovascular responses, the regulatory mechanisms involved in those cardiovascular responses, and whether those mechanisms involved the central nervous system (CNS) of embryonic chickens. Heart rate (f(H)) and blood pressure were measured in chicken embryos of different incubation ages during exposure to different levels of hypoxia (15, 10, and 5% O(2)). At all levels of hypoxia and at all developmental ages, a depression of f(H) and arterial pressure was observed, with the exception of day 20 embryos in 15 and 10% O(2). The intensity of the embryonic f(H) and blood pressure responses were directly related to the level of hypoxia used. Muscarinic and alpha-adrenergic receptor stimulation limited the hypoxic hypotension on days 15-19 and 15-21, respectively, as indicated after blockade with atropine and phentolamine. During the final 3 days of incubation, the intensity of the hypoxic hypotension was magnified due to alpha-vasodilation caused by beta-adrenergic and muscarinic receptor stimulation. In 19- to 21-day-old embryos, the f(H) response to hypoxia was limited by alpha-adrenergic receptor stimulation as indicated by the accentuated bradycardia after blockade with phentolamine. Furthermore, on day 21, atropine limited the hypoxic bradycardia, indicating that muscarinic receptors also play a role in the f(H) response at this age. In addition, the muscarinic actions on the heart and the adrenergic effects on the vasculature appeared to occur through a hypoxic-induced direct release from chromaffin tissue and autonomic nerve terminals. Thus, in embryonic chickens, the only cardiovascular response to hypoxia that involves the CNS was the cholinergic regulation of arterial pressure after day 15 of incubation. Therefore, although embryonic chickens and fetal sheep, the standard models of perinatal cardiovascular physiology, respond to hypoxia with a similar redistribution of cardiac output, the underlying mechanisms differ between these species.

Direct effects of acute hypoxia on the reactivity of peripheral arteries of the chicken embryo

In the chicken embryo, acute hypoxemia results in cardiovascular responses, including an increased peripheral resistance. We investigated whether local direct effects of decreased oxygen tension might participate in the arterial response to hypoxemia in the chicken embryo. Femoral arteries of chicken embryos were isolated at 0.9 of incubation time, and the effects of acute hypoxia on contraction and relaxation were determined in vitro. While hypoxia reduced contraction induced by high K(+) to a small extent (-21.8 +/- 5.7%), contractile responses to exogenous norepinephrine (NE) were markedly reduced (-51.1 +/- 3.2%) in 80% of the arterial segments. This effect of hypoxia was not altered by removal of the endothelium, inhibition of NO synthase or cyclooxygenase, or by depolarization plus Ca(2+) channel blockade. When arteries were simultaneously exposed to NE and ACh, hypoxia resulted in contraction (+49.8 +/- 9.3%). Also, relaxing responses to ACh were abolished during acute hypoxia, while the vessels became more sensitive to the relaxing effect of the NO donor sodium nitroprusside (pD(2): 5.81 +/- 0.21 vs. 5.31 +/- 0.27). Thus, in chicken embryo femoral arteries, acute hypoxia blunts agonist-induced contraction of the smooth muscle and inhibits stimulated endothelium-derived relaxation factor release. The consequences of this for in vivo fetal hemodynamics during acute hypoxemia depend on the balance between vasomotor influences of circulating catecholamines and those of the endothelium.

Influence of long-term changes in incubation temperature on catecholamine levels in plasma of chicken embryos (Gallus gallus f. domestica)

Catecholamine concentrations were determined from day 18 to 21 of incubation (D18, D21) in developing chicken embryos. The control group was continuously incubated at 37.5 degrees C. The eggs of the two other groups were incubated at 37.5 degrees C until day 14. In the cold group, temperature was decreased to 35.0 degrees C and in the warm group, incubation temperature was increased to 38.5 degrees C for the remainder of incubation. Plasma catecholamine concentrations were measured in eggs exposed to a change in incubation temperature for 4, 5, 6 and 7 days. Embryos in the warm group had dopamine (DA) and noradrenaline (NA) concentrations that were significantly higher than in the control group. On the contrary, eggs incubated at the cooler temperature had hormone levels that were significantly lower than in the control group. Adrenaline (A) levels in the two experimental treatments were significantly lower compared to control eggs. Temperature modulated the time needed for development. Chicken embryos are supposed to hatch on day 21. However, on day 20, NA concentration in the cold-incubated group was too low to fulfill its essential physiological function, whereas in the warm group, the NA concentration seems to be sufficient. Long-term exposure to altered incubation temperature affects the quantitative catecholamine concentration during development, but the relative proportion of each catecholamine remained constant.

Sympathetic control of the cardiovascular response to acute hypoxemia in the chick embryo

In response to an acute hypoxemic insult, the mammalian fetus shows a redistribution of the cardiac output in favor of the heart and brain. Peripheral vasoconstriction contributes to this response and is partly mediated by the release of catecholamines. Two mechanisms of catecholamine release in the fetus are reported: 1) neurogenic sympathetic stimulation and 2) a nonneurogenic mechanism via a direct effect of hypoxemia on chromaffin tissues. In the present study, the effects of sympathetic blockade on plasma catecholamine release and cardiac output distribution in response to acute hypoxemia were studied in the chick embryo at different stages of incubation. Only at the end of the incubation period, sympathetic blockade markedly attenuated the increase in plasma catecholamine concentrations and resulted in a greater fraction of the cardiac output distributed to the carcass. However, these effects did not prevent a significant increase in cardiac output to the brain and heart during acute hypoxemia. These data imply that in the chick embryo the contribution of neurogenic mechanisms to the catecholaminergic response to acute hypoxemia becomes greater by the end of the incubation period.

Vascular reactivity in intrapulmonary arteries of chicken embryos during transition to ex ovo life

The present study aimed to characterize pulmonary vascular reactivity in the chicken embryo from the last stage of prenatal development and throughout the perinatal period. Isolated intrapulmonary arteries from non-internally pipped embryos at 19 days of incubation and from internally and externally pipped embryos at 21 days of incubation were studied. Arterial diameter and contractile responses to KCl, endothelin-1, and U-46619 increased with incubation but were unaffected by external pipping. In contrast, the contractions induced by norepinephrine, phenylephrine, and electric field stimulation decreased with development. No developmental changes were observed in endothelium-dependent [acetylcholine (ACh) and cyclopiazonic acid] or endothelium-independent [sodium nitroprusside (SNP)] relaxation. These relaxations were abolished by the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. Endothelium-dependent relaxation was unaffected by blockade of cyclooxygenase or heme oxygenase but was significantly reduced by nitric oxide (NO) synthase inhibitors. Reduction of O2 concentration from 95 to 5% produced a marked reduction in ACh and SNP-induced relaxations. Chicken embryo pulmonary arteries show a marked endothelium-dependent relaxation that is unaffected by transition to ex ovo life. Endothelium-derived NO seems to be the main mediator responsible for this relaxation.

Restriction of prenatal gas exchange impairs memory consolidation in the chick

Our aim was to assess the effects of restricting gas exchange during incubation on postnatal memory formation and growth in the chick. Gas exchange across the eggshell was restricted by covering 50% of the eggshell with an impermeable membrane for 4 or 8 days, commencing at days 14 and 10, respectively, of a 21-day incubation. Memory formation was examined postnatally at 1-2 days using a one-trial discriminated bead task, and at 5-6 days using a discriminated wheat task. For both tasks, chicks from eggs wrapped from days 14 to 18 had impaired memory retention at 60 min after training, although learning and labile memory were not impaired. Chicks from eggs wrapped from days 10-18 appeared to be poorer in their ability to form memories, and did not discriminate as well as controls in any of the tasks. Body weights of chicks from wrapped eggs were reduced from 2 days after hatching; chicks from eggs wrapped from day 10 had lower body weights at hatching. We conclude that a period of altered prenatal gas exchange can impair memory consolidation in the chick soon after hatching. The ability to form memories may be permanently altered, as this impairment is still apparent at 5-6 days after hatching. Pre- and postnatal growth was also impaired in the chicks from wrapped eggs. Our results suggest that the extent to which postnatal neurological function and growth is impaired depends on the timing and possibly the duration of the prenatal insult.

Alpha-adrenergic contribution to the cardiovascular response to acute hypoxemia in the chick embryo

Fetal responses to acute hypoxemia include bradycardia, increase in blood pressure, and peripheral vasoconstriction. Peripheral vasoconstriction contributes to the redistribution of the cardiac output away from ancillary vascular beds toward myocardial, cerebral, and adrenal circulations. We investigated the effect of alpha-adrenergic receptor blockade on this fetal response. Fluorescent microspheres were used to measure cardiac output distribution during basal and hypoxemic conditions with and without phentolamine treatment. Phentolamine altered basal cardiac output distribution, indicating a basal alpha-adrenergic tone, but this was mainly noted at the earlier stages of incubation. During hypoxemia, phentolamine prevented vasoconstriction in the carcass. At day 19 of incubation, the percent cardiac output distributed to the carcass increased by 20% compared with a decrease in the control group by 17%. Phentolamine markedly attenuated the subsequent redistribution of the cardiac output toward the brain (from +102% in the control group to -25% in the phentolamine-treated group) and the heart (from +196% in the control group to +69% in the phentolamine-treated group). In the chick embryo, alpha-adrenergic mechanisms contribute to the maintenance of basal vascular tone and to the redistribution of the cardiac output away from the peripheral circulations toward the brain and heart during hypoxemic conditions.