Developmental changes in cardiac recovery from anoxia-reoxygenation

Proteomic analysis of chick embryonic heart in experimental hypoxia

Investigating prenatal hypoxia is difficult in mammals, as there are confounding factors stemming from maternal adaptations and compensatory mechanisms. We have thus established an avian model of hypoxic incubation (starting after 2 days of normoxia, 15% O2, normobaric, until the time of sampling at embryonic day 8) to study embryonic reactions to low oxygen concentration. Our previous studies have shown increased vascularization, oedema, and ventricular wall thinning preceding the lethality at mid-gestation. Analysis of the cardiac proteome after 6 days of hypoxic incubation showed strong upregulation of enzymes involved in anaerobic glycolysis as well as an increase in apoptosis-related proteins, cell adhesion proteins, and secretory activity.

A myocardial infarct border-zone-on-a-chip demonstrates distinct regulation of cardiac tissue function by an oxygen gradient

After a myocardial infarction, the boundary between the injured, hypoxic tissue and the adjacent viable, normoxic tissue, known as the border zone, is characterized by an oxygen gradient. Yet, the impact of an oxygen gradient on cardiac tissue function is poorly understood, largely due to limitations of existing experimental models. Here, we engineered a microphysiological system to controllably expose engineered cardiac tissue to an oxygen gradient that mimics the border zone and measured the effects of the gradient on electromechanical function and the transcriptome. The gradient delayed calcium release, reuptake, and propagation; decreased diastolic and peak systolic stress; and increased expression of inflammatory cascades that are hallmarks of myocardial infarction. These changes were distinct from those observed in tissues exposed to uniform normoxia or hypoxia, demonstrating distinct regulation of cardiac tissue phenotypes by an oxygen gradient. Our border-zone-on-a-chip model advances functional and mechanistic insight into oxygen-dependent cardiac tissue pathophysiology.

Myocardial development in crocodylians

BACKGROUND

Recent reports confirmed the notion that there exists a rudimentary cardiac conduction system (CCS) in the crocodylian heart, and development of its ventricular part is linked to septation. We thus analyzed myocardial development with the emphasis on the CCS components and vascularization in two different crocodylian species.

RESULTS

Using optical mapping and HNK-1 immunostaining, pacemaker activity was localized to the right-sided sinus venosus. The atrioventricular conduction was restricted to dorsal part of the atrioventricular canal. Within the ventricle, the impulse was propagated from base-to-apex initially by the trabeculae, later by the ventricular septum, in which strands of HNK-1 positivity were temporarily observed. Completion of ventricular septation correlated with transition of ventricular epicardial activation pattern to mature apex-to-base direction from two periapical foci. Despite a gradual thickening of the ventricular wall, no morphological differentiation of the Purkinje network was observed. Thin-walled coronary vessels with endothelium positive for QH1 obtained a smooth muscle coat after septation. Intramyocardial vessels were abundant especially in the rapidly thickening left ventricular wall.

CONCLUSIONS

Most of the CCS components present in the homeiotherm hearts can be identified in the developing crocodylian heart, with a notable exception of the Purkinje network distinct from the trabeculae carneae.

Cardiac Ischemia On-a-Chip: Antiarrhythmic Effect of Levosimendan on Ischemic Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Ischemic heart disease (IHD) is one of the leading causes of mortality worldwide. Preserving functionality and preventing arrhythmias of the heart are key principles in the management of patients with IHD. Levosimendan, a unique calcium (Ca2+) enhancer with inotropic activity, has been introduced into clinical usage for heart failure treatment. Human-induced pluripotent cell-derived cardiomyocytes (hiPSC-CMs) offer an opportunity to better understand the pathophysiological mechanisms of the disease as well as to serve as a platform for drug screening. Here, we developed an in vitro IHD model using hiPSC-CMs in hypoxic conditions and defined the effects of the subsequent hypoxic stress on CMs functionality. Furthermore, the effect of levosimendan on hiPSC-CMs functionality was evaluated during and after hypoxic stress. The morphology, contractile, Ca2+-handling, and gene expression properties of hiPSC-CMs were investigated in response to hypoxia. Hypoxia resulted in significant cardiac arrhythmia and decreased Ca2+ transient amplitude. In addition, disorganization of sarcomere structure was observed after hypoxia induction. Interestingly, levosimendan presented significant antiarrhythmic properties, as the arrhythmia was abolished or markedly reduced with levosimendan treatment either during or after the hypoxic stress. Moreover, levosimendan presented significant protection from the sarcomere alterations induced by hypoxia. In conclusion, this chip model appears to be a suitable preclinical representation of IHD. With this hypoxia platform, detailed knowledge of the disease pathophysiology can be obtained. The antiarrhythmic effect of levosimendan was clearly observed, suggesting a possible new clinical use for the drug.

Considerations for the Bioengineering of Advanced Cardiac In Vitro Models of Myocardial Infarction

Despite the latest advances in cardiovascular biology and medicine, myocardial infarction (MI) remains one of the major causes of deaths worldwide. While reperfusion of the myocardium is critical to limit the ischemic damage typical of a MI event, it causes detrimental morphological and functional changes known as "reperfusion injury." This complex scenario is poorly represented in currently available models of ischemia/reperfusion injury, leading to a poor translation of findings from the bench to the bedside. However, more recent bioengineered in vitro models of the human heart represent more clinically relevant tools to prevent and treat MI in patients. These include 3D cultures of cardiac cells, the use of patient-derived stem cells, and 3D bioprinting technology. This review aims at highlighting the major features typical of a heart attack while comparing current in vitro, ex vivo, and in vivo models. This information has the potential to further guide in developing novel advanced in vitro cardiac models of ischemia/reperfusion injury. It may pave the way for the generation of advanced pathophysiological cardiac models with the potential to develop personalized therapies.

Chicken embryos can maintain heart rate during hypoxia on day 4 of incubation

Acute exposure to hypoxic conditions is a frequent natural event during the development of bird eggs. However, little is known about the effect of such exposure on the ability of young embryos in which cardiovascular regulation is not yet developed to maintain a normal heart rate (HR). To address this question, we studied the effect of 10-20 min of exposure to moderate or severe acute hypoxia (10% or 5% O₂, respectively) on the HR of day 4 (D4) chicken embryos. In ovo, video recording of the beating embryo heart inside the egg revealed that severe, but not moderate, hypoxia resulted in significant HR changes. The HR response to severe hypoxia consisted of two phases: the first phase, consisting of an initial decrease in HR, was followed by a phase of partial HR recovery. Upon the restoration of normoxia, after an overshoot period of a few minutes, the HR completely recovered to its basal level. In vitro (isolated heart preparation), the first phase of the HR response to severe hypoxia was strengthened (nearly complete heart silencing) compared to that in ovo, and the HR recovery phase was greatly attenuated. Furthermore, neither an overshoot period nor complete HR recovery after hypoxia was observed. Thus, the D4 chicken embryo heart can partially maintain its rhythm during hypoxia in ovo, but not in vitro. Some factors from the egg, such as catecholamines, are likely to be critical for avian embryo responding to hypoxic condition and survival.

Impact of Chronic Fetal Hypoxia and Inflammation on Cardiac Pacemaker Cell Development

Chronic fetal hypoxia and infection are examples of adverse conditions during complicated pregnancy, which impact cardiac myogenesis and increase the lifetime risk of heart disease. However, the effects that chronic hypoxic or inflammatory environments exert on cardiac pacemaker cells are poorly understood. Here, we review the current evidence and novel avenues of bench-to-bed research in this field of perinatal cardiogenesis as well as its translational significance for early detection of future risk for cardiovascular disease.

Human Tissue-Engineered Model of Myocardial Ischemia-Reperfusion Injury

IMPACT STATEMENT

Reducing ischemia-reperfusion injury would significantly improve patient survival. Current preclinical models are inadequate because they rely on animals, which do not emulate human physiology and the clinical setting. We developed a human tissue platform that allowed us to assess the human cardiac response, and demonstrated the platform's utility by measuring injury during ischemia-reperfusion and the effects of cardioprotective strategies. The model provides a foundation for future studies on how patient-specific backgrounds may affect response to therapeutic strategies. These steps will be necessary to help translate therapies into the clinical setting.

In vitro Models of Ischemia-Reperfusion Injury

Timely reperfusion after a myocardial infarction is necessary to salvage the ischemic region; however, reperfusion itself is also a major contributor to the final tissue damage. Currently, there is no clinically relevant therapy available to reduce ischemia-reperfusion injury (IRI). While many drugs have shown promise in reducing IRI in preclinical studies, none of these drugs have demonstrated benefit in large clinical trials. Part of this failure to translate therapies can be attributed to the reliance on small animal models for preclinical studies. While animal models encapsulate the complexity of the systemic in vivo environment, they do not fully recapitulate human cardiac physiology. Furthermore, it is difficult to uncouple the various interacting pathways in vivo. In contrast, in vitro models using isolated cardiomyocytes allow studies of the direct effect of therapeutics on cardiomyocytes. External factors can be controlled in simulated ischemia-reperfusion to allow for better understanding of the mechanisms that drive IRI. In addition, the availability of cardiomyocytes derived from human induced pluripotent stem cells (hIPS-CMs) offers the opportunity to recapitulate human physiology in vitro. Unfortunately, hIPS-CMs are relatively fetal in phenotype, and are more resistant to hypoxia than the mature cells. Tissue engineering platforms can promote cardiomyocyte maturation for a more predictive physiologic response. These platforms can further be improved upon to account for the heterogenous patient populations seen in the clinical settings and facilitate the translation of therapies. Thereby, the current preclinical studies can be further developed using currently available tools to achieve better predictive drug testing and understanding of IRI. In this article, we discuss the state of the art of in vitro modeling of IRI, propose the roles for tissue engineering in studying IRI and testing the new therapeutic modalities, and how the human tissue models can facilitate translation into the clinic.

Ondansetron and teratogenicity in rats: Evidence for a mechanism mediated via embryonic hERG blockade

The potent hERG channel blocking drug ondansetron is used off-label for treatment of nausea and vomiting in early pregnancy. Some human epidemiological studies have associated ondansetron with fetal cardiovascular defects and orofacial clefts. This study investigated the effects of ondanestron on embryonic heart rhythm of gestational day (GD) 13 rat embryos in vitro and then integrated the results with published animal teratology, and animal and human pharmacokinetic studies to perform a risk evaluation. Ondansetron caused concentration dependent bradycardia and arrhythmia. Cardiovascular malformations in rats occurred at exposures slightly higher than those in early human pregnancy. Together the results suggest that ondansetron can have teratogenic potential in rats and humans mediated via hERG block and severe heart rhythm disturbances in the embryo. The risk may be increased in human pregnancy if additional risk factors are present such as hypokalemia.

Acute temperature effects on function of the chick embryonic heart

AIM

We analysed the effects of acute temperature change on the beating rate, conduction properties and calcium transients in the chick embryonic heart in vitro and in ovo.

METHODS

The effects of temperature change (34, 37 and 40 °C) on calcium dynamics in isolated ED4 chick hearts in vitro were investigated by high-speed calcium optical imaging. For comparison and validation of in vitro measurements, experiments were also performed in ovo using videomicroscopy. Artificial stimulation experiments were performed in vitro and in ovo to uncover conduction limits of heart segments.

RESULTS

Decrease in temperature from 37 to 34 °C in vitro led to a 22% drop in heart rate and unchanged amplitude of Ca(2+) transients, compared to a 25% heart rate decrease in ovo. Increase in temperature from 37 to 40 °C in vitro and in ovo led to 20 and 23% increases in heart rate, respectively, and a significant decrease in amplitude of Ca(2+) transients (atrium -35%, ventricle -38%). We observed a wide spectrum of arrhythmias in vitro, of which the most common was atrioventricular (AV) block (57%). There was variability of AV block locations. Pacing experiments in vitro and in ovo suggested that the AV blocks were likely caused by relative tissue hypoxia and not by the tachycardia itself.

CONCLUSION

The pacemaker and AV canal are the most temperature-sensitive segments of the embryonic heart. We suggest that the critical point for conduction is the connection of the ventricular trabecular network to the AV canal.

Susceptibility of murine induced pluripotent stem cell-derived cardiomyocytes to hypoxia and nutrient deprivation

Introduction

Induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) may be suitable for myocardial repair. While their functional and structural properties have been extensively investigated, their response to ischemia-like conditions has not yet been clearly defined.

Methods

iPS-CMs were differentiated and enriched from murine induced pluripotent stem cells expressing enhanced green fluorescent protein (eGFP) and puromycin resistance genes under the control of an α-myosin heavy chain (α-MHC) promoter. iPS-CMs maturity and function were characterized by microscopy, real-time PCR, calcium transient recordings, electrophysiology, and mitochondrial function assays, and compared to those from neonatal murine cardiomyocytes. iPS-CMs as well as neonatal murine cardiomyocytes were exposed for 3 hours to hypoxia (1% O₂) and glucose/serum deprivation, and viability, apoptosis markers, reactive oxygen species, mitochondrial membrane potential and intracellular stress signaling cascades were investigated. Then, the iPS-CMs response to mesenchymal stromal cell-conditioned medium was determined.

Results

iPS-CMs displayed key morphological and functional properties that were comparable to those of neonatal cardiomyocytes, but several parameters indicated an earlier iPS-CMs maturation stage. During hypoxia and glucose/serum deprivation, iPS-CMs exhibited a significantly higher proportion of poly-caspase-active, 7-aminoactinomycin D-positive and TUNEL-positive cells than neonatal cardiomyocytes. The average mitochondrial membrane potential was reduced in “ischemic” iPS-CMs but remained unchanged in neonatal cardiomyocytes; reactive oxygen species production was only increased in “ischemic” iPS-CMs, and oxidoreductase activity in iPS-CMs dropped more rapidly than in neonatal cardiomyocytes. In iPS-CMs, hypoxia and glucose/serum deprivation led to upregulation of Hsp70 transcripts and decreased STAT3 phosphorylation and total PKCε protein expression. Treatment with mesenchymal stromal cell-conditioned medium preserved oxidoreductase activity and restored pSTAT3 and PKCε levels.

Conclusion

iPS-CMs appear to be particularly sensitive to hypoxia and nutrient deprivation. Counteracting the ischemic susceptibility of iPS-CMs with mesenchymal stromal cell-conditioned medium may help enhance their survival and efficacy in cell-based approaches for myocardial repair.

Arrhythmias in the developing heart

Prevalence of cardiac arrhythmias increases gradually with age; however, specific rhythm disturbances can appear even prior to birth and markedly affect foetal development. Relatively little is known about these disorders, chiefly because of their relative rarity and difficulty in diagnosis. In this review, we cover the most common forms found in human pathology, specifically congenital heart block, pre-excitation, extrasystoles and long QT syndrome. In addition, we cover pertinent literature data from prenatal animal models, providing a glimpse into pathogenesis of arrhythmias and possible strategies for treatment.

A hypoxic episode during cardiogenesis downregulates the adenosinergic system and alters the myocardial anoxic tolerance

To what extent hypoxia alters the adenosine (ADO) system and impacts on cardiac function during embryogenesis is not known. Ectonucleoside triphosphate diphosphohydrolase (CD39), ecto-5'-nucleotidase (CD73), adenosine kinase (AdK), adenosine deaminase (ADA), equilibrative (ENT1,3,4), and concentrative (CNT3) transporters and ADO receptors A1, A2A, A2B, and A3 constitute the adenosinergic system. During the first 4 days of development chick embryos were exposed in ovo to normoxia followed or not followed by 6 h hypoxia. ADO and glycogen content and mRNA expression of the genes were determined in the atria, ventricle, and outflow tract of the normoxic (N) and hypoxic (H) hearts. Electrocardiogram and ventricular shortening of the N and H hearts were recorded ex vivo throughout anoxia/reoxygenation ± ADO. Under basal conditions, CD39, CD73, ADK, ADA, ENT1,3,4, CNT3, and ADO receptors were differentially expressed in the atria, ventricle, and outflow tract. In H hearts ADO level doubled, glycogen decreased, and mRNA expression of all the investigated genes was downregulated by hypoxia, except for A2A and A3 receptors. The most rapid and marked downregulation was found for ADA in atria. H hearts were arrhythmic and more vulnerable to anoxia-reoxygenation than N hearts. Despite downregulation of the genes, exposure of isolated hearts to ADO 1) preserved glycogen through activation of A1 receptor and Akt-GSK3β-GS pathway, 2) prolonged activity and improved conduction under anoxia, and 3) restored QT interval in H hearts. Thus hypoxia-induced downregulation of the adenosinergic system can be regarded as a coping response, limiting the detrimental accumulation of ADO without interfering with ADO signaling.

Developmental determinants of cardiac sensitivity to hypoxia

Cardiac sensitivity to oxygen deprivation changes significantly during ontogenetic development. However, the mechanisms for the higher tolerance of the immature heart, possibilities of protection, and the potential impact of perinatal hypoxia on cardiac tolerance to oxygen deprivation in adults have not yet been satisfactorily clarified. The hypoxic tolerance of an isolated rat heart showed a triphasic pattern: significant decrease from postnatal day 1 to 7, followed by increase to the weaning period, and final decline to adulthood. We have observed significant ontogenetic changes in mitochondrial oxidative phosphorylation and mitochondrial membrane potential, as well as in the role of the mitochondrial permeability transition pores in myocardial injury. These results support the hypothesis that cardiac mitochondria are deeply involved in the regulation of cardiac tolerance to oxygen deprivation during ontogenetic development. Ischemic preconditioning failed to increase tolerance to oxygen deprivation in the highly tolerant hearts of newborn rats. Chronic hypoxic exposure during early development may cause in-utero or neonatal programming of several genes that can change the susceptibility of the adult heart to ischemia–reperfusion injury; this effect is sex dependent. These results would have important clinical implications, since cardiac sensitivity in adult patients may be significantly affected by perinatal hypoxia in a sex-dependent manner.

Studying dynamic events in the developing myocardium

Differentiation and conduction properties of the cardiomyocytes are critically dependent on physical conditioning both in vitro and in vivo. Historically, various techniques were introduced to study dynamic events such as electrical currents and changes in ionic concentrations in live cells, multicellular preparations, or entire hearts. Here we review this technological progress demonstrating how each improvement in spatial or temporal resolution provided answers to old and provoked new questions. We further demonstrate how high-speed optical mapping of voltage and calcium can uncover pacemaking potential within the outflow tract myocardium, providing a developmental explanation of ectopic beats originating from this region in the clinical settings.

Concise review: maturation phases of human pluripotent stem cell-derived cardiomyocytes

Human pluripotent stem cell-derived cardiomyocytes (hPS-CM) may offer a number of advantages over previous cardiac models, however, questions of their immaturity complicate their adoption as a new in vitro model. hPS-CM differ from adult cardiomyocytes with respect to structure, proliferation, metabolism and electrophysiology, better approximating fetal cardiomyocytes. Time in culture appears to significantly impact phenotype, leading to what can be referred to as early and late hPS-CM. This work surveys the phenotype of hPS-CM, including structure, bioenergetics, sensitivity to damage, gene expression, and electrophysiology, including action potential, ion channels, and intracellular calcium stores, while contrasting fetal and adult CM with hPS-CM at early and late time points after onset of differentiation.

Optical coherence tomography captures rapid hemodynamic responses to acute hypoxia in the cardiovascular system of early embryos

BACKGROUND

The trajectory to heart defects may start in tubular and looping heart stages when detailed analysis of form and function is difficult by currently available methods. We used a novel method, Doppler optical coherence tomography (OCT), to follow changes in cardiovascular function in quail embryos during acute hypoxic stress. Chronic fetal hypoxia is a known risk factor for congenital heart diseases (CHDs). Decreased fetal heart rates during maternal obstructive sleep apnea suggest that studying fetal heart responses under acute hypoxia is warranted.

RESULTS

We captured responses to hypoxia at the critical looping heart stages. Doppler OCT revealed detailed vitelline arterial pulsed Doppler waveforms. Embryos tolerated 1 hr of hypoxia (5%, 10%, or 15% O(2) ), but exhibited changes including decreased systolic and increased diastolic duration in 5 min. After 5 min, slower heart rates, arrhythmic events and an increase in retrograde blood flow were observed. These changes suggested slower filling of the heart, which was confirmed by four-dimensional Doppler imaging of the heart itself.

CONCLUSIONS

Doppler OCT is well suited for rapid noninvasive screening for functional changes in avian embryos under near physiological conditions. Analysis of the accessible vitelline artery sensitively reflected changes in heart function and can be used for rapid screening. Acute hypoxia caused rapid hemodynamic changes in looping hearts and may be a concern for increased CHD risk.

Caffeine acts via A1 adenosine receptors to disrupt embryonic cardiac function

Background

Evidence suggests that adenosine acts via cardiac A1 adenosine receptors (A1ARs) to protect embryos against hypoxia. During embryogenesis, A1ARs are the dominant regulator of heart rate, and A1AR activation reduces heart rate. Adenosine action is inhibited by caffeine, which is widely consumed during pregnancy. In this study, we tested the hypothesis that caffeine influences developing embryos by altering cardiac function.

Methodology/Principal Findings

Effects of caffeine and adenosine receptor-selective antagonists on heart rate were studied in vitro using whole murine embryos at E9.5 and isolated hearts at E12.5. Embryos were examined in room air (21% O₂) or hypoxic (2% O₂) conditions. Hypoxia decreased heart rates of E9.5 embryos by 15.8% and in E12.5 isolated hearts by 27.1%. In room air, caffeine (200 µM) had no effect on E9.5 heart rates; however, caffeine increased heart rates at E12.5 by 37.7%. Caffeine abolished hypoxia-mediated bradycardia at E9.5 and blunted hypoxia-mediated bradycardia at E12.5. Real-time PCR analysis of RNA from isolated E9.5 and E12.5 hearts showed that A1AR and A2aAR genes were expressed at both ages. Treatment with adenosine receptor-selective antagonists revealed that SCH-58261 (A2aAR-specific antagonist) had no affects on heart function, whereas DPCPX (A1AR-specific antagonist) had effects similar to caffeine treatment at E9.5 and E12.5. At E12.5, embryonic hearts lacking A1AR expression (A1AR−/−) had elevated heart rates compared to A1AR+/− littermates, A1AR−/− heart rates failed to decrease to levels comparable to those of controls. Caffeine did not significantly affect heart rates of A1AR−/− embryos.

Conclusions/Significance

These data show that caffeine alters embryonic cardiac function and disrupts the normal cardiac response to hypoxia through blockade of A1AR action. Our results raise concern for caffeine exposure during embryogenesis, particularly in pregnancies with increased risk of embryonic hypoxia.

Differential contribution of mitochondria, NADPH oxidases, and glycolysis to region-specific oxidant stress in the anoxic-reoxygenated embryonic heart

The ability of the developing myocardium to tolerate oxidative stress during early gestation is an important issue with regard to possible detrimental consequences for the fetus. In the embryonic heart, antioxidant defences are low, whereas glycolytic flux is high. The pro- and antioxidant mechanisms and their dependency on glucose metabolism remain to be explored. Isolated hearts of 4-day-old chick embryos were exposed to normoxia (30 min), anoxia (30 min), and hyperoxic reoxygenation (60 min). The time course of ROS production in the whole heart and in the atria, ventricle, and outflow tract was established using lucigenin-enhanced chemiluminescence. Cardiac rhythm, conduction, and arrhythmias were determined. The activity of superoxide dismutase, catalase, gutathione reductase, and glutathione peroxidase as well as the content of reduced and oxidized glutathione were measured. The relative contribution of the ROS-generating systems was assessed by inhibition of mitochondrial complexes I and III (rotenone and myxothiazol), NADPH oxidases (diphenylene iodonium and apocynine), and nitric oxide synthases ( N-monomethyl-l-arginine and N-iminoethyl-l-ornithine). The effects of glycolysis inhibition (iodoacetate), glucose deprivation, glycogen depletion, and lactate accumulation were also investigated. In untreated hearts, ROS production peaked at 10.8 ± 3.3, 9 ± 0.8, and 4.8 ± 0.4 min (means ± SD; n = 4) of reoxygenation in the atria, ventricle, and outflow tract, respectively, and was associated with arrhythmias. Functional recovery was complete after 30–40 min. At reoxygenation, 1) the respiratory chain and NADPH oxidases were the main sources of ROS in the atria and outflow tract, respectively; 2) glucose deprivation decreased, whereas glycogen depletion increased, oxidative stress; 3) lactate worsened oxidant stress via NADPH oxidase activation; 4) glycolysis blockade enhanced ROS production; 5) no nitrosative stress was detectable; and 6) the glutathione redox cycle appeared to be a major antioxidant system. Thus, the glycolytic pathway plays a predominant role in reoxygenation-induced oxidative stress during early cardiogenesis. The relative contribution of mitochondria and extramitochondrial systems to ROS generation varies from one region to another and throughout reoxygenation.

Hypoxia and fetal heart development

Fetal hearts show a remarkable ability to develop under hypoxic conditions. The metabolic flexibility of fetal hearts allows sustained development under low oxygen conditions. In fact, hypoxia is critical for proper myocardial formation. Particularly, hypoxia inducible factor 1 (HIF-1) and vascular endothelial growth factor play central roles in hypoxia-dependent signaling in fetal heart formation, impacting embryonic outflow track remodeling and coronary vessel growth. Although HIF is not the only gene involved in adaptation to hypoxia, its role places it as a central figure in orchestrating events needed for adaptation to hypoxic stress. Although "normal" hypoxia (lower oxygen tension in the fetus as compared with the adult) is essential in heart formation, further abnormal hypoxia in utero adversely affects cardiogenesis. Prenatal hypoxia alters myocardial structure and causes a decline in cardiac performance. Not only are the effects of hypoxia apparent during the perinatal period, but prolonged hypoxia in utero also causes fetal programming of abnormality in the heart's development. The altered expression pattern of cardioprotective genes such as protein kinase c epsilon, heat shock protein 70, and endothelial nitric oxide synthase, likely predispose the developing heart to increased vulnerability to ischemia and reperfusion injury later in life. The events underlying the long-term changes in gene expression are not clear, but likely involve variation in epigenetic regulation.

The L-Type Ca+ and KATP channels may contribute to pacing-induced protection against anoxia-reoxygenation in the embryonic heart model

L-Type Ca(2+) and K(ATP) Channels in Pacing-Induced Cardioprotection.

AIMS

The L-type Ca(2+) channel, the sarcolemmal (sarcK(ATP)), and mitochondrial K(ATP) (mitoK(ATP)) channels are involved in myocardial preconditioning. We aimed at determining to what extent these channels can also participate in pacing-induced cardioprotection.

METHODS

Hearts of 4-day-old chick embryos were paced in ovo during 12 hour using asynchronous intermittent ventricular stimulation at 110% of the intrinsic rate. Sham operated and paced hearts were then submitted in vitro to anoxia (30 minutes) and reoxygenation (60 minutes). These hearts were exposed to L-type Ca(2+) channel agonist Bay-K-8644 (BAY-K) or blocker verapamil, nonselective K(ATP) channel antagonist glibenclamide (GLIB), mitoK(ATP) channel agonist diazoxide (DIAZO), or antagonist 5-hydroxydecanoate. Electrocardiogram, electromechanical delay (EMD) reflecting excitation-contraction (E-C) coupling, and contractility were determined.

RESULTS

Under normoxia, heart rate, QT duration, conduction, EMD, and ventricular shortening were similar in sham and paced hearts. During reoxygenation, arrhythmias ceased earlier and ventricular EMD recovered faster in paced hearts than in sham hearts. In sham hearts, BAY-K (but not verapamil), DIAZO (but not 5-hydroxydecanoate) or GLIB accelerated recovery of ventricular EMD, reproducing the pacing-induced protection. By contrast, none of these agents further ameliorated recovery of the paced hearts.

CONCLUSION

The protective effect of chronic asynchronous pacing at near physiological rate on ventricular E-C coupling appears to be associated with subtle activation of L-type Ca(2+) channel, inhibition of sarcK(ATP) channel, and/or opening of mitoK(ATP) channel.

Modulation of the c-Jun N-terminal kinase activity in the embryonic heart in response to anoxia-reoxygenation: involvement of the Ca2+ and mitoKATP channels

Whether the response of the fetal heart to ischemia-reperfusion is associated with activation of the c-Jun N-terminal kinase (JNK) pathway is not known. In contrast, involvement of the sarcolemmal L-type Ca2+ channel (LCC) and the mitochondrial KATP (mitoKATP) channel has been established. This work aimed at investigating the profile of JNK activity during anoxia-reoxygenation and its modulation by LCC and mitoK(ATP) channel. Hearts isolated from 4-day-old chick embryos were submitted to anoxia (30 min) and reoxygenation (60 min). Using the kinase assay method, the profile of JNK activity in the ventricle was determined every 10 min throughout anoxia-reoxygenation. Effects on JNK activity of the LCC blocker verapamil (10 nM), the mitoK(ATP) channel opener diazoxide (50 microM) and the blocker 5-hydroxydecanoate (5-HD, 500 microM), the mitochondrial Ca2+ uniporter (MCU) inhibitor Ru360 (10 microM), and the antioxidant N-(2-mercaptopropionyl) glycine (MPG, 1 mM) were determined. In untreated hearts, JNK activity was increased by 40% during anoxia and peaked fivefold relative to basal level after 30-40 min reoxygenation. This peak value was reduced by half by diazoxide and was tripled by 5-HD. Furthermore, the 5-HD-mediated stimulation of JNK activity during reoxygenation was abolished by diazoxide, verapamil or Ru360. MPG had no effect on JNK activity, whatever the conditions. None of the tested pharmacological agents altered JNK activity under basal normoxic conditions. Thus, in the embryonic heart, JNK activity exhibits a characteristic pattern during anoxia and reoxygenation and the respective open-state of LCC, MCU and mitoKATP channel can be a major determinant of JNK activity in a ROS-independent manner.

Differential effects of anoxia on heart rate in developmental stages of the annual killifish Austrofundulus limnaeus that differ in their tolerance of anoxia

Embryos of the annual killifish Austrofundulus limnaeus can experience oxygen deprivation as part of their normal developmental environment. We exposed embryos to anoxia and monitored heart activity for 48 hr, and subsequent aerobic recovery from anoxia for 40 hr. Embryos were tested at four different developmental stages that differ in their tolerance of anoxia. Our results indicate that high tolerance of anoxia is associated with an arrest of heart contractility during the first 24 hr of anoxia. These embryos recover to normoxic levels of heart rate within 16 hr of aerobic recovery. In contrast, embryos from later developmental stages that have a highly reduced ability to survive long-term anoxia experience a severe bradycardia but not an arrest of heart rate. These data illustrate a new and potentially powerful model for investigating the effects of anoxia on the developing cardiovascular system in vertebrates.

Magnetocardiography in the evaluation of fetuses at risk for sudden cardiac death before birth

BACKGROUND

We hypothesized that fetuses at risk for sudden death may have abnormal conduction or depolarization, ischemia, or abnormal heart rate variability (HRV) detectable by magnetocardiography.

METHODS

Using a 37-channel biomagnetometer, we evaluated 3 groups of fetuses at risk for sudden death: group 1, critical aortic stenosis (AS); group 2, arrhythmias; and group 3, heart failure and in utero demise. Five to 10 recordings of 10-minute duration were recorded, and signal was averaged to determine rhythm, conduction intervals, HRV, and T-wave morphology.

RESULTS

In group 1, 2 of 3 had atrial and ventricular strain patterns. In (n = 53) group 2, 15% had prolonged QTc and 17% had T-wave alternans (TWA). Of 23 group 2 fetuses with atrioventricular block, 74% had ventricular ectopy, 21% had junctional ectopic tachycardia, and 29% had ventricular tachycardia. Group 3 (n = 2) had abnormal HRV and TWA.

CONCLUSION

Repolarization abnormalities, unexpected arrhythmias, and abnormal HRV suggest an arrhythmogenic mechanism for "sudden cardiac death before birth."

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.

Arrhythmogenesis in the Developing Heart During Anoxia-Reoxygenation and Hypothermia-Rewarming: An In Vitro Model

INTRODUCTION

The spatio-temporal pattern of arrhythmias in the embryonic/fetal heart subjected to a transient hypoxic or hypothermic stress remains to be established.

METHODS AND RESULTS

Spontaneously beating hearts or isolated atria, ventricles, and conotruncus from 4-day-old chick embryos were subjected in vitro to 30-minute anoxia and 60-minute reoxygenation. Hearts were also submitted to 30-minute hypothermia (0-4 degrees C) and 60-minute rewarming. ECG disturbances and alterations of atrial and ventricular electromechanical delay (EMD) were systematically investigated. Baseline functional parameters were stable during at least 2 hours. Anoxia induced tachycardia, followed by bradycardia, atrial ectopy, first-, second-, and third-degree atrio-ventricular blocks and, finally, transient electromechanical arrest after 6.8 minutes, interquartile ranges (IQR) 3.1-16.2 (n = 8). Reoxygenation triggered also Wenckebach phenomenon and ventricular escape beats. At the onset of reoxygenation QT, PR, and ventricular EMD increased by 68%, 70%, and 250%, respectively, whereas atrial EMD was not altered. No fibrillations, no ventricular ectopic beats, and no electromechanical dissociation were observed. Arrhythmic activity of the isolated atria persisted throughout anoxia and upon reoxygenation, whereas activity of the isolated ventricles abruptly ceased after 5 minutes of anoxia and resumed after 5 minutes of reoxygenation. During hypothermia-rewarming, cardiac activity stopped at 17.9 degrees C, IQR 16.2-20.6 (n = 4) and resumed at the same temperature with no arrhythmias. All preparations fully recovered after 40 minutes of reoxygenation or rewarming.

CONCLUSION

In the embryonic heart, arrhythmias mainly originated in the sinoatrial tissue and resembled those observed in the adult heart. Furthermore, oxygen readmission was by far more arrhythmogenic than rewarming and the chronotropic, dromotropic, and inotropic effects were fully reversible.

Physiopathology of the embryonic heart (with special emphasis on hypoxia and reoxygenation)

The adaptative response of the developing heart to adverse intrauterine environment such as reduced O2 delivery can result in alteration of gene expression with short- and long-term consequences including adult cardiovascular diseases. The tolerance of the developing heart of acute or chronic oxygen deprivation, its capacity to recover during reperfusion and the mechanisms involved in reoxygenation injury are still under debate. Indeed, the pattern of response of the immature myocardium to hypoxia-reoxygenation differs from that of the adult. This review deals with the structural and metabolic characteristics of the embryonic heart and the functional consequences of hypoxia and reoxygenation. The relative contribution of calcium and sodium overload, pH disturbances and oxidant stress to the hypoxia-induced cardiac dysfunction is examined, as well as various cellular signaling pathways (e.g. MAP kinases) involved in cell survival or death. In the context of the recent advances in developmental cardiology and fetal cardiac surgery, a better understanding of the physiopathology of the stressed developing heart is required.

MCT-4, A511/Basigin and EF5 expression patterns during early chick cardiomyogenesis indicate cardiac cell differentiation occurs in a hypoxic environment

We have identified the presence of the hypoxia marker EF5 in the stage 4/5 chick heart fields. This suggests that cardiac cell differentiation occurs in a relatively anaerobic environment. Monocarboxylate transporter (MCT) studies in adult cardiac myocytes have demonstrated that MCTs catalyze proton-linked pyruvate and lactate transport activity. 5A11/Basigin is an ancillary protein that targets MCTs to the plasma membrane for their function. MCT-4 expression is most evident in cells with a high glycolytic rate associated with hypoxic energy production. Subsequent to the immunohistochemical localization of EF5 in the early heart field, we continued in our analysis during stages 5 to 12 for the expression of indicators of cellular glycolytic metabolism in the developing heart, such as MCT-4, MCT-1, and 5A11 (Basigin/CD147). Our observations indicate that MCT-4 and 5A11/Basigin are expressed early, in a differential left-right pattern, in the bi-lateral plate mesoderm, as the cardiac compartment is forming. At stage 11, MCT-4/5A11 continues to be highly expressed in the myocardial wall of the looping heart, but not in the dorsal mesocardium. RT-PCR analyses for MCT-1, -4, and 5A11 indicate that MCT-4 and 5A11 are expressed throughout precardiac, embryonic, and fetal stages in the heart. MCT-1 is first detected in the heart on embryonic day 3 and then remains expressed throughout development to hatching. These results indicate that cardiac precursor cells are equipped for differentiating in a hypoxic environment using anaerobic metabolism for energy production.

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.

Optical mapping of electrical activation in the developing heart

Specialized conduction tissues mediate coordinated propagation of electrical activity through the adult vertebrate heart. Following activation of the atria, the activation wave is slowed down in the atrioventricular canal or node, after which it spreads rapidly into the left and right ventricles via the His-Purkinje system (HPS). This results in the ventricles being activated from the apex toward the base, which is a hallmark of HPS function. The development of mature HPS function follows significant phases of cardiac morphogenesis. Initially, the cardiac impulse propagates in a slow, linear, and isotropic fashion from the sinus venosus at the most caudal portion of the tubular heart. Although the speed of impulse propagation gradually increases as it travels toward the anterior regions of the heart tube, the actual sequence of ventricular activation in the looped heart proceeds in the same direction as blood flow. Eventually, the immature base-to-apex sequence of ventricular activation undergoes an apparent reversal, changing to the mature apex-to-base pattern. Using an optical mapping approach, we demonstrate that the timing of this last transition shows striking dependence on hemodynamic loading of the ventricle, being accelerated by pressure overload and delayed in left ventricular hypoplasia. Comparison of chick and mammalian hearts revealed some striking similarities as well as key differences in the timing of such events during cardiac organogenesis.

mitoKATP channel activation in the postanoxic developing heart protects E-C coupling via NO-, ROS-, and PKC-dependent pathways

Whereas previous studies have shown that opening of the mitochondrial ATP-sensitive K+ (mitoKATP) channel protects the adult heart against ischemia-reperfusion injury, it remains to be established whether this mechanism also operates in the developing heart. Isolated spontaneously beating hearts from 4-day-old chick embryos were subjected to 30 min of anoxia followed by 60 min of reoxygenation. The chrono-, dromo-, and inotropic disturbances, as well as alterations of the electromechanical delay (EMD), reflecting excitation-contraction (E-C) coupling, were investigated. Production of reactive oxygen species (ROS) in the ventricle was determined using the intracellular fluorescent probe 2′,7′-dichlorofluorescin (DCFH). Effects of the specific mitoKATP channel opener diazoxide (Diazo, 50 μM) or the blocker 5-hydroxydecanoate (5-HD, 500 μM), the nitric oxide synthase (NOS) inhibitor NG-nitro-l-arginine methyl ester (l-NAME, 50 μM), the antioxidant N-(2-mercaptopropionyl)glycine (MPG, 1 mM), and the PKC inhibitor chelerythrine (Chel, 5 μM) on oxidative stress and postanoxic functional recovery were determined. Under normoxia, the baseline parameters were not altered by any of these pharmacological agents, alone or in combination. During the first 20 min of postanoxic reoxygenation, Diazo doubled the peak of ROS production and, interestingly, accelerated recovery of ventricular EMD and the PR interval. Diazo-induced ROS production was suppressed by 5-HD, MPG, or l-NAME, but not by Chel. Protection of ventricular EMD by Diazo was abolished by 5-HD, MPG, l-NAME, or Chel, whereas protection of the PR interval was abolished by l-NAME exclusively. Thus pharmacological opening of the mitoKATP channel selectively improves postanoxic recovery of cell-to-cell communication and ventricular E-C coupling. Although the NO-, ROS-, and PKC-dependent pathways also seem to be involved in this cardioprotection, their interrelation in the developing heart can differ markedly from that in the adult myocardium.

Postanoxic functional recovery of the developing heart is slightly altered by endogenous or exogenous nitric oxide

Nitric oxide synthase (NOS) is strongly and transiently expressed in the developing heart but its function is not well documented. This work examined the role, either protective or detrimental, that endogenous and exogenous NO could play in the functioning of the embryonic heart submitted to hypoxia and reoxygenation. Spontaneously beating hearts isolated from 4-day-old chick embryos were either homogenized to determine basal inducible NOS (iNOS) expression and activity or submitted to 30 min anoxia followed by 100 min reoxygenation. The chrono-, dromo- and inotropic responses to anoxia/reoxygenation were determined in the presence of NOS substrate (L-arginine 10 mM), NOS inhibitor L-NIO (1-5 mM), or NO donor (DETA NONOate 10-100 microM). Myocardial iNOS was detectable by immunoblotting and its activity was specifically decreased by 53% in the presence of 5 mM L-NIO. L-Arginine, L-NIO and DETA NONOate at 10 microM had no significant effect on the investigated functional parameters during anoxia/reoxygenation. However, irrespective of anoxia/reoxygenation, DETA NONOate at 100 microM decreased ventricular shortening velocity by about 70%, and reduced atrio-ventricular propagation by 23%. None of the used drugs affected atrial activity and hearts of all experimental groups fully recovered at the end of reoxygenation. These findings indicate that (1) by contrast with adult heart, endogenously released NO plays a minor role in the early response of the embryonic heart to reoxygenation, (2) exogenous NO has to be provided at high concentration to delay postanoxic functional recovery, and (3) sinoatrial pacemaker cells are the less responsive to NO.

Ectopic pacing at physiological rate improves postanoxic recovery of the developing heart

Recently, rapid and transient cardiac pacing was shown to induce preconditioning in animal models. Whether the electrical stimulation per se or the concomitant myocardial ischemia affords such a protection remains unknown. We tested the hypothesis that chronic pacing of a cardiac preparation maintained in a normoxic condition can induce protection. Hearts of 4-day-old chick embryos were electrically paced in ovo over a 12-h period using asynchronous and intermittent ventricular stimulation (5 min on-10 min off) at 110% of the intrinsic rate. Sham (n = 6) and paced hearts (n = 6) were then excised, mounted in vitro, and subjected successively to 30 min of normoxia (20% O(2)), 30 min of anoxia (0% O(2)), and 60 min of reoxygenation (20% O(2)). Electrocardiogram and atrial and ventricular contractions were simultaneously recorded throughout the experiment. Reoxygenation-induced chrono-, dromo-, and inotropic disturbances, incidence of arrhythmias, and changes in electromechanical delay (EMD) in atria and ventricle were systematically investigated in sham and paced hearts. Under normoxia, the isolated heart beat spontaneously and regularly, and all baseline functional parameters were similar in sham and paced groups (means +/- SD): heart rate (190 +/- 36 beats/min), P-R interval (104 +/- 25 ms), mechanical atrioventricular propagation (20 +/- 4 mm/s), ventricular shortening velocity (1.7 +/- 1 mm/s), atrial EMD (17 +/- 4 ms), and ventricular EMD (16 +/- 2 ms). Under anoxia, cardiac function progressively collapsed, and sinoatrial activity finally stopped after approximately 9 min in both groups. During reoxygenation, paced hearts showed 1) a lower incidence of arrhythmias than sham hearts, 2) an increased rate of recovery of ventricular contractility compared with sham hearts, and 3) a faster return of ventricular EMD to basal value than sham hearts. However, recovery of heart rate, atrioventricular conduction, and atrial EMD was not improved by pacing. Activity of all hearts was fully restored at the end of reoxygenation. These findings suggest that chronic electrical stimulation of the ventricle at a near-physiological rate selectively alters some cellular functions within the heart and constitutes a nonischemic means to increase myocardial tolerance to a subsequent hypoxia-reoxygenation.