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

In-Utero Low-Dose Irradiation Leads to Persistent Alterations in the Mouse Heart Proteome

Prenatal exposure to stress such as increased level of reactive oxygen species or antiviral therapy are known factors leading to adult heart defects. The risks following a radiation exposure during fetal period are unknown, as are the mechanisms of any potential cardiac damage. The aim of this study was to gather evidence for possible damage by investigating long-term changes in the mouse heart proteome after prenatal exposure to low and moderate radiation doses. Pregnant C57Bl/6J mice received on embryonic day 11 (E11) a single total body dose of ionizing radiation that ranged from 0.02 Gy to 1.0 Gy. The offspring were sacrificed at the age of 6 months or 2 years. Quantitative proteomic analysis of heart tissue was performed using Isotope Coded Protein Label technology and tandem mass spectrometry. The proteomics data were analyzed by bioinformatics and key changes were validated by immunoblotting. Persistent changes were observed in the expression of proteins representing mitochondrial respiratory complexes, redox and heat shock response, and the cytoskeleton, even at the low dose of 0.1 Gy. The level of total and active form of the kinase MAP4K4 that is essential for the embryonic development of mouse heart was persistently decreased at the radiation dose of 1.0 Gy. This study provides the first insight into the molecular mechanisms of cardiac impairment induced by ionizing radiation exposure during the prenatal period.

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.

Myocardial response in preterm fetal sheep exposed to systemic endotoxinaemia

Exposure of the fetus to antenatal inflammation can occur from chorioamnionitis, which may progress to a fetal inflammatory response syndrome (FIRS) and to fetal sepsis. We tested whether the fetal myocardium responded to systemic Gram-negative endotoxinaemia. We hypothesized that the myocardium would respond to inflammation by changes in hypoxia-inducible factor-α (HIF-1α), inducible NO-synthase (iNOS), Toll-like receptors 2 and 4 (TLR2 and TLR4), IL-6, and phosphorylated signal transducer and activator of transcription-3 (pSTAT3). To model systemic endotoxinaemia, fetal sheep were exposed to Gram-negative endotoxin or saline i.v. 3 d before preterm delivery at 113 d of gestation (term = 147 d). All endotoxin-exposed animals developed cardiac dysfunction within these 72 h. Cardiac mRNA and protein levels of HIF-1α and TLR2 and TLR4 mRNA increased, whereas STAT3 phosphorylation decreased significantly. IL-6 and iNOS mRNA remained unchanged. Fetal systemic endotoxinaemia induced myocardial inflammation by activating TLR2 and 4. The following cardiac dysfunction seems not to be mediated via cardiac iNOS.

Atrioventricular conduction and arrhythmias at the initiation of beating in embryonic mouse hearts

To investigate cardiac physiology at the onset of heart beating in embryonic mouse hearts, we performed optical imaging of membrane potential (Vm) and/or intracellular calcium (Ca(i)). Action potentials and Ca(i) transients were detected in approximately 50% of mouse embryo hearts at E8.5, but in all hearts at E9.0, indicating that beating typically starts between E8-E9. Beating was eliminated by Ca channel blocker nifedipine and the I(f) blocker ZD7288, unaffected by tetrodotoxin and only mildly depressed by disabling sarcoplasmic (SR) and endoplasmic (ER) reticulum Ca cycling. From E8.5 to E10, conduction velocity increased from 0.2-1 mm/s to >5 mm/s in first ventricular and then atrial tissue, while remaining slow in other areas. Arrhythmias included atrioventricular reentry induced by adenosine. In summary, at the onset of beating, I(f)-dependent pacemaking drives both AP propagation and Ca(i) transient generation through activation of voltage-dependent Ca channels. Na channels and intracellular Ca cycling have minor roles.

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.