Nitric oxide synthase expression and function in embryonic and adult cardiomyocytes

Protecting cardiomyocytes from hypoxia-reoxygenation injury, empaglifozin and liraglutide alone or in combination?

OBJECTIVES

Empagliflozin, a sodium-dependent glucose co-transporter 2 (SGLT2) inhibitor, and liraglutide, a GLP-1 receptor (GLP-1R) agonist, are commonly recognized for their cardiovascular benefits in individuals with type 2 diabetes (T2D). In prior studies, we have demonstrated that both drugs, alone or in combination, were able to protect cardiomyocytes from injury induced by diabetes. Mechanistic investigations also suggested that the cardioprotective effect may be independent of diabetes In this study, we utilized a hypoxia-reoxygenation (H/R) model to investigate the cardiovascular benefits of SGLT2 inhibitor empagliflozin and GLP-1 receptor (GLP-1R) agonist liraglutide, both alone and in combination, in the absence of T2D. Our hypothesis was that empagliflozin and liraglutide, either individually or in combination, would demonstrate cardioprotective properties against H/R-induced injury, with an additive and/or synergistic effect anticipated from combination therapy.

METHODS

In this study, the cardiac muscle cell line, HL-1 cells, were treated with vehicle, empagliflozin, liraglutide, or a combination of the two drugs. The cells were then subjected to a hypoxia-reoxygenation (H/R) protocol, consisting of 1 h of hypoxia followed by 24 h of reoxygenation. The effects of the treatments on cytotoxicity, oxidative stress, endothelial nitric oxide synthase (eNOS) activity, phospho-protein kinase C (PKC) beta and phospho-eNOS (Thr495) expression were subsequently evaluated at the end of the treatments.

RESULTS

We found that H/R increased cytotoxicity and reduces eNOS activity, empagliflozin, liraglutide or combination treatment attenuated some or all of these effects with the combination therapy showing the greatest improvement.

CONCLUSIONS

Empagliflozin, liraglutide or combination of these two have cardioprotective effect regardless of diabetes. Cardioprotective effects of SGLT2 inhibitor and GLP-1R agonist is additive and synergistic.

Fetoplacental endothelial dysfunction in gestational diabetes mellitus and maternal obesity: A potential threat for programming cardiovascular disease

Gestational diabetes mellitus (GDM) and maternal obesity (MO) increase the risk of adverse fetal outcomes, and the incidence of cardiovascular disease later in life. Extensive research has been conducted to elucidate the underlying mechanisms by which GDM and MO program the offspring to disease. This review focuses on the role of fetoplacental endothelial dysfunction in programming the offspring for cardiovascular disease in GDM and MO pregnancies. We discuss how pre-existing maternal health conditions can lead to vascular dysfunction in the fetoplacental unit and the fetus. We also examine the role of fetoplacental endothelial dysfunction in impairing fetal cardiovascular system development and the involvement of nitric oxide and hydrogen sulfide in mediating fetoplacental vascular dysfunction. Furthermore, we suggest that the L-Arginine-Nitric Oxide and the Adenosine-L-Arginine-Nitric Oxide (ALANO) signaling pathways are pertinent targets for research. Despite significant progress in this area, there are still knowledge gaps that need to be addressed in future research.

Evaluation of left ventricular myocardial mechanics in patients with normally functioning bicuspid aortic valves: A systematic review and meta-analysis

BACKGROUND

Studies have found conflicting results concerning the left ventricular (LV) myocardial deformation properties in patients with normally functioning bicuspid aortic valves (BAVs). Whether the remodeling process of LV occurs independently in patients with BAV is a matter of debate.

METHOD

We searched PubMed, Embase, Cochrane library, and Web of Science for cohort studies aiming to assess LV function in adults with isolated BAV compared with tricuspid aortic valve (TAV) controls. Standard mean difference (SMD) was calculated from random-effects meta-analyses.

RESULTS

Eight cohort studies were included. There were significantly lower global longitudinal strain (GLS), global radial strain (GRS), global circumferential strain (GCS) in BAV than in TAV controls (GLS: SMD = 0.64, 95% CI: 0.35 to 0.92, P < .0001; GRS: SMD = -0.71, 95% CI: -1.09 to -0.32, P = .0003; GCS: SMD = 0.70, 95% CI: 0.41 to 0.98, P < .00001) and significantly higher left atrial volume index (LAVI) and E/e' in BAV than in TAV controls (LAVI: SMD = 0.50, 95% Cl: 0.12 to 0.88, P = .01; E/e': SMD = 0.54, 95% CI: 0.30 to 0.77, P < .00001). There was significantly higher Left ventricular mass index（LVMI） in BAV than in TAV controls (SMD = 0.51, 95% CI: 0.24 to 0.79, P = .0003). Left ventricular ejection fraction (LVEF) was not significantly different between BAV patients and TAV controls (SMD = 0.05, 95% CI: -0.16 to 0.26, P = .63).

CONCLUSION

The impairment of LV myocardial mechanics, including LV systolic, diastolic dysfunction and LV hypertrophy, is present in patients with normally functioning BAV. This might support the hypothesis that BAV is not only a valvular disease but also a myocardial disease.

Vitamin C: historical perspectives and heart failure

Vitamin C (Vit C) is an ideal antioxidant as it is easily available, water soluble, very potent, least toxic, regenerates other antioxidants particularly Vit E, and acts as a cofactor for different enzymes. It has received much attention due to its ability in limiting reactive oxygen species, oxidative stress, and nitrosative stress, as well as it helps to maintain some of the normal metabolic functions of the cell. However, over 140 clinical trials using Vit C in different pathological conditions such as myocardial infarction, gastritis, diabetes, hypertension, stroke, and cancer have yielded inconsistent results. Such a divergence calls for new strategies to establish practical significance of Vit C in heart failure or even in its prevention. For a better understanding of Vit C functioning, it is important to revisit its transport across the cell membrane and subcellular interactions. In this review, we have highlighted some historical details of Vit C and its transporters in the heart with a particular focus on heart failure in cancer chemotherapy.

Two central pattern generators from the crab, Cancer borealis, respond robustly and differentially to extreme extracellular pH

The activity of neuronal circuits depends on the properties of the constituent neurons and their underlying synaptic and intrinsic currents. We describe the effects of extreme changes in extracellular pH – from pH 5.5 to 10.4 – on two central pattern generating networks, the stomatogastric and cardiac ganglia of the crab, Cancer borealis. Given that the physiological properties of ion channels are known to be sensitive to pH within the range tested, it is surprising that these rhythms generally remained robust from pH 6.1 to pH 8.8. The pH sensitivity of these rhythms was highly variable between animals and, unexpectedly, between ganglia. Animal-to-animal variability was likely a consequence of similar network performance arising from variable sets of underlying conductances. Together, these results illustrate the potential difficulty in generalizing the effects of environmental perturbation across circuits, even within the same animal.

Evaluation of myocardial strain and aortic elasticity in patients with bicuspid aortic valve

This study evaluated the myocardial strain and aortic elasticity in patients with bicuspid aortic valve (BAV) and then investigated the relation between them. Thirty-nine patients (30 males; mean age 44±19 years; range 6 to 75 years) with BAV were recruited as BAV group, and 29 age- and sex-matched healthy controls (21 males; mean age 42±11 years; range 20 to 71 years) served as control group. Aortic strain, distensibility and stiffness index were derived using M-mode echocardiography. Left ventricular global myocardial strain was acquired with speckle-tracking echocardiography. Correlation between aortic elasticity and myocardial strain was also analyzed. The results showed that aortic stiffness was higher (17.5±14.0 vs. 5.3±2.7, P<0.001), and aortic strain (4.9±4.7 vs. 11.0±4.1, P<0.001) and distensibility (1.8±2.1 vs. 3.7±1.6, P<0.001) were lower significantly in BAV group than in control group. Global circumferential strain (-19.1±4.2 vs.-22.5±3.7, P<0.001), radial stain (29.8±14.9 vs. 38.0±8.8, P<0.001) and longitudinal stain (-18.4±3.4 vs.-20.8±3.5, P<0.001) were significantly lower in BAV group than in control group. There was weak association between aortic elasticity and myocardial strain. These findings indicated BAV patients manifest reduced myocardial strain which had weak relationship with aortic elastic lesion.

Why Does Exercise "Trigger" Adaptive Protective Responses in the Heart?

Numerous epidemiological studies suggest that individuals who exercise have decreased cardiac morbidity and mortality. Pre-clinical studies in animal models also find clear cardioprotective phenotypes in animals that exercise, specifically characterized by lower myocardial infarction and arrhythmia. Despite the clear benefits, the underlying cellular and molecular mechanisms that are responsible for exercise preconditioning are not fully understood. In particular, the adaptive signaling events that occur during exercise to "trigger" cardioprotection represent emerging paradigms. In this review, we discuss recent studies that have identified several different factors that appear to initiate exercise preconditioning. We summarize the evidence for and against specific cellular factors in triggering exercise adaptations and identify areas for future study.

Parallel effects of β-adrenoceptor blockade on cardiac function and fatty acid oxidation in the diabetic heart: Confronting the maze

Diabetic cardiomyopathy is a disease process in which diabetes produces a direct and continuous myocardial insult even in the absence of ischemic, hypertensive or valvular disease. The β-blocking agents bisoprolol, carvedilol and metoprolol have been shown in large-scale randomized controlled trials to reduce heart failure mortality. In this review, we summarize the results of our studies investigating the effects of β-blocking agents on cardiac function and metabolism in diabetic heart failure, and the complex inter-related mechanisms involved. Metoprolol inhibits fatty acid oxidation at the mitochondrial level but does not prevent lipotoxicity; its beneficial effects are more likely to be due to pro-survival effects of chronic treatment. These studies have expanded our understanding of the range of effects produced by β-adrenergic blockade and show how interconnected the signaling pathways of function and metabolism are in the heart. Although our initial hypothesis that inhibition of fatty acid oxidation would be a key mechanism of action was disproved, unexpected results led us to some intriguing regulatory mechanisms of cardiac metabolism. The first was upstream stimulatory factor-2-mediated repression of transcriptional master regulator PGC-1α, most likely occurring as a consequence of the improved function; it is unclear whether this effect is unique to β-blockers, although repression of carnitine palmitoyltransferase (CPT)-1 has not been reported with other drugs which improve function. The second was the identification of a range of covalent modifications which can regulate CPT-1 directly, mediated by a signalome at the level of the mitochondria. We also identified an important interaction between β-adrenergic signaling and caveolins, which may be a key mechanism of action of β-adrenergic blockade. Our experience with this labyrinthine signaling web illustrates that initial hypotheses and anticipated directions do not have to be right in order to open up meaningful directions or reveal new information.

North American ginseng protects the heart from ischemia and reperfusion injury via upregulation of endothelial nitric oxide synthase

Emerging evidence suggests ginseng has therapeutic potential in cardiovascular disease. The aim of this study was to investigate the role of endothelial nitric oxide synthase (eNOS) in the cardioprotective effects of ginseng during myocardial ischemia and reperfusion (I/R). Treatment with ginseng extract significantly increased Akt phosphorylation and eNOS protein levels in cultured neonatal cardiomyocytes. Upregulation of eNOS was blocked by LY294002, a PI3-kinase inhibitor, suggesting a PI3-kinase/Akt-dependent mechanism. To simulate I/R, cultured neonatal cardiomyocytes from eNOS(-/-) and wild-type (WT) mice were subjected to anoxia and reoxygenation (A/R). Ginseng treatment inhibited A/R-induced apoptosis in WT, but not in either eNOS(-/-) cardiomyocytes or WT cardiomyocytes treated with LY294002. To further study the cardioprotective effects of ginseng in vivo, WT and eNOS(-/-) mice were pretreated with ginseng extract (50mg/kg/day, oral gavage) for 7 days before they were subjected to myocardial I/R. Treatment with ginseng significantly increased Akt phosphorylation and eNOS protein levels in the myocardium. Furthermore, ginseng-induced myocardial eNOS expression was inhibited by LY294002. Strikingly, ginseng treatment significantly decreased infarct size and myocardial apoptosis following I/R in WT mice, but not in either eNOS(-/-) mice or WT mice treated with LY294002. We conclude that ginseng treatment protects the heart from I/R injury via upregulation of eNOS expression. Our study suggests that ginseng may serve as a potential therapeutic agent to limit myocardial I/R injury.

Endothelial nitric oxide synthase promotes bone marrow stromal cell migration to the ischemic myocardium via upregulation of stromal cell-derived factor-1alpha

The aim of this study was to investigate the role of endothelial nitric oxide synthase (eNOS) in the host myocardium on bone marrow mesenchymal stromal cells (MSC) migration to the ischemic myocardium and whether stromal cell-derived factor-1alpha (SDF-1alpha) contributes to eNOS-mediated MSC migration. MSCs and coronary microvascular endothelial cells were isolated from adult wild-type (WT) mouse bone marrow and hearts, respectively. Cultured neonatal cardiomyocytes from WT, eNOS(-/-), and eNOS overexpressing transgenic (Tg) mice were subjected to anoxia and reoxygenation (A/R), and the conditioned medium was used as a chemoattractant for in vitro transendothelial migration assay. MSC migration was decreased in the presence of conditioned medium derived from eNOS(-/-) cardiomyocytes but increased in the presence of eNOS-Tg conditioned medium. SDF-1alpha expression was decreased in eNOS(-/-) but increased in eNOS-Tg cardiomyocytes following A/R and in the myocardium following ischemia/reperfusion (I/R). SDF-1alpha expression was cGMP-dependent as inhibition of soluble guanylyl cyclase decreased SDF-1alpha expression in WT cardiomyocytes. MSCs expressed very low levels of eNOS proteins compared with the adult myocardium. To examine MSC migration in vivo, MSCs derived from mice expressing enhanced green fluorescence protein (EGFP(+)) were intravenously administered to WT mice subjected to myocardial I/R. EGFP(+) cells in the ischemic region were decreased in eNOS(-/-) but increased in eNOS-Tg compared with WT hearts. MSC treatment improved cardiac function following I/R in WT but not in eNOS(-/-) mice. In conclusion, eNOS in the host myocardium promotes MSC migration to the ischemic myocardium and improves cardiac function through cGMP-dependent increases in SDF-1alpha expression.

Deficient BH4 production via de novo and salvage pathways regulates NO responses to cytokines in adult cardiac myocytes

Adult rat cardiac myocytes typically display a phenotypic response to cytokines manifested by low or no increases in nitric oxide (NO) production via inducible NO synthase (iNOS) that distinguishes them from other cell types. To better characterize this response, we examined the expression of tetrahydrobiopterin (BH4)-synthesizing and arginine-utilizing genes in cytokine-stimulated adult cardiac myocytes. Intracellular BH4 and 7,8-dihydrobiopterin (BH2) and NO production were quantified. Cytokines induced GTP cyclohydrolase and its feedback regulatory protein but with deficient levels of BH4 synthesis. Despite the induction of iNOS protein, cytokine-stimulated adult cardiac myocytes produced little or no increase in NO versus unstimulated cells. Western blot analysis under nonreducing conditions revealed the presence of iNOS monomers. Supplementation with sepiapterin (a precursor of BH4) increased BH4 as well as BH2, but this did not enhance NO levels or eliminate iNOS monomers. Similar findings were confirmed in vivo after treatment of rat cardiac allograft recipients with sepiapterin. It was found that expression of dihydrofolate reductase, required for full activity of the salvage pathway, was not detected in adult cardiac myocytes. Thus, adult cardiac myocytes have a limited capacity to synthesize BH4 after cytokine stimulation. The mechanisms involve posttranslational factors impairing de novo and salvage pathways. These conditions are unable to support active iNOS protein dimers necessary for NO production. These findings raise significant new questions about the prevailing understanding of how cytokines, via iNOS, cause cardiac dysfunction and injury in vivo during cardiac inflammatory disease states since cardiac myocytes are not a major source of high NO production.

Mouse ES cell-derived cardiac precursor cells are multipotent and facilitate identification of novel cardiac genes

Although the differentiation of ES cells to cardiomyocytes has been firmly established, the extent to which corresponding cardiac precursor cells can contribute to other cardiac populations remains unclear. To determine the molecular and cellular characteristics of cardiac-fated populations derived from mouse ES (mES) cells, we isolated cardiac progenitor cells (CPCs) from differentiating mES cell cultures by using a reporter cell line that expresses GFP under the control of a cardiac-specific enhancer element of Nkx2-5, a transcription factor expressed early in cardiac development. This ES cell-derived CPC population initially expressed genetic markers of both stem cells and mesoderm, while differentiated CPCs displayed markers of 3 distinct cell lineages (cardiomyocytes, vascular smooth muscle cells, and endothelial cells)--Flk1 (also known as Kdr), c-Kit, and Nkx2-5, but not Brachyury--and subsequently expressed Isl1. Clonally derived CPCs also demonstrated this multipotent phenotype. By transcription profiling of CPCs, we found that mES cell-derived CPCs displayed a transcriptional signature that paralleled in vivo cardiac development. Additionally, these studies suggested the involvement of genes that we believe were previously unknown to play a role in cardiac development. Taken together, our data demonstrate that ES cell-derived CPCs comprise a multipotent precursor population capable of populating multiple cardiac lineages and suggest that ES cell differentiation is a valid model for studying development of multiple cardiac-fated tissues.

Trypanosoma cruzi disrupts myofibrillar organization and intracellular calcium levels in mouse neonatal cardiomyocytes

Immunofluorescence studies of normal and Trypanosoma cruzi-infected primary cultures of heart muscle cells were performed to gather information about the arrangement of myofibrillar components during the intracellular life cycle of this parasite. By using a panel of monoclonal antibodies against various myofibrillar proteins, a progressive disruption and loss of contractile proteins (such myosin and actin) of the host cell was detected during infection. The host cell formed a loose network of myofibrillar proteins around the parasites. Breakdown of the myofibrils occurred in regions where the parasites were present, and heavily infected cells showed myofibrillar proteins at their periphery. In parallel, we investigated the effect of T. cruzi infection on intracellular calcium levels by using a Ca2+ fluorescent indicator (confocal microscopy). Infected cardiomyocytes displayed a marked impairment in contractility, and calcium influxes became irregular and less intense when compared with those of non-infected cells. Our results demonstrate that T. cruzi infection dramatically affects calcium fluxes and causes myofibrillar breakdown disturbing cardiomyocyte contractility.

Reactive oxygen species as signaling molecules in cardiovascular differentiation of embryonic stem cells and tumor-induced angiogenesis

Besides the well known pathophysiological impact of oxidative stress in cardiovascular disease, reactive oxygen species (ROS) generated at low concentrations exert a role as signaling molecules that are involved in signal transduction cascades of numerous growth factor-, cytokine-, and hormone-mediated pathways, and regulate biological effects such as apoptosis, cell proliferation, and differentiation. Embryonic stem cells have the capacity to differentiate into the cardiovascular cell lineage. Furthermore, upon confrontation culture with tumor tissue, they form blood vessel-like structures that induce tumor-induced angiogenesis within tumor tissues. The role of ROS in cardiovascular differentiation of embryonic stem cells appears to be antagonistic. Whereas continuous exposure to ROS results in inhibition of cardiomyogenesis and vasculogenesis, pulse chase exposure to low-level ROS enhances differentiation toward the cardiomyogenic as well as vascular cell lineage. This review summarizes the current knowledge of ROS-induced cardiovascular differentiation of embryonic stem cells as well as the role of ROS in tumor-induced angiogenesis.

Aquaporins in development -- a review

Water homeostasis during fetal development is of crucial physiologic importance. It depends upon maternal fetal fluid exchange at the placenta and fetal membranes, and some exchange between fetus and amniotic fluid can occur across the skin before full keratinization. Lungs only grow and develop normally with fluid secretion, and there is evidence that cerebral spinal fluid formation is important in normal brain development. The aquaporins are a growing family of molecular water channels, the ontogeny of which is starting to be explored. One question that is of particular importance is how well does the rodent (mouse, rat) fetus serve as a model for long-gestation mammals such as sheep and human? This is particularly important for organs such as the lung and the kidney, whose development before birth is very much less in rodents than in the long-gestation species.

Simvastatin Attenuates Expression of Cytokine-inducible Nitric-oxide Synthase in Embryonic Cardiac Myoblasts

Cardiac stem cells or myoblasts are vulnerable to inflammatory stimulation in hearts with infarction or ischemic injury. Widely used for the prevention and treatment of atherosclerotic heart disease, the cholesterol-lowering drugs statins may exert anti-inflammatory effects. In this study, we examined the impact of inhibition of hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase with simvastatin on the expression of inducible nitric-oxide synthase (iNOS) in embryonic cardiac myoblasts stimulated with the proinflammatory cytokines, interleukin-1 or tumor necrosis factor. Treatment with simvastatin significantly reduced the levels of iNOS mRNA and protein in cytokine-treated rat H9c2 cardiac embryonic myoblasts. Addition of the HMG-CoA reductase product, L-mevalonate, and the by-product of cholesterol synthesis, geranylgeranyl pyrophosphate, could reverse the statin inhibitory effect on iNOS expression. Simvastatin treatment lowered the Rho GTPase activities, whereas the Rho-associated kinase inhibitor Y27632 partially blocked the statin inhibitory effect on nitrite production in the cytokine-treated H9c2 cells. Treatment with simvastatin led to inactivation of NF-kappaB by elevation of the NF-kappaB inhibitor IkappaB and reduction of the NF-kappaB nuclear contents in the cytokine-stimulated H9c2 cells. Hence, treatment with simvastatin can attenuate iNOS expression and NO synthesis in cytokine-stimulated embryonic cardiac myoblasts. The statin inhibitory effect may occur through isoprenoid-mediated intracellular signal transduction, which involves several key signal proteins, such as Rho kinase and IkappaB/NF-kappaB. These data suggest that statin therapy may protect the cardiac myocyte progenitors against the cytotoxicity of cytokine-induced high output of NO production in infarcted or ischemic hearts with inflammation.

Heart mitochondrial nitric oxide synthase is upregulated in male rats exposed to high altitude (4,340 m)

Male rats exposed for 21 days to high altitude (4,340 m) responded with arrest of weight gain and increased hematocrit and testosterone levels. High altitude significantly (58%) increased heart mitochondrial nitric oxide (NO) synthase (mtNOS) activity, whereas heart cytosolic endothelial NOS (eNOS) and liver mtNOS were not affected. Western blot analysis found heart mitochondria reacting only with anti-inducible NOS (iNOS) antibodies, whereas the postmitochondrial fraction reacted with anti-iNOS and anti-eNOS antibodies. In vitro-measured NOS activities allowed the estimation of cardiomyocyte capacity for NO production, a value that increased from 57% (sea level) to 79 nmol NO.min(-1).g heart(-1) (4,340 m). The contribution of mtNOS to total cell NO production increased from 62% (sea level) to 71% (4340 m). Heart mtNOS activity showed a linear relationship with hematocrit and a biphasic quadratic association with estradiol and testosterone. Multivariate analysis showed that exposure to high altitude linearly associates with hematocrit and heart mtNOS activity, and that testosterone-to-estradiol ratio and heart weight were not linearly associated with mtNOS activity. We conclude that high altitude triggers a physiological adaptive response that upregulates heart mtNOS activity and is associated in an opposed manner with the serum levels of testosterone and estradiol.

Mechanisms of beta 3-adrenoceptor-induced eNOS activation in right atrial and left ventricular human myocardium

beta-Adrenoceptors are important modulators of cardiac function. The present study investigated beta(3)-adrenergic eNOS activation in human myocardium. We measured nitric oxide (NO) liberation (diaminofluorescence) and signal transduction (immunohistochemistry, phosphorylation of eNOS(Ser1177), eNOS(Thr495), eNOS(Ser114), Akt/protein kinase B (Akt/PKB), and eNOS translocation) in human right atrial (RA, aortocoronary-bypass OP) and left ventricular nonfailing (LV, rejected donor hearts) myocardium after application of BRL 37344 (BRL), a preferential beta(3)-adrenoceptor agonist. In both RA and LV, BRL (10 microl) induced a liberation of NO. An eNOS activation via translocation was only observed in RA after application of BRL (10 microM). Yet, the NO liberation in both LV and RA was accompanied by phosphorylation of eNOS(Ser1177) and Akt/PKB. BRL-induced eNOS phosphorylation was abolished by LY292004, a blocker of PI-3 kinase. eNOS-Ser(114) phosphorylation was unchanged in RA, but decreased in LV after beta(3)-adrenergic stimulation. BRL did not alter phosphorylation of eNOS(Thr495). In conclusion, receptor-dependent eNOS activation is differentially regulated in the human heart. In the left ventricle, eNOS activation via phosphorylation seems to be of major importance, whereas in human atrial myocardium eNOS translocation is the predominant mechanism induced by beta(3)-adrenergic activation.

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.