Positive and negative effects of nitric oxide on Ca(2+) sparks: influence of beta-adrenergic stimulation

Nitric Oxide Modulates Ca2+ Leak and Arrhythmias via S-Nitrosylation of CaMKII

BACKGROUND

Nitric oxide (NO) has been identified as a signaling molecule generated during β-adrenergic receptor stimulation in the heart. Furthermore, a role for NO in triggering spontaneous Ca2+ release via S-nitrosylation of CaMKIIδ (Ca2+/calmodulin kinase II delta) is emerging. NO donors are routinely used clinically for their cardioprotective effects on the heart, but it is unknown how NO donors modulate the proarrhythmic CaMKII to alter cardiac arrhythmia incidence. We test the role of S-nitrosylation of CaMKIIδ at the Cysteine-273 inhibitory site and cysteine-290 activating site in cardiac Ca2+ handling and arrhythmogenesis before and during β-adrenergic receptor stimulation.

METHODS

We measured Ca2+-handling in isolated cardiomyocytes from C57BL/6J wild-type (WT) mice and mice lacking CaMKIIδ expression (CaMKIIδ-KO) or with deletion of the S-nitrosylation site on CaMKIIδ at cysteine-273 or cysteine-290 (CaMKIIδ-C273S and -C290A knock-in mice). Cardiomyocytes were exposed to NO donors, S-nitrosoglutathione (GSNO; 150 μM), sodium nitroprusside (200 μM), and β-adrenergic agonist isoproterenol (100 nmol/L).

RESULTS

Both WT and CaMKIIδ-KO cardiomyocytes responded to isoproterenol with a full inotropic and lusitropic Ca2+ transient response as well as increased Ca2+ spark frequency. However, the increase in Ca2+ spark frequency was significantly attenuated in CaMKIIδ-KO cardiomyocytes. The protection from isoproterenol-induced Ca2+ sparks and waves was mimicked by GSNO pretreatment in WT cardiomyocytes but lost in CaMKIIδ-C273S cardiomyocytes. When GSNO was applied after isoproterenol, this protection was not observed in WT or CaMKIIδ-C273S but was apparent in CaMKIIδ-C290A. In Langendorff-perfused isolated hearts, GSNO pretreatment limited isoproterenol-induced arrhythmias in WT but not CaMKIIδ-C273S hearts, while GSNO exposure after isoproterenol sustained or exacerbated arrhythmic events.

CONCLUSIONS

We conclude that prior S-nitrosylation of CaMKIIδ at cysteine-273 can limit subsequent β-adrenergic receptor-induced arrhythmias, but that S-nitrosylation at cysteine-290 might worsen or sustain β-adrenergic receptor-induced arrhythmias. This has important implications for the administration of NO donors in the clinical setting.

Nitric Oxide modulates spontaneous Ca2+ release and ventricular arrhythmias during β-adrenergic signalling through S-nitrosylation of Calcium/Calmodulin dependent kinase II

Rationale

Nitric oxide (NO) has been identified as a signalling molecule generated during β-adrenergic receptor (AR) stimulation in the heart. Furthermore, a role for NO in triggering spontaneous Ca2+ release via S-nitrosylation of Ca2+/calmodulin kinase II delta (CaMKIIδ) is emerging. NO donors are routinely used clinically for their cardioprotective effects in the heart, but it is unknown how NO donors modulate the pro-arrhythmic CaMKII to alter cardiac arrhythmia incidence.

Objective

We test the role of S-nitrosylation of CaMKIIδ at the Cys-273 inhibitory site and Cys-290 activating site in cardiac Ca2+ handling and arrhythmogenesis before and during β-AR stimulation.

Methods and Results

We measured Ca2+-handling in isolated cardiomyocytes from C57BL/6J wild-type (WT) mice and mice lacking CaMKIIδ expression (CaMKIIδ-KO) or with deletion of the S-nitrosylation site on CaMKIIδ at Cys-273 or Cys-290 (CaMKIIδ-C273S and -C290A knock-in mice). Cardiomyocytes were exposed to NO donors, S-nitrosoglutathione (GSNO; 150 μM), sodium nitroprusside (SNP; 200 μM) and/or β-adrenergic agonist isoproterenol (ISO; 100 nM). WT and CaMKIIδ-KO cardiomyocytes treated with GSNO showed no change in Ca2+ transient or spark properties under baseline conditions (0.5 Hz stimulation frequency). Both WT and CaMKIIδ-KO cardiomyocytes responded to ISO with a full inotropic and lusitropic Ca2+ transient response as well as increased Ca2+ spark frequency. However, the increase in Ca2+ spark frequency was significantly attenuated in CaMKIIδ-KO cardiomyocytes. The protection from ISO-induced Ca2+ sparks and waves was mimicked by GSNO pre-treatment in WT cardiomyocytes, but lost in CaMKIIδ-C273S cardiomyocytes that displayed a robust increase in Ca2+ waves. This observation is consistent with CaMKIIδ-C273 S-nitrosylation being critical in limiting ISO-induced arrhythmogenic sarcoplasmic reticulum Ca2+ leak. When GSNO was applied after ISO this protection was not observed in WT or CaMKIIδ-C273S but was apparent in CaMKIIδ-C290A. In Langendorff-perfused isolated hearts, GSNO pre-treatment limited ISO-induced arrhythmias in WT but not CaMKIIδ-C273S hearts, while GSNO exposure after ISO sustained or exacerbated arrhythmic events.

Conclusions

We conclude that prior S-nitrosylation of CaMKIIδ at Cys-273 can limit subsequent β-AR induced arrhythmias, but that S-nitrosylation at Cys-290 might worsen or sustain β-AR-induced arrhythmias. This has important implications for the administration of NO donors in the clinical setting.

Gaseous Signaling Molecules in Cardiovascular Function: From Mechanisms to Clinical Translation

Carbon monoxide (CO), hydrogen sulfide (H₂S), and nitric oxide (NO) constitute endogenous gaseous molecules produced by specific enzymes. These gases are chemically simple, but exert multiple effects and act through shared molecular targets to control both physiology and pathophysiology in the cardiovascular system (CVS). The gases act via direct and/or indirect interactions with each other in proteins such as heme-containing enzymes, the mitochondrial respiratory complex, and ion channels, among others. Studies of the major impacts of CO, H₂S, and NO on the CVS have revealed their involvement in controlling blood pressure and in reducing cardiac reperfusion injuries, although their functional roles are not limited to these conditions. In this review, the basic aspects of CO, H₂S, and NO, including their production and effects on enzymes, mitochondrial respiration and biogenesis, and ion channels are briefly addressed to provide insight into their biology with respect to the CVS. Finally, potential therapeutic applications of CO, H₂S, and NO with the CVS are addressed, based on the use of exogenous donors and different types of delivery systems.

Recent advances in understanding cardiac contractility in health and disease

The aim of this review is to provide the reader with a synopsis of some of the emerging ideas and experimental findings in cardiac physiology and pathophysiology that were published in 2015. To provide context for the non-specialist, a brief summary of cardiac contraction and calcium (Ca) regulation in the heart in health and disease is provided. Thereafter, some recently published articles are introduced that indicate the current thinking on (1) the Ca regulatory pathways modulated by Ca/calmodulin-dependent protein kinase II, (2) the potential influences of nitrosylation by nitric oxide or S-nitrosated proteins, (3) newly observed effects of reactive oxygen species (ROS) on contraction and Ca regulation following myocardial infarction and a possible link with changes in mitochondrial Ca, and (4) the effects of some of these signaling pathways on late Na current and pro-arrhythmic afterdepolarizations as well as the effects of transverse tubule disturbances.

Effect of NO Synthase Blockade on Myocardial Contractility of Hypokinetic Rats during Stimulation of β-Adrenoreceptors

Stimulation of β-adrenoreceptors with low (10(-8) and 10(-7) M) or high (10(-6) M) doses of isoproterenol in hypokinetic rats treated with L-NAME (a non-selective blocker of NO synthases) decreased or increased myocardial contractility, respectively. In control rats, all examined doses of isoproterenol used under blockade of NO synthases inhibited myocardial contractility.

CaMKIIδ mediates β-adrenergic effects on RyR2 phosphorylation and SR Ca(2+) leak and the pathophysiological response to chronic β-adrenergic stimulation

Chronic activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) has been implicated in the deleterious effects of β-adrenergic receptor (β-AR) signaling on the heart, in part, by enhancing RyR2-mediated sarcoplasmic reticulum (SR) Ca(2+) leak. We used CaMKIIδ knockout (CaMKIIδ-KO) mice and knock-in mice with an inactivated CaMKII site S2814 on the ryanodine receptor type 2 (S2814A) to investigate the involvement of these processes in β-AR signaling and cardiac remodeling. Langendorff-perfused hearts from CaMKIIδ-KO mice showed inotropic and chronotropic responses to isoproterenol (ISO) that were similar to those of wild type (WT) mice; however, in CaMKIIδ-KO mice, CaMKII phosphorylation of phospholamban and RyR2 was decreased and isolated myocytes from CaMKIIδ-KO mice had reduced SR Ca(2+) leak in response to isoproterenol (ISO). Chronic catecholamine stress with ISO induced comparable increases in relative heart weight and other measures of hypertrophy from day 9 through week 4 in WT and CaMKIIδ-KO mice, but the development of cardiac fibrosis was prevented in CaMKIIδ-KO animals. A 4-week challenge with ISO resulted in reduced cardiac function and pulmonary congestion in WT, but not in CaMKIIδ-KO or S2814A mice, implicating CaMKIIδ-dependent phosphorylation of RyR2-S2814 in the cardiomyopathy, independent of hypertrophy, induced by prolonged β-AR stimulation.

Activation of Sonic hedgehog signaling in ventricular cardiomyocytes exerts cardioprotection against ischemia reperfusion injuries

Sonic hedgehog (SHH) is a conserved protein involved in embryonic tissue patterning and development. SHH signaling has been reported as a cardio-protective pathway via muscle repair–associated angiogenesis. The goal of this study was to investigate the role of SHH signaling pathway in the adult myocardium in physiological situation and after ischemia-reperfusion. We show in a rat model of ischemia-reperfusion that stimulation of SHH pathway, either by a recombinant peptide or shed membranes microparticles harboring SHH ligand, prior to reperfusion reduces both infarct size and subsequent arrhythmias by preventing ventricular repolarization abnormalities. We further demonstrate in healthy animals a reduction of QTc interval mediated by NO/cGMP pathway leading to the shortening of ventricular cardiomyocytes action potential duration due to the activation of an inward rectifying potassium current sharing pharmacological and electrophysiological properties with ATP-dependent potassium current. Besides its effect on both angiogenesis and endothelial dysfunction we demonstrate here a novel cardio-protective effect of SHH acting directly on the cardiomyocytes. This emphasizes the pleotropic effect of SHH pathway as a potential cardiac therapeutic target.

Interaction between neuronal nitric oxide synthase signaling and temperature influences sarcoplasmic reticulum calcium leak: role of nitroso-redox balance

RATIONALE

Although nitric oxide (NO) signaling modulates cardiac function and excitation-contraction coupling, opposing results because of inconsistent experimental conditions, particularly with respect to temperature, confound the ability to elucidate NO signaling pathways. Here, we show that temperature significantly modulates NO effects.

OBJECTIVE

To test the hypothesis that temperature profoundly affects nitroso-redox equilibrium, thereby affecting sarcoplasmic reticulum (SR) calcium (Ca(2+)) leak.

METHODS AND RESULTS

We measured SR Ca(2+) leak in cardiomyocytes from wild-type (WT), NO/redox imbalance (neuronal nitric oxide synthase-deficient mice-1 [NOS1(-/-)]), and hyper S-nitrosoglutathione reductase-deficient (GSNOR(-/-)) mice. In WT cardiomyocytes, SR Ca(2+) leak increased because temperature decreased from 37°C to 23°C, whereas in NOS1(-/-) cells, the leak suddenly increased when the temperature surpassed 30°C. GSNOR(-/-) cardiomyocytes exhibited low leak throughout the temperature range. Exogenously added NO had a biphasic effect on NOS1(-/-) cardiomyocytes; reducing leak at 37°C but increasing it at subphysiological temperatures. Oxypurinol and Tempol diminished the leak in NOS1(-/-) cardiomyocytes. Cooling from 37°C to 23°C increased reactive oxygen species generation in WT but decreased it in NOS1(-/-) cardiomyocytes. Oxypurinol further reduced reactive oxygen species generation. At 23°C in WT cells, leak was decreased by tetrahydrobiopterin, an essential NOS cofactor. Cooling significantly increased SR Ca(2+) content in NOS1(-/-) cells but had no effect in WT or GSNOR(-/-).

CONCLUSIONS

Ca(2+) leak and temperature are normally inversely proportional, whereas NOS1 deficiency reverses this effect, increasing leak and elevating reactive oxygen species production because temperature increases. Reduced denitrosylation (GSNOR deficiency) eliminates the temperature dependence of leak. Thus, temperature regulates the balance between NO and reactive oxygen species which in turn has a major effect on SR Ca(2+).

NO-dependent CaMKII activation during β-adrenergic stimulation of cardiac muscle

AIMS

During β-adrenergic receptor (β-AR) stimulation, phosphorylation of cardiomyocyte ryanodine receptors by protein kinases may contribute to an increased diastolic Ca(2+) spark frequency. Regardless of prompt activation of protein kinase A during β-AR stimulation, this appears to rely more on activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), by a not yet identified signalling pathway. The goal of the present study was to identify and characterize the mechanisms which lead to CaMKII activation and elevated Ca(2+) spark frequencies during β-AR stimulation in single cardiomyocytes in diastolic conditions.

METHODS AND RESULTS

Confocal imaging revealed that β-AR stimulation increases endogenous NO production in cardiomyocytes, resulting in NO-dependent activation of CaMKII and a subsequent increase in diastolic Ca(2+) spark frequency. These changes of spark frequency could be mimicked by exposure to the NO donor GSNO and were sensitive to the CaMKII inhibitors KN-93 and AIP. In vitro, CaMKII became nitrosated and its activity remained increased independent of Ca(2+) in the presence of GSNO, as assessed with biochemical assays.

CONCLUSIONS

β-AR stimulation of cardiomyocytes may activate CaMKII by a novel direct pathway involving NO, without requiring Ca(2+) transients. This crosstalk between two established signalling pathways may contribute to arrhythmogenic diastolic Ca(2+) release and Ca(2+) waves during adrenergic stress, particularly in combination with cardiac diseases. In addition, NO-dependent activation of CaMKII is likely to have repercussions in many cellular signalling systems and cell types.

Modulation of myocardial contraction by peroxynitrite

Peroxynitrite is a potent oxidant that is quickly emerging as a crucial modulator of myocardial function. This review will focus on the regulation of myocardial contraction by peroxynitrite during health and disease, with a specific emphasis on cardiomyocyte Ca²⁺ handling, proposed signaling pathways, and protein end-targets.

Posttranslational modifications of cardiac ryanodine receptors: Ca(2+) signaling and EC-coupling

In cardiac muscle, a number of posttranslational protein modifications can alter the function of the Ca(2+) release channel of the sarcoplasmic reticulum (SR), also known as the ryanodine receptor (RyR). During every heartbeat RyRs are activated by the Ca(2+)-induced Ca(2+) release mechanism and contribute a large fraction of the Ca(2+) required for contraction. Some of the posttranslational modifications of the RyR are known to affect its gating and Ca(2+) sensitivity. Presently, research in a number of laboratories is focused on RyR phosphorylation, both by PKA and CaMKII, or on RyR modifications caused by reactive oxygen and nitrogen species (ROS/RNS). Both classes of posttranslational modifications are thought to play important roles in the physiological regulation of channel activity, but are also known to provoke abnormal alterations during various diseases. Only recently it was realized that several types of posttranslational modifications are tightly connected and form synergistic (or antagonistic) feed-back loops resulting in additive and potentially detrimental downstream effects. This review summarizes recent findings on such posttranslational modifications, attempts to bridge molecular with cellular findings, and opens a perspective for future work trying to understand the ramifications of crosstalk in these multiple signaling pathways. Clarifying these complex interactions will be important in the development of novel therapeutic approaches, since this may form the foundation for the implementation of multi-pronged treatment regimes in the future. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Prolonged Action Potential and After depolarizations Are Not due to Changes in Potassium Currents in NOS3 Knockout Ventricular Myocytes

Ventricular myocytes deficient in endothelial nitric oxide synthase (NOS3(-/-)) exhibit prolonged action potential (AP) duration and enhanced spontaneous activity (early and delayed afterdepolarizations) during β-adrenergic (β-AR) stimulation. Studies have shown that nitric oxide is able to regulate various K(+) channels. Our objective was to examine if NOS3(-/-) myocytes had altered K(+) currents. APs, transient outward (I(to)), sustained (I(Ksus)), and inward rectifier (I(K1)) K(+) currents were measured in NOS3(-/-) and wild-type (WT) myocytes. During β-AR stimulation, AP duration (measured as 90% repolarization-APD(90)) was prolonged in NOS3(-/-) compared to WT myocytes. Nevertheless, we did not observe differences in I(to), I(Ksus), or I(K1) between WT and NOS3(-/-) myocytes. Our previous work showed that NOS3(-/-) myocytes had a greater Ca(2+) influx via L-type Ca(2+) channels with β-AR stimulation. Thus, we measured β-AR-stimulated SR Ca(2+) load and found a greater increase in NOS3(-/-) versus WT myocytes. Hence, our data suggest that the prolonged AP in NOS3(-/-) myocytes is not due to changes in I(to), I(Ksus), or I(K1). Furthermore, the increase in spontaneous activity in NOS3(-/-) myocytes may be due to a greater increase in SR Ca(2+) load. This may have important implications for heart failure patients, where arrhythmias are increased and NOS3 expression is decreased.

Decreased cardiac L-type Ca²⁺ channel activity induces hypertrophy and heart failure in mice

Antagonists of L-type Ca²⁺ channels (LTCCs) have been used to treat human cardiovascular diseases for decades. However, these inhibitors can have untoward effects in patients with heart failure, and their overall therapeutic profile remains nebulous given differential effects in the vasculature when compared with those in cardiomyocytes. To investigate this issue, we examined mice heterozygous for the gene encoding the pore-forming subunit of LTCC (calcium channel, voltage-dependent, L type, α1C subunit [Cacna1c mice; referred to herein as α1C⁻/⁺ mice]) and mice in which this gene was loxP targeted to achieve graded heart-specific gene deletion (termed herein α1C-loxP mice). Adult cardiomyocytes from the hearts of α1C⁻/⁺ mice at 10 weeks of age showed a decrease in LTCC current and a modest decrease in cardiac function, which we initially hypothesized would be cardioprotective. However, α1C⁻/⁺ mice subjected to pressure overload stimulation, isoproterenol infusion, and swimming showed greater cardiac hypertrophy, greater reductions in ventricular performance, and greater ventricular dilation than α1C⁺/⁺ controls. The same detrimental effects were observed in α1C-loxP animals with a cardiomyocyte-specific deletion of one allele. More severe reductions in α1C protein levels with combinatorial deleted alleles produced spontaneous cardiac hypertrophy before 3 months of age, with early adulthood lethality. Mechanistically, our data suggest that a reduction in LTCC current leads to neuroendocrine stress, with sensitized and leaky sarcoplasmic reticulum Ca²⁺ release as a compensatory mechanism to preserve contractility. This state results in calcineurin/nuclear factor of activated T cells signaling that promotes hypertrophy and disease.

cGMP signals modulate cAMP levels in a compartment-specific manner to regulate catecholamine-dependent signaling in cardiac myocytes

RATIONALE

cAMP and cGMP are intracellular second messengers involved in heart pathophysiology. cGMP can potentially affect cAMP signals via cGMP-regulated phosphodiesterases (PDEs).

OBJECTIVE

To study the effect of cGMP signals on the local cAMP response to catecholamines in specific subcellular compartments.

METHODS AND RESULTS

We used real-time FRET imaging of living rat ventriculocytes expressing targeted cAMP and cGMP biosensors to detect cyclic nucleotides levels in specific locales. We found that the compartmentalized, but not the global, cAMP response to isoproterenol is profoundly affected by cGMP signals. The effect of cGMP is to increase cAMP levels in the compartment where the protein kinase (PK)A-RI isoforms reside but to decrease cAMP in the compartment where the PKA-RII isoforms reside. These opposing effects are determined by the cGMP-regulated PDEs, namely PDE2 and PDE3, with the local activity of these PDEs being critically important. The cGMP-mediated modulation of cAMP also affects the phosphorylation of PKA targets and myocyte contractility.

CONCLUSIONS

cGMP signals exert opposing effects on local cAMP levels via different PDEs the activity of which is exerted in spatially distinct subcellular domains. Inhibition of PDE2 selectively abolishes the negative effects of cGMP on cAMP and may have therapeutic potential.

Low extracellular pH induces damage in the pancreatic acinar cell by enhancing calcium signaling

Low extracellular pH (pHe) occurs in a number of clinical conditions and sensitizes to the development of pancreatitis. The mechanisms responsible for this sensitization are unknown. Because abnormal Ca(2+) signaling underlies many of the early steps in the pathogenesis of pancreatitis, we evaluated the effect of decreasing pHe from 7.4 to 7.0 on Ca(2+) signals in the acinar cell. Low pHe significantly increased the amplitude of cerulein-induced Ca(2+) signals. The enhancement in amplitude was localized to the basolateral region of the acinar cell and was reduced by pretreatment with ryanodine receptor (RYR) inhibitors. Because basolateral RYRs also have been implicated in the pathogenesis of pancreatitis, we evaluated the effects of RYR inhibitors on pancreatitis responses in acidic conditions. RYR inhibitors significantly reduced the sensitizing effects of low pHe on zymogen activation and cellular injury. These findings suggest that enhanced RYR-mediated Ca(2+) signaling in the basolateral region of the acinar cell is responsible for the injurious effects of low pHe on the exocrine pancreas.

Nitric oxide/reactive oxygen species generation and nitroso/redox imbalance in heart failure: from molecular mechanisms to therapeutic implications

Adaptation of the heart to intrinsic and external stress involves complex modifications at the molecular and cellular levels that lead to tissue remodeling, functional and metabolic alterations, and finally to failure depending upon the nature, intensity, and chronicity of the stress. Reactive oxygen species (ROS) have long been considered as merely harmful entities, but their role as second messengers has gradually emerged. At the same time, our comprehension of the multifaceted role of nitric oxide (NO) and the related reactive nitrogen species (RNS) has been upgraded. The tight interlay between ROS and RNS suggests that their imbalance may implicate the impairment in physiological NO/redox-based signaling that contributes to the failing of the cardiovascular system. This review initially provides basic concepts on the role of nitroso/oxidative stress in the pathophysiology of heart failure with a particular focus on sources of ROS/RNS, their downstream targets, and endogenous modulators. Then, the role of NO/redox regulation of cardiomyocyte function, including calcium homeostasis, electrogenesis, and insulin signaling pathways, is described. Finally, an overview of old and emerging therapeutic opportunities in heart failure is presented, focusing on modulation of NO/redox mechanisms and discussing benefits and limitations.

Regulation of myocyte contraction via neuronal nitric oxide synthase: role of ryanodine receptor S-nitrosylation

The sarcoplasmic reticulum (SR) Ca(2+) release channel (ryanodine receptor, RyR2) has been proposed to be an end target of neuronal nitric oxide synthase (NOS1) signalling. The purpose of this study is to investigate the mechanism of NOS1 modulation of RyR2 activity and the corresponding effect on myocyte function. Myocytes were isolated from NOS1 knockout (NOS1(/)) and wild-type mice. NOS1(/) myocytes displayed a decreased fractional SR Ca(2+) release, NOS1 knockout also led to reduced RyR2 S-nitrosylation levels. RyR2 channels from NOS1(/) hearts had decreased RyR2 open probability. Additionally, knockout of NOS1 led to a decrease in [(3)H]ryanodine binding, Ca(2+) spark frequency (CaSpF) and a rightward shift in the SR Ca(2+) leak/load relationship. Similar effects were observed with acute inhibition of NOS1. These data are indicative of decreased RyR2 activity in myocytes with NOS1 knockout or acute inhibition. Interestingly, the NO donor and nitrosylating agent SNAP reversed the depressed RyR2 open probability, the reduced CaSpF, and caused a leftward shift in the leak/load relationship in NOS1(/) myocytes. SNAP also normalized Ca(2+) transient and cell shortening amplitudes and SR fractional release in myocytes with NOS1 knockout or acute inhibition. Furthermore, SNAP was able to normalize the RyR2 S-nitrosylation levels. These data suggest that NOS1 signalling increases RyR2 activity via S-nitrosylation, which contributes to the NOS1-induced positive inotropic effect. Thus, RyR2 is an important end target of NOS1.

Peroxynitrite Increases Protein Phosphatase Activity and Promotes the Interaction of Phospholamban with Protein Phosphatase 2a in the Myocardium

High levels of peroxynitrite have been shown to decrease cardiomyocyte contraction through a reduction in phospholamban (PLB) phosphorylation. However, previous reports did not examine the direct effect of peroxynitrite on protein phosphatase activity in the myocardium or the role of specific phosphatases. Here we test the effect of the peroxynitrite donor SIN-1 on protein phosphatase activity in whole heart homogenates, as well as the interaction of PLB with protein phosphatase 1 (PP1) and 2a (PP2a). SIN-1 (200 μmol/L) induced a significant increase in protein phosphatase activity, which was alleviated with the specific PP1/PP2a inhibitor okadaic acid. Conversely, lower concentrations of SIN-1 and the nitric oxide donor spermine NONOate (300 μmol/L) were both without effect on phosphatase activity. We next examined the effect of SIN-1 on the interaction of PLB with PP1 and PP2a using co-immunoprecipitation, since okadaic acid inhibited the effects of SIN-1 in our current and previous studies. SIN-1 significantly increased the interaction of PLB with PP2a, but had no effect on the interaction between PLB and PP1. Urate, a peroxynitrite scavenger, inhibited the effects of SIN-1 on phosphatase activity and the interaction of PLB with PP2a, thus implicating peroxynitrite as the causal species. The results of this study provide further insight into the mechanism through which high levels of peroxynitrite serve to decrease PLB phosphorylation and myocardial contraction. Therefore, peroxynitrite signaling could play a key role in the contractile dysfunction manifested in heart failure where peroxynitrite production and protein phosphatase activity are increased and PLB phosphorylation is decreased.

Current concepts of neurohormonal activation in heart failure: mediators and mechanisms

Neurohormonal activation is a commonly cited array of phenomena in the body's physiologic response to heart failure. Although various neurohormones and pharmacologic agents that moderate their pathophysiologic effects have been reviewed in the nursing literature, both the mechanisms of neurohormonal system activation and cellular and organ system effects have been described only in brief. Accordingly, this article reviews mechanisms of neurohormonal activation and describes cellular and cardiovascular effects of the (1) sympathetic nervous system, (2) renin-angiotensin-aldosterone system, (3) kallikrein-kininogen-kinin system, (4) vasopressinergic system, (5) natriuretic peptide systems, and (6) endothelin in the context of heart failure. This article implicitly details the physiologic basis for numerous current and potential future pharmacologic agents used in the management of heart failure. It is intended that this article be used as a reference for advanced clinical nursing practice, research, and education.

Nitric oxide signaling and the regulation of myocardial function

Nitric oxide, which is produced endogenously within cardiac myocytes by three distinct isoforms of nitric oxide synthase, is a key regulator of myocardial function. This review will focus on the regulation of myocardial function by each nitric oxide synthase isoform during health and disease, with a specific emphasis on the proposed end-targets and signaling pathways.

Biphasic effect of SIN-1 is reliant upon cardiomyocyte contractile state

Many studies have demonstrated a biphasic effect of peroxynitrite in the myocardium, but few studies have investigated this biphasic effect on beta-adrenergic responsiveness and its dependence on contractile state. We have previously shown that high 3-morpholinosydnonimine (SIN-1) (source of peroxynitrite, 200 micromol/L) produced significant anti-adrenergic effects during maximal beta-adrenergic stimulation in cardiomyocytes. In the current study, we hypothesize that the negative effects of high SIN-1 will be greatest during high contractile states, whereas the positive effects of low SIN-1 (10 micromol/L) will predominate during low contractility. Isolated murine cardiomyocytes were field stimulated at 1 Hz, and [Ca(2+)](i) transients and shortening were recorded. After submaximal isoproterenol (ISO) (beta-adrenergic agonist, 0.01 micromol/L) stimulation, 200 micromol/L SIN-1 induced two distinct phenomena. Cardiomyocytes undergoing a large response to ISO showed a significant reduction in contractility, whereas cardiomyocytes exhibiting a modest response to ISO showed a further increase in contractility. Additionally, 10 micromol/L SIN-1 always increased contractility during low ISO stimulation, but had no effect during maximal ISO (1 micromol/L) stimulation. SIN-1 at 10 micromol/L also increased basal contractility. Interestingly, SIN-1 produced a contractile effect under only one condition in phospholamban-knockout cardiomyocytes, providing a potential mechanism for the biphasic effect of peroxynitrite. These results provide clear evidence for a biphasic effect of peroxynitrite, with high peroxynitrite modulating high levels of beta-adrenergic responsiveness and low peroxynitrite regulating basal function and low levels of beta-adrenergic stimulation.

Cellular mechanisms of burn-related changes in contractility and its prevention by mesenteric lymph ligation

Major burn injury results in impairment of left ventricular (LV) contractile function. There is strong evidence to support the involvement of gut-derived factor(s) transported in mesenteric lymph in the development of burn-related contractile dysfunction; i.e., mesenteric lymph duct ligation (LDL) prevents burn-related contractile depression. However, the cellular mechanisms for altered myocardial contractility of postburn hearts are largely unknown, and the cellular basis for the salutary effects of LDL on cardiac function have not been investigated. We examined contractility, Ca2+ transients, and L-type Ca2+ currents ( ICa) in LV myocytes isolated from four groups of rats: 1) sham burn, 2) sham burn with LDL (sham + LDL), 3) burn (≈40% of total body surface area burn), and 4) burn with LDL (burn + LDL). Myocytes isolated from hearts at 24 h postburn had a depressed contractility (≈20%) at baseline and blunted responsiveness to elevation of bath Ca2+. Myocyte contractility was comparable in sham + LDL and sham burn hearts. LDL completely prevented burn-related changes in myocyte contractility. Mechanistically, the decrease in contractility in myocytes from postburn hearts occurred with a decrease in the amplitude of Ca2+ transients (≈20%) without changes in resting Ca2+ or Ca2+ content of the sarcoplasmic reticulum. On the other hand, ICa density was decreased (≈30%) in myocytes from postburn hearts, with unaltered voltage-dependent properties. Thus burn-related myocardial contractile dysfunction is linked with depressed myocyte contractility associated with a decrease in ICa density. These findings also provide strong evidence that mesenteric lymph is involved in the onset of burn-related cardiomyocyte dysfunction.

Nitroxyl improves cellular heart function by directly enhancing cardiac sarcoplasmic reticulum Ca2+ cycling

Heart failure remains a leading cause of morbidity and mortality worldwide. Although depressed pump function is common, development of effective therapies to stimulate contraction has proven difficult. This is thought to be attributable to their frequent reliance on cAMP stimulation to increase activator Ca(2+). A potential alternative is nitroxyl (HNO), the 1-electron reduction product of nitric oxide (NO) that improves contraction and relaxation in normal and failing hearts in vivo. The mechanism for myocyte effects remains unknown. Here, we show that this activity results from a direct interaction of HNO with the sarcoplasmic reticulum Ca(2+) pump and the ryanodine receptor 2, leading to increased Ca(2+) uptake and release from the sarcoplasmic reticulum. HNO increases the open probability of isolated ryanodine-sensitive Ca(2+)-release channels and accelerates Ca(2+) reuptake into isolated sarcoplasmic reticulum by stimulating ATP-dependent Ca(2+) transport. Contraction improves with no net rise in diastolic calcium. These changes are not induced by NO, are fully reversible by addition of reducing agents (redox sensitive), and independent of both cAMP/protein kinase A and cGMP/protein kinase G signaling. Rather, the data support HNO/thiolate interactions that enhance the activity of intracellular Ca(2+) cycling proteins. These findings suggest HNO donors are attractive candidates for the pharmacological treatment of heart failure.

Peroxynitrite and myocardial contractility: in vivo versus in vitro effects

Generation of peroxynitrite (ONOO-) as a result of altered redox balance has been shown to affect cardiac function; however, inconsistencies in the data exist, particularly for myocardial contractility. The hypothesis that the cardiac impact of ONOO- formation depends on its site of generation, intravascular or intramyocardial, was examined. Cardiac contractility was assessed by pressure-volume analysis to delineate vascular versus cardiac changes on direct infusion of ONOO- into the right atria of conscious dogs both with normal cardiac function and in heart failure. Additionally, ONOO- was administered to isolated murine cardiomyocytes to mimic in situ cardiac generation. When infused in vivo, ONOO- had little impact on inotropy but led to systemic arterial dilation, likely as a result of rapid decomposition to NO2- and NO3-. In contrast, infused ONOO- was long lived enough to abolish beta-adrenergic (dobutamine)-stimulated contractility/relaxation, most likely through catecholamine oxidation to aminochrome. When administered to isolated murine cardiomyocytes, ONOO- induced a rapid reduction in sarcomere shortening and whole cell calcium transients, although neither decomposed ONOO- or NaNO2 had any effect. Thus, systemic generation of ONOO- is unlikely to have primary cardiac effects, but may modulate cardiac contractile reserve, via blunted beta-adrenergic stimulation, and vascular tone, as a result of generation of NO2- and NO3-. However, myocyte generation of ONOO- may impair contractile function by directly altering Ca2+ handling. These data demonstrate that the site of generation within the cardiovascular system largely dictates the ability of ONOO- to directly or indirectly modulate cardiac pump function.

Isoprenaline enhances local Ca2+ release in cardiac myocytes

AIM

Contraction of cardiac myocytes is controlled by the generation and amplification of intracellular Ca2+ signals. The key step of this process is the coupling between sarcolemma L-type Ca2+ channels (LCCs) and ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR). Beta-adrenergic stimulation is an important regulatory mechanism for this coupling process. But the details underlied the global level, which require local Ca2+ release study are still unclear. The present study is to explore the effects of beta-adrenergic stimulation on local Ca2+ release.

METHODS

Using confocal microscopy combined with loose-seal patch-clamp approaches, effects of isoprenaline (1 micromol/L), a beta-adrenergic agonist, on local SR Ca2+ release triggered by Ca2+ influx through LCCs in intact rat cardiac myocytes were investigated.

RESULTS

Isoprenaline increased the intensity of ensemble averaged local Ca2+ transients, the peak of which displayed a typical bell-shaped voltage-dependence over the membrane voltages ranging from approximately -40 mV to approximately +35 mV. Further analysis showed that this enhancement could be explained by the increased coupling fidelity (which refers the increased probability of RyRs activation upon depolarization), and the increased amplitude of evoked Ca2+ sparks [due to more Ca2+ releases through local RyRs]. In addition, isoprenaline decreased the first latency, which displayed a typical 'U ' shaped voltage-dependence, showing the available acceleration and synchronization of beta-adrenergic stimulation on intracellular calcium release.

CONCLUSIONS

Isoprenaline enhances local Ca2+ release in cardiac myocytes. These results underscore the importance of regulation of beta-adrenergic stimulation on local intermolecular signals between LCCs and RyRs in heart cells.

Correction of Defective Interdomain Interaction Within Ryanodine Receptor by Antioxidant Is a New Therapeutic Strategy Against Heart Failure

Background— Defective interdomain interaction within the ryanodine receptor (RyR2) seems to play a key role in the pathogenesis of heart failure, as shown in recent studies. In the present study we investigated the effect of oxidative stress on the interdomain interaction, its outcome in the cardiac function in heart failure, and the possibility of preventing the problem with antioxidants.

Methods and Results— Sarcoplasmic reticulum (SR) vesicles were isolated from dog left ventricular (LV) muscle (normal or rapid ventricular pacing for 4 weeks with or without the antioxidant edaravone). In the edaravone-treated paced dogs (EV+), but not in the untreated paced dogs (EV−), normal cardiac function was restored almost completely. In the SR vesicles isolated from the EV−, oxidative stress of the RyR2 (reduction in the number of free thiols) was severe, but it was negligible in EV+. The oxidative stress of the RyR2 destabilized interdomain interactions within the RyR2 (EV−), but its effect was reversed in EV+. Abnormal Ca 2+ leak through the RyR2 was found in EV− but not in EV+. The amount of the RyR2-bound FKBP12.6 was less in EV− than in normal dogs, whereas it was restored almost to a normal amount in EV+. The NO donor 3-morpholinosydnonimine (SIN-1) reproduced, in normal SR, several abnormal features seen in failing SR, such as defective interdomain interaction and abnormal Ca 2+ leak. Both cell shortening and Ca 2+ transients were impaired by SIN-1 in isolated normal myocytes, mimicking the pathophysiological conditions in failing myocytes. Incubation of failing myocytes with edaravone restored the normal properties.

Conclusions— During the development of heart failure, edaravone ameliorated the defective interdomain interaction of the RyR2. This prevented Ca 2+ leak and LV remodeling, leading to an improvement of cardiac function and an attenuation of LV remodeling.

Nitric oxide induces [Ca2+]i oscillations in pituitary GH3 cells: involvement of IDR and ERG K+ currents

The role of nitric oxide (NO) in the occurrence of intracellular Ca2+ concentration ([Ca2+]i) oscillations in pituitary GH3 cells was evaluated by studying the effect of increasing or decreasing endogenous NO synthesis with L-arginine and nitro-L-arginine methyl ester (L-NAME), respectively. When NO synthesis was blocked with L-NAME (1 mM) [Ca2+]i, oscillations disappeared in 68% of spontaneously active cells, whereas 41% of the quiescent cells showed [Ca2+]i oscillations in response to the NO synthase (NOS) substrate L-arginine (10 mM). This effect was reproduced by the NO donors NOC-18 and S-nitroso-N-acetylpenicillamine (SNAP). NOC-18 was ineffective in the presence of the L-type voltage-dependent Ca2+ channels (VDCC) blocker nimodipine (1 microM) or in Ca2+-free medium. Conversely, its effect was preserved when Ca2+ release from intracellular Ca2+ stores was inhibited either with the ryanodine-receptor blocker ryanodine (500 microM) or with the inositol 1,4,5-trisphosphate receptor blocker xestospongin C (3 microM). These results suggest that NO induces the appearance of [Ca2+]i oscillations by determining Ca2+ influx. Patch-clamp experiments excluded that NO acted directly on VDCC but suggested that NO determined membrane depolarization because of the inhibition of voltage-gated K+ channels. NOC-18 and SNAP caused a decrease in the amplitude of slow-inactivating (IDR) and ether-à-go-go-related gene (ERG) hyperpolarization-evoked, deactivating K+ currents. Similar results were obtained when GH3 cells were treated with L-arginine. The present study suggests that in GH3 cells, endogenous NO plays a permissive role for the occurrence of spontaneous [Ca2+]i oscillations through an inhibitory effect on IDR and on IERG.

2APB- and JTV519(K201)-sensitive micro Ca2+ waves in arrhythmogenic Purkinje cells that survive in infarcted canine heart

OBJECTIVES/BACKGROUND

Studies from several laboratories have implicated intracellular Ca(2+) dynamics in the modulation of electrical activity. We have reported that abnormal Ca(2+) wave activity is the underlying cause of afterdepolarization-induced electrical activity in subendocardial Purkinje cells that survive in the 48-hour infarcted canine heart. These cells form the focus of arrhythmias at this time postcoronary artery occlusion.

METHODS

We studied the effects of agonists and antagonists on the abnormal Ca(2+) release activity of Purkinje cell aggregates dispersed from the subendocardium 48 hours postcoronary artery occlusion (IZPCs). Studies were completed using epifluorescent microscopy of Fluo-3 loaded Purkinje cells.

RESULTS

Similar to our previous report, highly frequent traveling micro Ca(2+) transients (muCaiTs) and cell-wide Ca(2+) waves were seen in IZPCs in the absence of any drug. Isoproterenol (ISO) increased muCaiTs and cell-wide Ca(2+) waves in Purkinje cells dispersed from the normal heart (NZPCs). In IZPCs, ISO increased cell-wide wave frequency but had no effect on the already highly frequent micro Ca(2+) wave transient activity, suggesting that ISO lowers the threshold of cell-wide generators responding to micro Ca(2+) transients. Drugs that block inward sodium or calcium currents (verapamil, tetrodotoxin) had no effect on Ca(2+) activity in Purkinje cells. Antagonists of intracellular Ca(2+) release channels [ryanodine, JTV519(K201)] greatly suppressed spontaneous Ca(2+) release events in IZPCs. 2APB, an agent that blocks IP(3) receptors, greatly reduced the frequency of Ca(2+) events in IZPCs.

CONCLUSIONS

In arrhythmogenic Purkinje cells that survive in the infarcted heart, agents that block or inhibit intracellular Ca(2+) release channel activity reduced Ca(2+) waves and could be antiarrhythmic.

Nitric oxide control of cardiac function: is neuronal nitric oxide synthase a key component?

Nitric oxide (NO) has been shown to regulate cardiac function, both in physiological conditions and in disease states. However, several aspects of NO signalling in the myocardium remain poorly understood. It is becoming increasingly apparent that the disparate functions ascribed to NO result from its generation by different isoforms of the NO synthase (NOS) enzyme, the varying subcellular localization and regulation of NOS isoforms and their effector proteins. Some apparently contrasting findings may have arisen from the use of non-isoform-specific inhibitors of NOS, and from the assumption that NO donors may be able to mimic the actions of endogenously produced NO. In recent years an at least partial explanation for some of the disagreements, although by no means all, may be found from studies that have focused on the role of the neuronal NOS (nNOS) isoform. These data have shown a key role for nNOS in the control of basal and adrenergically stimulated cardiac contractility and in the autonomic control of heart rate. Whether or not the role of nNOS carries implications for cardiovascular disease remains an intriguing possibility requiring future study.

A nitric oxide‐releasing derivative of enalapril, NCX 899, prevents progressive cardiac dysfunction and remodeling in hamsters with heart failure

Nitric oxide (NO) production is known to be impaired in heart failure. A new compound (NCX 899), a NO-releasing derivative of enalapril was characterized, and its actions were evaluated in Bio 14.6 cardiomyopathic (CM) hamsters with heart failure. The hamsters were randomized to oral treatment for 4 weeks with vehicle (n=11), NCX 899 (NCX, 25 mg/kg, n=10), or enalapril (25 mg/kg, n=10). In the vehicle group, fractional shortening by echocardiography decreased (-23.6+/-2.0%) and LV end-diastolic dimension) increased (+10.9+/-1.0%), whereas fractional shortening increased (+17.5+/-4.4%) in NCX and was unchanged in the enalapril group (both P<0.01 vs. vehicle). End-diastolic dimension decreased only in NCX. LV contractility (LVdP/dt max and Emax) was significantly greater in NCX than in enalapril or vehicle, while relaxation (Tau) was shortened in both NCX and enalapril vs. vehicle. ACE activity was inhibited equally by NCX and enalapril in the CM hamster, and plasma nitrate levels were increased only in NCX (P<0.05 vs. enalapril and vehicle). In aortic strips endothelium-independent relaxation occurred only with NCX. The superior effects of NO-releasing enalapril (NCX) vs. enalapril alone to enhance vascular effects, increase LV contractility and prevent unfavorable remodeling and are consistent with vascular delivery of exogenous NO. NCX 899 may hold promise for the future treatment of heart failure.

Gender differences in sarcoplasmic reticulum calcium loading after isoproterenol

Males exhibit enhanced myocardial ischemia-reperfusion injury versus females under hypercontractile conditions associated with increased sarcoplasmic reticulum (SR) Ca2+. We therefore examined whether there were gender differences in SR Ca2+. We used NMR Ca2+ indicator 1,2-bis(2-amino-5,6-difluorophenoxy)-ethane-N,N,N',N'-tetraacetic acid to measure SR Ca2+ in perfused rabbit hearts. Isoproterenol increased SR Ca2+ in males from a baseline of 1.13 +/- 0.07 to 1.52 +/- 0.24 mM (P < 0.05). Female hearts had basal SR Ca2+ that was not significantly different from males (1.04 +/- 0.03 mM), and addition of isoproterenol to females resulted in a time-averaged SR Ca2+ (0.97 +/- 0.07 mM) that was significantly less than in males. To confirm this difference, we measured caffeine-induced release of SR Ca2+ with fura-2 in isolated ventricular myocytes. Ca2+ release after caffeine in untreated male myocytes was 377 +/- 41 nM and increased to 650 +/- 55 nM in isoproterenol-treated myocytes (P < 0.05). Ca2+ release after caffeine addition in untreated females was 376 +/- 27 nM and increased to 503 +/- 49 nM with isoproterenol, significantly less than in male myocytes treated with isoproterenol (P < 0.05). Treatment of female myocytes with NG-nitro-l-arginine methyl ester, an inhibitor of nitric oxide synthase (NOS), resulted in higher SR Ca2+ release than that measured in females treated only with isoproterenol and was not significantly different from that measured in males with isoproterenol. Female myocytes also have significantly higher levels of neuronal NOS. This gender difference in SR Ca2+ handling may contribute to reduced ischemia-reperfusion injury observed in females.

Nitric oxide and cardiac function: ten years after, and continuing

Nitric oxide (NO) is produced from virtually all cell types composing the myocardium and regulates cardiac function through both vascular-dependent and -independent effects. The former include regulation of coronary vessel tone, thrombogenicity, and proliferative and inflammatory properties as well as cellular cross-talk supporting angiogenesis. The latter comprise the direct effects of NO on several aspects of cardiomyocyte contractility, from the fine regulation of excitation-contraction coupling to modulation of (presynaptic and postsynaptic) autonomic signaling and mitochondrial respiration. This multifaceted involvement of NO in cardiac physiology is supported by a tight molecular regulation of the three NO synthases, from cellular spatial confinement to posttranslational allosteric modulation by specific interacting proteins, acting in concert to restrict the influence of NO to a particular intracellular target in a stimulus-specific manner. Loss of this specificity, such as produced on excessive NO delivery from inflammatory cells (or cytokine-stimulated cardiomyocytes themselves), may result in profound cellular disturbances leading to heart failure. Future therapeutic manipulations of cardiac NO synthesis will necessarily draw on additional characterization of the cellular and molecular determinants for the net effect of this versatile radical on the cardiomyocyte biology.

Dissociation of cGMP accumulation and relaxation in myometrial smooth muscle: effects of S-nitroso-N-acetylpenicillamine and 3-morpholinosyndonimine

In guinea pig, primate and man, nitric oxide (NO)-induced regulation of myometrial smooth muscle contraction is distinct from other smooth muscles because cyclic guanosine 3',5'-cyclic monophosphate (cGMP) accumulation is neither necessary nor sufficient to relax the tissue. To further our understanding of the mechanism of action of NO in myometrium, we employed the NO donors, S-nitroso-N-acetylpenicillamine (SNAP), and 3-morpholinosyndonimine (SIN-1) proposed to relax airway smooth muscle by disparate mechanisms involving elevation in intracellular calcium ([Ca(2+)](i)) or cGMP accumulation, respectively. Treatment of guinea pig myometrial smooth muscle with either NO donor at concentrations thought to produce maximal relaxation of smooth muscles resulted in significant elevations in cGMP that were accompanied by phosphorylation of the cGMP-dependent protein kinase substrate vasodilator-stimulated phosphoprotein (VASP), shown here for the first time to be present and phosphorylated in myometrium. Stimulation of myometrial strips with oxytocin (OT, 1 microM) produced an immediate increase in contractile force that persisted in the continued presence of the agonist. Addition of SNAP (100 microM) in the presence of OT relaxed the tissue completely as might be expected of an NO donor. SIN-1 failed to relax the myometrium at any concentration tested up to 300 microM. In Fura-2 loaded myometrial cells prepared from guinea pig, addition of SNAP (100 microM) in the absence of other agonists caused a significant, reproducible elevation of intracellular calcium while SIN-1 employed under the same conditions did not. Our data further support the notion that NO action in myometrium is distinct from that in other smooth muscles and underscores the possibility that discrete regional changes in [Ca(2+)](i), rather than cGMP, signal NO-induced relaxation of the muscle.

Endothelial NO formation does not control myocardial O2 consumption in mouse heart

To test whether endothelium-derived nitric oxide (NO) regulates mitochondrial respiration, NO was pharmacologically modulated in isolated mouse hearts, which were perfused at constant flow to sensitively detect small changes in myocardial O2 consumption (MVO2). Stimulation of NO formation by 10 microM bradykinin (BK) increased coronary venous nitrite release fivefold to 58 +/- 33 nM (n = 17). Vasodilatation by BK, adenosine (1 microM), or papaverine (10 microM) decreased perfusion pressure, left ventricular developed pressure (LVDP), and MVO2. In the presence of adenosine-induced vasodilatation, stimulation of endothelial NO synthesis by BK had no effect on LVDP and MVO2. Also, inhibition of NO formation by NG-monomethyl-l-arginine (l-NMMA, 100 microM) did not significantly alter LVDP and MVO2. Similarly, intracoronary infusion of authentic NO <or=2 microM did not influence LVDP or MVO2 (-1 +/- 1%). Only when NO was >2 microM were contractile dysfunction and MVO2 reduction observed. Because BK-induced stimulation of endothelial NO formation and basal NO are not sufficient to impair MVO2 in the saline-perfused mouse heart, a tonic control of the respiratory chain by endothelial NO is difficult to conceive.

Modulation of cardiac contraction, relaxation and rate by the endothelial nitric oxide synthase (eNOS): lessons from genetically modified mice

The modulatory role of endothelial nitric oxide synthase (eNOS) on heart contraction, relaxation and rate is examined in light of recent studies using genetic deletion or overexpression in mice under specific conditions. Unstressed eNOS-/- hearts in basal conditions exhibit a normal inotropic and lusitropic function, with either decreased or unchanged heart rate. Under stimulation with catecholamines, eNOS-/- mice predominantly show a potentiation in their beta-adrenergic inotropic and lusitropic responsiveness. A similar phenotype is observed in beta 3-adrenoceptor deficient mice, pointing to a key role of this receptor subtype for eNOS coupling. The effect of eNOS on the muscarinic cholinergic modulation of cardiac function probably operates in conjunction with other NO-independent mechanisms, the persistence of which may explain the apparent dispensability of this isoform for the effect of acetylcholine in some eNOS-/- mouse strains. eNOS-/- hearts submitted to short term ischaemia-reperfusion exhibit variable alterations in systolic and diastolic function and infarct size, while those submitted to myocardial infarction present a worsened ventricular remodelling, increased 1 month mortality and loss of benefit from ACE inhibitor or angiotensin II type I receptor antagonist therapy. Although non-conditional eNOS gene deletion may engender phenotypic adaptations (e.g. ventricular hypertrophy resulting from chronic hypertension, or upregulation of the other NOS isoforms) potentially confounding the interpretation of comparative studies, the use of eNOS-/- mice has undoubtedly advanced (and will probably continue to improve) our understanding of the complex role of eNOS (in conjunction with the other NOSs) in the regulation of cardiac function. The challenge is now to confirm the emerging paradigms in human cardiac physiology and hopefully translate them into therapy.

Expression of inducible nitric oxide synthase depresses beta-adrenergic-stimulated calcium release from the sarcoplasmic reticulum in intact ventricular myocytes

BACKGROUND

beta-adrenergic hyporesponsiveness in many cardiomyopathies is linked to expression of inducible nitric oxide synthase (iNOS) and increased production of NO. The purpose of this study was to examine whether iNOS expression alters the function of the sarcoplasmic reticulum (SR) Ca(2+) release channel (ryanodine receptor, RyR) during beta-adrenergic stimulation.

METHODS AND RESULTS

Expression of iNOS was induced by lipopolysaccharide (LPS) injection (10 mg/kg) 6 hours before rat myocyte isolation. Confocal microscopy (fluo-3) was used to measure Ca(2+) spark frequency (CaSpF, reflecting resting RyR openings) and Ca(2+) transients. CaSpF was greatly increased by the adenylate cyclase activator forskolin (100 nmol/L) in normal myocytes (iNOS not expressed), but this effect was suppressed (by 77%) in LPS myocytes (iNOS expressed). When NO production by iNOS was inhibited by aminoguanidine (1 mmol/L), there was a further increase in the forskolin-induced CaSpF in LPS myocytes (to levels similar to the forskolin-stimulated CaSpF in normal myocytes). This effect was also seen in myocytes isolated from a failing human heart. There was no effect of aminoguanidine on forskolin-stimulated CaSpF in normal myocytes. ODQ (10 micromol/L), an inhibitor of NO stimulation of guanylate cyclase, did not restore the forskolin-induced rise in CaSpF in LPS myocytes. Aminoguanidine also increased twitch Ca(2+) transient amplitude in LPS myocytes after forskolin application (independent of changes in SR Ca(2+) load).

CONCLUSIONS

iNOS/NO depresses beta-adrenergic-stimulated RyR function through a cGMP-independent pathway (eg, NO- and/or peroxynitrite-dependent redox modification). This mechanism limits beta-adrenergic responsiveness and may be an important signaling pathway in cardiomyopathies, including human heart failure.