Factors released from endocardium of the ferret and pig modulate myocardial contraction

β3-Adrenergic receptor overexpression in cardiomyocytes preconditions mitochondria to withstand ischemia-reperfusion injury

β3-Adrenergic receptor (β3AR) agonists have been shown to protect against ischemia-reperfusion injury (IRI). Since β3ARs are present both in cardiomyocytes and in endothelial cells, the cellular compartment responsible for this protection has remained unknown. Using transgenic mice constitutively expressing the human β3AR (hβ3AR) in cardiomyocytes or in the endothelium on a genetic background of null endogenous β3AR expression, we show that only cardiomyocyte expression protects against IRI (45 min ischemia followed by reperfusion over 24 h). Infarct size was also limited after ischemia-reperfusion in mice with cardiomyocyte hβ3AR overexpression on top of endogenous β3AR expression. hβ3AR overexpression in these mice reduced IRI-induced cardiac fibrosis and improved long-term left ventricular systolic function. Cardiomyocyte-specific β3AR overexpression resulted in a baseline remodeling of the mitochondrial network, characterized by upregulated mitochondrial biogenesis and a downregulation of mitochondrial quality control (mitophagy), resulting in elevated numbers of small mitochondria with a depressed capacity for the generation of reactive oxygen species but improved capacity for ATP generation. These processes precondition cardiomyocyte mitochondria to be more resistant to IRI. Upon reperfusion, hearts with hβ3AR overexpression display a restoration in the mitochondrial quality control and a rapid activation of antioxidant responses. Strong protection against IRI was also observed in mice infected with an adeno-associated virus (AAV) encoding hβ3AR under a cardiomyocyte-specific promoter. These results confirm the translational potential of increased cardiomyocyte β3AR expression, achieved either naturally through exercise or artificially through gene therapy approaches, to precondition the cardiomyocyte mitochondrial network to withstand future insults.

Increased nitric oxide availability worsens the cardiac performance during early re-perfusion period in adult rats

OBJECTIVES

Re-perfusion is the standard therapy for acute myocardial infarction, despite the associated pathologies that may contribute to irreversible myocardial injury. The present study aims to clarify the alterations in cardiac activities in response to experimental cardiac ischemic arrest followed by re-perfusion in isolated hearts perfused with nitric oxide (NO) donor, l-arginine, or NO inhibitor, Nω-Nitro-l-arginine methyl ester hydrochloride (l-NAME), to shed light on the possible role of NO in the re-perfusion process.

METHODS

Hearts isolated from adult Wistar rats were studied on Langendorff preparation under basal conditions and during 30 min re-perfusion following 30 min of total global ischemia. Rats were randomly divided into three groups; control and l-arginine or l-NAME infused heart groups. Cardiac tissue content of malondialdhyde, catalase and nitrite was also measured.

RESULTS

Compared to the control group, both l-arginine and l-NAME infused hearts showed increased basal chronotropy and myocardial flow rate. Following ischemia and during the whole period of re-perfusion, the three groups demonstrated significant deterioration in the inotropic activity and compromised myocardial flow rate. l-arginine infused hearts revealed depressed inotropy and chronotropy, weak systolic and diastolic functions with compromised myocardial flow at early 5 min of re-perfusion, yet with significantly higher myocardial flow rate by the end of re-perfusion.

CONCLUSIONS

Reducing NO availability by l-NAME revealed mild impact on the ischemia re-perfusion induced contractile dysfunction, whereas excess NO worsens cardiac performance at the early re-perfusion period.

Endocardial TRPC-6 Channels Act as Atrial Mechanosensors and Load-Dependent Modulators of Endocardial/Myocardial Cross-Talk

Mechanoelectrical feedback may increase arrhythmia susceptibility, but the molecular mechanisms are incompletely understood. This study showed that mechanical stretch altered the localization, protein levels, and function of the cation-selective transient receptor potential channel (TRPC)-6 in atrial endocardial cells in humans, pigs, and mice. In endocardial/myocardial cross-talk studies, addition of media from porcine atrial endocardium (AE) cells altered the calcium (Ca²⁺) transient characteristics of human-induced pluripotent stem cell-derived cardiomyocytes. These changes did not occur with media from stretched AE cells. Our data suggested that endocardial TRPC-6-dependent paracrine signaling may modulate myocardial Ca²⁺ homeostasis under basal conditions and protect against stretch-induced atrial arrhythmias.

Naturally Engineered Maturation of Cardiomyocytes

Ischemic heart disease remains one of the most prominent causes of mortalities worldwide with heart transplantation being the gold-standard treatment option. However, due to the major limitations associated with heart transplants, such as an inadequate supply and heart rejection, there remains a significant clinical need for a viable cardiac regenerative therapy to restore native myocardial function. Over the course of the previous several decades, researchers have made prominent advances in the field of cardiac regeneration with the creation of in vitro human pluripotent stem cell-derived cardiomyocyte tissue engineered constructs. However, these engineered constructs exhibit a functionally immature, disorganized, fetal-like phenotype that is not equivalent physiologically to native adult cardiac tissue. Due to this major limitation, many recent studies have investigated approaches to improve pluripotent stem cell-derived cardiomyocyte maturation to close this large functionality gap between engineered and native cardiac tissue. This review integrates the natural developmental mechanisms of cardiomyocyte structural and functional maturation. The variety of ways researchers have attempted to improve cardiomyocyte maturation in vitro by mimicking natural development, known as natural engineering, is readily discussed. The main focus of this review involves the synergistic role of electrical and mechanical stimulation, extracellular matrix interactions, and non-cardiomyocyte interactions in facilitating cardiomyocyte maturation. Overall, even with these current natural engineering approaches, pluripotent stem cell-derived cardiomyocytes within three-dimensional engineered heart tissue still remain mostly within the early to late fetal stages of cardiomyocyte maturity. Therefore, although the end goal is to achieve adult phenotypic maturity, more emphasis must be placed on elucidating how the in vivo fetal microenvironment drives cardiomyocyte maturation. This information can then be utilized to develop natural engineering approaches that can emulate this fetal microenvironment and thus make prominent progress in pluripotent stem cell-derived maturity toward a more clinically relevant model for cardiac regeneration.

Simvastatin Promotes Cardiac Myocyte Relaxation in Association with Phosphorylation of Troponin I

The number of people taking statins is set to increase across the globe due to recent changes in prescription guidelines. For example, half the US population over 40 is now eligible for these drugs, whether they have high serum cholesterol or not. With such development in policy comes a stronger need for understanding statins’ myriad of effects. Surprisingly little is known about possible direct actions of statins on cardiac myocytes, although claims of a direct myocardial toxicity have been made. Here, we determine the impact of simvastatin administration (40 mg/kg/day) for 2 weeks in normocholesterolemic rats on cardiac myocyte contractile function and identify an underlying mechanism. Under basal conditions, statin treatment increased the time to half (t_0.5) relaxation without any effect on the magnitude of shortening, or the magnitude/kinetics of the [Ca²⁺]_i transient. Enhanced myocyte lusitropy could be explained by a corresponding increase in phosphorylation of troponin I (TnI) at Ser^23,24. Statin treatment increased expression of eNOS and Ser¹¹⁷⁷ phosphorylated eNOS, decreased expression of the NOS-inhibitory proteins caveolins 1 and 3, and increased (P = 0.06) NO metabolites, consistent with enhanced NO production. It is well-established that NO stimulates protein kinase G, one of the effectors of TnI phosphorylation at Ser^23,24. Trends for parallel changes in phospho-TnI, phospho-eNOS and caveolin 1 expression were seen in atrial muscle from patients taking statins. Our data are consistent with a mechanism whereby chronic statin treatment enhances TnI phosphorylation and myocyte lusitropy through increased NO bioavailability. We see no evidence of impaired function with statin treatment; the changes we document at the level of the cardiac myocyte should facilitate diastolic filling and cardiac performance.

Spatial phenotyping of the endocardial endothelium as a function of intracardiac hemodynamic shear stress

Despite substantial evidence for the central role of hemodynamic shear stress in the functional integrity of vascular endothelial cells, hemodynamic and molecular regulation of the endocardial endothelium lining the heart chambers remains understudied. We propose that regional differences in intracardiac hemodynamics influence differential endocardial gene expression leading to phenotypic heterogeneity of this cell layer. Measurement of intracardiac hemodynamics was performed using 4-dimensional flow MRI in healthy humans (n=8) and pigs (n=5). Local wall shear stress (WSS) and oscillatory shear indices (OSI) were calculated in three distinct regions of the LV - base, mid-ventricle (midV), and apex. In both the humans and pigs, WSS values were significantly lower in the apex and midV relative to the base. Additionally, both the apex and midV had greater oscillatory shear indices (OSI) than the base. To investigate regional phenotype, endocardial endothelial cells (EEC) were isolated from an additional 8 pigs and RNA sequencing was performed. A false discovery rate of 0.10 identified 1051 differentially expressed genes between the base and apex, and 321 between base and midV. Pathway analyses revealed apical upregulation of genes associated with translation initiation. Furthermore, tissue factor pathway inhibitor (TFPI; mean 50-fold) and prostacyclin synthase (PTGIS; 5-fold), genes prominently associated with antithrombotic protection, were consistently upregulated in LV apex. These spatio-temporal WSS values in defined regions of the left ventricle link local hemodynamics to regional heterogeneity in endocardial gene expression.

Pulling on my heartstrings: mechanotransduction in cardiac development and function

PURPOSE OF REVIEW

Endothelial cells line the surface of the cardiovascular system and display a large degree of heterogeneity due to developmental origin and location. Despite this heterogeneity, all endothelial cells are exposed to wall shear stress (WSS) imparted by the frictional force of flowing blood, which plays an important role in determining the endothelial cell phenotype. Although the effects of WSS have been greatly studied in vascular endothelial cells, less is known about the role of WSS in regulating cardiac function and cardiac endothelial cells.

RECENT FINDINGS

Recent advances in genetic and imaging technologies have enabled a more thorough investigation of cardiac hemodynamics. Using developmental models, shear stress sensing by endocardial endothelial cells has been shown to play an integral role in proper cardiac development including morphogenesis and formation of the conduction system. In the adult, less is known about hemodynamics and endocardial endothelial cells, but a clear role for WSS in the development of coronary and valvular disease is increasingly appreciated.

SUMMARY

Future research will further elucidate a role for WSS in the developing and adult heart, and understanding this dynamic relationship may represent a potential therapeutic target for the treatment of cardiomyopathies.

The concomitant coronary vasodilator and positive inotropic actions of the nitroxyl donor Angeli's salt in the intact rat heart: contribution of soluble guanylyl cyclase-dependent and -independent mechanisms

BACKGROUND AND PURPOSE

The NO redox sibling nitroxyl (HNO) elicits soluble guanylyl cyclase (sGC)-dependent vasodilatation. HNO has high reactivity with thiols, which is attributed with HNO-enhanced left ventricular (LV) function. Here, we tested the hypothesis that the concomitant vasodilatation and inotropic actions induced by a HNO donor, Angeli's salt (sodium trioxodinitrate), were sGC-dependent and sGC-independent respectively.

EXPERIMENTAL APPROACH

Haemodynamic responses to Angeli's salt (10 pmol-10 μmol), alone and in the presence of scavengers of HNO (L-cysteine, 4 mM) or of NO [hydroxocobalamin (HXC), 100 μM] or a selective inhibitor of sGC [1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), 10 μM], a CGRP receptor antagonist (CGRP8-37 , 0.1 μM) or a blocker of voltage-dependent potassium channels [4-aminopyridine (4-AP), 1 mM] were determined in isolated hearts from male rats.

KEY RESULTS

Angeli's salt elicited concomitant, dose-dependent increases in coronary flow and LV systolic and diastolic function. Both L-cysteine and ODQ shifted (but did not abolish) the dose-response curve of each of these effects to the right, implying contributions from HNO and sGC in both the vasodilator and inotropic actions. In contrast, neither HXC, CGRP8-37 nor 4-AP affected these actions.

CONCLUSIONS AND IMPLICATIONS

Both vasodilator and inotropic actions of the HNO donor Angeli's salt were mediated in part by sGC-dependent mechanisms, representing the first evidence that sGC contributes to the inotropic and lusitropic action of HNO in the intact heart. Thus, HNO acutely enhances LV contraction and relaxation, while concomitantly unloading the heart, potentially beneficial actions in failing hearts.

Endocardial endothelium is a key determinant of force-frequency relationship in rat ventricular myocardium

We tested the hypothesis that removing endocardial endothelium (EE) negatively impacts the force-frequency relationship (FFR) of ventricular myocardium and dissected the signaling that underlies this phenomenon. EE of rat trabeculae was selectively damaged by brief (<1 s) exposure to 0.1% Triton X-100. Force, intracellular Ca(2+) transient (iCa(2+)), and activity of protein kinase A (PKA) and protein kinase C (PKC) were determined. In control muscles, force and iCa(2+) increased as the stimulation frequency increased in steps of 0.5 Hz up to 3.0 Hz. However, EE-denuded (EED) muscles exhibited a markedly blunted FFR. Neither isoproterenol (ISO; 0.1-5 nmol/l) nor endothelin-1 (ET-1; 10-100 nmol/l) alone restored the slope of FFR in EED muscles. Intriguingly, however, a positive FFR was restored in EED preparations by combining low concentrations of ISO (0.1 nmol/l) and ET-1 (20 nmol/l). In intact muscles, PKA and PKC activity increased proportionally with the increase in frequency. This effect was completely lost in EED muscles. Again, combining ISO and ET-1 fully restored the frequency-dependent rise in PKA and PKC activity in EED muscles. In conclusion, selective damage of EE leads to significantly blunted FFR. A combination of low concentrations of ISO and ET-1 successfully restores FFR in EED muscles. The interdependence of ISO and ET-1 in this process indicates cross-talk between the β1-PKA and ET-1-PKC pathways for a normal (positive) FFR. The results also imply that dysfunction of EE and/or EE-myocyte coupling may contribute to flat (or even negative) FFR in heart failure.

Modulation of myocardial contraction by endocardial and coronary vascular endothelium

Endothelial cells in the heart, both endocardial endothelium and coronary vascular endothelium, influence myocardial contraction in isolated tissue and pump function in intact hearts by releasing diffusible agents that affect subjacent myocardium. Endocardial endothelium releases both nitric oxide (NO) and an unidentified "contraction-prolonging substance" ("endocardin") that respectively decrease and increase the duration of twitch contraction, probably by altering myofibrillar calcium sensitivity. These agents modulate the duration of ejection and the timing of relaxation, but without significantly altering early systolic behavior. Coronary vascular endothelium also releases NO, with similar effects on contraction, and in addition probably releases several other agents. Current work is aimed at identifying all of the agents involved in these novel endothelial influences and studying their potential physiologic and pathophysiologic roles in cardiac contractile and other functions.

Complex innervation patterns of the conus arteriosus in the heart of the longnose gar, Lepisosteus osseus

Anatomical and functional studies of the autonomic innervation in the conus arteriosus of the garfishes are lacking. This study reveals that the conus arteriosus of the longnose gar is primarily myocardial in nature, but additionally, large numbers of smooth muscle cells are present in the subendocardium. A well-developed system of adrenergic, cholinergic, substance P (SP) and neuronal nitric oxide synthase (nNOS) positive nerve terminals are found in the wall of the conus arteriosus. Coronary blood vessels running in the adventitia receive a rich supply of nNOS positive nerve fibers, thus suggesting their importance in the nitrergic control of blood flow in the conus arteriosus. The present data show that the patterns of autonomic innervation of the garfish conus arteriosus are more complex than previously appreciated.

L-lysine uptake in giant vesicles from cardiac ventricular sarcolemma: two components of cationic amino acid transport

Cationic L-amino acids enter cardiac-muscle cells through carrier-mediated transport. To study this process in detail, L-[(14)C]lysine uptake experiments were conducted within a 10(3)-fold range of L-lysine concentrations in giant sarcolemmal vesicles prepared from rat cardiac ventricles. Vesicles had a surface-to-volume ratio comparable with that of an epithelial cell, thus representing a suitable system for initial uptake rate studies. Two Na(+)-independent, N-ethylmaleimide-sensitive uptake components were found, one with high apparent affinity (K(m)=222+/-71 microM) and low transport capacity (V(max)=121+/-36 pmol/min per mg of vesicle protein) and the other with low apparent affinity (K(m)=16+/-4 mM) and high capacity (V(max)=4.0+/-0.4 nmol/min per mg of vesicle protein). L-Lysine uptake mediated by both components was stimulated by the presence of intravesicular L-lysine as well as by valinomycin-induced membrane hyperpolarization. Altogether, this behaviour is consistent with the functional properties of the CAT-1 and CAT-2A members of the system y(+) family of cationic amino acid transporters. Furthermore, mRNA transcripts for these two carrier proteins were identified in freshly isolated rat cardiac myocytes, the amount of CAT-1 mRNA, relative to beta-actin, being 33-fold larger than that of CAT-2A. These two transporters appear to function simultaneously as a homoeostatic device that supplies cardiac-muscle cells with cationic amino acids under a variety of metabolic conditions. Analysis of two carriers acting in parallel with such an array of kinetic parameters shows significant activity of the low-affinity component even at amino acid plasma levels far below its K(m).

Immortalization and Characterization of Porcine Ventricular Endocardial Endothelial Cells

Endocardial endothelial cells (EECs), which form the inner lining of the cavities of the heart, are a distinct cell population whose dysfunction can be critical in pathological conditions of heart. Insights into the role and organization of these cells in pathological states of the heart are limited mainly due to a dearth of experimental models. To date no endocardial endothelial cell line is available. The authors attempted to immortalize porcine ventricular EECs by transfecting the cells with human telomerase reverse transcriptase (hTERT). EECs immortalized by ectopic expression of hTERT exhibit phenotypic and functional characteristics similar to primary EECs. The EE cell line could be useful for the study of mechanisms involved in the interaction of EECs with the underlying myocardium and cardiac interstitium and as useful tools in understanding their role in diseased states of heart.

Effects of the NO donor sodium nitroprusside on oxygen consumption and energetics in rabbit myocardium

Nitric oxide (NO) has influence on various cellular functions. Little is known of the influence of NO on myocardial energetics. In the present study oxygen consumption and mechanical parameters of isometrically contracting rabbit papillary muscles (1 Hz stimulation frequency) were investigated at varying interventions while maintaining physiological conditions (37°C; 2.5 mM Ca²⁺) to study the effects of NO on energetics. The NO donor sodium nitroprusside (SNP) showed a negative inotropic effect. SNP decreased the maximal force in normal rabbit muscle strips by 30%, the force time integral (FTI) by 40% and the relaxation time by 20%. In addition the oxygen consumption decreased by 60%, a notably disproportional decrease compared to the mechanical parameters. Consequently, the economy as a ratio of FTI and oxygen consumption is significantly increased by SNP. In contrast the negative inotropic effect due to a reduction in extracellular Calcium (Ca²⁺) from 2.5 to 1.25 mM reduced FTI and oxygen consumption proportionally by 40% and did not change economy. The effect of NO on force and oxygen consumption could be reproduced by the application of the cyclic guanosine monophosphate (cGMP) analogue 8-bromo-cGMP. In summary, NO increased the economy of isometrically contracting papillary muscles. The improvement in contraction economy under NO seems to be mediated by cGMP as the secondary messenger and maybe due to alterations of the crossbridge cycle.

Effects of the beta3-adrenergic agonist BRL 37344 on endothelial nitric oxide synthase phosphorylation and force of contraction in human failing myocardium

BACKGROUND

In nonfailing myocardium, beta(3)-adrenergic signaling causes a decrease in contractility via endothelial nitric oxide synthase (eNOS) activation and nitric oxide (NO) release. This study investigates the hypothesis that beta(3)-adrenergic signaling undergoes alterations in failing myocardium.

METHODS

We compared eNOS- and beta(3)-adrenoceptor expression using Western blot analysis in human nonfailing myocardium versus failing myocardium. With the use of immunohistochemistry, we investigated the distribution of the beta(3)-adrenoceptor protein and eNOS translocation and phosphorylation under basal conditions. beta(3)-adrenergic, eNOS activation, and inotropy were measured in failing myocardium using BRL37344 (BRL, a beta(3)-adrenoceptor agonist).

RESULTS

beta(3)-adrenoceptor expression was increased in failing myocardium. Under basal conditions, Akt- and eNOS(Ser1177) phosphorylation were reduced in failing myocardium. During stimulation with BRL in failing myocardium, a further dephosphorylation of eNOS(Ser1177) and Akt was observed, whereas eNOS(Ser114) phosphorylation was increased. These results suggest a deactivation of eNOS via beta(3)-adrenergic stimulation. Nevertheless, BRL decreased contractility in failing myocardium, but this effect was not observed in the presence of the NO blocker L-NMA. In failing myocardium, the beta(3)-adrenoceptor was predominantly expressed in endothelial cells. In the cardiomyocytes, the beta(3)-adrenoceptor was mainly located at the intercalated disks.

CONCLUSION

In failing cardiomyocytes, beta(3)-adrenergic stimulation seems to deactivate rather than activate eNOS. At the same time, beta(3)-adrenergic stimulation induced a NO-dependent negative inotropic effect. Because beta(3)-adrenoceptors are expressed mainly in the endothelium in failing myocardium, our observations suggest a paracrine-negative inotropic effect via NO liberation from the cardiac endothelial cells.

An unexpected negative inotropic effect of prostaglandin F2alpha in the rat heart

Prostaglandin F(2alpha) (PGF(2alpha)) is produced during myocardial inflammation and many of the insults that trigger contractile dysfunction also activate prostaglandin synthesis and production. However, although PGF(2alpha) plays a significant role in the cardiac response to inflammation, the effect of this particular compound on the heart was largely studied at the cellular level and probably no due attention was paid to the effect of PGF(2alpha) on the whole heart contractility. Therefore, in the present study we have investigated the effect of PGF(2alpha) on isolated right ventricle of the rat heart. PGF(2alpha) (1nM-1microM) induced concentration-dependent decrease of the amplitude of contractions of the ventricular muscle. Real time RT-PCR has revealed that prostaglandin FP receptors are expressed in the rat myocardium and the level of expression was similar to those of creatine kinase and adenylate kinase, which are proteins abundantly present in the heart. An antagonist of FP receptors, PGF(2alpha) dimetilamide (10nM), abolished negative inotropic effect induced by PGF(2alpha). To examine the possibility that PGF(2alpha) could activate non-FP prostaglandin receptor, we have measured the level of expression of all known prostaglandin receptors in the rat heart. These experiments have shown that the order of expression of prostaglandin receptors in the rat heart is FP>>EP1=TP>EP4>EP3>DP=IP. Based on the obtained results we conclude that PGF(2alpha) induces negative inotropic effect on rat heart by activating FP prostaglandin receptors. This effect of PGF(2alpha) could contribute to cardiac dysfunction in conditions of systemic and myocardial inflammation.

Nitric oxide donors alter cardiomyocyte cytokine secretion and cardiac function

OBJECTS

The mechanisms by which nitric oxide produces beneficial/detrimental effects on physiologic function are unclear. In this study, we hypothesized that nitric oxide promotes cyclic guanosine monophosphate (cGMP) formation, which, in turn, promotes cardiomyocyte secretion of inflammatory cytokines as well as accumulation of intracellular Na+/Ca2+; these factors contribute to altered cardiac contractile function.

DESIGN

Laboratory study.

SETTING

Medical Center.

SUBJECTS

Adult Sprague Dawley rats weighing 325-350 g.

INTERVENTIONS

Cardiomyocytes were prepared by collagenase perfusion of rat hearts; cells were plated (5 x 10(4) cells/microtiter well) and challenged with either vehicle or nitric oxide donor (S-nitroso-N-acetyl-penicillamine [SNAP] or PAPA NONOATE, 3-[2-Hydroxy-2-nitroso-1-propythdrazinol]-1-propanamine], NOC-15 [PAPA-NO], 0.3 or 1.0 mM of each nitric oxide donor) in the presence/absence of methylene blue (10 microM/L to inhibit cGMP). After 3 hrs, supernatants were collected to measure nitrite/nitrate (nitric oxide), cytokines (tumor necrosis factor-alpha, interleukin-1beta, interleukin-6), and cGMP levels; cells were then loaded with a fluorescent indicator (Fura-2AM or sodium-binding benzofurzan isophthalate) to measure myocyte Ca2+ or Na+, respectively. Parallel experiments included the addition of nitric oxide donor (0.3 or 1.0 mM SNAP or PAPA-NO) to perfused hearts in presence or absence of the methylene blue to examine cGMP-mediated effects on myocardial contraction-relaxation, while other experiments determined a) potential lipopolysaccharide contamination of myocyte preparations; and b) whether a cGMP analogue recapitulated the effects of nitric oxide donors on cytokine secretion.

MEASUREMENTS AND MAIN RESULTS

Nitric oxide donors produced a dose-dependent increase in cGMP levels in myocyte supernatants as well as an increase in myocyte cytokine secretion, increased myocyte loading of Na+/Ca2+, and produced myocardial contractile dysfunction. Addition of the cGMP analog, 8-bromo-cGMP, recapitulated the effects of nitric oxide donors on myocyte cytokine secretion. Nitric oxide donor-related effects were ablated by pretreatment of myocytes or isolated hearts with methylene blue. Treatment of myocytes with recombinant bactericidal/permeability-increasing protein to scavenge lipopolysaccharide confirmed that cytokine responses to nitric oxide donors were not related to lipopolysaccharide contamination of myocyte preparations.

CONCLUSIONS

We suggest that nitric oxide synthesis in injury and disease promotes cGMP formation, which, in turn, modulates cardiac contraction/relaxation by a) altering cardiomyocyte secretion of inflammatory cytokines and b) altering myocyte handling of Na+/Ca2+.

Modulation of contractility by myocyte-derived arginase in normal and hypertrophied feline myocardium

L-Arginine, the sole substrate for the nitric oxide (NO) synthase (NOS) enzyme in producing NO, is also a substrate for arginase. We examined normal feline hearts and hearts with compensated left ventricular (LV) hypertrophy (LVH) produced by ascending aorta banding. Using Western blot analysis, we examined the abundance of arginase isozymes in crude homogenates and isolated cardiac myocytes obtained from the LVs of normal and LVH hearts. We examined the functional significance of myocyte arginase via measurement of shortening and intracellular calcium in isolated myocytes in the presence and absence of boronoethyl chloride (BEC), a specific pharmacological inhibitor of arginase. Both arginase I and II were detected in crude myocardial homogenates, but only arginase I was present in isolated cardiac myocytes. Arginase I was downregulated in LVH compared with normal. Inhibition of arginase with BEC reduced fractional shortening, maximal rate of shortening (+dL/dt) and relengthening (-dL/dt), and the peak of the free cytosolic calcium transient in normal myocytes but did not affect these parameters in LVH myocytes. These negative inotropic actions of arginase inhibition were associated with increases in cGMP generation. These studies indicate that only arginase I is present in cardiac myocytes where it tends to limit NO and cGMP production with the effect of supporting basal contractility. In experimental LVH induced by pressure overload, our studies demonstrate reduced arginase I expression and reduced functional significance, allowing greater arginine substrate availability for NO/cGMP signaling.

Cardiac neurobiology of nitric oxide synthases

Nitric oxide (NO) is a potent modulator of cardiac and vascular regulation. Its role in cardiac-autonomic neural signaling has received much attention over the last decade because of the ability of NO to alter cardiac sympathovagal balance to favor more anti-arrhythmic states. Complexity and controversy have arisen, however, because of the numerous sources of NO in the brain, peripheral nerves, and cardiomyocytes, all of which are potential regulators of cardiac excitability and calcium signaling. This review addresses the integrative role of NO as a relatively ubiquitous signaling molecule with respect to cardiac neurobiology. The present idea, that divergent NO-signaling pathways from multiple sources within the heart and nervous system converge to modulate cardiac excitability and impact on morbidity and mortality in health and disease, is discussed.

LACK OF INOTROPIC EFFECT OF NITRIC OXIDE ON THE RAT MYOCARDIUM

1. Nitric oxide (NO) is an important mediator of contractile function in the heart. However, isolated papillary muscle preparations appear to lack NO responsiveness in certain animal species. Although cat, guinea-pig and ferret models have been NO responsive, there have been mixed results in the rat papillary muscle. In null form, we tested three separate hypotheses in rat papillary muscle, specifically that the NO donor sodium nitroprusside (SNP) would not affect the contractility of: (i) the isolated papillary muscle; (ii) papillary muscle prestimulated with the beta-adrenoceptor agonist isoprenaline; and (iii) papillary muscle subjected to 15 min anoxia followed by 45 min reoxygenation. 2. Male Sprague-Dawley rats were used. The left ventricular papillary muscle was mounted and maintained at 30 degrees C and was stimulated at 10 b.p.m. Each experiment was performed in parallel with a control papillary muscle from the same animal. Papillary muscles were exposed to increasing concentrations of SNP (10(-9) to 10(-5) mol/L) either alone or following pretreatment with 10(-7) mol/L isoprenaline. Anoxia/reoxygenation was simulated by 15 min anoxia followed by 60 min reoxygenation in the presence or absence of 10(-7) mol/L SNP. 3. Both isometric and isotonic parameters were assessed. As expected, isoprenaline had a significant positive inotropic response. Similarly, contractility was impaired during anoxia and partially recovered during reoxygenation. Nitric oxide did not alter either isotonic or isometric parameters in the three experimental protocols. 4. The rat isolated papillary muscle has no measurable response to exogenous NO. The inotropic effects of beta-adrenoceptor stimulation and anoxia/reoxygenation are NO independent.

Both cGMP and peroxynitrite mediate chronic interleukin-6-induced negative inotropy in adult rat ventricular myocytes

We previously showed that chronic exposure to interleukin (IL)-6 decreases contractile and sarcoplasmic reticular (SR) function assessed by postrest potentiation (PRP) via a nitric oxide (NO)-dependent mechanism in adult rat ventricular myocytes (ARVM). Cyclic GMP (cGMP) has been associated with NO-associated negative inotropic effects of IL-6 during acute exposure; however, its role in chronic cardiac effects of IL-6 remains unclear. The present study examined the roles of cGMP and peroxynitrite (ONOO-) in chronic IL-6-induced negative inotropy in ARVM. After ARVM were exposed to IL-6 for 2-24 h, intracellular cGMP contents were time dependently increased; this was mimicked by a NO donor and abolished by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of soluble guanylyl cyclase (sGC), or Rp-8-Br-cGMP, an inhibitor of cGMP-dependent protein kinase G (PKG). Meanwhile, the IL-6-induced decrease in PRP at 2 h was blocked by ODQ or Rp-8-Br-cGMP. By contrast, ODQ or Rp-8-Br-cGMP only attenuated the inhibition of PRP induced by IL-6 after 24 h exposure. Furthermore, IL-6 time dependently increased superoxide anion production and ONOO- formation; the latter was abolished by 5,10,15,20-tetrakis-(4-sulphonatophenyl)-porphyrinato iron (III) (FeTPPS), an ONOO- decomposition catalyst. Interestingly, FeTPPS had no effect on the IL-6-elicited decrease in PRP at 2 h, but attenuated it after 24 h exposure. Moreover, inhibition of sGC/cGMP/PKG, but not ONOO- formation, abolished the IL-6-induced inhibition of kinetics of myocyte contraction during 24 h exposure. We conclude that while the sGC/cGMP/PKG pathway was the primary mechanism for chronic IL-6-induced negative inotropy at 2 h, both sGC/cGMP/PKG and ONOO-, at least in part, mediate the IL-6-induced inhibition of SR function after 24 h exposure.

Metabolic communication from cardiac myocytes to vascular endothelial cells

The endothelium releases substances that affect both vascular and cardiac myocytes. However, under conditions of augmented metabolic demands and cardiac work, signals from the cardiac myocytes may be critical for the endothelium to fulfill its secretory and regulatory function in the vascular bed. Therefore, we hypothesized that cardiac myocytes produce substances that alter the resting membrane potential of endothelial cells and thus vascular tone. Isolated rat cardiac myocytes were electrically stimulated at the rate of 0 and 400 beats/min (Po2 = 150 mmHg), and supernatants were collected from each group (Sup-0; control) and (Sup-400) and used within 6 mo. These supernatants were applied to human coronary endothelial cells that were subsequently analyzed by using the whole cell and cell-attached patch-clamp modes. Sup-0 had no effect on the whole cell current and the zero-current potential. The Sup-0 from myocytes treated with aprotinin, an inhibitor of kallikrein and serine protease, reduced whole cell current between −120 and −60 mV. Sup-400 depolarized endothelial cells from the resting membrane potential of −45 to −5 mV ( P < 0.05), increased the magnitude of an inward current, and activated an outward current. Moreover, Sup-400 cells assayed in cell-attached patches increased single channel amplitude and the probability of a channel being in the open state. These effects were reversed by the Sup-400 from aprotinin-treated cells. We conclude that under certain metabolic conditions, isolated cardiac myocytes produce and release vasoactive substances that alter the function of K+ channels in vascular endothelial cells. Thus cardiac myocytes seem to communicate metabolic information to the endothelium, which could potentially influence vascular tone.

Thyroid hormone stimulates myoglobin gene expression in rat cardiac muscle

T3 increases the heart activity, O2 consumption and the reactive O2 species production. Myoglobin (Mb) is highly expressed in the heart, where it facilitates O2 diffusion, mitochondrial respiration, and scavenges reactive O2 species. Here we investigate, by dose-response (0.3-100 microg/100 g BW, i.p., 5 days) and time-course studies (100 microg/100 g BW, i.v., from 0.5 to 24h), whether T3 affects the Mb mRNA and protein expression in atrium (A) and ventricle (V), by Northern and Western blot. We show that the Mb gene is controlled by T3 in A and V, as indicated by Mb mRNA and protein content decrease in thyroidectomized (Tx) rats, and restoration by T3 treatment. In the A, the different doses of T3 induced the Mb mRNA and protein recovery to the euthyroid levels; in the time-course study, this occurred only with the protein levels. In the V, T3 progressively increased the Mb mRNA above the euthyroid levels at a dose of 25 microg/100g BW; higher doses decreased it to the euthyroid levels. Mb protein increased only to the euthyroid levels at all T3 doses injected. The time-course study showed a progressive increase in the ventricular Mb mRNA and protein, which exceeded the euthyroid levels from 6 to 24h, and at 2 and 6 h of the T3 treatment, respectively. We conclude that heart Mb gene expression is influenced by thyroid status.

Molecular mechanisms in endothelial regulation of cardiac function

Endothelium is now recognized as a massive, regionally specific, multifunctional organ. Given its strategic anatomic location between the circulating blood components and the vascular smooth muscle or the cardiac muscle, it is a biologically significant interface whose dysfunction can be a critical factor in various pathological conditions. Two types of endothelial cells are recognized in the heart, the endocardial endothelial (EE) cells and the microvascular endothelial cells (MVE). Both produce common autacoids and share similar roles in signal transduction induced by neurotransmitters, hormones or mechanical stimuli. They are however two distinct cell populations with dissimilar embryological origin, cytoskeletal organization, receptor mediated functions and electrophysiological properties. Both the MVE and EE are modulators of cardiac performance. Myocardial contraction may be modulated by cardioactive agents such as nitric oxide, prostanoids, endothelin, natriuretic peptides, angiotensin II, kinins, reactive oxygen species and adenyl purines released from the cardiac endothelium. Two mechanisms have been proposed for the signal transduction from EE to the underlying myocytes: stimulus-secretion-contraction coupling and blood-heart barrier. Nitric oxide, bradykinin and myofilament desensitizing agent are probably important in short-term regulation of myocardial functions. Endothelin and Angiotensin II are probably involved in long-term regulation. Besides its sensory function and paracrine modulation of myocardial performance, EE as a blood-heart barrier could be of significance for the ionic homeostasis of the cardiac interstitium. In cardiac diseases, the damage to EE or MVE leading to failure of the endothelial cells to perform its regulatory and modulator functions may have serious consequences. A better understanding of the endothelial signaling pathways in cardiac physiology and pathophysiology may lead to the development of novel therapeutic strategies.

Inotropic effects of ETB receptor stimulation and their modulation by endocardial endothelium, NO, and prostaglandins

Endothelin (ET)-1 acts on ETA and ETB receptors. The latter include ETB1 (endothelial) and ETB2 (muscular) subtypes, which mediate opposite effects on vascular tone. This study investigated, in rabbit papillary muscles ( n = 84), the myocardial effects of ETB stimulation. ET-1 (10−9 M) was given in the absence or presence of BQ-123 (ETA antagonist). The effects of IRL-1620 (ETB1 agonist, 10−10–10−6 M) or sarafotoxin S6c (ETB agonist, 10−10–10−6 M) were evaluated in muscles with intact or damaged endocardial endothelium (EE); intact EE, in the presence of NG-nitro-l-arginine (l-NNA); and intact EE, in the presence of indomethacin (Indo). Sarafotoxin S6c effects were also studied in the presence of BQ-788 (ETB2 antagonist). ET-1 alone increased 64 ± 18% active tension (AT) but decreased it by 4 ± 2% in the presence of BQ-123. In muscles with intact EE, sarafotoxin S6c alone did not significantly alter myocardial performance. Sarafotoxin S6c (10−6 M) increased, however, AT by 120 ± 27% when EE was damaged and by 39 ± 8% or 23 ± 6% in the presence of l-NNA or Indo, respectively. In the presence of BQ-788, sarafotoxin S6c decreased AT (21 ± 3% at 10−6 M) in muscles with intact EE, an effect that was abolished when EE was damaged. IRL-1620 also decreased AT (22 ± 3% at 10−6 M) in muscles with intact EE, an effect that was abolished when EE was damaged or in the presence of l-NNA or Indo. In conclusion, the ETB-mediated negative inotropic effect is presumably due to ETB1 stimulation, requires an intact EE, and is mediated by NO and prostaglandins, whereas the ETB-mediated positive inotropic effect, observed when EE was damaged or NO and prostaglandins synthesis inhibited, is presumably due to ETB2 stimulation.

Nitric oxide's role in the heart: control of beating or breathing?

Beneficial actions of nitric oxide (NO) in failing myocardium have frequently been overshadowed by poorly documented negative inotropic effects mainly derived from in vitro cardiac preparations. NO's beneficial actions include control of myocardial energetics and improvement of left ventricular (LV) diastolic distensibility. In isolated cardiomyocytes, administration of NO increases their diastolic cell length consistent with a rightward shift of the passive length-tension relation. This shift is explained by cGMP-induced phosphorylation of troponin I, which prevents calcium-independent diastolic cross-bridge cycling and concomitant diastolic stiffening of the myocardium. Similar improvements in diastolic stiffness have been observed in isolated guinea pig hearts, in pacing-induced heart failure dogs, and in patients with dilated cardiomyopathy or aortic stenosis and have been shown to result in higher LV preload reserve and stroke work. NO also controls myocardial energetics through its effects on mitochondrial respiration, oxygen consumption, and substrate utilization. The effects of NO on diastolic LV performance appear to be synergistic with its effects on myocardial energetics through prevention of myocardial energy wastage induced by LV contraction against late-systolic reflected arterial pressure waves and through prevention of diastolic LV stiffening, which is essential for the maintenance of adequate subendocardial coronary perfusion. A drop in these concerted actions of NO on diastolic LV distensibility and on myocardial energetics could well be instrumental for the relentless deterioration of failing myocardium.

Inotropic effects on mammalian skeletal muscle change with contraction frequency

Over the last decade, we have attempted to determine if mammalian skeletal muscle's steady-level force development as established by mechanical and stimulation parameters can be increased or decreased by physiological signals. In these experiments, nitric oxide (NO), endothelin-1 (ET-1), adenosine (Ado), and beta-adrenergic agonists (beta) modified force production in the soleus and (or) the extensor digitorum longus (EDL) of the mouse. NO and beta increased the force produced by 0.5-s tetanic contractions at 0.6 contractions/min in both muscles. While EDL did not respond to either Ado or ET-1, the developed force of the soleus was amplified by Ado but attenuated by ET-1. Increased cAMP analogue concentrations amplified developed force in both muscles, but a cGMP analogue had no effect on either muscle. Following an increase in the contraction frequency of the soleus, the increased force in response to NO disappeared, as did the decreased force to ET-1. The increase in force due to a cAMP analogue disappeared during fatigue but reappeared quickly during recovery. Thus, steady-level developed force can be modified by a number of substances that can be released from locations in the body or muscle. The response to a given compound is determined by a complex interaction of metabolic and intracellular signals on the force-generating cascade.

Cardiac role of frog ANF: negative inotropism and binding sites in Rana esculenta

To elucidate the role of atrial natriuretic peptides (NPs) in the amphibian heart, the myotropic effects and the cardiac distribution of frog atrial natriuretic factor (fANF) have been studied in Rana esculenta. Spontaneously, beating in vitro isolated working heart preparations were treated with increased concentrations (10(-11)-10(-8) M) of fANF-(1-24). The peptide at 10(-9) and 10(-8) M significantly reduced heart rate (HR) and, on the electrically paced preparations, decreased cardiac output (CO), stroke volume (SV) and work. Such negative inotropism was abolished by pretreatment with the pertussis toxin or by blocking the particulate guanylate cyclase (GC) with anantin while it was independent both from the functional impairment of the endocardium-endothelium by Triton X-100 and the inhibition of the soluble guanylate cyclase by 1 H-(1,2,4,) oxadiazolo-(4,3-a) quinoxalin-1-one (ODQ). By autoradiography, two classes of high and low affinity NPs binding sites were detected in the ventricular endocardium and myocardium and in the bulbus arteriosus. The analysis of displacement binding data using the radioligand [125I]-rat atrial natriuretic peptide [125I-rANP-(1-28)], its cold counterpart and the fANF-(1-24) showed that in the ventricular myocardium, the low affinity NPs sites bound both the heterologous and the homologous ligands at a concentration close to that responsible for the negative inotropism and chronotropism.

Caveolin-1 null mice develop cardiac hypertrophy with hyperactivation of p42/44 MAP kinase in cardiac fibroblasts

Recently, development of a caveolin-1-deficient (Cav-1 null) mouse model has allowed the detailed analysis of caveolin-1's function in the context of a whole animal. Interestingly, we now report that the hearts of Cav-1 null mice are markedly abnormal, despite the fact that caveolin-1 is not expressed in cardiac myocytes. However, caveolin-1 is abundantly expressed in the nonmyocytic cells of the heart, i.e., cardiac fibroblasts and endothelia. Quantitative imaging studies of Cav-1 null hearts demonstrate a significantly enlarged right ventricular cavity and a thickened left ventricular wall with decreased systolic function. Histological analysis reveals myocyte hypertrophy with interstitial/perivascular fibrosis. Because caveolin-1 is thought to act as a negative regulator of the p42/44 MAP kinase cascade, we performed Western blot analysis with phospho-specific antibodies that only recognize activated ERK1/2. As predicted, the p42/44 MAP kinase cascade is hyperactivated in Cav-1 null heart tissue (i.e., interstitial fibrotic lesions) and isolated cardiac fibroblasts. In addition, endothelial and inducible nitric oxide synthase levels are dramatically upregulated. Thus loss of caveolin-1 expression drives p42/44 MAP kinase activation and cardiac hypertrophy.

Role of nitric oxide in the pathophysiology of heart failure

Nitric oxide (NO) plays critical roles in the regulation of integrated cardiac and vascular function and homeostasis. An understanding of the physiologic role and relative contribution of the three NO synthase isoforms (neuronal--NOS1, inducible--NOS2, and endothelial--NOS3) is imperative to comprehend derangements of the NO signaling pathway in the failing cardiovascular system. Several theories of NO and its regulation have developed as explanations for the divergent observations from studies in health and disease states. Here we review the physiologic and pathophysiologic influence of NO on cardiac function, in a framework that considers several theories of altered NO signaling in heart failure. We discuss the notion of spatial compartmentalization of NO signaling within the myocyte in an effort to reconcile many controversies about derangements in the influences of NO in the heart and vasculature.

Cardiac endothelial-myocardial signaling: its role in cardiac growth, contractile performance, and rhythmicity

Experimental work during the past 15 years has demonstrated that endothelial cells in the heart play an obligatory role in regulating and maintaining cardiac function, in particular, at the endocardium and in the myocardial capillaries where endothelial cells directly interact with adjacent cardiomyocytes. The emerging field of targeted gene manipulation has led to the contention that cardiac endothelial-cardiomyocytal interaction is a prerequisite for normal cardiac development and growth. Some of the molecular mechanisms and cellular signals governing this interaction, such as neuregulin, vascular endothelial growth factor, and angiopoietin, continue to maintain phenotype and survival of cardiomyocytes in the adult heart. Cardiac endothelial cells, like vascular endothelial cells, also express and release a variety of auto- and paracrine agents, such as nitric oxide, endothelin, prostaglandin I(2), and angiotensin II, which directly influence cardiac metabolism, growth, contractile performance, and rhythmicity of the adult heart. The synthesis, secretion, and, most importantly, the activities of these endothelium-derived substances in the heart are closely linked, interrelated, and interactive. It may therefore be simplistic to try and define their properties independently from one another. Moreover, in relation specifically to the endocardial endothelium, an active transendothelial physicochemical gradient for various ions, or blood-heart barrier, has been demonstrated. Linkage of this blood-heart barrier to the various other endothelium-mediated signaling pathways or to the putative vascular endothelium-derived hyperpolarizing factors remains to be determined. At the early stages of cardiac failure, all major cardiovascular risk factors may cause cardiac endothelial activation as an adaptive response often followed by cardiac endothelial dysfunction. Because of the interdependency of all endothelial signaling pathways, activation or disturbance of any will necessarily affect the others leading to a disturbance of their normal balance, leading to further progression of cardiac failure.

Myocardial contractile effects of nitric oxide

Recent experimental and clinical research solved some of the controversies surrounding the myocardial contractile effects of NO. These controversies were: (1) does NO exert a contractile effect at baseline? (2) is NO a positive or a negative inotrope? (3) Are the contractile effects of NO similar when NO is derived from NO-donors or from the different isoforms of NO synthases (NOS)? (4) Does NO exert the same effects in hypertrophied, failing or ischemic myocardium? Transgenic mice with cardioselective overexpression of NOS revealed NO to produce a small reduction in basal developed LV pressure and a LV relaxation-hastening effect mainly through myofilamentary desensitization. Similar findings had previously been reported during intracoronary infusions of NO-donors in isolated rodent hearts and in humans. The LV relaxation hastening effect was accompanied by increased diastolic LV distensibility, which augmented LV preload reserve especially in heart failure patients. This beneficial effect on diastolic LV function always overrode the small NO-induced attenuation in LV developed pressure in terms of overall LV performance. In most experimental and clinical conditions, contractile effects of NO were similar when NO was derived from NO-donors or produced by the different isoforms of NOS. Because expression of inducible NOS (NOS2) is frequently accompanied by elevated oxidative stress, NO produced by NOS2 can lead to peroxynitrite-induced contractile impairment as observed in ischemic or septic myocardium. Finally, shifts in isoforms or in concentrations of myofilaments can affect NO-mediated myofilamentary desensitization and alter the myocardial contractile effects of NO in hypertrophied or failing myocardium.

Myocardial dysfunction in the patient with sepsis

The nature of myocardial dysfunction during sepsis and septic shock has been investigated for more than half a century. This review traces the evolution of scientific thought regarding this phenomenon during this period with particular emphasis on the current understanding of both the clinical manifestations and the molecular/cellular basis of septic myocardial dysfunction in critically ill patients. Current data suggest, contrary to older literature, that patients with septic shock develop a hyperdynamic circulatory state after fluid resuscitation and maintain this hyperdynamic circulatory state until death or recovery. Overt myocardial depression, as manifested by decreased cardiac output, is decidedly uncommon, even in the preterminal phase. Nonetheless, myocardial depression, as evidenced by biventricular dilation and depression of the ejection fraction, can be demonstrated in most patients with septic shock by using either radionuclide cineangiography or echocardiography. Depression is reversible over the course of 7 to 10 days in survivors. Available evidence suggests that myocardial hypoperfusion is not responsible for septic myocardial depression, because examination of humans with septic shock demonstrates increased myocardial perfusion, and animal models of septic shock appear to maintain myocardial high-energy phosphates. A circulating factor or factors, including the cytokines tumor necrosis factor alpha and interleukin-1beta, appear to have a significant role in the phenomenon. In addition, septic myocardial depression appears to be mediated in part through combinations of nitric oxide-dependent and -independent alterations of basal and catecholamine-stimulated cardiac myocyte contractility.

Protective effects of antioxidative serotonin derivatives isolated from safflower against postischemic myocardial dysfunction

N-(p-Coumaroyl)serotonin (C) and N-feruroylserotonin (F) with antioxidative activity are present in safflower oil. The protective effects of C and F were investigated in perfused guinea-pig Langendorff hearts subjected to ischemia and reperfusion. Changes in cellular levels of high phosphorous energy, NO and Ca2+ in the heart together with simultaneous recordings of left ventricular developed pressure (LVDP) were monitored by an nitric oxide (NO) electrode, fluorometry and 31P-NMR. The rate of recovery of LVDP from ischemia by reperfusion was 30.8% in the control, while in the presence of C or F a gradual increase to 63.2 or 61.0% was observed. Changes of transient NO signals (TNO) released from heart tissue in one contraction (LVDP) were observed to be upside-down with respect to transient fura-2-Ca2+ signals (TCa) and transient O2 signals detected with a pO2 electrode. At the final stage of ischemia, the intracellular concentration of Ca2+ ([Ca2+]i) and the release of NO increased with no twitching and remained at a high steady level. The addition of C increased the NO level at the end of ischemia compared with the control, but [Ca2+]i during ischemia decreased. On reperfusion, the increased diastolic level of TCa and TNO returned rapidly to the control level with the recovery of LVDP. By in vitro EPR, C and F were found to directly quench the activity of active radicals. Therefore, it is concluded that the antioxidant effects of two derivatives isolated from safflower play an important role in ischemia-reperfusion hearts in close relation with NO.

Effects of nitric oxide donors on cardiac contractility in wild-type and myoglobin-deficient mice

1. The effects of the nitric oxide (NO) donors S-nitroso-N-acetylpenicillamine (SNAP), sodium(Z)-1-(N,N-diethylamino)diazen-1-ium-1,2-diolate (DEA-NONOate), and (Z)-1-[N-(2-Aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NONOate) on force of contraction (F(c)) were studied in atrial and ventricular muscle strips obtained from wild-type (WT) and myoglobin-deficient (myo(-/-)) mice. 2. SNAP slightly reduced F(c) in preparations from WT mice at concentrations above 100 microM; this effect was more pronounced in myo(-/-) mice. 3. DEA-NONOate reduced F(c) in preparations from myo(-/-) mice to a larger extent than those from WT mice. 4. DETA-NONOate reduced F(c) in preparations from myo(-/-) but not from WT mice. 5. Pre-incubation with an inhibitor of the soluble guanylyl cyclase (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; 100 microM) prevented the effects of SNAP, DEA-NONOate and DETA-NONOate on F(c) in myo(-/-) mice. 6. It is suggested that, in physiological conditions, myoglobin acts as intracellular scavenger preventing NO from reaching its intracellular receptors in cardiomyocytes, whereas, in myoglobin-deficient conditions, NO is able to reduce contractility via activation of the soluble guanylyl cyclase/cyclic GMP pathway.

Caveolin-1/3 double-knockout mice are viable, but lack both muscle and non-muscle caveolae, and develop a severe cardiomyopathic phenotype

The caveolin gene family consists of caveolins 1, 2, and 3. Caveolins 1 and 2 are co-expressed in many cell types, such as endothelial cells, fibroblasts, smooth muscle cells and adipocytes, where they form a heteroligomeric complex. In contrast, the expression of caveolin-3 is muscle-specific. Thus, the expression of caveolin-1 is required for caveolae formation in non-muscle cells, while the expression of caveolin-3 drives caveolae formation in striated muscle cell types (cardiac and skeletal). To create a truly caveolae-deficient mouse, we interbred Cav-1 null mice and Cav-3 null mice to generate Cav-1/Cav-3 double-knockout (Cav-1/3 dKO) mice. Here, we report that Cav-1/3 dKO mice are viable and fertile, despite the fact that they lack morphologically identifiable caveolae in endothelia, adipocytes, smooth muscle cells, skeletal muscle fibers, and cardiac myocytes. We also show that these mice are deficient in all three caveolin gene products, as caveolin-2 is unstable in the absence of caveolin-1. Interestingly, Cav-1/3 dKO mice develop a severe cardiomyopathy. At 2 months of age, analysis of Cav-1/3 dKO hearts via gated magnetic resonance imaging reveals a dramatic increase in left ventricular wall thickness, as compared with Cav-1-KO, Cav-3 KO, and wild-type mice. Further functional analysis of Cav-1/3 dKO hearts via transthoracic echocardiography demonstrates hypertrophy and dilation of the left ventricle, with a significant decrease in fractional shortening. As predicted, Northern analysis of RNA derived from the left ventricle of Cav-1/3 dKO mice shows a dramatic up-regulation of the atrial natriuretic factor message, a well-established biochemical marker of cardiac hypertrophy. Finally, histological analysis of Cav-1/3 dKO hearts reveals hypertrophy, disorganization, and degeneration of the cardiac myocytes, as well as chronic interstitial fibrosis and inflammation. Thus, dual ablation of both Cav-1 and Cav-3 genes in mice leads to a pleiotropic defect in caveolae formation and severe cardiomyopathy.

The inotropic effect of nitric oxide on mammalian papillary muscle is dependent on the level of beta1-adrenergic stimulation

We tested the hypothesis that nitric oxide has a positive inotropic effect on mammalian cardiac muscle contractility and that this effect sums with the positive inotropic effect of beta1-adrenergic agonists when both are present. Feline right ventricular papillary muscles were stimulated to contract isometrically at 0.2 Hz in Krebs-Henseleit bicarbonate buffer (KREBS) gassed with 95% O2 and 5% CO2 (26 degrees C; pH 7.34). The nitric oxide (NO) donor, S-nitroso-N-acetylpenicillamine (SNAP, 10(-5) M), and the membrane permeable cGMP analog 8-bromoguanosine-3',5'-cyclophosphate sodium (Br-cGMP, 10(-5) M), significantly increased developed force by 13.3+/-1.5% (n = 11) and 7.8+/-2.8% (n = 7), respectively. SNAP, at 10(-5) M, significantly increased the force developed by papillary muscle treated with 10(-11) M or 10(-9) M dobutamine hydrochloride (a beta1-adrenergic agonist) (n = 25, 11.3+/-2.9% and 10.0+/-3.6%, respectively) when compared with the addition of KREBS (n = 27, 2.6+/-0.9% and 5.5+/-0.9%), but the increase was less than predicted by the sum of inotropic effects of SNAP and dobutamine. SNAP at 10(-5) M did not change developed force in muscles treated with 10(-7) M dobutamine but it significantly decreased developed force in muscles challenged with 10(-5) M dobutamine (n = 18, 29.3+/-5.0%) when compared with KREBS (n = 10, 41.5+/-6.8%). Similarly, 10(-4) M 8-bromo-adenosine cyclic 3',5'-hydrogen phosphate monosodium (a membrane permeable cAMP analog) increased developed force 14.9+/-3.3% and the addition of 10(-5) M Br-cGMP to those muscles significantly reduced developed force by 3.5%+/-1.1% (n = 7). Thus, the positive inotropic effect of NO decreased and ultimately became an attenuation as the level of beta1-adrenergic stimulation increased due at least in part, to an interaction between the cAMP and cGMP second messenger pathways.

Role of cyclic GMP-dependent protein kinase in the contractile response to exogenous nitric oxide in rat cardiac myocytes

Nitric oxide (NO) can directly modulate cardiac contractility by accelerating relaxation and reducing diastolic tone. The intracellular mechanisms underlying these contractile effects are poorly understood. Here we investigate the role of cyclic GMP-dependent protein kinase (PKG) in the contractile response to exogenous NO in rat ventricular myocytes. Isolated ventricular myocytes were stimulated electrically and contractility was assessed by measuring cell shortening. Some cells were loaded with the fluorescent Ca(2+) probe indo-1 AM for simultaneous assessment of the intracellular Ca(2+) transient. The NO donor diethylamine NONOate (DEA/NO, 10 microM) significantly increased resting cell length, reduced twitch amplitude and accelerated time to 50 % relaxation (to 100.8 +/- 0.2, 83.7 +/- 3.0 and 88.9 +/- 3.7 % of control values, respectively). The contractile effects of DEA/NO occurred without significant changes in the amplitude or kinetics of the intracellular Ca(2+) transient, suggesting that the myofilament response to Ca(2+) was reduced. These effects were abolished by inhibition of either guanylyl cyclase (with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; ODQ, 10 microM) or PKG (with Rp-8-Br-cGMPs, 10 microM) suggesting that, at the concentration investigated, the effects of DEA/NO were mediated exclusively by PKG, following activation of guanylyl cyclase and elevation of cGMP. Direct activation of PKG with 8-pCPT-cGMP (10 microM) mimicked the effects of DEA/NO (resting cell length and time to 50 % relaxation were 100.6 +/- 0.1 and 90.5 +/- 1.5 % of control values, respectively).The reduced myofilament Ca(2+) responsiveness was not attributable to an intracellular acidosis since the small reduction in pH(i) induced by DEA/NO was found to be uncoupled from its contractile effects. However, hearts treated with DEA/NO (10 microM) showed a significant increase (1.4-fold; P < 0.01) in troponin I phosphorylation compared to control, untreated hearts. These results suggest that the reduction in myofilament Ca(2+) responsiveness produced by DEA/NO results from phosphorylation of troponin I by PKG.

Effects of adrenomedullin on hypertrophic responses induced by angiotensin II, endothelin-1 and phenylephrine

We examined whether adrenomedullin (AM), a vasoactive peptide with significant expression and binding sites in the heart, modulates the hypertrophic response in cultured neonatal rat ventricular myocytes. Myocyte hypertrophy was induced by treating the cells with angiotensin II (Ang II), endothelin-1 (ET-1) or alpha-adrenergic agonist, L-phenylephrine (PHE). All treatments resulted in a hypertrophic response as reflected by increased protein synthesis and expression of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) genes. AM treatment resulted in a complete inhibition of the Ang II-induced increase in ANP and BNP gene expression and secretion. In contrast, no inhibitory effect was seen in either ET-1-induced natriuretic peptide gene expression or PHE-induced ANP and BNP gene expression and secretion. AM had only a modest effect on basal levels of natriuretic peptide secretion and gene expression. When AM mRNA levels in isolated neonatal rat myocytes treated for 48 h with Ang II, ET-1 or PHE were measured, only Ang II induced a consistent increase in AM gene expression. These results indicate that AM is not invariably associated with attenuation of the hypertrophic response but its effect is dependent on the stimulus activating myocyte hypertrophy. AM may form an important autocrine/paracrine growth-inhibitory loop in Ang II-induced myocyte hypertrophy.

Nitric oxide: not just a negative inotrope

Nitric oxide (NO) appears to play a role in modulating cardiac function in both health and disease. Early studies in isolated rodent cardiac myocytes demonstrated a depressant effect of NO supplied by NO donors (exogenous) as well as NO generated within myocytes (endogenous). There is increasing evidence for a functional NO generating system within the human myocardium, which appears upregulated in certain disease states. Induction of the high output nitric oxide synthase isoform (iNOS) has been demonstrated in the failing myocardium, though its functional significance remains unproven. More recently published data have contradicted the notion that NO acts solely as a negative inotrope demonstrating positive inotropy in both isolated rodent and human ventricular myocytes in response to a range of NO donors. Different NO donors have different NO release kinetics and generate a range of NO species (NO., NO+ and NO-) which may interact at a number of subcellular targets. The observed response of any cardiac preparation to an NO donor represents the net effect of activation of different effector targets and may explain the contradictory reported effects of NO. To realise the therapeutic potential of NO will require specific targeting at a subcellular level.

Nitric oxide: does it play a role in the heart of the critically ill?

Nitric oxide regulates many aspects of myocardial function, not only in the normal heart but also in ischemic and nonischemic heart failure, septic cardiomyopathy, cardiac allograft rejection, and myocarditis. Accumulating evidence implicates the endogenous production of nitric oxide in the regulation of myocardial contractility, distensibility, heart rate, coronary vasodilation, myocardial oxygen consumption, mitochondrial respiration, and apoptosis. The effects of nitric oxide promote left ventricular mechanical efficiency, ie, appropriate matching between cardiac work and myocardial oxygen consumption. Most of these beneficial effects are attributed to the low physiologic concentrations generated by the constitutive endothelial or neuronal nitric oxide synthase. By contrast, inducible nitric oxide synthase generates larger concentrations of nitric oxide over longer periods of time, leading to mostly detrimental effects. In addition, the recently identified beta3-adrenoceptor mediates a negative inotropic effect through coupling to endothelial nitric oxide synthase and is overexpressed in heart failure. An imbalance between beta 1 and beta2-adrenoceptor and beta3-adrenoceptor, with a prevailing influence of beta3-adrenoceptor, may play a causal role in the pathogenesis of cardiac diseases such as terminal heart failure. Likewise, changes in the expression of endothelial nitric oxide synthase or inducible nitric oxide synthase within the myocardium may alter the delicate balance between the effects of nitric oxide produced by either of these isoforms. New treatments such as selective inducible nitric oxide synthase blockade, endothelial nitric oxide synthase promoting therapies, and selective beta3-adrenoceptor modulators may offer promising new therapeutic approaches to optimize the care of critically ill patients according to their stage and specific underlying disease process.

Endocardial expression and functional characterization of endothelin-1

Endothelin-1 (ET-1), a 21 amino acid peptide exerts a wide range of biological activities including vasoconstriction, mitogenesis and inotropic effects on the heart. In this study, we examined whether endocardial endothelial cells express ET-1 and evaluated its functional properties. Using immunofluorescence localization method, we demonstrated cytoplasmic staining of ET-1 in the human endocardial endothelial cells from the right atrium and left ventricle. Employing reverse transcriptase polymerase chain reaction (RT-PCR) expression of ET-1 mRNA and its receptors ET(A) and ET(B) mRNAs were found in human myocardial as well as in endocardial endothelial cells. Biological activity of endocardial endothelial cells derived ET-1 was established as the conditioned media obtained from cultured porcine endocardial endothelial cells induced a slowly developing, strong and long-lasting contraction of circular rat aortic segments, with similar characteristics to that obtained with exogenous ET-1. Furthermore, the selective endothelin-A receptor antagonist, FR 139317, blocked the conditioned media induced contractions. Our results suggest that endocardial endothelial cells express and release biologically active ET-1 which could play a pivotal role in the regulation of myocardial contractility as well as a circulatory peptide may further act in other peripheral target organs.

Cyclic GMP regulation of the L-type Ca(2+) channel current in human atrial myocytes

1. The regulation of the L-type Ca(2+) current (I(Ca)) by intracellular cGMP was investigated in human atrial myocytes using the whole-cell patch-clamp technique. 2. Intracellular application of 0.5 microM cGMP produced a strong stimulation of basal I(Ca) (+64 +/- 5 %, n = 60), whereas a 10-fold higher cGMP concentration induced a 2-fold smaller increase (+36 +/- 8 %, n = 35). 3. The biphasic response of I(Ca) to cGMP was not mimicked by the cGMP-dependent protein kinase (PKG) activator 8-bromoguanosine 3',5' cyclic monophosphate (8-bromo-cGMP, 0.5 or 5 microM), and was not affected by the PKG inhibitor KT 5823 (100 nM). 4. In contrast, cGMP stimulation of I(Ca) was abolished by intracellular perfusion with PKI (10 microM), a selective inhibitor of the cAMP-dependent protein kinase (PKA). 5. Selective inhibition of the cGMP-inhibited phosphodiesterase (PDE3) by extracellular cilostamide (100 nM) strongly enhanced basal I(Ca) in control conditions (+78 +/- 13 %, n = 7) but had only a marginal effect in the presence of intracellular cGMP (+22 +/- 7 % in addition to 0.5 microM cGMP, n = 11; +20 +/- 22 % in addition to 5 microM cGMP, n = 7). 6. Application of erythro-9-[2-hydroxy-3-nonyl]adenine (EHNA, 30 microM), a selective inhibitor of the cGMP-stimulated phosphodiesterase (PDE2), fully reversed the secondary inhibitory effect of 5 microM cGMP on I(Ca) (+99 +/- 16 % stimulation, n = 7). 7. Altogether, these data indicate that intracellular cGMP regulates basal I(Ca) in human atrial myocytes in a similar manner to NO donors. The effect of cGMP involves modulation of the cAMP level and PKA activity via opposite actions of the nucleotide on PDE2 and PDE3.