Inotropic response of rabbit ventricular myocytes to endothelin-1: difference from isolated papillary muscles

Tissue-specific and time-dependent regulation of the endothelin axis by the circadian clock protein Per1

AIMS

The present study is designed to consider a role for the circadian clock protein Per1 in the regulation of the endothelin axis in mouse kidney, lung, liver and heart. Renal endothelin-1 (ET-1) is a regulator of the epithelial sodium channel (ENaC) and blood pressure (BP), via activation of both endothelin receptors, ETA and ETB. However, ET-1 mediates many complex events in other tissues.

MAIN METHODS

Tissues were collected in the middle of murine rest and active phases, at noon and midnight, respectively. ET-1, ETA and ETB mRNA expressions were measured in the lung, heart, liver, renal inner medulla and renal cortex of wild type and Per1 heterozygous mice using real-time quantitative RT-PCR.

KEY FINDINGS

The effect of reduced Per1 expression on levels of mRNAs and the time-dependent regulation of expression of the endothelin axis genes appeared to be tissue-specific. In the renal inner medulla and the liver, ETA and ETB exhibited peaks of expression in opposite circadian phases. In contrast, expressions of ET-1, ETA and ETB in the lung did not appear to vary with time, but ET-1 expression was dramatically decreased in this tissue in Per1 heterozygous mice. Interestingly, ET-1 and ETA, but not ETB, were expressed in a time-dependent manner in the heart.

SIGNIFICANCE

Per1 appears to regulate expression of the endothelin axis genes in a tissue-specific and time-dependent manner. These observations have important implications for our understanding of the best time of day to deliver endothelin receptor antagonists.

MCP-1and IL-1β expression in the myocardia of two young patients with Type 1 diabetes mellitus and fatal diabetic ketoacidosis

Convincing evidence exists for the early onset of diabetic cardiomyopathy and coronary artery disease (CAD) as distinct forms of cardiac disease in young patients with Type 1 diabetes mellitus (T1DM) and the pre-stages of T2DM, forms of dysregulated insulin signaling. Progression of both chronic cardiac conditions is mediated by oxidative stress and low grade inflammation. This study reports the expression of monocyte chemotactic protein-1 (MCP-1) chemokine and the interleukin (IL)-1β inflammatory cytokine in two young patients with suboptimal metabolic control and fatal diabetic ketoacidosis (DKA), two age-matched overweight/obesity cases and two age-matched controls. In addition, markers of oxidative stress, apoptosis, collagen deposition and cardiomyocyte hypertrophy were studied. Significant expression of MCP-1 and IL-1β was seen in the myocardia of the T1DM/DKA cases, with lesser amounts expressed in the overweight/obesity myocardia. All of the other markers except cardiomyocyte hypertrophy were expressed to a significantly greater extent in the T1DM/DKA and overweight/obesity cases in comparison to the age-matched controls. Cardiomyocyte hypertrophy was significantly greater in the overweight/obesity cases than in the T1DM/DKA or the control cases.

The pathophysiology of endothelin in complications after solid organ transplantation: a potential novel therapeutic role for endothelin receptor antagonists

Although short-term allograft survival after solid organ transplantation has improved during the past two decades, improvement in long-term graft survival has been less pronounced. Common complications after transplantation include chronic allograft rejection, nephrotoxicity from calcineurin inhibitors (CNIs), and systemic hypertension, which all impact posttransplantation morbidity and mortality. Endothelin (ET)-1, a potent endogenous vasoconstrictor, inducer of fibrosis, and vascular smooth muscle cell proliferation, may play a key role in both the development of CNI-induced nephrotoxicity and endothelial vasculopathy in chronic allograft rejection. ET-1 levels increase after isograft implantation, and ET-1 plays a key role in CNI-induced renal vasoconstriction, sodium retention, and hypertension. Preclinical studies have demonstrated that endothelin receptor antagonists (ERAs) can reduce or prevent CNI-induced hypertension after renal transplantation. In addition, ERAs can ameliorate CNI-induced renal vasoconstriction and improve proteinuria and preserve renal function in animal models of renal transplantation. ET-1 may also play a significant role in cardiac allograft vasculopathy, and in animal models, ERAs improve pulmonary function and ischemic-reperfusion injury in lung transplantation and hepatic function and structure in liver transplantation. Emerging pharmacokinetic data suggest that the selective ERA ambrisentan may be used safely in conjunction with the most commonly used immunosuppressive agents tacrolimus and mycophenolate, albeit with appropriate dose adjustment. The weight of available evidence pointing toward a potential beneficial role of ERAs in ameliorating common complications after solid organ transplantation must be balanced with potential toxicities of ERAs but suggests that a randomized clinical trial of ERAs in transplant patients is warranted.

Physiological regulation of cardiac contractility by endogenous reactive oxygen species

Increased production of reactive oxygen species (ROS) has been linked to the pathogenesis of congestive heart failure. However, emerging evidence suggests the involvement of ROS in the regulation of various physiological cellular processes in the myocardium. In this review, we summarize the latest findings regarding the role of ROS in the acute regulation of cardiac contractility. We discuss ROS-dependent modulation of the inotropic responses to G protein-coupled receptor agonists (e.g. β-adrenergic receptor agonists and endothelin-1), the potential cellular sources of ROS (e.g. NAD(P)H oxidases and mitochondria) and the proposed end-targets and signalling pathways by which ROS affect contractility. Accumulating new data supports the fundamental role of endogenously generated ROS to regulate cardiac function under physiological conditions.

Endothelin and endothelin receptors in the renal and cardiovascular systems

Endothelin-1 (ET-1) is a multifunctional hormone which regulates the physiology of the cardiovascular and renal systems. ET-1 modulates cardiac contractility, systemic and renal vascular resistance, salt and water renal reabsorption, and glomerular function. ET-1 is responsible for a variety of cellular events: contraction, proliferation, apoptosis, etc. These effects take place after the activation of the two endothelin receptors ET(A) and ET(B), which are present - among others - on cardiomyocytes, fibroblasts, smooth muscle and endothelial cells, glomerular and tubular cells of the kidney. The complex and numerous intracellular pathways, which can be contradictory in term of functional response depending on the receptor type, cell type and physiological situation, are described in this review. Many diseases share an enhanced ET-1 expression as part of the pathophysiology. However, the use of endothelin blockers is currently restricted to pulmonary arterial hypertension, and more recently to digital ulcer. The complexity of the endothelin system does not facilitate the translation of the molecular knowledge to clinical applications. Endothelin antagonists can prevent disease development but secondary undesirable effects limit their usage. Nevertheless, the increasing understanding of the effects of ET-1 on the cardiac and renal physiology maintains the endothelin system as a promising therapeutic target.

cAMP- and Ca²(+) /calmodulin-dependent protein kinases mediate inotropic, lusitropic and arrhythmogenic effects of urocortin 2 in mouse ventricular myocytes

BACKGROUND AND PURPOSE

Urocortin 2 is beneficial in heart failure, but the underlying cellular mechanisms are not completely understood. Here we have characterized the functional effects of urocortin 2 on mouse cardiomyocytes and elucidated the underlying signalling pathways and mechanisms.

EXPERIMENTAL APPROACH

Mouse ventricular myocytes were field-stimulated at 0.5 Hz at room temperature. Fractional shortening and [Ca²(+)](i) transients were measured by an edge detection and epifluorescence system respectively. Western blots were carried out on myocyte extracts with antibodies against total phospholamban (PLN) and PLN phosphorylated at serine-16.

KEY RESULTS

Urocortin 2 elicited time- and concentration-dependent positive inotropic and lusitropic effects (EC₅₀ : 19 nM) that were abolished by antisauvagine-30 (10 nM, n= 6), a specific antagonist of corticotrophin releasing factor (CRF) CRF₂ receptors. Urocortin 2 (100 nM) increased the amplitude and decreased the time constant of decay of the underlying [Ca²(+)](i) transients. Urocortin 2 also increased PLN phosphorylation at serine-16. H89 (2 µM) or KT5720 (1 µM), two inhibitors of protein kinase A (PKA), as well as KN93 (1 µM), an inhibitor of Ca²(+)/calmodulin-dependent protein kinase II (CaMKII), suppressed the urocortin 2 effects on shortening and [Ca²(+)](i) transients. In addition, urocortin 2 also elicited arrhythmogenic events consisting of extra cell shortenings and extra [Ca²(+)](i) increases in diastole. Urocortin 2-induced arrhythmogenic events were significantly reduced in cells pretreated with KT5720 or KN93.

CONCLUSIONS AND IMPLICATIONS

Urocortin 2 enhanced contractility in mouse ventricular myocytes via activation of CRF₂ receptors in a cAMP/PKA- and Ca²(+)/CaMKII-dependent manner. This enhancement was accompanied by Ca²(+)-dependent arrhythmogenic effects mediated by PKA and CaMKII.

Wortmannin inhibits the increase in myofilament Ca(2+) sensitivity induced by cross-talk of endothelin-1 with norepinephrine in canine ventricular myocardium

Endothelin-1 (ET-1) modulates cardiac contractility by cross-talk with norepinephrine (NE) in canine ventricular myocardium. The present experiments were performed to investigate the influence of wortmannin that has inhibitory action on phosphatidylinositol 3-kinase (PI3-K) (IC50 = 3 nM) and myosin light chain kinase (MLCK) (IC50 = 200 nM) on Ca(2+) signaling and the inotropic effects of ET-1 induced by cross-talk with NE. Experiments were carried out in isolated canine ventricular trabeculae and indo-1/AM-loaded single ventricular cardiomyocytes. ET-1 alone elicited a transient small negative inotropic effect (NIE). In the presence of NE at low (1-10 nM) and high (100 nM) concentrations, ET-1 induced a long-lasting positive inotropic effect (PIE) or a marked sustained NIE, respectively. Wortmannin up to 300 nM did not affect the contractility; and at 1 microM and higher, it decreased the basal contraction without suppressing Ca(2+) transients. Wortmannin (1 microM) inhibited the long-lasting PIE of ET-1 without affecting the ET-1-induced increase in Ca(2+) transients. Wortmannin at the same concentration did not affect the ET-1-induced transient and sustained NIE and the PIE mediated by beta-adrenoceptor stimulation. These results imply that wortmannin exerts selective inhibitory action on the increase in myofilament Ca(2+) sensitivity induced by cross-talk of ET-1 with NE probably through an inhibition of MLCK in canine ventricular myocardium.

Cardiac Ca2+ signaling and Ca2+ sensitizers

The role of Ca2+ in cardiac excitation-contraction (E-C) coupling has been established by simultaneous measurements of contractility and Ca2+ transients by means of aequorin in intact myocardium and Ca2+ sensitive fluorescent dyes in single myocytes. The E-C coupling process can be classified into 3 processes: upstream (Ca2+ mobilization), central (Ca2+ binding to troponin C) and downstream mechanism (thin filament regulation and crossbridge cycling). These mechanisms are regulated differentially by various inotropic interventions. Positive force-frequency relationship and effects of beta-adrenoceptor stimulation, phosphodiesterase 3 inhibitors and digitalis are essentially exerted via upstream mechanism. Alpha-adrenoceptor stimulation, endothelin-1, angiotensin II, and clinically available Ca2+ sensitizers, such as levosimendan and pimobendan, act by a combination of the upstream and central/downstream mechanism. The Frank-Starling mechanism and effects of Ca2+ sensitizers such as EMD 57033 and Org 30029 are primarily induced via the central/downstream mechanism. Whereas the upstream and central mechanisms are markedly suppressed in failing myocytes and under acidotic conditions, Ca2+ sensitizers such as EMD 57033 and Org 30029 can induce cardiotonic effects under such conditions. Ca2+ sensitizers have high therapeutic potential for the treatment of contractile dysfunction in congestive heart failure and ischemic heart diseases, because they have energetic advantages and less risk of Ca2+ overload and can maintain effectiveness under pathological conditions.

Genistein stimulates myocardial contractility in guinea pigs by different subcellular mechanisms

The purpose of this study was to investigate the mechanisms involved in the excitatory effect induced by genistein in isolated guinea pig left ventricular papillary muscles and to determine relationship of genistein action with the tyrosine kinase pathway and phosphatidylinositol 3-kinase (PI3K) activity, the cyclic adenosine 5'-monophosphate (cAMP) signal system and the sarcoplasmic reticulum Ca2+ mobilization. Genistein (1-100 microM) significantly increased contraction of left ventricular papillary muscles from male and female guinea pigs in a concentration-dependent manner and its action had no obvious gender differences. Prior treatment with an L-type Ca2+ channel blocker verapamil hydrochloride, beta-adrenoceptor inhibitors propranolol and atenolol, an inhibitor of Na+-Ca2+ reverse exchanger Kb-r7943 or the blocker of estrogen receptor ICI 182,780 failed to alter the positive inotropic effect induced by genistein in papillary muscles. However, tyrosine phosphatase inhibitor, sodium orthovanadate or a potent phosphotyrosine phosphatase inhibitor bpV (phen) could partly but significantly reduce the stimulatory action of genistein. Interestingly, insulin-like growth factor-1, a known PI3K activator could also decrease the stimulatory action of genistein obviously, but the PI3K inhibitor LY294002 had no significant effect on the stimulatory action of genistein. The excitatory effect of genistein was markedly attenuated not only after treatment with an inhibitor of cAMP synthesis Sq 22536, carbachol or an inhibitor of specific protein kinase A H-89, but also after the inhibition of sarcoplasmic reticulum Ca2+ mobilization by ruthenium red, ryanodine or the inhibitor of sarcoplasmic reticulum Ca2+-ATPase thapsigargin. All these results indicate that the excitatory effects of genistein in papillary muscles are due to the inhibition of tyrosine kinase pathway and PI3K activity, thereby locally activating cAMP signal transduction and facilitating intracellular Ca2+ mobilization, but are not related to the activation of beta-adrenoceptor, the Na+-Ca2+ reverse exchange and the estrogen receptor.

The positive inotropic effect of endothelin-1 is mediated by mitochondrial reactive oxygen species

We have previously demonstrated the participation of reactive oxygen species (ROS) in the positive inotropic effect of a physiological concentration of Angiotensin II (Ang II, 1 nM). The objective of the present work was to evaluate the role and source of ROS generation in the positive inotropic effect produced by an equipotent concentration of endothelin-1 (ET-1, 0.4 nM). Isolated cat ventricular myocytes were used to measure sarcomere shortening with a video-camera, superoxide anion (()O(2)(-)) with chemiluminescence, and ROS production and intracellular pH (pH(i)) with epifluorescence. The ET-1-induced positive inotropic effect (40.4+/-3.1%, n=10, p<0.05) was associated to an increase in ROS production (105+/-29 fluorescence units above control, n=6, p<0.05). ET-1 also induced an increase in ()O(2)(-) production that was inhibited by the NADPH oxidase blocker, apocynin, and by the blockers of mitochondrial ATP-sensitive K(+) channels (mK(ATP)), glibenclamide and 5 hydroxydecanoic acid. The ET-1-induced positive inotropic effect was inhibited by apocynin (0.3 mM; 6.3+/-6.6%, n=13), glibenclamide (50 microM; 8.8+/-3.5%, n=6), 5 hydroxydecanoic acid (500 microM; 14.1+/-8.1, n=9), and by scavenging ROS with MPG (2 mM; 0.92+/-5.6%, n=8). ET-1 enhanced proton efflux (J(H)) carried by the Na(+)/H(+) exchanger (NHE) after an acid load, effect that was blocked by MPG. Consistently, the ET-induced positive inotropic effect was also inhibited by the NHE selective blocker HOE642 (5 microM; 9.37+/-6.07%, n=7). The data show that the effect of a concentration of ET-1 that induces an increase in contractility of about 40% is totally mediated by an intracellular pathway triggered by mitochondrial ROS formation and stimulation of the NHE.

IP3 receptor-dependent Ca2+ release modulates excitation-contraction coupling in rabbit ventricular myocytes

Inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)-dependent Ca(2+) signaling exerts positive inotropic, but also arrhythmogenic, effects on excitation-contraction coupling (ECC) in the atrial myocardium. The role of IP(3)R-dependent sarcoplasmic reticulum (SR) Ca(2+) release in ECC in the ventricular myocardium remains controversial. Here we investigated the role of this signaling pathway during ECC in isolated rabbit ventricular myocytes. Immunoblotting of proteins from ventricular myocytes showed expression of both type 2 and type 3 IP(3)R at levels approximately 3.5-fold less than in atrial myocytes. In permeabilized myocytes, direct application of IP(3) (10 microM) produced a transient 21% increase in the frequency of Ca(2+) sparks (P < 0.05). This increase was accompanied by a 13% decrease in spark amplitude (P < 0.05) and a 7% decrease in SR Ca(2+) load (P < 0.05) and was inhibited by IP(3)R antagonists 2-aminoethoxydiphenylborate (2-APB; 20 microM) and heparin (0.5 mg/ml). In intact myocytes endothelin-1 (100 nM) was used to stimulate IP(3) production and caused a 38% (P < 0.05) increase in the amplitude of action potential-induced (0.5 Hz, field stimulation) Ca(2+) transients. This effect was abolished by the IP(3)R antagonist 2-APB (2 microM) or by using adenoviral expression of an IP(3) affinity trap that buffers cellular IP(3). Together, these data suggest that in rabbit ventricular myocytes IP(3)R-dependent Ca(2+) release has positive inotropic effects on ECC by facilitating Ca(2+) release through ryanodine receptor clusters.

Contractile effects of angiotensin and endothelin in failing and non-failing human hearts

BACKGROUND

Angiotensin II (Ang II) and endothelin-1 (ET-1) share their effects on growth of myocardial cells but have been shown to elicit different effects on myocardial function. However, these effects vary markedly among species, cardiac regions (atrium or ventricle) and failing or non-failing myocardium. We therefore investigated the effects of both peptides on contractile function of isolated human myocytes from failing and non-failing hearts.

METHODS AND RESULTS

Cardiomyocytes were enzymatically isolated and electrically stimulated (15 V, 0.2 Hz). Ang II elicited a positive inotropic effect (PIE) mediated by activation of protein kinase C (PKC) in atrial but no effect in ventricular myocytes. ET-1 (10(-8) M) increased cell-shortening by 146+/-9.3% (p<0.05) in atrial myocytes, by 99.1+/-16.5% (p<0.05) in non-failing ventricular but only by 40.5+/-6.4% (p<0.05) in failing ventricular myocytes. The PIE of ET-1 in failing myocytes was more pronounced at low extracellular pH (+112.6+/-27% at pH 7.0 vs. +40.5+/-6.4% at pH 7.4, p<0.05). Amiloride, a sodium-hydrogen-exchange inhibitor, inhibited two thirds of the PIE of ET-1 in failing myocytes. The PKC-inhibitor decreased the PIE by 50% from 113% to 64% in ventricular myocytes under acidotic conditions.

CONCLUSION

Ang II and ET-1 elicited PIE in atrial myocytes, whereas in ventricular myocytes Ang II did not induce PIE in contrast to ET-1. The PIE of ET-1 was markedly attenuated in failing myocytes. Under acidotic conditions, the PIE of ET-1 was more pronounced in failing myocytes, indicating that ET-1 may activate signaling processes in failing myocytes, which are not activated in normal myocytes.

Signal transduction and Ca2+ signaling in intact myocardium

The experimental procedures to simultaneously detect contractile activity and Ca(2+) transients by means of the Ca(2+) sensitive bioluminescent protein aequorin in multicellular preparations, and the fluorescent dye indo-1 in single myocytes, provide powerful tools to differentiate the regulatory mechanisms of intrinsic and external inotropic interventions in intact cardiac muscle. The regulatory process of cardiac excitation-contraction coupling is classified into three categories; upstream (Ca(2+) mobilization), central (Ca(2+) binding to troponin C), and/or downstream (thin filament regulation of troponin C property or crossbridge cycling and crossbridge cycling activity itself) mechanisms. While a marked increase in contractile activity by the Frank-Starling mechanism is associated with only a small alteration in Ca(2+) transients (downstream mechanism), the force-frequency relationship is primarily due to a frequency-dependent increase of Ca(2+) transients (upstream mechanism) in mammalian ventricular myocardium. The characteristics of regulation induced by beta- and alpha-adrenoceptor stimulation are very different between the two mechanisms: the former is associated with a pronounced facilitation of an upstream mechanism, whereas the latter is primarily due to modulation of central and/or downstream mechanisms. alpha-Adrenoceptor-mediated contractile regulation is mimicked by endothelin ET(A)- and angiotensin II AT(1)-receptor stimulation. Acidosis markedly suppresses the regulation induced by Ca(2+) mobilizers, but certain Ca(2+) sensitizers are able to induce the positive inotropic effect with central and/or downstream mechanisms even under pathophysiological conditions.

Endothelin ETA receptors and endothelium partially mediate the positive inotropic and lusitropic effects of angiotensin II

We analyzed the influence of endothelin-1 and endocardial endothelium on the myocardial effects of angiotensin-II. Angiotensin-II (10(-9)-10(-5) M) was tested in rabbit right papillary muscles in absence (Protocol-A) or presence of PD-145065 (10(-7) M; Protocol-B), BQ-123 (10(-7) M; Protocol-C) or losartan (10(-6) M; Protocol-E), as well as, after removing the endocardial endothelium with Triton X-100 0.5% (Protocol-D). In Protocol-F increasing concentrations of endothelin-1 (10(-10)-10(-8) M) were added in presence of angiotensin-II (10(-7) M) after selective removal of the endocardial endothelium. In Protocol-A, angiotensin-II had dose-dependent positive inotropic and lusitropic effects, maximal at 10(-6) M increasing 122+/-13% active tension, 117+/-16% dT/dtmax and 86+/-9% dT/dtmin. In Protocols B, C and D the inotropic and lusitropic effects of angiotensin-II were significantly attenuated. The same concentration (10(-6) M) of angiotensin-II increased respectively 48+/-11%, 59+/-27% and 72+/-16% active tension; 54+/-14%, 54+/-20% and 32+/-9% dT/dtmax; and 39+/-8%, 48+/-19% and 59+/-11% dT/dtmin; and 40+/-10%. EC(50) for active tension significantly increased from -7.8+/-0.1 logM in Protocol A to -7.1+/-0.3, -6.7+/-0.4 and -6.8+/-0.3 logM in Protocols B, C and D respectively, while Emax decreased from 106+/-14% in Protocol A to 50+/-14 and 51+/-19% in Protocols B and C respectively, but did not significantly change in Protocol D (114+/-25%). Losartan completely blocked the inotropic and lusitropic effects of angiotensin-II, while the attenuation of these effects after the selective removal of the endocardial endothelium was blunted by concomitant administration of endothelin-1 (Protocol F). In conclusion, angiotensin-II has a dose-dependent positive inotropic effect that depends, to a great extent, on endothelin ETA receptor activation and intact endocardial endothelium.

Electrophysiological and haemodynamic effects of endothelin ETA and ETB receptors in normal and ischaemic working rabbit hearts

The aims of this study were to determine if endothelin-1 (ET-1) under normal and ischaemic conditions exhibits a direct arrhythmogenic effect that is independent of its ability to cause coronary vasoconstriction, and to determine the contribution of the ET(A) and ET(B) receptor subtype. ET(A/B) (with ET-1) and ET(A) (ET-1 in the presence of BQ-788) receptor activation resulted in a significant reduction in both epi- and endocardial monophasic action potential duration (MAPD(90)). ET(A) receptor activation reduced both epi- and endocardial effective refractory period (ERP). This MAPD(90) and ERP shortening were associated with a reduction in coronary flow, myocardial contractility and induction of ventricular fibrillation (VF) during ERP measurement. The ET(B) agonist sarafotoxin (S6c) had no marked, or concentration-dependent, effect on MAPD(90), ERP, myocardial contractility or induction of arrhythmias. Neither ET-1 nor S6c, given prior to coronary artery occlusion, significantly changed the ischaemia-induced dispersion of MAPD(90), ERP or the % incidence of VF. In conclusion, neither ET(A) nor ET(B) receptor stimulation has a direct arrhythmogenic effect in isolated rabbit hearts under normal or ischaemic conditions. The ET-1-induced arrhythmogenic effect observed in nonischaemic hearts is likely to be the result of the associated coronary vasoconstriction caused by ET(A) receptor stimulation resulting in myocardial ischaemia.

Differential action of a protein tyrosine kinase inhibitor, genistein, on the positive inotropic effect of endothelin-1 and norepinephrine in canine ventricular myocardium

Experiments were carried out in isolated canine ventricular trabeculae and acetoxymethylester of indo-1-loaded single myocytes to elucidate the role of protein tyrosine kinase (PTK) in the inotropic effect of endothelin-1 (ET-1) induced by crosstalk with norepinephrine (NE). The PTK inhibitor genistein was used as a pharmacological tool. Genistein but not daidzein inhibited the positive inotropic effect and the increase in Ca(2+) transients induced by ET-1 by crosstalk with NE at low concentrations. Genistein and daidzein antagonized the negative inotropic effect and the decrease in Ca(2+) transients induced by ET-1 by crosstalk with NE at high concentrations, but genistein did not affect the antiadrenergic effect of carbachol. Genistein but not daidzein enhanced the positive inotropic effect and the increase in Ca(2+) transients induced by NE via beta-adrenoceptors, while the enhancing effect of genistein was abolished by the protein tyrosine phosphatase inhibitor vanadate. These findings indicate that genistein (1) induces a positive inotropic effect in association with an increase in Ca(2+) transients, (2) inhibits the positive inotropic effect of ET-1 induced by crosstalk with NE, and (3) enhances the positive inotropic effect of NE induced via beta-adrenoceptors by inhibition of PTK. In addition, genistein inhibits the negative inotropic effect of ET-1 induced by crosstalk with NE through a PTK-unrelated mechanism. PTK may play a crucial role in the receptor-mediated regulation of cardiac contractile function in canine ventricular myocardium.

Intracellular signaling following myocardial stretch: an autocrine/paracrine loop

The stretch of adult papillary muscle elicits a chain of autocrine/paracrine events in which the Na(+)/H(+) exchanger (NHE-1) activation is the central step. This activation is induced by a sequential angiotensin II-endothelin (Ang II-ET) release and results in an increase in intracellular Na(+) (Na(+)(i)) without significant changes in intracellular pH. The increase in Na(+)(i) negatively shifts the reverse potential of the Na(+)/Ca(2+) exchanger (NCX) thus inducing cell Ca(2+) influx that augments myocardial contractility. This increase in force represents the mechanical counterpart of the autocrine/paracrine mechanism triggered by stretch and has been called the slow force response (SFR) to stretch.

Differential inhibition by the Rho kinase inhibitor Y-27632 of the increases in contractility and Ca2+ transients induced by endothelin-1 in rabbit ventricular myocytes

The role of Rho kinase activation in the regulation of cardiac contractility and Ca(2+) signaling remains unclear, whereas its role in smooth muscle regulation has been well documented. To study the potential role of Rho kinase in the regulation of cardiac contractility and Ca(2+) transients induced by endothelin-1 (ET-1) and isoproterenol, we used the Rho kinase inhibitor Y-27632 in rabbit ventricular myocardium and myocytes loaded with indo-1/AM. Y-27632 (3-30 microM) inhibited significantly the baseline contractility and Ca(2+) transients. Furthermore, Y-27632 suppressed the increase in contractility and Ca(2+) transients induced by ET-1 in a concentration-dependent manner, when it was used in a concentration at which it did not affect the effects of isoproterenol via beta-adrenoceptors. In the presence of Y-27632, ET-1 increased cell shortening in the absence of an increase in Ca(2+) transients. This is an indication that the increase in myofilament Ca(2+) sensitivity induced by ET-1 is less susceptible to the inhibitory action of Y-27632. These findings imply that the Rho kinase activation may partially contribute to the ET-1-induced regulation of contractility, primarily due to an ET-1-induced increase in Ca(2+) transients in rabbit ventricular myocardium.

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.

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.

The role of inositol 1,4,5-trisphosphate receptors in Ca(2+) signalling and the generation of arrhythmias in rat atrial myocytes

Various cardio-active stimuli, including endothelin-1 (ET-1), exhibit potent arrhythmogenicity, but the underlying cellular mechanisms of their actions are largely unclear. We used isolated rat atrial myocytes and related changes in their subcellular Ca(2+) signalling to the ability of various stimuli to induce diastolic, premature extra Ca(2+) transients (ECTs). For this, we recorded global and spatially resolved Ca(2+) signals in indo-1- and fluo-4-loaded atrial myocytes during electrical pacing. ET-1 exhibited a higher arrhythmogenicity (arrhythmogenic index; ratio of number of ECTs over fold-increase in Ca(2+) response, 8.60; n = 8 cells) when compared with concentrations of cardiac glycosides (arrhythmogenic index, 4.10; n = 8 cells) or the beta-adrenergic agonist isoproterenol (arrhythmogenic index, 0.11; n = 6 cells) that gave similar increases in the global Ca(2+) responses. Seventy-five percent of the ET-1-induced arrhythmogenic Ca(2+) transients were accompanied by premature action potentials, while for digoxin this proportion was 25 %. The beta-adrenergic agonist failed to elicit a significant number of ECTs. Direct activation of inositol 1,4,5-trisphosphate (InsP(3)) receptors with a membrane-permeable InsP(3) ester (InsP(3) BM) mimicked the effect of ET-1 (arrhythmogenic index, 14.70; n = 6 cells). Inhibition of InsP(3) receptors using 2 microM 2-aminoethoxydiphenyl borate, which did not display any effects on Ca(2+) signalling under control conditions, specifically suppressed the arrhythmogenic action of ET-1 and InsP(3) BM. Immunocytochemistry indicated a co-localisation of peripheral, junctional ryanodine receptors with InsP(3)Rs. Thus, the pronounced arrhythmogenic potency of ET-1 is due to the spatially specific recruitment of Ca(2+) sparks by subsarcolemmal InsP(3)Rs. Summation of such sparks efficiently generates delayed after depolarisations that trigger premature action potentials. We conclude that the particular spatial profile of cellular Ca(2+) signals is a major, previously unrecognised, determinant for arrhythmogenic potency and that the InsP(3) signalling cassette might therefore be a promising new target for understanding and managing atrial arrhythmia.