Signal transduction and Ca2+ signaling in contractile regulation induced by crosstalk between endothelin-1 and norepinephrine in dog ventricular myocardium

Peptide Lv augments L-type voltage-gated calcium channels through vascular endothelial growth factor receptor 2 (VEGFR2) signaling

We previously identified peptide Lv, a novel bioactive peptide that enhances the activity of L-type voltage-gated calcium channels (L-VGCCs) in cone photoreceptors. In this study, we verified that peptide Lv was able to augment L-VGCC currents in cardiomyocytes, as well as promote proliferation of endothelial cells. We used a proteomics approach to determine the specific receptors and binding partners of peptide Lv and found that vascular endothelial growth factor receptor 2 (VEGFR2) interacted with peptide Lv. Peptide Lv treatment in embryonic cardiomyocytes stimulated tyrosine autophosphorylation of VEGFR2 and activated its downstream signaling. Peptide Lv activity was blocked by DMH4, a VEGFR2 specific blocker, but not by SCH202676, an allosteric inhibitor of G protein-coupled receptors, suggesting that the activity of peptide Lv was mediated through VEGFR2 signaling. Inhibition of VEGFR tyrosine kinase or its downstream signaling molecules abolished the augmentation of L-VGCCs elicited by peptide Lv in cardiomyocytes. In addition, peptide Lv promoted cell proliferation of cultured human endothelial cells. Calcium entry through L-VGCCs is essential for excitation-contraction coupling in cardiomyocytes. Since peptide Lv was able to augment L-VGCCs through activation of VEGF signaling in cardiomyocytes and promote proliferation of endothelial cells, peptide Lv may play an important role in regulating the cardiovascular system.

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.

The relationships between plasma adrenomedullin and endothelin-1 concentrations and Doppler echocardiographic indices of left ventricular function during static exercise in healthy men

Our previous study showed a significant relationships between static exercise-induced changes in plasma adrenomedullin (ADM) and those in endothelin-1 (ET-1), noradrenaline (NA) and pre-ejection period/left ventricular ejection time ratio (PEP/LVET) in older healthy men. It is hypothesized that ADM, ET-1, NA and adrenaline (A) may function as endogenous regulators of cardiac function by modulating myocardial contractility during static exercise. The present study was undertaken to assess the relationships between exercise-induced changes in plasma ADM, ET-1, NA, A concentrations and those in ascending aortic blood flow peak velocity (PV) and mean acceleration (MA) measured by Doppler echocardiography in 24 healthy older men during two 3-min bouts of handgrip at 30% of maximal voluntary contraction, performed alternately with each hand without any break between the bouts. Plasma ADM, ET-1, NA and A as well as heart rate (HR), blood pressure (BP), PV and MA were determined. During handgrip, plasma ADM, ET-1, NA and A as well as HR, BP increased, whereas PV and MA decreased. The increases in plasma ADM correlated positively with those in ET-1, NA and diastolic BP, and correlated negatively with changes in PV (r= −0.68) and MA (r= −0.62). The increases in plasma ET-1 correlated positively with those in NA and BPs and correlated negatively with changes in PV (r= −0.67) and MA (r= −0.60). The results of this study suggest that in healthy older men the exercise-induced changes in plasma ADM, ET 1 and catecholamines are related to alterations in left ventricular contractile state and may co-operatively counteract age-related deterioration of cardiac performance in men.

Berbamine increases myocardial contractility via a Ca2+-independent mechanism

Berbamine (BM), a natural compound derived from Berberis vulgaris L, has been reported to inhibit cardiac contractile function at higher concentrations. Here, we report that BM had concentration-dependent biphasic effects on myocardial contraction in Langendorff-perfused rat hearts, that is, at lower concentrations (30-100 nM), it displayed positive inotropic and lusitropic effects, whereas at a higher concentration of 1 μM, it caused a negative inotropic effect after an initially weak increase. These effects were further confirmed in cardiomyocytes isolated from the left ventricles of rats. Moreover, the increased cell shortening by BM at concentrations from 0.1 to 100 nM was not associated with an alteration of intracellular Ca transients. Consistently, at 30 nM, BM shifted the cell shortening--Ca transient relationship curve induced by cumulative elevation of extracellular Ca concentration to the left. Furthermore, BM significantly increased membrane-bound but not filament-bound protein kinase C epsilon (PKCε) in the isolated hearts and cardiomyocytes. Such a translocation was inhibited by PKCε-specific inhibitor PKCε V1-2 concomitant with the abolishment of the BM-induced increase in contraction. These findings reveal the positive inotropic effect of BM in the myocardium and demonstrate that BM increases myocardial contractility by increasing myofilament Ca sensitivity via a PKCε-dependent signaling pathway.

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.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Role of reactive oxygen species in the regulation of cardiac contractility

Increased production of reactive oxygen species (ROS) has been linked to the pathogenesis of contractile dysfunction in heart failure. However, it is unclear whether ROS can regulate physiological cellular processes in the myocardium. Here, we characterized the role of endogenous ROS production in the acute regulation of cardiac contractility in the intact rat heart. In isolated perfused rat hearts, endothelin-1 (ET-1, 1nmol/L) stimulated ROS formation in the left ventricle, which was prevented by the antioxidant N-acetylcysteine and the NAD(P)H oxidase inhibitor apocynin. N-acetylcysteine, the superoxide dismutase mimetic MnTMPyP, and apocynin significantly attenuated ET-1-mediated inotropic effect, which was accompanied by inhibition of extracellular signal regulated kinase 1/2 (ERK1/2) phosphorylation. Moreover, the mitochondrial K(ATP) channel blocker 5-HD, and the mitochondrial large conductance calcium activated potassium channel blocker paxilline, but not the sarcolemmal K(ATP) channel blocker HMR 1098 attenuated the inotropic response to ET-1. However, ET-1-induced ROS generation was not abolished by inhibiting mitochondrial K(ATP) channel opening. In contrast to ET-1 stimulation, the positive inotropic effect of β(1)-adrenergic receptor agonist dobutamine (250nmol/L) was significantly augmented by N-acetylcysteine and apocynin. Moreover, dobutamine-induced phospholamban phosphorylation was markedly enhanced by apocynin. In conclusion, NAD(P)H oxidase-derived ROS play a physiological role in the acute regulation of cardiac contractility in the intact rat heart. Our results reveal that ET-1-induced increase in cardiac contractility is partially dependent on enhanced ROS generation, which in turn, activates the ERK1/2 pathway. On the other hand, β-adrenergic receptor-induced positive inotropic effect and phospholamban phosphorylation is enhanced by NAD(P)H oxidase inhibition.

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.

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.

Endothelin-1 regulates cardiac L-type calcium channels via NAD(P)H oxidase-derived superoxide

It has been shown that reactive oxygen species (ROS) are involved in the intracellular signaling response to G-protein coupled receptor stimuli in vascular smooth muscle cells and in neurons. In the present study, we tested the hypothesis that NAD(P)H oxidase-derived ROS are involved endothelin-1 (ET-1)-induced L-type calcium channel activation in isolated cardiac myocytes. ET-1 (10 nM) induced a 2-fold increase in L-type calcium channel open-state probability (NPo). This effect of ET-1 was abolished by the ET(A) receptor antagonist cyclo(D-Trp-D-Asp-Pro-D-Val-Leu) [BQ-123 (1 microM)] but was not altered in the presence of an ET(B) receptor antagonist N-cis-2,6-dimethylpiperidinocarbonyl-b-tBu-Ala-D-Trp(1-methoxycarbonyl)-D-Nle-OH [BQ-788 (1 microM)]. Pretreatment of cells with the ROS scavenger tempol (100 microM), polyethylene glycol-superoxide dismutase (SOD, 25 U/ml), or the NAD(P)H-oxidase inhibitor gp91ds-tat ([H]RKKRRQRRR-CSTRIRRQL[NH(3)]) (5 microM) significantly attenuated ET-1-induced increases in calcium channel NPo. Tempol, SOD, and gp91ds-tat alone had no effect on basal calcium channel activity. In addition, ET-1 significantly increased NAD(P)H oxidase activity and elevated intracellular superoxide levels in cultured cardiac myocytes. The superoxide generator, xanthine-xanthine oxidase (10 mM, 20 mU/ml), also increased calcium channel NPo in cardiac myocytes, mimicking the effect of ET-1. These observations provide the first evidence that ET-1 induces the activation of L-type Ca(2+) channels via stimulation of NAD(P)H-derived superoxide production in cardiac myocytes.

Overexpression of diacylglycerol kinase zeta inhibits endothelin-1-induced decreases in Ca2+ transients and cell shortening in mouse ventricular myocytes

Endothelin-1 (ET-1) is released in various cardiovascular disorders including congestive heart failure, and may modulate significantly the disease process by its potent action on vascular and cardiac muscle cell function and gene regulation. In adult mouse ventricular cardiomyocytes loaded with indo-1, ET-1 induced a sustained negative inotropic effect (NIE) in association with decreases in Ca(2+) transients. The ET-1-induced effects on Ca(2+) transients and cell shortening were abolished in diacylglycerol (DAG) kinase zeta-overexpressing mouse ventricular myocytes. A nonselective protein kinase C (PKC) inhibitor, GF109203X, inhibited the ET-1-induced decreases in Ca(2+) transients and cell shortening in concentration-dependent manners, whereas a selective Ca(2+)-dependent PKC inhibitor, Gö6976, did not affect the ET-1-induced effects. A phospholipase Cbeta inhibitor, U73122, and an inhibitor of phospholipase D, C(2)-ceramide, partially, but significantly, attenuated the ET-1-induced effects. Derivatives of the respective inhibitors with no specific effects, U73343 and dihydro-C(2)-ceramide, did not affect the ET-1-induced effects. Taken together, these results indicate that activation of a Ca(2+)-independent PKC isozyme by 1,2-DAG, which is generated by phospholipase Cbeta and phospholipase D activation and inactivated by phosphorylation via DAG kinase, is responsible for the ET-1-induced decreases in Ca(2+) transients and cell shortening in mouse ventricular cardiomyocytes.

Intracellular mechanisms and receptor types for endothelin-1-induced positive and negative inotropy in mouse ventricular myocardium

We examined the intracellular mechanisms for endothelin-1-induced positive and negative inotropic components that coexist in the mouse ventricular myocardium using isolated ventricular tissue and myocytes from 4-week-old mice. In the presence of SEA0400, a specific inhibitor of the Na+-Ca2+ exchanger, endothelin-1 produced positive inotropy. Endothelin-1, when applied to cardiomyocytes in the presence of SEA0400, did not change the peak amplitude of the Ca2+ transient but increased intracellular pH and Ca2+ sensitivity of contractile proteins. On the other hand, in the presence of dimethylamiloride (DMA), a specific inhibitor of the Na+-H+ exchanger, endothelin-1 produced negative inotropy. In cardiomyocytes, in the presence of DMA, endothelin-1 produced a decrease in peak amplitude of the Ca2+ transient. In the presence of both DMA and SEA0400, endothelin-1 produced neither positive nor negative inotropy. Positive inotropy was blocked by BQ-123 and negative inotropy by BQ-788. These results suggested that endothelin-1-induced positive inotropy is mediated by ET(A) receptors, activation of the Na+-H+ exchanger and an increase in intracellular pH and Ca2+ sensitivity and that the negative inotropy is mediated by ET(B) receptors, activation of the Na+-Ca2+ exchanger and decrease in Ca2+ transient amplitude.

Role of protein kinase C isozymes in the regulation of alpha1-adrenergic receptor-mediated contractions in ovine uterine arteries

Previously, we demonstrated that activation of protein kinase C (PRKC) enhanced alpha(1)-adrenergic receptor-induced contractions in nonpregnant ovine uterine arteries but inhibited the contractions in pregnant ovine uterine arteries. The present study tested the hypothesis that differential regulation of PRKC isozyme activities contributes to the different effects of phorbol 12, 13-dibutyrate (PDBu) on alpha(1)-adrenergic receptor-mediated contractions between the pregnant and nonpregnant ovine uterine arteries. Phenylephrine-induced contractions of ovine nonpregnant and pregnant uterine arteries were determined in the absence or presence of the PRKC activator PDBu and/or in combination with conventional and novel PRKC isozyme inhibitor GF109203X, PRKC isozyme-selective inhibitory peptides for conventional PRKC, PRKCB1, PRKCB2, and PRKCE. GF109203X produced a concentration-dependent inhibition of phenylephrine-induced contractions in both nonpregnant and pregnant uterine arteries, and it reversed the PDBu-mediated potentiation and inhibition of phenylephrine-induced contractions in nonpregnant and pregnant uterine artieries, respectively. In addition, PRKCB1, PRKCB2, and PRKCE inhibitory peptides blocked the PDBu-mediated responses in both nonpregnant and pregnant uterine arteries. Western blot analysis showed that PDBu induced a membrane translocation of PRKCA, PRKCB1, PRKCB2, and PRKCE in pregnant uterine arteries, and PRKCB1, PRKCB2, and PRKCE in nonpregnant uterine arteries. The results disprove the hypothesis that the dichotomy of PRKC mechanisms in the regulation of alpha(1)-adrenergic receptor-induced contractions in nonpregnant and pregnant uterine arteries is caused by the activation of different PRKC isozymes, and suggest downstream mechanisms of differential subcellular distributions for the distinct functional effects of PRKC isozymes in the adaptation of uterine arteries to pregnancy.

Ceramide mediates inhibition of the renal epithelial sodium channel by tumor necrosis factor-alpha through protein kinase C

To determine whether ceramide mediates regulation of the renal epithelial sodium channel (ENaC) by tumor necrosis factor-alpha (TNF-alpha), confocal microscopy and patch-clamp experiments were performed in A6 distal nephron cells. We found that TNF-alpha (100 ng/ml) had no effect on ENaC activity and ceramide level when the cells were grown in the presence of aldosterone, but significantly inhibited ENaC and induced ceramide production after the cells were pretreated with LY 294002, an inhibitor of phosphatidylinositol 3-kinase, for 24 h. The inhibition of ENaC induced by TNF-alpha was mimicked by exogenous sphingomyelinase (0.1 U/ml) and C(2)-ceramide (50 microM), but neither C(2)-dihydroceramide, a membrane-impermeable analog of C(2)-ceramide, nor choline, and abolished by pretreatment with GF109203X, a protein kinase C (PKC) inhibitor. C(2)-ceramide failed to affect ENaC in the cells pretreated with GF109203X, but not in the cells pretreated with PD-98059, a mitogen-activated protein kinase kinase inhibitor. C(2)-ceramide induced the externalization of phosphatidylserine (PS) in control A6 cells, but not in the cells pretreated with GF109203X. Together with our previous finding that cytosolic PS maintains ENaC activity in A6 cells, these data suggest that ceramide mediates TNF-alpha inhibition of the renal ENaC via a pathway associated with PKC-dependent externalization of PS.

Calcium signaling phenomena in heart diseases: a perspective

Ca(2+) is a major intracellular messenger and nature has evolved multiple mechanisms to regulate free intracellular (Ca(2+))(i) level in situ. The Ca(2+) signal inducing contraction in cardiac muscle originates from two sources. Ca(2+) enters the cell through voltage dependent Ca(2+) channels. This Ca(2+) binds to and activates Ca(2+) release channels (ryanodine receptors) of the sarcoplasmic reticulum (SR) through a Ca(2+) induced Ca(2+) release (CICR) process. Entry of Ca(2+) with each contraction requires an equal amount of Ca(2+) extrusion within a single heartbeat to maintain Ca(2+) homeostasis and to ensure relaxation. Cardiac Ca(2+) extrusion mechanisms are mainly contributed by Na(+)/Ca(2+) exchanger and ATP dependent Ca(2+) pump (Ca(2+)-ATPase). These transport systems are important determinants of (Ca(2+))(i) level and cardiac contractility. Altered intracellular Ca(2+) handling importantly contributes to impaired contractility in heart failure. Chronic hyperactivity of the beta-adrenergic signaling pathway results in PKA-hyperphosphorylation of the cardiac RyR/intracellular Ca(2+) release channels. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, protein kinase C, Gq, and the down stream effectors such as mitogen activated protein kinases pathways, and the Ca(2+) regulated phosphatase calcineurin. A number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocytes. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underlie heart diseases. Recent progress in molecular cardiology makes it possible to envision a new therapeutic approach to heart failure (HF), targeting key molecules involved in intracellular Ca(2+) handling such as RyR, SERCA2a, and PLN. Controlling these molecular functions by different agents have been found to be beneficial in some experimental conditions.

Acute heart failure: inotropic agents and their clinical uses

Inotropic agents are indispensable for the improvement of cardiac contractile dysfunction in acute or decompensated heart failure. Clinically available agents, including sympathomimetic amines (dopamine, dobutamine, noradrenaline) and selective phosphodiesterase-3 inhibitors (amrinone, milrinone, olprinone and enoximone) act via cAMP/protein kinase A (PKA)-mediated facilitation of intracellular Ca2+ mobilisation. Phosphodiesterase-3 inhibitors also have a vasodilatory action, which plays a role in improving haemodynamic parameters in certain patients, and are termed inodilators. The available inotropic agents suffer from risks of Ca2+ overload leading to arrhythmias, myocardial cell injury and ultimately, cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and cellular metabolism. Furthermore, they lose their effectiveness under pathophysiological conditions, such as acidosis, stunned myocardium and heart failure. Pimobendan and levosimendan (that act by a combination of an increase in Ca2+ sensitivity and phosphodiesterase-3 inhibition) appear to be more beneficial among existing agents. Novel Ca2+ sensitisers that are under basic research warrant clinical trials to replace available inotropic agents.

Effect of endothelins on the cardiovascular system

Endothelins (ETs) exert a persistent constrictor effect on the vessels via an increase in intracellular Ca2+ concentration due to the activation of Na+/H+ and Na+/Ca2+ exchangers of the vascular smooth muscle fibres. They also produce a transient dilator effect via the activation of endothelial nitric oxide synthase mediated by protein kinase B/Akt. ETA and ETB2 receptors are involved in vasoconstriction, whereas transient vasodilatation depends on the activation of ETB1 receptors. Depending on animal species and experimental conditions, ETs can also play a role in cardiac muscle contraction and induce either an increase or a decrease in contractility. It is likely that only ETA, and not ETB, receptors are involved in the ET-induced increase in myocardial contractility. As in the case of vasoconstriction, this inotropic effect depends on an increase in intracellular Ca2+ concentration induced by Na+/H+ and Na+/Ca2+ exchangers. Activation of the Na+/H+ exchanger is stimulated by protein kinase C, which is activated by diacylglycerol released in response to ET activity. It has also been proposed that the positive inotropic effect can occur without the contribution of the Na+/Ca2+ exchanger, if the cell alkalinisation produced by the Na/H exchanger improves myofibrillar Ca2+ sensitivity. A reduction in contractility has been attributed to the involvement of the Gi protein/protein kinase G pathway or to the activation of protein kinase C without an increase in intracellular Ca2+ concentration or in myofibrillar Ca2+ sensitivity. The chronic effect of ETs on the myocardium results in hypertrophy and prevention of apoptosis, two processes that are together responsible for the contradictory effect of ETs in heart failure.

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.

Synergistic activation of salmon cardiac function by endothelin and beta-adrenergic stimulation

The aim was to find out the effects of endothelin-1 (ET-1) in salmon (Salmo salar) cardiac contractile and endocrine function and its possible interaction with beta-adrenergic regulation. We found that ET-1 has a positive inotropic effect in salmon heart. ET-1 (30 nM) increased the contraction amplitude 17+/-4.7% compared with the basal level. beta-Adrenergic activation (isoprenaline, 100 nM) increased contraction amplitude 30+/-13.1%, but it did not affect the contractile response to ET-1. ET-1 (10 nM) stimulated the secretion of salmon cardiac natriuretic peptide (sCP) from isolated salmon ventricle (3.3+/-0.14-fold compared with control) but did not have any effect on ventricular sCP mRNA. Isoprenaline alone (0.1-1,000 nM) did not stimulate sCP release, but ET-1 (10 nM) together with isoprenaline (0.1 nM) caused a significantly greater increase of sCP release than ET-1 alone (5.4+/-0.07 vs. 3.3+/-0.14 times increase compared with control). The effects on the contractile and secretory function could be inhibited by a selective ETA-receptor antagonist BQ-610 (1 microM), whereas ETB-receptor blockage (by 100 nM BQ-788) enhanced the secretory response. Thus ET-1 is a phylogenetically conserved regulator of cardiac function, which has synergistic action with beta-adrenergic stimulation. The modulatory effects of ET-1 may therefore be especially important in situations with high beta-adrenergic tone.

Cardiovascular endothelins: essential regulators of cardiovascular homeostasis

The endothelin (ET) system consists of 3 ET isopeptides, several isoforms of activating peptidases, and 2 G-protein-coupled receptors, ETA and ETB, that are linked to multiple signaling pathways. In the cardiovascular system, the components of the ET family are expressed in several tissues, notably the vascular endothelium, smooth muscle cells, and cardiomyocytes. There is general agreement that ETs play important physiological roles in the regulation of normal cardiovascular function, and excessive generation of ET isopeptides has been linked to major cardiovascular pathologies, including hypertension and heart failure. However, several recent clinical trials with ET receptor antagonists were disappointing. In the present review, the authors take the stance that ETs are mainly and foremost essential regulators of cardiovascular function, hence that antagonizing normal ET actions, even in patients, will potentially do more harm than good. To support this notion, we describe the predominant roles of ETs in blood vessels, which are (indirect) vasodilatation and ET clearance from plasma and interstitial spaces, against the background of the subcellular mechanisms mediating these effects. Furthermore, important roles of ETs in regulating and adapting heart functions to different needs are addressed, including recent progress in understanding the effects of ETs on diastolic function, adaptations to changes in preload, and the interactions between endocardial-derived ET-1 and myocardial pump function. Finally, the potential dangers (and gains) resulting from the suppression of excessive generation or activity of ETs occurring in some cardiovascular pathological states, such as hypertension, myocardial ischemia, and heart failure, are discussed.

Flow-independent myocardial ischemia induced by endothelin-1: an NADH fluorescence analysis

The endothelin-1 (ET-1) is known to cause myocardial ischemia; however, whether this effect is entirely dependent on vasoconstriction is uncertain. The aim of this study was to characterize the myocardial ischemia after the intracoronary administration of endothelin-1, and compare it with that induced by coronary stenosis. In the left anterior descending coronary artery of 15 dogs, a mild inflow reduction (30%) was produced for 10 minutes using intracoronary endothelin-1 (46 +/- 33 pmol/min) or coronary stenosis. The hearts were rapidly cross-sectioned at short axial plane and freeze-clamped within 120 milliseconds using a specially developed device to visualize and quantify the area of ischemia (%IA) with NADH fluorescence photography. The %IA was larger in the endothelin-1 group than in the stenosis group (66 +/- 23 versus 18 +/- 18, P = 0.0005); furthermore, the ischemia was transmural in the ET-1 group, but limited to subendocardium in the stenosis group. ET-1 increased the coronary arterial resistance especially in subepicardial region and produced smaller ischemic foci in microcirculation. The mechanism of larger ischemia produced by ET-1 might depend on pro-ischemic effects on myocytes and vasoconstriction of the coronary microcirculation, predominantly in the subepicardium in vivo.

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.

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.

Calmodulin kinase II inhibition protects against structural heart disease

Beta-adrenergic receptor (betaAR) stimulation increases cytosolic Ca(2+) to physiologically augment cardiac contraction, whereas excessive betaAR activation causes adverse cardiac remodeling, including myocardial hypertrophy, dilation and dysfunction, in individuals with myocardial infarction. The Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) is a recently identified downstream element of the betaAR-initiated signaling cascade that is linked to pathological myocardial remodeling and to regulation of key proteins involved in cardiac excitation-contraction coupling. We developed a genetic mouse model of cardiac CaMKII inhibition to test the role of CaMKII in betaAR signaling in vivo. Here we show CaMKII inhibition substantially prevented maladaptive remodeling from excessive betaAR stimulation and myocardial infarction, and induced balanced changes in excitation-contraction coupling that preserved baseline and betaAR-stimulated physiological increases in cardiac function. These findings mark CaMKII as a determinant of clinically important heart disease phenotypes, and suggest CaMKII inhibition can be a highly selective approach for targeting adverse myocardial remodeling linked to betaAR signaling.

Receptor subtypes mediating the inotropic effects and Ca(2+) signaling induced by endothelin-1 through crosstalk with norepinephrine in canine ventricular myocardium

In canine ventricular myocardium, endothelin-1 (ET-1) alone induced only a weak transient negative inotropic effect (NIE). However, ET-1 induced a marked sustained positive inotropic effect (PIE) subsequent to a transient NIE in the presence of norepinephrine (NE) at low concentrations (0.1 - 1 nM) and elicited a pronounced sustained NIE in the presence of NE at high concentrations (around 100 nM). Thus, the extent of beta-adrenoceptor stimulation induced by NE played a crucial role in determining the characteristics of the inotropic effects of ET-1. The characteristics of ET receptor subtypes involved in contractile regulation and Ca(2+) signaling induced by ET-1 were determined. The ET-1-induced transient NIE and decrease in Ca(2+) transients were abolished by the selective ET(A)-receptor antagonist FR319317, but not by the selective ET(B)-receptor antagonist BQ-788. The sustained PIE and the increase in Ca(2+) transients induced by ET-1 were abolished by FR319317, but not inhibited by BQ-788. In contrast, the sustained NIE of ET-1 was abolished by the non-selective ET antagonist TAK-044, markedly attenuated by FR319317, and partially inhibited by BQ-788. ET-1 alone elicited a PIE in the presence of BQ-788, which indicates that the activation of ET(B)-receptors counteracts the development of the PIE of ET-1. The current findings indicate that both ET(A) and ET(B) receptors are involved in the regulation of Ca(2+) signaling and contractility in canine ventricular myocardium.

Cross-talk between beta(1)-adrenoceptors and ET(A) receptors in modulation of the slow component of delayed rectifier K(+) currents

Delayed rectifier K(+) currents (I(K)) play a critical role in determining cardiac action potential duration (APD). Modulation of I(K) affects cardiac excitability critically. There are three components of cardiac delayed rectifier, and the slowly activating component (I(Ks)) is influenced strongly by a variety of stimuli. Plasma levels of noradrenaline and endothelin are elevated in heart failure, and arrhythmias are promoted by such humoral abnormalities through modulation of ion channels. It has been reported that protein kinase A (PKA) and protein kinase C (PKC) modulate I(Ks) from human minK in a complex manner. In the present study, we coexpressed human minK with the human beta(1)-adrenoceptor (hbeta(1)AR) and the endothelin receptor subtype A (hET(A)R) in Xenopus oocytes and investigated the effects of receptor activation on the currents (I(Ks)) flowing through the oocytes. ET-1 modulated I(Ks) biphasically: a transient increase followed by a decrease. The PKC inhibitor chelerythrine completely inhibited the effects of ET-1. Intracellular EGTA abolished the transient increase by ET-1 and partially inhibited the subsequent decrease in the currents. When I(Ks) was increased by 10(-6) M isoproterenol (ISO), ET-1 did not increase but rather decreased the current to an even greater extent than under control conditions. In addition, the effects of ISO on I(Ks) were suppressed by ET(A)R stimulation. These data indicate that I(Ks) can be regulated by cross-talk between the ET(A)R and beta(1)AR systems in addition to direct regulation by each receptor system.

Wortmannin inhibits the myofilament Ca2+ sensitization induced by endothelin-1

Endothelin-1 induces a positive inotropic effect due to a combination of an increase in Ca2+ transients and myofilament Ca2+ sensitivity in rabbit ventricular myocardium. We carried out the experiments to examine the potential contribution of myosin light chain kinase to the Ca2+ sensitization induced by endothelin-1 by use of wortmannin that inhibits myosin light chain kinase at high concentrations (IC50=200 nM). Wortmannin at 3 microM suppressed the basal force of contraction, but did not affect the positive inotropic effect mediated by beta-adrenoceptors. Wortmannin at 1 and 3 microM markedly inhibited the positive inotropic effect of endothelin-1, but did not affect the increase in Ca2+ transients elicited by endothelin-1. The present findings imply that the increase in myofilament Ca2+ sensitivity induced by endothelin-1 may be in part due to activation of myosin light chain kinase in rabbit ventricular myocardium.

Differential inotropic effects of endothelin-1, angiotensin II, and phenylephrine induced by crosstalk with cAMP-mediated signaling process in dog ventricular myocardium

Endothelin-1 (ET-1), angiotensin II (Ang II), and phenylephrine, an alpha1-adrenoceptor agonist, share the common signaling process, resulting in activation of Gq protein-coupled receptor (GqPCR) to activate the hydrolysis of phosphoinositide (PI). They do not elicit any inotropic effect in isolated dog ventricular muscle. In the presence of forskolin or IBMX (3-isobutyl-1-methylxanthine), ET-1 produced a dual effect, that is, a positive inotropic effect (PIE) and/or a negative inotropic effect (NIE) depending on concentrations of forskolin or IBMX present simultaneously with ET-1. Phenylephrine produced a definite PIE and Ang II induced a small and transient PIE in the presence of forskolin or IBMX, but they did not elicit a NIE. Facilitation of Ca2+ influx via L-type Ca2+ channel may play a crucial role in the crosstalk because GqPCR agonists produced, likewise a PIE in the presence of Bay k 8644. GqPCR agonists failed to induce a PIE in the presence of dihydroouabain or elevated [Ca2+]o. These findings indicate that the accumulation of cAMP or activation of L-type Ca2+ channels markedly modulates the inotropic response to GqPCR agonists in a manner that leads to a PIE in dog ventricular myocardium. In addition, ET-1, but not Ang II or phenylephrine, activates the signal transduction process that results in a NIE.