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

Cardiovascular Effects of Snake Toxins: Cardiotoxicity and Cardioprotection

Snake venoms, as complex mixtures of peptides and proteins, affect various vital systems of the organism. One of the main targets of the toxic components from snake venoms is the cardiovascular system. Venom proteins and peptides can act in different ways, exhibiting either cardiotoxic or cardioprotective effects. The principal classes of these compounds are cobra cardiotoxins, phospholipases A2, and natriuretic, as well as bradykinin-potentiating peptides. There is another group of proteins capable of enhancing angiogenesis, which include, e.g., vascular endothelial growth factors possessing hypotensive and cardioprotective activities. Venom proteins and peptides exhibiting cardiotropic and vasoactive effects are promising candidates for the design of new drugs capable of preventing or constricting the development of pathological processes in cardiovascular diseases, which are currently the leading cause of death worldwide. For example, a bradykinin-potentiating peptide from Bothrops jararaca snake venom was the first snake venom compound used to create the widely used antihypertensive drugs captopril and enalapril. In this paper, we review the current state of research on snake venom components affecting the cardiovascular system and analyse the mechanisms of physiological action of these toxins and the prospects for their medical application.

Protective effects of endothelin receptor A and B inhibitors against doxorubicin-induced cardiomyopathy

The clinical efficiency of the highly potent antitumor agent doxorubicin is limited by cardiotoxic effects. In a murine doxorubicin cardiotoxicity model, increased endothelin-1 (ET-1) expression and cardioprotective effects of the dual ET-1 blocker bosentan were demonstrated. To date it is unclear if combined blocking of endothelin A/B receptors is necessary or whether selective inhibition of one of the ET-1 receptors is sufficient for the observed cardioprotection. Therefore, we investigated the impact of dual (bosentan) and single endothelin receptor antagonism through sitaxentan (receptor A blocker) or BQ788 (receptor B blocker) in a murine doxorubicin cardiotoxicity model (C57BL/6N). Simultaneous administration of each endothelin receptor antagonist (ERA) with doxorubicin resulted in a significantly improved hemodynamic performance in comparison to the impaired cardiac function in control mice with bosentan being most effective but closely followed by sitaxentan and also BQ788. This cardioprotection was not caused by diminished doxorubicin levels in heart since the doxorubicin content in cardiac tissue was not altered by ERAs significantly. However, whole transcript expression profiling showed partly different effects of the ERAs on doxorubicin-modulated cardiac gene expression of genes involved in signal transduction (e.g. Stat3, Pim1, Akt1, Plcb2), fibrosis (e.g. Myl4), energy production (e.g. Ant1) or oxidative stress (e.g. Aox1). Furthermore, doxorubicin-mediated gene regulations were verified in the murine cardiomyocyte model HL-1 showing partly reversed expression patterns after co-administration of the ERAs. In summary, our results demonstrate strong cardioprotective effects of blocking ET-1 receptors against the doxorubicin-related cardiomyopathy and provide evidence to potential underlying signaling pathways.

Ouabain stimulates atrial natriuretic peptide secretion via the endothelin-1/ET(B) receptor-mediated pathway in beating rabbit atria

AIMS

Ouabain has been reported to increase the secretion of atrial natriuretic peptide (ANP) in vitro. However, the mechanism by which ouabain increases ANP secretion is not well known. Therefore, the purpose of the present study was to investigate the underlying mechanism of ouabain-stimulated ANP secretion.

MAIN METHODS

A perfused beating rabbit atrial model was used. The ANP and ET-1 levels in the atrial perfusates were measured by radioimmunoassays.

KEY FINDINGS

Ouabain (1.0, 3.0 and 6.0 μmol/L) significantly increased atrial ANP secretion in a dose-dependent manner, while the endothelin (ET)-1 levels were increased by the higher doses (3.0 and 6.0 μmol/L) of ouabain. Ouabain-increased atrial ET-1 release was blocked by PD98059 (30.0 μmol/L), an inhibitor of mitogen-activated protein kinase (MAPK). Nifedipine (1.0 μmol/L), an inhibitor of L-type Ca(2+) channels, completely abolished ouabain-increased ANP secretion without changing the ouabain-induced atrial dynamics. KB-R7943 (3.0 μmol/L), an inhibitor of Na(+)-Ca(2+) exchangers, completely blocked the effects of ouabain-increased atrial dynamics, but did not modulate ouabain-increased ANP secretion. ET-1 significantly stimulated atrial ANP release in a dose-dependent manner. The effects of ET-1 and ouabain on ANP secretion were completely blocked by BQ788 (0.3 μmol/L), an inhibitor of ET-1 type B (ET(B)) receptors, but not by BQ123 (0.3 μM), an inhibitor of ET-1 type A receptors. Ouabain-increased atrial ANP secretion was blocked by PD98059 and indomethacin (30.0 μmol/L), an inhibitor of cyclooxygenase.

SIGNIFICANCE

Ouabain significantly stimulated atrial ANP secretion via an ET-1-ET(B) receptor-mediated pathway involving MAPK signaling pathway activation and prostaglandin formation.

Intermedin elicits a negative inotropic effect in rat papillary muscles mediated by endothelial-derived nitric oxide

Intermedin (IMD) is a novel vasoactive peptide from the calcitonin gene-related peptide (CGRP) implicated in cardiac regulation, yet the contractile effects of IMD remain controversial, since previous studies in vivo and isolated cardiomyocytes documented contradictory results. We hypothesized cardiac endothelial cells involvement in IMD modulation of cardiac function as an explanation for these opposing observations. With this in mind, we investigated the direct action of increasing concentrations of IMD (10(-8) to 10(-6)M) on myocardial performance parameters in rat left ventricular (LV) papillary muscles with and without endocardial endothelium (EE) and in presence of receptor antagonists and intracellular pathways inhibitors. In LV papillary muscles with intact EE, IMD induced a concentration-dependent negative inotropic action (%decrease relative to baseline, at IMD concentration of 10(-6)M, active tension of 14 ± 4%, and maximum velocity of tension rise of 10 ± 4%). These effects were blunted by EE removal, AM receptor antagonist (AM(22-52)), and CGRP receptor antagonist (CGRP(8-37)). Additionally, nitric oxide (NO) synthase inhibition with N(G)-nitro-l-arginine (l-NAME) in muscles with and without EE and guanylyl cyclase inhibition with {1H-[1,2,4]oxadiazole-[4,4-a]-quinoxalin-1-one} not only blunted the negative inotropic action of IMD but also unmasked IMD-positive inotropic effect dependent on CGRP receptor PKA activation. Western blot quantification of phosphorylated cardiac troponin I (P-cTnI) in IMD-treated papillary muscles revealed a significant increase in P-cTnI when compared with untreated muscles, while in l-NAME-pretreated papillary muscles IMD failed to increase P-cTnI. Finally, we found that stimulation of both EE and microvascular endothelial cells with IMD significantly increased NO production by 40 ± 3 and 38 ± 3%, respectively, suggesting the role of cardiac endothelial cells in NO production upon IMD stimulation. Our findings establish IMD negative inotropic effect in isolated myocardium due to NO/cGMP pathway activation with concomitant thin myofilament desensitization by increase in cTnI phosphorylation and provide a coherent explanation for the previously reported contradictory results.

Modulation of myocardial stiffness by β-adrenergic stimulation--its role in normal and failing heart

The acute effects of beta-adrenergic stimulation on myocardial stiffness were evaluated. New-Zealand white rabbits were treated with saline (control group) or doxorubicin to induce heart failure (HF) (DOXO-HF group). Effects of isoprenaline (10(-10)-10(-5) M), a non-selective beta-adrenergic agonist, were tested in papillary muscles from both groups. In the control group, the effects of isoprenaline were also evaluated in the presence of a damaged endocardial endothelium, atenolol (beta(1)-adrenoceptor antagonist), ICI-118551 (beta(2)-adrenoceptor antagonist), KT-5720 (PKA inhibitor), L-NNA (NO-synthase inhibitor), or indomethacin (cyclooxygenase inhibitor). Passive length-tension relations were constructed before and after adding isoprenaline (10(-5) M). In the control group, isoprenaline increased resting muscle length up to 1.017+/-0.006 L/L(max). Correction of resting muscle length to its initial value resulted in a 28.5+/-3.1 % decrease of resting tension, indicating decreased muscle stiffness, as confirmed by the isoprenaline-induced right-downward shift of the passive length-tension relation. These effects were modulated by beta(1)- and beta(2)-adrenoceptors and PKA. In DOXO-HF group, the effect on myocardial stiffness was significantly decreased. We conclude that beta-adrenergic stimulation is a relevant mechanism of acute neurohumoral modulation of the diastolic function. Furthermore, this study clarifies the mechanisms by which myocardial stiffness is decreased.

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.

Acute modulation of myocardial function by angiotensin 1-7

Angiotensin 1-7 is a bioactive heptapeptide of the renin-angiotensin system. Its cardiovascular actions have recently acquired growing relevance, mainly due to its counter-regulatory actions in the angiotensin cascade. The aim of the present study was to evaluate the actions of angiotensin 1-7 on myocardial function. Increasing concentrations of angiotensin 1-7 (10(-9) to 10(-5)M) were added to rabbit right papillary muscles: (1) in baseline conditions with intact endocardial endothelium (EE); (2) after selective removal of the EE with Triton X-100 (1s, 0.01%); (3) with intact EE in the presence of the Mas receptor antagonist A-779, the AT(1) receptor antagonist ZD-7155, the AT(2) receptor antagonist PD-123,319 or the nitric oxide synthesis inhibitor NG-nitro-l-arginine (l-NA). Concerning the effects on contractility, we observed a significant decrease on active tension, dT/dt(max), peak shortening and dL/dt(max) of -10.5+/-3.6%, -8.0+/-3.0%, -5.3+/-2.6% and -5.7+/-2.3%, respectively. There was no change on relaxation parameters, namely dT/dt(min) or dL/dt(min). Time to half relaxation was significantly decreased. The presence of ZD-7155 or PD-123,319 did not change these effects. However, angiotensin 1-7 effects on myocardial properties were abolished after selective EE removal and in the presence of A-779 or l-NA. In conclusion, in this animal species, angiotensin 1-7 through its binding to Mas receptor induces a negative inotropic effect modulated by the EE and nitric oxide and independent of AT(1) or AT(2) receptors activation. As the effects described in the present work were influenced by the endocardial endothelium, they may be disrupted in situations associated to endothelial dysfunction, as in heart failure or myocardial ischemia.

Changes in red cell ion transport, reduced intratumoral neovascularization, and some mild motor function abnormalities accompany targeted disruption of the Mouse Kell gene (Kel)

Kell (ECE-3), a highly polymorphic blood group glycoprotein, displays more than 30 antigens that produce allo-antibodies and, on red blood cells (RBCs), is complexed through a single disulfide bond with the integral membrane protein, XK. XK is a putative membrane transporter whose absence results in a late onset form of neuromuscular abnormalities known as the McLeod syndrome. Although Kell glycoprotein is known to be an endothelin-3-converting enzyme, the full extent of its physiological function is unknown. To study the functions of Kell glycoprotein, we undertook targeted disruption of the murine Kel gene by homologous recombination. RBCs from Kel(-/-) mice lacked Kell glycoprotein, Kell/XK complex, and endothelin-3-converting enzyme activity and had reduced levels of XK. XK mRNA levels in spleen, brain, and testis were unchanged. In Kel(-/-) mice RBC Gardos channel activity was increased and the normal enhancement by endothelin-3 was blunted. Analysis of the microvessels of tumors produced from LL2 cells indicated that the central portion of tumors from wild-type mice were populated with many mature blood vessels, but that vessels in tumors from Kel(-/-) mice were fewer and smaller. The absence of Kell glycoprotein mildly affected some motor activities identified by foot splay on the drop tests. The targeted disruption of Kel in mouse enabled us to identify phenotypes that would not be easily detected in humans lacking Kell glycoprotein. In this regard, the Kell knockout mouse provides a good animal model for the study of normal and/or pathophysiological functions of Kell glycoprotein.

Acute neurohumoral modulation of diastolic function

Diastole plays a central role in cardiovascular homeostasis. Its two main determinants, myocardial relaxation and passive properties of the ventricular wall, are nowadays regarded as physiological mechanisms susceptible of active modulation. Furthermore, diastolic dysfunction and heart failure with normal ejection fraction (previously called diastolic heart failure) are two subjects of major clinical relevance and an intense area of research. The role of several neurohumoral mediators like angiotensin-II and endothelin-1 on the modulation of diastolic function was systematically described as having only chronic deleterious effects such as cardiac hypertrophy and fibrosis. However, over the last years a growing body of evidence described a new role for several peptides on the acute modulation of diastolic function. In the acute setting, some of these mediators may have the potential to induce an adaptive cardiac response. In this review, we describe the role of angiotensin-II, endothelin-1, nitric oxide, urotensin-II and ghrelin on the acute modulation of diastolic function, emphasizing its pathophysiological relevance. Only a thorough understanding of diastolic physiology as well as its active modulation, both in the acute and chronic settings, will improve our knowledge on diastolic dysfunction and allow us to solve the enigmas of heart failure with normal ejection fraction.

Catestatin (chromogranin A344-364) is a novel cardiosuppressive agent: inhibition of isoproterenol and endothelin signaling in the frog heart

The catecholamine release-inhibitory catestatin [Cts; human chromogranin (Cg) A(352-372), bovine CgA(344-364)] is a vasoreactive and anti-hypertensive peptide derived from CgA. Using the isolated avascular frog heart as a bioassay, in which the interactions between the endocardial endothelium and the subjacent myocardium can be studied without the confounding effects of the vascular endothelium, we tested the direct cardiotropic effects of bovine Cts and its interaction with beta-adrenergic (isoproterenol, ISO) and endothelin-1 (ET-1) signaling. Cts dose-dependently decreased stroke volume and stroke work, with a threshold concentration of 11 nM, approaching the in vivo level of the peptide. Cts reduced contractility by inhibiting phosphorylation of phospholamban (PLN). Furthermore, the Cts effect was abolished by pretreatment with either nitric oxide synthase (N(G)-monomethyl-l-arginine) or guanylate cyclase (ODQ) inhibitors, or an ET(B) receptor (ET(BR)) antagonist (BQ-788). Cts also noncompetitively inhibited the positive inotropic action of ISO. In addition, Cts inhibited the positive inotropic effect of ET-1, mediated by ET(A) receptors, and did not alter the negative inotropic ET-1 influence mediated by ET(BR). Cts action through ET(BR) was further suggested when, in the presence of BQ-788, Cts failed to inhibit the positive inotropism of both ISO and ET-1 stimulation and PLN phosphorylation. We concluded that the cardiotropic actions of Cts, including the beta-adrenergic and ET-1 antagonistic effects, support a novel role of this peptide as an autocrine-paracrine modulator of cardiac function, particularly when the stressed heart becomes a preferential target of both adrenergic and ET-1 stimuli.

Upregulation of leptin pathway correlates with abnormal expression of SERCA2a, phospholamban and the endothelin pathway in heart failure and reversal by CPU86017

Emerging evidence indicates that leptin may be a potential new target in chronic heart failure (CHF) treatment. We hypothesized that hyperleptinemia may correlate with abnormal expression of SERCA2a, PLB (phospholamban), and the endothelin (ET) pathway in CHF. An activated ET pathway is involved in CHF that is suppressed by CPU86017 (p-chlorobenzyltetrahydroberberine chloride), a complex class III antiarrhythmic agent with an antioxidant effect. Thus, relief of CHF may be mediated by a reversal of abnormalities of the leptin system, the ET-reactive oxygen species (ROS) pathway, SERCA2a, and PLB by CPU86017. CHF was produced by coronary artery ligation for 6 weeks in rats. The rats were divided into 3 groups: sham, CHF untreated, and CHF+CPU86017 (4 mg/kg per day, s.c.). Hemodynamic changes, cardiac morphology, serum biochemistry, messenger ribonucleic acid (mRNA) and protein expression of the leptin pathway, ET pathway, and redox were measured. In CHF rats, hemodynamic abnormalities, cardiac remodeling, and histological changes with features of cardiac failure were associated with hyperlipidemia accompanied by oxidative stress and upregulated OB-Rb, ECE, pp-ET-1, ET(A)R, and ET(B)R mRNA expression in the myocardium. Protein expression of leptin and ET(A)R in the myocardium was markedly increased in CHF rats. An activated leptin pathway was associated with downregulation of SERCA2a and upregulation of PLB in mRNA and protein expression in CHF. CPU86017 downregulated the leptin system and reversed the above changes in the myocardium. An activated leptin pathway correlates with abnormal expression of SERCA2a and PLB and an activated ET-ROS system in the affected myocardium. The multi-ion-channel-blocking and antioxidative effects of CPU86017 downregulate the leptin pathway and ET system, resulting in reversal of the abnormalities of expression of SERCA2a and PLB and cardiac performance in CHF.

Myocardial dysfunction and neurohumoral activation without remodeling in left ventricle of monocrotaline-induced pulmonary hypertensive rats

In monocrotaline (MCT)-induced pulmonary hypertension (PH), only the right ventricle (RV) endures overload, but both ventricles are exposed to enhanced neuroendocrine stimulation. To assess whether in long-standing PH the left ventricular (LV) myocardium molecular/contractile phenotype can be disturbed, we evaluated myocardial function, histology, and gene expression of autocrine/paracrine systems in rats with severe PH 6 wk after subcutaneous injection of 60 mg/kg MCT. The overloaded RV underwent myocardial hypertrophy ( P < 0.001) and fibrosis ( P = 0.014) as well as increased expression of angiotensin-converting enzyme (ACE) (8-fold; P < 0.001), endothelin-1 (ET-1) (6-fold; P < 0.001), and type B natriuretic peptide (BNP) (15-fold; P < 0.001). Despite the similar upregulation of ET-1 (8-fold; P < 0.001) and overexpression of ACE (4-fold; P < 0.001) without BNP elevation, the nonoverloaded LV myocardium was neither hypertrophic nor fibrotic. LV indexes of contractility ( P < 0.001) and relaxation ( P = 0.03) were abnormal, however, and LV muscle strips from MCT-treated compared with sham rats presented negative ( P = 0.003) force-frequency relationships (FFR). Despite higher ET-1 production, BQ-123 (ETA antagonist) did not alter LV MCT-treated muscle strip contractility distinctly ( P = 0.005) from the negative inotropic effect exerted on shams. Chronic daily therapy with 250 mg/kg bosentan (dual endothelin receptor antagonist) after MCT injection not only attenuated RV hypertrophy and local neuroendocrine activation but also completely reverted FFR of LV muscle strips to positive values. In conclusion, the LV myocardium is altered in advanced MCT-induced PH, undergoing neuroendocrine activation and contractile dysfunction in the absence of hypertrophy or fibrosis. Neuroendocrine mediators, particularly ET-1, may participate in this functional deterioration.

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.