Effect of non-selective endothelin blockade, TAK-044, on the ischemic cellular injury of rat heart

Endothelin-1 stimulates the expression of L-type Ca2+ channels in neonatal rat cardiomyocytes via the extracellular signal-regulated kinase 1/2 pathway

The cardiac L-type Ca(2+) channel current (I(Ca,L)) plays an important role in controlling both cardiac excitability and excitation-contraction coupling and is involved in the electrical remodeling during postnatal heart development and cardiac hypertrophy. However, the possible role of endothelin-1 (ET-1) in the electrical remodeling of postnatal and diseased hearts remains unclear. Therefore, the present study was designed to investigate the transcriptional regulation of I(Ca,L) mediated by ET-1 in neonatal rat ventricular myocytes using the whole-cell patch-clamp technique, quantitative RT-PCR and Western blotting. Furthermore, we determined whether the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway is involved. ET-1 increased I(Ca,L) density without altering its voltage dependence of activation and inactivation. In line with the absence of functional changes, ET-1 increased L-type Ca(2+) channel pore-forming α1C-subunit mRNA and protein levels without affecting the mRNA expression of auxiliary β- and α2/δ-subunits. Furthermore, an actinomycin D chase experiment revealed that ET-1 did not alter α1C-subunit mRNA stability. These effects of ET-1 were inhibited by the ETA receptor antagonist BQ-123 but not the ETB receptor antagonist BQ-788. Moreover, the effects of ET-1 on I(Ca,L) and α1C-subunit expression were abolished by the ERK1/2 inhibitor (PD98059) but not by the p38 MAPK inhibitor (SB203580) or the c-Jun N-terminal kinase inhibitor (SP600125). These findings indicate that ET-1 increased the transcription of L-type Ca(2+) channel in cardiomyocytes via activation of ERK1/2 through the ETA receptor, which may contribute to the electrical remodeling of heart during postnatal development and cardiac hypertrophy.

Antagonism of endothelin-1 inhibits hypoxia-induced apoptosis in cardiomyocytes

Apoptosis is well documented to be a common feature of many pathological processes of the heart. Exogenous endothelin-1 (ET-1) has been shown to be proapoptotic or antiapoptotic, depending on ET-1 concentration, cell type, and the ratio of ETA/ETB receptor subtypes. The role of endogenous ET-1 in cardiomyocyte apoptosis, however, is not clarified. This study observed the effects of the ETA-receptor antagonists BQ610 and BQ123 and the ETB-receptor antagonist BQ788 on hypoxia-induced apoptosis in primary cultured neonatal rat cardiomyocytes. Hypoxic apoptosis was induced by incubating cardiomyocytes in serum-free medium under 3% O2 and 5% CO2 for 24 h and evaluated by TUNEL analysis and flow cytometry. TUNEL analysis showed that the apoptotic cardiomyocytes constituted 24.2% +/- 2.2% of the total cells under hypoxic conditions. Treatment with BQ610 (5 micromol/L) significantly reduced the apoptosis rate to 13.2% +/- 3.7% (data from 4 independent experiments, p < 0.01 vs. hypoxia). Flow cytometry showed that the percentage of apoptotic cells positively stained with annexin V and propidium iodide was 42.76% +/- 4.44% (n = 12) in cultures subjected to hypoxia. BQ123 at 0.04, 0.2, and 1.0 micromol/L dose-dependently reduced the apoptosis rate to 34.00% +/- 10.35% (n = 6, p < 0.05), 30.38% +/- 8.28% (n = 6, p < 0.01), and 22.89% +/- 4.19% (n = 6, p < 0.01), respectively. In contrast, BQ788 did not affect hypoxic apoptosis. These findings suggested that endogenous ET-1 contributed to hypoxia-induced apoptosis in cultured cardiomyocytes, which was mediated by ETA receptors, but not by ETB receptors.

Effects of dual endothelin receptor blockade on sympathetic activation and arrhythmogenesis during acute myocardial infarction in rats

The effects of dual (ETA and ETB) endothelin receptor blockade on ventricular arrhythmogenesis during acute myocardial infarction are not well defined. We randomly allocated Wistar rats to bosentan (100 mg/kg daily, n=24), a dual endothelin receptor antagonist, or vehicle (n=23). After 7 days of treatment, myocardial infarction was induced by permanent coronary ligation. Ventricular tachyarrhythmias were evaluated for 24 h following ligation, using a miniature telemetry electrocardiogram recorder. Action potential duration was measured from monophasic epicardial recordings and sympathetic activation was assessed by heart rate variability and catecholamine serum level measurements. Compared to controls (1012+/-185 s), bosentan (59+/-24 s) markedly decreased (P<0.00001) the total duration of ventricular tachyarrhythmias during the delayed (1-24 h) phase post-ligation, with a modest effect during the early (0-1 h) phase (132+/-38 s, versus 43+/-18 s, respectively, P=0.053). Treatment did not affect infarct size or total mortality. Action potential duration at 90% repolarization prolonged in controls (from 93.1+/-4.7 ms to 117.6+/-6.9 ms), displaying increased temporal dispersion (from 4.14+/-0.45 ms to 10.42+/-2.51 ms, both P<0.001), but was preserved in treated animals. Bosentan decreased norepinephrine, but increased epinephrine levels 24 h post-ligation. Low frequency spectra of heart rate variability, an index of net sympathetic tone, were lower in bosentan-treated rats. Dual endothelin-1 receptor blockade decreases ventricular tachyarrhythmias during myocardial infarction without reperfusion, by preventing repolarization inhomogeneity. Diverse treatment effects on sympathetic activation may ameliorate the antiarrhythmic action.

Endothelin receptors, localized in sympathetic nerve terminals of the heart, modulate norepinephrine release and reperfusion arrhythmias

Endothelin (ET)-1 is an endogenous vasoconstrictor which modulates norepinephrine (NE) release in myocardial ischemia reperfusion. Recent studies have demonstrated the pro- or anti-arrhythmic effects in reperfusion. The present studies were undertaken to test the hypothesis that ET receptors located in sympathetic nerve terminals modulate NE release associated with reperfusion arrhythmias (ventricular fibrillation; VF). Immunohistochemical studies showed that both ETA and ETB receptors exist in the sympathetic nerve varicosities, which were stained positive for tyrosine hydroxylase (TH) in the left ventricular wall in guinea pigs. Isolated guinea pig hearts were subjected to 20 min of normothermic global ischemia followed by 30 min reperfusion. Exogenously applied ET-1 (0.1 and 1 nM) dose-dependently increased NE release and the duration of VF, but these responses were significantly suppressed with the Na(+)/H(+) exchanger inhibitor, 5-(N-ethyl-N-isopropyl)-amiloride (10 microM). The ETA receptor antagonist (BQ123, 1 microM) and nonselective ET receptor antagonist (PD142893, 1 microM) significantly attenuated NE release and VF, whereas the ETB receptor antagonist (BQ788,300 nM) markedly elevated NE release but did not affect VF. These studies provide the first evidence that both ETA and ETB receptors, located in the sympathetic nerve varicosities, modulate NE release, at least in part, in association with reperfusion arrhythmias.

The therapeutic potential of endothelin-1 receptor antagonists and endothelin-converting enzyme inhibitors on the cardiovascular system

Clinical trials have established bosentan, an orally active non-selective endothelin (ET) receptor antagonist, as a beneficial treatment in pulmonary hypertension. Trials have also shown short-term benefits of bosentan in systemic hypertension and congestive heart failure. However, bosentan also increased plasma levels of ET-1, probably by inhibiting the clearance of ET-1 by endothelin type B (ET(B)) receptors, and this may mean its effectiveness is reduced with long-term clinical use. Preliminary data suggests that selective endothelin type A (ET(A)) receptor antagonists (BQ-123, sitaxsentan) may be more beneficial than the non-selective ET receptor antagonists in heart failure, especially when the failure is associated with pulmonary hypertension. Experimental evidence in animal disease models suggests that non-selective ET or selective ET(A) receptor antagonism may have a role in the treatment of atherosclerosis, restenosis, myocarditis, shock and portal hypertension. In animal models of myocardial infarction and/or reperfusion injury, non-selective ET or selective ET(A) receptor antagonists have beneficial or detrimental effects depending on the conditions and agents used. Thus clinical trials of the non-selective ET or selective ET(A) receptor antagonists in these conditions are not presently warranted. Several selective endothelin-converting enzyme inhibitors have been synthesised recently, and these are only beginning to be tested in animal models of cardiovascular disease, and thus the clinical potential of these inhibitors is still to be defined.

Endothelin receptor antagonism attenuates cardiomyocyte apoptosis after induction of ischemia in rats

OBJECTIVE

Apoptosis has recently been implicated as a process that may contribute to loss of cardiomyocytes and to adverse myocardial remodeling in congestive heart failure (CHF). We investigated to what extent myocardial ischemia and CHF lead to induction of apoptotic gene programs and loss of cardiomyocytes through apoptosis, and subsequently to what extent the beneficial effects of endothelin (ET) receptor antagonism after myocardial infarction (MI) could be attributed to reduction of apoptotic cell loss in the myocardium.

DESIGN

Northern blot analysis, analysis of DNA fragmentation, and immunohistochemical analysis were performed after induction of MI in rats.

RESULTS

After induction of MI, the mRNA levels of the pro-apoptotic genes FAS, BAX, P53, and CASPASE-1 were significantly increased in the non-ischemic region of the left ventricle (LV) with highest levels of expression in the peri-infarct area. High levels of FAS, BAX, P53, and CASPASE-1 mRNA were also observed in the infarcted region. Concomitantly, numerous TUNEL-positive cells and internucleosomal degradation of DNA were found in tissue from the peri-infarct area and in the infarcted zone, indicating apoptotic cell death. Treatment with bosentan, a mixed ET(A)/ET(B) receptor antagonist, during the first 24 h after induction of MI significantly reduced the area of TUNEL-positive myocardium in the ischemic region (64 +/- 2% of LV circumference in the vehicle group vs 55 +/- 2% of LV circumference in the bosentan group, p < 0.05). Consistently, bosentan also caused a similar reduction of infarct size.

CONCLUSION

Our data demonstrate activation of pro-apoptotic genes and provide evidence of cardiomyocyte apoptosis in the viable peri-infarct area and in the infarcted region after MI in rats. Intervention with bosentan may attenuate cardiomyocyte cell loss through apoptosis in the area at risk after induction of ischemia.

Role of endothelin in cardiovascular disease

Endothelins are a family of peptides, which comprises endothelin-1 (ET-1), endothelin-2 (ET-2) and endothelin-3 (ET-3), each containing 21 amino-acids. ET-1 is a peptide secreted mostly by vascular endothelial cells, the predominant isoform expressed in vasculature and the most potent vasoconstrictor currently known. ET-1 also has inotropic, chemotactic and mitogenic properties. In addition, it influences salt and water homeostasis through its effects on the renin-angiotensin-aldosterone system (RAAS), vasopressin and atrial natriuretic peptide and stimulates the sympathetic nervous system. The overall action of endothelin is to increase blood pressure and vascular tone. Therefore, endothelin antagonists may play an important role in the treatment of cardiac, vascular and renal diseases associated with regional or systemic vasoconstriction and cell proliferation, such as essential hypertension, pulmonary hypertension, chronic heart failure and chronic renal failure. Long-term anti-endothelin therapy may improve symptoms and favourably alter the progression of heart failure. Endothelin appears to participate in induction and progression of sclerotic renal changes, leading to progression to end-stage renal disease. Anti-endothelin therapy might offer additional benefits in the prevention of progression of chronic renal failure in addition to the known benefits of RAAS inhibition. Clinical trials have demonstrated potentially important benefits of endothelin antagonists for patients with essential hypertension, pulmonary hypertension and heart failure. Further studies are necessary to determine the role of anti-endothelin therapy in the treatment of cardiovascular diseases and determine the different roles of selective receptor antagonism vs. mixed ET(A/B)-receptor antagonism in human diseases.

Potential role of endothelin receptor antagonists in the setting of cardiopulmonary bypass: relevance to myocardial performance

The ET system is activated in cardiac surgical setting as evidenced by elevated systemic and myocardial ET-1 levels after coronary bypass grafting surgery which requires hypothermic cardioplegic arrest and cardiopulmonary bypass. Increased ET-1 may influence a number of clinical parameters in this setting. First, ET-1 may directly modulate myocardial contractile performance in the early postoperative period resulting in LV dysfunction and a complex postoperative course. Second, elevated ET-1 levels may exacerbate increased pulmonary vascular resistance and contribute to the development of transient pulmonary hypertension following bypass. Finally, augmented postoperative ET-1 levels could contribute to changes in the caliber and flow of vascular conduits used for coronary bypass. In this review, a current perspective on the ET system in the setting of cardiopulmonary bypass grafting surgery is provided and the potential use of ET receptor antagonists in this setting is discussed.

Effects of an endothelin receptor antagonist TAK-044 on myocardial energy metabolism in ischemia/reperfused rat hearts

The purpose of this study was to investigate the effects of an endothelin-receptor antagonist TAK-044 on functional defects and metabolic derangement in myocardial ischemia/reperfusion injury. We sequentially measured high-energy phosphate metabolites and intracellular pH by phosphorus magnetic resonance spectroscopy during 35-min global ischemia followed by 60-min reperfusion in Langendorff-perfused rat hearts. TAK-044 (initial loading by 3 mg/kg followed by perfusion with 100 nM solution) was administered in two different ways: before ischemia or immediately after reperfusion. In addition, we investigated the effects of TAK-044 on functional defects and metabolic alterations induced by hydrogen peroxide (200 microM, 30 min). The recoveries of left ventricular developed pressure after reperfusion in TAK-044 groups (51 +/-12% in TAK-I, 61 +/- 12% in TAK-R) were better than in control (10 +/- 5% in control; p < 0.01). Increases in left ventricular end-diastolic pressure (LVEDP) in TAK-044 groups (22 +/- 5 mm Hg in TAK-I, 24 +/- 5 mm Hg in TAK-R) were less than in control (38 +/- 3 mm Hg; p < 0.01). Adenosine triphosphate (ATP) (33 +/- 5% in TAK-I, 28 +/- 4% in TAK-R) in TAK-044 groups were higher than in control (13 +/- 3%; p < 0.01). The creatine phosphokinase (CPK) release during reperfusion in TAK-044 groups (3.3 +/- 1.5 IU/g wet wt/60 min in TAK-I, 3.5 +/- 2.5 IU/g wet wt/60 min in TAK-R) were lower than in control (13.8 +/- 3.9 IU/g wet wt/60 min; p < 0.05). In contrast, TAK-044 did not attenuate the myocardial injury induced by hydrogen peroxide. TAK-044, even if administered simultaneous with coronary reperfusion, attenuated myocardial ischemia/ reperfusion injury. The energy-preservative effect of TAK-044 could be associated with the good functional recovery in ischemia/reperfused rat hearts.