Inotropic effects of endothelin in ferret ventricular myocardium

Calcitonin gene-related peptide protects the myocardium from ischemia induced by endothelin-1: intravital microscopic observation and (31)P-MR spectroscopic studies

AIMS

Calcitonin gene-related peptide (CGRP) is a potent vasodilator neuropeptide. We investigated the ameliorating effect of CGRP in myocardial ischemia induced by endothelin-1 (ET-1), with special emphasis on myocardial microvascular hemodynamics and levels of energy-related metabolites.

MAIN METHODS

The Langendorff preparations of rat isolated heart were perfused at a constant flow rate. Microvascular blood flow was also visualized in the anterior epicardium of the left ventricle by means of an intravital fluorescence microscope system. Energy-related metabolite contents in the myocardium were measured by means of (31)P-magnetic resonance spectroscopy ((31)P-MRS).

KEY FINDINGS

Intracoronary bolus injections of CGRP caused dose-dependent decreases in coronary perfusion pressure (CPP) in the hearts exposed to ET-1 (30 pmol). The vasodilator potency of CGRP was about 10,000-fold greater than that of nitroglycerin and 1,000-fold greater than that of isobutylmethylxanthine. Vasodilation of the small-sized arterioles (10-40 μm in diameter) in response to CGRP (100 pmol) was confirmed by direct microscopic observation. After ET-1 (30 pmol) plus vehicle administration, high energy phosphates (phosphocreatine (PCr), ATP) were markedly reduced (p<0.05). CGRP administration significantly (p<0.05) attenuated the anaerobic changes in the myocardium (decrease in PCr) and macrohemodynamic alterations (increase in CPP, decrease in dP/dt etc.) induced by ET-1.

SIGNIFICANCE

We conclude that CGRP effectively confers hemodynamic and metabolic protections to isolated beating hearts against ET-1-induced myocardial ischemia.

The role of the paracrine/autocrine mediator endothelin-1 in regulation of cardiac contractility and growth

Endothelin-1 (ET-1) is a critical autocrine and paracrine regulator of cardiac physiology and pathology. Produced locally within the myocardium in response to diverse mechanical and neurohormonal stimuli, ET-1 acutely modulates cardiac contractility. During pathological cardiovascular conditions such as ischaemia, left ventricular hypertrophy and heart failure, myocyte expression and activity of the entire ET-1 system is enhanced, allowing the peptide to both initiate and maintain maladaptive cellular responses. Both the acute and chronic effects of ET-1 are dependent on the activation of intracellular signalling pathways, regulated by the inositol-trisphosphate and diacylglycerol produced upon activation of the ET(A) receptor. Subsequent stimulation of protein kinases C and D, calmodulin-dependent kinase II, calcineurin and MAPKs modifies the systolic calcium transient, myofibril function and the activity of transcription factors that coordinate cellular remodelling. The precise nature of the cellular response to ET-1 is governed by the timing, localization and context of such signals, allowing the peptide to regulate both cardiomyocyte physiology and instigate disease.

LINKED ARTICLES

This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Modulation of myocardial contraction by endocardial and coronary vascular endothelium

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

PKC-ε mediates multiple endothelin-1 actions on systolic Ca2+ and contractility in ventricular myocytes

Endothelin-1 (ET-1) induces positive inotropy (enhanced contractility) in cardiac muscle, but establishing underlying cellular mechanisms has been controversial in part because of a growing number of signaling pathways and end effectors targeted by ET-1. Here we present evidence that ET-1 induces positive inotropism in ventricular tissue by increasing both systolic Ca2+ and myofilament Ca2+ sensitivity. To examine the roles of PKC-δ and PKC-ε in these acute responses to ET-1, kinase inactive dominant negative PKC (dn-PKC) constructs were expressed in adult rat ventricular myocytes. Yellow fluorescent protein (YFP) was fused to dn-PKC constructs to visualize expression and localization of dn-PKC in living myocytes. Due to an alanine to glutamate mutation in the pseudosubstrate site, dn-PKCs constitutively translocated to anchoring sites and were unaffected by agonist or phorbol ester treatment. Dn-PKC-δ-YFP mainly distributed at Z-lines and at intercalated disks in adult myocytes, whereas dn-PKC-ε-YFP stained the surface sarcolemma, T-tubules/Z-lines and perinuclear region. Myocytes expressing dn-PKC-δ-YFP showed normal systolic Ca2+ and contractile responses to ET-1. In contrast, the entire ensemble of ET-1 responses was blocked in myocytes expressing dn-PKC-ε-YFP including increased Ca2+ transients, enhanced myofilament Ca2+ sensitivity, and positive inotropy. This report provides direct evidence that PKC-ε is activated early and robustly following ET-1 stimulation and thus mediates multiple intracellular changes underlying the acute actions of ET-1 on myocardium.

The pharmacology of bosentan

This article describes the pharmacological properties and the overall preclinical and clinical profiling of bosentan (Ro 47-0203), a non-peptide endothelin receptor antagonist with oral activity. Bosentan is a combined and competitive antagonist of both ETA and ETB receptors that is selective for the endothelin system. In vitro and in vivo, bosentan potently antagonises the vascular response elicited by the endothelins. Preclinical efficacy is demonstrated in a variety of pathological models including pulmonary and essential hypertension, renal failure of ischaemic and nephrotic origin and cerebral vasospasm following subarachnoid haemorrhage. Effects are particularly marked in experimental models of heart failure (HF) where bosentan acts as a potent vasodilator that improves overall left ventricular performance. After chronic treatment, bosentan also improves survival in rats with HF. As a result of the first encouraging clinical results that show pulmonary and systemic vasodilation, long-term studies are ongoing in the treatment of congestive heart failure (CHF).

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.

ET-1 increases distensibility of acutely loaded myocardium: a novel ETA and Na+/H+ exchanger-mediated effect

This study investigated, in rabbit papillary muscles (n = 61) and human auricular strips (n = 7), effects of endothelin-1 (ET-1; 0.1-10 nM) on diastolic myocardial properties. ET-1 (1 nM) was also given in the presence of selective ET(A) or ET(B) antagonism, nonselective ET(A)/ET(B) antagonism, and Na(+)/H(+) exchanger inhibition. Effects of 6.3 mM Ca(2+) were also studied. ET-1 dose dependently increased inotropism. In contrast to baseline, in the presence of ET-1, resting tension (RT) decreased, after an isometric twitch, 3.4 +/- 1.4, 6.9 +/- 1.5, and 12.5 +/- 3.1% with 0.1, 1, and 10 nM, respectively, reflecting an increase in myocardial distensibility. ET-1 effects were abolished with selective ET(A) as well as with nonselective ET(A)/ET(B) antagonism, whereas they were still present with ET(B) antagonism. Na(+)/H(+) exchanger inhibition abolished ET-1 effects on distensibility, whereas it only partially inhibited positive inotropic effect. Ca(2+) increased inotropism to a similar extent to ET-1 (1 nM) but did not affect distensibility. ET-1 therefore increased diastolic distensibility of acutely loaded human and nonhuman myocardium. This effect is mediated by ET(A) receptors, requires Na(+)/H(+) exchanger activation, and cannot be elicited by Ca(2+).

Endothelin-1 stimulates cardiomyocyte injury during mitochondrial dysfunction in culture

To understand the pathophysiological role of endothelin-1 in the failing heart, we constructed a cellular mitochondrial impairment model and demonstrated the effect of endothelin-1. Primary cultured cardiomyocytes from neonatal rats were pretreated with rotenone, a mitochondrial complex I inhibitor, and the cytotoxic effect of endothelin-1 on the cardiomyocytes was demonstrated. Rotenone gradually decreased the pH of the culture medium with incubation time and caused slight cell injury. Endothelin-1 markedly enhanced the effect of rotenone that decreased the pH of the medium and enhanced cellular injury. The enhancement of the decrease in pH and cell injury induced by endothelin-1 was counteracted by the endothelin ET(A) receptor antagonist BQ123 or by maintaining the pH of the medium by the addition of 50 mM HEPES. Endothelin-1 markedly increased the uptake of 2-deoxyglucose and lactic acid production when the cardiomyocytes were pretreated with rotenone. These findings suggest that the stimulation of glucose uptake and anaerobic glycolysis followed by the increase in lactic acid accumulation in cardiomyocytes under the condition of mitochondrial impairment may be involved, at least in part, in the cellular injury by endothelin-1. Moreover, these findings suggest the possibility that the effect of endothelin-1 on myocardium is reversed by the condition of the mitochondria, and endogenous endothelin-1 may deteriorate cardiac failure with mitochondrial dysfunction. This may contribute to clarify the beneficial effect of endothelin receptor blockade in improving heart failures.

Chelerythrine and genistein inhibit the endothelin-1-induced increase in myofilament Ca(2+) sensitivity in rabbit ventricular myocytes

We performed experiments to elucidate the cellular mechanism for the biphasic inotropic response to endothelin-1 of single rabbit ventricular myocytes loaded with a fluorescent dye, acetoxymethylester of indo-1. Endothelin-1 at 10 nM elicited a biphasic inotropic effect: a transient decrease in cell shortening and Ca(2+) transients followed by an increase in cell shortening without significant elevation of peak Ca(2+) transients. The selective endothelin ET(A) receptor antagonist FR139317 (2(R)-[2(R)-[2(S)-[(1-hexahydro-1H-azepinyl)]carbonyl]amino-4-methylpentanoyl]amino-3-[3-(1-methyl-1H-indolyl)propionyl]amino-3-(2-pyridyl)propionic acid) at 1 microM abolished the biphasic effect of endothelin-1 on cell shortening and Ca(2+) transients. The selective protein kinase C inhibitor chelerythrine at 1 microM and the tyrosine kinase inhibitor genistein at 5 microM inhibited the endothelin-1-induced increase in cell shortening without significantly affecting Ca(2+) transients and the transient decrease in cell shortening and Ca(2+) transients. The present results indicate that both protein kinase C and tyrosine kinase may contribute to the increase in myofilament Ca(2+) sensitivity induced by endothelin-1, whereas the decrease in Ca(2+) transients induced by endothelin-1 may be mediated by a signalling pathway different from that involved in the increase in cardiac contractility in rabbit ventricular myocytes.

The role of endothelin in the pathogenesis of Chagas' disease

Infection with Trypanosoma cruzi causes a generalised vasculitis of several vascular beds. This vasculopathy is manifested by vasospasm, reduced blood flow, focal ischaemia, platelet thrombi, increased platelet aggregation and elevated plasma levels of thromboxane A(2) and endothelin-1. In the myocardium of infected mice, myonecrosis and a vasculitis of the aorta, coronary artery, smaller myocardial vessels and the endocardial endothelium are observed. Immunohistochemistry studies employing anti-endothelin-1 antibody revealed increased expression of endothelin-1, most intense in the endocardial and vascular endothelium. Elevated levels of mRNA for prepro endothelin-1, endothelin converting enzyme and endothelin-1 were observed in the infected myocardium. When T. cruzi-infected mice were treated with phosphoramidon, an inhibitor of endothelin converting enzyme, there was a decrease in heart size and severity of pathology. Mitogen-activated protein kinases and the transcription factor activator-protein-1 regulate the expression of endothelin-1. Therefore, we examined the activation of mitogen-activated protein kinases in the myocardium by T. cruzi. Western blot demonstrated an extracellular signal regulated kinase. In addition, the activator-protein-1 DNA binding activity, as determined by electrophoretic mobility shift assay, was increased. Increased expression of cyclins A and cyclin D1 was observed in the myocardium, and immunohistochemistry studies revealed that interstitial cells and vascular and endocardial endothelial cells stained intensely with antibodies to these cyclins. These data demonstrate that T. cruzi infection of the myocardium activates extracellular signal regulated kinase, activator-protein-1, endothelin-1, and cyclins. The activation of these pathways is likely to contribute to the pathogenesis of chagasic heart disease. These experimental observations suggest that the vasculature plays a role in the pathogenesis of chagasic cardiomyopathy. Additionally, the identification of these pathways provides possible targets for therapeutic interventions to ameliorate or prevent the development of cardiomyopathy during T. cruzi infection.

A novel pharmacological action of ET-1 to prevent the cytotoxicity of doxorubicin in cardiomyocytes

We previously reported that cardiomyocytes produce endothelin (ET)-1 and that the tissue level of ET-1 markedly increased in failing hearts in rats with chronic heart failure. Because the level of plasma ET-1 also increased progressively in patients with breast cancer who received doxorubicin (Dox; Adriamycin), which possesses cardiotoxicity, we hypothesized that ET-1 plays a role in the pathophysiology of cardiomyocytes injured by Dox. In this study, we investigated the effect of ET-1 on the cytotoxicity of Dox in primary cultured neonatal rat cardiomyocytes. The results showed that ET-1 effectively attenuated Dox-induced acute cardiomyocyte cytotoxicity (24-h incubation with Dox) evaluated by in vitro cell toxicity assay [3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assay and lactate dehydrogenase release]. The cytoprotective effect of ET-1 was mediated via ET(A) receptors, because pretreatment with the ET(A)-receptor antagonist BQ123 completely suppressed the cytoprotective effect of ET-1, whereas the ET(B)-receptor antagonist BQ788 did not. The cytoprotective effect of ET-1 was abolished by pretreatment with cycloheximide or staurosporine. These results suggest that a protein molecule(s), which is synthesized de novo by the stimulation of protein kinase pathway, is involved in the cytoprotective effect of ET-1. ET-1 increased the expression of an endogenous antioxidant, manganese superoxide dismutase (Mn-SOD), in the cardiomyocytes, as demonstrated by a Western blotting analysis. Pretreatment with an antisense oligodeoxyribonucleotide of Mn-SOD markedly attenuated the cytoprotective effect of ET-1 on the Dox-induced cytotoxicity. However, under conditions of prolonged incubation with Dox (48 h), ET-1 did not affect Dox-induced cardiomyocyte cytotoxicity in culture. These results suggest that ET-1 prevents the early phase of Dox-induced cytotoxicity via the upregulation of the antioxidant Mn-SOD through ET(A) receptors in cultured cardiomyocytes.

Biphasic inotropic response to endothelin-1 in the presence of various concentrations of norepinephrine in dog ventricular myocardium

The present study was undertaken to investigate the interaction between endothelin-1 (ET-1) and norepinephrine (NE) on contractile regulation in dog ventricular myocardium. ET-1 alone did not elicit any inotropic response in isolated dog ventricular trabeculae (37 degrees C, 0.5 Hz). In the presence of NE at a high concentration (10(-7) M), ET-1 (10(-8) M) elicited a long-lasting negative inotropic effect, while in the presence of NE at a moderate concentration (3 x 10(-8) M) it produced a biphasic inotropic effect: a sustained positive inotropic effect subsequent to a short-lasting negative inotropic effect. In the presence of a lower concentration (10(-9) M) that affected scarcely the basal force of contraction, ET-1 produced a pronounced positive inotropic effect in association with negative lusitropic and negative clinotropic effects in a concentration-dependent manner subsequent to a small transient negative inotropic effect. The presented results indicate that not only the extent, but also the quality of the inotropic response to ET-1 is determined by the level of NE in the biophase. The crosstalk of ET-1 with NE may play a crucial role in pathophysiological regulation of cardiac contractility in intact dog ventricular myocardium.

Antiadrenergic effects of endothelin-1 on the L-type Ca2+ current in dog ventricular myocytes

The effect of endothelin-1 (ET-1) on L-type Ca2+ current (I(Ca)) and the interaction of ET-1 with beta-adrenergic stimulation were studied in dog ventricular myocytes by means of a whole-cell patch-clamp technique. ET-1 (10(-8) M) had no effect on the baseline I(Ca), but at 10(-9)-10(-7) M, it inhibited the isoproterenol (ISO)-induced increase in I(Ca). The maximal inhibition induced by ET-1 at 3 x 10(-8) M was approximately 30%, and the median inhibitory (IC50) value of ET-1 was 1.1 x 10(-9) M. The inhibitory action of ET-1 (10(-8) M) on the ISO-induced increase in I(Ca) was markedly attenuated by the ET(A) antagonist FR139317 (10(-6) M) and was partially inhibited by the ET(B) antagonist BQ-788 (10(-6) M). The inhibitory action of ET-1 was totally inhibited by the nonselective ET-receptor antagonist, TAK-044 (10(-6) M). These results indicate that ET-1 exerts an antiadrenergic effect on the ISO-induced increase in I(Ca), which is mediated mainly by ET(A), but activation of ET(B) receptors might contribute to the effect of ET-1 to a lesser extent.

Myocardial expression of endothelin-1 in murine Trypanosoma cruzi infection

Chagas' disease, caused by Trypanosoma cruzi, is an important cause of myocarditis and chronic cardiomyopathy and is accompanied by microvascular spasm and myocardial ischemia. We reported previously that infection of cultured endothelial cells with T. cruzi increased the synthesis of biologically active endothlein-1 (ET-1). In the present study, we examined the role of ET-1 in the cardiovascular system of CD1 mice infected with the Brazil strain of T. cruzi and C57BL/6 mice infected with the Tulahuen strain during acute infection. In the myocardium of infected mice myonecrosis and multiple pseudocysts were observed. There was also an intense vasculitis of the aorta, coronary artery, smaller myocardial vessels and the endocardial endothelium. Immunohistochemistry studies employing anti-ET-1 antibody revealed increased expression of ET-1 that was most intense in the endocardial and vascular endothelium. Elevated levels of mRNA for preproET-1, endothelin converting enzyme and ET-1 were observed in the same myocardial samples. Plasma ET-1 levels were significantly elevated in infected CD1 mice 10-15 days post infection. These observations suggest that increased levels of ET-1 are a consequence of the initial invasion of the cardiovascular system and provide a mechanism for infection-associated myocardial dysfunction.

Negative inotropic effect of endothelin-1 in the mouse right ventricle

Effects of endothelin-1 on the contraction and cytosolic Ca(2+) concentrations (¿Ca(2+)(i)) of the mouse right ventricle were investigated. Endothelin-1 (1-300 nM) elicited a negative inotropic effect in a concentration-dependent manner. The endothelin-1-induced negative inotropy was antagonized by a selective endothelin ET(A) receptor antagonist, BQ-123 (cyclo ¿Asp-Pro-Val-Leu-Trp-; 3, 10 microM). Endothelin-1 reduced the peak amplitudes of both the ¿Ca(2+)(i) transient and contraction without changing inward Ca(2+) current. The relationship between peak amplitude of ¿Ca(2+)(i) and peak force generated by changing the extracellular Ca(2+) concentration (¿Ca(2+)(o)) was not affected by endothelin-1. In addition, the trajectory of the ¿Ca(2+)(i)-contraction phase plane diagram obtained at 2 mM ¿Ca(2+)(o) in the absence of endothelin-1 was superimposable on that obtained at 4 mM ¿Ca(2+)(o) in the presence of endothelin-1 (300 nM). Endothelin-1 (300 nM) translocated protein kinase C from cytosol to membrane, suggesting activation of protein kinase C. Further, a selective protein kinase C inhibitor, bisindolylmaleimide I (10 microM), inhibited the endothelin-1-induced negative inotropy. These results suggest that endothelin-1 elicits negative inotropy by reducing the amplitude of the ¿Ca(2+)(i) transient without changing inward Ca(2+) current through the activation of the endothelin ET(A) receptor followed by protein kinase C activation in the mouse right ventricle.

ATP is involved in myocardial and vascular effects of exogenous bradykinin in ejecting guinea pig heart

It has recently been reported that bradykinin induces selective left ventricular (LV) relaxation in isolated guinea pig hearts via the release of nitric oxide. Exogenous bradykinin also induces vasodilation, which is only partly due to nitric oxide release. In the present study we investigated the role of adenyl purines on these bradykinin-induced effects. Isolated ejecting guinea pig hearts were studied. LV pressure was monitored by a 2-Fr micromanometer-tipped catheter. ATP concentrations were measured using a luciferin-luciferase assay. Bradykinin (1 and 100 nM) caused a progressive acceleration of LV relaxation together with a transient increase in coronary flow. These effects were inhibited by the nonselective P(2) purinoceptor antagonist suramin (1 microM, n = 6) but were unaffected by the selective P(2x) purinoceptor antagonist pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid (1 microM, n = 6). These myocardial and vascular effects of bradykinin were associated with increased ATP levels in coronary effluent. These data suggest that the selective enhancement of LV relaxation and rise in coronary flow induced by exogenous bradykinin involve endogenous ATP and the subsequent stimulation of P(2) purinoceptors.

The putative mechanistic basis for the modulatory role of endothelin-1 in the altered vascular tone induced by Trypanosoma cruzi

Chagas' disease, caused by Trypanosoma cruzi, is an important cause of heart disease in Latin America. T. cruzi-induced microvascular compromise, in turn, is thought to play a major role in chagasic heart disease. Previous in vitro studies have implicated endothelin-1 (ET-1) as a potentially important vasomodulator present in increased levels in the supernatant of T. cruzi infected cultured human umbilical vein endothelial cells (HUVEC). Thus, the goal of the present investigation was to further evaluate the potentially important contribution of ET-1 to T. cruzi-induced alterations in vascular tone in vitro. Bioassay studies once again documented that exposure of isolated rat aortic rings to infected HUVEC supernatants elicited contractile responses whose steady-state magnitude was significantly greater than contractile responses elicited by exposure of aortic rings to uninfected HUVEC supernatants. Furthermore, the increased aortic contractility was significantly attenuated by the presence of the ET(A) subtype selective antagonists BMS-182,874 or BQ-123. Additionally, incubation of HUVEC with either verapamil or phosphoramidon prior to infection was also associated with reduced aortic contractility, upon exposure to the supernatant. Phosphoramidon, but not verapamil, produced a significant decrease in the measured ET-1 levels in the HUVEC supernatant. Consistent with the bioassay results, preincubation of Fura-2-loaded cultured rat aortic vascular smooth muscle cells with verapamil resulted in a near complete ablation of ET-1-induced transmembrane Ca2+ flux. Taken together, these data are consistent with the hypothesis that ET-1-induced vasoconstriction may play an important modulatory role in the vascular compromise characteristic of T. cruzi infection.

Role of Na+/Ca2+ exchange in endothelin-1-induced increases in Ca2+ transient and contractility in rabbit ventricular myocytes: pharmacological analysis with KB-R7943

1. The effects of endothelin-1 (ET-1) on intracellular Ca2+ ion level and cell contraction were simultaneously investigated in rabbit ventricular cardiac myocytes loaded with indo-1/A1. The role of Na+/Ca2+ exchange in ET-1-induced positive inotropic effect (PIE) was examined by using KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulphonate), a selective inhibitor of reverse mode Na+/Ca2+ exchange. 2. ET-1 at 0.3 pM - 1 nM increased cell contraction and Ca2+ transient (CaT) with EC50 values of 2.9 pM and 1.2 pM, respectively, and the increase in amplitude of CaT was much smaller relative to the PIE: ET-1 at 1 nM increased peak cell shortening by 237%, while it increased peak CaT by 167%. For a given level of PIE, ET-1-induced increase in CaT was much smaller than that induced by elevation of [Ca2+]o and by isoprenaline. Therefore, ET-1 shifted the relationship between peak CaT and cell shortening to the left relative to the relationship for increase in [Ca2+]o, an indication that ET-1 increased myofibrillar Ca2+ sensitivity. 3. KB-R7943 at 0.1 microM and higher inhibited contraction and CaT induced by 0.1 nM ET-1 and at 0.3 microM it abolished the increase in CaT while inhibiting the PIE by 48.1%. Over concentration range of 0.1-0.3 microM, KB-R7943 neither inhibited baseline contraction and CaT nor the isoprenaline-induced response, although at 1 microM and higher it had a significant inhibitory action on these responses. 4. These results indicate that in rabbit ventricular myocytes both increases in CaT and myofibrillar Ca2+ sensitivity contribute to the ET-induced PIE, and the activation of reverse mode Na+/Ca2+ exchange may play a crucial role in increase in CaT induced by ET-1 in rabbit ventricular cardiac myocytes.

Species-dependent differences in inotropic effects and phosphoinositide hydrolysis induced by endothelin-3 in mammalian ventricular myocardium

1. Species-dependent variations in the positive inotropic effect (PIE) of endothelin-3 (ET-3), and the relationships between the PIE and specific binding sites for [125I]-ET-3 and the PIE and the acceleration of phosphoinositide hydrolysis by ET-3, were studied in ventricular muscles from the rat, guinea-pig, rabbit, ferret and dog. 2. ET-3 in the presence of (+/-)-bupranolol (0.3 microM) and prazosin (0.3 microM) elicited a concentration-dependent PIE in the ventricular muscle from the rat, guinea-pig, rabbit and ferret. The potency of ET-3 and its efficacy in inducing a PIE were highest in the rabbit, intermediate in the rat and guinea-pig and lowest in the ferret. ET-3 did not have any inotropic effect on ventricular muscle from the dog. 3. Specific high-affinity binding of [125I]-ET-3 was observed with membrane fractions derived from the ventricular muscle of the five species. The maximal specific binding (Bmax) of ET-3 was highest in the rat and guinea-pig, intermediate in the rabbit and ferret and lowest in the dog. The values of KD in the rabbit and dog (33 and 52 pM) were lower than those in the rat, guinea-pig and ferret (141-221 pM). 4. In slices of ventricular muscle from all five species, ET-3 increased the accumulation of [3H]-inositol monophosphate (IP1) in a concentration-dependent manner. The extent of accumulation of IP1 was highest in the rat, intermediate in the guinea-pig and rabbit and lowest in the ferret and dog. 5. The results demonstrate the wide range of variations in the PIE of ET-3 on mammalian ventricular muscles. The variations in the coupling processes subsequent to the acceleration of the hydrolysis of PI, triggered by the binding of ET-3 to its receptor, might be important in these species-dependent differences in the PIE of ET-3.

The cardiac endothelium: cardioactive mediators

Endothelial cells within the heart release a number of substances that modulate myocardial contractile function. These agents include nitric oxide, endothelin, prostanoids, adenylpurines, and other substances that have so far been characterized only in bioassay studies. A notable feature of many of these agents is that they influence contractile behavior predominantly by modifying cardiac myofilament properties rather than altering cytosolic Ca2+ transients. A consequence of this subcellular action is often a disproportionate effect on myocardial relaxation and diastolic tone. The paracrine modulation of cardiac myocyte function by endothelial cell factors is likely to be an important mechanism contributing to the overall regulation of cardiac contractile function, both physiologically and in pathological states.

Pharmacological characteristics of endothelin receptors in the rabbit ventricular myocardium: the nonselective endothelin receptor antagonist PD 145065 antagonizes the positive inotropic effect of endothelin-3 but not of endothelin-1

Endothelin-3 (ET-3) elicited a concentration-dependent positive inotropic effect on rabbit papillary muscle, the maximal response being approximately 65% of the maximal response to isoproterenol. ET-1 induced a positive inotropic effect over the concentration range below 10(-9) M, at which ET-3 did not produce a positive inotropic effect, but the maximal response to ET-1 was equivalent to or slightly lower than that of ET-3. The nonselective ET receptor antagonist PD 145065 effectively antagonized the positive inotropic effect of ET-3 in a concentration-dependent manner and abolished it at 10(-5) M. PD 145065 decreased the positive inotropic effect induced by ET1 at lower concentrations (< 10(-9) M) but it did not affect the main portion of the concentration-response curve for the positive inotropic effect, i.e., the effect induced by high concentrations (> 10(-9) M) of ET-1. PD 145065 antagonized also the positive inotropic effect of sarafotoxin S6c. PD 145065 inhibited the specific binding of [125I]ET-1 and of [125I]ET-3 with a high- and a low-affinity site for competition. ETB selective ligands, RES-701-1 and sarafotoxin S6c, displaced [125Iuc]ET-3 with high affinity but they scarcely affected the [125I]ET-1 binding. These findings indicate that different subtypes of the ET receptor are responsible for the induction of the positive inotropic effect of ET-3 and ET-1. ET receptors involved in the production of the positive inotropic effect in the rabbit ventricular myocardium have pharmacological characteristics that are different from those of conventional ET receptors originally classified based on the pharmacological findings in noncardiac tissues. The positive inotropic effect of ET-3 in the rabbit ventricular muscle may be mediated predominantly by ETA1 receptors that are susceptible to PD 145065 as well as BQ-123 and FR139317, and partially mediated by ETB receptors that are inhibitable with RES-701-1. ETA2 receptors that are resistant to ETA selective as well as nonselective antagonists may mainly be responsible for the positive inotropic effect of ET-1 in the rabbit ventricular muscle.

Mechanical effects of ET-1 in cardiomyocytes isolated from normal and heart-failed rabbits

Endothelin (ET-1) is found at elevated concentrations in the plasma of patients with heart failure and in animal models of cardiomyopathy. The peptide is a potent positive inotropic agent, the effects of which are mediated by increases in cytosolic Ca2+ in cardiomyocytes. The object of this study was to investigate at the cellular level, the actions of ET-1 on contractile function and on Ca2+ currents in heart-failed ventricular myocardium. Male New Zealand White rabbits (8 wks) were treated with twice weekly injections of epirubicin (4 mg/kg/wk, n = 7) or with saline (n = 7) for 6 wks, followed by a washout period of 2 wks. Ventricular cardiomyocytes were isolated from rabbit hearts using Langendorff perfusion with collagenase; contractile function was examined using a video microscopy method, and L-type Ca2+ currents were recorded using a whole-cell patch-clamp technique. ET-1 produced a concentration-dependent increase in contractile response (% increase from basal value) to a maximum at 1 nM ET-1 of 69 +/- 11% (mean +/- S.D.) in control cardiomyocytes and 33 +/- 6% in heart-failed cells. However, there was no significant change in the EC50 obtained with ET-1 for healthy (0.31 +/- 0.1 nM) and for failed cardiomyocytes (0.24 +/- 0.1 nM). The effects of ET-1 on L-type Ca2+ channels were similar with a peak amplitude at 1 nM ET-1 of -3.26 +/- 0.8 nA in control cardiomyocytes and -3.32 +/- 0.9 nA in heart-failed cells. The attenuation of the contractile response to ET-1 in heart-failed cells may reflect a desensitization of ET receptors as a consequence of elevated circulating levels of ET and was not reflected by alteration of transmembrane Ca2+ conductance. It is probable, therefore, that multiple signalling pathways are involved in the actions of ET on ventricular myocardium.

Influence of endothelin 1 on human atrial myocardium--myocardial function and subcellular pathways

The influence of endothelin 1 on isometrically contracting human atrial muscle strip preparations was investigated under physiological conditions (37 degrees C, 1 Hz, Ca2+ 2.5 mM). Endothelin dose-dependently increased isometric tension from 3 x 10(-10) M to 1 x 10(-7) M. At 1 x 10(-7) M the inotropic effect of endothelin was maximum with isometric tension being increased by 32 +/- 6% (n = 11, p < 0.05). At 1 x 10(-7) M endothelin the positive inotropic effect was preceded by a transient negative inotropic effect with a decline in tension by -5 +/- 1% (n = 11, p < 0.05). Endothelin prolonged time from peak tension to 50% relaxation (RT50) by 29 +/- 5%. With BQ123 a competitive antagonist of the ETA receptor positive inotropic effect and the prolongation of relaxation was significantly reduced and initial negative a inotropic effect was abolished, indicating a ETA receptor mediated effect. Preincubation with phorbolmyristateacetate (10(-5) M) to downregulate proteinkinase C (PKC) eliminated the positive inotropic effect of endothelin. Similarly, N-5,5-dimethylamiloride (10(-5) M) which inhibits Na+/H(+)-exchanger activity, abolished the positive inotropic effect of ET. However, with either PMA or DMA the initial transient negative inotropic effect was still present (-13 +/- 7%, n = 9, p < 0.05 and -3 +/- 1%, n = 6, p < 0.05). Furthermore, both substances did not abolish the prolongation of twitch time parameters observed under endothelin. After preincubation with PMA, endothelin prolonged RT50 by 18 +/- 6% and with DMA by 11 +/- 2%. Using the photoprotein aequorin as an indicator for intracellular calcium concentrations showed that the positive inotropic effect was mainly mediated by an increase of systolic intracellular calcium concentrations. Thus, the present data indicate that the positive inotropic effect of endothelin in human atrial myocardium results from activation of PKC with a subsequent activation of the Na+/H(+)-exchanger. However, the initial negative inotropic effects as well as the prolongation of relaxation seem to result from a different intracellular mechanism of endothelin.

Effects of endothelin-1 on the contractility of cardiomyocytes from the spontaneously hypertensive rat

1. Disturbances in cardiovascular responsiveness to endogenous endothelin-1 (ET-1) may play a significant role in the pathogenesis of essential hypertension. In this study the inotropic responses of cardiomyocytes derived from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rat (SHR) strains to ET-1 (10(-11)-10(-8) mol/L) were characterized. Isotonic contraction cycles of ventricular cardiomyocytes isolated from age-matched (11 week) WKY and SHR rats were recorded using a rapid digital imaging technique and evaluated by computation of a range of normalized parameters. 2. The maximum effect of ET-1, eliciting a 60-70% increase in myocyte shortening after 3 min, was observed at 10(-9) mol/L in both strains, and was associated with elevations in the rate of shortening and lengthening, abbreviated latency, contractile cycle prolongation and delayed time to peak shortening. 3. No evidence for a significant strain dependent difference in the relative responsiveness to ET-1 was detected. This finding indicates that altered sensitivity to ET-1 is unlikely to be a major factor underlying the development of hypertension in this model. 4. The distinct nature of the alterations in contractile parameters produced by ET-1 compared with angiotensin II (AII) suggests that the prevailing cellular mechanisms of action of these peptides are different and that ET-1 is not a paracrine or autocrine inotropic intermediate for AII.

Beneficial effects of the endothelin receptor antagonist bosentan on myocardial and endothelial injury following ischaemia/reperfusion in the rat

The effects of bosentan, a nonpeptide endothelin receptor antagonist, on endothelin-induced changes in coronary flow and myocardial ischaemic and reperfusion injury were investigated in the Langendorff perfused rat isolated heart. Endothelin-1 (0.012-0.4 nmol) evoked dose-dependent reduction in coronary flow, which was attenuated by bosentan (1.0-10 microM) in a concentration-related fashion. The inhibitory effect of bosentan lasted more than 30 min. The endothelin ETB receptor agonist Suc-[Glu9,Ala11,15]endothelin-1-(8-21) (IRL 1620) increased coronary flow in the absence but not in the presence of bosentan. In hearts subjected to 30 min of global ischaemia followed by 30 min of reperfusion, the recoveries of the left ventricular developed pressure, dP/dtmax, and coronary flow were significantly larger in a group given bosentan 10 microM at the start of ischaemia (92 +/- 7%, 98 +/- 8% and 83 +/- 5%, respectively) than in a vehicle-treated group (70 +/- 4%, 70 +/- 6% and 42 +/- 2%, respectively) at the end of the reperfusion period. During the reperfusion period, left ventricular end diastolic pressure was significantly lower in the bosentan group than in the vehicle group. The area of no-reflow in the bosentan group was 7 +/- 3% of left ventricle compared to 21 +/- 2% in the vehicle group (P < 0.01). Acetylcholine-induced endothelium-dependent vasodilatation was significantly reduced after ischaemia and reperfusion in the vehicle group but not in the bosentan group. It is concluded that bosentan attenuates the coronary vasoconstrictor effect elicited by endothelin and reduces ischaemia/reperfusion-induced myocardial and endothelial injury in the rat isolated heart.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelial cell products alter mammalian skeletal muscle function in vitro

We tested the hypothesis that endothelin and nitric oxide (NO) alter the force developed by fast-twitch and slow-twitch mammalian skeletal muscle, using a mouse skeletal muscle preparation trimmed to approximately 50% of the original diameter to decrease diffusion distances. We suspended trimmed soleus (SOL) and extensor digitorum longus (EDL) muscles in Krebs-Henseleit buffer (27 degrees C; pH 7.4) gassed with 95% O2 -5% CO2. Muscles were stimulated once every 90 s for 500 ms at 50 Hz for SOL and 100 Hz for EDL. The force developed by trimmed SOL was 223.8 +/- 9.1 mN/mm2 and by EDL was 247.3 +/- 9.4 mN/mm2. Endothelin 1 (ET-1) had no effect on EDL but significantly accelerated the rate of decrease of developed force of SOL at concentrations of 10(-10) mol/L and higher within 10 contractions. When ET-1 was removed, force returned toward control value. Endothelin 3 (ET-3) had no effect on either muscle. S-Nitroso-N-acetylpenicillamine (SNAP), a source of NO, increased developed force over time in both muscles, with a threshold of 10(-6) mol/L. The effect was evident within 5 contractions in both muscles. Force remained elevated above control values after the removal of SNAP. Thus ET-1 attenuated and NO amplified mammalian skeletal muscle function.

Novel technique to bioassay endocardium-derived nitric oxide from the beating heart

Nitric oxide is a potent vasodilator and antiplatelet substance released by the vascular endothelium. In the current study, isolated rabbit hearts were perfused retrograde in the aortic root with a balanced salt solution using a Langendorff technique. To perfuse the right cardiac chambers, an inflow cannula was placed in the superior vena cava and an outflow cannula in the right ventricular apex via the pulmonary artery. To detect endocardial vasodilator production, right heart perfusate was used to bathe a "bioassay" segment of canine coronary artery denuded of endothelium. Perfusate from unstimulated hearts did not alter smooth muscle tone in the bioassay tissue. Calcium inophore, a potent stimulus for endothelial nitric oxide production, produced relaxation of the bioassay smooth muscle when added to the cardiac perfusate but not when applied directly to the bioassay segment. Cardiac effluent vasodilator activity was abolished by removal of the endocardium or addition of nitric oxide synthesis inhibitors, but not by prostanoid inhibitors. These experiments describe a practical method to bioassay endocardial nitric oxide production in the beating heart.

Endothelin-1 does not phosphorylate phospholamban and troponin I in intact beating rat hearts

To determine a role of phosphorylation of specific cardiac regulatory proteins in the positive inotropic effect of endothelin-1, we examined phosphorylation of sarcoplasmic reticulum and myofibrillar proteins in perfused beating rat hearts treated with endothelin-1. In parallel experiments, the effects of isoprenaline and phorbol-12,13-dibutyrate (PDB) on protein phosphorylation were also tested. In 32Pi-labeled hearts, perfusion with isoprenaline (100 nM) caused 4.4- and 10.4-fold increases in the degree of phosphorylation of phospholamban in sarcoplasmic reticulum and of troponin I in myofibrils, respectively. In contrast, neither endothelin-1 (100 nM) nor PDB (1 microM) significantly changed the phosphorylation state of these proteins. These findings provide evidence that phosphorylation of major cardiac regulatory proteins is not responsible for the positive inotropic action of endothelin-1.

The effects of various drugs on the myocardial inotropic response

1. The signal transduction process mediated by cyclic AMP that leads to the characteristic positive inotropic effect (PIE) in association with a positive lusitropic effect (acceleration of rate of twitch relaxation) has been well established. Relationships between accumulation of cyclic AMP, changes in intracellular Ca2+ transients and the PIE differ, however, depending on the mechanism of particular drugs that affect different steps in the metabolism of cyclic AMP. Selective partial agonists of beta 1-adrenoceptors and inhibitors of phosphodiesterase (PDE) III cause the accumulation of less cyclic AMP for a given PIE than does isoproterenol. In addition, in aequorin-microinjected canine ventricular muscle, selective inhibitors of PDE III, OPC 18790 and Org 9731, produced smaller decreases in the responsiveness of myofilaments to Ca2+ ions than isoproterenol, while a partial agonist of beta 1-adrenoceptors, denopamine, elicits a decrease in Ca2+ responsiveness of the same extent as does isoproterenol. 2. Activation of myocardial alpha 1-adrenoceptors, as well as stimulation of receptors for endothelin and angiotensin II, which accelerates hydrolysis of phosphoinositide (PI) to result in production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) are associated with very similar inotropic regulation: (1) the dependence on the species of animals of induction of the PIE; (2) an excellent correlation between the extent of acceleration of hydrolysis of PI and the PIE; (3) isometric contraction curves associated with a negative lusitropic effect; (4) the PIE associated with increases in myofibrillar responsiveness to Ca2+ ions; and (5) the selective inhibition of the PIE by an activator of protein kinase C (PKC), phorbol 12,13-dibutyrate (PDBu), with little effect on the PIE of isoproterenol and Bay k 8644. 3. A novel class of cardiotonic agents, namely, Ca2+ sensitizers such as EMD 53998 and Org 30029, act on the Ca(2+)-binding site of troponin C, increasing the affinity of these sites for Ca2+ ions, or at the actin-myosin interface to facilitate the cycling of cross-bridges. These agents produce a PIE with little change or decrease in Ca2+ transients and may bring about a significant breakthrough in the development of drugs for reversal of myocardial failure in the treatment of congestive heart failure.

Role of endogenous endothelin in myocardial and coronary endothelial injury after ischaemia and reperfusion in rats: studies with bosentan, a mixed ETA-ETB antagonist

1. Previous studies suggested that endothelin-1 (ET-1) may play a role in myocardial ischaemia and reperfusion. This study was designed to test the effect of a new nonpeptide antagonist of endothelin ETA and ETB receptors, bosentan, on myocardial infarct size, ventricular arrhythmias, and coronary endothelial dysfunction after ischaemia and reperfusion. 2. Anaesthetized male Wistar rats were subjected to 20 min ischaemia (left coronary artery occlusion) followed by 1 h (for the evaluation of coronary endothelial dysfunction) or 2 h (for the evaluation of infarct size) reperfusion, or 5 min ischaemia followed by 15 min reperfusion (for the evaluation of reperfusion arrhythmias). Vascular studies were performed on 1.5-2 mm coronary segments (internal diameter 250-300 microns) removed distal to the site of occlusion and mounted in wire myographs for isometric tension recording. Area at risk and infarct size were determined by Indian ink injection and triphenyl tetrazolium staining, using computerized analysis of enlarged sections after colour video acquisition. 3. Bosentan, administered at a dose which virtually abolished the pressor response to big ET-1 (3 mg kg-1, i.v. before ischaemia) did not affect heart rate, arterial pressure or the rate pressure product before ischaemia, during ischaemia and during reperfusion. Bosentan did not affect the incidence of reperfusion-induced ventricular fibrillation (controls: 86%, n = 14; bosentan: 93%, n = 15), and did not modify infarct size (% of area at risk: controls: 63 +/- 4, n = 10; bosentan: 60 +/- 6, n = 8). Ischaemia followed by reperfusion markedly reduced the endothelium-dependent relaxations to acetylcholine(maximal response: sham: 59 +/- 4%, n = 9; ischaemia-reperfusion: 26+/- 6%, n = 8; P<0.01), characteristic of reperfusion-induced endothelial dysfunction, and this dysfunction was not prevented by bosentan (maximal response to acetylcholine: 25 +/-5%, n = 9; P<0.01 vs sham; P = NS vs ischaemia/reperfusion).4. These experiments suggest that endogenous endothelin does not contribute to myocyte or coronary endothelial injury in this rat model of ischaemia and reperfusion.

Endothelin and myocardial ischemia

Endothelin is a potent vasoconstrictor with a wide range of effects on the heart. Changes in myocardial and circulating levels of endothelin have been described in various experimental models of myocardial ischemia, and in humans with acute myocardial infarction and different forms of angina pectoris. The role played by endothelin in the different states of myocardial ischemia is unclear. However, myocardial damage has been shown to be reduced in several experimental models of myocardial infarction by administering agents that block the action of endothelin. The aim of this review article is to present the current literature concerning the interaction between endothelin and the various forms of myocardial ischemia, and to explore the significance of such interactions.

Role of endothelin receptor subtype B (ET-B) in myocardial ischemia

Previous work indicated that endothelin (ET) may be involved in the pathogenesis of myocardial ischemia, although the relative importance of the ET receptor subtypes is presently not clear. The purpose of this study was to determine the role of myocardial ET-B receptors in mediating ischemic/reperfusion damage in isolated rat hearts. Saturation binding analyses were conducted with [125I]ET-1 and [125I]IRL-1620 to assess changes in ET-A and ET-B receptor binding. Total ET receptor density (Bmax) was greater in atrial versus ventricular tissue. ET-A Bmax was 8 to 10-fold greater than ET-B Bmax. In ischemic and ischemic/reperfused atrial tissue neither the equilibrium dissociation constant (Kd) nor Bmax for ET-B receptors was changed. The ET-B receptor Kd in ischemic or ischemic/reperfused ventricular tissue was also unchanged. In ischemic ventricular tissue there was a trend towards an increased ET-B Bmax, which was accentuated after ischemia/reperfusion. No changes were found in ET-A Bmax or Kd in ischemic ventricular or atrial tissue. The physiological importance of this receptor subtype in ischemic myocardium was determined using the selective ET-B agonist, sarafotoxin S6c. In non-ischemic tissue no effect on coronary flow or function were observed with sarafotoxin S6c. Furthermore, no changes were seen in ischemic time to contracture or any of the reperfusion indexes of myocardial damage. The sarafotoxin S6c utilized was active as it inhibited [125I]ET-3 binding to ET-B receptors (Ki = 0.1 nM). Thus, the pro-ischemic effect of ET-1 seems to be mediated by ET-A receptors. ET-B receptors do not appear to play a role in the pathogenesis of myocardial ischemia.

Role of endothelial cells in cardiovascular function

Since the first description of vascular endothelium-dependent relaxation in response to acetylcholine, the role of endothelial cells in the regulation of cardiovascular function has been increasingly studied. The identification of endothelial releasing factors such as nitric oxide and endothelin has enabled us to better understand the mechanisms involved in autoregulation. It has also been shown that both vascular and endocardial endothelium can modify the contractile characteristics of their adjacent myocardium. In the heart, these modulating effects of endothelial cells are more widespread than previously thought and, can be the result of the direct effects of endocardial and vascular endothelial cells and their indirect effects, via modulation of the myocardial response to inotropic agents.

Contractile proteins in myocardial cells are regulated by factor(s) released by blood vessels

The importance of perfusion of the coronary vasculature in the regulation of ATPase activity of myosin in rat myocardial cells has been studied. Quantitative histochemistry was used to determine the activity of the enzyme among cells in tissues that had been either perfused through the coronary system or superfused over the surface of the tissue. Enzymatic activity was measured in cryostatic sections from three different preparations: 1) hearts frozen immediately after removal from the animal; 2) isolated hearts frozen after they had been perfused through the coronary circulation; and 3) isolated papillary muscles or trabeculae that had been superfused after dissection and then frozen. ATPase activity was measured in the isolated tissues at different times after dissection. Both calcium- and actin-activated myosin ATPase activities were uniform among cells in both the ventricles of the hearts frozen immediately after dissection and those that had been perfused through the coronary system. In the superfused tissues, although calcium-activated myosin ATPase activity was uniform, actin-activated ATPase activity was not uniform for about 90 minutes after the dissection, the period required for stabilization of the contraction. The pattern of nonuniformity was complex. In all bundles the lowest enzymatic activity was found in the most superficial cells. In very thin bundles, the cells in the center had the highest activity. In the medium and thicker bundles, there were three concentric zones of actin-activated ATPase activity, the superficial zone with the lowest activity, an intermediate zone with high activity, and a central zone with lower activity. Within each zone, the activity was often greatest in myocardial cells immediately next to blood vessels even though the blood vessels had not been perfused. The transverse distribution of ATPase activity of myosin could be explained by a mechanism in which cells in blood vessels (presumably endothelium) release a substance that upregulates myosin ATPase activity, with the rate of release being related to the local oxygen tension. A downregulating substance may also be produced. The period of stabilization of the contraction coincides with the time during which the pattern of actomyosin ATPase activity is nonuniform. These data suggest that the contractile proteins are regulated by a substance produced by blood vessels in proportion to the local PO2, and possibly in relation to shear force on the vascular endothelium.

The influence of endocardial endothelium on myocardial contraction

A novel unidentified agent, provisionally named 'endocardin', has been shown to be released from endocardial endothelium. Endocardin has a unique prolonging effect on myocardial contraction. In contrast, endothelium-derived relaxing factor released from endocardial endothelium has the opposite effect of abbreviating contraction. Jerry Smith and colleagues discuss the mechanisms of action of these agents and their possible physiology and pathophysiology.

Endothelial modulation of myocardial contraction: mechanisms and potential relevance in cardiac disease

Recent studies in isolated cardiac preparations and the intact heart demonstrate that the endocardial and coronary vascular endothelium modulate myocardial contractile behaviour and cardiac pump function in a novel manner, mainly by influencing the duration of contraction and the onset of relaxation but without major effect on early systolic contractile characteristics. These effects are mediated by the release of at least two diffusible substances from endothelial cells: a) endothelium-derived relaxing factor (EDRF) which shortens contractile duration by elevating myocardial cyclic GMP, and b) a novel substance, provisionally named "endocardin", which prolongs contractile duration. Under physiological conditions these endothelial influences may be particularly important for relaxation and early diastolic filling events in the heart. It is possible that they could influence myocardial growth, interact with other cardiac hormones, and via EDRF inhibit platelet adhesion to endothelial surfaces. The release of the endothelial factors is regulated by stimuli such as circulating neurohumoral substances, increased flow, products of platelet aggregation, and endogenous peptides stored in endothelial cells. Although experimental evidence is still limited, it seems likely that cardiac endothelium may play an important role in the pathophysiology of cardiac disease, e.g. overload-induced hypertrophy. The endothelium could a) influence the development of phenotype change by modulating and mediating transduction of extrinsic signals, b) contribute to contractile and other abnormalities (especially "diastolic" dysfunction) because of loss or impairment of its normal function, and c) be uniquely amenable to therapeutically useful pharmacological manipulation.

Endothelins. Myocardial actions of a new class of cytokines

There is growing evidence to support the existence of a dynamic interaction in vivo between cardiac myocytes and adjacent microvascular endothelial cells in the regulation of both cardiac myocyte and possibly endothelial cell phenotype and function. Endothelins may be only one of several endogenous cytokines or autocoids that are released by the cardiac microvascular and/or endocardial endothelium and transported vectorially to adjacent myocytes that could modify cardiac contractile state, perhaps in response to changes in microvascular blood flow. Similarly, cardiac myocytes themselves could release cytokines that could directly affect endothelial cell proliferation or angiogenesis and indirectly elicit or modify the release of endothelium-derived cytokines and autocoids. Thus, in addition to modifying function, endothelial cell-cardiac myocyte interactions may also be of importance in the dynamic events that lead to myocardial wall remodeling and angiogenesis during hypertrophic growth and in the response to cardiac injury.

Endothelin 1 enhances myofilament Ca2+ responsiveness in aequorin-loaded ferret myocardium

The influence of endothelin 1 on intracellular Ca2+ transients and isometric contractions was investigated in ferret papillary muscles loaded with the Ca(2+)-regulated bioluminescent indicator aequorin. In concentrations of 3 x 10(-9) to 1 x 10(-7) M, endothelin produced dose-dependent increases in the amplitudes of both aequorin light signals (maximum, 31 +/- 12%) and developed tension (maximum, 64 +/- 13%). The peak aequorin light [( Ca2+]i)-peak tension curve generated by increasing endothelin concentrations was steeper and shifted to the left of the curve generated by varying [Ca2+]o; however, the maximum developed tension produced by endothelin did not exceed that produced by 6 mM [Ca2+]o. The effect of endothelin on the amplitude of the aequorin light signal was less than the effect of [Ca2+]o for similar levels of tension development. Moreover, 1 x 10(-7) M endothelin caused an upward shift in the peak aequorin light-peak tension curve generated by varying [Ca2+]o and increased the maximum twitch force by about 12%. The contractions were prolonged, whereas the time course of the Ca2+ transient was not changed in the presence of endothelin. When the function of the sarcoplasmic reticulum was inhibited by 6 microM ryanodine, 10(-7) M endothelin still increased the force generation without increasing the intracellular peak Ca2+, either during isometric twitches or during tetani.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial contractile actions of endothelin-1 in rat and rabbit papillary muscles. Role of endocardial endothelium

The actions of endothelin on the contractile and twitch configuration characteristics of rat and rabbit papillary muscles were evaluated before and after endocardial endothelium removal. In rabbit papillary muscles, endothelin produced a dose-dependent (10(-11) to 10(-7) M) increase in tension (T) (from 2.7 +/- 0.2 to 8.9 +/- 0.7 g/mm2, p less than 0.01), dT/dt, and Vmax (from 1.14 +/- 0.03 to 3.0 +/- 0.13 Lmax/sec, p less than 0.01), a decrease in time to peak tension (from 251 +/- 9 to 216 +/- 7 msec, p less than 0.05), and an increase in time to half relaxation (from 168 +/- 9 to 293 +/- 14 msec, p less than 0.01). Increasing calcium concentration in the bath from 1.25 to 15 mM greatly attenuated these changes. In the presence of propranolol (10(-5) M), endothelin increased all indexes of contractility but to a lesser extent (T = 2.6 +/- 0.3 to 6.5 +/- 0.2 g/mm2, p less than 0.01; Vmax = 1.16 +/- 0.10 to 2.06 +/- 0.10 Lmax/sec, p less than 0.01). Myocardial catecholamine depletion with reserpine had effects similar to those of propranolol. The myocardial contractile and twitch configuration characteristics of endothelin were similar to those of phenylephrine (10(-4) M), a strong stimulator of the phosphatidylinositol pathway and a moderate beta-adrenergic stimulator. The effects of phenylephrine and endothelin were modified in a similar manner by propranolol (10(-5) M). The presence of nicardipine (3 X 10(-7) M) decreased the absolute increase in contractility caused by endothelin but did not alter the percent change or shift the dose-response curve of endothelin. The actions of endothelin in rat papillary muscles studied at physiological calcium concentrations (Ca2+ = 1.25 mM) were less marked than those of rabbit studied at physiological calcium (Ca2+ = 1.25 mM) but similar to those of rabbit studied at high calcium concentrations (Ca2+ = 15 mM). Removing the endocardial endothelial layer of rabbit papillary muscles did not alter the absolute changes in contractility caused by endothelin but shifted the dose-response curve to the left and markedly altered the effects of endothelin on twitch configuration characteristics. Thus, it would appear that endothelin increases contractility and modifies twitch configuration. It does so more at lower rather than higher extracellular calcium concentrations and in a species, such as rabbit, that responds more to interventions that increase intracellular calcium.(ABSTRACT TRUNCATED AT 400 WORDS)

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

1. In isolated heart muscle preparations, selective removal of the endocardium results in a characteristic and unusual negative inotropic effect. Possible mechanisms for this effect were investigated in this study. 2. In endocardium-intact preparations of ferret papillary muscle, 8-bromo-cyclic GMP, sodium nitroprusside, atrial natriuretic peptide (ANP) and substance P each induced changes in contractile behaviour similar to selective endocardial removal, and each significantly elevated myocardial cyclic GMP levels. Substance P failed to elevate myocardial cyclic GMP levels following removal of endocardium or in the presence of haemoglobin, suggesting that it may act by releasing endothelium-derived relaxing factor (EDRF) from endocardium. However, there was no change in myocardial cyclic GMP levels following endocardium removal alone. 3. In cascade bioassay experiments, it was confirmed that porcine cultured endocardial cells released an unstable humoral agent whose effects on an endothelium-denuded pig coronary artery were indistinguishable from EDRF. 4. The negative inotropic effects of endocardium removal were reversed in bioassay experiments where an endocardium-denuded papillary muscle was exposed to the effluent from a column of porcine cultured endocardial cells on microcarrier beads. This demonstrates for the first time the release of a 'contraction prolonging factor' from endocardium, the tonic release of which would explain the negative inotropic effect of endocardium removal. 5. It is concluded that elevation of ferret papillary muscle cyclic GMP (as for example with EDRF) produces changes in contractile performance similar to those induced by endocardium removal. We also demonstrate that superfused porcine cultured endocardial cells release a humoral agent (provisionally named 'endocardin') which causes reversal of the changes in mechanical properties seen after endocardial removal.

Endothelin activation of an inwardly rectifying K+ current in atrial cells

Various tissues including heart express specific binding sites for endothelin. Endothelins have been reported to increase the force of contraction of cardiac muscle, presumably via specific receptors. Specific binding of endothelin to atrial tissue is particularly high. In spontaneously contracting rat atrial cells used in this study, all three isoforms of endothelin (endothelin-1, endothelin-2, and endothelin-3) decreased the rate of beating and caused an increase in inwardly rectifying K+ current in voltage-clamped whole cells. Endothelin-3 was the most potent isoform, and its effects on beating rate and K+ current were present at a concentration as low as 100 pM (Kd, approximately 1 nM). the atrial cells did not have the hyperpolarization-activated current (the pacemaker current), If. In excised inside-out patches, all three isoforms of endothelin activated a population of K+ channels with kinetic properties identical to those of acetylcholine (muscarinic)-activated K+ channels, and this was GTP dependent. Endothelin failed to decrease the beating rate or to elicit the K+ current in pertussis toxin-treated cells. These results indicate that endothelin has a potent negatively chronotropic effect by activation of the inwardly rectifying, muscarinic K+ channel and therefore could be an important regulator of heart function.

Interactions between endothelin-1 and other chronotropic agents in rat isolated atria

In isolated spontaneously beating right and left atria and in electrically driven left atrium from rat, endothelin-1 increased the rate and force of contraction, but significantly decreased the positive chronotropic and inotropic responses to sympathetic nerve stimulation. The decrease may be partly dependent on the positive cronotropic and inotropic effects of endothelin-1, since other agents with chronotropic activity (noradrenaline, isoprenaline, serotonin and Bay k 8644) also decreased stimulation-induced chronotropic responses. Endothelin-1 caused a significant rightward shift of the linear portion of the log concentration-response curve for the chronotropic actions of noradrenaline and isoprenaline. The changes in the log concentration-response curve were not a consequence of the direct chronotropic effect of endothelin-1, since they were still evident when the chronotropic action of endothelin-1 was offset by carbachol. Furthermore, the chronotropic agent, Bay k 8644, did not shift the linear portion of the log concentration-response curves for noradrenaline and isoprenaline. The mechanism of the effects of endothelin-1 in rat atria is not known, but they were not changed by blockade of alpha-adrenoceptors or of L-type voltage-sensitive Ca2+ channels.

Vascular activities of the endothelins

The endothelins are a family of novel 21 amino-acid peptides and are the most potent vasoconstrictor substances yet discovered. The endothelins not only produce prolonged pressor responses in intact animals but they also constrict large and small arterial and venous vessels studied as isolated vascular preparations, influence autonomic transmission, exert positive inotropic effects on the heart and have been shown to be capable of releasing EDRF, prostanoids and atrial natriuretic factor. Release of endothelins occurs after de novo synthesis which may be stimulated by various agonists, fluid-flow and possibly hypoxia. The endothelins have been implicated in the pathophysiology of a variety of cardiovascular disorders but their precise role remains to be elucidated.

Endothelin and increased contractility in adult rat ventricular myocytes. Role of intracellular alkalosis induced by activation of the protein kinase C-dependent Na(+)-H+ exchanger

Endothelin, a 21-amino acid vasoactive peptide, is among the most potent positively inotropic agents yet described in mammalian heart. Having demonstrated that endothelin's inotropic effect is due, in part, to an apparent sensitization of cardiac myofilaments to intracellular calcium, we determined whether this could be due to a rise in intracellular pH (pHi). In isolated adult rat ventricular cells loaded with the H(+)-selective fluorescent probe BCECF, 100 pM endothelin increased contractile amplitude to 190 +/- 26% of baseline and pHi by 0.08 +/- 0.02 (n = 8), whereas 1 nM endothelin increased pHi by 0.13 +/- 0.03 with little further increase in contractility. Amiloride (10(-4)M) prevented the increase in pHi in response to endothelin and reduced the inotropic response by 45%, although the inotropic effect could be readily restored by subsequent NH4Cl-induced alkalinization. Similarly, inhibitors of protein kinase C (H-7 and sphingosine) diminished or abolished the rise in pHi after endothelin superfusion while causing a decline in its inotropic effect comparable with that observed with amiloride. Pretreatment with pertussis toxin, which we have demonstrated results in complete ADP-ribosylation of the alpha-subunits of Go and Gi GTP-binding proteins and abolition of endothelin's positive inotropic effect, only partially reduced the intracellular alkalinization induced by the peptide, suggesting a complex signal transduction mechanism. Thus, the positive inotropic action of endothelin is due in part to stimulation of the sarcolemmal Na(+)-H+ exchanger by a protein kinase C-mediated pathway, resulting in a rise in pHi and sensitization of cardiac myofilaments to intracellular Ca2+.

Endothelin enhances the contractile responsiveness of adult rat ventricular myocytes to calcium by a pertussis toxin-sensitive pathway

It has long been assumed that the primary influences regulating cardiac contractility are the extent of mechanical loading of muscle fibers and the activity of the autonomic nervous system. However, the vasoactive peptide endothelin, initially found in vascular endothelium, is among the most potent positively inotropic agents yet described in mammalian myocardium. In isolated adult rat ventricular cells, endothelin's action was slow in onset but very long lasting with an EC50 of 50 pM that approximates the reported KD of the peptide for its receptor in rat heart. When the calcium activity of the buffer superfusing isolated single fura-2-loaded myocytes paced at 1.5 Hz was varied from 0.1 to 0.9 mM [Ca2+]o, 100 pM endothelin increased contractile amplitude with no significant change in diastolic or systolic [Ca2+]i, thus appearing to sensitize the myofilaments to intracellular calcium. Pertussis toxin, or prior exposure to a beta-adrenergic agonist, reduced or abolished the increase in myocyte contractility induced by endothelin. This novel and potent pharmacologic action of endothelin points to the potential importance of local, paracrine factors, perhaps derived from microvascular endothelium or endocardium, in the control of the contractile function of the heart.

Direct positive inotropic and vasoconstrictor effects of endothelin

Endothelin, a newly discovered vasoconstrictor peptide, when added to isolated cat papillary muscles, induced a direct positive inotropic effect that was slow in onset but of long duration. The magnitude of the developed force was concentration dependent. Endothelin exerted a marked concentration-dependent vasoconstriction in isolated cat carotid arteries and rabbit aortic rings. In both the carotid arteries and the aortic rings, endothelin induced a similar vasoconstrictor effect in the presence or absence of an intact endothelium. Addition of propyl gallate, a 5-lipoxygenase inhibitor, ibuprofen, a cyclooxygenase inhibitor, or SKF-525A, a cytochrome P450 inhibitor, at 2 microM did not significantly attenuate the ability of endothelin to vasoconstrict aortic rings in the presence or absence of an intact endothelium. These results demonstrate that the vasoconstrictor activity of endothelin operates independently of all three pathways of arachidonic acid metabolism (i.e., lipoxygenase, cyclooxygenase, or cytochrome P450 pathways) and is not dependent upon other endothelium-derived mediators (e.g., endothelium-derived relaxing factor, or eicosanoids) in these preparations. Moreover, endothelin exerts a direct positive inotropic effect in isolated cat ventricular myocardial tissue.

The arrhythmogenic action of endothelin in rats

Endothelin (ET) was administered into the coronary ostia at doses of 0.1-1 microgram/kg in anesthetized rats. The ST segment was depressed at doses below 0.5 microgram/kg and was transiently elevated at 1 microgram/kg. Ventricular arrhythmias developed at doses above 0.5 microgram/kg. The arrhythmias that developed at 1 microgram/kg were precipitated into ventricular fibrillation. At the time when the arrhythmias developed, the ischemic changes had already subsided. These results suggest that ET may have an arrhythmogenic action, which is not solely attributable to myocardial ischemia.

Chronic hypertension produced by infusion of endothelin in rats

Endothelin, a potent vasoconstrictor peptide synthesized by the vascular smooth muscle endothelium, was chronically infused into male Sprague-Dawley rats to determine whether a long-term increase in circulating endothelin levels would cause a sustained elevation in mean arterial pressure. Rats were catheterized, housed in metabolic cages, and maintained on a fixed 6 meq/day sodium intake throughout the experiment with daily measurements including mean arterial pressure, heart rate, water intake, urine output, urinary sodium excretion, urinary potassium excretion, cardiac output, total peripheral resistance, and stroke volume. Infusion of endothelin-1 (ET-1) at rates of 3, 5, or 7.5 pmol/kg/min for 7 days was associated with significant, sustained, and dose-dependent increases in mean arterial pressure and smaller less consistent elevations in total peripheral resistance. Other parameters were unaffected. Similar results were observed in rats receiving endothelin-3 (ET-3), except that a higher dose of ET-3 was required. These results indicate that elevated blood levels of endothelin could produce a maintained hypertension without sodium or water retention and that the hemodynamic basis for the increased mean arterial pressure is similar to that seen in most other forms of experimental and clinical hypertension.

Cardiac output effects of endothelin-1, -2 and -3 and sarafotoxin S6b in conscious rats

Conscious, Long-Evans rats, chronically-instrumented for the direct measurement of intra-arterial and central venous pressures and ascending aortic blood flow (i.e. cardiac output), were given bolus, intravenous doses (4 and 40 pmol) of endothelin-1, -2, or -3 or sarafotoxin S6b in random order. The lower dose of endothelin-1 and -3 and sarafotoxin S6b caused a significant increase in cardiac output. The higher bolus dose of the same three peptides caused initial hypotension and increases in cardiac output, stroke volume and total peripheral conductance. Endothelin-2 did not have these initial effects in all animals. Subsequently, all peptides increased mean arterial blood pressure and decreased cardiac output and total peripheral conductance. Overall the effects of endothelin-1 and sarafotoxin S6b were very similar, consistent with them acting at the same site.