Endothelin has potent inotropic effects in rat atria

Endothelial Dysfunction in Heart Failure With Preserved Ejection Fraction: What are the Experimental Proofs?

Heart failure with preserved ejection fraction (HFpEF) has been recognized as the greatest single unmet need in cardiovascular medicine. Indeed, the morbi-mortality of HFpEF is high and as the population ages and the comorbidities increase, so considerably does the prevalence of HFpEF. However, HFpEF pathophysiology is still poorly understood and therapeutic targets are missing. An unifying, but untested, theory of the pathophysiology of HFpEF, proposed in 2013, suggests that cardiovascular risk factors lead to a systemic inflammation, which triggers endothelial cells (EC) and coronary microvascular dysfunction. This cardiac small vessel disease is proposed to be responsible for cardiac wall stiffening and diastolic dysfunction. This paradigm is based on the fact that microvascular dysfunction is highly prevalent in HFpEF patients. More specifically, HFpEF patients have been shown to have decreased cardiac microvascular density, systemic endothelial dysfunction and a lower mean coronary flow reserve. Importantly, impaired coronary microvascular function has been associated with the severity of HF. This review discusses evidence supporting the causal role of endothelial dysfunction in the pathophysiology of HFpEF in human and experimental models.

Expression of endothelin-1 system in a pig model of endotoxic shock

Endothelin (ET)-1 is a potent vasoconstrictive peptide and it is involved in the pathogenesis of septic shock. Blockade of ET-1 receptors abolishes the LPS-induced pulmonary hypertension and worsens the LPS-dependent systemic hypotension, but the role of ET-1 in sepsis remains uncertain. To determine the role of ET-1 in cardiovascular and respiratory derangement in a porcine model of endotoxemic shock we evaluated ET-1 plasma levels and ET-1 mRNA and protein levels in lung, liver, and heart as well as Endothelin Converting Enzyme-1, ET(A) and ET(B) receptors mRNA in the same tissues. Twelve piglets were randomised to sham operated or to LPS-treated (40 microg/kg/h for 4 h) groups. During the experiment, respiratory and circulatory parameters have been recorded and blood samples collected. At the end of the experiment the animals were sacrificed and tissue samples collected for real-time quantitative PCR and ELISA test. LPS infusion evokes a large increase in ET-1 plasma concentration, and in tissues mRNA levels, associated with an increase in pulmonary arterial pressure, as well as in pulmonary and systemic vascular resistances, and a decrease in stroke volume. LPS infusion caused also a derangement of respiratory mechanics, evidenced by an increase in resistance and a decrease in compliance of the respiratory system. ET(A) and ET(B) receptor mRNA levels were markedly decreased in liver and lung and slightly increased in heart, evidencing that ET receptor subtypes were differentially regulated in the major organs of endotoxin treated pigs. In conclusion our data show the presence of a continuative and differentially regulated stimulating mechanism of ET-1 expression during pig endotoxaemia as well as a fundamental role of ET-1 system in the cardiovascular and respiratory derangement.

Coronary effects of endothelin-1 and vasopressin during acute hypotension in anesthetized goats

Coronary effects of endothelin-1 and vasopressin during acute hypotension, and the role of NO and prostanoids in these effects were examined in anesthetized goats. Left circumflex coronary artery flow was measured electromagnetically, and hypotension was induced by constriction of the caudal vena cava in animals non-treated (7 goats) or treated with the inhibitor of NO synthesis N(w)-nitro-L-arginine methyl esther (L-NAME, 5 goats), the cyclooxygenase inhibitor meclofenamate (5 goats) or both drugs (5 goats). Under normotension (22 goats), mean arterial pressure averaged 93 +/- 3 mm Hg and coronary vascular conductance (CVC) 0.37 +/- 0.025 ml/min/mm Hg. Endothelin-1 (0.01-0.3 nmol) and vasopressin (0.03-1 nmol), intracoronarily injected, dose-dependently decreased CVC by up to 56% for endothelin-1 and 40% for vasopressin. During hypotension in every condition tested, mean arterial pressure decreased to approximately 60 mm Hg, and CVC only decreased during hypotension pretreated with L-NAME (23%) or L-NAME + meclofenamate (34%). Under non-treated hypotension, the decreases in CVC by endothelin-1 were augmented approximately 1.5 fold, and those by vasopressin were not modified. This increase in CVR by endothelin-1 was not affected by L-NAME and was reversed by meclofenamate or L-NAME + meclofenamate. The coronary effects of vasopressin were not modified by any of these treatments. Therefore, acute hypotension increases the coronary vasoconstriction in response to endothelin-1 but not to vasopressin. This increased response to endothelin-1 may be related to both inhibition of NO release and release of vasoconstrictor prostanoids.

Significance of the endothelin ETA receptor in the haemodynamic and inotropic effects of endothelin-1 in rats

The main aim of the present study was to investigate the direct inotropic effects of stimulation of the endothelin (ET) receptor ETA under in vivo conditions. It is well known that ETA receptor stimulation causes pronounced vasoconstriction. The ET-1-induced coronary vasoconstriction may lead to myocardial ischaemia and, consequently, to cardiodepressor effects that may mask the direct positive inotropic effect of ETA receptor stimulation. Thus, in the present study, steps were taken to avoid this possibility. In anaesthetized open-chest rats the haemodynamic and inotropic effects of ETA receptor stimulation were studied by monitoring responses evoked by ET-1 (1 nmol/kg of body weight) after ETB receptor blockade with BQ 788 (0.5 μmol/kg of body weight); these responses were compared with saline controls (after ETB receptor blockade). To avoid vasoconstrictor effects induced by ETA receptor stimulation, additional experiments were performed in the presence of the vasodilator adenosine (2.0 mg·kg−1 of body weight·min−1). Myocardial function was also examined during aortic clamping so as to circumvent the effect of changes in afterload. We studied further the effect of ETA receptor stimulation on myocardial energy metabolism. ETA receptor stimulation reduced cardiac output (−49% compared with control), raised total peripheral resistance (+173%) and reduced myocardial ATP content (−23%). Aortic clamping did not reveal a positive inotropic effect of ETA receptor stimulation. Furthermore, even though adenosine attenuated the decrease in cardiac output (−21%), the increase of total peripheral resistance (+48%) and prevented the fall of myocardial ATP content (+6%), this did not unmask a positive inotropic effect of ETA receptor stimulation. Thus we conclude that ETA receptor stimulation causes vasoconstriction and myocardial ischaemia, but has no positive inotropic effects in rats.

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.

Coronary action of endothelin-1 and vasopressin during acute hypertension in anesthetized goats. Role of nitric oxide and prostanoids

Coronary reactivity to endothelin-1 and vasopressin during acute, moderate hypertension, and the role of nitric oxide (NO) and prostanoids in this reactivity was examined in anesthetized goats. Left circumflex coronary flow was electromagnetically measured, and hypertension was induced by constriction of the thoracic aorta in animals nontreated (7 goats) or treated with the inhibitor of NO synthesis Nw-nitro-L-arginine methyl esther (L-NAME, 6 goats) or the cyclooxygenase inhibitor meclofenamate (6 goats). Under normotension (19 animals), basal mean values for mean arterial pressure and coronary vascular conductance (CVC) were 89+/-3 mm Hg and 0.36+/-0.038 ml/min/mm Hg, respectively. Endothelin-1 (0.01-0.3 nmol) and vasopressin (0.03-1 microg) dose-dependently decreased CVC, which, for endothelin-1 ranged from 5+/-1% (0.01 nmol; P<0.01) to 66+/-4% (0.3 nmol; P<0.001) and for vasopressin ranged from 9+/-1% (0.03 microg P<0.01) to 41+/-3% (1 microg; P<0.001). During nontreated and treated hypertension, mean arterial pressure increased to approximately 130 mmHg (P<0.01), and CVC decreased (17%) only during L-NAME-treated hypertension. The effects of endothelin-1 and vasopressin on CVC were decreased by approximately 50% during nontreated hypertension, and this was abolished by L-NAME and was not affected by meclofenamate. Therefore, during acute, moderate hypertension, the coronary vasoconstriction to endothelin-1 and vasopressin is attenuated, which may be related with increased NO release but not with prostanoids.

The Role of Endothelin in Mediating Ischemia/Hypoxia-Induced Atrial Natriuretic Peptide Release

The aim of the present study was to investigate the putative role of endothelin (ET) in mediating ischemia/hypoxia-induced ANP release utilizing exogenous ET-1 or ET receptor antagonists (BQ-123 or Bosentan). Isolated rat hearts with non-distended atria were perfused using a Langendorff apparatus and heart rate maintained constant via atrial pacing. Global ischemia was induced either by direct reduction in perfusion or by infusion of exogenous ET-1 (5 x 10(-10) M) for 30 minutes. Perfusion with the ET receptor antagonists, BQ-123 (10(-6) M) or Bosentan (10(-5) M) was initiated 10 minutes before onset of ischemia. Moderate or severe ischemia was induced by reduction (52-61% and 70-82%, respectively) in perfusate flow. Thirty minutes of ischemia/hypoxia (5% O2) was followed by 30 minutes of reperfusion/re-oxygenation. Both moderate and severe ischemia increased ANP release. BQ-123 and Bosentan did not affect basal or ischemia-induced ANP release. Exogenous ET-1 perfusion induced a late increase in ANP release (P < 0.01) that did not exceed the increase in ANP release associated with equivalent direct flow reduction. Hypoxia induced an 8-fold increase in ANP release rate. The ANP release rate returned toward basal levels after re-oxygenation. Bosentan, but not BQ-123, significantly attenuated (P < 0.01) hypoxia-induced ANP release. In conclusion, in this system, ANP release is stimulated by moderate (or severe) ischemia and severe hypoxia independent of change in atrial distension; endogenous ET does not mediate basal and ischemia-induced ANP release; and hypoxia-induced ANP release is partially modulated via interaction with endogenous ET.

Endothelins in chronic diabetic complications

Endothelins are widely distributed in the body and perform several vascular and nonvascular functions. Experimental data indicate abnormalities of the endothelin system in several organs affected in chronic diabetic complications. In support of this notion, it has been shown that endothelin-receptor antagonists prevent structural and functional abnormalities in target organs of diabetic complications in animal models. Alterations of plasma endothelin levels have also been demonstrated in human diabetes. This review discusses the role of endothelins in the pathogenesis of chronic diabetic complications. The current experimental evidence suggests that endothelin-receptor antagonism may potentially be an adjuvant therapeutic tool in the treatment of chronic diabetic complications.

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.

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.

Molecular regulation of the endothelin-1 gene by hypoxia. Contributions of hypoxia-inducible factor-1, activator protein-1, GATA-2, AND p300/CBP

Endothelin-1 (ET-1) is a peptide hormone with potent vasoconstrictor properties which is synthesized and secreted predominantly by vascular endothelial cells. Its production is regulated by numerous stimuli including ischemia and hypoxia, and the enhanced levels that occur during myocardial ischemia may contribute to the progression of heart failure. We reported previously a preliminary characterization of a hypoxia-inducible factor-1 (HIF-1) binding site in the human ET-1 promoter which contributed to the activation of ET-1 expression in endothelial cells. We report here that the HIF-1 binding site alone is not sufficient for the response to hypoxia but requires an additional 50 base pairs of flanking sequence that includes binding sites for the factors activator protein-1 (AP-1), GATA-2, and CAAT-binding factor (NF-1). Mutation of any one of these sites or the HIF-1 site eliminated induction by hypoxia. Mutations of the AP-1 and GATA-2 sites, but not the HIF-1 site, were complemented by overexpressing AP-1, GATA-2, HIF-1alpha, or the activator protein p300/CBP, restoring the response to hypoxia. Binding studies in vitro confirmed physical associations among GATA-2, AP-1, and HIF-1 factors. Overexpression or depletion of p300/CBP modulated the level of ET-1 promoter expression as well as the endogenous ET-1 transcript but did not change the fold induction by hypoxia in either case. Regulation of the ET-1 promoter by hypoxia in non-endothelial cells required overexpression of GATA-2 and HIF-1alpha. The results support essential roles for AP-1, GATA-2, and NF-1 in stabilizing the binding of HIF-1 and promoting recruitment of p300/CBP to the ET-1 hypoxia response complex.

Therapeutic potential of endothelin receptor antagonists in diabetes

Endothelins (ETs) are widely distributed in the body and perform several vascular and non-vascular functions. Experimental evidence indicates that abnormalities of the ET system occur in several organs affected in chronic diabetic complications. Furthermore, ET antagonists were found to prevent structural and functional changes in the target organs of chronic diabetic complications in animal models. Abnormalities of plasma ET levels have also been demonstrated in human diabetes. This review discusses the role of ET in the pathogenesis of chronic diabetic complications. The current experimental evidence suggests that ET antagonism may potentially represent an adjuvant therapeutic tool in the treatment of chronic diabetic complications.

Pharmacological analysis by HOE642 and KB-R9032 of the role of Na(+)/H(+) exchange in the endothelin-1-induced Ca(2+) signalling in rabbit ventricular myocytes

The role of Na(+)/H(+) exchange in endothelin-1 (ET-1)-induced increases in Ca(2+) transients and cell shortening was studied in rabbit ventricular myocytes loaded with indo-1/AM. Selective inhibitors of Na(+)/H(+) exchange HOE642 (4-isopropyl-3-methyl-sulphonylbenzoyl guanidine methanesulphonate) and KB-R9032 (N-(4-isopropyl-2,2-dimethyl-3-oxo-3, 4-dihydro-2H-benzo-[1,4]oxazine-6-carbonyl) guanidine methanesulphonate) were used as pharmacological tools for the analysis. ET-1 at 0.1 nM induced an increase in Ca(2+) transients by 45.6%, while it increased cell shortening by 109.6%. For a given increase in cell shortening, the ET-1-induced increase in Ca(2+) transients was much smaller than that induced by isoprenaline (ISO, 10 nM). Pretreatment with HOE642 and KB-R9032 (1 microM) inhibited the increase in cell shortening induced by 0.1 nM ET-1 by 51 and 65. 4%, respectively, without a significant alteration of ET-1-induced increase in Ca(2+) transients. HOE642 and KB-R9032 did not affect baseline levels of cell shortening and peak Ca(2+) transients, and the effects of ISO (10 nM). These results indicate that activation of Na(+)/H(+) exchange by ET-1 may play an important role in the positive inotropic effect and the ET-1-induced increase in myofilament Ca(2+) sensitivity in rabbit ventricular myocytes.

Diabetes-induced myocardial structural changes: role of endothelin-1 and its receptors

Several metabolic abnormalities may be triggered secondary to hyperglycemia in diabetes. Some of these abnormalities may alter expression of vasoactive factors in the target organs of diabetic complications. We investigated alterations of endothelin-1 (ET-1) and its receptors, ET(A) and ET(B), and associated structural changes in the myocardium of streptozotocin-induced diabetic rats after 6 months of hyperglycemia. We further assessed the preventive effects of an ET-receptor antagonist bosentan on these changes. Compared to the non-diabetic, age- and sex-matched control animals, diabetic rats showed hyperglycemia, glucosuria, reduced body weight gain and elevated glycated Hb levels. Measurement of ET-1, ET(A) and ET(B) mRNAs by semiquantitative RT-PCR showed significantly increased mRNA levels in the hearts of diabetic rats. Treatment with bosentan failed to reduce ET-1 or ET(B) mRNA expression in diabetes, however ET(A) mRNA expression was reduced. Immunocytochemically, ET-1 was detected in the cardiomyocytes, endothelium and smooth muscle cells of the larger blood vessels and was increased in diabetes. Autoradiographic localization of ET-1 receptors, using (125)I-ET-1, showed increased binding in the endothelium and myocardium of diabetic animals. Histologically, focal fibrous scarring with apoptotic cardiomyocytes, consistent with changes secondary to microvascular occlusion, was only present in the diabetic rats. In keeping with focal fibrosis, myocardium from diabetic rats further showed significantly increased mRNA expression of two extracellular matrix protein transcripts, fibronectin and collagen alpha 1(IV) which were completely prevented by treatment with bosentan. These data suggest that hyperglycemia-induced upregulation of the ET-system in the heart may be important in the pathogenesis of cardiac involvement in diabetes.

Cardiac swelling-induced chloride current is enhanced by endothelin

Endothelins (ETs) are a family of peptide hormones that act on G protein-coupled ET(A) and ET(B) receptors. ETs exert inotropic and chronotropic actions in the heart. Myocardial ischemia is associated with increased plasma levels of ET and cell swelling. We examined the effect of ETs on dog atrial swelling-induced chloride current (I(Cl,swell)). Whole-cell patch clamp was used; 10 nM ET-1 or ET-2 increased I(Cl,swell) by approximately twofold. ET-2 had no effect if I(Cl,swell) activation was prevented by hypertonic superfusate. Outward ET-2-induced current was blocked by 150 microM DIDS more effectively than inward current. Overnight pretreatment with phorbol 12-myristate 13-acetate (1.6 microM), pertussis toxin (100 ng/ml), or dialysis of the cell with 300 microM 2'-deoxyadenosine 3'-monophosphate, a P-site inhibitor of adenylyl cyclase, did not diminish the effect of ET-2. The effect of ET-2 was blocked by an ET(A1)- (BQ123), but not an ET(B)-selective (BQ788) antagonist. ET-2-induced currents were inhibited approximately 70% by PD 98059 (30 microM), a selective MAPK kinase (MEK) blocker. PD 98059 did not affect basal whole cell current or I(Cl,swell) before exposure to ET-2. The data suggest that MEK activity is not required for activation of atrial I(Cl,swell) but that ET-2 stimulates I(Cl,swell) by a MEK-dependent pathway.

Specificity of glycosaminoglycan suppression of endothelin-1 production by human umbilical vein endothelial cells

Endothelin-1 (ET-1) is the most potent vasoconstrictor peptide found in nature. Its production is stimulated by thrombin. By inhibiting thrombin we have previously shown that heparin, a highly negatively-charged glycosaminoglycan (GAG), suppresses the production of ET-1 by cultured human umbilical vein endothelial cells (HUVEC). The purpose of our study is to determine the effect of other GAGs and related compounds on ET-1 production. The GAGs and related compounds used in the study were: chondroitin sulfate A, chondroitin sulfate B, chondroitin sulfate C, fucoidin, low molecular weight dextran sulfate, high molecular weight dextran sulfate, and hyaluronan. HUVEC were incubated for 48 hr with media containing these GAGs and related compounds and with media without GAG as control. ET-1 levels were measured by radioimmunoassay. GAGs and related molecules with higher sulfate content, heparin, chondroitin sulfate B, low and high molecular weight dextran sulfates significantly suppressed ET-1 production by HUVEC. Fucoidin also suppressed ET-1 production despite its lower sulfate content, probably because of its structural similarity to heparin. These compounds may be useful for future in vivo studies.

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.

Regulation of natriuretic peptide secretion by the heart

Secreted by the heart, more specifically by atrial cardiomyocytes under normal conditions but also by ventricular myocytes during cardiac hypertrophy, natriuretic peptides are now considered important hormones in the control of blood pressure and salt and water excretion. Studies on natriuretic peptide secretagogues and their mechanisms of action have been complicated by hemodynamic changes and contractions to which the atria are constantly subjected. It now appears that atrial stretch through mechano-sensitive ion channels, adrenergic stimulation via alpha 1A-adrenergic receptors, and endothelin via its ETA receptor subtype are major triggering agents of natriuretic peptide release. With several other stimuli, such as angiotensin II and beta-adrenergic agents, modulation of natriuretic peptide release appears to be linked to local generation of prostaglandins. In all cases, intracellular calcium homeostasis, controlled by several ion channels, is considered a key element in the regulation of natriuretic peptide secretion.

Inotropic effects of endothelin-1: interaction with molsidomine and with BQ 610

-In vivo studies could not detect a positive inotropy of endothelin (ET)-1 as described in in vitro experiments. ET-induced direct positive inotropy, which seems to be mediated by ETB receptors, may be antagonized in vivo by an indirect cardiodepressive effect owing to an ET-induced coronary vasoconstriction via ETA receptors. This study compares the effects of a dose of 1 nmol/kg ET-1 alone on myocardial contractility and myocardial energy metabolism with the effects of 1 nmol/kg ET-1 after pretreatment with 5 mg/kg molsidomine or with 100 microg/kg of the ETA receptor antagonist BQ 610. We investigated the effects of ET-1 versus saline controls in open-chest rats. In addition to measurements in the intact circulation, myocardial function was examined by isovolumic registrations independent of peripheral vascular effects. We also studied the effect of ET-1 on myocardial high-energy phosphates. Pretreatment with molsidomine and BQ 610 attenuated the ET-induced reduction of cardiac output (ET-1: -62%; molsidomine+ET-1: -47%; BQ 610+ET-1: -27% different from controls). After a transient initial vasodilation, ET-1 raised total peripheral resistance (ET-1: +190%; molsidomine+ET-1: +171%; BQ 610+ET-1: +89%). BQ 610 was more effective in preventing ET-induced vasoconstriction. The increase of isovolumic peak first derivative of left ventricular pressure (ET-1: -2%; molsidomine+ET-1: +16%; BQ 610+ET-1: +19%) after pretreatment with molsidomine or BQ 610 indicates that these drugs unmask the positive inotropy of ET-1. ET-induced myocardial ischemia was abolished by molsidomine and BQ 610. Pretreatment with molsidomine or blockade of ETA receptors by BQ 610 can unmask the positive inotropy of ET-1 by preventing ET-induced myocardial ischemia. The positive inotropic effect of ET-1 seems to be mediated by ETB receptors.

Hemodynamic and inotropic effects of the endothelin A antagonist BQ-610 in vivo

The positive inotropy of endothelin-1 (ET-1) described by in vitro studies is not detectable in vivo because this effect is antagonized by cardiodepressive effects due to ET-induced vasoconstriction with subsequent myocardial ischemia. This vasoconstriction is mainly mediated by ETA receptors. In a previous in vivo study with a selective ETB receptor agonist, we showed that ETB receptors play an important role in the ET-induced positive inotropy. The present in vivo study examined whether selective ETA receptor blockade can unmask the ETB receptor-mediated positive inotropy of endogenous ET-1 by preventing its cardiodepressive effects via ETA receptors. In an open-chest rat model, we compared the acute hemodynamic and inotropic effects of the highly selective ETA receptor antagonist BQ-610 (100 micrograms/kg) with NaCl controls during and after a 7-min infusion. In addition to measurements in the intact circulation, the effects on myocardial contractility were studied by isovolumic registrations (peak LVSP, peak dP/dtmax), which are independent of peripheral vascular effects. Acute blockade of the ETA receptors by BQ-610 had no effect on blood pressure and heart rate. BQ-610 caused vasodilatation (total peripheral resistance -7.5% vs. control at the end of infusion; p < 0.01) with a consecutive increase in stroke volume (+15.3%; p < 0.01), cardiac output (+15.4%; p < 0.001), and ejection fraction (+10.4%; p < 0.01). The isovolumic measurements indicated a significant positive inotropic effect of BQ-610 (peak LVSP + 4.2%, p < 0.01; peak dP/dtmax + 5.5%, p < 0.01). Therefore, selective ETA receptor blockade by BQ-610 improves the hemodynamics in the intact circulation by causing a reduction in afterload and an increase in myocardial contractility. The positive inotropic effect of BQ-610 may be mediated by the positive inotropy of endogenous ET-1 via ETB receptors after selective ETA receptor blockade.

Endothelin: receptor subtypes, signal transduction, regulation of Ca2+ transients and contractility in rabbit ventricular myocardium

Endothelin (ET) isopeptides, ET-1, ET-2 and ET-3, elicit a positive inotropic effect (PIE) in association with a negative lusitropic effect, essentially with identical efficacies and potencies in the isolated rabbit papillary muscle, but with different concentration-dependent properties. Pharmacological analysis indicates that the PIE of ET-1 is mediated by an ETA2 subtype that is less sensitive to BQ-123 and FR139317, whereas the PIE of ET-3 is mediated by an ETA1 subtype that is highly sensitive to these ETA antagonists. ETs increased the amplitude of intracellular Ca2+ transient (CaT) in indo-1 loaded rabbit ventricular myocytes, but the increase was much smaller than that produced by elevation of [Ca2+]o or isoproterenol for a given extent of PIE, an indication of increased myofibrillar Ca2+ sensitivity. ETs stimulate phosphoinositide (PI) hydrolysis, which leads to production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Evidence for the role of IP3-induced Ca2+ release in cardiac E-C coupling is tenuous. Generation of IP3 induced by ET-1 was transient and returned to the baseline level when the PIE reached an elevated steady level. Protein kinase C (PKC) that is activated by DAG and also via other pathways triggered by ETs stimulates Na+-H+ exchanger to lead to an increased [Na+]i and alkalinization. The former may contribute to an increase in the amplitude of CaT through Na+-Ca2+ exchanger, and the latter, to an increase in myofibrillar Ca2+ sensitivity. A number of PKC inhibitors, such as staurosporine, H-7, calphostin C and chelerythrine, consistently and selectively inhibited the PIE of ET-3 without affecting the PIE of isoproterenol and Bay k 8644. The maximum inhibition was 20-30% of the total response. A Na+-H+ exchange inhibitor, [5-(N-ethyl-N-isopropyl) amiloride (EIPA)] or a Ca2+ antagonist, verapamil, could not completely inhibit the PIE of ET-3, but the combination of both inhibitors totally abolished the PIE of ET-3. These findings indicate that activation of PKC and subsequent activation of Na+-H+ exchanger and/or L-type Ca2+ channels may play a crucial role in the cardiac action of ET isopeptides in the rabbit ventricular myocardium.

Hypoxia regulates expression of the endothelin-1 gene through a proximal hypoxia-inducible factor-1 binding site on the antisense strand

Endothelin-1 (ET-1) is a peptide hormone with potent vasoconstrictor properties that is synthesized and secreted predominantly by vascular endothelial cells. Its production is regulated by numerous stimuli including ischemia and hypoxia, and the enhanced levels that occur during myocardial ischemia may contribute to the progression of heart failure. We previously reported that ET-1 expression was induced by both hypoxia and transition metals in endothelial cells (ECs). Here we define an element in the proximal promoter of the ET-1 gene that is responsible for this induction. By using deletions and site directed mutagenesis of the human ET-1 promoter, in combination with electrophoretic gel mobility shifts and transient expression assays in human ECs, we identified an active hypoxia-inducible factor 1 (HIF-1) binding site starting at position -118 upstream of the transcription start site on the non-coding DNA strand. Mutation of this site eliminated induction by hypoxia without affecting basal (aerobic) expression, and the mutated sequence did not display hypoxia-specific binding of HIF-1.

Cardiovascular and renal actions of the endothelin(B) receptor in pigs

Previously we showed that blocking the endothelin (ET)A receptor subtype with BQ-153 inhibited the vasoconstrictor effects of intravenously administered ET-1. In the presence of the ET(A) antagonist, ET-1 produced marked reductions in myocardial contractility and renal blood flow. We postulated that either the ET(B) receptor, or some other, as yet unidentified, ET-receptor subtype mediated the observed hemodynamic changes. In anesthetized pigs, this hypothesis was tested by using a recently developed selective, high-affinity antagonist to the ET(B) receptor, BQ-788, and sarafotoxin S6c, a selective ET(B) agonist, to determine the contribution of this receptor subtype to cardiovascular function. Endothelin-1 (0.4 nmol/kg, i.v.) produced the characteristic biphasic hemodynamic responses, consisting of an initial transient reduction in mean arterial pressure (MAP; 83 +/- 3 to 72 +/- 4 mm Hg; n = 9) followed by a prolonged increase (112 +/- 4 mm Hg; p < 0.01). As well, cardiac output (-58%; p < 0.05), myocardial contractility (-19%; p < 0.01), and renal blood flow (63%; p < 0.05) decreased. Sarafotoxin S6c produced marked but transient reductions in MAP (p < 0.001), cardiac output (p < 0.01), myocardial contractility (p < 0.001), and renal blood flow (p < 0.05). BQ-788 (1.0 mg/kg, i.v.), administered 3 min before sarafotoxin S6c, inhibited its effects. BQ-788 also inhibited the initial transient reduction in MAP seen after the injection of ET-1, but the subsequent sustained pressor responses were enhanced as reflected in the greater increases in left ventricular pressure (p < 0.02), myocardial contractility (p < 0.05), MAP (p < 0.01), and a larger reduction in cardiac output (p < 0.05). The heart rate was not changed after the initial ET injection, but it increased 54% when the peptide was administered in the presence of BQ-788. The reduction in renal blood flow was still evident, and its magnitude (64%) remained the same (p < 0.01) after treatment with BQ-788. Only the combined administration of both the ET(A) (BQ-123) and ET(B) (BQ-788) receptor antagonists blocked the effects of ET-1 on renal blood flow (p < 0.05). These data confirm that BQ-788 is a selective and effective antagonist of the ET(B) receptor and show that activation of this receptor subtype is involved in the transient vasodilation provoked by ET-1. Additionally, the ET(B) receptor appears to oppose the vasoconstrictor effects of the ET(A) receptor, which clearly mediates vasoconstriction. Combined treatment with BQ-123 and BQ-788 attenuated the reductions in renal blood flow produced by ET-1. Furthermore, some actions of ET-1 were not blocked by these antagonists and cannot be attributed to either the ET(A) or ET(B) receptors. We hypothesize the existence of an additional ET receptor or a subtype of the ET(B) receptor that is insensitive to BQ-788.

Endothelins and cardiac hypertrophy

Cardiac hypertrophy is a commonly observed complication of a variety of cardiovascular diseases. As well as mechanical stresses such as pressure overload, several humoral factors may contribute to the development of cardiac hypertrophy. In the early 1990s, endothelin-1 (ET-1) was found to induce cardiomyocyte hypertrophy in cultured neonatal rat cardiomyocytes. Furthermore, evidence is accumulating to date to support the postulate that ET-1 may function as an autocrine/paracrine factor in the development of cardiac hypertrophy in vivo as well as in vitro In this review article, I discuss physiological and pathophysiological roles of ET-1 and its related peptides in cardiac hypertrophy associated with various cardiovascular diseases.

Endothelin-1 inhibits L-type Ca2+ current enhanced by isoprenaline in rat atrial myocytes

Endothelin-1 (ET-1) was shown to exert direct cardiac effects by complex signaling pathways and to interact with neurotransmitter regulation of cardiac activity. The effect of ET-1 was investigated on the beta-adrenergic stimulation of cardiac L-type Ca2+ current (ICaL) on isolated rat atrial myocytes by using the patch-clamp technique. ET-1 (5 x 10(-8) M) reversed the increase in ICaL induced by isoprenaline (10(-6) M) but had no effect on basal ICaL and on (-) Bay K 8644-increased ICaL (10(-6) M); so ET-1 might exert an effect only when the Ca2+ channels are phosphorylated. The antiadrenergic action of ET-1, blocked by BQ-123 (10(-6) M) and unaffected by IRL 1038 (3.5 x 10(-8) M) should be mediated by ET-A receptors. The inhibitory action of ET-1 was still observed when ICaL was previously increased by forskolin (3 x 10(-6) M), 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP; 200 microM), or cAMP (100 microM) in presence of isobutyl methyl xanthine (IBMX; 10(-6) M), suggesting that the antiadrenergic action of ET-1 on ICaL was exerted independent of the cAMP-dependent phosphorylation pathway. ET-1 is known to be an activator of phosphoinositide hydrolysis, resulting in an increased production of IP3 and diacylglycerol (DAG). A Ca(2+)-dependent inhibition of ICaL consequently to an elevation of the intracellular Ca2+ pool via IP3 might be excluded in the action of ET-1, because of the presence of EGTA in the intrapipette medium. ET-1 reversed the isoprenaline-induced increase in ICaL in the presence of protein kinase C inhibitor [PKC(19-31); 100 microM), making unlikely the involvement of a DAG-dependent activation of PKC. Therefore the antiadrenergic action of ET-1 might also be independent on the phosphoinositide pathway.

Coronary vasoconstriction by endothelin-1 in anesthetized goats: role of endothelin receptors, nitric oxide and prostanoids

The role of endothelin ETA and ETB receptors as well as of nitric oxide (NO) and prostanoids in the effects of endothelin-1 on the coronary circulation was studied in anesthetized goats. Where blood flow in the left circumflex coronary artery (coronary blood flow) (electromagnetically measured), systemic arterial pressure, left ventricle pressure and d P/dt, and heart rate were recorded. Endothelin-1 (0.01-0.3 nmol), intracoronarily injected, produced marked, dose-dependent reductions in basal coronary blood flow, ranging from 5% for 0.01 nmol to 75% for 0.3 nmol; 0.1 and 0.3 nmol endothelin-1 also reduced systolic ventricle pressure and dP/dt. The effects of endothelin-1 on coronary blood flow were diminished during intracoronary infusion of BQ-123 (cyclo-(D-Asp-Pro-D-Val-Leu-D-Trp). specific antagonist for endothelin ETA receptors. 2-16 nmol/min) in a dose-dependent way, but not during the infusion of BQ-788 (N-[N-[N-[(2.6-dimethyl-1-piperidinyl)carbonyl]-4-methyl-1-leucyl]-1- (methoxycarbonyl)-D-tryptophyl]-D-norleucine monosodium, specific antagonist for endothelin ETB receptors. 2-4 nmol/min). IRL 1620 (Suc-[Glu9, Ala11.15]endothelin-1-(8-21), specific agonist for endothelin ETB receptors. 0.01-0.3 nmol), intracoronarily injected. slightly reduced basal coronary blood flow only when 0.1 and 0.3 nmol were applied (maximal reduction about 25%); 0.3 nmol IRL 1620 also reduced systolic ventricle pressure and dP/dt. The effects of IRL 1620 were not modified by BQ-123 or BQ-788. NG-nitro-1-arginine methyl ester (L-NAME, inhibitor of NO synthesis, 47 mg/kg by i.v. route) reduced resting coronary blood flow by 10% and increased mean systemic arterial pressure and systolic ventricle pressure by 22 and 20%. respectively, without changing systolic ventricle dP/dt and heart rate. With L-NAME, the reductions of coronary blood flow by endothelin-1 were potentiated (P < 0.05), and those by IRL 1620 were not changed (P > 0.05). Meclofenamate (cyclooxygenase inhibitor, 4-6 mg/kg by i.v. route) modified neither the basal values of hemodynamic variables nor the coronary effects of endothelin-1 and IRL 1620. Therefore, endothelin-1 produces marked coronary vasoconstriction, which may be mediated by endothelin ETA receptors, with no participation of endothelin ETB receptors. NO, but not prostanoids, may produce a basal coronary vasodilator tone and may inhibit endothelin-1-induced coronary vasoconstriction. Also, it is suggested that the coronary vasoconstriction by endothelin-1 may impair cardiac performance due to heart ischemia.

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.

Increased concentrations of endothelin-1 messenger RNA in tissues and endothelin-1 peptide in plasma in septic pigs: modulation by betamethasone

OBJECTIVES

To study the expression of preproendothelin-1 messenger RNA (mRNA) in tissue after Escherichia coli lipopolysaccharide challenge and to evaluate the possible effects of betamethasone both regarding endothelin-1 production as well as hemodynamic and vascular effects during E. coli lipopolysaccharide infusion in pigs in vivo.

DESIGN

Prospective trial.

SETTING

Laboratory at a university medical center.

SUBJECTS

Ten domestic pigs, weighing 18 to 25 kg.

INTERVENTIONS

Anesthetized pigs were given continuous infusions of E. coli lipopolysaccharide (15 micrograms/kg/hr for 3 hrs), with or without prior treatment with betamethasone (0.5 mg/kg im 12 hrs before the start of the surgical preparation and 0.5/kg iv at the start of the preparation).

MEASUREMENTS AND MAIN RESULTS

The E. coli lipopolysaccharide infusion evoked the characteristic cardiovascular changes observed in septic shock: decreased mean arterial pressure and cardiac output; increased heart rate and increased pulmonary vascular resistance. Large increases in both arterial plasma concentrations of endothelin-1-like immunoreactivity, as well as preproendothelin-1 mRNA concentrations in tissues, were also observed during the E. coli lipopolysaccharide infusion. Treatment with betamethasone significantly attenuated the E. coli lipopolysaccharide-induced increase in endothelin-1 plasma concentrations, whereas the increased mRNA concentrations were only slightly affected. Furthermore, betamethasone treatment also affected cardiovascular parameters, with significant attenuation of the E. coli lipopolysaccharide-induced increase in heart rate and a higher cardiac output after 60 mins of the E. coli lipopolysaccharide infusion. The urine production, which was markedly decreased during the E. coli lipopolysaccharide infusion, was significantly higher in the betamethasone-treated group compared with the control group.

CONCLUSIONS

The present results indicate that the increased concentrations of endothelin-1-like immunoreactivity that are observed in septic shock may have negative effects on both cardiovascular parameters as well as renal function, which is in agreement with a possible role for endothelin-1 in the pathogenesis of septic shock.

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.

Endothelin-1 inhibits L-type Ca currents enhanced by isoproterenol in guinea-pig ventricular myocytes

To investigate the action of endothelin-1 (ET-1) on L-type Ca currents (ICa,L) in guinea-pig ventricular cells, whole-cell currents were recorded at approximately 36-37 degrees C in enzymatically isolated myocytes. ET-1 (> or =10 nM) suppressed the basal ICa,L to 79+/-8% of control at 20 nM. Bath application of isoproterenol (ISO; 10 nM) enhanced ICa,L to 192+/-28% with about a -10-mV shift of its relationship with membrane potential. ET-1 concentration dependently inhibited this ISO-enhanced ICa,L with a half-maximally inhibitory concentration (IC50) of 168 pM. The inhibitory actions of ET-1 were antagonised by BQ-123 (300 nM), cyclo(D-Asp-L-Pro-D-Val-L-Leu-D-Trp), a specific ETA receptor antagonist. Histamine-enhanced ICa,L was also suppressed by ET-1, but ICa, L potentiated by internal adenosine 3',5'-cyclic monophosphate (cAMP) was unaffected. Preincubation of myocytes with pertussis toxin (PTX, at 5 microgram/ml for >60 min at 36 degrees C) completely occluded the ET-1 action. Thus, stimulation of ETA receptors by subnanomolar ET-1 inhibits ICa,L via PTX-sensitive G-proteins.

Inhibition of endothelin- and phorbol ester-stimulated tyrosine kinase activity by corticotrophin in the rat adrenal zona glomerulosa

1. The experiments described in this study were carried out to investigate the role of tyrosine kinase in the acute adrenal response to peptide hormone stimulation, and to determine whether the activity of this kinase may be subject to regulation by other intracellular signalling mechanisms in the adrenal zona glomerulosa. 2. Previous studies from this laboratory have shown that angiotensin II stimulates tyrosine kinase activity in the rat adrenal cortex. This study has shown, for the first time, that endothelin-1 also stimulates tyrosine kinase activity in this tissue. 3. Using the specific inhibitor of protein kinase C (PKC) activity, Ro 31-8220, we have shown that stimulation of tyrosine kinase activity, in response to endothelin-1, angiotensin II or the phorbol ester phorbol 12-myristate 13-acetate, is at least partly dependent on increased PKC activity. 4. The data presented also provide further evidence of cross-talk between signalling systems in the adrenal cortex. Corticotrophin and its intracellular second messenger, cyclic AMP, significantly attenuate the increment in tyrosine kinase activity seen in response to each of the effectors used. 5. The results of this study provide important new evidence for the regulation of protein kinases by other intracellular second messenger systems.

Effect of volume support, antibiotic therapy, and monoclonal antiendotoxin antibodies on mortality rate and blood concentrations of endothelin and other mediators in fulminant intra-abdominal sepsis in rats

OBJECTIVE

To study the therapeutic effects of volume support, antibiotics, and a monoclonal antiendotoxin antibody on the mortality rate and blood concentrations of endothelin and other mediators in fulminant intra-abdominal sepsis in rats.

DESIGN

Prospective, randomized, controlled trial.

SETTING

Research laboratory in a university hospital.

SUBJECTS

Adult male Wistar rats.

INTERVENTIONS

Fulminant polymicrobial intra-abdominal sepsis was induced by a 4-mm cecal perforation. Treatment was performed with saline volume support, the antibiotic imipenem/cilastatin, and the monoclonal antiendotoxin antibody E5, both as monotherapy and as a combined regimen. Mortality rates were recorded and concentrations of bacteria, endotoxin, tumor necrosis factor (TNF), big endothelin, and endothelin-1 (21 amino acids) in blood were determined.

MEASUREMENTS AND MAIN RESULTS

Substantial increases in circulating big endothelin and endothelin-1 concentrations were observed during sepsis. The combination of volume support with antibiotics reduced the mortality rate, but neither as monotherapy nor as a combined regimen did this intervention modify plasma endothelin-1 concentrations. This finding suggests that hypovolemia and bacteria per se are not important stimuli for endothelin synthesis and a high plasma level of endothelin-1 does not necessarily predict poor outcome in sepsis. The inactive big endothelin is enzymatically cleaved, leaving the biologically active 21-residue endothelin-1. Intervention with E5 substantially reduced the mortality rate and concentrations of endotoxin, TNF, and plasma endothelin-1, while big endothelin and total endothelin immunoreactivity did not decrease. This finding indicates a suppressed conversion of big endothelin to endothelin-1 after E5 treatment. Because E5 has no direct effect on endothelin metabolism, E5 probably reduces the synthesis of endothelin-1 by suppressing the endothelin-activators endotoxin and TNF. A triple combination of volume support, imipenem/cilastatin, and E5 was the only regimen that reduced all of the end points: mortality rate, hemoconcentration, bacteria, endotoxin, TNF, and endothelin-1.

CONCLUSIONS

The concentration of plasma endothelin was increased during fulminant intra-abdominal sepsis in rats. Combining volume support with antibiotic therapy reduced the mortality rate, but did not modify concentrations of plasma endothelin-1. The monoclonal antiendotoxin antibody E5 reduced the mortality rate and concentrations of endotoxin, TNF, and endothelin-1, but not big endothelin. This finding indicates that E5 therapy inhibits the conversion of big endothelin to 21-residue endothelin-1.

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.

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.

Negative chronotropic effect of endothelin 1 mediated through ETA receptors in guinea pig atria

Endothelins exert potent excitatory cardiac effects by acting on specific receptors on myocytes. In this study, we have examined the signal transduction mechanism for the chronotropic effect of endothelins in guinea pig atria. A competition binding of [125I]endothelin 1 ([125I]ET-1) using the recently developed ETA receptor-selective antagonist BQ123 showed the presence of almost equal populations of ETA (44%) and ETB (56%) receptors in the guinea pig right atria. In a concentration-response study, endothelin 3 (ET-3), an agonist with higher affinity to ETB receptors than to ETA receptors, and sarafotoxin S6c (STXS6c), an ETB receptor-selective agonist, increased the rate of spontaneous beating at all concentrations tested (10 pmol/L to 100 nmol/L). In contrast, ET-1, a nonselective agonist, increased the heart rate at lower concentrations (10 pmol/L to 10 nmol/L) but decreased it at higher concentrations (30 to 100 nmol/L). When ET-1 (100 nmol/L) was applied in a single amount, heart rate was strongly increased; however, this increase was followed by a rapid decline in the response. ET-1 (100 nmol/L) but not ET-3 or STXS6c significantly reduced the heart rate when it was raised by isoproterenol (ISO, 300 nmol/L) either in the absence or presence of a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). Correspondingly, ET-1 significantly reduced the ISO-induced elevation of cAMP accumulation (19.1 +/- 1.7 pmol/mg protein [n = 8] and 12.6 +/- 1.2 pmol/mg protein [n = 7] in the absence and presence of ET-1, respectively; P < .01), which was also observed even in the presence of IBMX.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of endothelin-1-induced vascular effects in the rat heart by using endothelin receptor antagonists

The coronary vasoconstrictor effect of endothelin-1 was characterized in the isolated rat heart by using the endothelin ETA receptor antagonist D-Asp-L-Pro-D-Val-L-Leu-D-Trp (BQ-123) and the endothelin ETB receptor antagonist [Cys11-Cys15]endothelin-1-(11-21) (IRL 1038). In addition, the involvement of nitric oxide and cyclooxygenase products was investigated. Endothelin-1 (0.012-0.4 nmol) dose dependently reduced coronary flow, which reached a maximum reduction of 83% at 0.4 nmol. BQ-123 (1 microM) attenuated the responses to all doses of endothelin-1, whereas a lower concentration of BQ-123 (0.1 microM) only reduced the vasoconstriction due to the lower doses of endothelin-1 (0.012-0.12 nmol). In contrast, IRL 1038, which markedly antagonized the vasodilator response to the endothelin ETB receptor agonist Suc-[Glu9,Ala11,15]endothelin-1-(8-21) (IRL 1620), significantly enhanced the endothelin-1-evoked coronary vasoconstriction. Endothelin-1 (0.04 nmol) reduced coronary flow by 61% in the presence of IRL 1038 as compared to 30% in the absence of the endothelin ETB receptor antagonist. The endothelin-1-evoked reduction in coronary flow was also significantly enhanced by the nitric oxide synthesis inhibitor NG-nitro-L-arginine but was unaffected by the cyclooxygenase inhibitor diclofenac. IRL 1038 did not affect the response to endothelin-1 after blockade of nitric oxide synthesis. These results demonstrate that the coronary vasoconstriction induced by endothelin-1 in the isolated rat heart is a net effect of the stimulation of both endothelin ETA and endothelin ETB receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

A low-affinity, low-molecular-mass endothelin-A receptor in neonatal rat heart

Endothelin receptors with endothelin-A (ETa) specificity were present in neonatal rat ventricle. However, in both receptor-binding studies and studies of inositol phosphate accumulation, these receptors had lower affinity for endothelin-1 than ETa receptors on isolated neonatal cardiomyocytes or adult left atria. Receptors in the three myocardial preparations were cross-linked to 125I-endothelin-1 and their molecular masses measured using SDS/PAGE. Receptors on left atria and neonatal cardiomyocytes had the expected molecular mass of 48 kDa, whereas the receptors in neonatal ventricle were smaller (38 kDa). Despite this, neonatal ventricles contained ETa receptor mRNA which was not different in size from that in the isolated cells (4.5 kb). Thus the 38 kDa ETa receptor present in neonatal ventricle appears to be transcribed from full-length ETa receptor mRNA and is possibly formed by processing of the 48 kDa receptor.

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.

Identification of a 65 kDa endothelin receptor in bovine cardiac sarcolemmal vesicles

Endothelin-1, an endothelial cell-derived vasoconstrictor peptide, also exerts a potent positive inotropic effect on cardiac tissue. Characterization of specific binding of endothelin-1 to bovine cardiac sarcolemmal vesicles is reported. In the presence of 1 mM CaCl2, the observed binding for 125I-endothelin-1 had a Kd of 6.2 nM with an observed Bmax of 14 pmol/mg sarcolemmal protein. In the presence of 1 mM EDTA (and no added Ca2+) Bmax was reduced to 9 pmol/mg sarcolemmal protein while the Kd remained unchanged. Binding affinity for sarafotoxin S6b was at least one order of magnitude less than for endothelin-1. 125I-Endothelin-1 covalently cross-linked to a sarcolemmal protein with an apparent molecular weight of 65 kDa. Site-directed polyclonal antibodies to a sequence located on the third extramembranal segment of a previously cloned endothelin ETA receptor from bovine lung were produced. Using Western blot analysis, the site-directed polyclonal antibody recognized a sarcolemmal protein at 65 kDa. We conclude that sarcolemmal membranes from bovine ventricular myocardium contain an endothelin binding site and that it is a protein with an apparent molecular weight of 65 kDa.

Hemodynamic and inotropic effects of endothelin-1 in vivo

Endothelin-1 (ET-1) is known to have strong vasoactive properties. Contradictory results have been reported with regard to its inotropic effects. This study examined the dose-dependent (500, 1000, 2500, 5000 and 10,000 ng ET-1/kg vs. NaCl controls) hemodynamic and inotropic effects of ET-1 in 53 open-chest rats during and after a 7-min infusion. Besides measurements in the intact circulation the myocardial function was examined by isovolumic registrations independent of peripheral vascular effects. A transient ET-1 induced (500, 1000, 2500, 5000 ng ET-1/kg) decrease of the left ventricular systolic pressure (LVSP) and the mean aortic pressure (AoPmean) was followed by a dose-related rise of these pressures (LVSP: -1%, -1%, +8%, +16% vs. preinfusion values; AoPmean: -11%, +9%, +39%, +52%). Heart rate (HR) was not influenced by ET-1. Due to the dose-dependent decrease of the stroke volume (SV) the cardiac output (CO) was reduced (CO: -8%, -23%, -40%, -50%). After an initial vasodilatation ET-1 elevates the total peripheral resistance (TPR: -1%, +49%, +139%, +215%) dose-dependently. 10,000 ng ET-1/kg was a lethal dose resulting in cardiac failure within minutes (low output). Since the maximum of the isovolumic LVSP (peak LVSP) and the corresponding dP/dtmax (peak dP/dtmax) were unchanged under ET-1, the isovolumic measurements do not indicate a positive inotropic effect of ET-1 in vivo in contrast to published results of in vitro experiments. It may be possible that a direct positive inotropic effect of ET-1 observed in in vitro studies is counterbalanced in vivo by an indirect negative inotropic effect due to the coronary-constrictive effect of ET-1.

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.

A low affinity, low molecular weight endothelin-A receptor present in neonatal rat heart

1. Addition of endothelin-1 (ET-1) to [3H]-inositol-labelled neonatal rat hearts stimulated the accumulation of [3H]-labelled inositol phosphates (InsP), but only at high concentrations; concentration at half maximum stimulation (EC50) > 0.1 mumol/L). When similar experiments were performed using isolated myocytes, the potency of endothelin-1 was higher and the EC50 value averaged 3.2 +/- 0.5 nmol/L (mean +/- s.e.m., n = 4). 2. The binding affinity of [125I]-endothelin-1 was higher for receptors on isolated cells than for receptors on membranes prepared from intact heart (72 +/- 16 pmol/L compared with 3.9 +/- 0.7 nmol/L, mean +/- s.e.m., n = 4, P < 0.01; Students' t test). 3. Receptors from both sources were cross-linked to [125I]-endothelin-1 and their molecular weights measured using sodium dodecylsulfate gradient polyacrylamide gel electrophoresis (SDS-PAGE). The receptors present on the isolated cells had a higher molecular weight (48 kD) than the receptor on the heart membranes (38 kD).

Effect of dysfunctional vascular endothelium on myocardial performance in isolated papillary muscles

Vascular endothelium has been shown to modify the contractile characteristics of vascular smooth muscle, and endocardial endothelium has been shown to modify the contractile characteristics of adjacent myocardium. In this study, whether vascular endothelium also modifies the contractile characteristics of adjacent myocardium and whether these effects are additive to those of endocardial endothelium were investigated. Rabbit hearts (n = 54) were excised and mounted in a Langendorff preparation. Vascular reactivity was verified by acetylcholine infusion. One group of these hearts had Triton X-100 injected as a bolus into the coronaries to render the vascular endothelium dysfunctional. The other portion served as control hearts. Triton X-100 bolus injection resulted in little or no pathological changes on morphological examination; however, the vasodilatory response to acetylcholine in these hearts was abolished, suggesting vascular endothelial dysfunction. Vascular smooth muscle reactivity was verified in Triton X-100-injected hearts by nitroprusside infusion. In the control Langendorff-perfused hearts, there was little evidence of vascular endothelial dysfunction, with the coronary perfusion rate increasing from 8.9 +/- 0.4 to 11.0 +/- 0.3 ml/g per minute (p < 0.01) in response to acetylcholine. All hearts were then removed, and right ventricular papillary muscles were excised for myocardial mechanical studies. Control Langendorff-perfused hearts had myocardial mechanical characteristics similar to those of muscles from 18 other control hearts without Langendorff perfusion, indicating that the Langendorff perfusion itself had little effect on myocardial mechanics. The muscles from the Triton X-100-injected Langendorff hearts had marked changes: a shortening of twitch duration (363 +/- 16 versus 449 +/- 9 msec, p < 0.01) and decreases in total tension (2.2 +/- 0.2 versus 2.9 +/- 0.2 g/mm2, p < 0.01), dT/dt (9 +/- 1 versus 12 +/- 1 g/mm2 per second, p < 0.05), and maximum velocity of unloaded muscle shortening (Vmax) (0.89 +/- 0.06 versus 1.14 +/- 0.07 length at which maximum developed tension occurred [Lmax]/sec, p < 0.05). Endocardial endothelial removal of the papillary muscles in the two control groups (with and without Langendorff perfusion) by Triton X-100 caused the same changes in twitch characteristics as occurred in muscles from the Langendorff-perfused hearts injected with Triton X-100 but with intact endocardial endothelium, suggesting that vascular endothelial dysfunction had similar effects on contractile characteristics as endocardial endothelial removal.(ABSTRACT TRUNCATED AT 400 WORDS)

The production of coronary vasoconstrictor substances by freshly harvested endothelial cells

The effects of cell free superfusates from freshly harvested bovine endothelial cells attached to microcarrier beads on the isolated rabbit and rat heart and on superfused rabbit jugular veins were observed. Cell free conditioned filtrates from freshly harvested cells caused marked diminution in coronary flow and cardiac output in the isolated rabbit heart; in the perfused rat heart an increase in coronary perfusion pressure and a decline in left ventricular systolic tension and maximal left ventricular contractility (dP/dt) were recorded. Marked differences were found between changes induced by conditioned filtrate as compared to synthetic endothelin. Endothelin as present in conditioned filtrate could not account for the pronounced effect on coronary perfusion pressure, dp/dt and cardiac output induced by conditioned filtrate; more than one hundred times that of synthetic endothelin was needed to achieve comparable cardiodynamic effects. This suggested that additional non-prostanoid vasoconstrictor substance or substances are produced by freshly harvested endothelial cells. This conclusion was supported by the observation that BQ-123, a specific inhibitor of endothelin A (ETA) receptor significantly prevented contractions by endothelin, while failing to inhibit those induced by freshly harvested endothelial cells. These constrictor substances may be leukotrienes.

Plasma endothelin, atrial natriuretic peptide (ANP) and uterine and umbilical artery flow velocity waveforms in hypertensive pregnancies

OBJECTIVE

To investigate the relation between concentrations of endothelin and atrial natriuretic peptide (ANP) in maternal plasma and vasospasm in the uterine and umbilical arteries as detected by duplex pulsed colour Doppler ultrasonography in hypertensive pregnancies.

DESIGN

An observational study.

SUBJECTS

32 women admitted consecutively to hospital with pregnancy induced hypertension (seven without proteinuria and 25 with proteinuria) and 78 healthy pregnant women examined at 28-40 weeks gestation.

MAIN OUTCOME MEASURES

Systolic/diastolic (S/D) ratio in flow velocity waveforms (FVWs) and plasma concentrations of endothelin and ANP in the 32 women with pregnancy induced hypertension; plasma concentrations of endothelin and ANP in 78 healthy pregnant women (controls).

RESULTS

Pathological FVWs suggesting vasospasm in the uterine or umbilical artery, or both arteries, were found in 12 women with hypertension. Plasma ANP was significantly higher (P = 0.03) in the women with hypertension and pathological FVWs (median 23.0, range 10.1-52.8 pmol/l) than in those with hypertension and normal FVWs, (median 13.8, range 5.3-42.3 pmol/l) but corresponding plasma endothelin levels did not show any significant difference (median 1.63, range 0.51-3.33 pmol/l and median 1.38, range 0.51-3.51 pmol/l, respectively).

CONCLUSION

Local release of endothelin from the vascular endothelium is thought to cause vasospasm in pregnancy induced hypertension but this does not seem to increase the concentration of endothelin in the maternal peripheral plasma, probably because of its rapid disappearance from the blood circulation. As ANP dilates the blood vessels, the increase of its release in hypertensive pregnancies may be a compensatory mechanism against vasospasm.

Endothelin peptides: biological activities, cellular signalling and clinical significance

Endothelins (ET-1, ET-2 and ET-3) are a family of 21 amino acid peptides produced by endothelial cells. They are thought to regulate the local vasomotor tone with endothelium-derived relaxing factors. ETs are the most potent vasoconstrictor substances yet identified and veins and renal vasculature are the most sensitive targets. They reduce cardiac output and have positive inotropic and chronotropic effects. ETs increase the secretion of atrial natriuretic peptide (ANP), aldosterone and catecholamines but reduce renal blood flow and glomerular filtration and they also have mitogenic properties. ETs bind to receptors (ETA and ETB), activate phospholipase C, modulate intracellular Ca2+ concentration and open Ca2+ channels. Vasoactive agents (adrenaline, angiotensin, vasopressin, thrombin, endotoxins) and hypoxia stimulate the release of ET and also ET gene expression. Raised concentrations of plasma ET have been found to occur in several clinical conditions such as hypertension, myocardial infarction, cardiogenic shock, pregnancy induced hypertension, arteriosclerosis, Raynaud's disease, subarachnoid haemorrhage, uraemia, ulcerative colitis, Crohn's disease and surgical operations suggesting that ETs have a role in several patophysiological processes.

Interaction between vasopressin and endothelin in renal papillary tubules: uncoupling following cell isolation and culture

1. In freshly prepared rat renal papillary tubules, endothelin-related peptides inhibited the vasopressin-stimulated accumulation of cAMP. No inhibition was observed when tubules were cultured overnight ex vivo. 2. Endothelin-1, endothelin-3 and sarafatoxin S6b had similar potencies as inhibitors of cAMP accumulation, indicating an endothelin (ETb) receptor. 3. These results demonstrate an interaction between ETb receptors and vasopressin receptors at the level of their signal transduction pathways, and show that this relationship is lost following cell culture.

Activity of sarafotoxin/endothelin peptides in the heart and brain of lower vertebrates

The effects of sarafotoxin-b (SRTX-b) and endothelin-1 (ET-1) were tested in the fish tilapia (Ore niloticus x O. aureus hybrids) and torpedo (Torpedo ocellata), the toad (Bufo viridis), the agama lizard (Agama stellio) and water snake (Natrix tessellata). In isolated heart preparations of the fish and agama, peptide doses of 0.05-0.5 micrograms/ml induced positive inotropic effects, reduction of the contraction rate and arrhythmia, leading to cardiac arrest. In the toad, a negative inotropic effect and a reduction of the contraction rate were observed, whereas the water snake was hardly affected by either SRTX-b or ET-1. In the agama, an i.v. injection of 15 micrograms of SRTX-b caused changes in the ECG, culminating in A-V block that led to cardiac arrest, while in the toad an injection of 45 micrograms induced only transient disturbances in the ECG. Binding studies with 125I-SRTX-b revealed specific binding sites for SRTX-b and ET-1 in the heart and brain preparations of fish (tilapia and torpedo) and agama, whereas no specific binding could be demonstrated in the toad or in the snake. These results suggest that most vertebrates tested are sensitive to SRTX/ET, while the snake may possess receptors that are of a different structure.