Effect of endothelin blockade on basal and stimulated forearm blood flow in patients with essential hypertension

Endothelin-1 Regulation of Exercise-Induced Changes in Flow: Dynamic Regulation of Vascular Tone

Although endothelin (ET)-1 is a highly potent vasoconstrictor with considerable efficacy in numerous vascular beds, the role of endogenous ET-1 in the regulation of vascular tone remains unclear. The perspective that ET-1 plays little role in the on-going regulation of vascular tone at least under physiologic conditions is supported by findings that potential ET-1 constriction is minimized by the release of the vasodilator and ET-1 synthesis inhibitor, nitric oxide (NO). Indeed, ET-1 release and constriction is self-limited by ET-1-induced, endothelial ET_B receptor-mediated release of NO. Moreover, even if the balance between ET-1 and NO were reversed as the result of lowered NO activity, as occurs in a number of pathophysiologies associated with endothelial dysfunction, the well-known resistance of ET-1 constriction to reversal (as determined with exogenous ET-1) precludes ET-1 in the dynamic, i.e., moment-to-moment, regulation of vascular tone. On the other hand, and as presently reviewed, findings of ET-1-dependent modulation of organ blood flow with exercise under physiologic conditions demonstrate the dynamic regulation of vascular tone by ET-1. We speculate that this regulation is mediated at least in part through changes in ET-1 synthesis/release caused by pulsatile flow-induced shear stress and NO.

Endothelin

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.

Contrasting actions of endothelin ET(A) and ET(B) receptors in cardiovascular disease

First identified as a powerful vasoconstrictor, endothelin has an extremely diverse set of actions that influence homeostatic mechanisms throughout the body. Two receptor subtypes, ET(A) and ET(B), which usually have opposing actions, mediate the actions of endothelin. ET(A) receptors function to promote vasoconstriction, growth, and inflammation, whereas ET(B) receptors produce vasodilation, increases in sodium excretion, and inhibit growth and inflammation. Potent and selective receptor antagonists have been developed and have shown promising results in the treatment of cardiovascular diseases such as pulmonary arterial hypertension, acute and chronic heart failure, hypertension, renal failure, and atherosclerosis. However, results are often contradictory and complicated because of the tissue-specific vasoconstrictor actions of ET(B) receptors and the fact that endothelin is an autocrine and paracrine factor whose activity is difficult to measure in vivo. Considerable questions remain regarding whether ET(A)-selective or nonselective ET(A)/ET(B) receptor antagonists would be useful in a range of clinical settings.

Contrasting actions of endothelin ET(A) and ET(B) receptors in cardiovascular disease

First identified as a powerful vasoconstrictor, endothelin has an extremely diverse set of actions that influence homeostatic mechanisms throughout the body. Two receptor subtypes, ET(A) and ET(B), which usually have opposing actions, mediate the actions of endothelin. ET(A) receptors function to promote vasoconstriction, growth, and inflammation, whereas ET(B) receptors produce vasodilation, increases in sodium excretion, and inhibit growth and inflammation. Potent and selective receptor antagonists have been developed and have shown promising results in the treatment of cardiovascular diseases such as pulmonary arterial hypertension, acute and chronic heart failure, hypertension, renal failure, and atherosclerosis. However, results are often contradictory and complicated because of the tissue-specific vasoconstrictor actions of ET(B) receptors and the fact that endothelin is an autocrine and paracrine factor whose activity is difficult to measure in vivo. Considerable questions remain regarding whether ET(A)-selective or nonselective ET(A)/ET(B) receptor antagonists would be useful in a range of clinical settings.

The endothelin system and its antagonism in chronic kidney disease

The incidence of chronic kidney disease (CKD) is increasing worldwide. Cardiovascular disease (CVD) is strongly associated with CKD and constitutes one of its major causes of morbidity and mortality. Treatments that slow the progression of CKD and improve the cardiovascular risk profile of patients with CKD are needed. The endothelins (ET) are a family of related peptides, of which ET-1 is the most powerful endogenous vasoconstrictor and the predominant isoform in the cardiovascular and renal systems. The ET system has been widely implicated in both CVD and CKD. ET-1 contributes to the pathogenesis and maintenance of hypertension and arterial stiffness and more novel cardiovascular risk factors such as oxidative stress and inflammation. Through these, ET also contributes to endothelial dysfunction and atherosclerosis. By reversal of these effects, ET antagonists may reduce cardiovascular risk. In particular relation to the kidney, antagonism of the ET system may be of benefit in improving renal hemodynamics and reducing proteinuria. ET likely also is involved in progression of renal disease, and data are emerging to suggest a synergistic role for ET receptor antagonists with angiotensin-converting enzyme inhibitors in slowing CKD progression.

Responses to neither exogenous nor endogenous endothelin-1 are altered in patients with hypercholesterolemia

There is some controversy regarding whether vascular responses to endothelin are altered in hypercholesterolemia. Studies performed to date have been compromised by the use of endothelin antagonists at inappropriate concentrations. In the current study, we examine the role of endothelin-1 in hypercholesterolemic patients using lower, more selective doses of specific endothelin antagonists. Twenty-two patients with hypercholesterolemia (total plasma cholesterol > 6.0 mmol/l) and 17 healthy controls were recruited. Forearm vascular responses to endothelin-1 (5 pmol/min), the endothelin A antagonist BQ-123 (10 nmol/min), and the endothelin B antagonist BQ-788 (1 nmol/min) were obtained. Endothelin-1 caused a significant vasoconstriction in both hypercholesterolemic and control subjects, an effect that was not significantly different between the two groups (P = 0.784). BQ-123 caused a significant vasodilatation that was not significantly different between the two groups (P = 0.899). Similarly, responses to BQ-788 (P = 0.774) and mean plasma endothelin-1 levels were not different (control vs. hypercholesterolemia, 1.16 +/- 0.18 vs. 1.06 +/- 0.15 fmol/ml; P = 0.64). Responses to neither exogenous nor endogenous endothelin are influenced by plasma cholesterol levels in humans. It is thus unlikely that the endothelin system contributes to early vascular disease pathology in patients with hypercholesterolemia.

Endothelial function and dysfunction. Part I: Methodological issues for assessment in the different vascular beds: a statement by the Working Group on Endothelin and Endothelial Factors of the European Society of Hypertension

An enormous number of studies in the last two decades have been devoted to investigating the role of the endothelium in cardiovascular diseases. Nonetheless, the optimal methodology for investigating the multifaceted aspects of endothelial dysfunction is still under debate. Biochemical markers, molecular genetic tests and invasive and non-invasive tools with and without pharmacological and physiological stimuli have been introduced. Furthermore newer pharmacological tools have been proposed. However, the application of these methodologies should fulfil a number of requirements in order to provide conclusive answers in this area of research. Thus, the most relevant methodological issues in the research on endothelial function and dysfunction are summarized in this paper.

[New expansion of endothelin research: perspectives for clinical application of endothelin-receptor antagonists]

Three isopeptides of endothelin (ET-1, -2, and -3) exert various actions through stimulation of two sub-types of receptor (ETA and ETB). Vascular endothelial cells produce only ET-1. In addition to its powerful vasoconstrictor action, ET-1 has direct mitogenic actions on cardiovascular tissues, as well as comitogennic actions with a wide variety of growth factors and vasoactive substances. ET-1 also promotes the synthesis and secretion of growth factors and various substances, including extracellular constituents. These effects of endogenous ET-1 would naturally be thought to be concerned with the development and/or aggravation of chronic cardiovascular diseases; e.g., hypertension, pulmonary hypertension, vascular remodeling (stenosis, atherosclerosis), renal failure, and heart failure. A large number of peptide and orally active non-peptide endothelin receptor antagonists have been developed, and utilized to analyze physiological and pathophysiological roles of endogenous ET-1. These antagonists have been shown to exert excellent therapeutic effects in animal models of various kinds of diseases by either acute or chronic treatment. Therapeutic treatment of patients suffering from the above-mentioned cardiovascular diseases with ET-receptor antagonists have also been taking place, and bosentan (ETA/ETB antagonist) was recently approved by the FDA as a formal therapeutic drug for pulmonary hypertension. In this review, perspectives for therapeutic applicability of ET-receptor antagonists will be explored.

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

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