Distinct endothelin-B receptors mediate the effects of sarafotoxin S6c and IRL1620 in the ileum

A novel neuroregenerative approach using ET(B) receptor agonist, IRL-1620, to treat CNS disorders

Endothelin B (ET(B)) receptors present in abundance the central nervous system (CNS) have been shown to have significant implications in its development and neurogenesis. We have targeted ET(B) receptors stimulation using a highly specific agonist, IRL-1620, to treat CNS disorders. In a rat model of cerebral ischemia intravenous administration IRL-1620 significantly reduced infarct volume and improved neurological and motor functions compared to control. This improvement, in part, is due to an increase in neuroregeneration. We also investigated the role of IRL-1620 in animal models of Alzheimer's disease (AD). IRL-1620 improved learning and memory, reduced oxidative stress and increased VEGF and NGF in Abeta treated rats. IRL-1620 also improved learning and memory in an aged APP/PS1 transgenic mouse model of AD. These promising findings prompted us to initiate human studies. Successful chemistry, manufacturing and control along with mice, rat and dog toxicological studies led to completion of a human Phase I study in healthy volunteers. We found that a dose of 0.6 microg/kg of IRL-1620 can be safely administered, three times every four hours, without any adverse effect. A Phase II clinical study with IRL-1620 has been initiated in patients with cerebral ischemia and mild to moderate AD.

Adaptive regulation of endothelin receptor type-A and type-B in vascular smooth muscle cells during pregnancy in rats

Normal pregnancy is associated with systemic vasodilation and decreased vascular contraction, partly due to increased release of endothelium-derived vasodilator substances. Endothelin-1 (ET-1) is an endothelium-derived vasoconstrictor acting via endothelin receptor type A (ETA R) and possibly type B (ETB R) in vascular smooth muscle cells (VSMCs), with additional vasodilator effects via endothelial ETB R. However, the role of ET-1 receptor subtypes in the regulation of vascular function during pregnancy is unclear. We investigated whether the decreased vascular contraction during pregnancy reflects changes in the expression/activity of ETAR and ETBR. Contraction was measured in single aortic VSMCs isolated from virgin, mid-pregnant (mid-Preg, day 12), and late-Preg (day 19) Sprague-Dawley rats, and the mRNA expression, protein amount, tissue and cellular distribution of ETAR and ETBR were examined using RT-PCR, Western blots, immunohistochemistry, and immunofluorescence. Phenylephrine (Phe, 10(-5)  M), KCl (51 mM), and ET-1 (10(-6)  M) caused VSMC contraction that was in late-Preg < mid-Preg and virgin rats. In VSMCs treated with ETB R antagonist BQ788, ET-1 caused significant contraction that was still in late-Preg < mid-Preg and virgin rats. In VSMCs treated with the ETAR antagonist BQ123, ET-1 caused a small contraction; and the ETBR agonists IRL-1620 and sarafotoxin 6c (S6c) caused similar contraction that was in late-Preg < mid-Preg and virgin rats. RT-PCR revealed similar ETAR, but greater ETBR mRNA expression in pregnant versus virgin rats. Western blots revealed similar ETAR, and greater protein amount of ETBR in endothelium-intact vessels, but reduced ETBR in endothelium-denuded vessels of pregnant versus virgin rats. Immunohistochemistry revealed prominent ETBR staining in the intima, but reduced ETAR and ETBR in the aortic media of pregnant rats. Immunofluorescence signal for ETAR and ETBR was less in VSMCs of pregnant versus virgin rats. The pregnancy-associated decrease in ETAR- and ETBR-mediated VSMC contraction appears to involve downregulation of ETAR and ETBR expression/activity in VSM, and may play a role in the adaptive vasodilation during pregnancy.

Modulation of Endothelin-A Receptor, Gα Subunit, and RGS2 Expression during H9c2 Cardiomyoblast Differentiation

In cardiac myocytes, growth responses depend on activation of G protein-coupled receptors interacting with Gq/11 protein subfamily members. Endothelin receptors of the ETA subtype belong to this receptor group inducing hypertrophic responses. To understand the role of ETA receptors and signal transduction proteins in modulating cell growth, we analyzed the pharmacological profile of this receptor, its level of expression together with those of Galpha subunits and the RGS2 protein in cardiomyoblasts differentiating into the cardiac phenotype. H9c2 rat cardiomyoblasts were grown in the presence of 10% fetal bovine serum (FBS) or 1% FBS plus all-trans-retinoic acid to induce the cardiac phenotype. The pharmacological properties of ETA receptors were investigated by competition-binding experiments, whereas the protein expression profile was analyzed by immunoblot and immunocytochemistry. The pharmacological profile of ETA receptors changed during differentiation of cardiomyoblasts into cardiomyocytes, and the amount of expressed receptor appeared to increase. Immunocytochemistry also showed a marked increase of receptor expression on cell membranes of differentiated cardiomyocytes. Among the other signaling proteins examined, both Galphaq/11 and RGS2 expression decreased in cells with the cardiac phenotype. Our results demonstrate that the expression of key proteins (ETA receptor, Galphaq/11, and RGS2) involved in signal transduction of hypertrophic stimuli is modulated during cell differentiation and correlates with the cardiac phenotype.

Function of the endothelinB receptor in cardiovascular physiology and pathophysiology

One of the two receptors by which the potent vasoactive effects of endothelin (ET)-1 are mediated is the ET(B) receptor (ET(BR)), which is found in several tissues, but, more importantly from a cardiovascular point of view, on the endothelial cell. The endothelial cell also has the unique capability of releasing ET-1, as well as other factors, such as the endothelial-derived relaxing factors and prostacyclin, which counteract the myotropic effects of the peptide. The secretory and contractile responses to ET-1 rely on G-protein-coupled ET(BR)s, as well as ET(A)-G-protein-coupled receptor-like proteins. The mitogenic properties of ET-1 via ET(A) receptors (ET(AR)s) coupled to mitogen-activated protein kinases and tyrosine kinases on the vascular smooth muscle may occur in conjunction with the anti-apoptotic characteristics of the endothelial ET(BR)s. Interestingly, most of the relevant antagonists and agonists for both ET(AR)s and ET(BR)s have been developed by the pharmaceutical industry. This highlights the therapeutical potential of compounds that act on ET receptors. In normal as well as in physiopathological conditions, the ET(BR) plays an important role in the control of vascular tone, and must be taken into account when using ET receptor antagonists for the treatment of cardiovascular diseases. For the management of congestive heart failure, renal failure and primary pulmonary hypertension, the most recent literature supports the use of selective ET(AR) antagonists rather than mixed antagonists of ET(AR)s and ET(BR)s. Nonetheless, validation of this view will have to await the first clinical trials comparing the actions of ET(A) to mixed ET(A)/ET(B) receptor antagonists.

The endothelin system in septic and endotoxin shock

The view of the endothelium as a passive barrier has gradually changed as a number of endothelium-derived substances have been discovered. Substances like nitric oxide, prostaglandins and endothelins have potent and important properties, involving not only the circulation as such but also the response to stimuli like inflammation and trauma. The endothelin system, discovered in 1988, has not only strong vasoconstrictor properties, but also immunomodulating, endocrinological and neurological effects exerted through at least two types of receptors. Septic shock, a condition with high mortality, is associated with vast cardiovascular changes, organ dysfunction with microcirculatory disturbances and dysoxia. In the experimental setting, endotoxaemia resembles these changes and is, as well as septic shock, accompanied by a pronounced increase in plasma endothelin levels. The pathophysiology in septic and endotoxin shock remains to be fully elucidated, but several studies indicate that endothelial dysfunction is one contributing mechanism. Activation of the endothelin system is associated with several pathological conditions complicating septic shock, such as acute respiratory distress syndrome, cardiac dysfunction, splanchnic hypoperfusion and disseminated intravascular coagulation. Through the development of both selective and nonselective endothelin receptor antagonists, the endothelin system has been the object of a large number of studies during the last decade. This review highlights systematically the findings of previous studies in the area. It provides strong indications that the endothelin system, apart from being a marker of vascular injury, is directly involved in the pathophysiology of septic and endotoxin shock. Interventions with endothelin receptor antagonists during septic and endotoxin shock have so far only been done in animal studies but the results are interesting and promising.

Effects on haemodynamics by selective endothelin ET(B) receptor and combined endothelin ET(A)/ET(B) receptor antagonism during endotoxin shock

The endothelin system is highly activated during endotoxin and septic shock. To investigate this matter the selective non-peptide endothelin ET(B) receptor antagonist A-192621 ([2R-(2alpha,3beta, 4alpha)]-4-(1,3-benzodioxol-5-yl)-1-[2-[2, 6-diethylphenyl)amino]-2-oxoethyl]-2-(4-propoxy-phenyl)-3-py rrolidine carboxylic acid) was administered alone and in combination with the selective non-peptide endothelin ET(A) receptor antagonist PD 155080 (sodium 2-benzo[1, 3]dioxol-5-yl-3-benzyl-4-(4-methoxy-phenyl)-4-oxobut-2-enoat e) during established porcine endotoxin shock. Cardiopulmonary vascular function, metabolic parameters and plasma endothelin-1-like immunoreactivity levels were compared to a control group only receiving endotoxin. Administration of A-192621 alone resulted in cardiovascular collapse and death whereas combining A-192621 with PD 155080 abolished endotoxin induced pulmonary hypertension, enhanced cardiac performance and improved systemic oxygen delivery and acid-base balance. The beneficial effects of mixed endothelin ET(A)/ET(B) receptor antagonisms on the pulmonary and cardiovascular systems may result from blockage of constrictive endothelin receptors in and pulmonary circulation, reduced afterload and a direct inotropic effect. Possible mechanisms for the devastating effects by selective endothelin ET(B) receptor antagonism include increased endothelin ET(A) receptor-mediated vasoconstriction due to lack of endothelin ET(B) receptormediated vasodilation and decreased endothelin clearance from endothelin ET(B) receptor blockade. In conclusion, selective endothelin ET(B) receptor antagonism is deleterious whereas combined endothelin ET(A) and ET(B) receptor antagonism has favourable effects on haemodynamics, suggesting participation of the endothelin system in cardiopulmonary dysfunction during endotoxin shock.