irl-1620 and Hypertension--Pulmonary

Since the discovery of endothelin (ET)-1 in 1988, the main components of the signalling pathway have become established, comprising three structurally similar endogenous 21-amino acid peptides, ET-1, ET-2 and ET-3, that activate two GPCRs, ETA and ETB . Our aim in this review is to highlight the recent progress in ET research. The ET-like domain peptide, corresponding to prepro-ET-193-166 , has been proposed to be co-synthesized and released with ET-1, to modulate the actions of the peptide. ET-1 remains the most potent vasoconstrictor in the human cardiovascular system with a particularly long-lasting action. To date, the major therapeutic strategy to block the unwanted actions of ET in disease, principally in pulmonary arterial hypertension, has been to use antagonists that are selective for the ETA receptor (ambrisentan) or that block both receptor subtypes (bosentan). Macitentan represents the next generation of antagonists, being more potent than bosentan, with longer receptor occupancy and it is converted to an active metabolite; properties contributing to greater pharmacodynamic and pharmacokinetic efficacy. A second strategy is now being more widely tested in clinical trials and uses combined inhibitors of ET-converting enzyme and neutral endopeptidase such as SLV306 (daglutril). A third strategy based on activating the ETB receptor, has led to the renaissance of the modified peptide agonist IRL1620 as a clinical candidate in delivering anti-tumour drugs and as a pharmacological tool to investigate experimental pathophysiological conditions. Finally, we discuss biased signalling, epigenetic regulation and targeting with monoclonal antibodies as prospective new areas for ET research.

Topics: Antineoplastic Agents; Aspartic Acid Endopeptidases; Benzazepines; Bosentan; Endothelin A Receptor Antagonists; Endothelin-1; Endothelin-2; Endothelin-3; Endothelin-Converting Enzymes; Endothelins; Epigenesis, Genetic; Humans; Hypertension, Pulmonary; Metalloendopeptidases; Neoplasms; Peptide Fragments; Phenylpropionates; Pyridazines; Pyrimidines; Receptor, Endothelin B; Sulfonamides; Vasodilator Agents

Production of reactive oxygen species (ROS) may be increased during hypoxia in pulmonary arteries. In this study, the role of ROS in the effect of hypoxia on endothelin (ET) type B (ETB) receptor-mediated vasocontraction in lungs was determined. In rat intrapulmonary (approximately 0.63 mm ID) arteries, contraction induced by IRL-1620 (a selective ETB receptor agonist) was significantly attenuated after 4 h of hypoxia (30 mmHg Po2) compared with normoxic control (140 mmHg Po2). The effect was abolished by tiron, a scavenger of superoxide anions, but not by polyethylene glycol (PEG)-conjugated catalase, which scavenges H2O2. The hypoxic effect on ETB receptor-mediated vasoconstriction was also abolished by endothelium denudation but not by nitro-L-arginine and indomethacin. Exposure for 4 h to exogenous superoxide anions, but not H2O2, attenuated the vasoconstriction induced by IRL-1620. Confocal study showed that hypoxia increased ROS production in pulmonary arteries that were scavenged by PEG-conjugated SOD. In endothelium-intact pulmonary arteries, the ETB receptor protein was reduced after 4 h of exposure to hypoxia, exogenous superoxide anions, or ET-1. BQ-788, a selective ETB receptor antagonist, prevented these effects. ET-1 production was stimulated in endothelium-intact arteries after 4 h of exposure to hypoxia or exogenous superoxide anions. This effect was blunted by PEG-conjugated SOD. These results demonstrate that exposure to hypoxia attenuates ETB receptor-mediated contraction of rat pulmonary arteries. A hypoxia-induced production of superoxide anions may increase ET-1 release from the endothelium and result in downregulation of ETB receptors on smooth muscle.

Topics: Animals; Down-Regulation; Endothelin-1; Endothelins; Endothelium, Vascular; Hypertension, Pulmonary; Hypoxia; Lung; Male; Muscle Contraction; Muscle, Smooth, Vascular; Peptide Fragments; Pulmonary Artery; Pulmonary Circulation; Rats; Rats, Sprague-Dawley; Reactive Oxygen Species; Receptor, Endothelin B; Superoxides; Vasoconstriction

Endothelin-1 (ET-1) is a potent vasoactive peptide, and is thought to play an important role in the regulation of pulmonary vascular tone. Previous studies suggested that ET-1 can act as a vasoconstrictor via the endothelin-A and -B 2 receptors located on smooth muscle cells, and also act as a vasodilator through the endothelin-B 1 receptor situated on endothelial cells in the pulmonary circulation. To determine the role of endothelin-B receptors in hypoxic pulmonary hypertension, we examined the hemodynamic effects of a selective endothelin-B receptor agonist (IRL 1620) in chronic hypoxic pulmonary hypertensive rats. In rat lungs perfused with a half-blood solution, vasoconstriction by KCl administration was gradually reversed by IRL 1620 in normoxic rats, but not in chronic hypoxic rats. In in vivo studies, small doses of IRL 1620 induced transient vasodilation in normoxic rats, but had no obvious effects in chronic hypoxic rats. A high dose of IRL 1620 mediated vasoconstriction only in chronic hypoxic rats. Endothelin-B receptor messenger-RNA expression was lower in the lungs of chronic hypoxic rats than in normoxic rats. These results suggested that although the vasodilatory response mediated by endothelin-B receptor can be expected to play a protective role in the development of pulmonary hypertension, that response is diminished in hypoxic pulmonary hypertension.

Topics: Animals; Endothelins; Hypertension, Pulmonary; Hypoxia; In Vitro Techniques; Male; Peptide Fragments; Pulmonary Circulation; Rats; Rats, Sprague-Dawley; Receptor, Endothelin B; Receptors, Endothelin; Vasodilation

To investigate whether chronic hypoxia affects endothelin-B (ETB) receptor-mediated pulmonary vasodilation, we compared the vasodilator responses to IRL-1620, a selective ETB-receptor agonist, in isolated perfused lungs from normoxic and chronically hypoxic adult male rats. IRL-1620 caused a dose-dependent vasodilation that was greater in the hypertensive lungs than in the normotensive lungs. In normotensive lungs, a nitric oxide (NO) synthase inhibitor, Nomega-nitro-L-arginine (L-NNA; 300 microM), and an ATP-sensitive potassium (KATP)-channel inhibitor, glibenclamide (Glib; 10 microM), each reduced the vasodilator response to IRL-1620 (1 nM), but the combination of L-NNA and Glib inhibited it more effectively than either drug alone. In contrast, L-NNA alone, but not Glib alone, completely blocked IRL-1620-induced vasodilation in hypertensive lungs. The vasodilator response to a KATP-channel opener, NIP-121 (1 microM), but not the response to sodium nitroprusside (1 microM), was enhanced in hypertensive lungs. We also found increased expression of mRNA for the ETB receptor in lung tissue after hypoxic exposure. In addition, semiquantitative immunohistochemistry demonstrated higher expression levels of ETB receptors in the endothelium of distal segments of the pulmonary artery in hypoxic than in normoxic rats. These results suggest that ETB receptor-mediated pulmonary vasodilation is augmented after chronic hypoxic exposure and that release of NO may be the sole mechanism of this vasodilation in hypertensive lungs, whereas both release of NO and activation of KATP channels are involved in normotensive lungs. We speculate that the underlying mechanism responsible for this augmentation may partly be related to upregulation of ETB receptors in the endothelium of pulmonary resistance arteries in hypertensive lungs.

Topics: Adenosine Triphosphate; Animals; Chronic Disease; Endothelins; Hypertension, Pulmonary; Hypertrophy, Right Ventricular; Hypoxia; In Vitro Techniques; Lung; Male; Nitric Oxide; Nitric Oxide Donors; Peptide Fragments; Perfusion; Potassium Channel Blockers; Potassium Channels; Pulmonary Circulation; Rats; Rats, Sprague-Dawley; Receptor, Endothelin B; Receptors, Endothelin; RNA, Messenger; Vasodilation; Vasodilator Agents

The hemodynamic effects of endothelin-1 (ET-1) are mediated by at least two distinct receptors: ETa and ETb receptors. Recently, ETb receptor agonists (4 Ala ET-1 and IRL 1620) were developed. To investigate the role of ETb receptor activation on the pulmonary and systemic circulations, we studied the hemodynamic effects of intrapulmonary arterial injections of these receptor agonists in 10 intact newborn lambs. At rest, 4 Ala ET-1 (290-1,725 ng/kg) changed no hemodynamic variables. IRL 1620 (180-1,095 ng/kg) decreased mean pulmonary arterial pressure (PAP, 16.8% +/- 15.0 and 17.8% +/- 8.5, p < 0.05) and left pulmonary artery blood flow (21.6% +/- 22.1 and 33.4% +/- 27.7, p < 0.05) at the two highest doses only. During U46619-induced pulmonary hypertension, both 4 Ala ET-1 (3.2% +/- 8.0 to 15.9% +/- 6.4, p < 0.05) and IRL 1620 (8.7% +/- 6.3 to 21.9% +/- 4.1, p < 0.05) produced selective dose-dependent decreases in PAP. The decrease in mean PAP induced by 4 Ala ET-1 and IRL 1620 was attenuated by N omega-nitro-L-arginine [an inhibitor of endothelium-derived nitric oxide (EDNO) synthesis] (16.6% +/- 3.5 vs. 5.9% +/- 2.3 and 16.2% +/- 3.4 vs. 6.6% +/- 2.8, p < 0.05) and by glybenclamide (a blocker of ATP-dependent potassium channels) (18.2% +/- 7.9 vs. 7.5% +/- 8.3 and 14.7% +/- 3.6 vs. 6.3% +/- 3.2, p < 0.05). ETb receptor activation produces selective pulmonary vasodilation during pulmonary hypertension in intact newborn lambs. The vasodilating properties are mediated in part by release of ENDO and by potassium channel activation.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Animals; Animals, Newborn; Arginine; Blood Pressure; Disease Models, Animal; Endothelins; Endothelium, Vascular; Glyburide; Hypertension, Pulmonary; Injections, Intra-Arterial; Nitroarginine; Peptide Fragments; Prostaglandin Endoperoxides, Synthetic; Pulmonary Artery; Pulmonary Circulation; Regional Blood Flow; Sheep; Thromboxane A2; Vascular Resistance; Vasoconstrictor Agents; Vasodilation