Systemic ETA receptor antagonism with BQ‐123 blocks ET‐1 induced forearm vasoconstriction and decreases peripheral vascular resistance in healthy men
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The effect on systemic haemodynamics of BQ‐123, a selective endothelin A (ETA) receptor antagonist, was investigated in healthy men by giving, on separate occasions, ascending intravenous doses of 100, 300, 1000 and 3000 nmol min −1 BQ‐123, each for 15 min, in a randomized, placebo‐controlled, double‐blind study. The response of forearm blood flow to brachial artery infusion of endothelin‐1 (ET‐1; 5 pmol min −1 for 90 min) was also studied using bilateral forearm plethysmography, after systemic pre‐treatment, on separate occasions, with one of two doses of BQ‐123 (300 and 1000 nmol min −1 for 15 min) or placebo. Systemic BQ‐123 dose‐dependently decreased systemic vascular resistance ( P <0.01 for all doses vs placebo) and mean arterial pressure ( P <0.05 for 300 nmol min −1 and P <0.01 for 1000 and 3000 nmol min −1 ) during the 60 min following infusion. There were concurrent increases in heart rate and cardiac index. BQ‐123, when infused systemically for 15 min, appeared to reach a maximum effect at 1000 nmol min −1 . Intra‐brachial ET‐1 infusion, after pre‐treatment with placebo, caused a slow onset progressive forearm vasoconstriction without systemic effects. This vasoconstriction was attenuated by pre‐treatment with BQ‐123 at 300 nmol min −1 and abolished by BQ‐123 at 1000 nmol min −1 ( P <0.01 vs placebo). These effects occurred at concentrations of BQ‐123 in the plasma (510±64 nmol l −1 ) that were ETA receptor selective, and were not accompanied by an increase in plasma ET‐1 that would have indicated ETB receptor blockade. We conclude that ETA‐mediated vascular tone contributes to the maintenance of basal systemic vascular resistance and blood pressure in healthy men. British Journal of Pharmacology (2001) 134 , 648–654; doi: 10.1038/sj.bjp.0704304Keywords:
Brachial artery
Plethysmograph
Even though the pharmacology of the endothelin receptor subtypes ETB and ETB is well characterized, their physiological role in the control of basal vascular tone is poorly uindertood. It has been proposed that a tonic vasoconstriction mediated by endothelin counterbalances a vasodilator tone mediated by nitric oxide, the major endothelium-derived relaxing factor. Recent evidence suggest that the endogenous endothelin-1, released from the endothelial cells, mediates both vasodilation and vasoconstriction and indicates the existence of a negative feedback system involving endothelin-1, the endothelial ETB receptor and nitric oxide
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The role of endothelin ETB receptors in mediating vasoconstriction in humans is unclear. As yet, there have been no in vivo studies in resistance vessels, and in vitro data have been contradictory. We therefore investigated the function of ETB receptors in vivo in human forearm resistance and hand capacitance vessels using endothelin-1 as a nonselective agonist at ETA and ETB receptors and endothelin-3 and sarafotoxin S6c as selective agonists at the ETB receptor.A series of single-blind studies were performed, each in six healthy men. Brachial artery infusion of endothelin-1 and endothelin-3 caused slow-onset dose-dependent forearm vasoconstriction. Although endothelin-3 caused significantly less forearm vasoconstriction than endothelin-1 at low doses, vasoconstriction was similar to the two isopeptides at the highest dose (60 pmol/min). Endothelin-3 caused transient forearm vasodilatation at this dose, whereas endothelin-1 showed only a nonsignificant trend toward causing early vasodilatation. Intra-arterial sarafotoxin S6c caused a progressive reduction in forearm blood flow, although less than that to endothelin-1 (P = .04). Dorsal hand vein infusion of sarafotoxin S6c caused local venoconstriction that was also less than that to endothelin-1 (P = .002).Selective ETB receptor agonists cause constriction of forearm resistance and hand capacitance vessels in vivo in humans, suggesting that both ETA and ETB receptors mediate vasoconstriction. Hence, antagonists at both ETA and ETB receptors, or inhibitors of the generation of endothelin-1, may be necessary to completely prevent vasoconstriction to endogenously generated endothelin-1.
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Endothelin-1, discovered in 1988, is a 21–amino-acid peptide and currently the most potent vasoconstrictor and pressor substance known. Generated by vascular endothelial cells in response to a variety of chemical and mechanical signals, endothelin-1 is known to potentiate the actions of other vasoconstrictor substances and act as a comitogen in addition to directly causing vasoconstriction. There is evidence that endothelin-1 may contribute to the pathophysiology of conditions associated with sustained vasoconstriction, such as hypertension and heart failure, vasospastic conditions, such as subarachnoid hemorrhage, and atherogenesis. Studies using endothelin receptor antagonists show that endothelin-1 plays an important role in the maintenance of vascular tone and blood pressure in healthy humans, predominantly via an effect on the vascular smooth muscle ETA receptors. The endothelin receptor antagonist bosentan also effectively lowers blood pressure in hypertensive subjects and produces sustained and favorable effects on systemic and pulmonary hemodynamics in patients with chronic heart failure. A good side-effect profile, together with a potential for inhibition of atherogenesis, makes the endothelin receptor antagonists a potentially interesting class of novel agents for the treatment of cardiovascular disease.
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The effects of endothelin-1 (ET-1) are mediated by two subclasses of the endothelin receptor, ETA and ETB. The Langendorff perfused rat heart was used to determine the endothelin receptor subtype mediating rat coronary vasoconstriction. ET-1 (EC50 = 1.5 x 10(-10) M) and endothelin-3 (ET-3) (10(-11)-3 x 10(-8) M) caused dose-dependent increases in coronary perfusion pressure. BQ-123, a selective antagonist of the ETA-receptor subtype, did not cause a parallel shift in the dose-response curves of ET-1 or ET-3. In the presence of 1-3 x 10(-6) M BQ-123, ET-1 and ET-3 exhibited biphasic dose-response curves, suggesting that vasoconstriction was caused by two receptors. The ETB-selective agonist Suc-[Glu9,Ala11,15]-ET-1(8-21) (IRL 1620) maximally increased perfusion pressure by approximately 50% of the maximal response to ET-1 and was not inhibited by BQ-123. These data suggest that the rat coronary vasoconstrictor effects of ET-1 and ET-3 are mediated by both ETA and ETB receptors.
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Background Endothelin-1 (ET-1) is an endothelium-derived vasoconstrictor peptide. Controversy persists regarding the predominant ET receptor that mediates coronary vasoconstriction at pathophysiological concentrations. The aim of the present study was to test the hypothesis that ET mediates local coronary vasoconstriction via the ET-A receptor at low concentrations of exogenous ET-1 designed to mimic pathophysiological states compared with pharmacological concentrations. Methods and Results ET-1 (group 1, n=5) or sarafotoxin, a specific ET-B receptor agonist (group 3, n=6) (each at 2 ng/kg per minute), was infused into the left circumflex coronary artery in the anesthetized dog. In group 2 dogs (n=5), the same dose of ET-1 was infused with 4 μg/kg per minute of the specific ET-A receptor antagonist FR-139317. In group 4 (n=5), the same dose of sarafotoxin was infused with 50 μg/kg per minute of the specific inhibitor of nitric oxide formation, N G -monomethyl- l -arginine (L-NMMA). No difference in hemodynamics, coronary blood flow (CBF), coronary vascular resistance (CVR), or coronary artery diameter (CAD) was observed at baseline between the groups. In group 1, intracoronary ET-1 significantly decreased CBF and CAD in association with an increase in CVR. The percentage decrease in CBF and CAD in the group that received ET-1 and the ET-A receptor antagonist (group 2) was significantly less than that in the group that received ET-1 alone (group 1) (−12±3% versus −48±6% [ P <.001] and −4.6±0.8 versus 1.0±0.3 [ P <.05], respectively). The administration of the ET-A receptor antagonist (group 2) abolished the ET-mediated increase in CVR (7±5% versus 105±21%, P <.005). There was no significant effect on CBF, CVR, or CAD in the group receiving sarafotoxin alone (group 3). The administration of L-NMMA and sarafotoxin (group 4) resulted in a significant percentage decrease in CBF compared with the group that received sarafotoxin alone (−28±7% versus −8±2% [ P <.05]). Conclusions The present study demonstrates that low concentrations of exogenous ET-1, which may mimic pathophysiological concentrations, result in coronary vasoconstriction mediated predominantly via the ET-A receptor because such vasoconstriction is significantly attenuated by blockade with FR-139317. The ET-B receptor may have a dual vasoconstrictive and vasodilatory effect.
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The contribution of the endothelin (ET) receptors ETA and ETB to basal vascular tone and ET-1-induced vasoconstriction in the renal and splanchnic vasculature was investigated in six healthy humans. ET-1 was infused alone and in combination with the selective ETA receptor antagonist BQ123 or the selective ETB receptor antagonist BQ788 on three different occasions. BQ123 did not affect basal arterial blood pressure, splanchnic vascular resistance (SplVR) or renal vascular resistance (RVR), but inhibited the increase in vascular resistance induced by ET-1 [64±18 versus -1±7% in SplVR (P<0.05); 36±6 versus 12±3% in RVR (P<0.0001)]. BQ788 increased basal SplVR and RVR [38±16% (P = 0.01) and 21±5% (P<0.0001) respectively], and potentiated the ET-1-induced vasoconstriction. Plasma ET-1 increased more after ETB blockade than under control conditions or after ETA blockade. These findings suggest that the ETA receptor mediates the splanchnic and renal vasoconstriction induced by ET-1 in healthy humans. The ETB receptor seems to function as a clearance receptor and may modulate vascular tone by altering the plasma concentration of ET-1.
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