Regional hemodynamic effects of endothelin-2 and sarafotoxin-S6b in conscious rats

Endothelin-2, the forgotten isoform: emerging role in the cardiovascular system, ovarian development, immunology and cancer

Endothelin-2 [ET-2; also known as vasoactive intestinal contractor (VIC), in rodents] differs from endothelin-1 (ET-1) by only two amino acids, and unlike the third isoform, endothelin-3 (ET-3), it has the same affinity as ET-1 for both ET(A) and ET(B) receptors. It is often assumed that ET-2 would mimic the actions of the more abundant ET-1 and current pharmacological interventions used to inhibit the ET system would also block the actions of ET-2. These assumptions have focused research on ET-1 with ET-2 studied in much less detail. Recent research suggests that our understanding of the ET family requires re-evaluation. Although ET-2 is very similar in structure as well as pharmacology to ET-1, and may co-exist in the same tissue compartments, there is converging evidence for an important and distinct ET-2 pathway. Specifically is has been demonstrated that ET-2 has a key role in ovarian physiology, with ET-2-mediated contraction proposed as a final signal facilitating ovulation. Furthermore, ET-2 may also have a pathophysiological role in heart failure, immunology and cancer. Comparison of ET-2 versus ET-1 mRNA expression suggests this may be accomplished at the level of gene expression but differences may also exist in peptide synthesis by enzymes such as endothelin converting enzymes (ECEs) and chymase, which may allow the two pathways to be distinguished pharmacologically and become separate drug targets. LINKED ARTICLES This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Characterization of receptors mediating vascular responses to endothelin-1 in the conscious rat

1. The present study has determined the receptors mediating the vascular responses (pressor and depressor actions and vascular permeability effect) to endothelin-1 (ET-1) in the conscious rat by using the novel non-peptide ETA/ETB receptor antagonist, bosentan (Ro 47-0203, 4-tert-butyl-N-[6-(2 hydroxyethoxy)-5-(2-methoxy-phenoxy)-2,2'-bipyrimidine- 4-yl]benzene-sulphonamide), the ETA receptor-selective antagonist, FR 139317 and the ETB receptor-selective peptide agonist, IRL 1620. 2. Bolus injection of ET-1 (1 nmol kg-1, i.v.) resulted in a prolonged pressor effect (maximum increase in mean arterial blood pressure (MABP) was 47 +/- 3 mmHg, n = 6) preceded by a transient depressor response (maximum decrease in MABP was 17 +/- 1 mmHg). Both these responses were inhibited by bosentan (1-20 mg kg-1, i.v. bolus) in a dose-dependent manner. The maximum inhibition of ET-induced depressor and pressor responses did not exceed 53 and 87%, respectively. FR 139317 (2.5 mg kg-1, i.v.) attenuated the pressor response to ET-1 by 75% without affecting the depressor response. Furthermore, FR 139317, but not bosentan, prolonged the depressor action of ET-1. Corresponding to changes in blood pressure, a small transient tachycardia (delta heart rate 15 +/- 5 beats min-1) followed by a sustained bradycardia (delta heart rate -48 +/- 10 beats min-1, n = 6) was observed following injection of 1 nmol kg-1 ET-1. FR 139317 and bosentan (10 mg kg-1) inhibited ET-1-induced bradycardia by 79% and 71%, respectively.ET-l-induced tachycardia was significantly attenuated by bosentan,but not FR 139317.3. The ETB receptor agonist, IRL 1620 (0.1-2 micro molkg-1, i.v.) produced biphasic dose-dependent changes in MABP with an initial transient fall followed by a prolonged pressor action. The maximum decrease and increase in MABP were 11 +/- 2 and 19 +/- 3 mmHg, respectively (n = 5). These changes in MABP were accompanied by a transient tachycardia (Delta heart rate 9+/- 3 beats min-1) and prolonged bradycardia (Delta heart rate -17+/-11 beats min-1), respectively. Pretreatment of the animals with FR139317 (2.5 mg kg-1, i.v.) did not affect IRL 1620 (1 nmol kg-1)-induced changes in MABP and heart rate, whereas both the depressor and pressor actions of IRL 1620 and the accompanying tachycardia and bradycardia were almost completely inhibited by bosentan (10mgkg-1).4. ET-1 (1 nmol kg-1) enhanced albumin extravasation in the upper and lower bronchi, spleen, kidney,stomach and duodenum (up to 246%) as measured by the extravasation of Evans blue dye. FR 139317(2.5mgkg-1) completely inhibited ET-l-induced protein extravasation in the stomach and duodenum,whereas 40-75% inhibition was observed in the other vascular beds studied. The permeability effect of ET-l was almost completely inhibited by bosentan (10mgkg-1) in all vascular beds studied.5. IRL 1620 (0.4 or 1 nmol kg-1, i.v.) enhanced albumin extravasation (up to 219%) in the upper and lower bronchi, spleen and kidney in a dose-dependent manner. Unlike ET-1, IRL 1620 failed to increase albumin extravasation in the stomach and duodenum.6. The present study demonstrates in the conscious rat that ETA and ETB receptors are responsible for mediating the majority of the pressor response to ET-l and suggest that a small component of the ET-l pressor response might be mediated via a non-ETA, non-ETB receptor, whereas ETB and perhaps a non-ETA, non-ETB receptor may mediate the depressor action of ET-1. Furthermore, the ET-1 induced albumin extravasation is mediated solely via ETA receptors in the stomach and duodenum, whereas both ETA and ETB receptors are involved in the permeability effect of ET-l in the bronchial, splenic and renal vascular beds.

Effects of bosentan (Ro 47-0203), an ETA-, ETB-receptor antagonist, on regional haemodynamic responses to endothelins in conscious rats

1. Regional haemodynamic responses to endothelin (ET)-1, -2 and -3 and big ET-1 (all at 500 pmol kg-1) were assessed in the same conscious Long Evans rats (n = 8) in the absence or presence of the mixed ETA-, ETB-receptor antagonist, Ro 47-0203 (bosentan; 30 mg kg-1). 2. Bosentan blocked the initial depressor, tachycardic and hindquarters hyperaemic vasodilator effects of ET-1, -2 and -3, and substantially curtailed the primary renal and secondary hindquarters vasoconstrictor responses. Bosentan did not inhibit the initial mesenteric vasoconstrictor action of ET-1, but reduced the duration of the later mesenteric vasoconstriction. In contrast, bosentan delayed the rate of onset, and reduced the duration, of the mesenteric vasoconstrictor actions of ET-2 and ET-3. The most likely explanation of this finding is that ET-1, but not ET-2 or ET-3, triggered a covert mesenteric vasodilator mechanism which was antagonized by bosentan. 3. Bosentan blocked all the effects of big ET-1, and, in a separate group of rats (n = 7), blocked all the haemodynamic effects of a lower dose of ET-1 (50 pmol kg-1), with the exception of a slight mesenteric vasoconstriction. 4. The most straightforward explanation of the results is that the major haemodynamic effects of ET-1, -2 and -3, and all the effects of big ET-1, are mediated through ETA- and/or ETB-receptors that are effectively antagonized by bosentan.

Inhibition by phosphoramidon of the regional haemodynamic effects of proendothelin-2 and -3 in conscious rats

1. Regional haemodynamic studies were carried out in conscious, Long Evans rats, chronically-instrumented with pulsed Doppler flow probes and intravascular catheters. 2. In the first experiment, proendothelin-2 and -3 (0.1 and 1.0 nmol kg-1, i.v. boluses) were found to cause dose-dependent pressor, bradycardic, and renal and, particularly, mesenteric vasoconstrictor effects. The hindquarters showed an initial vasodilatation (which was not dose-dependent) followed by a vasoconstriction (which was dose-related). The pressor and renal and mesenteric vasoconstrictor effects of proendothelin-3 were greater than those of proendothelin-2. 3. In the second experiment, it was demonstrated that phosphoramidon (10 mumol kg-1, i.v. bolus) abolished the pressor, bradycardiac, and hindquarters vasoconstrictor effects of proendothelin-2 (1.0 nmol kg-1), and inhibited significantly the renal and mesenteric vasoconstrictor actions of this peptide. Phosphoramidon had similar effects on the responses to proendothelin-3 (1.0 nmol kg-1), although a slight pressor effect of this peptide remained in the presence of phosphoramidon. 4. In the third experiment, it was found that phosphoramidon had no significant effect on the pressor or vasoconstrictor responses to endothelin-2 or -3 (0.1 nmol kg-1). 5. Collectively, the results indicate that the haemodynamic effects of proendothelin-2 and -3 in vivo in conscious rats are probably due to their conversion to endothelin-2 and -3, respectively, by an enzyme(s) that is inhibited by phosphoramidon. There appears to be no obvious difference between proendothelin-2, proendothelin-3 and proendothelin-1 in this respect.

Endothelium-dependent mesenteric vasorelaxant effects and systemic actions of endothelin (16-21) and other endothelin-related peptides in the rat

1. The rat isolated superior mesenteric bed, perfused with Krebs-Henseleit solution containing 10 microM indomethacin and precontracted with 100 microM methoxamine, was used to study the vasorelaxation produced by some fragments of endothelin-1, by two alanyl-substituted analogues, and by human and porcine proendothelins. The systemic cardiovascular effects of some of these peptides were also studied in anaesthetized rats. 2. Endothelin (16-21) was an endothelium-dependent vasodilator about 10 times less potent than acetylcholine and its amide was about 500 times less potent than the free acid. Human proendothelin-1 and porcine proendothelin-1 also caused endothelium-dependent relaxations but neither [Ala3,11]endothelin-1 nor [Ala1,3,11,15]endothelin-1 produced significant reductions in the methoxamine-induced tone. Sodium nitroprusside was approximately 200 times less potent than acetylcholine in the presence of the endothelium and was the only vasorelaxant to be active after destruction of the endothelium by perfusion with 0.3% CHAPS; in the absence of the endothelium it was 3.7 times more potent as a vasodilator than in its presence. 3. Porcine proendothelin-1 had weak contractile activity in the isolated mesenteric bed but porcine endothelin (22-39), endothelin (16-21) and endothelin (16-21) amide did not have vasoconstrictor actions. 4. Endothelin-3, [Ala3,11]endothelin-1, [Ala1,3,11,15]endothelin-1 and endothelin (16-21) all had systemic blood pressure effects in the anaesthetized rat. Although endothelin (16-21) did not cause vasoconstriction in vitro, it increased mean arterial pressure in vivo but was at least 100 times less potent than endothelin-3. Despite causing no vasorelaxation in vitro, [Ala1,3,11,15]endothelin-1 and [Ala3,11]endothelin-1 induced short-lived systemic depressor responses which were followed by pressor responses. Endothelin-3 was more potent as a systemic depressor agent than as a pressor agonist and, as a vasodepressor agent, it was 3-4 times more potent than either of the alanyl-substituted analogues. Endothelin-3, [Ala3,11]endothelin-1 and [Ala1,3,11,15]endothelin-1 were equipotent vasopressor agents. Porcine endothelin (22-39) had no systemic blood pressure effects. 5. This study shows directly that the presence of both disulphide bonds is required in the endothelins for them to be able to cause endothelium-dependent relaxation in the mesenteric vascular bed and that this response is mediated by different receptors from those causing contraction of the vascular smooth muscle in the mesentery. Vasorelaxation caused by endothelin (16-21) and its amide suggests that there is a receptor on the endothelium similar to one reported in the guinea-pig trachea. Finally, the ability of endothelin peptides to cause systemic vasodepressor responses is not related to their ability to cause endothelium-dependent relaxation, at least in the mesenteric circulation, and, for endothelin (16-21), its systemic pressor response is not likely to be the result of vasoconstriction in the mesenteric bed.

Regional and cardiac haemodynamic effects of NG-nitro-L-arginine methyl ester in conscious, Long Evans rats

1. Regional haemodynamic responses to i.v. bolus doses (0.1-10.0 mg kg-1) of NG-nitro-L-arginine methyl ester (L-NAME) were measured in conscious, Long Evans rats (n = 8) chronically instrumented with renal, mesenteric and hindquarters pulsed Doppler flow probes and intravascular catheters. 2. L-NAME caused dose-dependent pressor effects associated with renal, mesenteric and hindquarters vasoconstrictions. The mesenteric vascular bed showed earlier onset with more rapid, and greater, maximum vasoconstrictions than the renal or hindquarters vascular beds; however, the hindquarters vasoconstriction was more persistent. D-NAME was without significant effects (n = 2). 3. Primed infusion of L-arginine (100 mg kg-1 bolus followed by 100 mg kg-1 h-1 infusion), starting 10 min after an i.v. bolus injection of L-NAME (10 mg kg-1), caused significant reversal of the pressor responses, and renal and mesenteric vasoconstrictions, but not of the hindquarters vasoconstriction. Primed infusions of L-arginine (100 mg kg-1, 100 mg kg-1 h-1) starting 5 min after L-NAME (1 mg kg-1) additionally caused some reversal of the hindquarters vasoconstriction, but this effect was transient. 4. Primed infusion of L-arginine (100 mg kg-1, 100 mg kg-1 h-1) starting 30 min before i.v. bolus injection of L-NAME (10 mg kg-1) caused significant attenuation of the pressor effects and the renal and mesenteric vasoconstrictions but not of the hindquarters vasoconstriction. 5. In a separate group of rats (n = 8) chronically instrumented with thoracic aortic electromagnetic flow probes for the measurement of cardiac haemodynamics, i.v. bolus injection of L-NAME (10mgkg-1) produced significant reductions in total peripheral conductance, cardiac output, stroke volume, peak thoracic aortic flow and the maximum rate of rise of aortic flow; these were coincident with the maximum pressor and vasoconstrictor effects. 6. These results, collectively, are consistent with L-NAME interfering with L-arginine-nitric oxide pathways that have important influences on regional vascular conductances in vivo. The pressor effect resulting from L-NAME-induced vasoconstrictions is offset by a substantial reduction in cardiac function that may depend on direct and/or indirect effects of L-NAME on the heart.