Tissue concentrations of endothelins and functional effects of endothelin-receptor activation in human arteries and veins

Endothelin-1 and its receptors on haemorrhoidal tissue: a potential site for therapeutic intervention

BACKGROUND AND PURPOSE

Haemorrhoids is a common anorectal condition affecting millions worldwide. We have studied the effect of endothelin-1 (ET-1) and the role of endothelin ET_A and ET_B receptors in haemorrhoid tissue.

EXPERIMENTAL APPROACH

Protein expression of ET-1, ET_A and ET_B receptors were compared between haemorrhoids and normal rectal submucosa using Western blot analysis, with the localization of proteins determined by autoradiography and immunohistochemistry. Effects of ET-1 and sarafotoxin 6a on human colonic and rectal arteries and veins was assessed by wire myography and the involvement of receptor subtypes established by selective antagonists.

KEY RESULTS

Dense binding of [¹²⁵ I]-ET-1 to haemorrhoidal sections was reduced by selective receptor antagonists. A higher density of ET_B than ET_A receptors was found in haemorrhoidal, than in control rectal tissue and confirmed by Western blot analysis. ET_A and ET_B receptors were localized to smooth muscle of haemorrhoidal arteries and veins, with ET_B receptors on the endothelium. Human colonic and rectal arteries and veins were similarly sensitive to ET-1 and affected by the ET_A selective antagonist, but sarafotoxin S6a-induced contractions were more pronounced in veins and antagonized by a selective ET_B receptor antagonist.

CONCLUSIONS AND IMPLICATIONS

ET_A and ET_B receptors are present in human haemorrhoids with ET_B receptors predominating. ET_A receptors are activated by ET-1 to mediate a contraction in arteries and veins, but the latter are selectively activated by sarafotoxin S6a - a response that involves ET_B receptors at low concentrations. Selective ET_B agonists may have therapeutic potential to reduce congestion of the haemorrhoidal venous sinusoids.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Cigarette smoke exposure up-regulates endothelin receptor B in human pulmonary artery endothelial cells: molecular and functional consequences

BACKGROUND AND PURPOSE

Pulmonary arteries from smokers and chronic obstructive pulmonary disease patients show abnormal endothelium-dependent vascular reactivity. We studied the effect of cigarette smoke extract (CSE) on endothelin receptor B (ET(B) ) expression in human pulmonary artery endothelial cells (HPAECs) and its role in endothelial dysfunction.

EXPERIMENTAL APPROACH

ET(B) receptor expression was measured by real time RT-PCR, Western blot and immunofluorescence. Cell contraction, intracellular Ca(2+) , F/G-actin, RhoA activity, myosin light chain phosphorylation, ET, NO, thromboxane (Tx)A(2) and reactive oxygen species (ROS) were measured by traction microscopy, fluorescence microscopy, phalloidin fluorescence, colorimetric assay, Western blot, elisa and DCFDA fluorescence respectively.

KEY RESULTS

Cigarette smoke extract dose-dependently increased ET(B) receptor expression in HPAECs after 24h incubation. CSE-induced ET(B) expression was attenuated by bosentan, the ET(B) receptor antagonist BQ788, the Rho kinase antagonist Y27632 and the antioxidant N-acetylcysteine. A monoclonal antibody to ET-1 prevented CSE-induced ET(B) receptor overexpression. Twenty-four hour exposure to ET-1 dose-dependently increased ET(B) receptor expression, mimicking the effect of CSE. CSE-induced ET(B) receptor overexpression caused greater cell contraction; increased intracellular Ca(2+) ; increased F/G-actin and RhoA activity; increased myosin light chain phosphorylation; augmented TxA(2) and ROS production; and decreased NO after acute ET-1 (10nM). These effects were attenuated by bosentan, BQ788, Y27632 and N-acetylcysteine.

CONCLUSIONS AND IMPLICATION

Cigarette smoke extract induced ET(B) receptor overexpression by a feed forward mechanism mediated partly by ET release, promoting HPAEC dysfunction and attenuated by ET(B) receptor blockade, Rho kinase and ROS inhibition. These results provide support for the use of bosentan in CS-related endothelial dysfunction.

Endothelin-1 and vein graft occlusion in patients undergoing bypass surgery

The saphenous vein is the most commonly used graft for revascularization procedures in patients with coronary artery disease and critical limb ischaemia. However, the patency rate of this vessel is poor, with a high proportion of patients requiring further surgery. Early graft occlusion is caused by vasoconstriction or thrombus formation, with later stages of graft failure being due to neointimal formation or atherosclerosis. Apart from its potent constrictor action, endothelin-1 is also a potent proliferative and proinflammatory peptide that is implicated in a number of vascular diseases. The surgical trauma caused during preparation of the saphenous vein as a bypass graft stimulates the release of a number of factors affecting vascular reactivity and structure, including endothelin-1. Endothelin-1 not only constricts animal and human isolated saphenous vein segments but also causes vascular smooth muscle proliferation and neointimal thickening in vitro, actions that are mediated via endothelin (A and B) receptors. Experimentally, the effects of subtype-selective and dual receptor antagonists have been shown to inhibit endothelin-1-mediated constriction and cell proliferation of the saphenous vein. In this review, data supporting a role of endothelin-1 in vein graft occlusion are presented, and the therapeutic potential of endothelin receptor antagonists in improving graft performance is discussed.

Spasmogenic action of endothelin-1 on isolated equine pulmonary artery and bronchus

REASONS FOR PERFORMING STUDY

There is currently little published information about the effects of endothelin-1 (ET-1), a potent endogenous spasmogen of vascular and airway smooth muscle, on pulmonary vasculature and airways or which ET receptor subtypes mediate ET-1-induced vasoconstrictive and bronchoconstrictive action in the horse.

OBJECTIVES

To investigate the effect of endothelin-1 (ET-1) on smooth muscle from isolated equine pulmonary artery and bronchus. In addition, the roles of ETA and ETB receptors in ET-1 mediated contraction in these tissues were assessed.

METHODS

The force generation of ring segments from pulmonary arteries or third-generation airways (obtained from horses subjected to euthanasia for orthopaedic reasons) were studied in an organ bath at 37 degrees C in response to exogenous endothelin and selective endothelin A (BQ123) or B receptor (BQ788) antagonists.

RESULTS

ET-1 produced concentration-dependent contractions of the equine pulmonary artery and bronchus. The threshold for contraction was 10(-10) and 10(-9) mol/l ET-1 for pulmonary artery and bronchus, respectively. The maximal contraction induced by the highest ET-1 concentration (10(-7) mol/l) was 173 and 194% of the contraction obtained with 100 mmol/l KCl in pulmonary artery and bronchus, respectively. ET-1 potency was 25 times greater in equine pulmonary artery than in equine bronchus (concentration of ET-1 producing 50% of maximal contraction [EC50] = 5.6 10(-9) mol/l and 2.2 10(-8) mol/l, respectively). In pulmonary artery, ET-1 induced contractions were significantly inhibited by the ETA receptor antagonist BQ123 (1 micromol/l; dose-response curve to ET-1 was shifted to the right by 5.4-fold), but not by the ETB antagonist BQ788. In bronchus, dose-responses curves to ET-1 were shifted to the right by BQ123 (1 micromol/l; 2.5-fold), but not by BQ788 (1 micromol/l). In the presence of both antagonists, the dose-response curve to ET-1 was shifted to the right by 4.5-fold.

CONCLUSIONS

These functional studies demonstrate that ET-1 is a potent spasmogen of equine third generation pulmonary artery and bronchus, and that contractions are mediated via ETA receptors in the former and both ETA and ETB receptors in the latter.

POTENTIAL CLINICAL RELEVANCE

Endothelin receptor antagonists may have potential for treating equine pulmonary hypertension or bronchoconstriction.

Is there a role for endothelin-blockade early after coronary artery bypass grafting?

BACKGROUND

Diverse results exist regarding myocardial release of endothelin after coronary artery bypass grafting. Because endothelin may be involved in regulation of coronary blood flow, postoperative endothelin-blockade could influence the surgical outcome. In this study, we have evaluated the cardiac outflow of endothelin and effects on coronary flow by endothelin-blockade immediately after completion of the coronary bypass grafting.

METHODS

Thirty patients were subjected to infusions of endothelinA blocker (BQ-123, 260 nmoL/min for up to 30 minutes) or endothelinA blocker and endothelinB blocker (BQ-123 and BQ-788, 260 and 250 nmol/min, respectively, for up to 30 minutes) into a veingraft anastomosed to a coronary vessel, and the coronary blood flow was measured. Plasma levels of endothelin from the coronary sinus and the periphery were determined.

RESULTS

There were no significant changes in flow caused by endothelinA blockade alone or in combination with endothelinB blockade. There were no immediately increased levels of endothelin after surgery or after infusions of the endothelin blockers.

CONCLUSIONS

Endothelin blockade does not influence the immediate perioperative myocardial blood flow after coronary bypass grafting. There is no significantly increased myocardial outflow of endothelin, and endothelin does not have any influence on the basal tone of the coronary vessels in the early phase after coronary bypass grafting.

Plasma levels and vascular effects of endothelin and big endothelin in patients with stable and unstable angina pectoris undergoing coronary bypass grafting

OBJECTIVES

The aim of this study was to determine the plasma and pericardial levels of endothelin-1 (ET-1) and its precursor big endothelin-1 (Big ET-1) in patients with unstable and stable angina prior to and following coronary bypass surgery. To further investigate the content of ET-1, tissue levels were studied in the internal mammary artery (IMA) in patients with stable and unstable angina pectoris. Finally, the difference in reactivity of the IMA to ET-1 and Big ET-1 in stable and unstable patients was evaluated.

METHODS

Plasma and pericardial levels of ET-1 and Big ET-1 were determined with radioimmunoassay in 81 patients (33 unstable) immediately before coronary bypass surgery, and at 6, 14, 40 and 64 h following the procedure. Specimens of the distal IMA from 12 patients (six unstable) were collected at the beginning of surgery for determination of tissue levels of ET-1. Additionally, distal internal mammary arteries were obtained from another 24 patients (12 unstable). These vessels were mounted in organ baths for functional studies on vascular reactivity to ET-1 and Big ET-1.

RESULTS

The peripheral plasma levels of ET-1 in unstable patients were significantly lower in patients with unstable angina compared with patients with stable angina pectoris at all points of measurement. The levels of Big ET-1 were significantly higher pre-operatively in the unstable group, but decreased to similar levels to those of stable patients following coronary bypass grafting. There was no difference in ET-1 tissue content in the IMA between the patients. ET-1 and Big ET-1 caused an endothelin(A) (ET(A))-receptor blocker sensitive, concentration-dependent contraction of the IMA obtained from stable as well as unstable patients.

CONCLUSIONS

It is concluded that unstable angina pectoris is associated with an increased ET-1 turnover. This increased turnover may participate in the local regulation of coronary vascular tone with subsequent influence of the condition of the patients. The present investigation also implies that ET(A)-blockade may be useful as an additional pharmacological principal in the treatment of unstable angina pectoris prior to revascularization, as well as to prevent post-operative arterial graft spasm.

Off-pump coronary bypass surgery causes less immediate postoperative coronary endothelial dysfunction compared to on-pump coronary bypass surgery

OBJECTIVE

In this study a comparison of the vascular reactivity in the coronary circulation was investigated by injection of acetylcholine (ACh) and adenosine (ADO) in coronary bypass patients, operated on with or without the assistance of heart-lung machine. The patients operated on with heart-lung machine were further divided into subjects with stable or unstable angina pectoris.

METHODS

Nine patients with stable angina pectoris subjected to off-pump surgery (target arterial occlusion time of 11+/-0.5 min) and 18 patients subjected to on-pump surgery (nine patients with stable angina and nine patients with unstable angina; cross-clamp time of 43+/-3 and 32+/-2 min, respectively), received ACh (10 microg) and ADO (18 microg) given as bolus injections into a vein-graft anastomosed to a coronary vessel. The blood flow in the vein-graft (i.e. indirectly the flow in the targeted coronary circulation) and hemodynamics were observed.

RESULTS

In the off-pump group, ACh evoked an increase with +14+/-12% of control in coronary blood flow, while in the stable on-pump group ACh decreased the blood flow with -60+/-7% of baseline and in the unstable on-pump group the flow was decreased with -38+/-8% of baseline (P<0.001 between the stable on- and off-pump groups, no significant difference between the stable and unstable on-pump groups). ADO significantly increased the coronary blood flow in all three groups; with +81+/-14% in the off-pump patients; with +95+/-14% in the stable on-pump group and with +74+/-13% in the unstable on-pump group (P<0.01 compared to baseline for all three groups). Neither ACh nor ADO injection caused any changes in hemodynamics.

CONCLUSIONS

The present study demonstrates that on-pump coronary bypass surgery appears to be more harmful to the coronary endothelium, in terms of ACh-induced vasoconstriction, compared to off-bypass pump surgery. Furthermore, there is no significant difference in direct smooth muscle vascular reactivity between off-pump and on-pump coronary bypass surgery. No apparent dissimilarities in endothelial dysfunction were observed in the stable and unstable on-pump groups suggesting other causes for differences in post-operative outcome for these patients.

Vasoconstrictor activity of novel endothelin peptide, ET-1(1 - 31), in human mammary and coronary arteries in vitro

1. The ability of the putative chymase product of big endothelin-1 (big ET-1), ET-1(1 - 31), to constrict isolated endothelium-denuded preparations of human coronary and internal mammary artery was determined. 2. pD2 values in coronary and mammary artery respectively were 8.21+/-0.12 (n=14) and 8.55+/-0.11 (n=12) for ET-1, 6.74+/-0.11 (n=16) and 7.10+/-0.08 (n=16) for ET-1(1 - 31) and 6.92+/-0.10 (n=15) and 7.23+/-0.11 (n=12) for big ET-1. ET-1(1 - 31) was significantly less potent than ET-1 (P<0.001, Student's t-test) and equipotent with big ET-1. 3. Vasoconstrictor responses to 100 - 700 nM ET-1(1 - 31) were significantly (P<0.05, Student's paired t-test) attenuated by the ET(A) antagonist PD156707 (100 nM). 4. There was no effect of the ECE inhibitor PD159790 (30 microM), the ECE/NEP inhibitor phosphoramidon (100 microM) or the serine protease inhibitor chymostatin (100 microM) on ET-1(1 - 31) responses in either artery. 5. Radioimmunoassay detected significant levels of mature ET in the bathing medium of coronary (1.6+/-0.5 nM, n=14) and mammary (2.1+/-0.6 nM, n=14) arteries, suggesting that conversion of ET-1(1 - 31) to ET-1 contributed to the observed vasoconstriction. 6. ET-1(1 - 31) competed for specific [(125)I]-ET-1 binding to ET(A) and ET(B) receptors in human left ventricle with a pooled K(D) of 71.6+/-7.0 nM (n=3). 7. Therefore, in human arteries the novel peptide ET-1(1 - 31) mediated vasoconstriction via activation of the ET(A) receptor. The conversion of ET-1(1 - 31) to ET-1, by an as yet unidentified protease, must contribute wholly or partly to the observed constrictor response. Chymase generated ET-1(1 - 31) may therefore represent an alternative precursor for ET-1 production in the human vasculature.

The endothelin system in human saphenous vein graft disease

Saphenous vein graft stenosis is a significant clinical complication for coronary artery bypass patients. Endothelin-1, a peptide synthesised by vascular endothelial cells, is the most potent known vasoconstrictor and has mitogenic properties. Recent advances in our knowledge of endothelin-1 synthesis and endothelin receptor expression and function in normal and atherosclerotic human saphenous vein imply a role for the peptide in the progression of vein graft failure. Manipulation of the endothelin system, by selective receptor antagonism or inhibition of the specific endothelin-converting enzymes may, therefore, represent a novel therapeutic target for treating vein graft disease.

Augmentation of endothelial function by endothelin antagonism in human saphenous vein conduits

OBJECT

Cerebral revascularization with saphenous vein (SV) conduits is used in the management of hard-to-treat lesions that require deliberate arterial occlusion and in selected patients with occlusive vascular disease. Endothelial dysfunction is thought to contribute to acute perioperative vasospasm and chronic graft atherosclerosis. In the present study the authors examined the contribution of the potent vasoconstrictor endothelin-1 (ET-1) to endothelial dysfunction in human SVs.

METHODS

The effects of an ET(A/B) receptor antagonist (bosentan), an ET(A) receptor antagonist (BQ-123), and an ET(B) receptor antagonist (BQ-788) on in vitro endothelium-dependent and -independent responses were studied in human SVs. Vascular segments were obtained in 34 patients who had undergone revascularization procedures, and isometric dose-response curves (DRCs) were constructed using the isolated tissue bath procedure as follows: 1) cumulative DRCs to norepinephrine; and 2) DRCs to acetylcholine (ACh) and sodium nitroprusside in the absence and presence of bosentan, BQ-123, or BQ-788. Maximal vasodilatory responses and sensitivity were compared between groups. In the presence of bosentan (Experiment 1) and BQ-123 or BQ-788 (Experiment 2), ACh responses were significantly augmented (percent maximum relaxation values: 7+/-2 [control] compared with 17+/-3 [bosentan], p < 0.002 [Experiment 1]; and 12+/-2 [control] compared with 29+/-2 [BQ-123] and 25+/-2 [BQ-788], p < 0.003 and p < 0.002, respectively [Experiment 2]). The sensitivity of SVs to ACh was unaffected by treatment. These beneficial effects were specific for the endothelium.

CONCLUSIONS

Blockade of ET receptors significantly improves endothelial function in SVs. Furthermore, these effects appear to be independently and maximally mediated by antagonism of either ET(A) or ET(B) receptors. Interventions aimed at improving endothelial function may serve to counter perioperative vasospasm and impede atherosclerosis in SVs used for revascularization procedures.

Short ischemia causes endothelial dysfunction in porcine coronary vessels in an in vivo model

BACKGROUND

The aim of this study was to evaluate the effects of a short period of ischemia (10 mins) and a prolonged period of ischemia (60 mins) followed by reperfusion on coronary flow changes induced by acetylcholine (ACh), adenosine (ADO), and endothelin (ET).

METHODS

The left anterior descending coronary artery in anesthetized pigs was occluded for 10 or 60 minutes followed by 120 minutes reperfusion. Thereafter, the flow changes in the left anterior descending coronary artery were studied after intracoronary infusion of ACh, ADO, and ET.

RESULTS

Short-term ischemia (10 minutes) caused a decrease in vasodilatation, but not the vasoconstriction response to ACh. Prolonged ischemia (60 minutes) impaired ADO induced vasodilatation and aggravated ET evoked vasoconstriction.

CONCLUSIONS

The present findings suggest that a short period of ischemia (10 minutes) causes disturbances of the endothelial regulation of coronary vascular tone and that this endothelial regulation is more sensitive, and precedes changes in vascular smooth muscle function after ischemia and reperfusion.

Effect of endothelin-1(1-31) on intracellular free calcium in cultured human mesangial cells

We found that human chymase selectively produces 31-amino-acid length endothelins (1-31) (ETs(1-31)). We investigated the effect of synthetic ET-1(1-31) on intracellular free Ca2+ concentration ([Ca2+]i) in cultured human mesangial cells. ET-1(1-31) increased [Ca2+]i in a concentration-dependent manner to a similar extent as ET-1. The ET-1 (1-31)-induced [Ca2+]i increase was not influenced by removal of extracellular Ca2+ but was inhibited by thapsigargin. ET-1(1-31)-induced [Ca2+]i increase was not affected by phosphoramidon. It was inhibited by BQ123, but not by BQ788. These results suggest that ET-1(1-31) by itself exhibits bioactive properties probably through endothelin ET(A) or ET(A)-like receptors. Since human chymase has been reported to exist in the kidney, ET-1(1-31) may be a candidate substance for mesangium-relevant diseases.

Comparison of the effects of endothelin-1, -2 and -3 (1-31) on changes in [Ca2+]i in human coronary artery smooth muscle cells

We have previously found that human chymase selectively cleaves big endothelins (ETs) at the Tyr31-Gly32 bond to produce 31-amino-acid endothelins, ETs (1-31). In the present study, we investigated the effects of ETs (1-31) on changes in intracellular free Ca2+ ([Ca2+]i) in cultured human coronary artery smooth muscle cells (HCASMCs) using confocal laser microscopy. ETs (1-31) increased [Ca2+]i in a concentration-dependent manner. Phosphoramidon did not inhibit the increases in [Ca2+]i caused by ETs (1-31). The [Ca2+]i increases induced by ETs (1-31) were compared to those of ETs (1-21) and big ETs. ET-1 (1-21) was about 10-times more potent than big ET-1 or ET-1 (1-31), whereas big ET-2 was 10-times less potent than ET-2 (1-21) or ET-2 (1-31). ETs (1-31) may induce [Ca2+]i increase through ET(A)-type or ET(A)-type-like receptors. The 10(-12) M ET (1-31)-induced increases in [Ca2+]i were not affected by removal of extracellular Ca2+, but were inhibited by thapsigargin. These results suggested that ET-1, -2 and -3 (1-31) showed similar potencies in increasing [Ca2+]i and mechanisms of ET (1-31)-induced increases in [Ca2+]i may be similar among the three ETs (1-31).

Mechanism of endothelin-1-(1-31)-induced calcium signaling in human coronary artery smooth muscle cells

We have found that human chymase produces a 31-amino acid endothelin [ET-1-(1-31)] from the 38-amino acid precursor (Big ET-1). We examined the mechanism of synthetic ET-1-(1-31)-induced intracellular Ca2+ signaling in cultured human coronary artery smooth muscle cells. ET-1-(1-31) increased the intracellular free Ca2+ concentration ([Ca2+]i) in a concentration-dependent manner (10(-14)-10(-10) M). This ET-1-(1-31)-induced [Ca2+]i increase was not affected by phosphoramidon, Bowman-Birk inhibitor, and thiorphan. The ET-1-(1-31)-induced [Ca2+]i increase was not influenced by removal of extracellular Ca2+ but was inhibited by thapsigargin. ET-1-(1-31) at 10(-12) M did not cause Ca2+ influx, whereas 10(-7) M ET-1-(1-31) evoked marked Ca2+ influx, which was inhibited by nifedipine. ET-1-(1-31) also increased inositol trisphosphate formation. These results suggest that the ET-1-(1-31)-induced [Ca2+]i increase at relatively low concentrations is attributable to the release of Ca2+ from inositol trisphosphate-sensitive intracellular stores, whereas Ca2+ influx into the cells evoked by high concentration of ET-1-(1-31) probably occurs through voltage-dependent Ca2+ channels. We concluded that the physiological activity of ET-1-(1-31) may be attributable to Ca2+ mobilization from intracellular stores rather than influx of Ca2+ from extracellular space.

Augmented response of endothelin-A and endothelin-B receptor stimulation in coronary arteries of hypertensive hearts

To determine whether the vasoconstrictor response to endothelin-1 (ET-1) is altered in coronary vessels of hypertensive hearts and the role of ETA and ETB receptors in these responses to ET-1, the vasoconstrictor response to ET-1 in coronary vessels was measured with or without ETA and ETB receptor antagonists. In isolated hearts of spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto (WKY) rat, the coronary perfusion pressure was measured on a Langendorff apparatus with constant pressure (75 mm Hg). Coronary perfusion resistance (CPR) (mm Hg/ml/min/g) was calculated. ET-1 elicited dose-dependent increases of CPR in both normotensive and SHR rat hearts. However, the responses were significantly greater in SHR than those of WKY. Pretreatment with the ETA antagonist FR139317 and the ETB antagonist BQ788 inhibited CPR increases with ET-1 infusion. However, vasoconstrictor responses to ET-1 were still greater in SHR than in WKY after FR139317 or BQ788 infusion. These findings suggest that the augmented vasoconstrictor response of coronary artery to ET-1 is mediated by both ETA and ETB receptors. These changes may contribute to the impaired coronary circulation in hypertension.

Atypical antagonism observed with BQ-123 in human saphenous vein

At present, endothelin (ET) receptor classification remains controversial. We investigated the presence of atypical ETA receptors in human saphenous veins (SV). Human SV was obtained from 24 patients undergoing coronary artery bypass grafting. Vessels were set up in organ baths, stretched to a tension of 6 g, and left to relax before being challenged with 90 mM KCl. After KCl challenge, tissues were incubated with 2.8 microM indomethacin and 100 microM L-NMMA for 30 min followed by 30 min in the presence of antagonist before a concentration-response curve to ET-1 or sarafotoxin 6b (S6b) (10(-10)-10(-7) M) was constructed. In endothelium-intact vessels, incubated with indomethacin and L-NMMA, BQ-123 (1 microM) caused nonparallel shifts, with lower concentrations of ET-1 being antagonized more than higher concentrations. This antagonism with BQ-123 was unaffected by BQ-788 (0.1 microM; n = 6) or by desensitization of ETB receptors with S6c (0.1 microM; n = 8). Blocking the Ca2+ channels with nifedipine (1 microM; n = 5) did not affect the antagonism, nor did denuding the endothelium or leaving the endothelium intact (n = 5). When S6b was used as an agonist, BQ-123 (0.3-3 microM) caused concentration-dependent biphasic shifts, with low concentrations of S6b not being antagonized. In conclusion, the antagonism observed with BQ-123 is not due to the action of ET-1 at ETB receptors, changes in Ca2+ handling, or endothelium. This unusual action of BQ-123 suggests subtypes of the ETA receptor.

Endothelin-1-(1-31), a novel vasoactive peptide, increases [Ca2+]i in human coronary artery smooth muscle cells

We have previously found that human chymase cleaves big endothelins at the Tyr31-Gly32 bond and produces 31-amino acid long endothelins-(1-31), without any further degradation products. In this study, we investigated the effect of synthetic endothelin-1-(1-31) on the intracellular free Ca2+ concentration ([Ca2+]i) in cultured human coronary artery smooth muscle cells. Endothelin-1-(1-31) increased [Ca2+]i in a concentration-dependent manner (10(-14) to 10(-10) M). This endothelin-1-(1-31)-induced [Ca2+]i increase was not affected by phosphoramidon (N-(alpha-Rhamnopyranosyloxyhydroxyphosphinyl)-L-Leucyl-L-Tryptoph an), an inhibitor of endothelin-converting enzyme. It was, however, inhibited by 10(-10) M BQ123 (Cyclo-(-D-Trp-D-Asp(ONa)-Pro-D-Val-Leu-)), an endothelin ET(A) receptor antagonist, but not by 10(-10) M BQ788 (N-cis-2,6-dimethylpiperidinocarbonyl-L-yMeLeu-D-Trp(COOM e)-D-Nle-ONa), an endothelin ET(B) receptor antagonist. These results suggest that endothelin-1-(1-31) by itself exhibits vasoactive properties probably through endothelin ET(A) receptors. Since human chymase has been reported to play a role in atherosclerosis, endothelin-1-(1-31) may be one of the candidate substances for its cause.

Cardiopulmonary bypass and circulatory arrest increase endothelin-1 production and receptor expression in the lung

BACKGROUND

Endothelin-1 has been shown to be a mediator of pulmonary hypertension after cardiopulmonary bypass and deep hypothermic circulatory arrest. It is not known whether the mechanism is increased production of endothelin-1 or alterations in expression of endothelin-1 receptors in the lung. This study was designed to test the hypothesis that circulatory arrest increases endothelin-1 mRNA levels and endothelin-1 receptor expression in the lung.

METHODS AND RESULTS

Twenty-four piglets (7 to 30 days old) were studied randomly either at baseline (controls, n = 12) or after cardiopulmonary bypass with 30 minutes of circulatory arrest (deep hypothermic circulatory arrest, n = 12). Lungs and pulmonary arteries were harvested immediately after hemodynamic data collection. Deep hypothermic circulatory arrest significantly increased pulmonary vascular resistance (p < 0.01). Deep hypothermic circulatory arrest also produced a significant increase in endothelin-1 mRNA levels in the pulmonary arteries (149 +/- 55 pg vs 547 +/- 111 pg, p = 0.007). There was no significant change in the pulmonary parenchymal endothelin-1 mRNA levels (4102 +/- 379 pg vs 4623 +/- 308 pg, p = 0.32). Ligand binding studies of the lung parenchyma revealed a single specific endothelin-1 binding site with an EC50 value (effective concentration causing 50% of the maximum response) of about 1 x 10(-8) mol/L, consistent with the endothelin B subtype. Deep hypothermic circulatory arrest resulted in a significant increase in the number of endothelin-1 receptors in the lung (109 +/- 6 fmol/mg total protein to 135 +/- 9 fmol/mg total protein, p = 0.02).

CONCLUSIONS

Deep hypothermic circulatory arrest increases production of endothelin-1 by the pulmonary vascular endothelium. Endothelin-1 production in the pulmonary parenchyma does not change. Expression of endothelin B receptors in the pulmonary parenchyma also increases after cardiopulmonary bypass with deep hypothermic circulatory arrest. This study supports the hypothesis that deep hypothermic circulatory arrest results in pulmonary vascular endothelial activation with increased endothelin-1 mRNA production.

The endothelin ETB receptor antagonist BQ-788 reduces the pulmonary vasodilator effect of endothelin-1 during acute hypoxia in pigs

Pigs were subjected to acute, intermittent hypoxia (fraction of inhaled O2 0.1, n = 10). The increase in mean pulmonary artery pressure during hypoxia was not altered after i.v. administration of the selective endothelin ETB receptor antagonist BQ-788 (N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D -1- methoxycarbonyltryptophanyl-D-norleucine) (1 mg). However, the vasodilatory effect of endothelin-1 (25 ng/kg per min) infused into the pulmonary artery during hypoxia was attenuated by BQ-788. The present results suggest that the pulmonary vasodilator effect of exogenously administered endothelin-1 during acute hypoxia is mediated by endothelin ETB receptors in the pig. Furthermore, endothelin ETB receptor antagonism with BQ-788 does not influence the pulmonary vascular response to acute hypoxia.