Comparison of selective ET(A) and ET(B) receptor antagonists in patients with chronic heart failure

BACKGROUND: The vasoconstrictor action of endothelin-1 (ET-1) is mediated through ET(A) and ET(B) receptor subtypes on vascular smooth muscle. ET(B) receptors are also present on the vascular endothelium where they mediate vasodilation. Animal studies suggest that the ET(B) receptor also acts as a clearance receptor for endothelin. AIMS: To investigate the effects of a selective ET(A) and a selective ET(B) receptor antagonist alone and in combination on haemodynamics and circulating concentrations of ET-1 in patients with chronic heart failure. RESULTS: Infusion of BQ-123 (n=10), a selective ET(A) receptor antagonist, led to systemic vasodilation and did not change plasma ET-1 concentrations (1.38+/-0.82 to 1.38+/-0.91 fmol/ml, ns). Infusion of BQ-788 (n=8) led to systemic vasoconstriction with a rise in plasma ET-1 (1.84+/-1.06 to 2.73+/-0.99 fmol/ml, p<0.01). The addition of BQ-123 to BQ-788 led to systemic and pulmonary vasodilation with no further increase in plasma ET-1 concentrations (2.80+/-1.14 to 2.90+/-1.20 fmol/ml, ns). CONCLUSION: The rise in plasma ET-1 concentrations in response to selective blockade of ET(B) receptors and the associated adverse haemodynamic effects suggest that ET(B) receptors have a role in the clearance of ET-1 in man and that their blockade may not be advantageous for patients with heart failure.     

Endothelin-1 potentiates cholinergic nerve-mediated contraction in human isolated bronchus.

That endothelin-1(ET-1) plays a mediator role in asthma is consistent with reports of ET-1-induced potentiation of cholinergic nerve-mediated contraction in airways from various animal species. This study examined the effect of ET-1 on cholinergic contractions in human isolated bronchus. Macroscopically nondiseased human bronchial tissue was obtained from 23 patients with respiratory tumours. An electrical field stimulation (EFS) frequency that produced one third of the contraction at 30 Hz (EFS30) was estimated. The effect of ET-1 on these EFS-evoked contractions was assessed. EFS-evoked contractions were frequency-dependent and abolished by either atropine or tetrodotoxin. Thus, EFS-induced contractions were mediated by acetylcholine from cholinergic nerves. ET-1 (3 nM) potentiated EFS-evoked contractions by 10+/-2% EFS30 (p<0.05) without any significant effect on contractions induced by exogenous acetylcholine. Neither the ET(A) receptor-selective antagonist BQ-123 (3 microM) nor the ET(B) receptor-selective antagonist BQ-788 (10 microM) alone significantly altered ET-1-induced potentiation of EFS-evoked contractions. However, in the combined presence of both BQ-123 and BQ-788, ET-1-induced potentiation of EFS-evoked contractions was abolished. Thus, prejunctional endothelinA and endothelinB receptors appear to mediate endothelin-1-induced potentiation of electrical field stimulation-evoked cholinergic contractions in human bronchus. This suggests another potentially important mechanism through which endothelin-1 could increase bronchial tone in asthma.     

Mechanisms underlying endothelin-1 effects on myocardial function]

INTRODUCTION: Endothelin-1 (ET-1) has potent vasoconstrictor, growth promoting and positive inotropic properties. Its effects on the intrinsic properties of the myocardium were recently described. The present study investigated the mechanisms underlying those effects. METHODS: The myocardial effects of 1 and 10 nM of ET-1 were evaluated in isolated rabbit papillary muscles (n = 9) and human atrial trabecula from CABG patients (Krebs-Ringer; 1.8 mM CaCl2; 35 degrees C). In papillary muscles the effects of 1 nM ET-1 were also studied in the presence of: (i) a selective ETA receptor antagonist, BQ-123 (0.1 microM; n = 9); (ii) a selective ETB receptor antagonist, BQ-788 (0.1 microM; n = 6); and (iii) an Na+/H+ exchanger inhibitor, methyl-isobutyl-amiloride (MIA; 1 microM; n = 6). Only significant results (mean +/- SE, p < 0.05) are given, expressed as delta % baseline. RESULTS: In AT by papillary muscles, 1 nM of ET-1 increased 64 +/- 16%, dT/dtmin 39 +/- 13% and decreased PT by 11 +/- 2%. The analysis of atrial strip contractions yielded similar results. In papillary muscles the effects of ET-1 were not affected by BQ-788, yet they were abolished by BQ-123, and reduced by 44% by MIA. CONCLUSIONS: The action of ET-1 on myocardial function is similar in human and non-human myocardium. The myocardial effects observed in the present study are mediated by the binding to ETA receptors, and partially dependent on Na+/H+ exchanger activation.     

Biochemical and pharmacological profile of a potent and selective endothelin B-receptor antagonist, BQ-788.

We describe the characteristics of a potent and selective endothelin (ET) B-receptor antagonist, BQ-788 [N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D -1- methoxycarbonyltryptophanyl-D-norleucine]. In vitro, this compound potently and competitively inhibits 125I-labeled endothelin 1 (ET-1) binding to ETB receptors on human Girardi heart cells (IC50, 1.2 nM) but only poorly inhibits the binding to ETA receptors on human neuroblastoma cell line SK-N-MC cells (IC50, 1300 nM). In isolated rabbit pulmonary arteries, BQ-788 shows no agonist activity up to 10 microM and competitively antagonizes the vasoconstriction induced by an ETB-selective agonist, BQ-3020 (pA2, 8.4). In rat, an ETA-selective antagonist, BQ-123 (1 mg/kg, i.v.), does not affect transient depressor response to ET-1 (0.3 nmol/kg, i.v.) but potently inhibits following sustained pressor response; vice versa, BQ-788 (1 mg/kg, i.v.) abolishes the depressor response, resulting in a rapid onset of apparently enhanced pressor response. Thus, being a potent and selective ETB receptor antagonist, BQ-788 may be considered as a powerful tool for investigating the role of ET in physiological and pathological processes.     

Endothelin-3 induces both human and opossum gallbladder contraction mediated mainly by endothelin-B receptor subtype in vitro

BACKGROUND: Endothelins are produced by gallbladder epithelial cells, suggesting a role in the regulation of gallbladder function. AIMS: To characterize the effect of endothelin-3 (ET-3) on human and Australian possum gallbladder contractility and identify the receptor(s) involved. METHODS: Human and possum gallbladder muscle strips were exposed to cumulative concentrations of ET-3 (10 pmol/L-100 nmol/L). Strips were pretreated with either tetrodotoxin (TTX) (1 micro mol/L), the selective ET receptor antagonists BQ-123 (ET(A)), BQ-788 (ET(B)), alone or together, or the mixed ET antagonist tezosentan (all 1 micro mol/L). Maximal changes in tone were measured and expressed as percentage of carbachol (100 micro mol/L)-induced tone. ANOVA was used for statistical analysis. RESULTS: Endothelin-3 induced a concentration-dependent increase in tone in both human and pos-sum strips (P < 0.05) and at 100 nmol/L represented 44.2 +/- 4.5% and 40.3 +/- 4.6% of carbachol-induced tone, respectively. The effect on human strips was TTX insensitive, whereas the possum concentration-response curve was shifted to the right. Individually, BQ-123 and BQ-788 shifted the human concentration-response curve to the right, but a greater inhibition by BQ-788 was achieved in the possum (P < 0.05). However, BQ-123 plus BQ-788 further reduced the ET-3 effect (P < 0.001) to a level comparable to that observed in the presence of tezosentan in both human and possum strips. CONCLUSION: Endothelin-3 produces potent gallbladder contraction in vitro, acting mainly via ET(B) receptors and also interacting with ET(A)receptors. The receptors are located on the smooth muscle, but in possum gallbladder, neural receptors may also be involved. These findings suggest that ET-3 may regulate motility of possum and human gallbladder.     

Endothelin-1 induces vasoconstriction on portal-systemic collaterals of portal hypertensive rats

Portal hypertension is associated with increased hepatic and collateral resistance to an increased portal blood flow. Endothelin-1 (ET-1) can induce intrahepatic vasoconstriction and consequently increase portal pressure. It is unknown if ET-1 also modulates portal pressure by a direct vasoconstrictive effect on collaterals. This study investigated the collateral vascular responses to ET-1, the receptors in mediation, and the regulation of ET-1 action by nitric oxide and prostaglandin. The portal-systemic collaterals of partially portal vein-ligated rats were tested by in situ perfusion. The concentration-response curves of collaterals to graded concentrations of ET-1 (10(-10)-10(-7) mol/L) with or without BQ-123 (ET(A) receptor antagonist, 2 x 10(-6) mol/L), BQ-788 (ET(B) receptor antagonist, 10(-7) mol/L) or both were recorded. In addition, the collateral responses to ET-1 with preincubation of n(omega)-nitro-L-arginine (NNA; 100 mol/L), indomethacin (INDO; 10 mol/L), or in combination were performed. ET-1 increased the perfusion pressure of collaterals and its effect was significantly suppressed by BQ-123 alone and BQ-123 plus BQ-788, but not BQ-788 alone (P <.05). Incubation with NNA, INDO, or both significantly enhanced the response of collaterals to ET-1 (P < .05). These results show that ET-1 produces a direct vasoconstrictive effect on the collateral vessels of portal hypertensive rats. This effect is mediated by ET(A,) but not ET(B), receptors. Both nitric oxide and prostaglandin modulate the collateral vascular response to ET-1 and may therefore participate in the development and maintenance of portal hypertension.     

Altered peripheral vascular responses to exogenous and endogenous endothelin-1 in patients with well-compensated cirrhosis

Plasma endothelin concentrations are elevated in cirrhosis and correlate with disease severity. This study assessed forearm vascular responses to exogenous endothelin-1 (ET-1), and evaluated the contribution of endogenous ET-1 to the maintenance of basal peripheral vascular tone in patients with well-compensated cirrhosis (n = 11) and matched healthy controls (n = 8). Bilateral forearm blood flow (FBF) was measured at baseline and following unilateral, subsystemic, intrabrachial artery infusions of ET-1 (2 and 6 pmol/min); BQ-123, a selective ET(A) receptor antagonist (3 and 10 nmol/min); and BQ-788, a selective ET(B) receptor antagonist (0.3 and 1 nmol/min) using venous occlusion plethysmography. Baseline systemic hemodynamics and plasma ET-1 and big ET-1 concentrations were measured using electrical bioimpedance and radioimmunoassay, respectively. Patients and controls had similar baseline FBF, systemic hemodynamics, and plasma ET-1 and big ET-1 concentrations. In both groups, ET-1 and BQ-788 caused significant vasoconstriction (P < .001) and BQ-123 caused significant vasodilatation (P < .001). Compared with controls, cirrhotic patients had attenuated ET-1 responses (P < .001), augmented BQ-123 responses (P < .001), and similar BQ-788 responses (P = .62). Despite normal systemic hemodynamics and plasma ET-1 concentrations, forearm vascular responses to exogenous ET-1 are reduced in cirrhotic patients. The augmented vasodilatation to BQ-123 in cirrhotic patients is consistent with a compensated vasodilated state, and a greater contribution of ET-1 to the maintenance of basal vascular tone acting through the ET(A) receptor.     

Endothelin receptor blockade lowers plasma aldosterone levels via different mechanisms in primary aldosteronism and high-to-normal renin hypertension

BACKGROUND: Endothelin (ET)-1 contributes to raising blood pressure (BP) and inducing cardiovascular disease by vasoconstriction and potent stimulation of aldosterone secretion. In the rat this latter effect occurs via ET(B) receptors; in humans in vitro studies implicated both ET(A) and ET(B) receptors, but there is no conclusive evidence in vivo. METHODS: We recruited 13 consenting hypertensive patients: six with primary aldosteronism (PA) and seven with high-to-normal renin hypertension (HNRH). They were infused with a low dose (200 nmol/min for 5 min followed by 100 nmol/min for 10 min) of the ET(A)-selective antagonist BQ-123 either alone or, on a different day, together with an identical dose of the ET(B)-selective antagonist BQ-788. Plasma aldosterone, cortisol and ACTH concentration and plasma renin activity (PRA) were measured with radioimmunoassay at -15, 0, 30, 60, 120, 240, 360 min, while BP was recorded non-invasively. RESULTS: BQ-123 alone and combined with BQ-788 significantly lowered mean BP in both PA and HNRH patients (by 6-10 mmHg at nadir; P<0.01). In PA patients, a short-lived decrease of aldosterone was elicited by combined BQ-123 and BQ-788 (-14%; P<0.05), but not by BQ-123 alone; cortisol, ACTH, and PRA were unaffected by either treatment. In HNRH patients, BQ-123 both alone and combined with BQ-788 lowered aldosterone (-39 and -28%, respectively) and PRA (-43 and -16%, respectively), while cortisol and ACTH were unaffected. CONCLUSIONS: Endogenous ET-1 contributes to maintaining the high BP values and the aldosterone secretion in both PA and HNRH patients. In the former patients, the aldosterone secretagogue effect of ET-1 is mediated via ET(B) receptors, while in the latter it occurs mainly via ET(A)-mediated stimulation of renin production.     

Myocardial effects of ETB receptor stimulation.

INTRODUCTION: Endothelin-1 (ET-1) is an endogenous peptide whose effects are mediated by two distinct types of receptors, ETA and ETB. Whereas the ETA receptors promote vasoconstriction and mitogenesis and increase inotropism, ETB receptors have vasodilatory and anti-mitogenic properties mediated by nitric oxide release. In this study we investigated the myocardial effects of selective ETB receptor stimulation. METHODS: The study was performed on right papillary muscles (n = 30) from New Zealand white rabbits (Krebs-Ringer; 1.8 mM CaCl2; 35 degrees C). The effects of selective ETB receptor activation by Sarafotoxin S6c (0.2 microM; n = 6) and of non-selective ETA and ETB receptor activation by ET-1 (1 nM; n = 9) were studied. The effects of ET-1 were also evaluated in the presence of a selective ETA receptor antagonist, BQ-123 (0.1 microM; n = 9) and of a selective ETB receptor antagonist BQ-788 (0.1 microM; n = 6). Only significant results (mean +/- SE; p < 0.05) are given, expressed as % baseline. RESULTS: Sarafotoxin S6c reduced active tension (AT) by 8.1 +/- 5.5% and peak rate of tension development (dT/dtmax) by 8.6 +/- 5.6%. Alone, ET-1 increased AT by 64.2 +/- 18.2% and dT/dtmax by 58.6 +/- 20.2%. These effects of ET-1 were exacerbated in the presence of BQ-788 (AT increased by 82.6 +/- 17.5% and dT/dtmax by 121.3 +/- 26.6%) and inverted in the presence of BQ-123 (AT decreased by 12.8 +/- 2.7% and dT/dtmax by 16.1 +/- 3.0%). CONCLUSIONS: This study demonstrates, for the first time, that selective stimulation of ETB receptors has a negative inotropic effect. Detailed characterization of the effects of stimulation of each type of ET-1 receptor is of particular relevance as selective and non-selective inhibitors of these receptors are currently being tested for treatment of heart failure.     

Importance of local production of endothelin-1 and of the ET(B)Receptor in the regulation of pulmonary vascular tone

Interaction between locally released endothelin-1 (ET-1) and the endothelial ET(B)receptor could modulate pulmonary vascular tone. We evaluated pulmonary ET-1 clearance and ET-1-ET(B)receptor interaction in the modulation of pulmonary vascular tone. Controls and rats with Monocrotaline (MCT)-induced pulmonary hypertension (PH) were studied. Lungs were isolated and perfused under constant pressure. The effect of the selective ET(B)antagonist BQ-788 (10(-12)-10(-8)mole) on perfusion flow rate and(125)I-ET-1 extraction was determined. Baseline(125)I-ET-1 extraction was reduced from 62+/-5% in controls to 49+/-10% in PH (P=0.012). BQ-788 inhibited extraction with a higher half-inhibitory dose in the MCT group (-Log ID(50)= 8.9+/-0.4 vs. 9.5+/-0.1, P=0.03). BQ-788 induced a mild reduction in perfusion flow rate of 0.7+/-0.3 ml/min in controls. In the MCT group, this occurred at a lower dose and was more pronounced with a maximal reduction of 3.3+/-0.7 ml/min (P<0.01 vs. control). ET-1 was undetectable in the effluent at baseline but was present in similar concentrations in both groups after ET(B)blockade. Addition of 2 pg/ml ET-1 to lung perfusate did not modify pulmonary ET-1 clearance or the effect of BQ788 on perfusion flow rate in control lungs. In normal rat lungs, the ET(B)receptor plays a minor regulatory role on vascular tone. In MCT hypertension however, despite a reduction in ET(B)mediated extraction, luminal production of ET-1 attenuates the increase in pulmonary vascular tone.     

Endothelin-a receptor in rat skeletal muscle microvasculature

Although the effect of endothelin-1 (ET-1) on vascular tone has been studied extensively at the arterial/arteriolar level, little is known about the direct effect of ET-1 at the level of the capillary. Using intravital microscopy, we determined capillary red blood cell velocity and arteriolar diameter responses to ET-1, ET(A)-receptor blocker BQ-123, and ET(B)-receptor blocker BQ-788 applied locally on capillaries in rat extensor digitorum longus (EDL) muscle. Using immunohistochemistry, we examined capillaries in this muscle and microvascular endothelial cells isolated from this muscle for immunoreactivity with ET(A)-receptor antibody. ET-1 (10(-9) to 10(-5) M in micropipette) caused quick reductions (i.e., within several seconds), whereas BQ-123 (10(-8) to 10(-4) M) and BQ-788 (10(-6) and 10(-4) M) caused quick increases, in both velocity and diameter. Capillaries and endothelial cells showed ET(A)-receptor immunoreactivity. We conclude that the microvasculature of the rat EDL muscle is sensitive to ET-1 and its receptor blockers and that the ET(A) receptor may be present in the capillary wall of this muscle, including the endothelium.     

Mechanosensitive nonselective cation channel facilitation by endothelin-1 is regulated by protein kinase C in arterial myocytes

OBJECTIVE: The mechanosensitive nonselective cation channel (NSC(MS)) and endothelin-1 (ET-1) play critical roles in the regulation of vascular tone. This study was undertaken to investigate the effect of ET-1 on NSC(MS) and on the myogenic response of arteries. METHODS: Cell-attached patch-clamp techniques were applied to rabbit pulmonary and cerebral arterial smooth muscle cells using a 140 mM CsCl pipette and bath solutions (Ca(2+)-free, 1 mM EGTA). Myogenic responses were determined by video analysis of pressurized arteries. RESULTS: The application of negative pressures through the pipette activated NSC(MS), and this was augmented by bath application of ET-1 (1 pM-30 nM). ET-1 lowered the lowest pressure required for NSC(MS) activation. NSC(MS) facilitation by ET-1 was prevented by BQ-123 (1 microM, an ET(A) antagonist) but not by BQ-788 (1 microM, an ET(B) antagonist). Phorbol 12-myristate 13-acetate (PMA, 100 nM), a protein kinase C activator, also increased the activity of NSC(MS). ET-1- or PMA-induced facilitation of NSC(MS) was abolished by GF109203X (10 microM), a protein kinase C inhibitor. Video analysis of pressurized cerebral artery showed inhibition of the myogenic response by the NSC(MS) channel blockers GsMTx-4 (5 microM) and DIDS (3-100 microM). Treatment with ET-1 (10 pM) augmented the myogenic response and this was inhibited by DIDS (30 microM). CONCLUSION: Stimulation of ET-1 receptor (ET(A)) facilitates NSC(MS) via a protein kinase C-dependent signaling pathway in rabbit arterial myocytes. Our findings suggest that NSC(MS) play a role in the myogenic response and its augmentation by ET-1.     

Endogenous endothelin in human coronary vascular function: differential contribution of endothelin receptor types A and B

Endothelin 1 mediates coronary vasoconstriction and endothelial dysfunction via endothelin receptor type A (ET(A)) activation. However, the effects of selective endothelin receptor type B (ET(B)) and combined ET(A+B) receptor blockade on coronary vasomotion are unknown. We measured coronary vascular tone and endothelium-dependent and -independent vasomotor function before and after selective infusion of BQ-788 (an ET(B) receptor antagonist) or combined infusion of BQ-788+BQ-123 (an ET(A) antagonist) into unobstructed coronary arteries of 39 patients with coronary atherosclerosis or risk factors undergoing cardiac catheterization. BQ-788 did not affect epicardial diameter but constricted the microcirculation (P<0.0001), increased coronary sinus endothelin, and reduced nitrogen oxide levels. In contrast, BQ-123+BQ-788 dilated epicardial (P<0.0001) and resistance (P=0.022) arteries. Responses to acetylcholine and sodium nitroprusside were unaffected by BQ-788 alone. Epicardial endothelial dysfunction improved after BQ-123+BQ-788 (P=0.007). Coronary microvascular responses to acetylcholine and sodium nitroprusside were unaffected by BQ-123+BQ-788. We conclude that selective ET(B) receptor antagonism causes coronary microvascular constriction, without affecting epicardial tone or endothelial function, via reduced endothelin clearance and NO availability. Combined ET(A+B) blockade dilates coronary conduit and resistance vessels and improves endothelial dysfunction of the epicardial coronary arteries. Thus, endogenous endothelin, predominantly via ET(A) receptor stimulation, contributes to basal constrictor tone and endothelial dysfunction, whereas ET(B) activation mediates vasodilation in human coronaries. Our data suggest that selective ET(A) blockade may have greater therapeutic potential than nonselective agents, particularly for treatment of endothelial dysfunction in atherosclerosis.     

Modulation of the myocardial effects of selective ETB receptor stimulation and its implications for heart failure.

INTRODUCTION: Endothelin-1 (ET-1) acts on two types of receptors, ET(A) and ET(B). Recent functional studies suggest the existence of two ET(B) receptor subtypes in the heart: ET(B1), located on endocardial endothelial cells and responsible for negative inotropism, and ET(B2), located on myocardial cells and responsible for positive inotropism. The aim of the present study was to investigate the mechanisms underlying the myocardial effects of selective ET(B) receptor stimulation. METHODS: The study was performed on right papillary muscles from New Zealand white rabbits (n = 39; Krebs-Ringer; 1.8 mM CaCl2; 35 degrees C). The effects of sarafotoxin S6c (SRTXc, ET(B) agonist; 0.2 microM) were evaluated in muscles with: (i) intact endocardial endothelium (EE) (n = 6); (ii) damaged EE (Triton X100; 0.5%; n = 6); (iii) intact EE, in the presence of N(G)-nitro-L-arginine (L-NNA, nitric oxide synthase inhibitor; n = 6); (iv) intact EE, in the presence of indomethacin (INDO, cyclooxygenase inhibitor; n = 6); (v) intact EE, in the presence of BQ-123 (ET(A) antagonist; n = 7); and (vi) damaged EE, in the presence of BQ-123 (n = 8). Only significant results (mean +/- SEM, p < 0.05) are given, expressed as % change from baseline. RESULTS: In muscles with intact EE, SRTXc alone induced negative inotropic and lusitropic effects, decreasing active tension (AT) by 11.0 +/- 5.6%, maximum velocity of tension rise (dT/dt(max)) by 11.2 +/- 5.9% and maximum velocity of tension decline (dT/dt(min)) by 11.5 +/- 6.2%. However, after removal of EE, or in the presence of L-NNA or INDO, SRTXc increased AT by 35.2 +/- 11.7%, 22.8 +/- 2.9% and 15.2 +/- 3.4%, dT/dt(max) by 29.5 +/- 7.9%, 20.1 +/- 2.1% and 13.3 +/- 5.0%, and dT/dt(min) by 28.2 +/- 8.1%, 21.2 +/- 3.8% and 12.3 +/- 2.2%, respectively. In muscles with intact EE and in the presence of BQ-123, the negative inotropic and lusitropic effects of SRTXc were enhanced: AT decreased by 27.0 +/- 7.4%, dT/dt(max) by 13.3 +/- 4.9% and dT/dt(min) by 31.1 +/- 7.9%. On the other hand, the positive inotropic and lusitropic effects of SRTXc in the absence of intact EE were reversed in the presence of ET(A) blockade: AT decreased by 9.0 +/- 1.8%, dT/dt(max) by 4.1 +/- 3.5% and dT/dt(min) by 8.1 +/- 3.6%. CONCLUSIONS: The present study shows that the inotropic and lusitropic effects mediated by ET(B) receptors are modulated by endocardial endothelium and by ET(A) receptor activity. These results may have pathophysiological and therapeutic implications in heart failure, a condition in which ET-1 levels are increased and endothelial dysfunction may be present.     

Regulation of guinea pig intestinal peristalsis by endogenous endothelin acting at ET(B) receptors

BACKGROUND & AIMS: Endothelins are expressed in many enteric neurons of the gut. Because activation of endothelin ET(A) and ET(B) receptors is known to alter intestinal muscle activity, the effect of ET(A) and ET(B) receptor agonists and antagonists on propulsive peristalsis was examined. METHODS: Repetitive peristalsis in fluid-perfused segments of the guinea pig isolated small intestine was elicited by a rise of the intraluminal pressure and recorded via the pressure changes generated by the peristaltic waves. RESULTS: Endothelin 1 (0.3-10 nmol/L added to the organ bath) stimulated peristalsis as shown by a decrease in the pressure threshold at which peristaltic waves were triggered, whereas the endothelin analog sarafotoxin 6c (0.3-10 nmol/L) inhibited peristalsis as reflected by an increase in the pressure threshold. The ET(A) receptor antagonist BQ-123 (3 micromol/L) converted the properistaltic action of endothelin 1 to an antiperistaltic action, whereas the ET(B) receptor antagonist BQ-788 (3 micromol/L) prevented the antiperistaltic action of sarafotoxin 6c. BQ-788, but not BQ-123, facilitated peristalsis on its own. Additional experiments indicated that the properistaltic action of endothelin 1 is mediated by enteric neurons, whereas the peristaltic motor effects of sarafotoxin 6c and BQ-788 are caused by a direct action on the muscle. CONCLUSIONS: ET(A) receptor activation stimulates, whereas ET(B) receptor activation inhibits, intestinal peristalsis. The ability of BQ-788 to facilitate peristalsis per se points to a physiologic role of ET(B) receptors in peristaltic motor regulation.     

Exaggerated coronary vasoreactivity to endothelin-1 in aged rats: role of protein kinase C

OBJECTIVE: The interaction between advanced age and increased susceptibility to ischemic insult is well documented. Age-related increases in coronary vascular resistance, in part due to impaired dilator responses, have been reported. Our aim was to determine the role of endothelin-1 (ET-1) on enhanced constrictor responses in aged coronary arteries (CAs) and whether protein kinase C (PKC) signaling mechanisms impact ET-1 responses. METHODS: Vasoreactivity was assessed in CAs isolated from aged (24 months; n=16) and adult (4 months; n=21) male F344 rats following ET-1 (10(-10)-10(-8)) with and without specific ETA/ETB receptor antagonists (BQ-123, 1 microM; BQ-788, 30 nM) or the PKC inhibitor bisindolylmaleimide (Bis; 10(-6) M). Constrictor responses to KCl (80 mM) were also measured and voltage-gated Ca2+ channel (VGCC) determined in isolated coronary smooth muscle cells. Dilator responses to acetylcholine (ACH) and sodium nitroprusside (SNP) were assessed. RESULTS: Passive diameter was greater (357+/-19 vs. 309+/-9; p<0.02) while spontaneous tone was similar in 24 months vs. 4 months. ET-1 resulted in greater constriction in 24 months vs. 4 months (79% vs. 67%; p<0.01). Group differences persisted following selective ETB inhibition with BQ-788 (p<0.02), while BQ-123 abolished contractile responses to ET-1. Importantly, inhibition of ET-1 constriction by Bis occurred in 24 months but not 4 months (p<0.01). Constrictor responses to KCl and peak VGCC current density were similar in 24 months vs. 4 months (48% vs. 50%). No age-related differences were observed in ACH- or SNP-mediated dilation. Western blotting revealed increases in Ca2+-sensitive PKCalpha, -betaI, and -betaII levels with age, while eNOS and ETA receptor protein levels were unchanged. CONCLUSION: Aberrant ETA constrictor responses and directional changes in PKC are likely to contribute to coronary vascular pathology with advanced age.     

Endothelin inhibits histamine-induced cyclic AMP accumulation in bovine brain vessels

We have studied whether endothelin isopeptides have any effects on histamine-induced cyclic AMP in [(3)H]adenine-prelabeled brain vessels isolated from bovine brain. Basal levels of [(3)H]cyclic AMP were enhanced by histamine in a concentration-dependent manner (EC(50) = 1.1 +/- 0.3 microM). Endothelin-1 inhibited histamine-elicited [(3)H]cyclic AMP generation with an IC(50) value of 3 +/- 2.5 nM. Sarafotoxin 6c, an ET-B receptor agonist, had no effect. ET-1 inhibition of histamine-induced [(3)H]cyclic AMP was reversed by the ET-A receptor antagonist BQ-123 while the ET-B receptor antagonist BQ-788 had no effect. The levels of [(3)H]cyclic AMP induced by isoprenaline were not altered by endothelin-1. Taken together, these results show that endothelins modulate the actions of histamine on the blood-brain barrier, probably by type A endothelin receptors.     

Endothelin-1[1-31]: a novel autocrine-paracrine regulator of human adrenal cortex secretion and growth.

Endothelin (ET)-1[1-21] stimulates steroid secretion and zona glomerulosa growth and is expressed in the human and rat adrenal cortex together with its receptor subtypes A and B (ETA and ETB). Although ET-1[1-21] is generated from bigET-1 by an ET-converting enzyme (ECE-1), there is evidence of an alternative chymase-mediated biosynthetic pathway leading to the production of an ET-1[1-31] peptide, the role of which in adrenal pathophysiology is largely unknown. Gene expression and immunohistochemical studies allowed localization of chymase in the normal human adrenal cortex. Sizable amounts, not only of ET-1[1-21] but also of ET-1[1-31], were found in the adrenal vein plasma of three patients. ET-1[1-21] and ET-1[1-31] elicited a clear-cut secretory response by dispersed human adrenocortical cells, ET-1[1-31] being significantly less potent than ET-1[1-21]. The secretagogue effect of ET-1[1-31] was abolished by the ETA receptor antagonist BQ-123 and was unaffected by the ETB receptor antagonist BQ-788. Because, in humans, the secretagogue effect of ET-1[1-21] involves both ETA and ETB receptors, the weaker action of ET-1[1-31] could be attributable to a selective ETA receptor activation. Two lines of evidence support this contention: 1) ET-1[1-31] was more effective than ET-1[1-21] in stimulating ETA-mediated cell proliferation of human adrenocortical cells cultured in vitro; and 2) autoradiography showed that a) ET-1[1-31] displaced in vitro [(125)I]ET-1[1-21] binding to the ETA, but not ETB receptors, in human internal thoracic artery rings; and b) BQ-123, but not BQ-788, eliminated [(125)I]ET-1[1-31] binding in the rat adrenal cortex.