The role of nitric oxide in the regional vasodilator effects of endothelin-1 in the rat

SR141716A-sensitive enhancement of ET-1 hypotensive effect by chronic NOS inhibition

The present study evaluated the potential mechanism involved in the hypotensive effect induced by ET-1 in rats treated with the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) in the drinking water during 7 days. Hypertension developed in the L-NAME-treated rats (164+/-3 versus 112+/-1 mm Hg in untreated control rats), and the hypotensive effect of ET-1 (100 pmol/kg IV) was significantly enhanced compared with control rats (32+/-2% versus 20+/-1% fall in mean arterial pressure). The enhanced ET-1 hypotensive effect in L-NAME-treated rats was abolished by the ETB receptor antagonist BQ-788 but was unaltered by the cyclooxygenase inhibitor diclofenac, the cytochrome P450 inhibitor fluconazole, or the potassium channel blockers apamin, glibenclamide, tetraethylammonium, and 4-aminopyridine. Pretreatment with the cannabinoid CB1 receptor antagonist SR141716A significantly reduced the hypotensive response to ET-1 in L-NAME-treated rats (20+/-1%), although it did not modify the response in untreated control rats (17+/-1%). These findings indicate that in rats under chronic NOS inhibition, the hypotensive effect of ET-1 is unexpectedly enhanced and appears to be mediated by a non-NO/non-prostanoid mechanism and involves an SR141716A-sensitive mechanism triggered by ETB receptor activation.

Upregulation of endothelin B receptors in kidneys of DOCA-salt hypertensive rats

Experiments were designed to elucidate the role of endothelin B receptors (ET(B)) on arterial pressure and renal function in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Male Sprague-Dawley rats underwent uninephrectomy and were treated with either DOCA and salt (0.9% NaCl to drink) or placebo. DOCA-salt rats given the ET(B)-selective antagonist, A-192621, for 1 wk (10 mg. kg(-1). day(-1) in the food) had significantly greater systolic arterial pressure compared with untreated DOCA-salt rats (208 +/- 7 vs. 182 +/- 4 mmHg) whereas pressure in placebo rats was unchanged. In DOCA-salt, but not placebo rats, A-192621 significantly decreased sodium and water excretion along with parallel decreases in food and water intake. To determine whether the response in DOCA-salt rats was due to increased expression of ET(B) receptors, endothelin receptor binding was performed by using membranes from renal medulla. Maximum binding (B(max)) of [(125)I]ET-1, [(125)I]ET-3, and [(125)I]IRL-1620 increased from 227 +/- 42, 146 +/- 28, and 21 +/- 1 fmol/mg protein, respectively, in placebo rats to 335 +/- 27, 300 +/- 38, and 61 +/- 6 fmol/mg protein, respectively, in DOCA-salt hypertensive rats. The fraction of receptors that are the ET(B) subtype was significantly increased in DOCA-salt (0.88 +/- 0.07) compared with placebo (0.64 +/- 0.01). The difference between [(125)I]ET-3 and [(125)I]IRL-1620 binding is consistent with possible ET(B) receptor subtypes in the kidney. These results indicate that ET(B) receptors in the renal medulla are up-regulated in the DOCA-salt hypertensive rat and may serve to maintain a lower arterial pressure by promoting salt and water excretion.

Cardiovascular responses to angiotensins I and II in normotensive and hypertensive rats; effects of NO synthase inhibition or ET receptor antagonism

1. We compared the cardiovascular responses to angiotensins (I and II), and any possible modulatory influences thereupon of nitric oxide (NO) or endothelin (ET) in conscious male, normotensive, Hannover Sprague-Dawley (SD) rats, and hypertensive, heterozygous ((mRen-2)27), transgenic (TG) rats. 2. The pressor effects of angiotensin I or of angiotensin II were not consistently different in SD and TG rats. The accompanying absolute reductions in renal and mesenteric vascular conductances were smaller in TG rats, but probably due to the baseline vasoconstriction in those animals. 3. Inhibition of NO synthase with L-NAME had no significant effects on the pressor responses to angiotensin I or angiotensin II in either SD or TG rats. L-NAME reduced the absolute, but not percentage, reductions in renal and mesenteric vascular conductances in response to angiotensin I and angiotensin II. L-NAME abolished the hindquarters vasodilator effects of angiotensin I and angiotensin II in both strains of rat. 4. ET receptor antagonism (with SB209670) had no significant influence on the pressor or renal or mesenteric vasoconstrictor effects of angiotensin II in SD rats. In TG rats, the pressor responses to angiotensin II were unaffected by SB209670; the accompanying falls in renal and mesenteric vascular conductances were enhanced in absolute, but not in percentage terms. 5. These results provide no evidence for a buffering action of NO, or a modulatory influence of ET, on the pressor or vasoconstrictor effects of angiotensin I and/or angiotensin II in SD rats. Furthermore, there is no evidence for an altered sensitivity to angiotensin I or angiotensin II, and no evidence for a differential modulatory influence of either NO or ET in TG, compared to SD, rats.

Dilatation by angiotensin II of the rat femoral arterial bed in vivo via pressure/flow-induced release of nitric oxide and prostaglandins

1. The haemodynamic effects of angiotensin II (AII) and, for comparison, arginine vasopressin (AVP) in the femoral and superior mesenteric artery of urethane-anaesthetized rats were analysed with the ultrasonic transit time shift technique. 2. I.v. bolus injection of AII (0.1-3 nmol kg-1) and AVP (0.03-1 nmol kg-1) increased blood pressure which was accompanied by a decrease in blood flow through the superior mesenteric artery and an increase in femoral blood flow. The femoral hyperaemia was in part due to vasodilatation as indicated by a rise of femoral vascular conductance up to 200% relative to baseline. The femoral vasodilatation caused by AVP, but not AII, was followed by vasoconstriction. 3. Blockade of angiotensin AT1 receptors by telmisartan (0.2-20 mumol kg-1) prevented all haemodynamic responses to AII. 4. The femoral dilator responses to AII and AVP depended on the increase in vascular perfusion pressure since vasodilatation was reversed to vasoconstriction when blood pressure was maintained constant by means of a gravity reservoir. However, the AII-evoked femoral vasodilatation was not due to an autonomic or neuroendocrine reflex because it was not depressed by hexamethonium (75 mumol kg-1), prazosin (0.25 mumol kg-1) or propranolol (3 mumol kg-1). 5. The AII-induced femoral vasodilatation was suppressed by blockade of nitric oxide (NO) synthesis with NG-nitro-L-arginine methyl ester (L-NAME, 40 mumol kg-1) and reversed to vasoconstriction when L-NAME was combined with indomethacin (30 mumol kg-1), but was left unaltered by antagonism of endothelin ETA/B receptors with bosentan (37 mumol kg-1). 6. These results demonstrate that the effect of AII to increase systemic blood pressure and the resulting rise of perfusion pressure in the femoral artery stimulates the formation of NO and prostaglandins and thereby dilates the femoral arterial bed. This local vasodilator mechanism is sufficient to mask the direct vasoconstrictor response to AII.

Differential release of prostacyclin and nitric oxide evoked from pulmonary and systemic vascular beds of the pig by endothelin-1

The vascular effects of endothelin-1 (ET-1) and the release of prostacyclin and nitric oxide (NO) evoked by this peptide were analyzed in anesthetized, mechanically ventilated pigs. ET-1 induced biphasic responses in both the pulmonary and systemic vascular beds characterized by a transient hypotension followed by a long-lasting hypertension. To evaluate the involvement of prostacyclin and NO in the ET-1-dependent vascular response, we used indomethacin to block cyclooxygenase and NG-nitro-L-arginine methyl ester (L-NAME) to block NO synthase. The results show that the systemic hypotensive response to ET-1 is mediated by the release of prostanoids and NO, but these are not responsible for the pulmonary hypotension. Indomethacin reduced the hypertensive effect of ET-1, showing that this peptide can also activate release of vasoconstrictor cyclooxygenase metabolites. When L-NAME was administered after indomethacin, the pulmonary vasoconstrictor activity of ET-1 was counterbalanced by NO. By contrast, in pigs pretreated with indomethacin plus L-NAME ET-1 caused transient systemic vasoconstriction, followed by progressive reduction of vascular tone, probably because of release of vasodilator agents other than prostanoids or NO.

Endothelin-1 produces heterogeneous regional haemodynamic effects in conscious rabbits

Blood flow in the renal artery, superior mesenteric artery and infra-renal abdominal aorta of conscious rabbits was measured by Doppler ultrasound. Arterial pressure, heart rate and blood flow responses were assessed following 0.2 and 0.8 nmol/kg intravenous endothelin-1. The effects of the following antagonists on these responses were examined: phentolamine, propranolol, scopolamine, captopril, nifedipine, indomethacin, the V1-vasopressin receptor antagonist d(CH2)5Tyr(Me)AVP and the competitive nitric oxide (NO) synthase inhibitor NG-nitro L-arginine (NOLA). Hindlimb resistance and arterial pressure responded in two phases, initial vasodilatation followed by vasoconstriction. Renal and mesenteric vasoconstriction occurred without initial vasodilatation. Following 0.2 nmol/kg endothelin-1, arterial pressure decreased by 18.5 +/- 0.8 mmHg, then increased by 25.2 +/- 1.7 mmHg (n = 27). Heart rate changed reciprocally. Renal resistance increased by 533 +/- 73% (n = 12). Mesenteric resistance increased by 420 +/- 34%. Hindlimb resistance decreased 54 +/- 2% (n = 12, all P < 0.01) then increased slightly (P < 0.05). All changes were greater at 0.8 nmol/kg, particularly the hindlimb vasoconstriction. The only antagonist to alter significantly these responses was NOLA, which in the hindlimb attenuated the vasodilatation and accentuated the vasoconstriction. We conclude that most of the haemodynamic effects of endothelin-1 are direct, but that NO generated by NO synthase causes part of the hindlimb vasodilatation, and that endothelin-1-induced vasoconstriction is attenuated by release of NO.

The endothelin system and its potential as a therapeutic target in cardiovascular disease

Endothelin (ET)-1, an endothelium-derived peptide, is the most potent vasoconstrictor agent described to date. ET-1 also has positive inotropic and chronotropic effects in the heart and is a co-mitogen in both cardiac and vascular myocytes. The major elements of the system involved in formation of ET-1 and its isopeptides, as well as the receptors mediating their effects, have been cloned and characterised. Antagonists of the ET receptors are now available, and selective inhibitors of the ET-converting enzymes are being developed. Early studies using receptor antagonists support the involvement of ET-1 in the pathophysiology of several cardiovascular diseases. The relative merits of ET-converting enzyme inhibitors and receptor antagonists for the treatment of cardiovascular disease are discussed.

The role of nitric oxide and other endothelium-derived vasoactive substances in vascular disease

Alteration in the release and action of endothelium-derived vasoactive factors is responsible for changes in vascular reactivity early in the course of vascular disease. These factors include nitric oxide, eicosanoids, endothelium-derived hyperpolarizing factor, endothelin, and angiotensin II. Because endothelial dysfunction occurs at early stages of disease, it may reflect physiological changes that, if allowed to become chronic, are responsible for changes in vascular structure and growth and adhesivity to platelets and leukocytes, ultimately leading to atherosclerosis and thrombosis. Each of the major risk factors predisposing to vascular disease are associated with endothelial cell dysfunction, suggesting a direct etiologic link between the effects of the risk factors on the endothelium and their propensity to accelerate vascular disease. Restoration or replacement of endothelium-derived factors such as nitric oxide and prostacyclin, which impede the progression of vascular disease, or preventing the action of mediators such as vasoconstrictor eicosanoids, angiotensin II, or endothelin, which accelerate the progression of vascular disease, has become a useful paradigm in the treatment and prevention of vascular disease. Thus, understanding the physiology of endothelium-derived vasoactive factors is a necessary part of every physician's education.

Evidence for ETA and ETB receptors in rat skin and an investigation of their function in the cutaneous microvasculature

1. The relative contribution of ETA and ETB receptors in the response of rat skin to endothelins was investigated by use of the selective ETB agonist IRL-1620 and the selective ETA antagonist BQ-123. 2. Binding data suggest the presence of ETA and ETB receptors as preincubation with [Ala3,11,18Nle7]-endothelin-1 reduced ET-1 binding by approximately 40%. 3. Intradermal injection of endothelin-1 (ET-1, 1-10 pmol/site) and ET-3 (3-100 pmol/site) induced a dose-dependent decrease in local blood flow assessed by 133Xe clearance at test sites in rat skin. 4. The endothelin analogue [Ala3,11,18Nle7]-ET-1 (30-1000 pmol/site) induced significant vasoconstriction (P < 0.05) at the highest doses used and the selective ETB receptor agonist, IRL-1620 [Suc[Glu9,Ala11,15] endothelin (8-21)], (0.01-100 pmol/site) acted in a potent manner to induce a significant (P < 0.01) dose-dependent decrease in 133Xe clearance. 5. Co-injection with the selective ETA receptor antagonist, BQ-123 (1 nmol/site), completely abolished the vasoconstriction to ET-1 and partially to ET-3, but had no effect on IRL-1620-induced vasoconstriction. In addition, IRL-1620 responses were not altered at sites treated with submaximal doses of a nitric oxide synthase inhibitor or a prostaglandin synthase inhibitor. 6. ET-1 and IRL-1620 (100 fmol-1 pmol/site) did not induce oedema formation as measured by [125I]-albumin accumulation in the presence or absence of the vasodilator, calcitonin gene-related peptide (CGRP). ET-1 (1-3 pmol/site) inhibited substance P-induced oedema formation and this effect,suggested to be secondary to a vasoconstrictor effect, was significantly reversed by BQ-123 (1 nmol/site).7. The findings in this study indicate that there are ETA and ETB receptors in rat skin and agents which activate either receptor act to mediate a decrease in cutaneous blood flow, but have no effect on increased microvascular permeability.

Endothelin receptor antagonism

Following the original report by Yanagisawa et al. (1988) more than 7 years ago, compelling evidence that ET plays an important role in the local regulation of smooth muscle tone and cell growth has been reported. In addition, many studies point to a significant role for endothelin in nonvascular function. The investigation of the endothelin system has been greatly advanced in the last 2 to 3 years through significant advances in the development of potent and selective ET receptor antagonists. These agents have proven to be essential tools for elucidating the biological significance of the ET system, leading to the realization that antagonism of the ET system may have significant therapeutic potential. As emphasized in this review, the importance of chronic blockade of the ET system may be a critical aspect of future research in this exciting area. Confounding issues remain the lack of information about the role of the ETB receptor, the apparent pharmacological evidence for additional ET receptor subtypes, and species variation in the tissue distribution of ET isoforms and receptor subtypes. Along with the greater ability to understand the endothelin system provided by potent and selective pharmacological agents, is the important contribution of modern molecular biology techniques, highlighted by the insights gained from recent reports of results from ET gene disruption studies. Kurihara et al. (1994) found that ET-1-deficient homozygous mice die at birth of apparent respiratory failure secondary to severe craniofacial abnormalities. Subsequently, Yanagisawa's laboratory has presented and published a series of complementary gene disruption studies. First, Hosoda et al. (1994) demonstrated remarkably, that ETA receptor knockout mice bear morphological abnormalities nearly identical to ET-1 knockout mice. Second, they found that disruption of the ET-3 peptide and ETB receptor genes result in homozygous mice that share identical phenotypic traits (i.e., coloration changes and aganglionic megacolon) which are similar to a previously known natural mutation, the Piebald-Lethal mouse (Hosoda et al., 1994; Baynash et al., 1994). This phenotype has a human corollary known as Hirschsprung's Disease and it is now known that the disease, though multigenic, results from a missense mutation of the ETB receptor gene in some individuals (Puffenberger et al., 1994). Taken together these data indicate that the endothelin system is essential to correct embryonic neural crest development, a completely novel finding within the superfamily of guanine-protein-linked receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Coronary vasoconstriction mediated by endothelin-1 in neonates. Reversal by nitroglycerin

To determine the role of the vasoconstrictor peptide endothelin-1 in cardiopulmonary bypass in neonates, we measured plasma endothelin-1 concentrations in infants before and after cardiopulmonary bypass for arterial switch procedures and studied the effects of endothelin-1 on coronary tone and contractility in normal and reperfused neonatal pig hearts. Endothelin-1 blood concentrations (picograms per milliliter, mean +/- standard error) were significantly higher in neonates with arterial transposition and in umbilical venous blood (22.9 +/- 2.3 and 19.2 +/- 2.9, respectively) than in older children with atrial septal defects (13.2 +/- 1.6) or in healthy adults (10.7 +/- 2.5). After cardiopulmonary bypass, endothelin-1 concentrations increased 29% in neonates undergoing arterial switch procedure and 28% in children undergoing atrial septal defect repair (p < 0.05 versus before bypass). In isolated, blood-perfused neonatal pig hearts, endothelin-1 had dose-related coronary constrictor and inotropic effects between 25 and 100 pmol. Endothelin-1 concentrations that did not increase coronary perfusion pressure (5 to 10 pmol) caused significant coronary constriction in the presence of norepinephrine (10 nmol/L). During reperfusion after 30 minutes of global normothermic ischemia, the coronary vasoconstrictor effects of both endothelin-1 alone and endothelin-1 plus norepinephrine were significantly enhanced. Nitroglycerin reversed vasoconstriction produced by endothelin-1 and endothelin-1 plus norepinephrine both before and after ischemia-reperfusion. We conclude that endothelin-1 concentrations are significantly elevated in neonates and are further increased after cardiopulmonary bypass. Coronary vasoconstriction caused by endothelin-1 is enhanced by ischemia-reperfusion and by norepinephrine present in concentrations typically observed after neonatal cardiopulmonary bypass. Nitroglycerin reverses coronary vasoconstriction induced by endothelin-1 and may therefore be beneficial in the postoperative management of neonates after cardiac operations.

Endothelins induce prostacyclin release in both vascular and non-vascular tissue

The effects of an i.v. administration of endothelin-1, -2 and -3 (0.25-3 nmol kg-1) or their corresponding proendothelins (1-20 nmol kg-1) on blood pressure and 6 keto-prostaglandin F1 alpha (6 keto-PGF1 alpha) release in the anaesthetized ganglion-blocked rat were evaluated. The same peptides were tested for their ability to release 6 keto-PGF1 alpha from the rat vas deferens in vitro. Endothelins and proendothelins showed a transient hypotensive effect followed by a potent, long lasting vasopressor response. Blood pressure increase induced by endothelins was found to be dose-dependently correlated with 6 keto-PGF1 alpha plasma level increases. On the other hand proendothelins produced similar pressor responses, but their effect on 6 keto-PGF1 alpha plasma levels was much less intense at equipressor doses. The effects of endothelins on arterial pressure and 6 keto-PGF1 alpha release were phosphoramidon-insensitive, while the activities of proendothelins were reduced by phosphoramidon (10 mg kg-1 i.v.). Both endothelins (5-15 nmol/l) and proendothelins (100-300 nmol/l) were able to increase to a similar extent 6 keto-PGF1 alpha levels in the rat vas deferens incubation buffer. The releasing activity of endothelins was not modified by the pretreatment with phosphoramidon (50 mumol/l). This pretreatment strongly inhibited proendothelin-1 and -2 effects, but not that of proendothelin-3.

Functional evidence that balloon angioplasty results in transient nitric oxide synthase induction

Since serious vasospastic episodes limit the efficacy of percutaneous transluminal coronary angioplasty, this study has examined the time-dependent changes in vascular reactivity that occur following rat carotid artery balloon angioplasty. Relative to vessels from sham-operated animals, angioplasty caused an immediate increase in endothelin-1 contractile potency, an observation not made with noradrenaline or KCl, implicating endothelin-1 in the pathogenesis of acute arterial vasospasm. This hyperreactivity, possibly resulting from the loss of endothelin-1-induced nitric oxide release from the endothelium, was transient and was followed by a non-specific decrease in reactivity to all three spasmogens. Since the delayed reduction in endothelin-1 contractile potency was restored by N omega-nitro-L-arginine methyl ester, this hyporeactivity appeared to result from nitric oxide synthase induction. Furthermore, since the regenerating endothelium was dysfunctional, the generation of nitric oxide was from a non-endothelial source (possibly the smooth muscle or infiltrating macrophages). This response may function to ameliorate the spasmogenic and proliferative actions of chronically acting vasoactive factors and oppose platelet aggregation.

Endothelin receptors mediating functional responses in human small arteries and veins

1. In the present study, responses of human omental small arteries and veins to endothelin-1 and endothelin-3 were characterized by use of the ETB receptor selective agonist, sarafotoxin S6c, the ETA receptor antagonist, BQ123, the ETB receptor antagonist, IRL1038, the NO-synthase inhibitor NG-monomethyl-L-arginine (L-NMMA, 300 microM) and indomethacin (10 microM). 2. Small arteries (internal diameter 413 +/- 22 microns) and parallel running veins (646 +/- 35 microns) were mounted in a myograph under a normalized tension equivalent to 90% of a transmural pressure of 100 mmHg and 19 mmHg in vivo, respectively. 3. In small arteries and veins, endothelin-1 caused a concentration-dependent increase in wall tension (Emax = 3.90 +/- 0.56 mN mm-1 and 1.90 m +/- 0.32 mN mm-1 respectively, P < 0.05) and was equipotent (arteries: pD2 = 8.91 +/- 0.11; veins: pD2 = 8.63 +/- 0.08, NS). In endothelium intact arteries, L-NMMA significantly enhanced the sensitivity to endothelin-1 (pD2 control: 8.92 +/- 0.16; pD2 L-NMMA: 9.37 +/- 0.11; P < 0.05). L-NMMA did not affect the sensitivity of veins to endothelin-1. Indomethacin was without effect in arteries and veins. In veins, endothelin-3 was about a hundred times less potent than endothelin-1 and showed a biphasic response curve. Small arteries did not contract to endothelin-3. Neither small arteries nor veins contracted to sarafotoxin S6c. Furthermore, no relaxation to endothelin-1 or sarafotoxin S6c was seen in any precontracted vessels. 4. BQ123 (0.03-3 MicroM) produced a concentration-dependent rightward parallel displacement of the endothelin-l concentration-response curve in small arteries and veins yielding pA2 values of 7.09 and 7.48 respectively. The slope of the Schild plot in arteries and veins was 1.26 +/- 0.24 (NS from unity) and 0.61 +/- 0.13 (P <0.05 compared to unity) respectively. IRL1038 (3 MicroM) did not affect the potency of endothelin-1 in arteries and veins. In veins, the low sensitivity component (pD2 = 7.16 +/- 0.08) of the biphasic response curve to endothelin-3 was completely blocked by BQ123 (3 MicroM), whereas the high sensitivity component (pD2 = 8.66 +/- 0.08) was resistant to BQ123 (3 MicroM) and IRL1038 (3 MicroM).5. These results indicate that contractions of human small vessels to endothelin-l are predominantly mediated by ETA receptors and that nitric oxide modulates the response to endothelin-l in small arteries but not in veins. The different antagonistic potency of BQ123 against endothelin-l and the differential endothelin-1/endothelin-3 potency ratios in arteries and veins provide evidence for the hypothesis that ETA receptors in human small arteries are different from ETA receptors in human small veins. There is no evidence of contractions mediated by 'classical' ETB receptors in these vessels, but small veins appear to contain a functional non ETA/non ETB receptor with a high affinity for endothelin-3.

Vascular responses to endothelin-1 following inhibition of nitric oxide synthesis in the conscious rat

1. The objectives of the present experiments were to assess the role of endogenous nitric oxide (NO) in mediating and/or modulating the effects of endothelin-1 (ET-1) on blood pressure and microvascular permeability in conscious rats. 2. Intravenous administration of the NO synthesis inhibitors, NG-monomethyl-L-arginine (L-NMMA) or NG-nitro-L-arginine methyl ester (L-NAME) at a dose (25 mg kg-1 or 2 mg kg-1, respectively) which evoked maximum increase in mean arterial blood pressure (MABP) significantly attenuated (by about 40%) the vasodepressor response and potentiated (by 100-180%) the pressor response to ET-1 (1 nmol kg-1, i.v.) compared to the effects of ET-1 in animals where the peripheral vasoconstrictor effects of L-arginine analogues were mimicked by an infusion of noradrenaline (620-820 ng kg-1 min-1). Similar inhibition of the depressor and potentiation of the pressor actions of ET-1 were observed when the MABP which had been elevated by L-NMMA or L-NAME was titrated to normotensive levels with hydralazine or diazoxide before injection of ET-1. 3. L-NAME (2 mg kg-1) increased the vascular permeability of the large airways, stomach, duodenum, pancreas, liver, kidney and spleen (up to 280%) as measured by the extravasation of Evans blue dye. The permeability of pulmonary parenchyma, skeletal muscle and skin was not affected significantly by L-NAME treatment. Elevation of MABP by noradrenaline infusion did not evoke protein extravasation in the vascular beds studied with the exception of the lung. In the large airways, tissue Evans blue content was similar following noradrenaline infusion and L-NAME.4. Both the pressor and permeability effects of L-NAME (2 mg kg-1) were effectively reversed by L-arginine (300 mg kg- 1) but not by D-arginine (300 mg kg-1 ). The D-enantiomer of L-NAME, D-NAME(2 mg kg-1) had no effect on the parameters studied.5. Protein extravasation was significantly enhanced by ET-1 (1 nmol kg-1) in the upper and lower bronchi, stomach, duodenum, kidney and spleen (up to 285%). This was potentiated by L-NAME(2 mg kg-1), resulting in marked increases in tissue Evans blue accumulation (up to 550%) in these tissues. The effects of L-NAME and ET-1 were additive in the trachea, duodenum, pancreas and liver.Combined administration of L-NAME plus ET-1 significantly increased protein extravasation in the pulmonary parenchyma, where neither L-NAME nor ET-1 alone caused significant increases.6. Noradrenaline infusion (620-820 ng kg-1 min-1) potentiated the permeability action of ET-1(1 nmol kg-1) in the pulmonary circulation, whereas it did not modify ET-1-induced protein extravasation in the other vascular beds.7. These results indicate that endogenous NO mediates, in part, the vasodepressor effect and attenuates the vasopressor action of ET-1 and modulates the effects of ET-1 on vascular permeability. These findings confirm the role of NO in the maintenance of blood pressure and suggest an important role for NO in the regulation of microvascular permeability.

Mediation of endothelin-1-induced inhibition of platelet aggregation via the ETB receptor

1. The effects of FR139317 (ETA antagonist) or PD145065 (non-selective ETA/ETB antagonist) on endothelin-1 (ET-1)-induced changes in blood pressure and inhibition of ex vivo platelet aggregation were investigated in the anaesthetized rabbit. 2. ET-1 (1 nmol kg-1, i.a. bolus) caused a sustained increase in mean arterial pressure (MAP) (peak increase 47 +/- 5 mmHg, n = 8). Intravenous infusion of FR139317 at 0.2 (n = 4) or 0.6 mg kg-1 min-1 (n = 4) inhibited the ET-1 pressor response by 83 or 89%, respectively. Infusion of PD145065 at 0.2 (n = 4) or 0.6 mg kg-1 min-1 (n = 4) inhibited the ET-1-induced increase in MAP by 79 or 75%, respectively. 3. The transient depressor response (-16 +/- 3 mmHg) which preceded the rise in blood pressure induced by ET-1 (1 nmol kg-1, i.a., n = 8) was enhanced by an intravenous infusion of FR139317 (0.6 mg kg-1 min-1) to -35 +/- 5 mmHg (P < 0.05, n = 4). This enhancement was abolished by indomethacin (5 mg kg-1, i.v.) pretreatment (-17 +/- 1 mmHg, n = 4). PD145065 (0.2 mg kg-1 min-1, i.v.) attenuated the ET-1-induced fall in blood pressure to -9 +/- 1 mmHg (n = 4), while a higher dose of this antagonist (0.6 mg kg-1 min-1, i.v.) completely abolished the ET-1-mediated depressor response. 4. ET-1 (1 nmol kg-1, n = 8) inhibited ex vivo platelet aggregation by 96% at 5 min after injection of the peptide. FR139317 (0.2 or 0.6 mg kg-1 min-1, i.v.) or PD145065 (0.2mg kg-1 min-1, i.v.) did not affect the inhibition of ex vivo platelet aggregation in response to ET-1. In contrast, intravenous infusion of PD145065 (0.6 mg kg-1 min-1) abolished the anti-aggregatory effects of ET-1.5. Thus, FR139317 inhibits the pressor, but not the depressor actions of ET-1 and has no effect on the ET-l-induced inhibition of ex vivo platelet aggregation. In contrast, PD145065 antagonizes the pressor and depressor responses to ET-1 and abolishes the anti-aggregatory effects of the peptide.6. These results strongly suggest that ET-1-induced vasoconstriction in the anaesthetized rabbit is primarily mediated via the ETA receptor while the depressor and antiaggregatory actions of ET-1 are due to activation of the ETB receptor.

Endothelin-3-induced relaxation of rat thoracic aorta: a role for nitric oxide formation

1. Endothelin-3 (ET-3) at concentrations below those which caused contraction (30 nM) elicited endothelium-dependent relaxation followed by rebound contraction in rat isolated thoracic aorta. 2. Endothelin-1 also relaxed the rat aorta with a similar potency. 3. The nitric oxide (NO) synthase inhibitor, NG-nitro L-arginine, the radical scavenger, haemoglobin and the soluble guanylate cyclase inhibitor, methylene blue, each inhibited the ET-3-induced relaxation. 4. The calmodulin inhibitor, calmidazolium, considerably attenuated the relaxation caused by ET-3 without affecting that to nitroprusside. 5. Concentrations of ET-3 that were necessary to induce the relaxation also caused concentration-dependent elevation of guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels. 6. NG-nitro L-arginine, haemoglobin, methylene blue, calmidazolium and removal of the endothelium completely abolished ET-3-stimulated cyclic GMP production. 7. These results suggest that ET-3 triggers NO formation possibly via ETB receptors on the endothelium to activate soluble guanylate cyclase, which in turn stimulates cyclic GMP production and smooth muscle relaxation. The enzyme contributing to the NO formation may be of the calcium/calmodulin-dependent, constitutive type.

Discrimination between ETA- and ETB-receptor-mediated effects of endothelin-1 and [Ala1,3,11,15]endothelin-1 by BQ-123 in the anaesthetized rat

1. The influence of BQ-123 (a selective ETA-receptor antagonist) on the haemodynamic response elicited by endothelin-1 (ET-1) and [Ala1,3,11,15]ET-1 (a selective ETB-receptor agonist) was studied in anaesthetized rats instrumented with ultrasonic Doppler flow probes on the carotid, coeliac, mesenteric, renal and iliac arteries. 2. BQ-123 alone (1.6 mumol kg-1, i.v.) induced a decrease in femoral mean arterial pressure (AP), accompanied by a systemic vasodilatation. The response was maximal after 3 min and then returned slowly to baseline. None of these effects was observed after a 0.016 mumol kg-1 dose of BQ-123. 3. ET-1 (1 nmol kg-1, i.v.) induced a biphasic response characterized by a transient initial decrease in AP accompanied by regional vasodilatation (mainly in the carotid and iliac beds) and by immediate mesenteric and renal vasoconstrictions. This was followed, within 1 min, by a marked and prolonged increase in AP accompanied by systemic vasoconstriction. Pretreatment with BQ-123 (1.6 mumol kg-1, i.v., 8 min before ET-1) increased and prolonged the vasodilator effect of ET-1 (mainly in the carotid, coeliac, mesenteric and iliac beds) and reduced its systemic vasoconstrictor effects with marked regional differences (the coeliac, mesenteric and renal beds being poorly affected). 4. [Ala1,3,11,15]ET-1 (3 nmol kg-1, i.v.) induced an initial and marked decrease in AP accompanied by regional vasodilatation (mainly in the carotid, coeliac and iliac beds) and by mesenteric and renal vasoconstrictions. This was followed, within 5 min, by a small increase in AP and systemic vasoconstriction. All these effects were dose-dependent. Pretreatment with BQ-123 (1.6 tmol kg'; 8 min before ET-1) did not modify the early effect of [Ala'3""5]ET-l, but abolished its secondary vasoconstrictor effect except in the mesenteric bed.5. This study demonstrates that pretreatment with BQ-123 not only reduced a large part of the sustained vasoconstrictor activity of ET-1, suggesting the involvement of ETA-receptors, but also enhanced the early vasodilator activity of ET-1 revealing a functional antagonism between the two effects. The vasodilator effect of [Ala1"3""l '5]ET-1 was not affected by BQ-123 and ET-1 induced a similar vasodilatation, that was potentiated by BQ-123, suggesting the involvement of ETB-receptors in this vasodilator response. Marked regional differences were however observed which might be partly related to different levels of functional antagonism between ETB- and ETA-mediated effects, but differences in receptor types, or subtypes, cannot be excluded, mainly in the mesenteric and renals beds.

Regional vasodilation to endothelin-1 is mediated by a non-ETA receptor subtype in the anaesthetized rat: effect of BQ-123 on systemic haemodynamic responses

The sustained infusion of 75 nmol/kg per min BQ-123 selectively attenuates the systemic pressor responses to i.v. bolus endothelin-1 in the anaesthetized rat; the initial systemic vasodepression is only attenuated when a 10-fold higher dose of this ETA-selective antagonist is infused. These doses of BQ-123 do not antagonize the haemodynamic actions of angiotensin II or calcitonin gene-related peptide nor do they alter basal systemic haemodynamics. The secondary increase in carotid vascular resistance associated with the systemic pressor actions of endothelin-1 is selectively attenuated by the bolus i.v. administration of BQ-123 (1.6 mumol/kg); the maximum degree of the vasorelaxation that precedes this more sustained constriction is unaltered. Thus, the initial carotid vasodilation observed to endothelin-1, and the associated systemic vasodepression, is mediated by a non-ETA receptor subtype. Furthermore, subtle differences might exist between the receptors mediating mesenteric and carotid vasoconstriction since only the latter is sensitive to BQ-123.