Comparison of the bronchopulmonary and pressor activities of endothelin isoforms ET-1, ET-2, and ET-3 and characterization of their binding sites in guinea pig lung

A simple tool for monitoring nebulized amikacin treatments based on a single urine assay

Aerosolized aminoglycosides have demonstrated their efficacy in the treatment of P. aeruginosa pneumonia in cystic fibrosis (CF) patients. There is wide interpatient variability in the deposited and systemic drug doses that depend on both the nebulization and inhalation conditions and result in a risk of inefficacy or toxicity. We have developed a tool to provide a simple method for individual dose monitoring by estimating the total quantity of amikacin excreted, which corresponds to the dose absorbed systemically. It is based on a single urine assay. Thirty-seven urinary pharmacokinetic time courses in healthy volunteers (groups A and B) or in CF patients (groups C and D) were used. The rules for extrapolating the total dose excreted on the basis of 6-, 8-, 10-, and 12-h urine samples, were determined from group A. The accuracy of these rules was then tested in the other three groups. The total amount excreted was poorly predictable, with a coefficient of variation (CV) of 36 and 30% in the healthy volunteers, and of 48 and 82% in the CF group, whereas the CV of the estimated amount, based on 8- to 12-h samples, was only 10-15% in the healthy volunteers and 4-8% in the CF patients. Collecting a single sample over an 8- to 12-h period requires overnight sampling. The very low circadian variations in renal function, ranging from -2% to +5%, demonstrated the absence of any significant bias resulting from overnight sampling. A single urine assay can therefore be proposed as a simple, noninvasive, low cost, and reliable method for the clinical monitoring of nebulized amikacin in CF patients. Further studies are needed before this method can be extended to aerosol treatments with other aminoglycosides.

Endothelin-1-induced airway and parenchymal mechanical responses in guinea-pigs: the roles of ETA and ETB receptors

Endothelin-1 (ET-1) has been shown to have a constrictor effect on the airways and parenchyma; however, the roles of the ETA and ETB receptors in the ET-1-induced changes in the airway and tissue compartments have not been fully explored. Low-frequency pulmonary impedance (ZL) was measured in anaesthetized, paralysed, open-chest guinea-pigs. ZL spectra were fitted by a model to estimate airway resistance (Raw) and inertance (Iaw), and coefficients of tissue damping (G) and elastance (H), and hysteresivity (eta = G/H). Two successive doses of ET-1 (0.05 and 0.2 nmol x kg(-1)) each evoked significant dose-related increases in Raw, G, H and eta. Pretreatment with 20 nmol x kg(-1) BQ-610 (a highly selective ETA receptor antagonist) resulted in a significantly decreased elevation only in H after the lower dose of ET-1. However, all parameters changed significantly less on the administration of ET-1 after pretreatment with 80 nmol-kg(-1) BQ-610, with 20 nmol x kg(-1) ETR-P1/fl (a novel ETA receptor antagonist) or with 20 nmol x kg(-1) IRL 1038 (an ETB receptor antagonist). The results of the separate assessments of the airway and tissue mechanics demonstrate that endothelin-1 induces airway and parenchymal constriction via stimulation of both receptor types in both compartments.

Antigen-induced elevation of immunoreactive endothelin-1 (ET-1) levels in ovalbumin-sensitized guinea pig airway tissue

Changes in the immunoreactive ET-1 levels during the anaphylactic reaction of airway tissue from ovalbumin-sensitized guinea pigs were investigated. ET-1-immunoreactivity (ET-IR) was detected in the epithelial and smooth muscle layers of tracheal sections from normal guinea pigs and it was enhanced slightly by phosphoramidon (1 microM) treatment. The ET-IR level of the epithelial layer of ovalbumin-treated tissue from actively sensitized animals was slightly higher than that from normal animals, but it was enhanced markedly by phosphoramidon (1 microM) treatment. Furthermore, the mean ET-IR level of homogenates of antigen-treated tracheal tissues from sensitized guinea pigs (22.8 +/- 1.55 fmol mg-1 protein, n = 5) was significantly higher than the corresponding normal level (12.3 +/- 1.21 fmol mg-1 protein, n = 5). These results suggest that increased epithelial airway ET-1 levels contribute to the anaphylactic reaction of guinea pig airways.

Nitric oxide limits the eicosanoid-dependent bronchoconstriction and hypotension induced by endothelin-1 in the guinea-pig

1. This study attempts to investigate if endogenous nitric oxide (NO) can modulate the eicosanoid-releasing properties of intravenously administered endothelin-1 (ET-1) in the pulmonary and circulatory systems in the guinea-pig. 2. The nitric oxide synthase blocker N(omega)-nitro-L-arginine methyl ester (L-NAME; 300 microM; 30 min infusion) potentiated, in an L-arginine sensitive fashion, the release of thromboxane A2 (TxA2) stimulated by ET-1, the selective ET(B) receptor agonist IRL 1620 (Suc-[Glu9,Ala11,15]-ET-1(8-21)) or bradykinin (BK) (5, 50 and 50 nM, respectively, 3 min infusion) in guinea-pig isolated and perfused lungs. 3. In anaesthetized and ventilated guinea-pigs intravenous injection of ET-1 (0.1-1.0 nmol kg(-1)), IRL 1620 (0.2-1.6 nmol kg(-1)), BK (1.0-10.0 nmol kg(-1)) or U 46619 (0.2-5.7 nmol kg(-1)) each induced dose-dependent increases in pulmonary insufflation pressure (PIP). Pretreatment with L-NAME (5 mg kg(-1)) did not change basal PIP, but increased, in L-arginine sensitive manner, the magnitude of the PIP increases (in both amplitude and duration) triggered by each of the peptides (at 0.25, 0.4 and 1.0 nmol kg(-1), respectively), without modifying bronchoconstriction caused by U 46619 (0.57 nmol kg(-1)). 4. The increases in PIP induced by ET-1, IRL 1620 (0.25 and 0.4 nmol kg(-1), respectively) or U 46619 (0.57 nmol kg(-1)) were accompanied by rapid and transient increases of mean arterial blood pressure (MAP). Pretreatment with L-NAME (5 mg kg(-1); i.v. raised basal MAP persistently and, under this condition, subsequent administration of ET-1 or IRL 1620, but not of U-46619, induced hypotensive responses which were prevented by pretreatment with the cyclo-oxygenase inhibitor indomethacin. 5. Thus, endogenous NO appears to modulate ET-1-induced bronchoconstriction and pressor effects in the guinea-pig by limiting the peptide's ability to induce, possibly via ET(B) receptors, the release of TxA2 in the lungs and of vasodilatory prostanoids in the systemic circulation. Furthermore, it would seem that these eicosanoid-dependent actions of ET-1 in the pulmonary system and on systemic arterial resistance in this species are physiologically dissociated.

Role of endothelin in endotoxin-induced sustained pulmonary hypertension in sheep

BMS182874, an endothelin receptor antagonist, blocks the effects of exogenously administered endothelins in chronically instrumented awake sheep. A possible role for endothelin in endotoxin-induced pulmonary hypertension in sheep was investigated by studying animals given intravenous endotoxin with and without pretreatment with BMS182874. BMS182874 administration alone caused a reduction in pulmonary artery pressure (P[PA]) and systemic arterial pressure (P[SA]). Endotoxin alone caused an acute, nearly threefold increase in P(PA) which was followed, from 2-5 h after endotoxin, by a sustained but less severe increase in P(PA). These changes were accompanied by a threefold increase in lung lymph flow and dramatic increases in plasma and lung lymph thromboxane B2 concentrations. Pretreatment with BMS182874 significantly attenuated the early endotoxin-induced acute increase in P(PA) and completely blocked the late sustained pulmonary hypertension (p < 0.05), while having no affect on the increases in thromboxane levels. BMS182874 shifts the dose response curve for U46619, a prostaglandin H2 analogue, to the right. BMS182874, in addition to functioning as an endothelium receptor antagonist, appears to counteract the action of thromboxane at the receptor level. We theorize that BMS182874 attenuates the early endotoxin-induced pulmonary hypertension by counteracting the effects of thromboxane, since previous studies demonstrated that the early acute rise in P(PA) is caused by thromboxane. The late sustained pulmonary hypertension of endotoxemia, on the other hand, appears to be mediated by endothelin.

The induction of a biphasic bronchospasm by the ETB agonist, IRL 1620, due to thromboxane A2 generation and endothelin-1 release in guinea-pigs

1. IRL 1620 (0.01-0.1 mg kg-1, i.v.), a selective endothelin B (ETB) receptor agonist, induced a dose-dependent biphasic increase in total lung resistance and a decrease in dynamic compliance in anaesthetized and artificially ventilated guinea-pigs. After intravenous injection of IRL 1620 (0.03 mg kg-1), the first phase was observed within 2 min whereas the second phase started between 5 and 10 min after injection and was long lasting. 2. In order to characterize which endothelin receptors are involved in both phases of bronchoconstriction, we studied the effect of ETA and ETB receptor antagonists (BQ 123 and BQ 788, respectively). BQ 788 (0.1-1 mg kg-1, i.v.) inhibited, in a dose-dependent manner, both phases of bronchoconstriction. BQ 123 (3 mg kg-1, i.v.) markedly inhibited (by 76%) the second phase of bronchoconstriction but had no effect on the early component of the response. 3. The effect of atropine, neurokinin-I (NK1) and neurokinin-2 (NK2) receptor antagonists (SR140333 and SR48968, respectively) were tested to investigate the possible involvement of cholinergic and sensory nerve activation, respectively, in the response to IRL 1620. Likewise, the role of arachidonic acid metabolites (leukotriene D4 antagonist, ONO-1078 and thromboxane A2 (TXA2) inhibitor, OKY-046) in this response was also investigated. OKY-046 (1 mg kg-1, i.v.) and atropine (1 mg kg-1, i.v.) partially inhibited the first phase (by 80% and 20%, respectively) without affecting the late phase of bronchoconstriction. Neither ONO-1078 (1 mg kg-1, i.v.) nor the combination of SR140333 (0.2 mg kg-1, i.v.) and SR 48968 (0.2 mg kg-1, i.v.) modified IRL 1620-induced bronchoconstriction. 4. A low dose of IRL 1620 (0.005 mg kg-1, i.v.) induced a monophasic bronchoconstriction. Pretreatment by phosphoramidon (100 mumol kg-1, i.v.) restored the second phase of bronchoconstriction. In this condition, BQ 123 (3 mg kg-1, i.v.) was able to inhibit partially the second phase of bronchoconstriction. 5. These results suggest that both phases of bronchoconstriction induced by IRL 1620 were mediated primarily by ETB receptor activation, the first phase being a consequence of TXA2 and acetylcholine release. The inhibition by an ETA receptor antagonist and the restoration by a neutral endopeptidase (NEP) inhibitor of the second phase of bronchoconstriction suggests that primary activation of ETB receptors leads to autocrine/paracrine endothelin-1 (ET-1) release that would subsequently cause profound bronchoconstriction through both ETA and ETB receptor activation.

Role of the neutral endopeptidase 24.11 in the conversion of big endothelins in guinea-pig lung parenchyma

1. We have studied the conversion of big endothelin-1 (big ET-1), big endothelin-2 (big ET-2) and big endothelin-3 (big ET-3) and characterized the enzyme involved in the conversion of the three peptides in guinea-pig lung parenchyma (GPLP). 2. Endothelin-1 (ET-1), endothelin-2 (ET-2) and endothelin-3 (ET-3) (10 nM to 100 nM) caused similar concentration-dependent contractions of strips of GPLP. 3. Big ET-1 and big ET-2 also elicited concentration-dependent contractions of GPLP strips. In contrast, big ET-3, up to a concentration of 100 nM, failed to induce a contraction of the GPLP. 4. Incubation of strips of GPLP with the dual endothelin converting enzyme (ECE) and neutral endopeptidase (NEP) inhibitor, phosphoramidon (10 microM), as well as two other NEP inhibitors thiorphan (10 microM) or SQ 28,603 (10 microM) decreased by 43% (P < 0.05), 42% (P < 0.05) and 40% (P < 0.05) the contractions induced by 30 nM of big ET-1 respectively. Captopril (10 microM), an angiotensin-converting enzyme inhibitor, had no effect on the contractions induced by big ET-1. 5. The incubation of strips of GPLP with phosphoramidon (10 microM), thiorphan (10 microM) or SQ 28,603 (10 microM) also decreased by 74% (P < 0.05), 34% and 50% (P < 0.05) the contractions induced by 30 nM big ET-2 respectively. As for the contractions induced by big ET-1, captopril (10 microM) had no effect on the concentration-dependent contractions induced by big ET-2. 6. Phosphoramidon (10 microM), thiorphan (10 microM) and SQ 28,603 (10 microM) significantly potentiated the contractions of strips of GPLP induced by both ET-1 (30 nM) and ET-3 (30 nM). However, the enzymatic inhibitors did not significantly affect the contractions induced by ET-2 (30 nM) in this tissue. 7. These results suggest that the effects of big ET-1 and big ET-2 result from the conversion to ET-1 and ET-2 by at least one enzyme sensitive to phosphoramidon, thiorphan and SQ 28,603. This enzyme corresponds possibly to EC 3.4.24.11 (NEP 24.11) and could also be responsible for the degradation of ETs in the GPLP.

Different pressor and bronchoconstrictor properties of human big-endothelin-1, 2 (1-38) and 3 in ketamine/xylazine-anaesthetized guinea-pigs

1. In the present study, the precursors of endothelin-1, endothelin-2 and endothelin-3 were tested for their pressor and bronchoconstrictor properties in the anaesthetized guinea-pig. In addition, the effects of big-endothelin-1 and endothelin-1 were assessed under urethane or ketamine/xylazine anaesthesia. 2. When compared to ketamine/xylazine, urethane markedly depressed the pressor and bronchoconstrictor properties of endothelin-1 and big-endothelin-1. 3. Under ketamine/xylazine anaesthesia, the three endothelins induced a biphasic increase of mean arterial blood pressure. In contrast, big-endothelin-1, as well as big-endothelin-2 (1-38), induced only sustained increase in blood pressure whereas big-endothelin-3 was inactive at doses up to 25 nmol kg-1. 4. Big-endothelin-1, but not big-endothelin-2, induced a significant increase in airway resistance. Yet, endothelin-1, endothelin-2 and endothelin-3 were equipotent as bronchoconstrictor agents. 5. Big-endothelin-1, endothelin-1 and endothelin-2, but not big-endothelin-2, triggered a marked release of prostacyclin and thromboxane A2 from the guinea-pig perfused lung. 6. Our results suggest the presence of a phosphoramidon-sensitive endothelin-converting enzyme (ECE) which is responsible for the conversion of big-endothelin-1 and big-endothelin-2 to their active moieties, endothelin-1 and 2. However, the lack of bronchoconstrictor and eicosanoid-releasing properties of big-endothelin-2, as opposed to endothelin-2 or big-endothelin-1, suggests the presence of two distinct phosphoramidon-sensitive ECEs in the guinea-pig. The ECE responsible for the systemic conversion of big-endothelins possesses the same affinity for big-endothelin-l and 2 but not big-endothelin-3. In contrast, in the pulmonary vasculature is localized in the vicinity of the sites responsible for eicosanoid release, an ECE which converts more readily big-endothelin-1 than big-endothelin-2.

Relation between endothelin-1 spillover in the lungs and pulmonary vascular resistance in patients with chronic heart failure

OBJECTIVES

The aim of this study was to clarify the origin of plasma endothelin-1 and to determine the relation between pulmonary vascular resistance and endothelin-1 secretion in the pulmonary circulation in patients with chronic congestive heart failure.

BACKGROUND

Plasma levels of endothelin-1, a potent endothelium-derived vasoconstrictor peptide, are increased in congestive heart failure, but the source has not been clarified. Recent studies have indicated a relation between endothelin-1 and pulmonary hypertension. We therefore evaluated the contribution of endothelin-1 secretion in the pulmonary circulation to the regulation of pulmonary vascular resistance in patients with chronic heart failure.

METHODS

A comparison was made of the plasma levels of endothelin-1 between the main pulmonary artery and the pulmonary capillary wedge region, as well as between the femoral artery and the femoral vein in 62 patients with chronic heart failure. Stepwise multivariate regression analysis was used to detect independent predictors of pulmonary vascular resistance among the various vasoconstrictor hormones in these patients.

RESULTS

There was no significant difference in plasma endothelin-1 levels between the femoral artery and vein. In contrast, plasma endothelin-1 increased significantly from the main pulmonary artery to the pulmonary capillary wedge region ([mean +/- SEM] 3.1 +/- 0.23 vs. 4.6 +/- 0.36 pg/ml, p < 0.01), and the increase was related to the severity of heart failure. Among the various vasoconstrictor factors, such as plasma active renin concentration, plasma angiotensin II, plasma norepinephrine, femoral venous plasma endothelin-1 and pulmonary endothelin-1 spillover, only endothelin-1 spillover in the lungs showed an independent and significant correlation with pulmonary vascular resistance (r = 0.82, p < 0.001).

CONCLUSIONS

The main source of circulating endothelin-1 is not the peripheral vascular bed but the pulmonary vascular bed in patients with chronic heart failure. In addition, endothelin-1 secretion in the lungs may regulate the pulmonary vascular resistance in patients with chronic heart failure. These findings are consistent with a significant role for endogenous endothelin-1 in the pathophysiology of heart failure, especially in the pulmonary circulation.

Characterization of ETB receptors mediating contractions induced by endothelin-1 or IRL 1620 in guinea-pig isolated airways: effects of BQ-123, FR139317 or PD 145065

1. We have characterized the receptors mediating contractions to endothelin-1 (ET-1) or IRL 1620, an ETB receptor selective agonist, in isolated strips of tissue prepared from different parts of the guinea-pig airways. We used as antagonists BQ-123 and FR139317 (ETA receptor-selective) and PD 145065 (ETA/ETB receptor non-selective). 2. ET-1 and IRL 1620 (10(-10) M to 10(-6) M) caused similar concentration-dependent contractions of strips of guinea-pig trachea and upper bronchus. In the guinea-pig trachea without epithelium or lung parenchyma, IRL 1620 was less potent than ET-1. 3. In the trachea, contraction to ET-1 (< 10(-8) M) was preceded by a transient relaxation which was inhibited by BQ-123 (10(-5) M) or FR 139317 (10(-5) M) or by the removal of the epithelium. The concentration-response curve to ET-1 in the trachea was shifted to the right by PD 145065 (10(-5) M to 10(-4) M). PD 145065 (10(-4) M) also inhibited the response to ET-1 (3 x 10(-7) M) by 55%. Contractions induced by IRL 1620 were not affected by BQ-123 (10(-6) M) or FR139317 (10(-6) M) but were significantly attenuated by 10(-5) M of either antagonist. PD 145065 at 10(-6) M strongly attenuated and at 10(-5) M abolished contractions induced by IRL 1620. 4. In the trachea, removal of the epithelium potentiated the effects of both agonists. BQ-123 (10-5 M)had no effect on contractions of the trachea without epithelium induced by ET-1, but FR139317 (10-5 M)caused a significant inhibition. PD 145065 (10-5 M to 10-4 M) caused a shift to the right of the ET-1 concentration-response curve without affecting the contractile effect at 3 x 10-7 M. All three antagonists inhibited contractions induced by IRL 1620.5. In the upper bronchus, BQ-123 (10-5 M) did not affect contractions induced by ET-1, whileFR139317 (10-5 M) attenuated (20-26%) only contractions induced by 1-3 x 10-7 M ET-1. PD 145065(10-5 M to 10-4 M) caused a shift to the right of the ET-1 concentration-response curve. The contractions induced by IRL 1620 were inhibited by BQ-123 or FR139317 (10-5M to 10-4 M). PD 145065(10-6 M) strongly inhibited contractions induced by IRL 1620 and PD 145065 (10-5 M) totally abolished them.6. The contractile action of ET-1 in the lung parenchyma was significantly and similarly attenuated by BQ-123 (10-5 M) or indomethacin (10-5 M), while FRI39317 (10-5 M) was less effective. PD 145065(10-6 to 10-5 M) inhibited contractions to ET-1. IRL 1620, which is less potent than ET-1 in this preparation, was antagonized by PD 145065 (10-5 to 10-6 M) but unaffected by BQ-123 (10-6 M to10-5M) or FR139317 (10-6 M).7. Thus, ETB receptors mediate contractions to ET-1 in all four guinea-pig airway preparations. In addition, contractions to ET-1 in the trachea and lung parenchyma are mediated in part by ETA receptors. In the latter tissue, these ETA receptors mediate contraction through the release of cyclooxygenase metabolites. Similarly, ETA receptors located on the epithelial cells also mediate the release of prostanoids in the trachea with epithelium but they are responsible for transient relaxations. Interestingly,contractions induced by IRL 1620 were more susceptible to inhibition by the different antagonists,most probably because it binds to the endothelin receptors in a reversible manner. High concentrations(10-5 M) of ETA-selective antagonists also inhibit responses to IRL 1620, most probably by an effect at ETB receptors in both the trachea and the upper bronchus.

Mechanical and biochemical responses to endothelin-1 and endothelin-3 in bovine bronchial smooth muscle

1. In this study, mechanical responses to endothelin-1 and endothelin-3 were examined in bovine bronchial smooth muscle. In addition, the involvement of phosphatidylinositol 4,5-bisphosphate hydrolysis (PIP2) in the responses to these peptides was assessed by measurement of inositol (1,4,5) trisphosphate (I(1,4,5)P3) production using a specific mass assay. 2. ET-1 evoked contractions of bovine bronchi which were concentration-dependent and initiated at between 10(-9) M and 10(-8) M. ET-1-evoked responses were unaffected by slight elevation of tone with potassium chloride (3 x 10(-2) M), methacholine (10(-6) M) or U46619 (10(-7) M). 3. Contractions to ET-1 were not altered by pre-incubation with atropine (10(-5) M), indomethacin (10(-5) M), nifedipine (10(-5) M), phosphoramidon (3.67 x 10(-5) M) or by removal of the epithelium. 4. ET-3 evoked small contractions which were not concentration-dependent. In the presence of phosphoramidon (3.67 x 10(-5) M) however, concentration-dependent contractions were obtained to ET-3 which were unaffected by atropine (10(-5) M) or by removal of the epithelium, but were significantly attenuated by indomethacin (10(-5) M). Nifedipine (10(-5) M) virtually abolished this response. 5. Both ET-1 and ET-3 (in the presence of phosphoramidon)-evoked contractions were significantly enhanced by the presence of the phorbol ester phorbol 12,13-dibutyrate (10(-8) M). Neither ET-1-, nor ET-3-mediated responses were antagonized by the protein kinase C (PKC) inhibitor, Ro 31-8220 (3 x 10(-9) - 3 x 10(-8) M). 6. ET-1 (3 x 10(-7) M) evoked a biphasic rise in levels of I(1,4,5)P3 which was unaltered by preincubation with atropine, whilst ET-3 (10(-10) - 3 x 10(-7) M) failed to alter levels of I(1,4,5)P3 at any time point examined, even in the presence of phosphoramidon (3.67 x 10(-5) M). 7. These results suggest that, in bovine bronchial smooth muscle, ET-l does not evoke contraction via cyclo-oxygenase metabolites, does not evoke release of the neurotransmitter substance acetylcholine, or require calcium influx via dihydropyridine-sensitive channels. ET-1 evokes 1(1,4,5)P3 production, but stimulation of protein kinase C may not be critical for the associated contraction. In contrast,ET-3-evoked contractions are partly mediated by cyclo-oxygenase metabolites. ET-3 does not stimulate PIP2 hydrolysis, nor activate PKC, but may, either directly or as a requirement of intermediates released in response to ET-3, rely upon extracellular calcium.

Different modes of endothelin-1 action in pressor response in vivo and pulmonary parenchymal contraction in vitro in the guinea pig

Intravenously administered endothelin-1 (ET-1) (2 x 10(-11)-6 x 10(-10) mol/kg) induced dose-dependent pressor responses in anesthetized guinea pigs. Pretreatment with indomethacin (5 mg/kg, i.v.) or with a thromboxane A2/prostaglandin endoperoxide receptor antagonist, ONO-3708 (0.5 and 1.0 mg/kg, i.v.) significantly attenuated the pressor responses. ET-1 (10(-11)-10(-7) M) dose-dependently contracted guinea pig pulmonary parenchymal strips in vitro. However, neither pretreatment with indomethacin (10(-5) M) nor one with ONO-3708 (10(-6) M and 10(-5) M) significantly affected the ET-1-induced guinea pig pulmonary parenchymal contraction in vitro. Moreover, pretreatment with a platelet activating factor receptor antagonist, CV-3988 (2 x 10(-5) M) did not significantly affect the contraction. Thus, in guinea pigs, the mechanism of ET-1-induced pressor response in vivo mediated via cyclooxygenase-generated-eicosanoid(s), possibly, thromboxane A2 is not identical to that of ET-1-induced contraction of pulmonary parenchymal strips in vitro.

Role of endothelin ETB receptors in bronchoconstrictor and vasoconstrictor responses in guinea-pigs

In anesthetized and ventilated guinea-pigs, intravenous injections of endothelin (ET)-1 (0.5 nmol/kg), ET-3 (0.5 nmol/kg), and [Ala1,3,11,15]ET-1 (20 nmol/kg), an ETB-selective receptor agonist, induced bronchoconstrictor and transient vasoconstrictor responses. Only the ET-1-induced transient vasoconstriction was followed by a secondary sustained pressor response. The ETA-selective receptor antagonist, BQ-123 (1 mg/kg i.v.), attenuated only the sustained pressor response. These results indicate that the bronchoconstrictor and transient vasoconstrictor responses to endothelins in guinea-pigs are mediated by ETB receptors, whilst the sustained pressor response is mediated by ETA receptors. The thromboxane A2 receptor antagonist, L-670,596 (0.5 mg/kg, i.v.) and a high dose of BQ-123 (30 mg/kg i.v.) abolished the bronchoconstriction only without affecting the transient pressor response to endothelin isopeptides. These results suggest that the ETB-mediated bronchoconstriction depends on thromboxane A2 formation. The different sensitivity of these ETB-mediated transient responses to BQ-123 suggests the possible existence of distinct ETB receptor subtypes.

Endothelin and the respiratory system

Pharmacological research involving the endothelin peptides has emphasized their activities in vascular systems, from both physiological and pathophysiological perspectives. However, the endothelins are known also to be synthesized and released from respiratory epithelial cells and to have potent effects in nonvascular components of the respiratory tract. Douglas Hay, Peter Henry and Roy Goldie summarize present understanding of the pharmacology of the endothelins in the respiratory system and assess the potential pathophysiological role in asthma.

Endothelin peptides: biological actions and pathophysiological significance in the lung

Endothelins (ETs) are a family of novel regulatory peptides. Besides their effects on the cardiovascular system, which have been extensively described, several lines of evidence suggest an important role for ETs in regulating pulmonary functions. ETs are present in the pulmonary tissues, bronchoalveolar space and pulmonary circulation. Release of ETs from macrophages, endothelial and epithelial cells is modulated by a variety of chemical and physical stimuli and is regulated at the level of transcription or translation. Specific endothelin receptors have been identified in the airways as well as in the pulmonary vasculature. ETs are among the most potent bronchoconstrictors yet described. In the pulmonary circulation, ETs can elicit both vasodilation and vasoconstriction and can enhance vascular permeability. ETs could also modulate activation of inflammatory cells. Enhanced ET expression and/or production have been detected in asthma, certain pulmonary tumors, shock states associated with adult respiratory distress syndrome and pulmonary hypertension. These findings suggest that by regulating pulmonary vascular and airway tone, activation of inflammatory cells and cellular growth and/or differentiation, ETs may play an important role in pulmonary pathophysiology.

Plasma endothelin correlates with the extent of pulmonary hypertension in patients with chronic congestive heart failure

BACKGROUND

Endothelin is a family of potent vasoconstrictor peptides of vascular endothelial origin. Although it has been proposed that the vasoconstrictor effects of endothelin are produced at the local vascular level, increased plasma concentration of endothelin has been identified in cardiovascular disorders.

METHODS AND RESULTS

We tested whether immunoreactive endothelin-1 could be detected by radioimmunoassay in plasma of congestive heart failure patients and whether levels correlated with hemodynamic characteristics. Twenty congestive heart failure patients (New York Heart Association class II-IV) were sampled in the morning after an overnight fast, before medication. Cardiac index was decreased to 2.14 +/- 0.45 l/m/m2, and pulmonary wedge pressure was increased to 22 +/- 7 mm Hg. The ranges of pulmonary pressures were: systolic, 22-100 mm Hg, mean, 13-61 mm Hg, and diastolic, 8-42 mm Hg. The endothelin-1 level was 9.07 +/- 4.13 pg/ml (range, 4-19 pg/ml), which was increased compared with 12 normals (3.7 +/- 0.6 pg/ml; range, 2.8-4.7 pg/ml); the difference was statistically significant (p less than 0.0001). Endothelin-1 significantly correlated with pulmonary pressures (systolic, r = 0.78; mean, r = 0.80; diastolic, r = 0.77; all p less than 0.003) and pulmonary vascular resistance (r = 0.65, p less than 0.01). Endothelin-1 strongly correlated with the resistance ratio (pulmonary vascular resistance/systemic vascular resistance) (r = 0.88, p less than 0.0001). Stepwise multiple regression analysis confirmed the significance of these observations.

CONCLUSIONS

Elevated immunoreactive endothelin-1 specifically correlated with the extent of pulmonary hypertension in congestive heart failure patients. Whether endothelin-1 is a regional mediator of pulmonary hypertension or a marker for its occurrence requires additional evaluation.

Different pharmacological profiles of big-endothelin-3 and big-endothelin-1 in vivo and in vitro

1. Human big-endothelin-1 (big-ET-1) and endothelin-1 (ET-1) are equipotent as pressor agents and produce a significant change in mean arterial blood pressure (MAP) in anaesthetized guinea-pigs (2 nmol kg-1: peak delta MAP: 23 +/- 6 mmHg and 26 +/- 5 mmHg, respectively). 2. Unlike big-ET-1, big-endothelin-3 (big-ET-3) (10 and 20 nmol kg-1) induces no pressor responses whereas endothelin-3 (ET-3) at 2 nmol kg-1 induces a significant increase of blood pressure in anaesthetized guinea-pigs (peak delta MAP: 27 +/- 5 mmHg) with a shorter duration than ET-1 and big-ET-1. 3. Big-ET-1 at concentrations 40 times higher than those required for ET-1 (2.5 nM) releases prostacyclin (PGI2) (maximal release: 2.7 +/- 0.8 ng ml-1; 2.9 +/- 0.9 ng ml-1, respectively) and thromboxane B2 (TxB2) (maximal release: 6.7 +/- 1.3 ng ml-1; 6.8 +/- 1.1 ng ml-1, respectively) from guinea-pig perfused lungs. ET-3 (2.5 nM) is also a potent releaser of PGI2 and TxB2 from the guinea-pig lungs (maximal release: PGI2: 2.4 +/- 1.0 ng ml-1; TxB2: 3.8 +/- 0.6 ng ml-1). Conversely, big-ET-3 (100 nM) does not increase basal release of eicosanoids. 4. Phosphoramidon (50 microM), a metalloprotease inhibitor, markedly reduced the eicosanoid releasing properties of big-ET-1 (n = 4, P less than 0.01) in guinea-pig perfused lungs without affecting the release stimulated by ET-1. 5. Our results suggest that big-ET-1 is converted to ET-1 via a phosphoramidon-sensitive endothelin converting enzyme (ECE) to release eicosanoids. The ECE is present in the guinea-pig pulmonary vasculature. Furthermore, our results suggest that the ECE activity is specific for big-ET-1 and may not convert big-ET-3 to its active metabolite, ET-3.