János G. Filep

ENDOTHELIN-1 INDUCES PROSTACYCLIN RELEASE FROM BOVINE AORTIC ENDOTHELIAL CELLS

The effects of endothelin-1 (ET-1) on the release of prostacyclin from cultured bovine aortic endothelial cells were studied. ET-1 induced a time- and dose-dependent release of 6-keto PGF1 alpha, the stable metabolite of prostacyclin, with an apparent EC50 value of 3.0 +/- 0.9 nM (n = 6). ET-1 up to a concentration of 500 nM did not affect cellular integrity. Preincubation of the cells for 30 min with 10 microM indomethacin inhibited ET-1 (100 nM) - induced prostacyclin release by 90%. These findings indicate that ET-1 can directly stimulate prostacyclin release from endothelial cells probably through a receptor mediated mechanism.

Enhancement by endothelin-1 of microvascular permeability via the activation of ETA receptors

1 The objective of the present experiments was to assess the involvement of endothelin‐A (ETA) receptors in mediating the effects of endothelin‐1 on microvascular permeability in conscious rats. 2 Bolus injection of endothelin‐1 (0.1 and 1 nmol kg−1, i.v.) resulted in a dose‐dependent prolonged pressor effect preceded by a transient depressor response. These changes were accompanied by a dose‐dependent loss of plasma volume. Endothelin‐1 (1 nmol kg−1) enhanced the vascular permeability of the upper and lower bronchi, kidney, stomach, duodenum and spleen (up to 270%) as measured by the extravasation of Evans blue dye. 3 Pretreatment of the animals with the selective ETA receptor antagonist, BQ‐123 (1 mg kg−1, i.v.) significantly blunted the pressor response to endothelin‐1 without affecting the depressor response. BQ‐123 inhibited by 87% the endothelin‐1 (1 nmol kg−1)‐induced plasma volume loss. BQ‐123 markedly attenuated protein extravasation elicited by endothelin‐1 in the upper and lower bronchi and kidney, whereas it completely inhibited the permeability effect of endothelin‐1 in the stomach and duodenum. BQ‐123 by itself had no significant effect on the parameters studied. 4 The endothelin‐1 analogue, [Trp(For)21]‐endothelin‐1, in which Trp21 is formylated, was as potent a pressor agent as endothelin‐1, but had no depressor action. Bolus injection of [Trp(For)21]‐endothelin‐l (0.1 and 1 nmol kg−1, i.v.) evoked similar plasma volume losses to those observed following administration of equimolar doses of endothelin‐1. Furthermore, 1 nmol kg−1 [Trp(For)21]‐endothelin‐l evoked increases in protein extravasation similar to endothelin‐1, 1 nmol kg−1. 5 The present findings suggest that endothelin‐1 enhances microvascular permeability, in part, via the activation of ETA receptors.

Nitric oxide modulates vascular permeability in the rat coronary circulation

1 The objective of the present study was to assess whether inhibition of nitric oxide (NO) production could modulate vascular permeability in the coronary circulation in conscious rats. 2 Intravenous injection of NG‐nitro‐l‐arginine methyl ester (l‐NAME, 2 mg kg−1) resulted in a slowly developing hypertension and evoked twofold increases in vascular permeability in the left ventricle and right atrium as measured by the extravasation of Evans blue dye. Maintenance of mean arterial blood pressure at the level observed following l‐NAME injection by infusion of noradrenaline (620–820 ng kg−1 min−1) did not induce significant protein extravasation in the coronary circulation. 3 l‐NAME treatment markedly enhanced (up to 490%) protein extravasation both in the left ventricle and right atrium in response to platelet‐activating factor (PAF, 1.9 nmol kg−1, i.v.) and endothelin‐1 (1 nmol kg−1, i.v.). Noradrenaline infusion potentiated (up to 69%) endothelin‐1‐induced protein extravasation. The permeability effect of PAF was only slightly enhanced by noradrenaline. 4 The present findings indicate that inhibition of endogenous NO synthesis leads to an increase in protein extravasation and to potentiation of the permeability effects of PAF and endothelin‐1 in the coronary circulation. These results also suggest that NO may be an important regulator of vascular permeability under physiological and pathological conditions.

Endothelin-1 enhances vascular permeability in conscious rats: role of thromboxane A2

The purpose of the present experiments was to study the effects of endothelin-1 (ET-1) on vascular permeability and the involvement of the cyclooxygenase metabolites in the vascular responses to ET-1. Bolus intravenous injection of ET-1 (0.1-1.0 nmol/kg) into conscious rats induced immediate hypotension lasting for 30 s followed by sustained dose-dependent hypertension. A low dose of ET-1 (0.1 nmol/kg) did not modify the hematocrit value but the 1.0-nmol/kg dose increased the hematocrit value from 39.7 to 44.4%. Pretreatment of the animals with BM-13505 (1 mg/kg), a thromboxane A2 (TxA2) receptor antagonist, prolonged the duration of the hypotensive response to ET-1 (1.0 nmol/kg) but had no effect on the pressor response. Pretreatment with OKY-046 (10 mg/kg), a TxA2 synthesis inhibitor, or indomethacin (10 mg/kg), a cyclooxygenase inhibitor, had no significant effect on ET-1-induced changes in blood pressure. Evans blue dye extravasation, a marker of vascular permeability, increased up to 235% over control levels in specific vascular beds including the upper and lower bronchi, stomach, duodenum and kidney of ET-1 (1.0 nmol/kg)-treated animals. Pretreatment of the animals with BM-13505, OKY-046 or indomethacin reduced by 60-100% the Evans blue extravasation in these tissues. These results suggest that the effect of ET-1 on vascular permeability is partly mediated and/or modulated by the secondary release of TxA2, whereas its action on arterial blood pressure appears to be independent from prostanoid release in conscious rats.

Induction by endothelin-1 of epithelium-dependent relaxation of guinea-pig trachea in vitro: role for nitric oxide.

1 The purpose of the present experiments was to study the underlying mechanisms responsible for the relaxant action of endothelin‐1 (ET‐1) in the guinea‐pig trachea in vitro. 2 In tracheal strips precontracted (60–70% of the maximum) with carbachol, ET‐1 (1–100 nm) evoked slowly developing concentration‐dependent relaxations. Preincubation of the tissues with the thromboxane A2/prostaglandin H2 receptor antagonist, BM 13505 (5 μm) significantly potentiated the relaxant response to ET‐1. 3 Removal of the epithelium changed the response of precontracted tracheal preparations to ET‐1 from a relaxation to a sustained contraction. 4 ET‐1‐induced relaxations were abolished by methylene blue (10 μm) and were almost completely attenuated by oxyhaemoglobin (5 μm) and NG‐monomethyl‐l‐arginine (l‐NMMA, 100 μm), an inhibitor of nitric oxide synthesis, but were not altered by indomethacin (10 μm). 5 In tracheal strips under passive tension, ET‐1 (1–100 nm) elicited dose‐dependent contractions. The sensitivity of tissues to ET‐1 was significantly enhanced by removal of the epithelium (apparent EC50 values were 28.1 ± 4.1 and 12.5 ± 0.8 nm in intact and rubbed trachea, respectively, n = 7, P < 0.01). 6 Preincubation of intact tracheal strips with methylene blue, oxyhaemoglobin or l‐NMMA did not mimic the effect of epithelium removal on ET‐1‐induced contractions. 7 There was a concentration‐dependent increase in thromboxane A2 but not in PGE2 and prostacyclin release from intact tracheal strips following stimulation with ET‐1 (5–100 nm). 8 These results show that ET‐1 exerts a dual action on guinea‐pig isolated trachea: it evokes contractions at low resting tone, whereas it induces relaxations at higher resting tone. The relaxant action of ET‐1 may be mediated by nitric oxide released from epithelial cells and resultant activation of smooth muscle guanylate cyclase.

ENDOTHELIN INDUCES THROMBOXANE RELEASE AND CONTRACTION OF ISOLATED GUINEA-PIG AIRWAYS

The possible role of thromboxane (TX) A2 in mediating the bronchopulmonary effect of endothelin (ET) was studied in isolated guinea pig airways (trachea, upper bronchi and parenchyma). ET (1-100 nM) evoked a concentration-dependent contraction of all three tissues. The contractile response was significantly attenuated by pretreatment of the tissues with the specific thromboxane receptor blockers, BM 13177 (10-50 microM) and BM 13505 (1-5 microM). Furthermore, ET caused a dose-dependent TXB2 release from trachea and parenchyma. These findings suggest that the bronchopulmonary action of ET is mediated, in part, by the release of TXA2.

Endothelin-1 enhances vascular permeability in the rat heart through the ETA receptor

Intravenous injection of endothelin-1 (ET-1, 0.1 and 1 nmol/kg) resulted in a dose-dependent increase in vascular permeability in the coronary circulation of conscious rats. The increase was almost completely abolished by the selective ETA receptor antagonist, BQ-123 (1 mg/kg). The ET-1 analogue, [Trp(For)21]ET-1, which is devoid of the depressor but not the pressor activity, evoked changes in protein extravasation similar to those with ET-1. These data indicate that the permeability effect of ET-1 is mediated through the ETA receptor.

Pharmacological modulation of endothelin‐induced contraction of guinea‐pig isolated airways and thromboxane release

1 The aim of the present experiments was to study the possible involvement of known bronchoconstrictor substances in mediating the myotropic action of endothelin‐1 (ET‐1, human‐porcine endothelin) in guinea‐pig isolated airways. 2 ET‐1 (1–100 nm) caused a dose‐dependent contraction of guinea‐pig trachea, upper bronchus and parenchyma. The contractions developed slowly, reaching maximal values 4–6 min after addition of the peptide. 3 The contractile action of ET‐1 was significantly attenuated by indomethacin (10 μm), a cyclo‐oxygenase blocker, BM 13505 (5 μm), a thromboxane receptor antagonist, FPL 55712 (19 μm) and YM 16638 (1 μm), antagonists of the sulphidopeptide leukotrienes, BN 52021 (10 μm) and WEB 2086 (1 μm), platelet‐activating factor receptor antagonists in all three tissue preparations studied. 4 Pretreatment of the airway tissues with compound U 75302 (3 μm), a selective leukotriene B4 receptor antagonist, or with a mixture of antagonists containing methysergide (0.75 μm), phentolamine (0.4 μm), propranolol (13 μm), atropine (0.4 μm) and diphenhydramine (0.45 μm) did not modify the myotropic action of ET‐1. 5 ET‐1, 10 and 100 nm induced three, and nine fold increases in thromboxane A2 release from lung parenchymal strips. 6 ET‐1‐induced thromboxane A2 release was completely abolished by indomethacin, and was significantly attenuated by BN 52021, WEB 2086 and FPL 55712. Neither BM 13505 nor YM 16638 exerted a significant effect on thromboxane release. 7 The present findings show that contraction of guinea‐pig airway smooth muscle by ET‐1 is mediated, in part, by the release of thromboxane A2, sulphidopeptide leukotrienes and platelet‐activating factor, and suggest that the increased thromboxane A2 release following ET‐1 is partly a consequence of enhanced synthesis of sulphidopeptide leukotrienes and platelet‐activating factor.

Potent thromboxane-mediated in vitro bronchoconstrictor effect of endothelin in the guinea-pig

The present data both confirm that endothelin is a potent constrictor of the guinea pig airways and suggest that this action of ET is mediated at least in part by the release of thromboxane A 2 , a well known bronchoconstrictor

Endothelin-induced constriction of guinea pig airways: role of platelet-activating factor.

Endothelin-1 (ET-1, 1-100 nM) evoked a concentration-dependent contraction of guinea pig trachea and upper bronchi in vitro. The response was significantly attenuated by BN 52021 (ginkgolide B, 10 microM), a specific receptor antagonist to platelet-activating factor. Precontraction of the tissues with endothelin (50 nM) desensitized tissues to platelet-activating factor (200 nM), while preincubation of the tissues with platelet-activating factor slightly inhibited the response to endothelin. These findings suggest that platelet-activating factor may mediate endothelin-induced airway contraction in the guinea pig.