Endothelin-induced mobilization of Ca2+ and the possible involvement of platelet activating factor and thromboxane A2

Platelet-activating factor enhanced the pressor response of endothelin-1

Both endothelin-1 (ET-1) and platelet-activating factor (PAF) have been suggested to play a role in the regulation of the cardiovascular system. In view of the limited data regarding the interaction between ET-1 and PAF, the hemodynamic effects of ET-1 and PAF, either alone or in combination, were investigated in the current study. Anesthetized male Sprague Dawley rats received bolus intravenous injections of ET-1 (1 and 2 nmol/kg) and/or PAF (0.075, 0.15 and 0.3 nmol/kg). In some experiments, the ET receptor antagonist, FR-139317 (2.5 or 5 mg/kg), were injected 5 min before the administration of ET-1 or PAF. ET-1 caused a biphasic response consisting of an initial depressor followed by a delayed but sustained pressor response. Injection of PAF to anesthetized rats resulted only in a decrease in arterial blood pressure. Interestingly, the pressor effect of ET-1 was significantly enhanced in the concomitant presence of PAF. Pretreatment with FR-139317 inhibited the magnitude of ET-1-induced hypertension and increased the duration of the depressor action of ET-1. The time-course of PAF-induced decrease of arterial blood pressure was also prolonged in rats pretreated with FR-139317. These data therefore suggested that ET receptors were activated, either directly or indirectly, by PAF, possibly to facilitate the return of blood pressure to resting level following a depressor response. Thus the activation of ET receptors by PAF might result in the enhancement of the pressor response of ET-1 observed in the current study.

Endothelin-1 increases cholinergic nerve-mediated contraction of human bronchi via tachykinin synthesis induction

1. In some asthmatics, muscarinic receptor antagonists are effective in limiting bronchoconstrictor response, suggesting an abnormal cholinergic drive in these subjects. There is a growing body of evidences indicating that cholinergic neurotransmission is also enhanced by endothelin-1 (ET-1) in rabbit bronchi, mouse trachea and in human isolated airway preparations. 2. We investigated the role of secondary mediators in ET-1 induced potentiation of cholinergic nerve-mediated contraction in human bronchi, in particular the possible role of neuropeptides in this phenomenon. 3. Bronchial tissues after endothelin treatment were exposed to a standard electrical field stimulation (EFS) (30% of EFS 30 Hz)-induced contraction. In addition, in some experiments, preparations were treated with a tachykinin NK(2) receptor antagonist and subsequently exposed to the same protocol. HPLC and RIA were performed on organ bath fluid samples. Moreover, the human bronchi were used for the beta-PPT (preprotachykinin) mRNA extraction and semiquantitative reverse transcription polymerase chain reaction (RT - PCR), prior to and 30-40 min following ET-1 challenge. 4. The selective tachykinin NK(2) receptor antagonist, SR48968, was effective to reduce ET-1 potentiation of EFS mediated contraction. HPLC or RIA showed significant increased quantities of NKA in organ bath effluents after EFS stimulation in bronchi pretreated with ET-1. Finally, beta-PPT mRNA level after stimulation of bronchi with ET-1 was increased about 2 fold respect to control untreated bronchi. 5. In conclusion, this study demonstrated that, at least in part, the ET-1 potentiation of cholinergic nerve-mediated contraction is mediated by tachykinin release, suggesting that in addition to nerves, several type of cells, such as airway smooth muscle cell, may participate to neuropeptide production.

Endothelin stimulates phosphoinositide hydrolysis and PAF synthesis in brain microvessels

Treatment of brain microvessels with the three endothelin (ET) isoforms resulted in an increase of phosphoinositide turnover by activation of phospholipase C in a dose- and time-dependent manner. Both ET-1 and ET-2 are maximally effective, whereas the effect evoked by ET-3 was smaller. Concomitantly, there was an enhanced production of a platelet-activating factor (PAF)-like material. This was identified by standard and biological probes in platelets, such as induction of aggregation, phosphatidic acid (PA) production, increase of endogenous protein phosphorylation, and reversal of these responses by a PAF antagonist. The effects evoked by endothelins on phosphoinositide metabolism and PAF production were, to a certain extent, dependent on the presence of extracellular Ca2+. In addition, ET induced changes in Ca2+ dynamics, evoking an initial and rapid intracellular mobilization and influx of Ca2+ and, later, a maintained Ca2+ influx. These findings contribute to the understanding of the pathophysiological role of ET in the blood-brain barrier (BBB).

Role of asphyxia and feeding in a neonatal rat model of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a common gastrointestinal disorder affecting premature infants. To investigate critically the importance of the purported risk factors of NEC (formula feeding, asphyxia, bacteria, and prematurity), we developed a neonatal rat model that closely mimics the human disease. Full-term and premature newborn rats were stressed with formula feeding, asphyxia, and/or exogenous bacterial colonization and subsequently evaluated grossly and histologically for the development of intestinal injury. We found that most animals treated with asphyxia, formula feeding, and bacteria developed NEC (77%) and died (86%) by 96 h. All maternally fed animals treated with asphyxia and bacterial colonization survived and had normal intestinal histology. Furthermore, asphyxia was a critical instigating factor, because formula and bacterial exposure without asphyxia resulted in normal intestine and minimal mortality (12%). Enteral bacterial colonization was not a significant determinant of NEC in this model. We conclude that the neonatal rat model is an excellent test system for the study of NEC. As in the human disease, asphyxia and formula feeding play an important role in the pathophysiology of experimental NEC.

Endothelin-1-induced myocardial ischaemia and oedema in the rat: involvement of the ETA receptor, platelet-activating factor and thromboxane A2

1. The objectives of the present experiments were to assess the role of ETA receptors in mediating endothelin-1 (ET-1)-induced myocardial ischaemia and oedema and to study the involvement of platelet-activating factor (PAF) and thromboxane A2 (TxA2) in these actions of ET-1 in rats. 2. Intravenous bolus injection of ET-1 (0.1-2 nmol kg-1) into anaesthetized rats induced ST segment elevation of the electrocardiogram in a dose-dependent manner without causing arrhythmias. ST segment elevation developed within 20-90 s and persisted for at least 10-20 min following administration of ET-1. 3. Pretreatment of the animals with the selective endothelin ETA receptor antagonist, FR 139317 (2.5 mg kg-1, i.v.) inhibited by 86% the ST segment elevation elicited by ET-1 (1 nmol kg-1). Pretreatment with intravenous administration of BM 13505 (1 mg kg-1), a TxA2 receptor antagonist, OKY-046 (10 mg kg-1), a thromboxane synthase inhibitor or the specific PAF receptor antagonist, WEB 2086 (1 mg kg-1) or BN 52021 (10 mg kg-1) markedly suppressed ST segment elevation in response to ET-1. Infusion of indomethacin (3 mg kg-1 bolus plus 2 mg kg-1 h-1) did not significantly affect ET-1-induced ST segment elevation. 4. Bolus injection of ET-1 (1 nmol kg-1, i.v.) to conscious rats resulted in a prolonged pressor effect preceded by a transient depressor response. Corresponding to changes in blood pressure, a small transient tachycardia was followed by a sustained bradycardia. ET-l enhanced albumin leakage by 87 and 120% in the left ventricle and right atrium, respectively, as measured by the extravasation of Evans blue dye.5. The selective ETA receptor antagonist, FR 139317 (2.5 mg kg-1) significantly blunted the pressor action of ET-1 and the accompanying bradycardia without affecting the depressor response. Furthermore,FR 139317 almost completely abolished the permeability effect of ET-l in both vascular beds studied.6. Pretreatment of the animals with BM 13505 (1 mg kg-1), OKY-046 (10mg kg-1), WEB 2086(1 mg kg-1) or BN 52021 (10mg kg-1) significantly reduced ET-1 (1 nmol kg-1)-induced albumin extravasation both in the left ventricle and right atrium. The PAF receptor antagonists, WEB 2086 and BN 52021 were equally potent inhibitors in the left ventricle, whereas BN 52021 appeared to be a more potent inhibitor than WEB 2086 in the right atrium. Pretreatment with indomethacin (3 mg kg-1 plus 2 mg kg-1 h-1) did not modify the permeability response to ET-1. None of these compounds affected significantly ET-l-induced changes in mean arterial blood pressure and heart rate.7. These results indicate that intravenous administration of ET-1 provokes ST segment elevation and myocardial oedema and suggest that these events are mediated, in part, through release of secondary mediators, such as PAF and TxA2 via the activation of ETA receptors.

A role for platelet-activating factor in endothelin-1-induced rat mesangial cell proliferation

Endothelin-1 stimulated the release of bioassayable platelet-activating factor-like material and the incorporation of acetate to PAF in rat mesangial cells, in a dose-dependent manner, with an EC50 of about 10(-9) M. Endothelin-1 also stimulated dose dependently [3H]thymidine incorporation into DNA, protein synthesis and cell growth. The platelet-activating factor antagonists BN-52021 (10(-5) M), and alprazolam (10(-5) M), reduced significantly endothelin-1-stimulated thymidine incorporation into DNA, protein synthesis and cell growth. Platelet-activating factor also stimulated significantly thymidine incorporation into DNA, cell proliferation and protein synthesis. In conclusion, the present results suggest that endothelin-1 induces mesangial cell proliferation through a mechanism involving among others, synthesis and release of platelet-activating factor by these cells.

Effect of endothelin 1 on ion transport in isolated rat colon

BACKGROUND

Endothelin may play a significant role in the regulation of gastrointestinal function because it has a variety of biological activities and because endothelin-like immunoreactivity as well as its specific binding sites have been found in the gastrointestinal tract. This study investigated the secretory effect and mechanism of action of endothelin 1 in mammalian large intestine.

METHODS

Distal colonic segments from Sprague-Dawley rats were stripped of their muscle layers and mounted in Ussing chambers. The effects of endothelin 1 on short-circuit current in rat colonic mucosa were studied in the absence or presence of specific inhibitors. Transmural unidirectional 22Na+ and 36Cl- fluxes and endothelin 1-induced prostacyclin release were also measured.

RESULTS

Serosal addition of endothelin 1 evoked a sustained increase in short-circuit current that was significantly reduced by tetrodotoxin or atropine, and virtually abolished by a selective endothelin A receptor antagonist (BQ-123), furosemide, piroxicam, d,I-verapamil, or removal of serosal calcium. Hexamethonium, amiloride, diphenhydramine, or a specific platelet-activating factor antagonist (CV-6209) did not influence the response to endothelin 1. Endothelin 1 significantly decreased net sodium and net chloride absorption and induced a marked increase in prostacyclin release from the serosal surface of stripped colonic mucosa.

CONCLUSIONS

Endothelin 1 has a secretory effect in rat colon. Its action seems to be mediated by cyclo-oxygenase products and enteric nerves via the activation of an endothelin A receptor.

Endothelin-1-induced prostaglandin E2 production: modulation of contractile response to endothelin-1 in porcine coronary artery

Endothelin-1 (ET-1, 1 nM) increased the release of prostaglandin E2 (PGE2) in endothelium-denuded smooth muscle strips of porcine coronary arteries. Indomethacin enhanced the amplitude of contraction induced by ET-1 and inhibited the stimulated release of PGE2. PGE2 (0.1-100 nM) attenuated the amplitude of contraction induced by 1 nM ET-1. These results suggest that in the smooth muscle of porcine coronary arteries, ET-1 increased the synthesis of PGE2, which functionally antagonizes the direct vasoconstrictor actions of ET-1.

Interaction between endothelium-derived relaxing factors, S-nitrosothiols, and endothelin-1 on Ca2+ mobilization in rat vascular smooth muscle cells

S-Nitrosothiols (S-nitrosocysteine, S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine), which belong to the group of endothelium-derived relaxing factors (EDRFs), caused decreases of cytosolic free Ca2+ concentrations ([Ca2+]i) in cultured rat vascular smooth muscle cells (VSMCs). The endothelin-1 (ET-1)-induced sustained increase of [Ca2+]i in rat VSMCs was completely abolished by preaddition of at least an equal molar quantity of S-nitrosocysteine (Cys-SNO). Also exposure of VSMCs to a mixture of Cys-SNO and ET-1 at the same time resulted in the transient increase only. These results suggest that S-nitrosothiols may have no significant effect on ET-1-induced Ca2+ release from intracellular stores via inositol 1,4,5-triphosphate production but do affect Ca2+ influx through Ca2+ channels in the plasma membrane.

Effects of endothelin-1 on vascular permeability in the conscious rat: interactions with platelet-activating factor

1. The objectives of the present experiments were to assess the effects of endothelin-1 on the macrovascular permeability in selected vascular beds, to study the involvement of platelet-activating factor (PAF) in vascular responses to endothelin-1 and to examine the vascular effects of combined administration of endothelin-1 and PAF in conscious rats. 2. Intravenous bolus injection of endothelin-1 (0.1-2 nmol kg-1) resulted in a dose-dependent biphasic change in mean arterial blood pressure (MABP) with initial transient hypotension followed by a prolonged pressor action. These changes were accompanied by a dose-dependent increase in haematocrit values. 3. Endothelin-1 (0.1 and 1 nmol kg-1) increased dose-dependently the vascular permeability of the trachea, upper and lower bronchi, stomach, duodenum, spleen and kidney (up to 240%) as measured by the extravasation of Evans blue dye. The permeability of pulmonary parenchyma, liver and pancreas was not affected significantly by endothelin-1 treatment. 4. Pretreatment of animals with the specific PAF receptor antagonist, WEB 2086 (1 mg kg-1, i.v.) or BN 52021 (10 mg kg-1, i.v.) reduced the endothelin-1 (1 nmol kg-1)-induced rise in haematocrit by about 50 and 30%, respectively. Both antagonists were highly effective at inhibiting protein extravasation in the stomach, duodenum and kidney. On the other hand, BN 52021, but not WEB 2086, significantly attenuated the effect of endothelin-1 on permeability in the lower bronchi and spleen. Neither WEB 2086 nor BN 52021 modified the changes in MABP evoked by endothelin-1.5. When low doses of endothelin-1 (0.1 nmolkg-')and PAF (0.19nmolkg-')were administered simultaneously, enhanced protein extravasation was detected in the upper and lower bronchi, whereas neither endothelin-1 nor PAF by themselves affected vascular permeability in these tissues. These changes occurred in the absence of significant changes in MABP.6. Combined administration of higher doses of endothelin-1 (1nmolkg-') and PAF (1.9nmolkg-') resulted in marked increases (up to 530%) in protein extravasation in the airways, pancreas, stomach and duodenum. The effect of endothelin-1 on permeability was not affected by PAF in the spleen, whereas it was completely inhibited by PAF in the kidney. Combined injection of endothelin-1 and PAF resulted in a slight, but significant increase in MABP.7. The present findings show that endothelin-1 is capable of increasing vascular permeability in selected vascular beds including the airways, gastrointestinal tract and kidney, and suggest that PAF may mediate, in part, its action on permeability, but not its hypotensive action. The present data also suggest that endothelin-1 and PAF can act in concert to increase vascular permeability in rat airways and gastrointestinal tract.

Endothelin: a potent stimulator of intestinal ion secretion in vitro

Effects of endothelin (ET) on electrical properties and Na+ and Cl- fluxes in stripped rabbit ileal mucosa were investigated in vitro in Ussing chambers. Results demonstrate that serosal addition of ET-1, ET-2, ET-3 or the precursor 38 amino acid 'big endothelin' produce dose-dependent increases in short-circuit current (Isc) with maximal effects at approx. 100 nM, 100 nM, 10 nM and 100 nM, respectively and half-maximal effects at 1.4 nM, 5 nM, 1.4 nM and 20 nM, respectively. Mucosal addition of ET-3 failed to elicit a response. Changes in Isc elicited by ET-3 are accompanied by decreases in net fluxes of both Na+ and Cl-. The cyclooxygenase inhibitors, indomethacin and piroxicam, inhibited the increase in Isc produced by ET-3 and indomethacin also abolished the changes in Na+ and Cl- fluxes produced by ET-3. However, no changes in the release of PGE2, thromboxane B2 or 6-keto-prostaglandin F1 alpha could be detected up to 20 min after the addition of ET-3. Preincubation of tissues with neuronal agonists or antagonists, antihistamines or an LTD4/LTE4 receptor antagonist, SKF 104353, failed to alter the response to ET-3. Furthermore, removal of serosal Ca2+ also failed to inhibit the change in Isc produced by ET-3. These results indicate that endothelin is a potent intestinal secretagogue which does not appear to elicit its response through stimulation of PGE2, thromboxane A2 or prostacyclin.

Renal effects and mesangial cell contraction induced by endothelin are mediated by PAF

The recently discovered vasoactive peptide endothelin and platelet activating factor (PAF) share similar renal effects. The purpose of the present series of experiments has been to analyze the functional relations between the effect of endothelin on renal function and glomerular and mesangial cell contraction and the production and actions of PAF in kidney. Endothelin 1 nmol/kg body wt induced a transient decrease of glomerular filtration rate (from 1.99 +/- 0.17 to 0.56 +/- 0.18 ml/min) and renal blood flow (from 10.8 +/- 1.3 to 2.7 +/- 0.3 ml/min). Endothelin also induced a marked reduction of planar cell surface area of cultured mesangial cells (30 +/- 5%) and of cross sectional area of isolated glomeruli (from 1.51 +/- 0.02 to 1.31 +/- 0.02 m2 x 10(-8]. BN-52021 or WEB-2170, two potent specific inhibitors of PAF receptor binding, blocked the effects of endothelin on renal function and on the contraction of isolated glomeruli and mesangial cells. In addition, endothelin induced a significant increase in the production of PAF by isolated glomeruli (Basal, 81 +/- 10 pg/mg protein; endothelin, 10(-7) M, 140 +/- 18 pg/mg protein). Endothelin also stimulated the incorporation of [3H]acetate into PAF, both in glomeruli (264.27 +/- 27.7%) and mesangial cells (251 +/- 41%). These effects were blocked by EGTA and by verapamil. Our results suggest that PAF may be a mediator of the effects of endothelin on renal function and glomerular and mesangial cell contraction.

Endothelin-1 and platelet activating factor stimulate thromboxane A2 biosynthesis in rat vascular smooth muscle cells

The effect of endothelin-1 (ET-1) on the release of thromboxane A2 (TXA2) was examined in cultured rat vascular smooth muscle cells (VSMC). ET-1 (10(-11) to 10(-6) M) significantly stimulated the release of thromboxane B2 (TXB2), a stable metabolite of TXA2. These effects of ET-1 were blocked by a cyclooxygenase inhibitor (indomethacin), a TXA2 synthetase inhibitor (CV-1451) and a specific platelet activating factor (PAF) antagonist (CV-6209). Additionally, PAF (10(-11) to 10(-6) M) stimulated the TXB2 release. Pretreatment with the phospholipase A2 inhibitor dexamethasone potently inhibited both ET-1 and PAF-induced elevation of cytosolic free Ca2+ concentrations [( Ca2+]i) in fura-2-loaded VSMC. These results clearly demonstrate that both ET-1 and PAF stimulate TXA2 biosynthesis in cultured rat VSMC, and TXA2 may contribute to the elevation of [Ca2+]i induced by ET-1 or PAF in VSMC. Furthermore, the stimulation of TXA2 biosynthesis may be a result of PLA2 activation by not only ET-1 but also PAF.

Receptor-linked early events induced by vasoactive intestinal contractor (VIC) on neuroblastoma and vascular smooth-muscle cells

Vasoactive intestinal contractor (VIC) caused a series of biochemical events, including the temporal biphasic accumulation of 1,2-diacylglycerol (DAG), transient formation of Ins(1,4,5)P3, and increase in intracellular free Ca2+ [( Ca2+]i) in neuroblastoma NG108-15 cells. In these cellular responses, VIC was found to be much more potent in NG108-15 cells than in cultured rat vascular smooth-muscle cells. The single cell [Ca2+]i assay revealed that in the presence of nifedipine (1 microM) or EGTA (1 mM), the peak [Ca2+]i declined more rapidly to the resting level in VIC-stimulated NG108-15 cells, indicating that the receptor-mediated intracellular Ca2+ mobilization is followed by Ca2+ influx through the nifedipine-sensitive Ca2+ channel. Pretreatment with pertussis toxin only partially decreased Ins(1,4,5)P3 generation as well as the [Ca2+]i transient induced by VIC, whereas these events induced by endothelin-1 were not affected by the toxin, suggesting involvement of distinct GTP-binding proteins. The VIC-induced transient Ins(1,4,5)P3 formation coincident with the first early peak of DAG formation suggested that PtdIns(4,5)P2 is a principal source of the first DAG increase. Labelling studies with [3H]myristate, [14C]palmitate and [3H]choline indicated that in neuroblastoma cells phosphatidylcholine (PtdCho) was hydrolysed by a phospholipase C to cause the second sustained DAG increase. Down-regulation of protein kinase C (PKC) by prolonged pretreatment with phorbol ester markedly prevented the VIC-induced delayed DAG accumulation. Furthermore, chelation of intracellular CA2+ completely abolished the second sustained phase of DAG production. These findings suggest that PtdCho hydrolysis is responsible for the sustained production of DAG and is dependent on both Ca2+ and PKC.

Role of thromboxane receptor activation in the bronchospastic response to endothelin

The selective TxA2/PGH2 (TP) receptor antagonist, SQ 30,741, was used to test the hypothesis that TP-receptor activation contributes to the reactivity of airways and isolated trachea to endothelin-1 (ET-1). Dose-dependent contractions of guinea pig tracheal strips to ET-1 in vitro were unaffected by either SQ 30,741 (1 microM) or indomethacin (2.8 microM). In contrast, maximal bronchospastic responses (increases in airways resistance and decreases in dynamic lung compliance) of anesthetized guinea pigs to ET-1 (0.5 and 1.5 nmole/kg i.v.) in vivo were blocked greater than 90% by SQ 30,741 (1 mg/kg i.v.). Concurrent increases in arterial blood pressure and decreases in leukocyte counts induced by ET-1 were unaffected by SQ 30,741. In rats, ET-1 (1.5 nmole/kg i.v.) did not affect lung mechanics, but did cause biphasic blood pressure and leukopenia responses which were unaltered by SQ 30,741. These data demonstrate that there is considerable species variability in the bronchospastic response to ET-1, and that in guinea pigs, this response is caused predominantly by the activation of TP-receptors.

Endothelin-1 increases intracellular calcium mobilization but not calcium uptake in rabbit vascular smooth muscle cells

Conflicting evidence has been reported regarding the role of endothelin-1, a potent vasconstrictor peptide, in stimulating extracellular calcium influx in rabbit vascular smooth muscle. The objective of this study was to elucidate the effects of endothelin-1 on transmembrane 45Ca2+ influx and intracellular calcium mobilization in cultured rabbit aortic smooth muscle cells. In calcium containing buffer, endothelin-1 induced a concentration-dependent 45Ca2+ efflux response over the range of 10 pM to 100 nM with an EC50 of approximately 60 pM. Maximum endothelin-stimulated 45Ca2+ efflux was not affected by the absence of extracellular calcium or the presence of 1 microM verapamil. Endothelin-1 did not induce transplasmalemmal 45Ca2+ uptake at times up to 30 min. These findings suggest that an alteration in intracellular calcium handling, rather than extracellular calcium influx, is responsible for the endothelin-stimulated increase in intracellular calcium concentration in rabbit aortic smooth muscle cells.

Endothelin-induced sudden death and the possible involvement of platelet activating factor (PAF)

Endothelin (5 nmol/kg, i.v.) caused a transient hypotension followed by a lasting hypertension in rats. However, an abrupt fall in the blood pressure was observed in most rats 6 to 30 min after the injection of endothelin and sudden death followed with lethality noted over 60 min. An abnormal electrocardiogram (ECG) (ventricular arrhythmias) was observed in rats injected with endothelin. Endothelin (i.v.) also caused sudden death in mice. Pretreatment (5 or 60 min) with specific PAF antagonists, CV-6209 (0.1-3 mg/kg, i.v.) and WEB 2086 (30 mg/kg, p.o.), and a calcium channel blocker, diltiazem (60 mg/kg, p.o.) prevented death and attenuated the ECG changes induced by endothelin, but CV-6209 did not prevent the blood pressure changes induced by endothelin. CV-6209 (0.5-3 mg/kg, i.v.), WEB 2086, diltiazem and dexamethasone (5 mg/kg, i.v.) protected mice against the death induced by endothelin. On the other hand, aspirin (cyclooxygenase inhibitor, 100 mg/kg, p.o.) did not protect mice from the death. Thus, endothelin is a highly toxic peptide with cardiotoxic effects, and PAF may be involved in the pathogenesis of the sudden death.