Functional evidence for the presence of a phosphoramidon-sensitive enzyme in rat brain that converts big endothelin-1 to endothelin-1

Endothelin-1 (ET-1) is produced from its precursor, big endothelin-1 (BigET-1), by a putative endothelin-converting enzyme (ECE), but it is not known whether the enzyme is present in the brain. This study was conducted to examine the central hemodynamic effects of BigET-1 and to indirectly determine the presence of an ECE in rat brain. Cardiovascular effects of centrally administered BigET-1 and ET-1 were examined in anesthetized, ventilated rats. BigET-1 (100 pmol) or ET-1 (10 pmol) applied to the IV ventricle produced similar prolonged decreases in mean arterial pressure (MAP) and renal blood flow (RBF). Thus, peak decreases with BigET-1 were (mean +/- S.E.): MAP = -35 +/- 4%; RBF = -27 +/- 5%, while those with ET-1 were: MAP = -36 +/- 5%; RBF = -29 +/- 9%. Pretreatment with phosphoramidon, a metalloprotease inhibitor (90 nmol), abolished the hemodynamic responses elicited by BigET-1 (MAP = -9 +/- 2%; RBF = -3 +/- 2%) but not those produced by ET-1. These data indicate that; i) conversion of BigET-1 to ET-1 in the brain is essential for the expression of hemodynamic actions and ii) a metalloprotease capable of converting BigET-1 to ET-1 is present in rat brain.     

Phosphoramidon inhibits the vasoconstrictor effects evoked by big endothelin-1 but not the elevation of plasma endothelin-1 in vivo

Intravenous injections of big endothelin (ET)-1 (700 pmol/kg) in the pig increased arterial plasma levels of ET-1-like immunoreactivity (ET-1-LI) from 11.1 +/- 0.7 pM to 46.3 +/- 6.7 pM in the control situation and from 11.5 +/- 0.4 pM to 58.2 +/- 17 pM in the presence of the neutral endopeptidase inhibitor phosphoramidon (3 mg/kg). Big ET-1 increased splenic vascular resistance by 29% in the control situation. The vasoconstriction evoked by big ET-1 in the spleen was reduced after phosphoramidon treatment whereas the elevation of arterial ET-1-LI was not influenced. Furthermore the splenic vasoconstriction evoked by ET-1 was reduced after phosphoramidon without influencing plasma ET-1-LI. Also in rats the pressor effect of big ET-1 (1 nmol/kg) was inhibited by phosphoramidon (5 mg/kg) whereas the elevation of plasma ET-1 was not influenced. It is concluded that the vasoconstrictor effects of both big ET-1 and ET-1 are inhibited, but the increase in plasma ET-1 is unaffected by phosphoramidon.     

Detection and characterization of endothelin-1-like immunoreactivity in rat plasma

Using two radioimmunoassays (RIAs) for endothelin-1 (ET-1) with and without a substantial cross-reactivity with ET-3, we have measured the plasma ET-1-like immunoreactivity (-LI) level in rat plasma. ET-1-LI was detected in plasma from male Wistar rats. ET-1-LI in rat plasma consisted of three components with molecular weights of 6K, 4K and 2.5K daltons by gel permeation chromatography. Two of the components were eluted at positions of big ET (4K) and synthetic ET-1 (2.5K). The remaining component was eluted at the preceding fraction (6K). No difference was observed in ET-1-LI of the small molecular form of ET (2.5K) between the two RIAs. Thus, there is little or no ET-3 in rat plasma, which has the sequence found originally in the rat genome. The concentration of the small molecular form of ET, presumably ET-1, in rat plasma was about 4 pg/ml.     

Prevention of endothelin-1-induced increases in blood pressure: role of endogenous CGRP

To determine the role of endothelin-1 (ET-1) and its receptors in the regulation of calcitonin gene-related peptide (CGRP) release, male Wistar rats were divided into six groups and subjected to the following treatments for 1 wk with or without ABT-627 (an ET(A) receptor antagonist, 5 mg.kg(-1).day(-1) in drinking water) or A-192621 (an ET(B)-receptor antagonist, 30 mg.kg(-1).day(-1) by oral gavage): control (Con), ET-1 (5 ng.kg(-1).min(-1) iv), Con + ABT-627, Con + A-192621, ET-1 + ABT-627, and ET-1 + A-192621. Baseline mean arterial pressure (MAP, mmHg) was higher (P < 0.05) in Con + A-192621 (122 +/- 4) and ET-1 + A-192621 (119 +/- 4) groups compared with Con (104 +/- 6), ET1 (106 +/- 3), Con + ABT-627 (104 +/- 3), and ET1 + ABT-627 (100 +/- 3) groups. Intravenous administration of CGRP(8-37) (a CGRP receptor antagonist, 1 mg/kg) increased MAP (P < 0.05) in ET-1 (13 +/- 1), Con + A-192621 (12 +/- 1), and ET-1 + A-192621 (15 +/- 3) groups compared with Con (4 +/- 1), Con-ABT-627 (4 +/- 1), and ET-1 + ABT-627 (5 +/- 1) groups. Plasma CGRP levels (in pg/ml) were increased (P < 0.05) in ET-1 (57.5 +/- 6.1), Con + A-192621 (53.9 +/- 3.4), and ET-1 + A-192621 (60.4 +/- 3.0) groups compared with Con (40.4 +/- 1.6), Con + ABT-627 (40.0 +/- 2.9), and ET-1 + ABT-627 (42.6 +/- 1.9) groups. Plasma ET-1 levels (in pg/ml) were higher (P < 0.05) in ET-1 (2.8 +/- 0.2), ET-1 + ABT-627 (3.2 +/- 0.4), Con + A-192621 (3.3 +/- 0.4), and ET-1 + A-192621 (4.6 +/- 0.3) groups compared with Con (1.1 +/- 0.2) and Con-ABT-627 (1.3 +/- 0.2) groups. Therefore, our data show that ET-1 infusion leads to increased CGRP release via activation of the ET(A) receptor, which plays a compensatory role in preventing ET-1-induced elevation in blood pressure.     

Obligatory role of the endocardial endothelium in the increase of myocardial distensibility induced by endothelin-1

This study investigated how the endocardial endothelium (EE) and particularly endothelial type B (ET(B)) receptors influence the effects of endothelin-1 (ET-1) on diastolic distensibility. ET-1 (0.1, 1, and 10 nM) was tested in rabbit papillary muscles (Krebs-Ringer; 1.8 mM CaCl2, 35 degrees C) (i) with intact EE (n = 10), (ii) with damaged EE (0.5% Triton X-100, n = 11), and (iii) in the presence of RES-701-1 (selective endothelial ET(B1) receptor antagonist, 1 microM, n = 6). Additionally, increasing doses (0.1 nM to 1 microM) of Sarafotoxin S6c (SRTXc, a selective ET(B) receptor agonist) and IRL-1620 (a selective endothelial ET(B1) agonist) were studied (i) in muscles with intact EE (n = 7 and n = 6, respectively) and (ii) after damaging the EE (n = 8 and n = 7, respectively). In papillary muscles with intact EE, ET-1 induced dose-dependent positive inotropic and lusitropic effects. At 10 nM, active tension (AT) increased 78% +/- 17%, maximum velocity of tension rise (dT/dt(max)) increased 82% +/- 10%, and maximum velocity of tension decline (dT/dt(min)) increased 77% +/- 17%. These effects were maintained when ET-1 was given after damaging the EE (AT increased 70% +/- 12%, dT/dt(max) increased 93% +/- 14%, and dT/dt(min) increased 56% +/- 14%), but were significantly reduced in the presence of RES-701-1 (AT increased 30% +/- 6%, dT/dt(max) increased 37% +/- 7%, and dT/ dt(min) increased 29% +/- 9%). ET-1 reduced resting tension (RT) and increased diastolic distensibility by 3% +/- 1%, 5% +/- 1%, and 9% +/- 2% (at 0.1, 1, and 10 nM, respectively) in muscles with intact EE. This effect was abolished after damaging the EE or in the presence of RES-701-1. In muscles with intact EE, SRTXc had no significant effects, whereas, when given after damaging the EE, SRTXc (1 microM) increased inotropy and lusitropy (AT increased 116% +/- 24%, dT/dt(max) 110% +/- 28%, and dT/dt(min) 88% +/- 19%) without affecting RT. IRL-1620 dose-dependently decreased AT, dT/dt(max), and dT/dt(min) in muscles with intact EE-effects that were abolished after EE damage. No significant effects were elicited by IRL-1620 in RT. ET-1-induced increase in myocardial distensibility, previously shown to be mediated by ET(A) receptor stimulation, requires an intact EE and active endothelial ET(B1) receptors.     

Enhanced endothelin-1 response and receptor expression in small mesenteric arteries of insulin-resistant rats

Hyperinsulinemia, a primary feature of insulin resistance, is associated with increased endothelin-1 (ET-1) activity. This study determined the vascular response to ET-1 and receptor binding characteristics in small mesenteric arteries of insulin-resistant (IR) rats. Rats were randomized to control (C) (n = 32) or IR (n = 32) groups. The response to ET-1 was assessed (in vitro) in arteries with (Endo+) and without (Endo-) endothelium. In addition, arteries (Endo+) were pretreated with the ET(B) antagonist A-192621 or the ET(A) antagonist A-127722. Finally, binding characteristics of [(125)I]ET-1 were determined. Results showed that in Endo+ arteries the maximal relaxation (E(max)) to ET-1 was similar between C and IR groups; however, the concentration at 50% of maximum relaxation (EC(50)) was decreased in IR arteries. In Endo- arteries, the E(max) to ET-1 was enhanced in both groups. Pretreatment with A-192621 enhanced the E(max) and EC(50) to ET-1 in both groups. In contrast, A-127722 inhibited the ET-1 response in all arteries in a concentration-dependent manner; however, a greater ET-1 response was seen at each concentration in IR arteries. Maximal binding of [(125)I]ET-1 was increased in IR versus C arteries although the dissociation constant values were similar. In conclusion, we found the vasoconstrictor response to ET-1 is enhanced in IR arteries due to an enhanced expression of ET receptors and underlying endothelial dysfunction.     

Solubilization and characterization of endothelin-1 receptors in rat cardiac tissue

Adult rat cardiac endothelin-1 (ET-1) receptors were solubilized with 0.5% digitonin and then characterized. The receptors retained binding activity after solubilization. Binding was saturable (KD of 0.065 +/- 0.004 nM, Bmax of 94.6 +/- 4.5 fmol/mg protein; Hill coefficient of 0.987 +/- 0.017 n = 6) and pH dependent, with the binding increasing as the pH was decreased from 10 to 4, but decreasing dramatically as pH dropped to 2. Specifically bound [125I]-ET-1 was not dissociated by 2 x 10(-7) M unlabelled ET-1, but was dissociated by pH 10 and 2. Returning the pH to 7.4 restored the binding activity of the receptors. Unlabelled ET-1 (10(-12) - 10(-7) M) and sarafotoxin S6b(10(-12) - 10(-7) M) competed with [125I]-ET-1 for binding to the receptors.     

Plasma and urinary endothelin-1 concentrations in asphyxiated newborns [corrected

The aim of this study was to determine if there is any correlation between the hypoxia induced deterioration of renal functions and urinary excretions of endothelin (ET). Therefore using a sensitive and specific radioimmunoassay, we have investigated plasma ET-1 concentrations and urine ET-1 excretions in healthy and asphyxiated newborns. Sixteen newborns (10 boys, 6 girls) with perinatal asphyxia or hypoxia of variable seriousness which were followed at Newborn Intensive Care Unit in Eskisehir Osmangazi University Faculty of Medicine were enrolled. Simultaneously, gestation and weight matched 10 newborns (6 boys, 4 girls) with no asphyxia (first minute Apgar score >7) were enrolled as controls. Plasma ET-1 concentrations of the asphyxiated infants (61.8+/-79.3 pg/ml, between 23.4-125.2 pg/ml) were higher than in the control group (29.3+/-22.1 pg/ml, between 12.3 and 50.8 pg/ml, p<0.05). However creatinine clearance values were not different between the two groups (p>0.05), mean fractional excretion of sodium levels (FeNa%) were higher in the study group than the controls (p<0.01). Urinary ET-1 concentrations in the asphyxiated infants were 144.6+/-63.4 pg/ml versus 70.1+/-27.7 pg/ml in the control group (p<0.001). The ET clearance were more elevated in the asphyxiated newborns than in the healthy infants (p<0.05). Urinary ET-1/Cr ratio in the hypoxic infants were significantly elevated in the first day of life when compared with those of healthy infants (p<0.05). Total ET excretion was negatively correlated with FeNa (%) (r=-0.603, p<0.05). Plasma ET-1 concentrations of the asphyxiated infants reduced at 48 hours of age (p<0.001). Fifth minute Apgar score was negatively correlated with urinary ET-1 levels (r=-0.615, p<0.01), urinary Na excretion (r=-0.583, p<0.01), FeNa (%) (r=-0.597, p<0.01) and total ET excretion (r=-0.560, p<0.01) and positively correlated with ET clearance (r=0.559, p<0.05). Urinary ET-1 levels were negatively correlated with umbilical artery BE levels (r=-0.612, p<0.05). To our study, elevated urinary ET-1 levels were observed during perinatal asphyxia and urinary ET-1 levels were negatively correlated with 5th minute Apgar score and cord blood base excess levels. For this reason urinary ET-1 levels could be a marker of perinatal asphyxia as cord blood ET-1 levels. With investigations showing renal production is independent from plasma and increased urinary ET-1/Cr levels in newborn with perinatal asphyxia and also negative correlation between the total ET excretion and FeNa, urinary ET-1 levels could be served as a useful marker to detecting also impaired renal functions in infants with perinatal asphyxia.     

Partial characterisation and subcellular distribution patterns of endothelin-1, -2 and -3 binding sites in human liver.

For the first time, endothelin-1, -2 and -3 (ET-1, -2, -3) binding sites were characterized in human liver and shown to differ significantly in their respective dissociation constants and densities. In addition, subcellular distribution patterns of these binding sites in biochemically analysed fractions obtained after differential centrifugation were shown to be heterogeneous. Thus, the bulk of ET-1 and ET-3 binding sites seemed to be present in plasma membranes, although their partial presence in a compartment sedimenting together with the endoplasmic reticulum cannot be excluded. In contrast, a major proportion of the ET-2 binding sites appeared to be associated with a compartment sedimenting together with mitochondria, suggesting a special accumulation of ET-2 binding sites in human liver. A significant portion of ET-1, -2 and -3 binding sites seems to be localized also in lysosomes, presumably indicating their participation in the internalisation process.     

Endothelin-3 concentrations in human plasma: the increased concentrations in patients undergoing haemodialysis

Plasma immunoreactive endothelin-3 (ir-ET-3) concentrations were measured by a sandwich-enzyme immunoassay (sandwich-EIA) for endothelin-3 (ET-3). The assay method consists of two antibodies directed against N-terminal and C-terminal portions of ET-3. It detects as little as 0.1 pg/well of ET-3 without the crossreaction with endothelin-1, endothelin-2 and big ET-3. Plasma ir-ET-3 concentrations were found to be 0.45 +/- 0.07 pg/ml (mean +/- SD) in healthy volunteers, and were increased in patients undergoing haemodialysis (0.83 +/- 0.26 pg/ml, p less than 0.001). In reverse-phase HPLC, ir-ET-3 in normal plasma and in plasma of haemodialysis patients was eluted at the position of authentic ET-3, indicating that ir-ET-3 in plasma detected by the EIA was ET-3 itself. These results suggest that circulating ET-3 exists in normal human plasma and that production and/or metabolism of ET-3 may be altered in patients undergoing haemodialysis.     

Chronic endothelin-1 blockade reduces sympathetic nerve activity in rabbits with heart failure

Endothelin-1 (ET-1) is elevated in chronic heart failure (CHF). In this study, we determined the effects of chronic ET-1 blockade on renal sympathetic nerve activity (RSNA) in conscious rabbits with pacing-induced CHF. Rabbits were chronically paced at 320--340 beats/min for 3--4 wk until clinical and hemodynamic signs of CHF were present. Resting RSNA and arterial baroreflex control of RSNA were determined. Responses were determined before and after the ET-1 antagonist L-754,142 (a combined ET(A) and ET(B) receptor antagonist, n = 5) was administered by osmotic minipump infusion (0.5 mg. kg(-1) x h(-1) for 48 h). In addition, five rabbits with CHF were treated with the specific ET(A) receptor antagonist BQ-123. Baseline RSNA (expressed as a percentage of the maximum nerve activity during sodium nitroprusside infusion) was significantly higher (58.3 +/- 4.9 vs. 27.0 +/- 1.0, P < 0.001), whereas baroreflex sensitivity was significantly lower in rabbits with CHF compared with control (3.09 +/- 0.19 vs. 6.04 +/- 0.73, P < 0.001). L-754,142 caused a time-dependent reduction in arterial pressure and RSNA in rabbits with CHF. In addition, BQ-123 caused a reduction in resting RSNA. For both compounds, RSNA returned to near control levels 24 h after removal of the minipump. These data suggest that ET-1 contributes to sympathoexcitation in the CHF state. Enhancement of arterial baroreflex sensitivity may further contribute to sympathoinhibition after ET-1 blockade in heart failure.     

Antagonism of endothelin-1 inhibits hypoxia-induced apoptosis in cardiomyocytes

Apoptosis is well documented to be a common feature of many pathological processes of the heart. Exogenous endothelin-1 (ET-1) has been shown to be proapoptotic or antiapoptotic, depending on ET-1 concentration, cell type, and the ratio of ETA/ETB receptor subtypes. The role of endogenous ET-1 in cardiomyocyte apoptosis, however, is not clarified. This study observed the effects of the ETA-receptor antagonists BQ610 and BQ123 and the ETB-receptor antagonist BQ788 on hypoxia-induced apoptosis in primary cultured neonatal rat cardiomyocytes. Hypoxic apoptosis was induced by incubating cardiomyocytes in serum-free medium under 3% O2 and 5% CO2 for 24 h and evaluated by TUNEL analysis and flow cytometry. TUNEL analysis showed that the apoptotic cardiomyocytes constituted 24.2% +/- 2.2% of the total cells under hypoxic conditions. Treatment with BQ610 (5 micromol/L) significantly reduced the apoptosis rate to 13.2% +/- 3.7% (data from 4 independent experiments, p < 0.01 vs. hypoxia). Flow cytometry showed that the percentage of apoptotic cells positively stained with annexin V and propidium iodide was 42.76% +/- 4.44% (n = 12) in cultures subjected to hypoxia. BQ123 at 0.04, 0.2, and 1.0 micromol/L dose-dependently reduced the apoptosis rate to 34.00% +/- 10.35% (n = 6, p < 0.05), 30.38% +/- 8.28% (n = 6, p < 0.01), and 22.89% +/- 4.19% (n = 6, p < 0.01), respectively. In contrast, BQ788 did not affect hypoxic apoptosis. These findings suggested that endogenous ET-1 contributed to hypoxia-induced apoptosis in cultured cardiomyocytes, which was mediated by ETA receptors, but not by ETB receptors.     

Induction of the oxLDL receptor LOX-1 by endothelin-1 in human endothelial cells.

In this study, we analyzed the effect of endothelin-1 (ET-1) on expression of the lectin-like oxidized low-density lipoprotein (oxLDL) receptor-1 LOX-1 and on oxLDL uptake in primary cultures of human umbilical vein endothelial cells (HUVEC). LOX-1 mRNA was quantified by standard-calibrated competitive RT-PCR, LOX-1 protein expression by Western analysis and endothelial oxLDL uptake using DiI-labeled oxLDL. ET-1 induces LOX-1 mRNA expression, reaching its maximum after 1 h (160 +/- 14% of control, 100 nM ET-1, P < 0.05). This increased ET-1-mediated LOX-1 mRNA expression could be inhibited by endothelin receptor B antagonist BQ-788. In addition, ET-1 stimulates LOX-1 protein expression and oxLDL uptake in HUVEC. The augmented oxLDL uptake by ET-1 is mediated by endothelin receptor B, but not by protein kinases. These data support a new pathophysiological mechanism how locally and systemically increased ET-1 levels could promote LOX-1-mediated oxLDL uptake in human endothelial cells and the development and progression of endothelial dysfunction and atherosclerosis.     

Phosphoramidon inhibits the conversion of intracisternally administered big endothelin-1 to endothelin-1

It is suggested that endothelin-1 (ET-1), a potent vasoconstrictor peptide, is involved in the pathogenesis of cerebral vasospasm following subarachnoid hemorrhage (SAH). We examined the effects of intracisternal administration of big ET-1 on the cerebral arteries in the absence or presence of pretreatment with phosphoramidon, an inhibitor of ET converting enzyme, in anesthetized dogs. After intracisternal administration of big ET-1 (10 micrograms/dog), the caliber of the basilar artery on the angiogram was decreased to about 59% of the control. This was accompanied by a marked increase in immunoreactive ET in the cerebrospinal fluid. Systemic arterial pressure was markedly elevated following big ET-1 injection. All changes induced by big ET-1 were effectively prevented with phosphoramidon. These data suggest that intracisternally administered big ET-1 is converted to ET-1 and that the generated ET-1 produces cerebral vasospasm and hypertension. A phosphoramidon-sensitive metalloproteinase appears to contribute to this conversion.     

Presence of endothelin-1-like immunoreactivity in human cerebrospinal fluid

By use of radioimmunoassay (RIA) specific for endothelin-1 (ET-1), we have studied whether ET-1-like immunoreactivity (LI) exists in human cerebrospinal fluid (CSF). Human CSF from patients with old cerebrovascular disease contained far greater amounts of ET-1-LI (28 +/- 1.3 pg/ml, n = 9) than normal human plasma. Reverse-phase HPLC coupled with ET-1 RIA revealed a single major ET-1-LI component in human CSF extract coeluting with standard ET-1. These data suggest ET-1 is a neuropeptide secreted from neural tissues in humans.     

Distribution and molecular form of immunoreactive big endothelin-1 in porcine tissue

In the present study a specific and sensitive radioimmunoassay for big endothelin-1 was developed. Half maximal inhibition of binding of radioiodinated big endothelin-1 was observed at 58 pg/tube and big endothelin-1 was detectable as low as 2 pg/tube. With this assay, the regional distribution of big endothelin-1 was determined in porcine tissue and compared to the distribution of an immunoreactive endothelin. Considerable amount of immunoreactive big endothelin-1 was observed in the aortic intima (0.84 +/- 0.094 pg/mg wet tissue; mean +/- S.D.) and the lung (0.47 +/- 0.055), but there was a low concentration in other tissue including the kidney inner medulla, which has been shown to be abundant in immunoreactive endothelin. Furthermore the molecular form of immunoreactive big endothelin-1 in aortic intima was found to be big ET-1[1-39], but the molecular form of major immunoreactive big endothelin-1 in the lung is big endothelin-1[22-39] with big endothelin-1[1-39] being minor.     

Intracoronary endothelin-1 infusion combined with systemic isoproterenol treatment: antagonistic arrhythmogenic effects

Endothelin-1 secretion and sympathetic activation may play important role in cardiovascular pathophysiology. In vivo interactions between these systems are not defined. We aimed to study the electrophysiological and haemodynamic effects of simultaneous intracoronary endothelin-1 and intravenous isoproterenol infusions. 18 anaesthetised open chest dogs were studied after AV-ablation. Mean arterial blood pressure, coronary blood flow, left ventricular contractility, standard electrocardiograms, right and left ventricular epi- and endocardial monophasic action potential (MAP) signals were recorded. Intracoronary endothelin-1 (30 pmol/min) was given to Group ET (n=6), intravenous isoproterenol (0.2 microg/kg/min) to Group ISO (n=6), both endothelin-1 and isoproterenol to Group ET+ISO (n=6) for 30 min. MAP duration increased in all studied regions of Group ET, decreased in all studied regions of Group ISO and ET+ISO (control vs. maximal changes of left ventricular epicardial MAP 90% duration, Group ET: 296+/-22 vs 369+/-20 ms, p<0.05, Group ISO: 298+/-18 vs 230+/-27 ms, p<0.01, Group ET+ISO: 302+/-18 vs 231+/-10 ms, p<0.01). In Group ET, early after depolarisations (3/6), polymorphic non-sustained ventricular tachycardias (6/6), and ventricular fibrillation (3/6) could be observed. In Group ISO, monomorphic non-sustained ventricular tachycardias (5/6) and atrial fibrillation (3/6) appeared. In Group ET+ISO, mono- and polymorphic non-sustained ventricular tachycardias occurred (5/6), neither ventricular fibrillation nor atrial fibrillation developed. An additive effect of endothelin-1 and isoproterenol on left ventricular contractility was observed. Isoproterenol treatment showed antagonistic effect against endothelin-1 induced MAP duration prolongation, early after depolarisation and ventricular fibrillation formation, while endothelin-1 showed protective effect against the development of isoproterenol induced atrial fibrillation.     

Endothelin-1 and endothelin-3 stimulate prostaglandin E2 secretion from the rat median eminence.

The in vitro effects of endothelin-1 (ET-1) and endothelin-3 (ET-3) on the release of prostaglandin (PG)E2 from the rat median eminence were investigated. The addition of ET-1 from 10(-9) M to 10(-6) M stimulated PGE2 release in a dose-dependent manner (from 10.5 +/- 2.1 to 54.4 +/- 5.6 pg/ME fragment/30 min; mean +/- SEM, p less than 0.001). ET-3 also stimulated the release of PGE2 from 10(-7) M to 10(-5) M dose dependently (from 18.1 +/- 0.7 to 60.9 +/- 17.4 pg/ME fragment/30 min p less than 0.05). The time course effect of ET-3 (10(-6) M) showed that PGE2 release was stimulated within five minutes (control, 1.5 +/- 0.5; ET-3, 15.8 +/- 3.0 pg/ME fragment/5 min, p less than 0.01). These results suggest that ET-1 and ET-3 have some physiological effects on the rat median eminence.     

Intrapericardial angiotensin II stimulates endothelin-1 and atrial natriuretic peptide formation of the in situ dog heart

Angiotensin (AT) II, endothelin (ET)-1, and atrial natriuretic peptide (ANP) play an important role in cardiovascular regulatory processes under physiologic and pathophysiologic conditions. All of these agents are present in the pericardial fluid, and alteration of their pericardial concentrations mirror changes in the myocardial interstitium. Moreover, the composition the pericardial fluid may also reflect the myocardial interaction of these agents. The local myocardial effects of AT II on cardiac ET-1 and ANP production, as well as on cardiovascular function, was studied by intrapericardial (ip) administration of AT II (0.125-1.0 microg/kg) to the in situ dog heart (n = 8). Big ET, ET-1, and ANP [1-28] fragment concentrations were measured by enzyme-linked immunosorbent assay in pericardial infusate samples and in peripheral blood before and after an AT II treatment of 15 mins. Systemic blood pressure (BP), heart rate (HR), and left ventricular contractility (dP/dt) were also recorded. In our studies, the pericardial big ET (but not ET-1) concentration was increased to a maximum value of 139 +/- 28 versus 74 +/- 12 pg/ml (control; P < 0.02) with ip AT II administration, with parallel elevations of the pericardial ANP levels (36.8 +/- 7.2 vs. 24.4 +/- 3.6 ng/ml; P < 0.05). The ip administration of AT II did not influence HR, and it elicited moderate changes in BP (BP(max), +14 +/- 2 mm Hg, P < 0.001; dP/dt(max), +10 +/- 3%, P < 0.02). The plasma levels of big ET, ET-1, and ANP did not change significantly. The results suggest that AT II promotes production of big ET and ANP in the heart. However, no detectable conversion of big ET-1 to ET-1 was observed within 15 mins. The myocardial formation of big ET-1 and ANP occurred, at least in part, independently of the changes in cardiovascular function.