Endothelin Isoforms Research Articles

Endothelium-mediated regulation of renin secretion

The aim of the present study was to investigate the endothelial influence on renin secretion of isolated juxtaglomerular cells. Specifically the role of nitric oxide (NO) and of endothelin was studied. Coculture of primary cultures of juxtaglomerular cells with aortic and microvascular endothelial cells decreased renin secretion. Inhibition of NO formation by absence of l-arginine or presence of N omega-nitro-l-arginine caused a marked decrease in cGMP accumulation and a reduction in renin secretion in cocultures. Exogenous NO (NO liberators sodium nitroprusside/SIN 1) stimulated the 20-hour renin secretion from juxtaglomerular cells markedly, too. The effect of NO on renin secretion was biphasic: short-time inhibition and long-time stimulation of renin release. NO's stimulatory effect on renin secretion is dependent on extracellular calcium, but independent on cAMP or cGMP accumulation. Endothelin 1, 2, and 3 did not affect basal renin secretion, but inhibited cAMP stimulated renin release to a similar extent. Endothelin's action is not mediated via the subtype A endothelin receptor, but seems to involve calcium mobilization in juxtaglomerular cells that is dependent on extracellular calcium and associated with prominent calcium activated chloride channels. Taken together, coculture of juxtaglomerular cells with endothelial cells inhibits renin secretion despite the stimulatory effect of native NO released from endothelial cells. cAMP stimulated renin secretion is inhibited by all three endothelin isoforms thus contributing to the inhibition of renin secretion in coculture.

Role of Na(+)-H+ exchange in mediating effects of endothelin-1 on normal and ischemic/reperfused hearts.

Endothelin (ET) has been shown to be elevated under conditions of cardiac pathology and to produce diverse cardiac effects, including coronary constriction and a positive inotropic influence. We characterized the concentration- and time-dependent effects of the most potent of the ET isoforms, ET-1 (0.4, 2, and 4 nmol/L), on myocardial contractility and coronary resistance and assessed its effects on the ischemic and reperfused heart. Because ET-1 has been shown to activate the Na(+)-H+ exchanger in cardiac myocytes, we determined the contribution of the antiport by examining the effects of ET-1 in the presence of the Na(+)-H+ exchange inhibitor methylisobutyl amiloride (MIA). At all three concentrations, ET-1 produced an initial positive inotropic effect that was reversed with continued perfusion, the degree of the reversal being dependent on ET-1 concentration. With 0.4 nmol/L, contractility returned to pre-ET-1 values, whereas after 75 minutes of perfusion with 4 nmol/L ET-1, contractility was depressed by 75%. At all concentrations, ET-1 produced a coronary-constricting effect, whereas an elevation in resting tension was observed only with 4 nmol/L ET-1. MIA significantly prevented the positive inotropic effect of ET-1 but had no effect on loss in function or elevation in resting tension produced by 4 nmol/L ET-1. Furthermore, MIA partially, but not significantly, attenuated the constricting effects of all ET-1 concentrations. In the ischemic heart, 0.4 nmol/L ET-1 appeared to delay the loss in contractility produced by cessation of flow, although the effect was not significant. Higher concentrations of ET-1 were without effect on ischemia-induced contractile depression, although their presence produced a marked elevation in resting tension during ischemia that was attenuated by MIA. Recovery in contractility was reduced by all concentrations of ET-1, although the effects of the lowest concentration were associated primarily with defective relaxation. The depressant effects of ET-1 either in normal or ischemic/reperfused hearts were irreversible. The inhibitory effects of ET-1 on contractile recovery were associated with diminished tissue glycogen and elevated lactate levels. High-energy phosphates after reperfusion were depressed in hearts treated with 4 nmol/L ET-1. The attenuation in contractile recovery and alterations in metabolite content were prevented by MIA. These results provide evidence that ET-1 produces complex effects on heart function that are likely mediated via different mechanisms and demonstrate its ability to aggravate ischemic and reperfusion injury through a mechanism possibly involving Na(+)-H+ exchange activation.

Involvement of angiotensin II and endothelin in matrix protein production and renal sclerosis

To review the evidence that links angiotensin II and endothelin as growth factors and as modifiers of extracellular matrix synthesis in renal cells, with particular reference to the effects of angiotensin converting enzyme (ACE) inhibition in models of renal injury. In cultured mesangial cells, both angiotensin II and endothelin promote contraction, proliferation/hypertrophy, signal transduction pathways, the activation of early growth genes, and the generation of inflammatory mediators, cytokines and growth factors. Both hormones have been shown to promote the synthesis of fibronectin and collagen in a dose-dependent manner. ACE inhibition attenuates the effect of endothelin-1, one of three isoforms of endothelin. In experimental models of renal injury, chiefly in those characterized by increased intraglomerular pressure, ACE inhibition has reduced proteinuria and glomerular and interstitial sclerosis. ACE inhibition has been shown to have major beneficial effects in patients with diabetic nephropathy, even in those with normal blood pressure. Although the renal-protective effects of ACE inhibitors in experimental and human renal injury may reflect systemic and/or local hemodynamic effects of these drugs, their modulatory actions on extracellular matrix synthesis and proteinuria may contribute to the benefit of ACE-inhibitor therapy.

Isolation and Characterization of Vascular Endothelial Cells Derived from Mice Lacking Endothelin-1

Microvascular endothelial cells were isolated from endothelin (ET)-1 knockout mice. The ET-1-lacking endothelial cells represent normal shape and activity of acetyl-LDL uptake. The growth of ET-1-lacking endothelial cells was not different from that of control cells in the presence of serum. Compensatory production of other ET isoforms does not occur in these cells. Conversion of big ET-1 to ET-1 is not different between ET-1-lacking and control cells. These results suggest that ET-1 is not essential to endothelial morphology and growth, that the expression of ET isoforms is not redundant in endothelial cells, and that the machinery processing ET-1 is preserved despite of the absence of its substrate.

Production of endothelin-1 and thrombomodulin by human pancreatic cancer cells.

Analysis of bioactive substances produced by cancer cells is one approach to understanding the biological features of human cancer. One of these bioactive substances is endothelin (ET)-1, a peptide with potent vasoconstrictive activity produced by vascular endothelial cells. We have previously reported the production of ET-1 by several types of human cancer, especially pancreatic cancer cells. To elucidate whether these cancer cells might share biological characteristics with vascular endothelial cells, we investigated the production of three ET isoforms in pancreatic cancer cells, using a specific radioimmunoassay. Further, we also investigated whether these cells produce thrombomodulin (TM), another product of endothelial cells functioning as a modulator of procoagulant activity. ET-1 was detected in 11 of 12 pancreatic cancer cell lines (92%) while ET-2 and ET-3 were detectable in only one cell line. Gel filtration analysis confirmed the presence of ET-1. Moreover, TM was detected in the cell lysates of 11 of the 12 cell lines (92%) and it was released into the culture medium in the majority (58%) of these cell lines. TM mRNA was also detected in these cells. In addition, TM was demonstrated immunocytochemically along the cell surface. These results suggest that pancreatic cancer cells share two characteristics with endothelial cells: the production of ET-1 and TM.

Pharmacological properties of endothelin receptor subtypes mediating positive inotropic effects in rabbit heart

The positive inotropic effect (PIE) of endothelin (ET) isoforms, ET-1 and ET-3, was similar in that 1) the PIE was associated with prolongation of isometric contractions, 2) the maximal response was approximately 60% of that to isoproterenol (Isomax), 3) the PIE was associated with acceleration of PI hydrolysis, and 4) it was selectively antagonized by phorbol 12,13-dibutyrate. Because the concentration-response curve for ET-1 was biphasic (whereas that for ET-3 was monophasic), ET-1 had a PIE greater than ET-3 up to 10(-8) M. ET-1 induced a PIE at 3 x 10(-14) M and higher, which reached a plateau of 10-20% of Isomax at 10(-12) M (first phase); the curve became steeper at 10(-9) M and higher (second phase), achieving the maximal response at 10(-7) M to 3 x 10(-7) M. An ETA-selective antagonist, BQ-123, did not affect the PIE of ET-1 up to 10(-7) M; it abolished the first phase at 10(-6) M but did not affect the second phase. BQ-123 at 10(-8) to 10(-6) M antagonized the PIE of ET-3, [Thr2]sarafotoxin S6b, and [Glu9]sarafotoxin S6b in a concentration-dependent manner. The PIE of ET-3 was abolished by 10(-6) M BQ-123. An ETB-selective partial agonist IRL-1620 neither elicited a PIE nor affected the PIE of ET-3. These findings indicate that the PIE of ET receptor agonists on rabbit ventricular myocardium cannot be totally explained by occupancy of the ETA or ETB receptor.

Endothelins. A potential target for pharmacological intervention in diseases of the elderly.

The existence of vasoconstrictive factors originating from the endothelium was confirmed by the description of endothelin, a 21-amino-acid peptide derived from a series of precursors, preproendothelin and a 38-amino-acid big endothelin. Three isoforms of endothelin, endothelin-1, -2 and -3, and 3 receptors (ETA, ETB and ETC) have been described and cloned. The cellular mode of action of endothelin seems to involve the modulation of intracellular calcium (through inositol trisphosphate, diacylglycerol and phospholipase C) and activation of calcium channels. The effects of endothelin are predominantly on the cardiovascular system. Its major effect is vasoconstriction, both systemic and pulmonary, with additional positive chronotropic and inotropic effects on the heart. It has also been implicated in homeostatic regulation of kidney microcirculation, and has powerful mitogenic effects on fibroblasts and smooth muscle cells. Many additional effects have been described on the endocrine system and on other systems. However, the clinical relevance of such effects is uncertain. Increased plasma endothelin levels have been reported in many diseases, but as yet it is not certain whether they are a cause or a consequence of the pathology. Pathologies most probably related to endothelin dysfunction are the vasospastic diseases, especially vasospasm after subarachnoid haemorrhage. Endothelin could be implicated to a lesser measure in diseases typical of the elderly population, such as hypertension or atherosclerosis. Drugs are being developed which act on endothelin metabolism, the most promising of which appear to be the inhibitors of endothelin converting enzyme and endothelin receptor antagonists. Some already existing drugs, such as calcium channel blockers or angiotensin converting enzyme inhibitors, probably act at least in part by interfering with endothelin metabolism or effects.

Endothelin in normal lung tissue of newborn mammals: immunocytochemical distribution and co-localization with serotonin and calcitonin gene-related peptide.

We demonstrated immunoreactivity for endothelins (ET)-1, -2, and -3 and for the precursor, big-ET-1, in the pulmonary diffuse neuroendocrine system (PDNES) of newborn cat, rat, hamster, and mouse. ET-like positive neuroepithelial bodies (NEB) were numerous in the intrapulmonary airways and the alveolar parenchyma. Single neuroendocrine cells (NEC) were less often labeled and mainly localized in the larger bronchi. ET-3-reactive neuronal elements were rarely observed. The intensity and number of immunostained NEB were highest for ET-3, followed in declining order by big-ET-1, ET-1, and ET-2. ET-like possessing NEB displayed interspecies differences. We conclude that ET-3 represents a neuroendocrine form of the ET peptide family. NEB expressing several ET isoforms can be grouped into NEB containing either big-ET-1 and ET-1 or ET-3 only. ET-like immunoreactivity was present in a subpopulation of serotonin (5HT)- and/or calcitonin gene-related peptide (CGRP)-positive NEB. As ET, 5HT, and CGRP have potent pulmonary vaso- and/or bronchomotor effects, our observations suggest that they play a separate or synergistic role in regulatory function of the mammalian PDNES, exerting their influence by paracrine, endocrine, and neurocrine pathways or a combination of these.

Sarafotoxin expression in the bronchopulmonary tract: immunohistochemical occurrence and colocalization with endothelins

The immunohistochemical occurrence of sarafotoxin (SRTX), a snake venom peptide under strong evolutionary control, was investigated in the pulmonary diffuse neuroendocrine system (PDNES) of newborn cats and rats. By applying the avidin-biotin-peroxidase complex method on serial lung sections, we have demonstrated its distribution and colocalization with different endothelin (ET) isoforms. A light microscopic study revealed apparent immunostaining for SRTX in neuronal components and smooth muscle tissue and in neuroepithelial bodies (NEB), while isolated neuroendocrine cells (NEC) remain unlabelled. Comparison of the SRTX reactivity pattern with that of different ET peptides on adjacent lung sections showed colocalization of SRTX-b with ET-3 in NEB, intrapulmonary ganglion cells and nerve fibres, on the one hand, and with ET-1 in airway and vascular smooth muscle cells, on the other. These findings, in addition to the remarkable functional and structural similarities between SRTX and ET peptides, suggest a common evolutionary origin and biological significance of sarafotoxin and endothelins. Moreover, this is the first time that a toxic peptide has been demonstrated in the PDNES.

Identification and characterization of endothelin receptor subtype B in rat retina.

The presence of immunoreactive (IR) endothelin (ET)-1 and ET-1 receptors in rat retina has been studied by radioimmunoassay and receptor assay, respectively. The specific binding of 125I-ET-1 to rat retinal particulate preparations was saturable. Apparent equilibrium conditions were established within 120-140 min. Scatchard analysis of binding data indicated a single class of high-affinity binding sites with a KD of 35 +/- 11 pM and a Bmax of 168 +/- 60 fmol/mg of protein. 125I-ET-1 binding to retinal particulate preparations was not inhibited by 1 microM concentrations of somatostatin, atrial natriuretic factor, brain natriuretic peptide, thyroid-stimulating hormone, growth hormone, or insulin. The three endothelin isoforms, ET-1, -2, and -3, had similar affinity for the receptor. Cross-linking of 125I-ET-1 to retinal particulate preparations with disuccinimidyl suberate resulted in the labeling of two bands with apparent molecular masses of 52 and 34 kDa. We have established a highly sensitive and specific radioimmunoassay for ET-1. The concentration of IR-ET-1 in rat retina was 35 +/- 10 fmol/g wet weight. The demonstration of specific high-affinity ETB receptors and the presence of IR-ET-1 suggest that the peptide may act as a neurotransmitter or neuromodulator in the retina.

Identification from a subtracted CDNA library of novel genes associated with differentiation of cytotrophoblast in vitro

Binding sites for iodinated endothelin (ET)-2, ET-3 and vasoactive intestinal contractor (VIC) were visualised in the adult rat brain using quantitative autoradiography and have a similar anatomical distribution to that of ET-1 and sarafotoxin S6b. Highest densities of binding sites for all 5 labelled peptides were present in the granular layer of the cerebellum. Cross-competition experiments show that at a concentration of 1 μM, unlabelled ET-1, ET-2, ET-3, VIC and sarafotoxin S6b were able to compete for the binding sites detected by each of the iodinated peptides. Binding sites for the ET isoforms were also present after 7–14 days in vitro in neurone-enriched primary cultures derived from embryonic rat cerebellum (16–18 days gestation) in which more than 90% of cells stained with an anti-neurofilament antibody. Using micro-autoradiography to detect the binding sites, an average of 14% of cells in these cultures with a diameter of9.2 ± 0.6 μm were associated with high silver grain densities (> 400grains/100 μm2). With some of these cells, silver grains were localised over cell bodies and branching processes characteristic of a neuronal phenotype. A second group of cells with high grain densities were more difficult to classify using morphological criteria and may be non-neuronal. The density of silver grains over the remaining cells was low(< 20grains/100 μm2) and was similar to that measured in nuclear emulsion overlying cultures used to assess non-specific binding. These results indicate that binding sites for all ET peptides are present in both adult rat brain and embryonic cerebellar cultures. In these cultures, binding does not appear to be confined to a single class of cells but may be present on both neuronal and non-neuronal cells. The results suggest that one or more of the ET isoforms could have actions in the rat CNS in addition to the proposed vasoactive role in the peripheral circulation.

Functional evidence for different endothelin receptors in the lung

The present study was undertaken to compare and contrast the characteristics of the pulmonary and systemic vascular responses to endothelin (ET) isoforms in the intact spontaneously breathing cat under conditions of constant pulmonary blood flow and left atrial pressure. When pulmonary vasomotor tone (PVT) was actively increased by intralobar infusion of U-46619, intralobar arterial bolus injections of 1 microgram ET-1, 1 microgram ET-2, or 3 micrograms ET-3 markedly decreased lobar arterial pressure, systemic arterial pressure, and systemic vascular resistance. After seven repeated injections of ET-1 or ET-2 to separate groups of cats, pulmonary and systemic responses were largely reversed from vasodilation to vasoconstriction. In contrast, the pulmonary vasodilator response to ET-3 remained intact after multiple ET-3 injections, whereas its systemic vasodilator response was lost. Repeated intralobar arterial bolus injections of ET-1, ET-2, or ET-3 also caused the loss of pulmonary vasodilation to subsequent doses of ET-1, ET-2, or sarafotoxin 6b but not to ET-3. The present data suggest that the pulmonary and systemic vasodilator responses to ET-1 and ET-2 undergo tachyphylaxis and cross-tachyphylaxis. In contrast, the pulmonary vasodilator response to ET-3, unlike its systemic vasodilator response, is resistant to tachyphylaxis and cross-tachyphylaxis. The present data provide a functional correlate for the existence of at least two ET receptor subtypes, ETA-like and ETC-like receptors, in the adult pulmonary vascular bed.

Identification of endothelin-1, endothelin-2 and endothelin-3 in human endometrium

This study determined the presence of specific endothelin isoforms in human endometrium using high performance liquid chromatography (HPLC) combined with radioimmunoassay, and immunocytochemistry to detect the endothelin precursors (proendothelin-1, proendothelin-2 and proendothelin-3). Endothelin-like immunoreactivity was measured in HPLC eluates using antisera raised in rabbits against the carboxy-terminal heptapeptide of endothelin-1, which is common to the three endothelin isoforms. Of eight endometrial samples analysed by HPLC, three were in the proliferative phase of the cycle, and five in the secretory phase. Endothelin-1 was detected in seven samples, whereas endothelin-2 and endothelin-3 were seen in four and five specimens, respectively. No relationship was seen between endothelin isoforms and the stage of the cycle. Immunocytochemistry was performed on five proliferative and three secretory phase tissues. When present, staining for the precursor proendothelins was localized to endometrial glandular and luminal epithelium (proendothelin-1, 5 of 8; proendothelin-2, 5 of 8; proendothelin-3, 6 of 8). In two sections, staining was also seen in vascular endothelium using antibody raised against proendothelin-1 (n = 1) and proendothelin-3 (n = 1). These data provide evidence that the three endothelin isoforms are present in human endometrium, and suggest that these potent vasoactive agents may play a role in the paracrine control of the uterine vascular bed.

Expression of Human Endothelin Receptor ET B by Escherichia coli Transformants

Membrane fractions from Escherichia coli transformed with an expression vector for the human endothelin receptor ET B were found to exhibit specific and saturable binding sites for labeled ET-1. Binding properties were similar to those observed in eukaryotic cells: K D was 110 pM and the three endothelin isoforms were equipotent in competition experiments. The importance of the cysteines located in each of the three extracellular loops was assessed by replacing them individually with alanine residues. Impaired binding was observed in each case but the effect was far more pronounced for the cysteine mutants of the first and second loop pointing to their criticalimportance for active ET B expression.

Characterization of endothelin isoforms in human heart: endothelin-2 demonstrated.

In humans, three endothelin (ET) isoforms are predicted to exist by analysis of genomic DNA. However, evidence for the presence of all three mature ET peptides and their precursors remains unclear. Our aim was to identify the ET isoforms present in human heart, using radioimmunoassay (RIA) and reverse-phase high-performance liquid chromatography (RP-HPLC). Antisera raised against the ET-1[15-21] terminal sequence were specific for mature ETs, showing no cross-reactivity with their precursor pro-ETs. Antisera raised against the pro-ET-1[31-38] terminal sequence was specific for pro-ET-1, showing no cross-reactivity with other ET peptides. In extracts of human cardiac tissues, the concentrations of immunoreactive (IR) mature ET and pro-ET-1 were found to be as follows: left atrium (n = 3): 282.3 +/- 113.0, 21.9 +/- 11.0, respectively; right atrium (n = 5): 308.3 +/- 95.4, 43.1 +/- 12.8, respectively; left ventricle (n = 6): 218.5 +/- 64.6, 47.9 +/- 11.9, respectively; right ventricle (n = 4): 215.1 +/- 79.8, 53.9 +/- 13.0, respectively (fmol/g wet weight, mean +/- SEM, for total IR mature ET and pro-ET-1, respectively). RP-HPLC showed peaks of immunoreactivity that coeluted with authentic ET-1 in all extracts of human left atria and ventricle tested. In addition, peaks were also present corresponding to ET-2, ET-3, and pro-ET-1. These results suggest that in addition to ET-1 and pro-ET-1, ET-2 and ET-3 are present in the human heart.

Molecular and cellular biology of endothelin and its receptors--Part I.

The endothelins (ET) are 21-amino-acid peptides produced by endothelial cells and many other tissues [1]. Three isoforms of ET have been characterized (ET-1, 2 and 3) which differ in two and five amino-acid residues, respectively [1–4]. In humans and other mammals three distinct genes encode the three isopeptides of the ET family [5].

Human Kidney: Endothelin Isoforms Detected by HPLC with Radioimmunoassay and Receptor Subtypes Detected Using Ligands BQ123 and BQ3020

Animal kidneys are exquisitely sensitive to the effects of endothelin (ET), but little is known of its binding characteristics, isoform prevalence, or receptor subtype distribution in human kidney. We investigated these parameters using high-performance liquid chromatography, radioimmunoassay (RIA), and the recently synthesized ETA and ETB receptor-selective peptide ligands BQ123 (cyclo[D-Asp-L-Pro-D-Val-L-Leu-D-Trp-]) and BQ3020 (Ala11,15-Ac-ET-1[6-21]). Fresh-frozen normal segments of kidneys excised for carcinoma were solid-phase-extracted using Amprep C2 columns, and parallel eluates were oxidized. All were subjected to RIA for ET and pro-ET-1, in triplicate. ET isoforms were characterized by RP-HPLC and subsequent RIA of eluates. Total amounts of immunoreactive ET were 6.9 +/- 3.8 and 4.5 +/- 1.6 pmol/g wet weight in medulla and cortex, respectively. Reverse-phase high performance liquid chromatography showed peaks of immunoreactivity with retention times identical to synthetic ET-1 and metsulphoxide ET-1. ET-2, ET-3, and pro-ET-1 were not detected. Saturation assays using 0.01-8.0 nM 125I-ET-1 or 125I-BQ3020, gave Kd values (mean +/- SEM) of 0.17 +/- 0.04 and 0.36 +/- 0.06 nM, respectively, with Bmax values of 57.7 +/- 15.4 and 30.0 +/- 5.0 fmol/mg protein, respectively. Hill coefficients were 0.86 +/- 0.03 and 0.77 +/- 0.04, but a two-site fit was not preferred. Receptor autoradiography has detected both subtypes, mainly present in medulla, with ETB predominating.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against a pathogenetic role for endothelin in pre-eclampsia.

To assess whether increased placental or systemic endothelin synthesis has a pathogenic role in pre-eclampsia (gestational proteinuric hypertension). Prospective observations study. 19 women with pre-eclampsia and 10 healthy pregnant women were studies. All were in the last trimester. Preproendothelin-1 gene expression by Northern blot analysis and generation of endothelin-1 precursor, big-endothelin-1, and endothelin isoforms, namely endothelin-1, 2 and 3, were assessed by specific radio-immunoassays, in placental tissue. Plasma endothelin-1 levels and urinary excretion of big-endothelin-1 and endothelin-1 were measured. Placental preproendothelin-1 gene expression and immunoreactive big-endothelin-1 and endothelin-1, 2 and 3, were comparable in placental tissue from pre-eclamptic and normal pregnant women. Plasma levels of endothelin-1 did not differ between pre-eclamptic and normal pregnancies. In contrast, urinary excretion of endothelin-1, which is likely to reflect the renal synthesis of the peptide, was significantly decreased in pre-eclamptic, as compared with normal pregnant women. This was not due to a decreased renal generation of endothelin-1 precursor, since urinary excretion of big-endothelin-1 did not differ between pre-eclamptic and normal pregnancies. These data suggest an increased renal endothelin-1 breakdown in pre-eclampsia. Endothelin is unlikely to play a role in the pathogenesis of pre-eclampsia. Instead, an increased renal breakdown may have a role in limiting the negative effects of other vasoactive factors on the renal circulation.

Endothelin in the Urinary Bladder. II. Characterization of Endothelin Receptor Subtypes

Endothelins (ET-1, ET-2, ET-3) are a family of regulatory peptides with diverse biological functions, including modulation of smooth muscle tone. To evaluate the possible role of endothelins in the control of the detrusor smooth muscle of urinary bladder, we have investigated the responses of isolated strips of bladder dome and base to endothelins, in organ chambers, and characterized the receptors for these isopeptides in bladder membranes. ET-1, ET-2 and to a lesser extent ET-3 caused sustained concentration-dependent contraction of bladder dome and base. The contraction to endothelins, unlike the short-lived contraction produced by cholinergic agents, was long lasting and difficult to wash out. Equilibrium binding studies demonstrated that endothelin isoforms bound to bladder membrane receptors specifically, with high affinity (KD of 0.4–0.6nM) and limited capacity (60-420 fmol/mg protein). Competition analysis showed two populations of receptors: one with high affinity for ET-1 and ET-2 and low affinity for ET-3 and another with high affinity for ET-1, ET-2 and ET-3. Chemical affinity labeling of endothelins to bladder membranes demonstrated that ET-1 and ET-2 were cross-linked to three proteins (75, 52 and 34kDa, respectively), whereas ET-3 was cross-linked mainly to a 34kDa protein. The data obtained from equilibrium binding studies, competition analysis and cross-linking experiments suggest that at least two endothelin receptor subtypes exist in bladder tissue. These observations further suggest that bladder smooth muscle tone may be modulated by endothelin or an endothelin-like substance via interaction with specific receptor sites.

Endothelins Inhibit Junctional Permeability in Cultured Mouse Astrocytes.

Endothelins, a family of potent vasoconstrictor peptides initially characterized in peripheral tissues, have also been reported to be synthesized in the brain. In this structure several cell types, including astrocytes, endothelial cells and certain neurons, are potential targets for these peptides. In astrocytes, endothelins induce changes in the concentration of several second messengers (calcium, diacylglycerol, arachidonic acid, cAMP) known to be involved in the regulation of gap junction channels. Using the scrape loading/dye transfer technique we have observed that two isoforms of endothelin, endothelin-1 and endothelin-3, strongly inhibit the extent of dye-coupling between confluent astrocytes, suggesting that gap junction permeability was reduced. This inhibitory effect on dye coupling was reproduced by the snake venom sarafotoxin. When used at 10-7 M, these three compounds had inhibitory effects on gap junction channels which were comparable to those induced by the well known uncoupling agents octanol and halothane. In the absence of extracellular calcium, the effects of endothelins were largely prevented, suggesting that second messengers linked to the activation of phospholipases C and/or A2, which both are dependent on external calcium, could be involved in the uncoupling mechanism.