Identification and characterization of endothelin binding sites in rat renal papillary and glomerular membranes

Diuretic Action of the Novel Loop Diuretic Torasemide in the Presence of Angiotensin II or Endothelin-1 in Anaesthetized Dogs

The effects of torasemide (0·1 and 1 mg kg−1, i.v.) and furosemide (3 mg kg−1) on renal haemodynamics and excretory responses in the presence of angiotensin II and endothelin-1 was examined in anaesthetized dogs. Angiotensin II or endothelin-1 was continuously infused into the renal artery throughout the experiment and a bolus of torasemide or furosemide was injected into the bracheal vein. Continuous intrarenal arterial (i.r.a.) infusion of angiotensin II, at a dose of 5 ng kg−1 min−1, increased renal vascular resistance (RVR) and decreased renal blood flow (RBF) and glomerular filtration rate (GFR), but had no effect on systemic mean arterial pressure (MAP). Urinary excretion of sodium (UNaV) and urine flow (UF) were significantly decreased during angiotensin II infusion. Intravenous injections of torasemide in the presence of angiotensin II caused a dose-dependent increase in UF, UNaV and urinary excretion of potassium (UKV), while a decrease in RVR was accompanied by an increase in RBF. UKV was greater in the furosemide group than in the torasemide group, despite both groups having the same degree of aquaresis and natriuresis. Continuous i.r.a. infusion of endothelin-1, 1·5 ng kg−1 min−1, produced effects similar to those of angiotensin II on renal haemodynamics; however, the onset of action was extremely slow compared with the effects produced by angiotensin II. Endothelin-1 caused a significant decrease in UF, UNaV and UKV only at a later period, despite a relatively early depression of renal haemodynamics. Torasemide and furosemide also produced a sufficient diuretic action in this model. Overall, kaliuresis was greater in the furosemide group than in the torasemide group. The present study demonstrates that torasemide exhibited a significant diuretic action in the angiotensin II- or endothelin-1 -induced renal impairment model, with less kaliuresis than furosemide at a concentration which caused the same degree of natriuresis.

Unique endothelin receptor binding in kidneys of ETB receptor deficient rats

Gariepy and colleagues (Gariepy CE, Williams SC, Richardson JA, Hammer RE, and Yanagisawa M. J Clin Invest 102: 1092-1101, 1998.) developed rescued spotting-lethal rats that carry a naturally occurring deletion of the endothelin (ET) type B receptor gene resulting in a lack of functional renal ETB receptor expression. It has been shown that rats homozygous (sl/sl) for the deletion have elevated plasma ET-1 levels; thus, the purpose of this study was to determine whether this deletion would result in a downregulation of ETA receptors in renal tissue. ET-1 and ET-3 binding experiments were performed with cortex, outer medullary, and inner medullary membranes of heterozygous (sl/+) and sl/sl ETB receptor-deficient rats. 125I-labeled ET-1 binding in sl/sl cortex and outer medulla was significantly lower than cortex and outer medulla from sl/+ rats. In contrast to sl/+ rats, [125I]ET-3 binding was not detected in the cortex and outer medulla of sl/sl rats, indicating a lack of ETB receptor expression. The inner medulla of sl/+ rats also demonstrated an abundance of ETB receptors. Surprisingly, however, we also observed significant [125I]ET-3 binding in the sl/sl inner medulla. Furthermore, ET-3 binding in the inner medulla could be blocked with an ETA receptor antagonist in sl/sl rats but not in tissue from sl/+ rats. These studies indicate that rats deficient in ETB receptors have decreased renal cortical and outer medullary ETA receptor number, most likely in response to elevated plasma ET-1 levels. In addition, homozygous ETB-deficient rats express a novel inner medullary ET-3 binding site.

Solution structure of a novel ETB receptor selective agonist ET1-21 [Cys(Acm)1,15, Aib3,11, Leu7] by nuclear magnetic resonance spectroscopy and molecular modelling

The solution structure of a biologically active modified linear endothelin-1 analogue, ET1-21[Cys(Acm)1,15, Aib3,11, Leu7], has been determined for the first time by two-dimensional nuclear magnetic resonance spectroscopy in a methanol-d3/water solvent mixture. Out of approximately one hundred linear peptide analogues tested by biological assay, this peptide, together with a dozen others, showed significant ETB selective agonist activity. Here we report the solution structure of an ETB selective agonist of a full-length, synthetic linear endothelin analogue. The calculated structures indicate that the peptide adopts an alpha-helical conformation between residues Ser5-His16, whilst both N- and C-termini show no preferred conformation. These results suggest that the disulphide bridges normally associated with endothelin and sarafotoxin peptides may not necessarily be important for either ETB receptor binding activity or the formation of a helical conformation in solution.

Luminal and basolateral endothelin inhibit chloride reabsorption in the mouse thick ascending limb via a Ca(2+)-independent pathway

1. The recent localization of endothelin synthesis and receptors in the thick ascending limb (TAL) prompted us to investigate a possible autocrine and/or paracrine effect of this agent. The net chloride flux (JCl) has been determined in isolated cortical and medullary TALs by the in vitro microperfusion technique. 2. In both segments, endothelin 1 (ET-1) at 10(-8) M in the bath significantly decreased JCl, an effect which was partially reversible and observed at concentrations equal to or greater than 10(-13) M. 3. This JCl inhibition (by 33.9 +/- 3.2%) was blocked by BQ788 and was also observed with sarafotoxin 6C and ET-3, indicating that endothelin receptor B (ETB) are present in TAL. 4. ET-1 did not affect cAMP content under basal or hormone-stimulated conditions. The presence of a prostaglandin synthesis inhibitor also did not prevent the ET-1 action on JCl. 5. The ET-1-induced inhibition of JCl was prevented by protein kinase C inhibitors (staurosporine or GF 109203) and was reproduced by diacylglycerol analogues (OAG and DiC8). However, ET-1 failed to increase intracellular Ca2+ concentration. 6. Addition of ET-1 or ET-3 to the apical surface induced a decrease of JCl throgh ETB receptors, an effect which was not additive with that induced by basolateral ET-1, and was not concomitant with an increase in intracellular Ca2+ concentration. 7. It is concluded that the basolateral and luminal inhibitions of JCl by ET-1 in TAL, through ETB receptors, is mediated by a protein kinase C activation which is independent of intracellular Ca2+ increase.

Characterization of human endothelin B receptor and mutant receptors expressed in insect cells

Endothelin type-B receptor (ET(B)R) forms a stable complex with its ligand, endothelin-1. To facilitate biochemical and biophysical studies of human ET(B)R, several ET(B)R mutants carrying a hexahistidine tag sequence at the N or C terminus were expressed in Sf9 cells and were purified by a combination of biotinylated endothelin-1-ligand-affinity and nickel-affinity chromatographies. The ligand-free receptor was purified by dissociating the ligand x receptor complex with 2 M NaSCN, whereas the ligand-bound ET(B)R was purified by the use of thiol-sensitive biotinylated endothelin-1. While the wild-type ET(B)R was expressed at about 100 pmol 125I-endothelin-1-binding activity/mg membrane protein, the deletion of 36 residues from the N-terminus reduced the expressed activity to about 30%. On the other hand, the lack of glycosylation and the replacement of 2-9 residues in the N-terminal tail resulted in a 20-40% reduction in the expressed activity. Among the mutant proteins, [H57-H62, G63-G65]ET(B)R, carrying six His residues in the N-terminal tail, was studied extensively because it was purified most effectively. Ligand-free [H57-H62, G63-G65]ET(B)R, purified in digitonin, retained full ligand-binding activity, while other detergents led to partial denaturation of the receptor after solubilization or after elution with NaSCN. On the other hand, ligand-bound [H57-H62, G63-G65]ET(B)R could be purified in various detergents, such as n-octyl-beta-D-glucopyranoside or n-decyl-beta-D-maltopyranoside. Ligand-free [H57-H62, G63-G65]ET(B)R reconstituted in phospholipid vesicles stimulated the binding of guanosine 5'-3-O-(thio)triphosphate by Gq in the presence of endothelin-1. Ligand-bound [H57-H62, G63-G65]ET(B)R showed similar catalytic activity in nucleotide exchange by Gq. These results indicate that the ligand x receptor complex in a detergent-micellar solution retained the biologically active structure, and that the presence of ligand, endothelin-1, in the receptor molecule reinforces the stable assembly of a helical bundle and therefore the active structure.

Effects of anti-endothelial cell antibody in pre-eclampsia on endothelin-1 release from cultured endothelial cells

In order to investigate the pathophysiological significance of anti-endothelial cell antibody (AECA) in pre-eclampsia, the effects of AECA on endothelin-1 (ET-1) and prostaglandin I2 (PGI2) release from cultured human umbilical vein endothelial cells (HUVEC) was evaluated. Serum samples were taken from 85 pre-eclamptic and 20 normal pregnant women. Anti-endothelial cell antibody was measured by ELISA using HUVEC. The release of ET-1 and 6-keto PGF1-alpha, a stable metabolite of PGI2, from HUVEC were evaluated after incubation with IgG-AECA-positive sera and IgG isolated from AECA-positive sera. The incidence of IgG- and IgM-AECA was 24.7 and 8.2%, respectively. The release of ET-1, in the medium containing IgG-AECA-positive sera was significantly greater than in the medium containing IgG-AECA-negative sera. There was significant correlation between the levels of IgG-AECA and the release of ET-1 from endothelial cells. The ET-1 release by IgG isolated from AECA-positive sera was greater than that from AECA-negative sera. However, the release of 6-keto PGF1-alpha by AECA-positive sera was not significantly different from that of AECA-negative sera. It is concluded that IgG-AECA in pre-eclampsia increases ET-1 release from endothelial cells and that AECA may affect local vascular function in this disorder.

Inhibitors of endothelin

With the advent of the first generation of both selective and nonselective endothelin antagonists being a relatively recent event, the manifold therapeutic potentials of these compounds are only now being explored clinically. Undoubtedly, numerous clinical utilities for these compounds will soon be realized.

High resolution localization of endothelin receptors in rat renal medulla

The cellular localization of endothelin receptors in the inner medulla of the rat kidney was investigated by using high resolution light and electron microscopic autoradiography, with the microwave irradiation fixation methods. Kidney slices were incubated with 125I-endothelin-1 alone or with selective ligands for the endothelin ETB and/or ETA receptors for light microscopic autoradiography. At the microscopic level, 125I-endothelin-1 was found to bind specifically to the glomeruli, arterioles and peritubular spaces in the cortex and vasa recta and surrounding tissues in the inner medulla. These bindings were also observed when the tissue slices were incubated in the presence of IRL1620 (ETB receptor agonist) or 97-139 (ETA receptor antagonist). Electron microscopic autoradiography using 125I-endothelin-1 in the inner medulla revealed silver grains over endothelial cells of the vasa recta and interstitial and collecting duct cells. No grains were detected over inner lining cells of the thin limbs of Henle's loop. These interstitial cells contained abundant microorganelles and lipid droplets, and had extensive cytoplasmic processes that closely related to the basement membranes of the vasa recta and loop of Henle. These findings demonstrate that type 1 interstitial cells are also primary sites for endothelin receptors as well as endothelial cells of the vasa recta and collecting duct cells in the inner medulla.

Failure of BQ123, a more potent antagonist of sarafotoxin 6b than of endothelin-1, to distinguish between these agonists in binding experiments

1. In homogenates of human saphenous vein, [125I]-ET-1 and [125I]-S6b each labelled a single population of high affinity binding sites with K(D) values of 0.64 +/- 0.11 nM and 0.55 +/- 0.08 nM respectively. Hill slopes were close to one. However, the density of receptors labelled by [125I]-ET-1 was significantly greater than that by [125I]-S6b (187.6 +/- 23.0 compared to 91.7 +/- 23.6 fmol mg-1 protein, P < 0.02). 2. BQ123, an ET(A-)selective antagonist, inhibited specific [125I]-ET-1 and [125I]-S6b binding with equal affinity. BQ123 competed in a biphasic manner for both [125I]-ET-1 (0.1 nM) and [125I]-S6b (0.1 nM) with ET(A) K(D) values of 0.55 +/- 0.17 nM and 0.52 +/- 0.02 nM and ET(B) K(D) values of 14.4 +/- 2.60 microM and 11.2 +/- 0.31 microM respectively. S6b monophasically inhibited 0.1 nM [125I]-ET-1 (K(D) 1.16 +/- 0.9 nM) but competed for 0.25 nM [125I]-ET-1 in a biphasic manner (K(D) high affinity site 1.99 +/- 0.84 nM, K(D) low affinity site 0.68 +/- 0.63 microM, ratio 67% : 33%). 3. BQ123 antagonized the vasoconstrictor responses of ET-1 with a pK(B) value of 6.47 whereas BQ123 exhibited 50 fold higher affinity against S6b-mediated vasoconstriction with a pK(B) value of 8.18. Regression slopes were 0.80 +/- 0.13 and 1.08 +/- 0.11 respectively. 4. In desensitization experiments, S6b (300 nM) did not contract preparations which were no longer responsive to ET-1 whereas a small contraction to ET-1 (300 nM) was obtained in preparations rendered unresponsive to S6b. 5. Medial sections of non-diseased human aorta, which express only ET(A) receptors, were used to compare dissociation rates of the two agonists. The time course for the dissociation of [125I]-ET-1 and [125I]-S6b was similar with 20-30% of each ligand dissociating at 4 h. 6. These data suggest that whilst BQ123, in common with other endothelin antagonists, is a much more potent blocker of S6b contractile responses than of ET-1 contractile responses, this is not reflected by the equal affinity of BQ123 determined in competition binding experiments against both [125I]-ET-1 and [125I]-S6b. This discrepancy in antagonist potency is probably not due to a marked difference in the rate of dissociation of [125I]-ET-1 and [125I]-S6b from endothelin receptors. One possible explanation is that ET-1 is activating an additional population of receptors which may have lower affinity for BQ123. This is suggested by the discrepancy in receptor density identified by [125I]-ET-1 and [125I]-S6b.

Cardiac and renal endothelin-1 binding sites in streptozotocin-induced diabetic rats

The aim of this work was to study cardiac and renal endothelin binding sites during the progression of diabetes. Male Crl:CD (BR) rats were made diabetic by injection of streptozotocin (STZ, 45 mg kg-1 i.v.). Only rats with a glycaemia of 500 mg per 100 ml or higher, were used. The hearts were taken at 2, 4 or 6 weeks and kidneys at 2 and 6 weeks, after diabetes induction, for binding studies. In the heart, the number of Et-1 binding sites was significantly increased 2 weeks after STZ-induction of diabetes (449 +/- 13 vs. 345 +/- 18 fmol (mg protein) -1, in controls; p < 0.05) without modification of KD value (104 +/- 5 vs 101 +/- 7 pM). Comparable results were obtained 4 and 6 weeks after STZ-induction. In the kidney both the parameters were unchanged at all the times tested.

IN CONCLUSION

a specific increase in cardiac Et-1 binding sites, without change in affinity of the peptide, was found 2, 4 and 6 weeks after diabetes induction; while renal Et-1 binding sites were not modified.

Isolation of the endothelin B receptor from bovine lung. Structure, signal sequence, and binding site

Bovine lung endothelin-B receptor has been isolated in good yield with a new procedure involving the use of endothelin-1 coupled to iminobiotin with a long spacer and avidin-agarose affinity chromatography. Contrary to previous reports, evidence has been obtained that the native form of this receptor corresponds to the full-length transcript expected on the basis of cDNA clones. The binding of endothelin to a variety of shortened fragments of the full receptor suggests that the long N-terminal sequence of this receptor has very little influence on the binding of endothelin and that the main determinants of the endothelin binding site might be constituted by residues in the sixth, and possibly the seventh, transmembrane helices.

Visualization of renal microcirculation in isolated Munich-Wistar rat kidneys: effects of endothelin-1 on renal hemodynamic activity

The aim of the present study was to visualize the superficial glomeruli of the Munich-Wistar (MW) rat and to characterize the responses of the renal microvasculature to endothelin-1 (ET-1). We first examined the distribution of superficial glomeruli of the MW rat compared to that in a control strain (Wistar rat). Secondly, we examined the effects of ET-1 on the renal microcirculation of the MW rat. The right kidney was perfused with a Krebs-Ringer solution containing fluorescein isothiocyanate dextran (FITC-dextran) and was visualized under an epi-illuminated fluorescence microscope system. Changes in perfusion pressure and diameter of the microvessels accompanying the administration of ET-1 (10 fmole-300 pmole) were measured. The number of superficial glomeruli was greater in the MW rat than in the Wistar rat. ET-1 had long-lasting and dose-dependent pressor effects. Perfusion pressure showed a 3.5-fold increase compared with the control, and the afferent arterioles showed greater dose-dependent vasoconstriction than the efferent arterioles. These findings suggest that the MW rat is a useful animal model for the study of renal microcirculation and that the renal microcirculation is extremely sensitive to ET-1.

Mechanism of vasoconstriction induced by chronic inhibition of nitric oxide in rats

Either acute or chronic inhibition of nitric oxide synthesis by L-arginine analogues results in increases in mean arterial pressure and reductions in renal blood flow. The role of endogenous vasoconstrictors in mediating these effects is not entirely clear. In the present study, nitric oxide was inhibited in male Sprague-Dawley rats by oral administration of nitro-L-arginine for 3 weeks. At the end of this time, mean arterial pressure was 30 to 40 mm Hg higher than in normal controls, renal blood flow and glomerular filtration rate were 25% to 30% lower, and renal vascular resistance was markedly increased. Intravenous infusion of receptor antagonists for angiotensin II, thromboxane, epinephrine, and endothelin-1 had no significant effect on the hypertension. Inhibition of prostaglandin synthesis and furosemide-induced diuresis in the presence of angiotensin blockade also had no effect on blood pressure. Renal vascular resistance was also unaffected by these interventions, except that saralasin did reduce renal resistance in both control and nitric oxide-inhibited groups. However, the absolute level of renal vascular resistance remained higher in the latter group. Calcium channel blockade partially corrected blood pressure and renal resistance, but the levels remained significantly higher than in control animals. The findings are consistent with the view that the increase in vascular smooth muscle tone caused by inhibition of nitric oxide synthesis cannot be accounted for by overexpression of common endogenous vasoconstrictors. Rather, the generalized increase in vascular smooth muscle tone appears to be due to a direct effect of reduced nitric oxide availability, which may lead to an increase in intracellular calcium concentration or sensitivity.

Termination of endothelin signaling: role of nitric oxide

Cellular mechanisms responsible for the termination of ET-1 signal are poorly understood. In order to examine the hypothesis that nitric oxide serves as a physiological brake of ET-1 signaling, Chinese hamster ovary (CHO) cells stably transfected with the ETA receptor cDNA (CHO-ET) were studied. CHO-ET responded to ET-1 with robust [Ca2+]i transients and developed a long-lasting homologous desensitization. Donors of nitric oxide (NO), 3-morpholino-sydnonimine HCl (SIN-1), or sodium nitroprusside (SNP) reduced the amplitude of these responses, accelerated the rate of [Ca2+]i recovery, and counteracted the development of homologous desensitization by a cyclic GMP-independent mechanism, suggesting an alternative mode for NO modulation of ET-1 responses. Stimulation of CHO-ET cells with mastoparan, a wasp venom acting directly on G proteins (bypassing receptor activation), was inhibited by NO, revealing a postreceptoral target for NO-induced modulation of [Ca2+]i mobilization. Using a lys9-biotinylated ET-1 (ET-1 [BtK9]), binding sites were "mapped" in CHO-ET cells. Receptor-ligand complexes did not exhibit spontaneous dissociation during 60-min observations. Quantitative fluorescence microscopy revealed that SNP or SIN-1 caused a rapid, concentration-dependent, and reversible dissociation of biotinylated ET-1 from ETA receptor (EC50 = 75 microM and 6 microM, respectively), an effect that was not mimicked by 8-bromo-cyclic GMP. "Sandwich" co-culture of endothelial cells with CHO-ET showed that activation of NO production by endothelial cells similarly resulted in dissociation of ET-1 [BtK9] from ETA receptors. We hypothesize that NO plays a role in physiological termination of ET-1 signalling by dual mechanisms: (1) displacement of bound ET-1 from its receptor, thus preventing homologous desensitization, and (2) interference with the postreceptoral pathway for [Ca2+]i mobilization, hence inhibiting end-responses to ET-1.

The receptors for endothelins and their analogues in SK-N-MC neuroblastoma cells

The potency order of peptides to inhibit [125I]endothelin-1 binding and to stimulate phosphatidylinositol phosphate (PtdInsP) turnover in SK-N-MC cells was consistent with the presence of ETA-endothelin receptors. Divalent cations enhanced [125I]endothelin-1 binding by, in the case of Mn2+, increasing radioligand affinity. Mn2+ did not induce conformational changes in endothelin-1, and its effect was maintained in solubilized receptors. Hence, metal ions may directly interact with endothelin receptors. The effects of BQ-123 and [Ala1,3,11,15]endothelin-1 on PtdInsP turnover were investigated. Concentration-response curves of endothelins were modeled by a second-order equation that assumes pseudoirreversible ligand binding.

Renal endothelin mechanism in altered thyroid states

Endothelin (ET) mechanisms were studied in hyper- and hypo-thyroid states in rats. Hyperthyroidism was induced by daily administration of thyroxine (0.1 mg/kg, i.p.) for 8 weeks, while hypothyroidism was induced by daily administration of methimazole (10 mg/kg, i.p.) for 8 weeks. The concentration of endogenous ET-1 was determined in the kidneys using radioimmunoassay. Systemic hemodynamics and renal blood circulation was measured using a radioactive microsphere technique. A significant increase in systolic and diastolic blood pressure, heart rate and cardiac output was observed in hyperthyroid rats as compared to eu- and hypo-thyroid rats. Total peripheral resistance was found to be similar in eu-, hyper- and hypo-thyroid rats. The endogenous concentration of ET-1 in the kidneys was significantly lower in hyper- as compared to eu- and hypo-thyroid rats. The blood flow to the kidneys was significantly increased in hyper- as compared to eu- and hypo-thyroid rats. Infusion of ET-1 (100 ng/kg/min i.v. for 45 min) produced a significant decrease in blood flow to the kidneys of eu-, hyper- and hypo-thyroid rats. The decrease in blood flow was similar in eu-, hyper- and hypo-thyroid rats, indicating that the response of renal blood vessels to exogenous ET-1 is not altered during thyroid dysfunction. Since endogenous ET-1 is involved in the regulation of vascular tone, it may be concluded that in hyper-thyroid rats decrease in concentration of the renal ET-1 could be contributing to an increase in blood flow to the kidney.

Endothelin ETA and ETB receptors mediate vasoconstriction and prostanoid release in the isolated kidney of the rat

Endothelin-1 or sarafotoxin 6c (10(-12) to 10(-9) M) induced concentration-dependent increases in perfusion pressure in the isolated perfused kidney of the rat, and increased the release of 6-keto-prostaglandin F1 alpha, prostaglandin E2 and prostaglandin F2 alpha. The pressor effects of endothelin-1 were partially antagonised by BQ-123 (10(-6) M) and more strongly so by the endothelin ETA/B receptor non-selective antagonist PD 145065 (10(-6) M). PD 145065, but not BQ-123, also completely blocked the pressor effects of sarafotoxin 6c. The releases of prostanoids induced by endothelin-1 were greatly reduced by infusion of either BQ-123 or PD 145065, whereas PD 145065, but not BQ-123, strongly antagonised the releases induced by sarafotoxin 6c. These results indicate that the vasoconstrictions and the prostanoid releases induced by the endothelin/sarafotoxin peptides are mediated by the activation of both endothelin ETA and endothelin ETB receptors in the isolated perfused kidney of the rat.

Role of endogenous endothelin in renal haemodynamics of newborn rabbits

Endothelin is a potent vasoconstrictor peptide produced by vascular endothelial cells which could play a role in the physiological regulation of the renal microcirculation. To test this hypothesis, experiments were performed in 24 anaesthetized and mechanically-ventilated newborn rabbits. In 8 newborn rabbits (group 1), a bolus injection of 5 nmol/kg endothelin caused a marked increase in mean blood pressure (MBP) and renal vascular resistance (RVR), leading to a significant fall in glomerular filtration rate (GFR) (by 12% +/- 4%) and renal blood flow (RBF) by 16% +/- 3%). A second group of animals (n = 8) was used for testing the in vivo neutralizing activity of an endothelin-1 antiserum. The antiserum was thereafter infused into 8 additional newborn rabbits (group 3) in order to define the role of endogenous endothelin in modulating the function of the immature kidney. The antiserum induced a surprising increase in RVR (by 34% +/- 9%, P < 0.05) associated with a fall in GFR (by 21% +/- 4%, P < 0.05) and RBF (by 25% +/- 4%, P < 0.05), while the filtration fraction and MBP remained unchanged. The occurrence of a vasoconstrictive response to both high-dose endothelin and to its antiserum could be explained by the recent demonstration that high levels of endothelin lead to renal vasoconstriction, while lower levels induce renal vasodilatation. The present results suggest that endogenous endothelin is active at low levels under normal conditions and that this peptide plays a role in the physiological control of renal function, but not MBP.

Endothelin-receptor interactions. Role of a putative sulfhydryl on the endothelin receptor

The mechanism of action of endothelin-receptor interactions was studied, using radioligand binding assays and SDS-PAGE, to investigate the possibility of disulfide interchange. Electrophoretic analysis suggested involvement of disulfide bond(s) in the receptor-ligand complex. Treatment of Et receptors with sulfhydryl-specific alkylating reagents (NEM or others) resulted in decreased ability to bind [125I]Et-1. [Dpr1-Asp15]Et-1, an antagonist homologous to Et but with an amide link replacing one of the disulfides, bound to Et receptors reversibly, but binding of Et-1 was less reversible. Preincubation of receptors with Et-1, but not with [Dpr1-Asp15]Et-1, protected receptors from alkylation with [14C]NEM. The data suggest that the Et receptor has a sulfhydryl group at or near the Et binding site. A model is proposed in which the role of the putative sulfhydryl group is discussed.

Regulation of cortical collecting duct function: effect of endothelin

We recently showed that endothelin-1 (ET-1) increases cell Ca2+ in the mouse cortical collecting duct. To clarify the cellular action and target cell of ET-1, electrophysiologic techniques and cell Ca2+ measurement were applied to rabbit cortical collecting ducts perfused in vitro. When 10(-8) mol/L ET-1 was added to the bath, a transient increase followed by a sustained increase in cell Ca2+ was observed. A sustained increase in cell Ca2+ lasted 10 to 20 minutes and was associated with a decrease in lumen-negative transepithelial voltage. To confirm the target cell type of ET-1, confocal laser microscopy was used. An increase in cell Ca2+ was observed in the same cell, which also showed an increase in cell Ca2+ in response to arginine vasopressin (AVP), which indicated that the principal cell has ET-1 receptors in the basolateral membrane. When ET-1 was applied to the bath, total cellular membrane resistance (Ri) decreased initially and then gradually increased because of inhibition of the luminal Na+ channel. An initial decrease in Ri was considered an influx of Ca2+ from the basolateral membrane. To further determine the source of an increase in cell Ca2+, the effect of ET-1 was tested in the absence of external Ca2+ and in the presence of a Ca2+ channel blocker in the bath. Cell Ca2+ did not respond to ET-1 in the absence of external Ca2+, a condition in which an AVP-stimulated increase in cell Ca2+ was preserved.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic effects of endothelin. Receptor characterization and endothelin-induced signal transduction in hepatocytes

Endothelin, a potent vasoactive peptide originally isolated from the vascular endothelial cells, exerts glycogenolytic and vasoconstrictive actions in the perfused rat liver. In this paper we demonstrate high-affinity binding sites for endothelin-1 (ET-1) on rat hepatocytes. Upon incubation at 37 degrees C, association of ET-1 with hepatocytes occurred in a time-dependent manner, was maximal between 3 and 6 h, and subsequently declined; at this temperature ET-1 was rapidly internalized with the internalized ligand exceeding the surface-bound ligand at all time points. The rate of association of 125I-ET-1 with hepatocytes was much slower when the binding assay was performed at 4 degrees C; sequestration of ET-1 in hepatocytes was also substantially reduced at this temperature. ET-1 was extremely potent in stimulating phosphoinositide metabolism in hepatocytes, with significant activation of this signal transduction process occurring at ET-1 concentrations as low as 0.1 pM, with an EC50 of 1 pM. The effect of ET-1 was coupled via a pertussis toxin-sensitive G-protein. Cholera toxin did not affect ET-1-mediated phosphoinositide metabolism and neither toxin influenced the association of 125I-ET-1 with hepatocytes. PAGE of hepatocyte membranes following exposure of the cells to 125I-ET-1 and cross-linking revealed labelling of three major proteins with apparent molecular masses of 32, 49 and 72 kDa. 125I-ET-1 labelling of each of these proteins was inhibited by unlabelled ET-1, whereas unlabelled ET-3 inhibited the labelling of only the 32 and 49 kDa proteins. 125I-ET-3 labelled the 49 kDa protein and this labelling was inhibited by both unlabelled ET-1 and ET-3. Each of these receptors appears to be functional, since both ET-1 and ET-3 stimulated phosphoinositide metabolism in hepatocytes. Down-regulation of ET-1 association and desensitization of ET-1-induced phosphoinositide metabolism occurred upon incubation of hepatocytes with the homologous ligand. Following down-regulation, the ET-1 receptor was restored to the surface of the hepatocyte by prolonged incubation, although the ET-1-stimulated phosphoinositide response remained inhibited even after complete recovery of the ET-1 association capability. These results demonstrate the presence of multiple high-affinity receptors for ET-1 on hepatocytes and the direct action of this peptide on hepatic parenchymal cells via the phosphoinositide signal transduction pathway.

Evidence for a direct vasoconstrictor effect of big endothelin-1 in the rat kidney

Inhibition of endothelin-1 (ET-1)-converting enzyme has been suggested as a strategy for blocking ET-1-mediated vasoconstriction. However, it is unclear whether its putative substrate, bigET-1, is an inactive precursor. Thus, we compared in the rat the effects of ET-1 and bigET-1 on renal vascular resistance (RVR) in vitro (isolated perfused kidney, n = 15) and in vivo (Doppler shift technique, n = 23) when injected i.v. or in the rat renal artery (i.a.), before and after metalloprotease inhibition with phosphoramidon (30 mg/kg i.v.). In vitro, the ET-1/bigET-1 potency ratio for the RVR increase was 175; in vivo (i.v.) it was approximately 7 (ED50: 99 and 692 pmol/kg, respectively; P < 0.01). Unlike that of ET-1, the bigET-1 effect started slowly (peak effect at 15 min). On i.a. injection, the ED50 of ET-1 was lower but that of big ET-1 was unchanged (ED50: 28 and 706 pmol/kg, respectively). Moreover, the effect of i.a. bigET-1 on RVR was biphasic, with a dose-related rapid increase followed by a slowly developing further rise. Phosphoramidon completely inhibited the hemodynamic effects of i.v. bigET-1, but abolished only the second phase of the response when given i.a. It also significantly enhanced the effect of ET-1. We conclude that in the rat: (1) bigET-1 may affect RVR by both a direct effect and through phosphoramidon-sensitive conversion to ET-1; (2) the direct vasoconstrictor effect of bigET-1 might be expressed during endothelin-converting enzyme inhibition; (3) metalloproteases are involved in ET-1 degradation.

Endothelin inhibits vasopressin-stimulated water permeability in rat terminal inner medullary collecting duct

Renal tubule solute and water transport is subject to regulation by numerous factors. To characterize direct effects of the recently discovered peptide endothelin (ET) on renal tubule transport, we determined signaling mechanisms for ET effects on vasopressin (AVP)-stimulated water permeability (PF) in rat terminal inner medullary collecting duct (IMCD) perfused in vitro. ET caused a rapid, dose-dependent, and reversible fall in AVP- but not cyclic AMP-stimulated PF, suggesting that its effect on PF is by inhibition of cyclic AMP accumulation. Indomethacin did not block ET actions, ruling out a role for prostaglandins in its effect. The protein kinase C (PKC) inhibitor calphostin, or pretreatment of perfused tubules with pertussis toxin, blocked ET-mediated inhibition of AVP-stimulated PF. ET caused a transient increase in intracellular calcium ([Ca2+]i) in perfused tubules, an effect unchanged in zero calcium bath or by PT pretreatment. ET effects on PF and [Ca2+]i desensitized rapidly. Inhibition of PF was transient and largely abolished by 20 min ET preexposure, and repeat exposure to ET did not alter [Ca2+]i. In contrast, PGE2-mediated inhibition of AVP-stimulated PF and increase of [Ca2+]i were sustained and unaltered by prior exposure of IMCD to ET. Thus desensitization to ET is homologous. We conclude that ET is a potent inhibitor of AVP-stimulated water permeability in rat terminal IMCD. Signaling pathways for its effects involve both an inhibitory guanine nucleotide-binding protein and phospholipase-mediated activation of PKC. Since ET is synthesized by IMCD cells, this peptide may be an important autocrine modulator of renal epithelial transport.

Sepsis-associated renal vasoconstriction: potential targets for future therapy

Systemic sepsis is associated with acute deterioration in renal function despite normal or increased cardiac output. The kidney is often structurally normal, but severe renal vasoconstriction underlies a marked decrease in the glomerular filtration rate (GFR). The mechanisms underlying renal vasoconstriction in sepsis include locally and systemically released vasoconstrictors. Novel peptide and lipid-derived mediators that have been implicated in experimental models of sepsis include endothelin (ET)-1, thromboxane A2 (TXA2), leukotrienes (LTs), and, most recently, noncyclooxygenase-derived prostaglandin F2 (PGF2) analogues. Plasma ET-1 levels are elevated in septic patients and following endotoxin administration in experimental animals; antagonism of the endogenous actions of ET-1 is associated with improvement in renal perfusion and function during experimental endotoxemia. Antagonists of the TXA2 receptor and/or TXA2 synthesis in vivo have been associated with selective improvement in renal vascular tone and preservation of GFR during experimental endotoxemia in several mammalian species. Furthermore, antagonism of the TXA2 receptor inhibits the actions of endogenously released free radical-generated PGF compounds. The latter are the most potent renal vasoconstrictors among the family of arachidonic acid derivatives. Sulfidopeptide LTs, in particular LTC4 and LTD4, are also implicated in the renal vasoconstriction that attends sepsis in rats and other experimental animals. LT hepatobiliary elimination is suppressed during sepsis and endogenous LT production is enhanced. Antagonism of LTD4 receptors is associated with amelioration of renal vasoconstriction. Taken together, these novel potential mediators of renal vasoconstriction during sepsis constitute specific molecular targets for future therapeutic interventions.

Transcriptional activation of the urokinase receptor gene by endothelin-1

Using an immortalized human glomerular epithelial cell line (E71 A1), we studied the effect of endothelin-1 (ET-1), a potent vasoconstrictor peptide, on the synthesis of urokinase type plasminogen activator (u-PA) and its receptor (u-PAR). The results show that ET-1 had no effect on u-PA synthesis but induced an increase in u-PAR number (2.8 +/- 0.6 x 10(4) vs 1.2 +/- 0.5 x 10(4) sites per cell, p less than 0.001) without change in receptor affinity (280 +/- 80 pM vs 250 +/- 50 pM, NS), maximal effect being observed at 10(-7) M. Time course shows that a plateau was reached after a 24 hour incubation. ET-1 induced-increase in binding capacity was abolished by cycloheximide. ET-1 also induced an increase in u-PAR mRNA level, which was completely blocked by alpha-amanitin (5 micrograms/ml). Cycloheximide (1 microgram/ml) alone induced an increase in u-PAR mRNA level and this effect was enhanced when cycloheximide was combined with ET-1. Our data show that ET-1 can induce an increase in membrane expression of u-PAR through activation of the transcription of the u-PAR gene and de novo protein synthesis.

Role of neutral endopeptidase in the metabolism of endothelin

Endothelin is a potent vasoconstrictor produced by endothelial cells. Although endothelin has been studied extensively, little is known about its metabolism in vivo. Neutral endopeptidase EC.3.4.24.11 is reported to degrade endothelin in vitro. Therefore, we studied the effect of neutral endopeptidase inhibition by SQ29,072 on plasma levels and urinary excretion of endogenous and exogenous endothelin. Injection of 30 or 60 mg/kg SQ29,072 into anesthetized rats increased the urinary excretion of endothelin nearly 14-fold. The response was maximal during the first 30 minutes of collection and lasted for 90 minutes. The larger dose of inhibitor caused a 37-43% increase (p less than or equal to 0.05) in the plasma concentration of endothelin. Only 0.20 +/- 0.04% of the total radioactivity injected as 125I-endothelin (1 microCi; 1,308 pg) into normal rats was recovered in the urine within 30 minutes. Urinary radioactivity increased to 0.54-0.63% (p less than or equal to 0.05) of the total infused in rats pretreated with SQ29,072. Chromatographic analysis of radioactivity in the urine revealed that intact endothelin accounted for only 6-9% of the total counts in control rats but 50-56% in rats pretreated with the inhibitor. We also studied the effects of another inhibitor of neutral endopeptidase, SQ28,063, on the distribution of radioactivity in the urine, kidney, and lung of rats injected with 125I-endothelin. SQ28,063 increased urinary excretion of labeled endothelin and increased total radioactivity accumulated in the lung and kidney from 157 and 105 pg to 234 and 157 pg, respectively. Intact endothelin accounted for 90% or more of the accumulated counts in both tissues. These results indicate that 1) little circulating endothelin is cleared into the urine, 2) endothelin in the urine is likely of renal origin, and 3) neutral endopeptidase EC.3.4.24.11 plays a major role in the inactivation of endothelin.

The endothelin receptor antagonist, BQ-123, inhibits angiotensin II-induced contractions in rabbit aorta

The purpose of this study was to examine the specificity of the cyclic pentapeptide ET(A) receptor antagonist BQ-123. BQ-123 competitively antagonized endothelin-1-induced contractions in rabbit aorta, increases in inositol phosphates in cultured rat vascular smooth muscle A10 cells, and binding of [125I]endothelin-1 to the cloned ETA receptor cDNA expressed in Cos 7 cells. In contrast, BQ-123 was a weak antagonist of [125I]endothelin-3 binding to rat cerebellar membranes and to membranes from Cos 7 cells transfected with the cloned ETB receptor cDNA. BQ-123 shifted concentration-response curves in isolated rabbit aorta elicited by angiotensin II, but did not bind to angiotensin II receptors nor affect angiotensin II-induced increases in inositol phosphates. BQ-123 also did not affect contractions induced by KCl or norepinephrine. These data suggest that endothelin may play a role in angiotensin II-induced contractions of rabbit aorta.

Intracellular distribution of endothelin-1 receptors in rat liver cells

We studied the binding of (125I)-endothelin-1 as well as that of the vasopressin analogue (125I)-[8-phenylpropionyl]-LVP to purified plasma membranes, Golgi cisternae and cell nuclei from rat liver. Cell organelles were isolated by differential centrifugation and discontinuous sucrose gradients. Endothelin-1 exhibited specific binding to plasma membranes, Golgi cisternae and nuclei, while the binding of (125I)-[8-phenylpropionyl]-LVP was restricted to the plasma membranes. The number of receptors (Bmax) and the binding constants (Kd) were determined by Scatchard analysis of competition binding studies. In all cases only one class of Et-1 binding sites could be detected. The presence of Et-1 receptors on the Golgi complex either indicates that the receptor is glycosylated within the cisternae or alternatively, there exists a recycling pathway. The unexpected finding of Et-1 receptors on highly purified nuclei suggests that this peptide may exert part of its biological functions intracellularly via the nucleus.

Plasma endothelin levels in preeclampsia: elevation and correlation with uric acid levels and renal impairment

OBJECTIVE

The purpose of this study was to determine if endothelin levels are elevated in women with preeclampsia and if these levels correlated with other laboratory features of disease severity.

STUDY DESIGN

Parameters were compared in four groups of women volunteers by means of analysis of variance: (1) 16 women with preeclamptic pregnancies, (2) 11 pregnant women without preeclampsia, of similar lengths of gestation, (3) six otherwise normal women with pregnancies at term or beyond (greater than 38 weeks), and (4) 22 normotensive young women.

RESULTS

Endothelin levels were elevated in women with preeclampsia as compared with those of gestation-matched pregnant and nonpregnant controls (22.6 +/- 2.0 vs 12.0 +/- 1.0 vs 10.4 +/- 1.3 pmol/L, p less than 0.005, preeclampsia vs controls) and also were increased in late gestation (17.7 +/- 2.0 pmol/L). Endothelin correlated positively with plasma levels of uric acid (r = 0.698, p less than 0.005) and inversely with creatinine clearance (r = -0.659, p less than 0.05).

CONCLUSION

Circulating endothelin levels are elevated in women with preeclampsia and correlate closely with serum uric acid levels and measures of renal dysfunction. These observations suggest that endothelin may contribute to renal vasoconstriction in preeclampsia.

Endothelin-1 stimulates eicosanoid production in cultured human nasal mucosa

Endothelin (ET) has been shown to contract both vascular and nonvascular smooth muscle and to stimulate human nasal glandular secretion of serous and mucous cell products. Some effects of ET are thought to be mediated by eicosanoid production. To explore the direct effect of ET on arachidonate metabolism in cultured human nasal mucosal explants, eicosanoids were measured after ET-1 stimulation. After labeling the explants with [3H]arachidonic acid (AA), supernatant from control and ET-1-treated explants were fractionated by reverse-phase high-performance liquid chromatography (HPLC). The resulting elution pattern suggested the release of prostaglandin (PG) E2 and AA in response to ET-1 stimulation. Radioimmunoassay after HPLC resolution confirmed that ET-1 induced a significantly increased release of PGE2 as well as PGD2, PGF2 alpha, thromboxane B2, and 15-hydroxyeicosatetraenoic acid (15-HETE). Although significant amounts of 15-HETE were generated, cyclooxygenase product generation was most remarkable. Eicosanoid release after ET-1 exposure (10 to 0.1 microM) is concentration dependent and occurs within 1 h. Whereas 15-HETE release was maximal at 4 h, prostanoid production was maximal 1 h after exposure to ET-1. Other assayed AA metabolites, including the peptidoleukotrienes, did not significantly change after ET-1 stimulation. We conclude that ET-1 induces the release of predominantly cyclooxygenase products from cultured human nasal mucosal explants.

A novel radioligand [125I]BQ-3020 selective for endothelin (ETB) receptors

A linear endothelin (ET) analog, N-acetyl-LeuMetAspLysGluAlaValTyrPheAlaHisLeu-AspIleIleTrp (BQ-3020), is highly selective for ETB receptors. BQ-3020 displaces [125I]ET-1 binding to ETB receptors (nonselective to ET isopeptides) in porcine cerebellar membranes (IC50: 0.2nM) at a concentration 4,700 times lower than that to ETA receptors (selective to ET-1) on aortic vascular smooth muscle cells (VSMC) (IC50: 940nM). BQ-3020 as well as ET-1 and ET-3 elicits vasoconstriction in the rabbit pulmonary artery. The ETA antagonist BQ-123 failed to inhibit this BQ-3020-induced vasoconstriction. Furthermore, BQ-3020 elicits endothelium-dependent vasodilation. These data indicate that BQ-3020 has ETB agonistic activity. The radioligand [125I]BQ-3020 binds to cerebellar membranes at single high affinity sites (Kd = 34.4pM), whereas it scarcely binds to VSMC. [125I]BQ-3020 binding to the cerebellum was displaced by BQ-3020, ET-1 and ET-3 in a nonselective manner (IC50: 0.07-0.17nM). However, the binding of [125I]BQ-3020 was insensitive to the ETA antagonist BQ-123 and other bioactive peptides. Both [125I]ET-1 and [125I]BQ-3020 show slow onset and offset binding kinetics to ETB receptors. These data indicate that the radioligand [125I]BQ-3020 selectively labels ETB receptors and that the slow binding kinetics of ET-1 are dependent on the peptide sequence from Leu6 to Trp21, but not on the structure formed by its two disulfide bridges.

Endothelin receptors in rat renal papilla with a high affinity for endothelin-3

A high density of binding sites for endothelin has been described in rat renal papilla but the nature and significance of papillary endothelin receptors have not yet been evaluated. In the current study, the effect of endothelin peptides on phosphatidylinositol turnover in papillary tubules has been investigated. Endothelin-1, endothelin-3 and the endothelin-related peptide sarafatoxin S6b all stimulated the accumulation of inositol phosphates in [3H]inositol-labelled papillary tubule preparations. However, at these papillary receptors endothelin-3 was more potent than endothelin-1. In other tissues, endothelin-1 is more potent than endothelin-3 at endothelin receptors coupled to phosphatidylinositol turnover. The EC50 value for endothelin-3 expressed as the negative logarithm was 9.3 +/- 0.13 compared with 8.42 +/- 0.11 for endothelin-1 (mean +/- S.E.M., n = 5 in each case, P less than 0.01). The affinity of sarafatoxin S6b was similar to that for endothelin-3 (9.2 +/- 0.15, n = 3). These findings raise the possibility of a direct tubular function of endothelin and suggest that endothelin-3 rather than endothelin-1 may be the natural agonist for these papillary receptors.

Characteristics of binding of endothelin-1 and endothelin-3 to rat hearts. Developmental changes in mechanical responses and receptor subtypes

Endothelin-1 (ET-1) and endothelin-3 (ET-3) produced positive inotropic effects on electrically stimulated left atria and increased the frequency of spontaneously beating right atria of adult rats. The potency of the inotropic effect of ET-1 was greater than that of ET-3, but the potencies of the chronotropic effects of ET-1 and ET-3 were not significantly different. In the neonatal atria, ET-1 and ET-3 also induced positive inotropic and chronotropic responses. ET-1 and ET-3 showed weak or no cardiotonic effects on the adult ventricles, whereas they caused marked positive inotropy in the neonatal ventricles. The characteristics of binding sites for ET-1 and ET-3 were very similar between the atria and the ventricles of the rat neonate. Saturation and competition binding experiments have shown that neonatal cardiac membranes from both atria and ventricle have two distinct binding sites for endothelin, that is, a low-affinity and a high-affinity site. ET-1 was found to bind to the low-affinity sites with a significantly lower Kd than ET-3, whereas the estimated Kd values for ET-1 and ET-3 at the high affinity sites were similar. In contrast, the binding sites in adult atria were different from those of the ventricles: only a single binding site for both ET-1 and ET-3 was detected. Adult atrial membranes, on the other hand, had two distinct binding sites similar to those of neonatal membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelets stimulate expression of endothelin mRNA and endothelin biosynthesis in cultured endothelial cells

Modulation of the biosynthesis of the vasoconstrictor peptide endothelin was studied in cultured endothelial cells. Immunoreactive endothelin (irET) levels were significantly elevated in conditioned medium from bovine pulmonary artery endothelial (BPAE) or human umbilical vein endothelial cells when coincubated with washed human platelets. Platelets (approximately 200,000 cells/microliters) enhanced irET levels approximately 250% over basal levels. Stimulation of irET levels in BPAE cell-conditioned medium by platelets was time and platelet number dependent. Platelets, as well as thrombin and transforming growth factor-beta 1, stimulated the expression of preproendothelin-1 mRNA in a time-dependent manner. Coincubation of low doses of thrombin (0.1 unit/ml) and subthreshold concentrations of platelets with BPAE cells resulted in a further enhancement of irET levels in conditioned medium. Platelet-mediated stimulation of irET production was not significantly affected by indomethacin (1 microM) or the platelet-activating factor receptor antagonist WEB 2086 (1 microM); however, coincubation of endotoxin (100 ng/ml) with platelets and BPAE cells resulted in significantly higher levels of irET. Whether direct contact or adhesion between platelets and endothelial cells is necessary for stimulating irET release was studied by separating platelets from BPAE cells with a 0.4 microns permeable membrane. Under these conditions, platelets still produced significant elevations (approximately 190% over basal levels) in irET levels in BPAE cell-conditioned medium. In addition, platelet-free buffer from agonist-induced platelet aggregation also significantly enhanced irET production (200% over basal values). These data indicate that a platelet-derived regulatory factor can induce the biosynthesis of endothelin from cultured endothelial cells and also suggest that platelets might play a role in vasomotor regulation via a novel intercellular interaction with the endothelium.

Design and synthesis of a specific endothelin 1 antagonist: effects on pulmonary vasoconstriction

The 21-amino acid vasoconstrictor peptide endothelin (Et) contains two disulfide bonds. We investigated the importance of the outer disulfide bond in Et-1 by replacing it with an amide linkage. Bioactivity was assessed in an isolated guinea pig lung preparation (perfused at constant flow with Ringer's solution/0.5% albumin) in which pulmonary artery pressure was monitored. Et-1 produced concentration-dependent pulmonary vasoconstriction at concentrations of 1 x 10(-10) M and higher. [Dpr1, Asp15]Et-1 (where Dpr is diaminopropionic acid), in which the outer disulfide was replaced by an amide bond and the inner disulfide was left intact, showed no agonist activity at 1 x 10(-6) M but 1 x 10(-7) M [Dpr1, Asp15]Et-1 inhibited Et-1-induced pulmonary vasoconstriction: effects of 1 x 10(-10) M 2 x 10(-10) M, and 1 x 10(-9) M Et-1 were inhibited by 98%, 75%, and 65%, respectively. Furthermore, this analog did not alter pulmonary vasoconstriction induced by thrombin, norepinephrine, or, most significantly, Et-3. A monocyclic Et-1 analog with the same sequence but in which the amide bond was not formed showed weak pulmonary vasoconstrictor activity (300-500 times less potent than Et-1) but had no antagonist activity. In addition, both the monocyclic control peptide and [Dpr1, Asp15]Et-1 competed effectively with 125I-labeled Et-1 for binding to cultured rat pulmonary artery smooth muscle cells. Thus, an Et-1 structural analog produced by replacement of the outer disulfide bond with an amide linkage displayed potent and specific Et-1 antagonism.

Affinity labeling of endothelin receptors in bovine and rat lung membranes by N epsilon 9-azidobenzoyl-125I-endothelin-1

Endothelin-1 (ET-1) is a potent, vasoconstrictive peptide isolated from culture media of vascular endothelial cells. The binding of ET-1 to membrane preparations from rat and bovine lung was studied using radioiodinated ET-1 (125I-ET-1). With both membrane preparations, 125I-ET-1 showed saturable binding to a single class of high affinity sites. Scatchard analysis of the binding data gave dissociation constants (Kd) for ET-1 of 0.22 nM and 0.15 nM, and receptor densities (Bmax) of 6.1 pmol/mg and 2.7 pmol/mg for rat and bovine lung membranes, respectively. Photo-reactive radioiodinated ET-1, N epsilon 9-azidobenzoyl-125I-ET-1, was synthesized and purified as a mono-reactive affinity labeling reagent. This reagent was used for affinity labeling of ET-1 receptor in bovine and rat lung membranes. Photoaffinity labeling followed by sodium dodecyl sulfate gel electrophoresis and autoradiography gave a radiolabeled protein band with an apparent Mr of 34,000 in both membrane preparations. The labeling of this protein band was inhibited by cold ET-1 in a concentration-dependent manner. Labeling was not abolished by unrelated peptides such as angiotensin II and [Arg8]-vasopressin, or by structurally related bee venom apamin. These results indicate that the ET-1 receptor or its ligand binding subunit consists of a 34,000 Da polypeptide.

Renal glomerular endothelin receptors in rats with high-output heart failure

Plasma immunoreactive endothelin (irET) concentration and renal glomerular ET receptors were investigated in rats with chronic high-output heart failure. Plasma irET was higher (0.67 +/- 0.03 fmol/ml vs. 0.52 +/- 0.04 fmol/ml) and the density of glomerular ET receptors lower (Bmax: 420 +/- 20 fmol/mg protein vs. 510 +/- 12 fmol/mg protein) in rats with heart failure than in controls. Our results indicate that circulating ET levels are responsive to changes in cardiovascular hemodynamics and suggest a potential role for ET as a vasoactive regulatory peptide during heart failure.

Localization of endothelinlike immunoreactivity and endothelin binding sites in human colon

The potent vasoconstrictor endothelin was originally isolated from vascular endothelial cells but has since been found in several other tissues. The aim of this study was to establish whether endothelinlike immunoreactivity occurs in human enteric nerves and to identify endothelin binding sites using immunocytochemical and in vitro autoradiographic techniques. Endothelinlike immunoreactivity was localized to nerve bundles throughout the colon and to most of the ganglion cells of the two major plexuses, many of which costored vasoactive intestinal polypeptide. High-affinity (dissociation constant = 0.35 +/- 0.014 nmol/L; mean +/- SEM) binding sites for endothelin 1, with an apparent binding capacity of 92 +/- 6.3 amol/mm2 (mean +/- SEM), were demonstrated in the myenteric plexus, with less dense binding being found in the submucous plexus, mucosa, muscle layers, and blood vessel walls. Competition data suggested two populations of binding sites, both showing high affinities for endothelins 1 and 2, vasoactive intestinal constrictor, and sarafatoxin b but differentiated by their affinity for endothelin 3 and sarafatoxin c. This study provides evidence that endothelin is a neuropeptide in the human intestine with binding sites on neural plexuses and mucosa, suggesting a role in the modulation of intestinal motility and secretion.

Role of endothelium-derived relaxing factor in regulation of vascular tone and remodeling. Update on humoral regulation of vascular tone

In addition to preserving the permselectivity of the vascular wall and providing an antithrombogenic surface, the vascular endothelium contributes importantly to the regulation of vasomotor tone. Indeed, the endothelium participates in the conversion of angiotensin I to angiotensin II; the enzymatic inactivation of several plasma constituents such as bradykinin, norepinephrine, serotonin, and ADP; and the synthesis and release of vasodilator substances such as prostacyclin and the recently discovered endothelium-derived relaxing factor (EDRF). The diffusible EDRF released from the endothelium is nitric oxide or a substance closely related to it such as nitrosothiol. The endothelium also synthesizes and releases vasoconstrictive factors, including products derived from arachidonic acid metabolism and the recently discovered peptide endothelin. An increasing body of evidence from experimental and clinical studies indicates that EDRF and endothelium-derived contracting factors play an important role in vascular physiology and pathology. It has become apparent that the balance of these factors may be a major determinant of systemic and regional hemodynamics. Moreover, through generally opposite effects on growth-related vascular changes, contracting factors such as endothelin and relaxing factors such as EDRF also may be important determinants of the vascular response to injury in various disease states such as atherosclerosis and hypertension. It is clear that the vascular endothelium is a complex and dynamic organ. Understanding endothelium function in normal physiology and disease states is of potential clinical importance and should be the focus of future investigation.

Intracellular ramification of endothelin signal

Effects of porcine 1-21 endothelin (ET-1) on [Ca2+]i, [Na+]i, and [Cl-]i and on membrane potential were studied in individual mesangial (MC) and vascular smooth muscle (VSMC) cells using microspectrofluorimetry of fura-2, SBFI, SPQ, and bis-oxonol, respectively. ET-1 increased [Ca2+]i by fivefold, showing an immediate and a sustained phase of response. Ca(2+)-free medium and nifedipine pretreatment significantly curtailed the sustained phase of response to ET-1. These findings were confirmed in studies of vascular ring preparations, demonstrating that Ca2+ influx may account for at least 50% of contraction. ET-1 caused immediate and sustained depolarization of MC and VSMC. This could not be attributed to Na+ influx, since fluorescence of SBFI was not affected by ET-1 and Na(+)-free medium did not abolish the ET-1-induced membrane depolarization. Studies of SPQ fluorescence changes induced by ET-1 revealed an increase in fluorescence intensity consistent with the decrease in [Cl-]i. A Cl- channel blocker, IAA-94, abolished changes in SPQ fluorescence and curtailed sustained phases of membrane depolarization and [Ca2+]i elevation in response to ET-1, but did not affect KCl-induced [Ca2+]i transients. IAA-94 also attenuated the ET-1-induced contraction of aortic rings. Microinjection of either calcium gluconate or inositol 1,4,5-trisphosphate (IP3) in SPQ-loaded cells resulted in an increase in fluorescence mimicking the effect of ET-1. These changes were blunted by pretreatment of cells with BAPTA and incubation in Ca(2+)-free medium. When IP3 was microinjected into fura-2-loaded MC, this resulted in immediate and sustained elevation of [Ca2+]i. In conclusion, generation of IP3 results in mobilization of intracellular Ca2+ stores and activation of Cl- channels. Ensuing Cl- efflux causes membrane depolarization and, in turn, activation of voltage-dependent Ca2+ channels, resulting in sustained elevation of [Ca2+]i which is indispensable for the full-scale contraction produced by ET-1.

The bovine endothelin receptor has an apparent molecular weight of 43,000

In crosslinking experiments, [125I]endothelin-1 was treated with N-hydroxysuccinimidyl-4-azidobenzoate, purified by HPLC, allowed to bind to bovine aortic membranes and then photoactivated. Autoradiography of sodium dodecyl sulfate polyacrylamide gel electrophoretograms of the products of this reaction showed that a component of apparent Mr = 42,000 was specifically labelled by endothelin-1 under reducing conditions. Under nonreducing conditions, a small amount of 125I-labelled endothelin-1 specifically labelled a component of apparent Mr = 45,900 in the absence of crosslinking agent. Non-radiolabelled endothelin analogues with a wide range of binding affinities inhibited specific labelling of the Mr = 42,000 and 45,900 components in parallel over the concentration ranges which inhibited binding of radiolabelled endothelin. Specific labelling of these components was also observed in parallel in membranes from bovine heart and kidney. The components labelled in the presence and absence of crosslinker appear to be the same, and the small difference in apparent Mr in the labelled components is likely due to a difference in conformational constraints arising from the two labelling processes, with a true, corrected Mr of 43,400. Since the specific labelling of this component is related to physiologically relevant binding in several bovine tissues, we conclude that it is a component of the bovine endothelin receptor.

Detection of an endothelin-1-binding protein complex by low temperature SDS-PAGE

We found that the complex of ET-1 and its binding protein was stable enough to be separated by SDS-PAGE when electrophoresis was run at a low temperature. Cross-linking was not necessary for the detection of [125I]-ET-1 and its binding protein complex by autoradiography. This simple method could be used in qualitative (estimation of apparent molecular weight of ET-1 binding protein) and quantitative (determination of relative content of ET-binding protein) analysis of the ET-binding protein complex. ET-binding protein complexes of various animal species and organs were investigated by this method.

Endothelin-induced intracellular Ca2+ mobilization through its specific receptors in murine peritoneal macrophages

We studied the presence of specific binding sites for endothelin (ET) and the effect of ET on cytosolic free Ca2+ concentration ([Ca2+]i) in murine thioglycolate-activated peritoneal macrophages. Scatchard analysis for binding experiments using [125I]ET-1 or [125I]ET-3 revealed the existence of a single class of binding sites. The binding parameters (Kd and Bmax) for [125I]ET-1 were almost identical to those for [125I]ET-3. In addition, unlabeled 3 ET isopeptides (ET-1, ET-2 and ET-3) inhibited the specific binding of both ET-1 and ET-3 with similar inhibitory potencies. All 3 ET isopeptides caused an increase in [Ca2+]i in the same dose-dependent manner (0.01-100 nM). These results demonstrate the existence of an ET receptor with the same affinity for all isoforms that mediates the ET-induced intracellular Ca2+ mobilization in murine peritoneal macrophages.

Multiple subtypes of endothelin receptors in porcine tissues: characterization by ligand binding, affinity labeling and regional distribution

To clarify the existence and the distribution of endothelin (ET) receptor subtypes, we have examined the pharmacological properties and the molecular weight (Mr) of 125I-ET-1 and 125I-ET-3 binding sites in various tissues of pigs. ET-1 and ET-2 showed almost identical potencies in displacing the bound 125I-ET-1 in all the tissues examined. ET-3, sarafotoxin S6b (SRT-b) and sarafotoxin S6c (SRT-c) displaced the 125I-ET-1 with the same sensitivity as ET-1 (IC50 = 0.1-1.4 nM) in brain, kidney, liver and adrenal, whereas the three peptides showed very weak competition (IC50 = 40-500 nM) against 125I-ET-1 binding in cardiac atria, aorta, lung, stomach and uterus. The computer analyses of the binding data suggested the presence of high (Kd1 = 0.04-0.29 nM) and low (Kd2 = 60-190 nM) affinity binding sites for ET-3 and SRT-b in lung and stomach. 125I-ET-3 bound to the high affinity sites in lung and stomach was displaced by ET/SRT isopeptides almost equipotently. Two proteins with Mr of 47,000 and 35,000 were affinity-labeled with 125I-ET-1 in cerebellum, while a protein with Mr of 123,000, in addition to the two proteins, was predominantly labeled in lung. The above findings indicated that two distinct subclasses of ET receptors, namely, ET-1-specific and ET/SRT family-common receptors were distributed in various proportions in mammalian tissues, and suggested that their molecular forms are also different.