Contractile responses of the human urinary bladder, renal pelvis and renal artery to endothelins and sarafotoxin S6b

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Expression of the ET(A) and ET(B) endothelin receptor subtype mRNA in human detrusor cultured smooth muscle cells

The aim of this study was to determine the expression of the endothelin receptor subtype mRNAs in human detrusor cultured smooth muscle cells using reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization (ISH). First strand cDNA was made from human detrusor cultured smooth muscle cells total RNA and used for PCR with primers designed to amplify fragments of the ET(A) and ET(B) endothelin receptor subtype cDNA sequences. Subcloned fragments of the ET(A) and ET(B) endothelin receptor cDNAs were used to synthesize digoxigenin-labeled cRNA probes by in vitro transcription. COS-7 cells transfected with the ET(A) and ET(B) receptor cDNAs were used as positive control and to confirm the absence of cross-hybridization due to sequence homology. Both ET(A) and ET(B) receptor mRNAs were detected by RT-PCR analysis. By ISH, both ET(A) and ET(B) receptor subtype mRNAs were detected. However, ET(A) signal was much more intense than ET(B) signal. These results indicate that mRNAs for both ET(A) and ET(B) receptors are expressed in detrusor smooth muscle cells of human urinary bladder. The ET(A) receptor is the predominant detrusor ET receptor.

Potential role of endothelin and nitric oxide in physiology and pathophysiology of the lower urinary tract

Endothelium-derived vasoactive mediators (endothelin-1 with its vasoconstrictive and mitogenic properties and nitric oxide with its vasodilatory and antiproliferative properties) play an important role in the regulation of vascular smooth muscle tone and cellular proliferation. Several recent studies have now demonstrated the presence of these vasoactive agents in the urinary tract where they are thought to play a prominent role in urinary tract physiology and disease. This article reviews the synthesis, localisation and actions of endothelin and nitric oxide in the lower urinary tract and examines the possible role of these mediators in disease.

Molecular pharmacology and pathophysiological significance of endothelin

Since the discovery of the most potent vasoconstrictor peptide, endothelin, in 1988, explosive investigations have rapidly clarified much of the basic pharmacological, biochemical and molecular biological features of endothelin, including the presence and structure of isopeptides and their genes (endothelin-1, -2 and -3), regulation of gene expression, intracellular processing, specific endothelin converting enzyme (ECE), receptor subtypes (ETA and ETB), intracellular signal transduction following receptor activation, etc. ECE was recently cloned, and its structure was shown to be a single transmembrane protein with a short intracellular N-terminal and a long extracellular C-terminal that contains the catalytic domain and numerous N-glycosylation sites. In addition to acute contractile or secretory actions, endothelin has been shown to exert long-term proliferative actions on many cell types. In this case, intracellular signal transduction appears to converge to activation of mitogen-activated protein kinase. As a recent dramatic advance, a number of non-peptide and orally active receptor antagonists have been developed. They, as well as current peptide antagonists, markedly accelerated the pace of investigations into the true pathophysiological roles of endogenous endothelin-1 in mature animals; e.g., hypertension, pulmonary hypertension, acute renal failure, cerebral vasospasm, vascular thickening, cardiac hypertrophy, chronic heart failure, etc. Thus, the interference with the endothelin pathway by either ECE-inhibition or receptor blockade may provide an exciting prospect for the development of novel therapeutic drugs.

Endothelin ETA and ETB receptors cause vasoconstriction of human resistance and capacitance vessels in vivo

BACKGROUND

The role of endothelin ETB receptors in mediating vasoconstriction in humans is unclear. As yet, there have been no in vivo studies in resistance vessels, and in vitro data have been contradictory. We therefore investigated the function of ETB receptors in vivo in human forearm resistance and hand capacitance vessels using endothelin-1 as a nonselective agonist at ETA and ETB receptors and endothelin-3 and sarafotoxin S6c as selective agonists at the ETB receptor.

METHODS AND RESULTS

A series of single-blind studies were performed, each in six healthy men. Brachial artery infusion of endothelin-1 and endothelin-3 caused slow-onset dose-dependent forearm vasoconstriction. Although endothelin-3 caused significantly less forearm vasoconstriction than endothelin-1 at low doses, vasoconstriction was similar to the two isopeptides at the highest dose (60 pmol/min). Endothelin-3 caused transient forearm vasodilatation at this dose, whereas endothelin-1 showed only a nonsignificant trend toward causing early vasodilatation. Intra-arterial sarafotoxin S6c caused a progressive reduction in forearm blood flow, although less than that to endothelin-1 (P = .04). Dorsal hand vein infusion of sarafotoxin S6c caused local venoconstriction that was also less than that to endothelin-1 (P = .002).

CONCLUSIONS

Selective ETB receptor agonists cause constriction of forearm resistance and hand capacitance vessels in vivo in humans, suggesting that both ETA and ETB receptors mediate vasoconstriction. Hence, antagonists at both ETA and ETB receptors, or inhibitors of the generation of endothelin-1, may be necessary to completely prevent vasoconstriction to endogenously generated endothelin-1.

Contractile activity of endothelin precursors in the isolated gallbladder of the guinea-pig: presence of an endothelin-converting enzyme

A single addition of 3 x I0-7 M ET-1, ET-2 or ET-3 produced contractions that reached a steady state in 28.2 +/- 4.2, 21.1 +/- 1.3 and 24.0 +/- 3.8 min, respectively and took 2.7 +/- 0.4, 2.1 + 0.1 and 1.6 +/- 0.1 min to reach half of this steady-state response.4. Contractions induced by 3 x I0-7 M big ET-11-38 or big ET-11- 39 reached a plateau in 38.5 +/- 3.6 and 35.6 +/- 3.3 min, respectively, and half of these responses were attained in 12.0 +/- 2.5 and 7.1 +/- 1.1 min.Thus, these contractions developed more slowly than those induced by ET-1. Contractions induced by 3 x 10-7 M big ET-21-38 were also much slower to develop than those to ET-2, for these took 49 +/- 2 min to reach plateau and 19.4 +/- 2.1 min to attain half that response. Contractions induced by 3 x 10-7 M big ET-31-41 amide took 50.2 +/- 3.7 min to reach a plateau and 27.3 +/- 3.0 min to reach half of this response.5. Phosphoramidon (0.1, 1 and 3 x 10-4 M) inhibited contractions induced by big ET-11.39. For instance,the contractions induced by 3 x 10-7 M big ET-11-39 were inhibited by 10-4 M or 3 x 10-4 M of phosphoramidon by 62.8 +/- 6.7% or 74.5 +/- 4.6%, respectively. Similarly, contractions induced by ET-21-38 were inhibited by 91.3 +/- 5.4% and the small response induced by big ET-3l-4l amide was abolished by 3 x 10-4M phosphoramidon. Conversely, the neutral endopeptidase (EC 24.11) inhibitor DL-thiorphan(3 x 10-4 M) had no effect. Captopril (10-5 M), pepstatin A (10-5 M), phenylmethylsulphonylfluoride(PMSF, 10-3 M), aprotinin (10-5 M), E-64 (10-5 M), cystatin (10-6 M), leupeptin (10-4 M),chymostatin (10-4 M), or bestatin (10-5 M) did not inhibit but rather increased to a similar, but small degree the contractions induced by 3 to 30 x 10-9 M big ET-11-39. Only captopril (10-5 M) or leupeptin(10-4 M) increased the contraction induced by 3 x 10-7 M big ET-11-39. Phosphoramidon (10-4 M),pepstatin (10-5 M) or PMSF (10-3 M) did not affect contractions induced by ET-1.6. Removal of the epithelium increased by 70% the size of the contraction induced by 5 microM histamine(1.08 +/- 0.05 g; n = 160 to 1.84 +/- 0.14 g; n = 12) but did not affect, in absolute terms, the contraction induced by ET-1 (as a % of the response to histamine, these responses were, of course, apparently depressed). Epithelium removal did, however, increase the size of the contractions induced by 3 to 30 x 10-9 M big ET-1 -39 which was very similar to the effect of the protease inhibitors.7. In competition binding studies on membranes prepared from the guinea-pig gallbladder, 10-11 MET-1 inhibited by 76.9 +/- 3.1% the binding of [125]-ET-I while porcine big ET-11-39 caused no inhibition(0.7 +/- 3.0; n = 3). ET-1 (10-6 M) inhibited binding by 95.7 =/- 1.1% (n = 3) while at this much higher concentration, big ET-11-39 inhibited binding by only 16.8 +/- 4.2% (n = 3). This clearly suggests that big ET-11-39 does not bind directly to ET receptors.8. Thus, a phosphoramidon-sensitive endothelin-converting enzyme (ECE), different from neutral endopeptidase (NEP; EC 24.11) and not located on the epithelium, converts big ET-1 into ET-1 in the gallbladder of the guinea-pig. This ECE appears to act preferentially on big ET-1 or big ET-2 over bigET-3.

Characterization of endothelin receptors in rat renal artery in vitro

1. The aim of this study was to investigate the function and characteristics of endothelin receptors in rat main branch renal artery in vitro. 2. Endothelin(ET)-1 (mean EC50 = 9.8 nM) was approximately 12 fold more potent than ET-3 (mean EC50 = 120 nM) as a contractile agonist and produced a greater maximum response. In contrast, neither of the ETB receptor-selective agonists, alanine[1,3,11,15]ET-1 nor sarafotoxin S6c, (0.1 nM-1 microM), induced any contractile effect, or any relaxant effect in endothelium-intact preparations pre-contracted with the thromboxane A2 mimetic, U-46619. Sarafotoxin S6c (30 nM) also failed to induce any further contraction in tissues pre-contracted with an EC50 concentration of ET-1. 3. The ETA receptor-selective antagonist, BQ123, behaved as a weak and variable antagonist of the contractile effects of ET-1 (mean pA2 estimates in the range 5.8-6.3). In contrast, BQ123 antagonized ET-3 with a potency (mean pA2 = 7.6) consistent with its affinity for ETA receptors. Co-incubation of BQ123 (3 microM) with the putative ETB receptor-selective antagonist, IRL1038 (10 microM), produced no greater antagonism of ET-1 responses than was induced by BQ123 (3 microM) alone. 4. In conclusion, ETB receptors do not appear to be present in rat main branch renal artery. The contractile effects of ET-3 in this tissue seem to be mediated by ETA receptors. While ETA receptors partly mediate the contractile effects of ET-1, these data raise the possibility that a population of novel BQ123-insensitive endothelin receptors may also contribute to this response.

[125I]-PD151242: a selective ligand for endothelin ETA receptors in human kidney which localizes to renal vasculature

1. The linear tetrapeptide radioligand, [125I]-PD151242 was used to characterize ETA receptors in human kidney which is an ETB-rich tissue. Saturation binding assays with [125I]-PD151242 revealed a single population of high affinity endothelin receptors: KD = 0.75 +/- 0.07 nM and Bmax = 48.4 +/- 1.6 fmol mg-1 protein (n = 3 individuals +/- s.e.mean). Hill slopes were close to unity and a one site fit was preferred to a two site model. 2. ETA-receptor-selective ligands competed for [125I]-PD151242 binding with sub-nanomolar affinity: BQ123 KD = 0.43 +/- 0.10 nM, Bmax = 46.6 +/- 7.9 fmol mg-1 protein; FR139317, KD = 0.37 +/- 0.06 nM, Bmax = 39.5 +/- 6.5 fmol mg-1 protein (n = 3 individuals +/- s.e.mean). In each case, monophasic inhibition curves were obtained and a one site fit was preferred to a two site model. The ETB-selective agonist, BQ3020 at the highest concentration tested (10 microM) inhibited binding by only 50%. The non-selective RO462005 competed for the binding of [125I]-PD151242: KD = 1.31 +/- 1.38 microM, Bmax = 33.0 +/- 9.7 fmol mg-1 protein. Endothelin-2 and sarafotoxin S6B inhibited [125I]-PD151242 binding to renal tissue whereas ET-3 and sarafotoxin S6C were less effective. Non-endothelin and non-sarafotoxin peptides did not compete. 3. No degradation of [125I]-PD151242 was detected following incubation of the ligand with renal tissue under the conditions of the binding assay. 4. Polymerase chain reaction products corresponding to the expected size for mRNA encoding ETA and ETB receptor sub-types were detected in cortex and medulla in each of the five individuals examined.5. Autoradiographical studies showed that ETA receptors visualised with ['25I]-PD151242 were mainly localized to blood vessels including interlobular and arcuate arteries, arterioles and adjacent arcuate veins. ETB receptors localized with ['251]-BQ3020 were concentrated in the medulla and the density of binding to vessels was low.6. These data suggest [251I]-PDl51242 is selective for ETA receptors in human kidney and this sub-type is mainly localized to the renal vasculature. The results provide further evidence that the human vasculature mainly expresses the ETA receptor.

Characterization of endothelin (ET) receptors in the isolated gall bladder of the guinea-pig: evidence for an additional ET receptor subtype

1. We have characterized the receptors mediating contractions induced by endothelin-1 (ET-1), ET-2, ET-3 and the ETB-selective receptor agonists, sarafotoxin 6c (SX6c), IRL 1620, BQ-3020, [Ala1,3,11,15]ET-1 and ET (16-21) in strips of the isolated gall bladder of the guinea-pig (GPGB). We used as antagonists BQ-123 (ETA receptor selective) and PD 145065 (ETA/ETB receptor non-selective). 2. ET-1, ET-2 and ET-3 (10(-10) M to 3 x 10(-7) M) caused similar slowly-developing concentration-dependent contractions of the GPGB. Contractile effects induced by ET-1, ET-2 or ET-3 (at 3 x 10(-7) M) were also similar (230 +/- 25, 241 +/- 7 and 287 +/- 37% of that to histamine at 5 x 10(-6) M, n = 7, 6, 12, respectively). However, the threshold concentration for ET-1 or ET-2 was 10(-10) M whereas it was 3 x 10(-9) M for ET-3. 3. SX6c (10(-10) M to 3 x 10(-7) M) also caused slowly-developing concentration-dependent contractions at a threshold concentration of 10(-10) M (n = 16). However, the contraction caused by SX6c at 3 x 10(-7) M was 116 +/- 9% of that to histamine at 5 x 10(-6) M, which was half of that induced by the same concentration of the ET isopeptides. The contraction induced by IRL 1620 at 3 x 10-7 M (n = 9) was 43 +/- 9% of that to histamine at 5 x 10-6 M, which was one fifth of that produced by the same concentration of ET-1. Contractions induced by BQ-3020 or [Ala1,3,11,15]ET-I at 3 x 10-7 M were even less than those produced by IRL 1620. ET (16-21) was inactive up to 10-5 M. Addition of a concentration of 3 x 10-7 M of ET-1 to tissues with developed contractions induced by the bolus addition of 3 x 10-7 M SX6c caused a further contraction of the GPGB to the level observed with ET-1 alone at 3 x 10-7M (n = 8).4. BQ-123 (10-5 M) did not affect the concentration-response curve to ET-1 and the contraction induced by 3 x 10- M was also not affected (n = 5; 239 +/- 19% of histamine at 5 x 10-6 M). PD 145065(10-5 M) shifted the ET-1 concentration-response curve to the right and the contraction induced by ET-1at 3 x 10-7 M was inhibited by 15% (n = 6; NS). A higher concentration of BQ-123 (10-4 M) caused a significant shift to the right of the ET-1 concentration-response curve similar to that caused by PD 145065 (10-s M) and caused a 24% (n = 6; NS) inhibition of the contractions induced by ET-1 at 3 x 10-7 M. PD 145065 (10-4 M) abolished contractions induced by ET-1 (up to 10- M) and inhibited the response to ET-1 at 3 x 10-7 M by 52% (n = 4; P< 0.05).5. Contractions induced by ET-3 were more sensitive to inhibition by the antagonists. BQ-123 (10-6,10-5 or 10-4 M) inhibited responses to 3 x 10-7 M ET-3 by 66, 71 and 83%, respectively (n = 5, 5, 3;P< 0.05). PD 145065 (10-6, 10-5 or 10-4 M) attenuated more strongly than did BQ-123 the contractions induced by ET-3. For instance, the contractions caused by ET-3 at 3 x 10-7 M were decreased by 73 and 80% (n = 5, 5; P<0.05) in the presence of PD 145065 (10-6 or 10-5 M, respectively). PD 145065(10-4 M) completely abolished contractions to ET-3 (n = 4; up to 3 x 10-7M).6. Contractions induced by SX6c, especially those observed at concentrations lower than 10-8 M, were attenuated by BQ-123 (up to 10-4 M). PD 145065 (10-5M) shifted to the right the concentration response curve to SX6c and inhibited by 38% (P<0.05) the contractions induced by 3 x 10-7M.However, the contractions induced by a bolus addition of a high concentration of SX6c (3 x 10-7 M)and the subsequent addition of an identical concentration of ET-1 on top of SX6c were not affected byBQ-123 (10-6 or 10-5 M).7. These results suggest that ETB receptors are involved in the contractions induced by endothelins in the GPGB. However, SX6c and other selective ETB agonists produced only half or less than half of the contractile response induced by non-selective agonists. In addition, the responses to ET-1 but not to ET-3, were insensitive to the antagonist action of BQ-123 at 10-5 M whereas BQ-123 or PD 145065 at 10-5 M strongly antagonized contractions induced by ET-3. Finally, BQ-123 at 10-4 M inhibited contractions to ET-1 and SX6c. Thus, within the GPGB there may well be additional ET receptor(s) not conforming to the established ETA/ETB receptor subtype classification, as well as ETB receptors.

Human saphenous vein contains both endothelin ETA and ETB contractile receptors

We have investigated the endothelin receptor subtypes mediating contraction in isolated preparations of human saphenous vein. Endothelin-1 (EC50: 17.8 nM), endothelin-3 (EC50: 82.3 nM) and the endothelin ETB receptor-selective agonists, [Ala1,3,11,15]endothelin-1 (EC50: 63 nM) and sarafotoxin S6c (EC50: 0.75 nM) all produced concentration-dependent contractions of human saphenous vein, although [Ala1,3,11,15]endothelin-1 and sarafotoxin S6c only produced a contraction in approximately 50% of the preparations tested. The endothelin ETA receptor antagonist, BQ123 (D-Val,Leu,D-Trp,D-Asp,Pro; 10 microM), antagonized endothelin-1-induced contractions with an estimated potency (pKB approximately 6.0) which was an order of magnitude lower than reported previously for non-human isolated vascular tissues from other species (pA2 values approximately 7.0). These data suggest that both endothelin ETA and endothelin ETB receptors can mediate vascular smooth muscle contraction in human saphenous vein.

Identification of a ligand-binding site of the human endothelin-A receptor and specific regions required for ligand selectivity

To investigate the ligand-binding site of the human endothelin-A-receptor subtype (ETA), we have produced various chimeric and mutated receptors in chinese hamster ovary cells. The substitution of Lys140 with Ile located in the C-terminus of the second transmembrane region caused a 13-fold reduction in affinity for endothelin-1 (ET-1) and 3.6-fold lower Bmax than those values for the original receptor. Correspondingly, the mutated ETA receptor with the Lys140-->Ile substitution failed to induce an increase in the intracellular calcium concentration in the presence of 1 nM ET-1. Thus, the Lys140 in the ETA receptor is important in ligand binding. ETA and ETB receptors possess the ET isopeptides selective and non-selective binding activities, respectively. Displacement experiments and the binding of 125I-ET-3 to various chimera receptors demonstrated that both the third and fourth extracellular regions, including the flanking transmembrane regions, are responsible for the ligand-binding selectivity of the ETA receptor.

Involvement of ETA receptors in the facilitation by endothelin-1 of non-adrenergic non-cholinergic transmission in the rat urinary bladder

1. Endothelin-1 (ET-1; 3-10 nM) raised the tone of rat bladders bathed in buffer containing atropine (1 microM) plus guanethidine (3.4 microM). In addition, ET-1 potentiated, in a concentration-dependent fashion (1-10 nM), the contractions evoked by both transmural nerve stimulation and applications of exogenous adenosine 5'-triphosphate (ATP). 2. The threshold concentration of ET-1 required to facilitate non-adrenergic non-cholinergic (NANC) transmission and potentiate ATP-induced contractions, was about 10 fold lower than that required to increase the bladder tone (3 nM). 3. The ET-1-induced increase in basal tension reached its maximal effect within 60-90 s. In contrast, the 7.8 microM ATP-induced contractions increased by 50% within the first minute following incubation with 10 nM ET-1 but required about 5 min to develop the maximal effect. 4. The ET-1-induced potentiation of NANC or ATP responses was long-lasting and persisted in spite of extensive washing. The recovery of the bladder excitability depended on the concentration of ET-1. Following the application of 3 nM ET-1, recovery required 30 min; applications of 10 nM ET-1 required at least 60 min for full recovery. 5. The ET-1-induced potentiation of responses was selective for ATP and related structural analogues. ET-1 did not modify the contractions induced by acetylcholine, 5-hydroxytryptamine, prostaglandin F2 alpha or bradykinin. 6. The potency of ET-2 was similar to that of ET-1. ET-3 and ET-C-terminal hexapeptide were inactive up to 100 M. Sarafotoxin S6b was 2 to 3 fold less potent than ET-1 whereas sarafotoxin S6c (100 nM) was inactive. AGETB-9 and AGETB-89, two ETB receptor agonists, were also inactive (up to 100 nM). 7. Removal of one or both disulphide bonds in ET-1 and tryptophan-21 formylation of ET-1, resulted in inactive peptides (up to 100 nM). 8. The ET-1 receptor antagonists, BE-18257B and FR 139317, blocked both the ET-1-induced rise in tone and the potentiation of ATP responses in a concentration-dependent fashion. FR 139317 was at least 30 fold more potent than BE-18257B. Both antagonists blocked at lower concentrations the ET-1 increase in bladder tone as compared to the ATP potentiation. The antagonism was slowly reversible. 9. Results are consistent with the presence of ETA receptors in the rat bladder, which mediate both actions of ET-1. The interaction of ET-1 with purinergic mechanisms is discussed.

Endothelin-1 in the human prostate: tissue levels, source of production and isometric tension studies

Endothelins mediate contractile responses in many types of vascular and nonvascular smooth muscle. The present study represents the first detailed characterization of endothelins in the human prostate. The objectives of this study were to determine the tissue levels and source of endogenous endothelin-1 (ET1) in the human prostate. The contractile effects of ET1 were also investigated using in vitro isometric tension studies. The mean tissue level of ET1 was 0.58 +/- 0.08 pg./mg. tissue wet weight. Endothelin-like activity was markedly prominent in the glandular epithelium of the human prostate, whereas minimal endothelin-like activity was observed in the prostatic stroma. Strips of human prostatic tissue were suspended in isolated tissue chambers and challenged to a concentration response of ET1. The mean EC50 and Emax for ET1 was 3.2 x 10(-8) M. and 0.12 +/- 0.02 gm. force per mm.2 cross-sectional area (CSA), respectively. Preincubation with indomethacin, terazosin, or nifedipine did not alter the concentration-dependent response to ET1. A calcium-free buffer abolished the contractile response to ET1. Thus, ET1 mediates a potent contraction of human prostatic smooth muscle that is not mediated via alpha 1 adrenergic or dihydropyridine sensitive calcium channels or prostaglandin synthesis. The presence of marked endothelin-like immunoreactivity strongly suggests a biological significance for endogenous endothelins in the human prostate.

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 0.4-0.6 nM) 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 34 kDa, respectively), whereas ET-3 was cross-linked mainly to a 34 kDa 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.

Endothelin in the urinary bladder. I. Synthesis of endothelin-1 by epithelia, muscle and fibroblasts suggests autocrine and paracrine cellular regulation

The synthesis and localization of endothelin-1 were studied in human and rabbit bladder. In addition, the effects of endothelin-1 on smooth muscle tone and cholinergic neurotransmission were investigated in rabbit bladder. Endothelin-like immunoreactivity was localized in the transitional epithelium, serosal mesothelium, and vascular endothelium; smooth muscle of the bladder (non-vascular) and that of blood vessels; and fibroblasts. With in situ hybridization, transcripts of endothelin messenger ribonucleic acid (mRNA) were localized with the same cellular distribution as endothelin-like immunoreactivity, in bladder tissue. Northern blot analysis of bladder RNA confirmed the expression of preproendothelin-1 mRNA. Rabbit bladder strips in organ chambers contracted when exposed to endothelin-1 and this response was partially attenuated by calcium channel blockers or by removal of extracellular calcium. Transmural electrical stimulation of rabbit bladder strips elicited contractions that were greatly reduced by atropine. The remaining atropine resistant component was blocked by alpha, beta-methylene ATP, which desensitizes purinergic receptors. Endothelin-1 caused a small but consistent attenuation of the atropine sensitive component of the neurogenic contraction, while it had no effect on the atropine resistant component. The localization of endothelin synthesis in epithelia, smooth muscle, and fibroblasts suggests that endothelin may act as an autocrine hormone in the regulation of the bladder wall structure and smooth muscle tone. In addition, endothelin-1 may regulate cholinergic neurotransmission by a paracrine mechanism.

Effect of endothelin (ET)-1, ET-3 and ET-(16-21) on the isolated and perfused rat kidney from normotensive and spontaneously hypertensive rats

We have investigated the effect of endothelin (ET)-1, ET-3 and ET-(16-21) on isolated and perfused rat kidney (IPK). ET-1 and ET-3 produced a similar dose-dependent increase in perfusion pressure of IPK, while ET-(16-21) was completely inactive. The ET-1 effects were greater in spontaneously hypertensive rats (SHR) as compared to normotensive rats (WKYR), whereas the ET-3 effects were greater in the SHR group only at the lowest doses. The pressure response of IPK induced by ET-1 was partially modified by prior application of the nitric oxide synthetase inhibitor L-nitroarginine in both WKYR and SHR.