Endothelins in the urinary tract

Endothelin-1 overexpression: a potential biomarker of unfavorable prognosis in non-metastatic muscle-invasive bladder cancer

Endothelin-1 (ET-1) is a multifunctional peptide exerting its effects via receptors A and B. ET-1 and its receptors, endothelin axis (ET axis), play a promoting role in cancer biology. Alterations of proteins of ET axis have been detected in non-metastatic muscle-invasive bladder cancer (NMMIBC). The objective of this study is to investigate the potential role of ET-1 tumor expression as a biomarker of prognosis, compared to other prognostic parameters (epidemiologic and pathologic), in NMMIBC. We prospectively included 40 consecutive, primary, high-grade NMMIBC patients. Tumor specimens after initial transurethral resection were stained immunohistochemically for ET-1. The ET-1 evaluation of expression was based on staining intensity (SI) of ET-1. SI was classified according to an arbitrary four-tiered scale (negative = 0, mild = 1, moderate = 2, strong = 3). Epidemiologic and pathologic parameters were analyzed, using univariate and multivariate statistics, for disease progression, progression-free survival (PFS), and overall survival (OS). ET-1 overexpression (SI = 3) was the unique parameter which associated significantly, both in univariate (log-rank test, p = 0.033) and multivariate (Cox regression analysis, p = 0.045, HR = 4.849, 95 % CI: 1.039-22.624) analysis, with an increased hazard ratio of progression. ET-1 overexpression (SI = 3) was also the unique parameter which associated, marginally significantly in univariate analysis (log-rank test, p = 0.056) and highly significantly in multivariate analysis (Cox regression analysis, p = 0.005, HR = 7.001, 95 % CI: 1.782-27.501), with an increased hazard ratio of death. Overexpression of ET-1 may be a potential biomarker of unfavorable prognosis in NMMIBC patients.

The endothelin axis in urologic tumors: mechanisms of tumor biology and therapeutic implications

Endothelin (ET)-1 and its receptors ET-A and ET-B, referred to commonly as the endothelin axis, have been identified in various human cancers, especially gynecologic tumors, such as breast cancer or ovarian cancer, but also including urologic tumor entities. They play a key role in tumor growth and progression by influencing critical cancer pathways, such as apoptosis, angiogenesis and proliferation. In prostate cancer, overexpression of the ET-A receptor increases with tumor progression, and clinical trials with selective ET-A receptor antagonists, such as atrasentan (ABT-627), have shown promising early results. In preclinical models of bladder cancer, overexpression of the ET axis has been demonstrated and ET-targeting agents are under investigation. This paper reviews the role of the ET axis in human cancers and focuses on preclinical and clinical studies in urologic tumor entities to further define the role of ET-targeting agents as targeted molecular therapy.

Efficacy of the specific endothelin a receptor antagonist zibotentan (ZD4054) in colorectal cancer: a preclinical study

Endothelin 1 (ET-1) is overexpressed in cancer, contributing to disease progression. We previously showed that ET-1 stimulated proliferative, migratory, and contractile tumorigenic effects via the ET(A) receptor. Here, for the first time, we evaluate zibotentan, a specific ET(A) receptor antagonist, in the setting of colorectal cancer, in cellular models. Pharmacologic characteristics were further determined in patient tissues. Colorectal cancer lines (n = 4) and fibroblast strains (n = 6), isolated from uninvolved areas of colorectal cancer specimens, were exposed to ET-1 and/or ET(A)/(B) receptor antagonists. Proliferation (methylene blue), migration (scratch wounds), and contraction (gel lattices) were assessed. Receptor distribution and binding characteristics (K(d), B(max)) were determined using autoradiography on tissue sections and homogenates and cytospun cells, supported by immunohistochemistry. Proliferation was inhibited by ET(A) (zibotentan > BQ123; P < 0.05), migration by ET(B) > ET(A), and contraction by combined ET(A) and ET(B) antagonism. Intense ET-1 stromal binding correlated with fibroblasts and endothelial cells. Colorectal cancer lines and fibroblasts revealed high density and affinity ET-1 binding (B(max) = 2.435 fmol/1 × 10(6) cells, K(d) = 367.7 pmol/L; B(max) = 3.03 fmol/1 × 10(6) cells, K(d) = 213.6 pmol/L). In cancer tissues, ET(A) receptor antagonists (zibotentan; BQ123) reduced ET-1 binding more effectively (IC(50): 0.1-10 μmol/L) than ET(B) receptor antagonist BQ788 (∼IC(50), 1 mmol/L). ET-1 stimulated cancer-contributory processes. Its localization to tumor stroma, with greatest binding/affinity to fibroblasts, implicates these cells in tumor progression. ET(A) receptor upregulation in cancer tissues and its role in tumorigenic processes show the receptor's importance in therapeutic targeting. Zibotentan, the most specific ET(A) receptor antagonist available, showed the greatest inhibition of ET-1 binding. With its known safety profile, we provide evidence for zibotentan's potential role as adjuvant therapy in colorectal cancer.

Targeting the endothelin system: novel therapeutic options in gynecological, urological and breast cancers

The endothelin system comprises the three peptide hormones endothelin (ET)-1, -2, -3, their G protein-coupled receptors, endothelin-A-receptor (ET(A)R) and endothelin-B-receptor (ET(B)R), and the enzymes of endothelin biosynthesis and degradation. In the past two decades, an impressive amount of data has been accumulated investigating the role of the endothelin system in a variety of malignancies. In many cancers, ET-1/ET(A)R interaction induces proliferation, angiogenesis, antiapoptosis and resistance to chemotherapy. Data indicate a pivotal role of the endothelin system in tumorigenesis, local progression and metastasis. Subsequently, novel drugs have been designed inhibiting ET-1 biosynthesis or ET(A)R interaction. A wide range of preclinical data is available on the role of ET(A)R antagonists in gynecological, urological and breast cancers providing evidence for their antiangiogenic, proapoptotic and growth inhibitory effects. Of particular interest is the anti-invasive and antimetastatic efficacy of ET(A)R antagonists and synergism when co-administered with established cancer therapies. Data indicate a future role of ET(A)R antagonists in oncologic therapies.

Altered endothelin receptor subtypes in colorectal cancer

BACKGROUND

The vasoactive peptide endothelin-1 (ET-1) acts via two endothelin receptor subtypes, ETA (ETAR) and ETB (ETBR). ET-1 and ETAR are overexpressed in colorectal cancer tissues. In vitro, ET-1 acting via ETAR, is a mitogen for colorectal cancer cells. To identify other potential stimulatory loops, we investigated the distribution and cell-specific localization of both ETAR and ETBR in tissue sections from patients with colorectal cancer.

METHODS

Frozen sections from specimens of colorectal cancer (n=9) and normal colon (n=9) were cut and subjected to either (i) autoradiography or (ii) a combination of cell type-specific immunohistochemistry, using antibodies against fibroblasts (AS02), endothelial cells (CD31) or nerve fibres (NF200) and in-vitro receptor microautoradiography, using ETAR-specific and ETBR-specific radioligands.

RESULTS

ETARs were upregulated in all cell types, apart from nerve, in cancer compared with normal colon (1:1.59 normal to cancer). Specifically, ETAR binding was highest in cancer-associated blood vessels and fibroblasts and to a lesser extent in epithelial cancer cells. In contrast, ETBRs were the predominant receptors in normal colon (1:0.59 normal to cancer) and were markedly down-regulated in cancer-associated blood vessels, fibroblasts and to a lesser extent in epithelial cells. Nerve colocalization was demonstrated, but remained unchanged for all tissues.

CONCLUSION

The shift in ET receptor binding observed in epithelial cancer cells and cancer-associated fibroblasts and endothelial cells may favour ET-1 signals contributing to colorectal cancer growth and neovascularization via ETAR. This may provide the basis for therapeutic use of specific ETAR antagonists as adjuvant treatment of colorectal cancer.

Endothelin-A-receptor antagonist LU 302146 inhibits electrostimulation-induced bladder contractions in vivo

OBJECTIVES

Endothelin (ET) is a strong constrictor of smooth muscle structures. The relevance of Endothelin-A receptors in the bladder was demonstrated in several in vitro studies. The aim of this functional study was to evaluate the acute effect of the selective ET-A-antagonist LU 302146 (LU) on neurostimulation-induced bladder contractions in vivo.

METHODS

Eight male mini pigs were anesthesized. The bladder was exposed and a double lumen catheter was inserted to perform intravesical pressure (pves) measurements. Laminectomy was performed for sacral anterior root stimulation (SARS) of S2. Four animals received the selective ET-A-antagonist LU, three atropine and one animal was treated with vehicle. Pves was recorded before and after drug administration as well as before and during neurostimulation. At the end of each LU trial, a supplementary application of 4 mg atropine was administered followed by a final SARS.

RESULTS

In all experiments reproducible pves values were elicited during electrostimulation before administration of the test substance. The selective ET-A-antagonist reduced stimulation-induced bladder contraction by a mean of 57%. Additional administration of atropine inhibited the detrusor contraction almost completely during SARS. The vehicle had no effect on bladder contraction.

CONCLUSIONS

In the presented animal model, ET-1 inhibition with the selective ET receptor-A-antagonist LU 302146 decreases stimulation-induced bladder contraction in vivo. The results suggest that the selective ET-A antagonist LU acts on the atropine-resistant component of efferent detrusor activation since additional administration of atropine almost completely abolish detrusor contraction. This observation in addition to the involvement of ET-1 in bladder smooth muscle proliferation, raises the possibility that ET-receptor antagonists might be beneficial in patients with neurogenic bladder dysfunction or in patients with functional or anatomical BOO.

Endothelin-1 promotes cell survival in renal cell carcinoma through the ET(A) receptor

Endothelin-1 (ET-1) is a potent vasoconstrictor that has been shown to significantly impact many benign and malignant tissues by signaling through its two cognate receptors: ET(A) and ET(B). As ET-1 has a role in both normal and diseased kidney, we initiated studies to investigate endothelin axis expression and function in renal cell carcinoma (RCC). In this study, relatively high levels of ET-1 were detected in all six human RCC cell lines investigated. RT-PCR and Southern analyses revealed that all six RCC cell lines expressed ET(A) receptor mRNA, while 3/6 cell lines also expressed ET(B) mRNA. High affinity ET-1 binding occurred in all but one RCC cell line and quantitative RT-PCR demonstrated ET(A) mRNA expression in all six cell lines. Methylation of the ET(B) promoter (EDNRB) in 4/6 RCC cell lines was observed, suggesting a mechanism for repressed ET(B) expression. Moreover, methylation occurred in 32/48 of renal tumors and in 27/55 of histologically normal adjacent tissue samples studied, while no methylation was evident in any normal tissue isolated from nephrectomy or at autopsy. Functionally, ET-1 significantly inhibited paclitaxel-induced apoptosis in RCC cells through binding ET(A) with the ET-1 signaling mediated via the PI3-kinase/Akt pathway. Collectively, these data support the therapeutic targeting of the ET(A) receptor as a novel treatment strategy for RCC.

Expression of endothelin-1 is related to poor prognosis in non-small cell lung carcinoma

The endothelin (ET) system influences tumourigenesis and tumour progression by various mechanisms, including angiogenesis. The aim of this study was to determine whether the expression of endothelin-1 (ET-1) is related to the angiogenic phenomenon in lung cancer and whether it could be involved in its clinical behaviour. Expression of ET-1, endothelin-converting enzyme-1 (ECE-1) and endothelin-receptors ETA and ETB was examined in 201 non-small cell lung carcinoma (NSCLC) and corresponding normal tissues using real-time polymerase chain reaction (RT-PCR). Forty NSCLC were also analysed for vascular endothelial growth factor (VEGF) expression by a competitive-PCR approach to assess whether ET-1 expression was related to this angiogenic factor. A higher number of cases with ET-1, ECE-1 and ETA mRNA expression was observed in malignant lung tumours compared with normal lung tissues (45.7% versus 33% for ET-1 (P < 0.0001); 38.3% versus 16.5% for ECE-1 (P = 0.004); and 42.8% versus 28.5% for ETA (P < 0.0001)). On the other hand, ETB mRNA was higher in normal lung tissues than in tumour samples (58.5% versus 52.8% (P < 0.0001)). Immunohistochemical analysis was also performed in 78 cases, selected from among those with high ET-1 mRNA, to confirm the presence of ET-1 protein and to determine its distribution and localisation. Moreover, an interesting relationship was observed between ET-1 and VEGF mRNA levels (P = 0.02). At univariate analysis, clinical-pathological parameters, such as sex, nodal metastatic involvement and stage, and ET-1 expression were seen to be significant predictors of worse prognosis regarding both overall survival (P = 0.001, P = 0.0003, P = 0.001 and P = 0.03, respectively) and disease-free interval (P = 0.0005, P = 0.0007, P = 0.001 and P = 0.04, respectively). We conclude that ET-1 could be involved in angiogenic phenomena in NSCLC and may represent a further indicator of progression and poor prognosis in this type of cancer, with interesting therapeutic implications.

Pharmacology of the lower urinary tract: basis for current and future treatments of urinary incontinence

The lower urinary tract constitutes a functional unit controlled by a complex interplay between the central and peripheral nervous systems and local regulatory factors. In the adult, micturition is controlled by a spinobulbospinal reflex, which is under suprapontine control. Several central nervous system transmitters can modulate voiding, as well as, potentially, drugs affecting voiding; for example, noradrenaline, GABA, or dopamine receptors and mechanisms may be therapeutically useful. Peripherally, lower urinary tract function is dependent on the concerted action of the smooth and striated muscles of the urinary bladder, urethra, and periurethral region. Various neurotransmitters, including acetylcholine, noradrenaline, adenosine triphosphate, nitric oxide, and neuropeptides, have been implicated in this neural regulation. Muscarinic receptors mediate normal bladder contraction as well as at least the main part of contraction in the overactive bladder. Disorders of micturition can roughly be classified as disturbances of storage or disturbances of emptying. Failure to store urine may lead to various forms of incontinence, the main forms of which are urge and stress incontinence. The etiology and pathophysiology of these disorders remain incompletely known, which is reflected in the fact that current drug treatment includes a relatively small number of more or less well-documented alternatives. Antimuscarinics are the main-stay of pharmacological treatment of the overactive bladder syndrome, which is characterized by urgency, frequency, and urge incontinence. Accepted drug treatments of stress incontinence are currently scarce, but new alternatives are emerging. New targets for control of micturition are being defined, but further research is needed to advance the pharmacological treatment of micturition disorders.

Age-related changes in the properties of the endothelin receptor system at protein and mRNA levels in the rat vas deferens

As age-related changes occur in the properties of the endothelin (ET) receptor system in several mammalian tissues, and as there are significant amounts of functional ET receptors in the vas deferens, we investigated the age-related changes in the ET receptor system at the protein and mRNA levels in the rat vas deferens. The ET system was investigated in the vasa deferentia of 3 weeks, 3 months and 22 months old rats. ET receptors were characterized and quantified at the protein level by radioligand receptor binding, and gene transcript levels of ET-1, ET-3, ET converting enzyme-1 (ECE-1), and ET(A) and ET(B) receptor subtypes were quantified by real-time reverse transcription polymerase chain reaction (RT-PCR). The results of radioligand receptor binding assays demonstrate that there is a higher density of total ET receptors in the vas deferens of 3 weeks old rats than in 3 months and 22 months old rats, and that the predominant ET receptor is of the ET(A) subtype in all three ages. Real-time RT-PCR data show that the predominant mRNA expression of ETs and their receptors in all age groups studied are ET-1 and the ET(A) receptor subtype, respectively. Furthermore, ET-1, ET-3, ECE-1, and ET(A) and ET(B) receptor subtype mRNAs are expressed at higher levels in the 3 weeks old rats as compared with the other two age groups. These results demonstrate the presence of age-related changes in the properties of the ET receptor system at both protein and mRNA levels in the rat vas deferens.

Endothelin axis expression is markedly different in the two main subtypes of renal cell carcinoma

BACKGROUND

The endothelin axis has been implicated in cancer growth, angiogenesis, and metastasis, but to the authors' knowledge the expression of endothelin genes has not been defined in renal cell carcinoma (RCC).

METHODS

Tissue specimens were harvested from both normal and tumor-affected regions at the time of radical nephrectomy from 35 patients with RCC (22 with clear cell RCC [ccRCC] and 13 with papillary RCC [PRCC]). Real-time reverse transcriptase-polymerase chain reaction analysis determined the expression profile of the preproendothelins (PPET-1, PPET-2, and PPET-3), the endothelin receptors (ET(A) and ET(B)), and the endothelin-converting enzymes (ECE-1 and ECE-2).

RESULTS

PPET-1 was found to be up-regulated in ccRCC tumor specimens and down-regulated in PRCC tumor specimens. ET(A) was significantly down-regulated in PRCC tumor specimens. ECE-1 was expressed in all tissue specimens at comparable levels, with moderate but significant elevation in normal tissue specimens associated with PRCC. Of the other genes, PPET-2 and ET(B) were expressed in all tissue specimens and no differences were observed between tumor subtypes or tumor-affected and normal tissue specimens, whereas PPET-3 and ECE-2 were present in all tissue specimens but were barely detectable.

CONCLUSIONS

The endothelin axis was expressed differently in the two main subtypes of RCC and appeared to match macroscopic features commonly observed in these tumors (i.e., high expression of PPET-1 in hypervascular ccRCC contrasted against low PPET-1 and ET(A) expression in hypovascular PRCC). The presence of ECE-1 mRNA in these tissue specimens suggested that active endothelin ligands were present, indicating endothelin axis activity was elevated in ccRCC compared with normal kidney, but impaired in PRCC. The current study provided further evidence that it is not appropriate to consider ccRCC and PRCC indiscriminately in regard to treatment.

Quantification of endothelins, their receptors, and endothelin-converting enzyme mRNAs in rat genitourinary tract using real-time RT-PCR

INTRODUCTION

Quantification of mRNA expression is essential for the assessment of endothelin (ET) receptor-mediated mechanisms. Recently, a novel technique for the determination of mRNA expression, termed real-time reverse transcription polymerase chain reaction (RT-PCR), has been developed. We therefore applied real-time PCR using SYBR Green I to quantify ET-1, ET-3, ET-converting enzyme-1 (ECE-1), and ETA and ETB receptor subtype mRNA expression in the rat genitourinary tract.

METHODS

The cDNA was synthesized by RT of RNA extracted from the rat bladder, ventral prostate, dorsolateral prostate, and vas deferens. All steps subsequent to the RT reaction were carried out by the thermal cycler/detector and computer-assisted programs processed a quantitative result.

RESULTS

Designing optimal primer sequences that minimized primer-dimer formation and adjusting annealing temperatures that prevented nonspecific product amplification have made it possible to identify a single peak in the melt curve and to obtain an appropriate standard curve for each gene transcript. In our experiments, input cDNA levels as low as 100 copies of the product could be detected.

DISCUSSION

We demonstrated that significant quantitative variations existed in the expression levels of ET-1, ET-3, ECE-1, and ETA and ETB receptor subtype mRNAs within a tissue and between different regions of the genitourinary tract and that the predominant expression of ETs and their receptor mRNAs in all tissues studied were ET-1 and the ETA receptor subtype, respectively.

Role of endothelin ETA and ETB receptors in the guinea-pig urinary bladder contraction

The distribution and function of endothelin receptors in the guinea-pig urinary bladder were examined. Specific [125I]endothelin-1 binding sites with both the endothelin ET(A) and ET(B) receptor subtypes were distributed in the muscle layer. Endothelin-1 elicited a tonic contraction which was inhibited by cyclo(D-Asp-Pro-D-Val-Leu-D-Trp) (BQ123) but not by N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D-1-methoxycarbonyltryptophanyl-D-norleucine (BQ788) and which was inhibited more strongly by a combination of BQ123 and BQ788. Sarafotoxin S6c elicited a contraction which was abolished by BQ788. The concentration of endothelin-1 in the muscle layer was 707.0+/-67.5 pg/g wet weight. Thus, endothelin-1 may regulate muscle tone via both subtypes of endothelin receptors in an autocrine manner in the guinea-pig urinary bladder.

Developmental changes in the functional, biochemical and molecular properties of rat bladder endothelin receptors

The effect of aging on functional, biochemical, anatomical and molecular properties of endothelin (ET) receptors in bladder smooth muscle of the 3-week-, 3-month- and 22-month-old rats was examined using isolated muscle bath techniques, radioligand binding on membrane particulates and slide mounted tissue sections, and real-time reverse transcription polymerase chain reaction (RT-PCR). ET-1 induced significantly larger contractile responses in bladder dome muscle strips from 3-week- than from 3-month- and 22-month-old rats. The expression level of total ET receptors, determined by saturation binding experiments with [(125)I]ET-1, was higher in detrusor from 3-week- than 22-month-old rats. Inhibition studies with BQ123, a selective ET(A) receptor antagonist, indicated the predominance of the ET(A) receptor subtype and a similar proportion of ET(A) to ET(B) receptor subtypes in the rat detrusor at all ages studied. Autoradiographic data support the age-dependent decrease in the density of ET receptors and also indicate that the ET(A) receptor subtype is primarily located in the smooth muscle layer, whereas the ET(B) receptor subtype is located in both the urothelial and smooth muscle layers. Determined by real-time RT-PCR, ET 1, ET-3, ECE-1 and ET receptor subtype (ET(A) and ET(B)) mRNAs were shown to be higher in bladders of 3-week- compared to 3-month- or 22-month-old rats. This study indicates age-dependent alterations in the ET receptor system at both gene transcript and protein levels in the Fischer rat detrusor.

Endothelin-1 as a target for therapeutic intervention in prostate cancer

The endothelins, a family of potent vasoconstricting peptides, have been implicated in the pathophysiology of advanced prostate cancer. Two endothelin receptors, ET-A and ET-B are found in normal prostate tissue. Malignant prostate cells are notable for the loss of ET-B receptors and increased levels of endothelin-1 [ET-1]; this distortion of the endothelin system may be a significant factor in the progression of prostate cancer. Proposed roles for endothelin in prostate cancer include growth promotion, apoptosis inhibition, bone formation, and stimulation of nociceptive receptors. ET-1 can act alone as a mitogen, but its effects are greatest as a comitogen with a variety of growth factors, including basic fibroblast growth factor, insulin-like growth factors, and platelet derived growth factor. Although their exact functions are unclear, ET-1, in conjunction with vascular endothelial growth factor, appears to play a major role in tumor angiogenesis. By a variety of methods, ET-1 alters the balance of osteoblast and osteoclasts to the favor new bone formation that is characteristic of metastatic disease. Several studies indicate that the refractory pain of metastatic cancer is related to the direct nociceptive effects ET-1. These findings suggest that ET receptors are promising therapeutic targets for pharmacologic intervention. Early clinical trials indicate that the ET-A receptor antagonist used in prostate cancer is reasonably well tolerated with mild but pervasive symptoms related to ET-1's vasoconstrictive effects. Results of ongoing clinical trials are eagerly awaited in order to see if the hypothetical promise of ET antagonism will result in clinical success.

Developmental changes in the biochemical and functional properties of endothelin receptors in rabbit renal pelvis

OBJECTIVE

To examine the effect of age on the biochemical and functional properties, and regional distribution of endothelin (ET) receptors in the rabbit renal pelvis.

MATERIALS AND METHODS

The properties of ET receptors in 6-week-old and 6-month-old male rabbit renal pelves were examined using isolated muscle-bath and radioligand receptor-binding techniques.

RESULTS

ET-1 caused a significant increase in the contractile force in muscle strips from all regions of the renal pelvis from both age groups, with the following rank order: upper=middle>lower. The magnitude of the ET-1-induced contractile responses were similar in the lower pelvic regions in both ages, but the responses in the upper and middle regions were significantly greater in younger rabbits. ET-1 increased the frequency of spontaneous activity in a concentration-dependent manner in the upper and middle pelvic regions in both age groups, with significantly smaller ED50 values in the younger than in the older rabbits. In both age groups the lower pelvic region lacked spontaneous activity. The density of total ET receptors was higher in the upper and middle regions of the renal pelvis than in the lower renal pelvis of both ages, with the density in the upper and middle regions being greater in older than in younger rabbits. In all regions, ET subtype selective compounds inhibited [125I]ET-1, binding consistent with the predominance of the ETA receptor subtype, except in the lower region of the older rabbits, in which the densities of ETA and ETB subtypes were similar. In all regions, the younger renal pelvis contained a higher proportion of ETA receptors than in older tissues. Light microscopic autoradiographic data indicated the presence of ETA and ETB receptors in smooth muscle and epithelial cells, respectively.

CONCLUSION

These data indicate the presence of regional differences in the density of ET receptors and in the contractile responses to ET-1 in rabbit renal pelvis, and that although older rabbit renal pelvis contains more total ET receptors than younger renal pelvis, the latter had a higher portion of the ETA receptor subtype and the younger tissues were more responsive to ET-1.