125I-endothelin-1 and 125I-big endothelin-1 in rat tissues: autoradiographic localization and receptor binding

Binding characteristics of endothelin ET(A) receptors in normal and post-mortem rat lung

In control lung homogenates, optimal specific binding of [(125)I]endothelin-1 and minimal filter binding was achieved using 50 microg/ml bacitracin, 30 microM phenylmethylsulphonyl fluoride (PMSF) and 10 mM EDTA. In post-mortem tissue (8, 16, and 32 h), no significant changes were seen in ET(A) receptor affinity (K(d)) or number (B(max)): control and 32 h K(d) = 309 +/- 75, 225 +/- 32 pM and B(max) = 173 +/- 42, 185 +/- 17 fmol/mg protein, respectively. Autoradiographic binding sites for [(125)I]endothelin-1 were densely expressed on bronchiolar smooth muscle and parenchyma with moderate binding on epithelium and blood vessels. Histologic sections of post-mortem lung showed minimal deterioration of structures expressing ET(A) binding sites. Hence the ET(A) receptor is stable in the rat lung for up to 32 h post-mortem.

Topographical distribution of neurons containing endothelin type A receptor in the rat brain

Endothelin (ET) was originally identified as a vasoactive peptide biosynthesized in vascular endothelial cells. Because ET has also been found in the brain as a neuropeptide, it has been thought to belong to the group of brain-vascular peptide hormones. To date, type A and type B receptors for ET have been found. To elucidate the topographic distribution of type A receptor (ET-AR) in the brain, we raised a specific antibody to the C-terminal (64 amino acids) peptide of rat ET-AR and immunostained rat brain sections with this antibody. Immunoreactivity for ET-AR was detected in neuronal cell bodies and also in the many proximal and some distal parts of their fibers. Nerve cell bodies containing strong ET-AR-immunoreactivity were distributed in the lateral part of the reticular formation, the nucleus of the solitary tract and its surrounding area, the dorsal midline area and medial longitudinal fasciculus, the subependymal layer of the fourth ventricular roof, the caudolateral area of the pontine tegmentum, the locus coeruleus, the rostral pontine area of the lateral reticular formation, the retrorubral area, the substantia nigra, the ventral tegmental area, the periventricular region lateral to the rostral mesencephalic aqueduct and caudal third ventricle, the arcuate hypothalamic nucleus, the caudomedial area of the zona incerta, the periventricular hypothalamic nucleus, the parvocellular portion of the paraventricular hypothalamic nucleus, and the periglomerular region of the olfactory bulb. In addition, the Purkinje cells of the cerebellar cortex, the nerve cells in the mesencephalic trigeminal nucleus, and the magnocellular neurons of the supraoptic and paraventricular hypothalamic nuclei showed weak immunoreactivity. The distribution of highly ET-AR-immunoreactive neurons is quite similar to that ofcatecholamine neurons.

Pulmonary big endothelin affects coronary tone and leads to enhanced, ET(A)-mediated coronary constriction in early endothelial dysfunction

BACKGROUND

Lung tissue produces a variety of mediators; however, little is known regarding how these mediators affect coronary regulation and myocardial contractility. In a novel rabbit lung-heart model, we investigated the possible influence exerted by pulmonary mediators on coronary tone both under normal conditions and in early endothelial dysfunction.

METHODS AND RESULTS

In our model, the effluent from the isolated lung is used to serially perfuse the coronary vessels of the isolated heart of the same animal. Compared with the hearts of control rabbits, isolated hearts of Watanabe rabbits revealed pharmacological evidence of endothelial dysfunction and a significant steeper decrease of coronary flow during serial perfusion of the coronary vessels with lung effluent (75+/-6% versus 89+/-3%). This decline in coronary flow was prevented by the nonselective endothelin (ET) antagonist PD-145065, the ET(A) antagonists BQ-123 and A-127722, and the endothelin-converting enzyme inhibitor phosphoramidon. The concentration of big ET in lung effluent ranged from 5.5 to 5.8 pmol/L in both control and Watanabe groups, with levels in corresponding coronary effluent falling to 0.9 to 1.1 pmol/L in controls and to 1.0 to 1.2 pmol/L in the Watanabe group. In either group, ET was not detected in lung effluent, but it rose significantly in coronary effluent during serial perfusion.

CONCLUSIONS

Pulmonary big ET, locally converted into ET during coronary passage, causes an ET(A)-mediated elevation in coronary tone under basal conditions as well as an enhanced coronary constriction when early endothelial dysfunction is present.

The role of endothelins in the paracrine control of the secretion and growth of the adrenal cortex

Endothelins (ETs) are a family of vasoactive peptides (ET-1, ET-2, and ET-3) mainly secreted by vascular endothelium and widely distributed in the various body systems, where they play major autocrine/paracrine regulatory functions, acting via two subtypes of receptors (ETA and ETB): Adrenal cortex synthesizes and releases ETS and expresses both ETA and ETB. Zona glomerulosa possesses both ETA and ETB, whereas zona fasciculata/reticularis is almost exclusively provided with ETB. ETS exert a strong mineralocorticoid and a less intense glucocorticoid secretagogue action, mainly via ETB receptors. ETS also appear to enhance the growth and steroidogenic capacity of zona glomerulosa and to stimulate its proliferative activity. This trophic action of ETS is likely to be mediated mainly by ETA receptors. The intraadrenal release of ETS undergoes a multiple regulation, with the rise in blood flow rate and the local release of nitric oxide being the main stimulatory factors. Data are also available that indicate that ETS may also have a role in the pathophysiology of primary aldosteronism caused by adrenal adenomas and carcinomas.

Evidence that both ETA and ETB receptor subtypes are involved in the in vivo aldosterone secretagogue effect of endothelin-1 in rats

Endothelins (ET) are a family of vasoconstrictor peptides, secreted by vascular endothelium, which act through two main subtypes of receptors: ETA and ETB. ET-1 is known to stimulate aldosterone (ALDO) secretion by adrenal zona glomerulosa (ZG), and in vitro its effect was recently found to be exclusively mediated by ETB receptors. In this study the involvement of ETA and ETB in the mediation of the in vivo acute ALDO secretagogue action of ET-1 was investigated by the use of their selective antagonists BQ-123 and BQ-788, respectively. The bolus intraperitoneal administration of ET-1 dose-dependently raised both basal and angiotensin II (ANG II)-enhanced plasma ALDO concentration (PAC) in rats. Both antagonists counteracted the stimulatory effect of ET-1 on basal PAC, and when administered together completely annulled it. Conversely, only BQ-788 reversed the effect of ET-1 on ANG II-enhanced PAC. ET-1 increased systolic blood pressure (BP) in normal rats, but not in animals simultaneously administered ANG II. The hypertensive effect of ET-1 was completely abolished by BQ-123, and not affected by BQ-788. In light of these findings the following conclusions can be drawn: (i) the in vivo ALDO secretagogue action of ET-1 is mediated by both ETA and ETB, this latter subtype of ET receptors playing a major role; and (ii) the mechanism whereby ETA participates in this in vivo effect of ET-1 is indirect, and probably connected with the ET-1-induced rise in BP and adrenal blood flow.

Endothelin adrenocortical secretagogue effect is mediated by the B receptor in rats

We investigated the gene expression and localization of endothelin-1 (ET-1) receptor subtypes ET(A) and ET(B) in the rat adrenal cortex as well as their involvement in the corticosteroid secretagogue effect of ET-1 in vitro. Reverse transcription-polymerase chain reaction with primers specific for ET(A) and ET(B) cDNAs demonstrated the expression of both receptor genes in homogenates of adrenocortical tissue. However, in isolated zona glomerulosa and zona fasciculata cells, only ET(B) mRNA was detected. Autoradiographic examination of the selective displacement of 125I-ET-1 binding by BQ-123 and BQ-788 (specific ligands for ET(A) and ET(B), respectively) indicated that zona glomerulosa possesses both ET(A) and ET(B), whereas zona fasciculata is exclusively provided with ET(B). ET-1 enhanced in a concentration-dependent manner aldosterone and corticosterone secretions of dispersed zona glomerulosa and zona fasciculata cells, respectively. The ET(B) antagonist BQ-788 markedly reduced the secretory response of zona glomerulosa cells and completely suppressed that of zona fasciculata cells, whereas the ET(A) antagonist BQ-123 was ineffective. These findings indicate that in the rat, the adrenocortical secretagogue action of ET-1 is mediated by the ET(B) receptor subtype and that the ET(A) receptor is not directly involved in such an effect.

Perichondrial localization of ETA receptor in rat tracheal and xiphoid cartilage and in fetal rat epiphysis

Autoradiographic studies using 125I-labeled endothelin-1 (ET-1) on sections of rat cartilage tissues, including the trachea, xiphisternum, and fetal rat epiphysis, revealed dense localization of endothelin receptors in the perichondrium. In contrast, the binding of ET-1 was not detected in the chondrocytes, cartilage matrix, and other connective tissues of the cartilage tissues tested. The perichondrial binding of 125I-ET-1 was completely abolished with BQ-123 [an endothelin receptor subtype A (ETA) antagonist] but not with BQ-3020 (an ETB agonist), and we demonstrated the perichondrial localization of ETA receptors. [3H]thymidine incorporation in vitro was significantly increased in rat xiphoid cartilage tissues exposed to ET-1. These findings suggest that the ET-1/ETA receptor system plays an important role in regulating cartilage metabolism and endochondral bone formation.

Characterization of endothelinA receptors in cerebral and peripheral arteries of the rat

We have characterized and quantified endothelin receptors in rat brain (anterior cerebral) and peripheral (aorta, carotid, and caudal) arteries, with the use of [125I]endothelin and quantitative autoradiography. Endothelin binding was saturable, of high affinity, and totally displaced by the selective endothelin ETA antagonist BQ 123. A single class of ETA receptors is located in the medial layer of peripheral and cerebral arteries, and its quantification by autoradiography allows study of their regulation and function.

Endothelin derivatives showing potent effects in the guinea pig trachea

Endothelin (ET-1) was synthesized by solid-phase peptide synthesis. To prevent side reactions with tryptophan residue, a formyl group was used to protect the beta-indole moiety. The two disulfide bridges were formed selectively using S-methylbenzyl for the cysteine residues in positions 3 and 11, and S-acetamidomethyl for positions 1 and 15. Crude synthetic ET-1 was obtained after deprotection and cleavage from the resin by HF, followed respectively by potassium ferricyanide and iodine oxidations, in order to generate sequentially the monocyclic and bicyclic forms of the peptide. Finally, Trp(For) was deprotected by a treatment with piperidine. After each step, a sample of peptide was isolated and purified by RP-HPLC. The biological activity of these synthetic peptides was evaluated on guinea pig tracheal strips. The results suggest that oxidation of Met-7 and/or formylation of Trp-21 do not affect the biological activity of ET-1 in the guinea pig trachea. Moreover, although a decrease of potency and intrinsic activity was observed, the monocyclic analog containing Cys(Acm) residues at positions 1 and 15 exhibited substantial constrictor activity. This result, contrasting with that observed with several vascular preparations, suggests that the guinea pig trachea contains a distinct ET receptor population.

Endothelin peptides: biological actions and pathophysiological significance in the lung

Endothelins (ETs) are a family of novel regulatory peptides. Besides their effects on the cardiovascular system, which have been extensively described, several lines of evidence suggest an important role for ETs in regulating pulmonary functions. ETs are present in the pulmonary tissues, bronchoalveolar space and pulmonary circulation. Release of ETs from macrophages, endothelial and epithelial cells is modulated by a variety of chemical and physical stimuli and is regulated at the level of transcription or translation. Specific endothelin receptors have been identified in the airways as well as in the pulmonary vasculature. ETs are among the most potent bronchoconstrictors yet described. In the pulmonary circulation, ETs can elicit both vasodilation and vasoconstriction and can enhance vascular permeability. ETs could also modulate activation of inflammatory cells. Enhanced ET expression and/or production have been detected in asthma, certain pulmonary tumors, shock states associated with adult respiratory distress syndrome and pulmonary hypertension. These findings suggest that by regulating pulmonary vascular and airway tone, activation of inflammatory cells and cellular growth and/or differentiation, ETs may play an important role in pulmonary pathophysiology.

Structure-function relationships of endothelins, sarafotoxins, and their receptor subtypes

The endothelins (ETs) and sarafotoxins (SRTXs) are two structurally related families of potent vasoactive peptides. Although their physiological functions have yet to be precisely elucidated, it seems likely that the ETs are involved in pathophysiological conditions such as hypertension and heart failure. In this minireview, recent advances in the biochemical characterization of the ET/SRTX system, with special reference to structure-function relationships and ET/SRTX receptor subtypes, are described, as well as the recent cloning and expression of ET receptors.

Endothelins and sarafotoxins: physiological regulation, receptor subtypes and transmembrane signaling

The endothelins and sarafotoxins are two structurally related families of potent vasoactive peptides. Although the physiological functions of these peptides are not entirely clear, the endothelins are probably involved in pathophysiological conditions such as hypertension and heart failure. This review summarizes the state of the art in some areas of this intensively studied subject, including: (1) structure-function relationships of ET/SRTX, (2) ET concentrations in plasma, (3) ET/SRTX receptor subtypes and (4) signaling events mediated by the activation of ET/SRTX receptors.