Agonist and antagonist characterization of a putative adrenoceptor with distinct pharmacological properties from the alpha- and beta-subtypes

Mechanical responses to catecholamines in the longitudinal muscle of guinea-pig gastric fundus

1. In the longitudinal muscle of guinea-pig gastric fundus, adrenaline and phenylephrine (1-30 microM) both produced a slow contraction preceded by a relaxation. The slow contraction was strongly inhibited by prazosin (0.1 microM), but only weakly by yohimbine (1 microM), suggesting main contribution of alpha 1-adrenoceptors. 2. Most of the slow contraction was blocked by meclofenamate or indomethacin (0.1-0.3 microM). Both these drugs also inhibited spontaneously generated muscle tone. In some preparations, obtained from the apical fundus, a small contraction remained in the presence of meclofenamate. 3. During contraction induced by prostaglandin E2, adrenaline produced sustained relaxation and phenylephrine often transient relaxation, in the presence of meclofenamate. The transient relaxation, but not the sustained relaxation, was suppressed by prazosin. 4. In the presence of prostaglandin E2 (5 nM), after treating with phenoxybenzamine (30 microM) for 30 min, isoprenaline and adrenaline produced concentration-dependent relaxation, with IC50 s of 3.9 nM and 64 nM, respectively. Propranolol shifted these concentration-response curves to the right, with apparent pA2 s of 8.15 and 7.34, respectively. 5. It is suggested that in the fundic longitudinal muscle, adrenaline-induced contraction is mediated mainly by an increase in endogenous prostaglandin production through activation of alpha 1-adrenoceptors and that adrenaline produces transient relaxation through alpha 1-adrenoceptors and sustained relaxation through beta-adrenoceptors. The beta-adrenoceptors in the longitudinal muscle are more sensitive to adrenaline and isoprenaline than those in the circular muscle.

Characterization of catecholamine-mediated relaxations in rat isolated gastric fundus: evidence for an atypical beta-adrenoceptor

1. Experiments were carried out in order to characterize the receptors mediating relaxant responses to catecholamines in the rat gastric fundus. The effects of noradrenaline, isoprenaline and the 'atypical' or beta 3-adrenoceptor agonist, BRL 37344, on methacholine-induced tone were measured. Prazosin, propranolol and cyanopindolol were used as antagonists. 2. Relaxant responses to noradrenaline, in the presence of propranolol (1 microM) were antagonized in a concentration-dependent manner by prazosin (0.01 to 1 microM), although this antagonism was weak and non-competitive in nature. Relaxant responses to isoprenaline, in the presence of prazosin (0.1 microM), were antagonized only by the highest concentration of propranolol (1 microM) giving a pKB of 6.3 BRL 37344 also relaxed the rat gastric fundus in the presence of prazosin (0.1 microM), and the responses to BRL 37344 were unaffected by propranolol (1 microM). 3. Tachyphylaxis to BRL 37344 was observed, a second concentration-response curve being significantly shifted to the right. Exposure of tissues to BRL 37344 (1 microM) between concentration-response curves also caused an 11 fold rightward shift in the response to isoprenaline. 4. In the presence of prazosin (0.1 microM) and propranolol (1 microM), the rank order of potency of the agonists was: (-)-isoprenaline (1.0) greater than (-)-noradrenaline (0.39) greater than BRL 37344 (0.10). 5. Responses to BRL 37344 in the presence of prazosin (0.1 microM) and propranolol (1 microM) were antagonized by (+/-)-cyanopindolol (1 microM), with a pKB of 6.56. Responses to isoprenaline, under the same conditions, were antagonized in a competitive manner by (+/-)-cyanopindolol (0.1-1 microM), with the slope of a Schild plot close to unity and a pA2 value of 7.44. 6. The resistance to blockade by prazosin and propranolol and the antagonism by cyanopindolol of the responses mediated by isoprenaline and BRL 37344 suggest that atypical beta-adrenoceptors similar to 'atypical',beta-adrenoceptors in rat adipocytes and other tissues are present in the rat gastric fundus.

Adrenergic and cholinergic regulation of cortisol secretion from the zona fasciculata/reticularis of bovine adrenal cortex

Inner zone cells, isolated from bovine adrenal cortex, secrete cortisol in response to both adrenergic and cholinergic agonists. The response to adrenaline (and other catecholamines) appears during culture, is evident by 24 h and reaches a maximum by 48-72 h, but is absent in freshly isolated cells. Pre-incubation of cultured cells with adrenaline leads to homologous desensitisation; the possibility that this may explain the absent response in freshly isolated cells is discussed. Cells show a dose-dependent cyclic AMP response but no increased membrane phosphoinositide turnover. In agreement, cortisol secretion is blocked by beta-receptor, but not alpha-receptor, antagonists. Schild analysis established that the response occurs through binding to a beta 1-receptor subtype, consistent with adrenergic innervation as opposed to an effect of circulating catecholamines. In contrast, cortisol secretion to AcCh was present in both freshly isolated cells and those in culture, reaching a maximum by 48-72 h in culture. The response was specifically blocked by muscarinic, but not nicotinic, antagonists. No effect on cyclic AMP formation was observed, but dose-dependent stimulation of phosphoinositide turnover occurred. HPLC analysis of the time-course of appearance of 3H-inositol labelled head groups (from cells pre-labelled with 3H-inositol) confirmed that AcCh activates a phosphoinositidase C. Intracellular Ca2+ oscillations were also measured from fura-2 loaded single cells in response to AcCh. Together with other pharmacological studies, these observations establish that AcCh acts through a M3 muscarinic receptor subtype in these cells. The possible significance of these findings in vivo is discussed.

Receptors involved in mechanical responses to catecholamines in the circular muscle of guinea-pig stomach treated with meclofenamate

1. In circular muscle strips of the fundus and corpus of guinea-pig stomach, mechanical responses to catecholamines were studied mainly in the presence of a prostaglandin biosynthesis inhibitor, meclofenamate. 2. Normal preparations developed considerable muscle tone, and adrenaline (10-100 microM) in the presence of 3-5 microM propranolol produced a multiphasic response, generally consisting of transient relaxation and contraction, followed by slow relaxation and then contraction. Responses to phenylephrine were similar to those of adrenaline. 3. Meclofenamate (0.3 microM) nearly abolished the muscle tone and under this condition, both adrenaline and phenylephrine produced a simple contraction. This response was strongly inhibited by prazosin, but only weakly by yohimbine. 4. When muscle tone was maintained by prostaglandin E2 (10 nM) in the presence of meclofenamate, phenylephrine (30 microM) produced transient relaxation followed by slow contraction in most preparations. These were strongly inhibited by prazosin. Adrenaline produced a similar response, but the relaxation was only partially reduced by prazosin. The remaining relaxation was more dominant in the middle fundic region and this was considered to be mediated through beta-adrenoceptors. 5. It is concluded that in the circular muscle of the fundic region of guinea-pig stomach, endogenous prostaglandins are involved in maintaining muscle tone and in modifying the response to catecholamines and that both contraction and relaxation are mediated by alpha 1-adrenoceptors.

Evidence for the existence of 'atypical' beta-adrenoceptors (beta 3-adrenoceptors) mediating relaxation in the rat distal colon in vitro

1. Experiments were carried out to characterize the adrenoceptors mediating relaxant responses in the rat distal colon. Three agonists were used: noradrenaline, isoprenaline and the beta 3-adrenoceptor agonist BRL 37344. Phentolamine, propranolol and (+/- )-cyanopindolol were tested as antagonists. Tone in the rat distal colon was induced with KCl (30-40 mM) as a spasmogen, and relaxations of this KCl-induced tone produced by the agonists were measured. 2. Relaxant responses to noradrenaline that were obtained in the presence of propranolol (1 microM) were not antagonized by phentolamine (0.01 to 1 microM). Relaxant responses to isoprenaline that were obtained in the presence of phentolamine (1 microM) were antagonized in a concentration-dependent manner by propranolol (0.01 to 3 microM), although this antagonism was weak and non-competitive. Relaxant responses to BRL 37344 that were obtained in the presence of phentolamine (1 microM) were only weakly antagonized by high (1 microM) concentrations of propranolol. 3. Tachyphylaxis to BRL 37344 was observed, a second concentration-response curve being shifted to the right by 15 fold. Exposure of the tissues to BRL 37344 (1 microM) between concentration-response curves also caused rightward shifts in the responses to noradrenaline (18 fold) and isoprenaline (19 fold) but not to papaverine. 4. In the presence of phentolamine (1 microM) and propranolol (1 microM), the rank order of potency of the agonists was: (-)-isoprenaline (1.0) greater than or equal to BRL 37344 (0.93) greater than (-)-noradrenaline (0.3). 5. Responses to BRL 37344 in the presence of phentolamine (1 microM) and propranolol (1 microM) were antagonized by (+/-)-cyanopindolol (I microM), with an apparent pA2 value of 6.67. Responses to isoprenaline, under the same conditions, were antagonized in a competitive manner by (+/-)-cyanopindolol (0.1 to 10 microM), with the slope of the Schild plot close to unity and a pA2 value of 7.12. 6. The resistance of the relaxant responses to antagonism by phentolamine and propranolol, along with the relatively high potency of the beta 3-adrenoceptor agonist BRL 37344 and the antagonism of 'resistant' responses by (+/-)-cyanopindolol would suggest that 'atypical' beta 6-adrenoceptors, similar to the beta 3-adrenoceptors of rat adipocytes and other tissues, exist in the rat distal colon.

A novel action of isoproterenol to inactivate a cardiac K+ current is not blocked by beta and alpha adrenergic blockers

The K+ current iKl sets the resting potential in cardiac cells. Here we report that isoproterenol (ISO), a prototypical beta agonist, increases inactivation of iKl. This action of ISO on iKl is mimicked by permeant analogues of cAMP but is not blocked by the beta blockers propranolol and pindolol or the alpha blockers prazosin or yohimbine. We suggest that this novel action of ISO may contribute to pacemaker activity in the Purkinje strand and be mediated through a class of receptors different from classical beta's or alpha's.

Evidence that an atypical beta-adrenoceptor mediates the inhibition of spontaneous rhythmical contractions of rabbit isolated jejunum induced by ritodrine and salbutamol

1. The nature of the adrenoceptors mediating the inhibitory action of noradrenaline, ritodrine and salbutamol on the spontaneous activity of longitudinal muscle of the rabbit jejunum in vitro was investigated by use of a range of adrenoceptor antagonists. 2. The actions of ritodrine and salbutamol were antagonized competitively by propranolol. The pA2 values of 6.4 and 6.6 respectively were smaller than those found elsewhere for beta 1- and beta 2-adrenoceptors. 3. In contrast, the responses to ritodrine and salbutamol were antagonized only by high concentrations (greater than 2.7 microM) of phentolamine and were unaffected by yohimbine (2.6 microM), mepyramine (2.5 microM) or cimetidine (4.0 microM). 4. Ritodrine which is less potent than salbutamol in tissues with typical beta 2-adrenoceptors was found to be 8 times more potent than salbutamol in the rabbit jejunum. 5. It is suggested that in the rabbit jejunum ritodrine and salbutamol may act at an atypical beta-adrenoceptor, at which propranolol is a competitive but not very potent antagonist.

Biotechnology of beta-adrenergic receptors

The emergence of Biotechnology has provided pharmacologists with a variety of methods for investigating the structure, the function, and the regulation of membrane-bound receptors with a precision that was not imagined even five years ago. These new tools have been developed and used to analyze the known catecholamine beta 1- and beta 2-adrenergic receptors and to discover and study a new subtype, the beta 3 receptor. We review here the salient features of each of these three receptors, compare their structural and functional properties, and propose models to explain their differential regulation in time and space. A whole family of proteins has now been found to share with the beta-adrenergic receptors their most prominent features, including seven transmembrane domains and coupling with GTP-binding "G" proteins. We therefore propose that the biotechnology-based procedures developed for the beta-adrenergic receptors will be well applicable to the other members of this "R7G" family of receptors.

Central and peripheral alpha-adrenergic control of ileal, caecal and proximal colonic myoelectrical activity in sheep

The role of central and peripheral alpha-adrenoceptors in regulation of ileal, caecal and proximal colonic myoelectrical activity was studied in five conscious ewes chronically fitted with intraparietal electrodes and a cannula in a lateral ventricle of the brain. About 5 min after the end of a regular spiking activity (RSA) phase of a migrating motor complex (MMC) in the distal ileum, selective alpha 2- and alpha 1-agonists and antagonists were administered intravenously (i.v.) or intracerebroventricularly (i.c.v.). The alpha 2-antagonist yohimbine (600 micrograms/kg i.v. or 60 micrograms/kg i.c.v.) induced an RSA phase in the ileum. The alpha 2-agonist naphazoline (30 micrograms/kg i.v. or 3 micrograms/kg i.c.v.) stopped or greatly reduced the frequency of caecal and proximal colonic contractions, respectively, and prevented any MMC in the ileum, effects which were abolished when the animals were either i.v. or i.c.v. pretreated with yohimbine. The alpha 1-antagonist prazosin (600 micrograms/kg i.v., but not 60 micrograms/kg i.c.v.) selectively reduced the frequency of contractions in the cranial caecum and proximal colon. The alpha 1-agonist phenylephrine (40 micrograms/kg i.v., but not 4 micrograms/kg i.c.v.) provoked an RSA phase in the ileum, decreased the frequency of contractions in the blind pole and the middle of the caecum and increased that in the cranial caecum and the proximal colon. These effects of phenylephrine were blocked by i.v., but not i.c.v., prazosin. The present results suggest that alpha-adrenoceptors are involved in the control of the intestinal motility as follows: I) central alpha 2-inhibitory and peripheral alpha 1-excitatory receptors for the ileum, II) central and peripheral alpha 2- and peripheral alpha 1-inhibitory receptors for the blind pole and the middle of caecum and III) central and peripheral alpha 2-inhibitory and peripheral alpha 1-excitatory receptors for the cranial caecum and the proximal colon.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro inhibition of intestinal motility by phenylethanolaminotetralines: evidence of atypical beta-adrenoceptors in rat colon

1. The new compounds phenylethanolaminotetralines (PEAT), unlike the reference beta-adrenoceptor agonists isoprenaline (Iso), ritodrine (Ri) and salbutamol (Sal), produced half-maximal inhibition of spontaneous motility of rat isolated proximal colon at substantially lower concentrations (EC50 2.7-30 nM) than those inducing beta 2-adrenoceptor-mediated responses (relaxation of guinea-pig isolated trachea and rat uterus) and had virtually no chronotropic action (EC50 greater than 3 x 10(5) M) on the guinea-pig isolated atrium (a beta 1-adrenoceptor-mediated response). 2. The nonselective beta-adrenoceptor antagonists alprenolol and propranolol prevented the inhibition of rat colon motility by the PEAT with low and different potencies (pA2 values around 7.5 and 6.5 respectively). Conversely alprenolol and propranolol had a higher and similar potency (pA2 values around 9.0) in preventing typical beta 1 or beta 2-responses (increase in atrial frequency by Iso or tracheal relaxation by Ri or Sal). 3. The selective beta-adrenoceptor antagonists CGP 20712A (beta 1) and ICI 118,551 (beta 2) either alone or in combination, did not prevent rat colon motility inhibition by the representative PEAT SR 58611A, which was also fully resistant to alpha-adrenoceptor, acetylcholine, dopamine, histamine, opioid and 5-hydroxytryptamine antagonists. 4. These results indicate that the PEAT are a new class of beta-adrenoceptor agonists and suggest that their preferential intestinal action may be accounted for by selectivity for atypical beta-adrenoceptors, abundant in the rat colon and distinct from the currently recognized beta 1 and beta 2 subtypes.

Subclassification of beta-adrenoceptors responsible for steroidogenesis in primary cultures of bovine adrenocortical zona fasciculata/reticularis cells

1. Forty eight hour primary cultures of purified bovine adrenocortical zona fasciculata/reticularis cells secreted hydrocortisone in response to stimulation with beta-adrenoceptor agonists. The observed order of potency was isoprenaline greater than noradrenaline greater than dobutamine greater than salbutamol greater than BRL37344. 2. Salbutamol acted as a partial agonist on these cells hence suggesting the presence of a beta 1-adrenoceptor. 3. Schild analysis of the hydrocortisone response to isoprenaline showed that the selective beta 1-antagonist practolol and the selective beta 2-antagonist ICI118,551 gave pA2 values of 6.85 and 7.17, respectively. These values were in close agreement with corresponding pA2 values previously obtained for the beta 1-adrenoceptor. 4. We conclude that beta 1-adrenoceptors are responsible for mediating catecholamine-stimulated hydrocortisone secretion from primary cultures of bovine zona fasciculata/reticularis cells.

Effects of endothelin on renal hemodynamics and excretory functions in anesthetized dogs

The effects of endothelin on renal hemodynamics and excretory functions were investigated in anesthetized dogs. Infusion of endothelin at a rate of 1 ng/kg.min resulted in a slight but significant decrease in renal blood flow and an increase in renal vascular resistance and filtration fraction. Endothelin at doses higher than 10 ng/kg.min significantly decreased cardiac output, glomerular filtration rate, urine volume, and urinary sodium and potassium excretion, whereas it increased systemic vascular resistance. Mean arterial pressure and heart rate showed a transient decrease and increase, respectively, at doses higher than 50 ng/kg.min. Plasma renin activity and plasma aldosterone concentrations were increased only at the dose of 100 ng/kg.min. These effects lasted for more than 60 min. These results suggest that endothelin may have an important role in the modulation of renal functions as well as in the modulation of systemic hemodynamics.

Relaxant effects of alpha-adrenoceptor agonists in the rat isolated gastric fundus

In rat gastric fundus preparations with tone raised by the addition of barium chloride or carbachol, and in the presence of propranolol (2 microM) to prevent beta-adrenoceptor mediated effects, the adrenoceptor agonists noradrenaline, adrenaline, alpha-methylnoradrenaline, isoprenaline, cirazoline and phenylephrine all caused concentration-related relaxant responses. Relaxations to the catecholamines were poorly antagonized by prazosin (0.01-1 microM) resulting in the slopes of Schild plots being less than unity, low pA2 values for prazosin against the catecholamines and a clear relaxant effect of the catecholamines even in the presence of 1 microM prazosin. The prazosin-resistant relaxations were unaffected by higher concentrations of prazosin (2 microM) and propranolol (30 microM) or by further additions of idazoxan (1 microM) or haloperidol (30 microM). The relaxations were not due to a non-specific effect of the catechol nucleus since neither dihydroxyphenylethylene glycol (DOPEG) nor dihydroxyphenylacetic acid (DOPAC) produced relaxant effects at concentrations up to 300 microM. In contrast to the results with the catecholamines, prazosin was a potent antagonist of the relaxant effect of cirazoline and phenylephrine, although the antagonism was difficult to quantify due to a lowering of the slope of the concentration response curves to cirazoline and phenylephrine with the higher concentrations of prazosin (0.1 and 1.0 microM). In conclusion postjunctional relaxatory effects of catecholamines in the rat gastric fundus are mediated partly via alpha 1-adrenoceptors and partly via an atypical adrenoceptor.

Molecular characterization of the human beta 3-adrenergic receptor

Since the classification of beta-adrenergic receptors (beta-ARs) into beta 1 and beta 2 subtypes, additional beta-ARs have been implicated in the control of various metabolic processes by catecholamines. A human gene has been isolated that encodes a third beta-AR, here referred to as the "beta 3-adrenergic receptor." Exposure of eukaryotic cells transfected with this gene to adrenaline or noradrenaline promotes the accumulation of adenosine 3',5'-monophosphate; only 2 of 11 classical beta-AR blockers efficiently inhibited this effect, whereas two others behaved as beta 3-AR agonists. The potency order of beta-AR agonists for the beta 3-AR correlates with their rank order for stimulating various metabolic processes in tissues where atypical adrenergic sites are thought to exist. In particular, novel beta-AR agonists having high thermogenic, antiobesity, and antidiabetic activities in animal models are among the most potent stimulators of the beta 3-AR.

Is there a third heart beta-adrenoceptor?

A series of partial agonists with high affinity for myocardial beta 1- and beta 2-adrenoceptors cause stimulant effects in heart that are resistant to blockade of beta 1- and beta 2-adrenoceptors. The concentrations of partial agonist that cause stimulant effects greatly exceed those that cause blockade. Alberto Kaumann suggests that such non-conventional partial agonists, often analogues of pindolol, may act through a third heart beta-adrenoceptor, which resembles the beta 3-adrenoceptor of white adipocytes and smooth muscle of airways and ileum.

The pharmacological properties of the peptide, endothelin

1. The effect of endothelin (ET-1) has been studied on isolated vascular and non-vascular preparations, using both functional and competition radioligand binding techniques. The effects of endothelin on blood pressure were studied in both anaesthetised, chemically denervated normotensive and spontaneously hypertensive rats (SHR). 2. Endothelin elicited contractile responses in the rat thoracic aorta, perfused mesenteric bed, rabbit mesenteric artery and portal vein. The maximal responses in the rat aorta were enhanced by removal of the endothelium, and were reduced in the presence of either a cyclo-oxygenase inhibitor (indomethacin) or a thromboxane receptor antagonist (SQ 29,548). In terms of potency, the most sensitive preparation was the rat endothelium-denuded aorta and rat perfused mesenteric bed (-log EC50 values = 8.2 +/- 0.07 and 8.2 +/- 0.12, mean +/- s.e.mean, n = 4, respectively). In the perfused mesenteric bed of the rat the maximum response to endothelin (219 +/- 12 mmHg, n = 4) was greater than that to either phenylephrine (maximal response = 67 +/- 9 mmHg; n = 4) or KCl (maximal response = 110 +/- 6 mmHg, n = 4). 3. Endothelin elicited contractile responses of the guinea-pig isolated ileum, oesophageal muscularis mucosae and uterus. Responses were also observed in the rat fundic strip and paced left atria. The guinea-pig urinary bladder, trachea, rat vas deferens and anococcygeus exhibited little or no response to endothelin at the concentrations studied (1 x 10(-12)-3.2 x 10(-8) M). Of the above preparations, the ileum and oesophageal muscularis mucosae were the most sensitive to endothelin (-log EC50 = 8.5 +/- 0.11 and 8.4 +/- 0.06, n = 6, respectively), exhibiting potencies similar to those observed in the endothelium-denuded aorta of the rat. 4. In competition-radioligand binding studies, endothelin did not displace either [3H]-PN 210-100 or [125I]-(-)-omega-conotoxin GVIA from binding sites in membranes from rat cerebral cortex and, skeletal muscle or from guinea-pig cerebral cortex and hippocampus, respectively. This indicates a lack of direct interaction of endothelin at the dihydropyridine binding site and the N-type calcium channel, respectively. However, in functional studies, contractile responses to endothelin (1 x 10(-8) M) in the endothelium-denuded aorta of the rat were potently reversed by nifedipine, verapamil, and prenylamine (-log IC50 values = 8.0 +/- 0.13, 7.2 +/- 0.09 and 6.6 +/- 0.08, n = 4-8, respectively). In addition, the responses to endothelin were virtually abolished in the presence of Krebs physiological salt solution containing no calcium but with 1 x 10-M EDTA added. Preequilibration with either (-)-w-conotoxin (1 x 10-6M) or tetrodotoxin (1 x 10-6M) did not affect responses to endothelin. 5. In chemically denervated rats, endothelin (1pmolkg-'-10nmolkg- , i.v.) exhibited pressor responses, which were unaffected by a 3 h pretreatment with indomethacin. In the SHR, the effects on blood pressure were not significantly different from those observed in normotensive animals at any of the doses studied. A transient (duration < 30 s) depressor response was also observed in all groups studied at a dose of 0.1-1 nmol kg-1 i.v. 6. In conclusion, endothelin is a potent contractile agonist in both vascular and non-vascular muscle. It appears to elicit responses partly via the entry of extracellular calcium (by a mechanism distinct from that of other calcium facilitators) and partly by release of endoperoxides.

Endothelin binding sites in porcine aortic and rat lung membranes

High-affinity binding sites for endothelin were identified on porcine aortic and rat lung membranes. Interaction of 125I-labelled endothelin with its binding site was specific, saturable, time- and temperature-dependent but dissociation of receptor-bound ligand was minimal. Maximal binding was observed at pH 7.0 in porcine aorta and at pH 3.1 in the rat lung. Treatment of membranes with trypsin destroyed the binding site in both tissues. Porcine endothelin showed a higher affinity for receptors in both tissues compared to rat endothelin. Vasoactive peptides and Ca2+ channel antagonists did not interact with this site suggesting high specificity of binding. Analysis of saturation binding showed that the number of binding sites was 1250 +/- 104 and 1650 +/- 170 fmol/mg protein and the affinity of binding sites was 0.47 +/- 0.15 and 0.16 +/- 0.07 nM in the aorta and the lungs respectively (n = 5). Presence of protease inhibitors did not alter binding suggesting that the label was stable under the incubation conditions. This was further confirmed by HPLC. Removal of the endothelium from the aorta did not change the binding characteristics of this tissue. Ca2+ and Mg2+ ions caused an increase in binding by increasing the affinity. Binding was completely abolished in the presence of Triton and dithiothreitol. The binding sites identified in this study may be responsible for the actions of endothelin in the aorta and the lung.