The hypotensive activity and side effects of methyldopa, clonidine, and guanfacine.

Clonidine (Catapres, Catapresan), guanfacine (Estulic), and methyldopa (Aldomet) are the prototypes of centrally acting antihypertensive drugs. Clonidine and guanfacine are lipophilic drugs that readily penetrate into the brain, where they stimulate alpha-adrenergic receptors in the pontomedullary region. The stimulation of these central alpha-adrenergic receptors has been shown to activate an inhibiting neuron, which causes a reduction of peripheral sympathetic tone and a subsequent fall in arterial blood pressure and heart rate. Both a centrally initiated reduction of vagus reflex activity and the activation of presynaptic alpha 2-adrenergic blocking agents in the heart may contribute to the bradycardia. Studies indicate that methyldopa also penetrates into the brain, where it is converted into alpha-methylnorepinephrine. This amine may stimulate the same central alpha-adrenergic receptors as those activated by clonidine, which will result in a hypotensive effect. Possibly, alpha-methyldopamine might also play a role. Accordingly, the modes of action of clonidine and alpha-methyldopa probably are very similar at a basic level. The central adrenergic receptors probably are located postsynaptically. Their receptor demand corresponds more closely to that of the alpha 2-subtype. Central alpha 1-adrenergic receptors might possibly play a part in the modulation of vagally induced baroreflex bradycardia. A discussion on the pharmacological basis of the side effects of the centrally acting antihypertensives has been limited to those adverse reactions that are somehow related to alpha-adrenergic receptors. Sedation, a common side effect, appears to be mediated by central alpha 2-adrenergic receptors, at least in animal models.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonpeptide angiotensin II receptor antagonists. IV. EXP6155 and EXP6803.

EXP6155 (2-n-butyl-1-[4-carboxybenzyl]-4-chloroimidazole-5-acetic acid) and EXP6803 (methyl 2-n-butyl-1-[4-(2-carboxybenzamido)benzyl]-4-chloroimidazole -5-acetate, sodium salt) are shown to be novel, nonpeptide, antihypertensive, specific angiotensin II receptor antagonists. In rabbit aorta, they competitively inhibited the contractile response to angiotensin II with pA2 values of 6.54 and 7.20 and did not alter the response to norepinephrine or KCl. In guinea pig ileum, both agents blocked the responses to angiotensin I and II and did not alter the responses to bradykinin and acetylcholine. A similar specific angiotensin II antagonism was shown in vivo in the spinal pithed rat model. In renal artery-ligated rats, a high renin hypertensive model, EXP6155 and EXP6803 given intravenously, decreased blood pressure with ED30 of 10 and 11 mg/kg, respectively. Both compounds did not alter blood pressure when given orally at 100 mg/kg. Unlike saralasin, EXP6155 and EXP6803 given intravenously did not cause a transient increase in blood pressure in the renal artery-ligated and normotensive rats. Our results indicate that EXP6155 and EXP6803 are selective angiotensin II receptor antagonists and antihypertensive agents. Since neither compound had partial agonist activities or bradykinin potentiation effects, unlike the existing peptide angiotensin II receptor antagonists and angiotensin converting enzyme inhibitors, respectively, they may represent preferred probes for studying the physiological roles of angiotensin II.

Quantitative relationships between alpha-adrenergic activity and binding affinity of alpha-adrenoceptor agonists and antagonists.

Quantitative relationships between in vitro affinity for alpha 1- and alpha 2-adrenoceptors (specific binding sites in rat brain membranes of [3H]prazosin and [3H]clonidine, respectively) and in vitro and in vivo alpha 1/alpha 2-adrenoceptor agonist/antagonist activities were derived for a series of 11 alpha-adrenergic antagonists and 35 agonists of dissimilar chemical structure. For the antagonists, the alpha 1/alpha 2-binding selectivity ratio most significantly correlated with the functional alpha 1/alpha 2-blocking selectivity ratios assessed in vitro (rabbit isolated pulmonary artery: antagonism of alpha 1-adrenoceptor-induced vasoconstriction and alpha 2-adrenoceptor-evoked facilitation of transmitter release) and in vivo (antagonism of alpha 1- and alpha 2-adrenoceptor-mediated vasoconstriction in pithed normotensive rats). These results show that the in vitro alpha 1- and alpha 2-adrenoceptor binding affinities of the antagonists provide adequate information concerning their functional alpha 1- and alpha 2-adrenoceptor blocking potencies against agonists. For the agonists, the central, alpha 2-adrenoceptor-elicited, hypotensive activity was not correlated with alpha 1-adrenoceptor binding affinity but was most significantly described in terms of affinity for alpha 2-adrenoceptors and a parabolic dependence on log P' (octanol/buffer; pH 7.4; 37 degrees C). The relevance of log P' in the regression is explained by the difference in accessibility to the membrane-bound alpha 2-adrenoceptors in the radioligand displacement experiments and the central medullary (hypotensive) alpha 2-adrenoceptors in the intact animal. In contrast, the affinity parameters for alpha 1- and alpha 2-adrenoceptors were found to be poor descriptors of the hypertensive potency of the agonists in which alpha 1- and alpha 2-adrenoceptors are known to play a role. The correlations in which the individual binding parameters and the combination of both variables were included reached only a moderate significance level.(ABSTRACT TRUNCATED AT 250 WORDS)

Design and synthesis of isoxazoline derivatives as factor Xa inhibitors. 1.

Thrombosis is a major cause of mortality in the industrialized world. Therefore, the prevention of blood coagulation has become a major target for new therapeutic agents. One attractive approach is the inhibition of factor Xa (FXa), the enzyme directly responsible for prothrombin activation. We report a series of novel biaryl-substituted isoxazoline derivatives in which the biaryl moiety was designed to interact with the S(4) aryl-binding domain of the FXa active site. Several of the compounds herein have low nanomolar affinity for FXa, have good in vitro selectivity for FXa, and show potent antithrombotic efficacy in vivo. The three most potent compounds (33, 35, and 37) have inhibition constants for human FXa of 3.9, 2.3, and 0.83 nM, respectively, and ID(50)'s ranging from 0.15 to 0.26 micromol/kg/h in the rabbit arterio-venous thrombosis model.

Orally active isoxazoline glycoprotein IIb/IIIa antagonists with extended duration of action.

Modification of the alpha-carbamate substituent of isoxazoline GPIIb/IIIa (alphaIIb beta3) antagonist DMP 754 (7) led to a series of alpha-sulfonamide and alpha-sulfamide diaminopropionate isoxazolinylacetamides which were found to be potent inhibitors of in vitro platelet aggregation. Aryl- and heteroaryl-alpha-sulfonamide groups, in conjunction with (5R)-isoxazoline (2S)-diaminopropionate stereochemistry, were found to impart a pronounced duration of antiplatelet effect in dogs, potentially due to high affinity for unactivated platelets. Isoxazolylsulfonamide 34b (DMP 802), a highly selective GPIIb/IIIa antagonist, demonstrated a prolonged duration of action after iv and po dosing and high affinity for resting and activated platelets. The prolonged antiplatelet profile of DMP 802 in dogs and the high affinity of DMP 802 for human platelets may be predictive of clinical utility as a once-daily antiplatelet agent.

Discovery of an orally active series of isoxazoline glycoprotein IIb/IIIa antagonists.

Using isoxazoline XR299 (1a) as a starting point for the design of highly potent, long-duration GPIIb/IIIa antagonists, the effect of placing lipophilic substituents at positions alpha and beta to the carboxylate moiety was evaluated. Of the compounds studied, it was found that the n-butyl carbamate 24u exhibited superior potency and duration of ex vivo antiplatelet effects in dogs. Replacement of the benzamidin-4-yl moiety with alternative basic groups, elimination of the isoxazoline stereocenter, and reversal of the orientation of the isoxazoline ring resulted in lowered potency and/or duration of action.

Early scintigraphic detection of experimental myocardial infarction in dogs with technetium-99m-glucaric acid.

Recent data have generated some interest in technetium-99m-(99mTc) glucaric acid as an in vivo viability marker. We studied 99mTc-glucaric acid retention in canine models of myocardial ischemia (20-min occlusion of the LAD/40-min reperfusion), acute myocardial infarction (MI) (90-min LAD occlusion/3-hr reperfusion), and chronic MI (90-min occlusion and either 48-hr or 10-day reperfusion). Regional myocardial blood flow was measured by radiolabeled microspheres. No preferential uptake of glucaric acid was observed in ischemic but viable myocardium. The compound showed high affinity for necrotic myocardial tissue for several days following injury. The preferential uptake in infarcted tissue disappeared by 10 days following injury. This study shows that 99mTc-glucaric acid acts exclusively as a marker of necrosis in canine models of MI. Technetium-99m-glucaric acid may have clinical utility in early cardiac imaging of myocardial infarction and in differentiating recent from old injuries.

Inhibitory effect of calcium antagonist drugs on vasoconstriction induced by vascular alpha 2-adrenoceptor stimulation.

A survey is given of the mechanisms of the antihypertensive effect of calcium entry blockers. The main background of the antihypertensive/hypotensive action is dilatation of precapillary arterioles (resistance vessels that cause a reduction in total peripheral resistance and, hence, a decrease in blood pressure). The vascular relaxation is caused by an inhibition of the transmembranous calcium influx and, probably less so, by interference with calmoduline. Calcium entry blockers significantly reduce the vasoconstriction induced by the excitation of vascular postsynaptic alpha 2 adrenoceptors. The inhibitory effect of calcium entry blockers is reversed by the calcium entry promoter Bay k 8644. The vasoconstriction induced by alpha 1-adrenoceptor stimulation is less generally influenced by calcium entry blockers than the alpha 2 effects. The interference with alpha 2-adrenoceptor-induced vasoconstriction may contribute to the vasodilator action of the calcium entry blockers, especially in hypertensive patients who show a hyperreactivity to pressor responses toward catecholamines.

Evaluation of the effect of azapropazone on neutrophil migration in regional myocardial ischaemia/reperfusion injury in rabbits.

1. The purpose of the present study was to determine the myocardial cytoprotective efficacy of azapropazone (AZA) and its potential site of action on neutrophil infiltration into reperfused/ischaemic myocardium with or without in vivo activation of neutrophils in rabbits. 2. AZA, 100 mg kg-1, was administered i.v. 10 min after occlusion of the left circumflex (LCX) artery in rabbits with and without pretreatment with phorbol myristate acetate ester (PMA). The LCX occlusion was then released at 10 min after AZA administration. Haemodynamic parameters (heart rate, LV pressure, mean arterial blood pressure and dp/dt) were monitored throughout the experiment. After 60 min reperfusion, the area at risk was delineated and the heart was then excised and divided into epi- and endocardial pieces for analysis of myeloperoxidase activity. 3. AZA inhibited neutrophil infiltration into the reperfused/ischaemic rabbit myocardium with and without PMA treatment. The inhibition of neutrophil infiltration was more apparent in the epicardium than in the endocardium. Additionally, AZA inhibited to a similar extent the in vivo PMA-stimulated neutrophil migration into the epicardium and endocardium area at risk. AZA had no significant effect on the haemodynamic parameters as compared to control. 4. AZA administered in an anaesthetized rabbit model of LCX occlusion/reperfusion resulted in the reduction of infarct size. 5. It is concluded that AZA has significant inhibitory effects on neutrophil migration which might contribute to its myocardial cytoprotective effect.

Sgd 101/75: cardiovascular effects in various animal preparations; interactions with vascular postjunctional alpha 1- and alpha 2-adrenoceptors.

The effect of the imidazolidine Sgd 101/75 (2-[2-methylindazol-4-imino]-imidazolidine HCl) on blood pressure, as well as its alpha-adrenoceptor agonist activity and affinity for these receptors, were examined in various animal preparations. After both intravenous administration to conscious spontaneously hypertensive rats and intravenous injection or infusion via the vertebral artery in chloralose-anaesthetized cats, Sgd 101/75 (1-10 mg kg-1) elicited pressor responses. Intracisternal application of Sgd 101/75 (1 mg kg-1) to chloralose-anaesthetized cats did not affect blood pressure. In the pithed rat and pithed cat the vasopressor responses to i.v. Sgd 101/75 were effectively antagonized by prazosin (0.1-1.0 mg kg-1, i.v.) but much less by yohimbine (1 mg kg-1, i.v.). Sgd 101/75 proved a less potent and less selective displacing agent of [3H]-clonidine- and [3H]-prazosin-binding in rat brain membranes than clonidine. The results suggest that Sgd 101/75 is a selective alpha 1-adrenoceptor agonist, devoid of any centrally or peripherally mediated hypotensive activity; this is probably caused by the low capacity of Sgd 101/75 for stimulating alpha 2-adrenoceptors.

Effects of the nonpeptide angiotensin II receptor antagonist DuP 753 on blood pressure and renal functions in spontaneously hypertensive PH dogs.

A colony of genetic hypertensive dogs with systolic blood pressure of 140 to 220 mm Hg and diastolic blood pressure greater than 100 mm Hg in the trained state was used. The objective of this study was to investigate the hemodynamic and renal effects of the novel angiotensin II receptor antagonist DuP 753 given intravenously to these dogs. Renal functions and blood pressure were measured 45 to 75 min after the intravenous administration of DuP 753 at 1, 3, 10, and 30 mg/kg and were compared to control (placebo) treatment. Arterial pressure was slightly but significantly and dose-dependently reduced by DuP 753. Glomerular filtration rate increased significantly in a dose-dependent manner. Similarly, effective renal plasma flow was dose-dependently increased. Filtration fraction was unchanged. Renal vascular resistance was significantly reduced in a dose-dependent manner at 3, 10, and 30 mg/kg of DuP 753. DuP 753 increased fractional sodium excretion at all doses and increased fractional potassium excretion only at the highest doses. The vasopressor effects of angiotensin I and II were dose-dependently inhibited by DuP 753. These data show that DuP 753 has beneficial renal hemodynamic effects and lowers arterial pressure in this canine model of essential hypertension.

Equal potency of nifedipine to inhibit alpha 1-(dobutamine and BDF 6143) and alpha 2-adrenoceptor (B-HT 920) induced pressor responses in pithed rats; lack of effect of phenoxybenzamine.

Intravenous (i.v.) dobutamine and BDF 6143 were partial agonists in increasing diastolic pressure in beta-adrenoceptor-blocked pithed rats. The log dose-pressor effect curves were not influenced by yohimbine (1 mg/kg i.v., -15 min) but were markedly shifted to the right by prazosin (0.1 mg/kg i.v., -15 min) indicating the exclusive involvement of alpha 1-adrenoceptors. Nifedipine (0.1-1 mg/kg i.a., -15 min) non-competitively inhibited the pressor effects of dobutamine and BDF 6143 as well as of the alpha 2-adrenoceptor agent B-HT 920 with equal potency. The -log ED50 values calculated for nifedipine amounted to 6.25 +/- 0.12, 6.16 +/- 0.14 and 6.20 +/- 0.10, respectively. Phenoxybenzamine (3 or 10 micrograms/kg i.v., -60 min) did not affect the effectiveness of nifedipine (0.1 mg/kg) to inhibit the pressor effects of dobutamine and BDF 6143. Following treatment with Bay k 8644 (1 mg/kg i.a., -15 min), the log dose-pressor effect curves for dobutamine and BDF 6143 were shifted to the left and the maximum responses were elevated. Our findings suggest that the alpha 1-adrenoceptor-induced pressor effects of dobutamine and BDF 6143 rely heavily on the influx of Ca2+, and are indistinguishable in this respect from the effects initiated by alpha 2-adrenoceptor stimulation. The data further support the view that the sensitivity of alpha-adrenoceptor-mediated pressor effects to inhibition by Ca2+ entry blockers depends on the extent to which Ca2+ influx contributes to the overall response and is not determined by the intrinsic activity or by the receptor reserve of the alpha-adrenoceptor agonist.

Effect of azapropazone and allopurinol on myocardial infarct size in rats.

The effect of the xanthine oxidase inhibitor allopurinol and the non-steroidal antiinflammatory agent azapropazone on infarct size in rats, subjected to 48 h of occlusion of the left anterior descending coronary artery, were studied. Allopurinol (50 mg/kg i.p., twice daily from 24 h before to 48 h after LAD occlusion) and azapropazone (100 mg/kg i.p twice daily from 24 h before to 48 h after LAD occlusion) significantly reduced infarct size when compared to saline-treated rats. These data point towards involvement of xanthine oxidase derived free radicals in evolving myocardial infarction in rats; beneficial effect of azapropazone in this model may be related to the drug's ability to inhibit xanthine oxidase as well as various key neutrophil functions.

Heterogeneity of the interaction between alpha 1- and alpha 2-adrenoceptor agonists with their respective receptors in the vascular system of the pithed rat.

The subdivision of alpha 1- and alpha 2-adrenoceptor-mediated pressor responses to different agonists based upon the influence of beta 2-adrenoceptor-mediated vasodilatation was further investigated in the pithed normotensive rat. The effect of salbutamol (4.18 X 10(-6) mol/kg) on the alpha 1-adrenoceptor-mediated increase in diastolic pressure due to dopamine and amidephrine as well as on the alpha 2-adrenoceptor-mediated pressor response to azepexole, DP-6,7-ADTN, M-7, TL-99 and dopamine was assessed. The alpha 1-pressor responses to amidephrine and dopamine were only slightly attenuated by salbutamol. The alpha 2-adrenoceptor-mediated increase in diastolic pressure due to B-HT 933 was strongly antagonized by salbutamol in contrast to the effect of dopamine, DP-6,7-ADTN and M-7. TL-99 occupied in intermediate position. The data do not support the existence of distinctly different subtypes of alpha 1- and alpha 2-adrenoceptors but favor the hypothesis that both alpha 1- and alpha 2-adrenoceptors are activated in a unique way by each of their respective agonists.

Lack of effect of D600 on alpha 1-adrenoceptor-mediated contractions of rat isolated anococcygeus muscle.

The experiments concerned the contractile responses of rat anococcygeus muscle to the selective alpha 1-adrenoceptor agonists cirazoline, phenylephrine, methoxamine, St 587, Sgd 101/75, amidephrine and SK&F 89748-A and the effect of the calcium entry blocker D600 on the responses. The effects of noradrenaline, adrenaline, K+, tyramine and electrical field stimulation were studied as well. The potency series of the various agonists on rat anococcygeus muscle differed from the series for rat and guinea-pig aorta, indicating differences in the structure of the alpha 1-adrenoceptors on these tissues. D600 was ineffective in inhibiting contractions of rat anococcygeus muscle to the various agonists, but effectively attenuated the responses to K+ in anococcygeus muscle from rats pretreated with reserpine or in prazosin-induced preparations. These data indicate that alpha 1-adrenoceptor activation in rat anococcygeus muscle triggers contractions which do not primarily require the influx of extracellular calcium. In this respect, the smooth muscle from the rat anococcygeus muscle differs from vascular smooth muscle from this species.

Role of ganglionic M-1 and M-2 receptors in the neuronal control of the cardiovascular system of the normotensive rat as determined with pilocarpine.

The adrenoceptors involved in the increase in diastolic pressure and heart rate elicited by i.v. administration of pilocarpine to the pithed rat were assessed using as pharmacological tools the alpha 1-adrenoceptor antagonist prazosin, the alpha 2-adrenoceptor antagonist rauwolscine, the beta 1-adrenoceptor blocker atenolol and the beta 2-adrenoceptor blocker ICI 118,551. Pilocarpine indirectly activated vascular alpha 1- and cardiac beta 1-adrenoceptors. By using the M-1 antagonist pirenzepine and the mixed M-1/M-2 antagonist dexetimide, pilocarpine was shown to be a mixed M-1/M-2 agonist. Pilocarpine initiated antagonistic effects on intrasynaptic alpha 2-adrenoceptor-mediated pressor responses and not on those triggered by extrasynaptic alpha 2-adrenoceptors. During vasopressin infusion to counteract a possible vasodilator action of pilocarpine, it was demonstrated that pilocarpine indirectly activated alpha 1- and alpha 2-adrenoceptors. The results support the hypothesis that intra- and extrasynaptic alpha 2-adrenoceptors comprise different populations and that the neuronal control of alpha 2-adrenoceptors is mediated by ganglionic M-2 receptors.

The contractions induced in rat and guinea-pig aortic strips by the alpha 2-adrenoceptor selective agonists B-HT 920 and UK 14,304 are mediated by alpha 1-adrenoceptors.

The alpha-adrenergic action of the selective alpha 2-adrenoceptor agonists B-HT 920 and UK 14,304 was investigated on helically cut preparations of rat and guinea-pig isolated aorta. The alpha 1-adrenoceptor selective stimulant (-)-phenylephrine was included for comparison. All agonists induced concentration-dependent contractions in both preparations. Calcium entry blockade by D 600 almost abolished the contractions evoked by B-HT 920 and UK 14,304 in rat aorta while those evoked in guinea-pig aorta were less strongly affected. Contractions elicited by (-)-phenylephrine were moderately impaired by D 600 in rat aorta whereas there was only a limited effect in guinea-pig aorta. Analysis of the prazosin and yohimbine antagonism of B-HT 920- and UK 14,304-evoked contractions showed the involvement of alpha 1-like adrenoceptors in rat and guinea-pig aorta, prazosin being approximately 1000 times more potent that yohimbine. The results show that B-HT 920 and UK 14,304 contract rat and guinea-pig aorta via alpha 1-like adrenoceptors which are not identical. It is submitted that rat and guinea-pig alpha 1-adrenoceptors activate different contractile processes.

Sgd 101/75 is distinguished from other selective alpha 1-adrenoceptor agonists by the inhibition of its pressor responses by calcium entry blockade and vasodilatation in pithed rats and cats.

The vasopressor effects of the selective alpha 1-adrenoceptor agonist Sgd 101/75 (2-[2-methylindazol-4-imino]-imidazolidine HCl) were analyzed in pithed rats and cats. Vasodilatation by the beta 2-adrenoceptor agonist salbutamol (1 mg/kg i.v.) or by the converting enzyme inhibitor captopril (5 mg/kg i.v.) antagonized the vasoconstriction by Sgd 101/75 in pithed rats. The effect of salbutamol was abolished by restoration of the baseline diastolic pressure by infusion of vasopressin. Calcium entry blockade by nifedipine (0.1-3 mg/kg i.v.) and (-)-verapamil (0.3 and 1 mg/kg i.v.) dose dependently inhibited the rise in the diastolic pressure induced by Sgd 101/75 pithed rats. This inhibition could not be attenuated by an infusion of vasopressin. In pithed cats, nifedipine most effectively antagonized the pressor effects of Sgd 101/75. In this respect, Sgd 101/75 is different from other alpha 1-adrenoceptor agonists, which are known to elicit a vasoconstriction which is virtually insensitive to vasodilatory measures and calcium entry blockade. These findings may be explained on the basis of a further subdivision of vascular postjunctional alpha 1-adrenoceptors.

Differential selectivities of RU 24969 and 8-OH-DPAT for the purported 5-HT1A and 5-HT1B binding sites. Correlation between 5-HT1A affinity and hypotensive activity.

RU 24969 and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) inhibited the specific binding of [3H]5-HT (2 nM) to rat brain membranes with shallow displacement curves. The displacement data were best fitted with a model of two independent, high and low affinity binding sites. Following addition of spiperone (1 microM) as a selective ligand for the putative 5-HT1A recognition site of [3H]5-HT, the displacement curve of RU 24969 underwent a leftward shift, whereas spiperone induced a shift to the right for the displacement curve of 8-OH-DPAT. In contrast to spiperone, pindolol (1 microM) shifted the displacement curve of RU 24969 to the right. These results suggest that RU 24969 possesses preference for the purported 5-HT1B subtype of central 5-HT1 recognition site. The reported significant linear correlation between hypotensive activity following intravenous (i.v.) administration to anesthetized rats and affinity for the central 5-HT1 binding site could only be maintained by incorporation of the affinity of RU 24969 for its low and 8-OH-DPAT for its high affinity binding site. Based on the proposal that the 5-HT1A site corresponds to the high affinity site of 8-OH-DPAT and the low affinity site of RU 24969, it is hypothesized that the late depressor phase of 5-HT agonists in rats is mediated by activation of peripheral (vascular) 5-HT receptors which have similarities with the 5-HT1A subtype of central 5-HT1 recognition site.

Non-peptide angiotensin II receptor antagonists. II. Pharmacology of S-8308.

2-Butyl-4-chloro-1-(2-nitrobenzyl)imidazole-5-acetic acid, sodium salt (S-8308), inhibited the specific binding of labeled angiotensin II (AII) to its receptor sites in rat adrenal cortical microsomes and in cultured aortic smooth muscle cells with IC50S of 15 and 4.5 microM, respectively. In the presence of S-8308 (15 microM) the dissociation constant for AII was increased 2-fold and the total number of binding sites was unaltered. In a concentration-dependent manner S-8308 blocked the 45Ca2+ influx induced by AII (3 X 10(-8) M) in rat aortic rings (IC50 7 microM) and the contractile response in rabbit aorta was competitively inhibited (pA2 = 5.74). This agent was highly specific for AII: it showed no affinity for alpha 1-adrenoceptors or Ca2+ channels and in addition, it did not alter the contractile responses to norepinephrine (10(-7) M) or KCl (55 mM). In conscious renal artery-ligated rats, S-8308 (30 mg/kg i.v.) elicited a rapid decrease of mean arterial pressure with a duration of about 30 min. The results demonstrate that S-8308 is a weak, but specific and competitive, non-peptide antagonist of AII exerting its inhibitory action at the receptor level.