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Serotonin (5-HT2) antagonists show high affinity for the α1-adrenoceptor (α1-AR) in addition to the 5-HT2 receptor. In the present study we compared the pharmacological characteristics of a new 5-HT2 antagonist sarpogrelate and its active metabolite M-1 with those of ketanserin on human recombinant α1-AR subtypes. In the binding study, sarpogrelate, M-1 and ketanserin produced concentration-dependent inhibition of 3H-prazosin binding to α1-ARs. Among the three drugs, ketanserin showed the highest affinity for α1a-, α1b- and α1d-ARs (pKi 8.0, 8.3 and 7.6, respectively). Sarpogrelate had a relatively low affinity for the three subtypes (6.3 , 6.4 and 6.3, respectively) and M-1 showed medium affinity (7.1, 7.1 and 6.1, respectively). Chinese hamster ovary (CHO) cells expressing each α1-AR subtype showed concentration-dependent inositol phosphate (IP) accumulation in response to phenylephrine. The concentration response curves were shifted to the right by three drugs, and the pKb values were close to the pKi values in the binding study. In addition to these effects, sarpogrelate and M-1, but not ketanserin produced an increase in the basal IP level of α1d-expressed CHO cells, although the increase was less than that of phenylephrine. The present results indicate that sarpogrelate and M-1 have antagonistic activity to the three α1-AR subtypes, but their affinities are significantly lower than those of ketanserin.

Electrical transmural stimulation evoked a sympathetic contraction in the isolated dog saphenous vein. This contraction consisted of three components (α1-adrenergic, α2-adrenergic and purinergic), which were separately observed under combined treatments either with yohimbine (blockade of α2-adrenoceptor) and α,β-methylene ATP (desensitization of P2X-purinoceptors), with prazosin (blockade of α1-adrenoceptor) and α,β-methylene ATP, or with prazosin and yohimbine, respectively. The α1-adrenergic and purinergic contractions immediately developed after the start of stimulation and reached a peak rapidly. In contrast, the α2-adrenergic contraction developed slowly, thus the time to peak contraction was longer than the other two components. The relationship between the peak amplitudes of contraction and stimulus frequencies were similar between α1- and α2-adrenergic components, but the purinergic contraction was smaller than the other components at all frequencies (0.1–30 Hz). Cocaine, a neuronal uptake inhibitor of noradrenaline, significantly potentiated α1- and α2-adrenergic components and prolonged their duration with a relatively greater effect on the α2-adrenergic component. In contrast, the purinergic component was not affected by cocaine. Exogenous noradrenaline produced concentration-dependent contraction, which was inhibited more effectively after combined treatment with combination of prazosin and yohimbine than either blocker given alone. Cocaine potentiated the attenuated contractile response to noradrenaline in the presence of prazosin, resulting in the recovery of response to the control level. Exogenous ATP produced a transient contraction, which was abolished under conditions where postjunctional P2X-purinoceptors were desensitized with α,β-methylene ATP. Cocaine did not affect the ATP-induced contraction. These results suggest that sympathetic contraction of the dog saphenous vein is caused through three distinct routes (α1- and α2-adrenoceptors and P2X-purinoceptors).

The dissociating and/or residual inhibitory effects of bopindolol from β-adrenoceptors (ARs) of atria strips pretreated with this drug and washed out with buffers on isoprenaline-induced chronotropic actions were determined. The effects of this drug were compared with those of its active metabolite 18-502, propranolol, and pindolol. Our results are as follows: (1) Lower concentrations of bopindolol (10–9 and 10–8 mol/l) and the active metabolite 18-502 (10–9 mol/l), propranolol (10–8 and 10–6 mol/l) and pindolol (10–8 mol/l) produced rightward shifts of concentration-response curves of isoprenaline. (2) Higher concentrations of bopindolol (10–7 mol/l) and 18-502 (10–8 and 10–7 mol/l) produced a reduced maximum response by isoprenaline. (3) Bopindolol (10–9–10–7 mol/l), 18-502 (10–9–10–7 mol/l) and propranolol (10–7 and 10–6 mol/l) did not recover to control levels at 180 min even after washout with buffers. In conclusion, bopindolol and 18-502 may slowly dissociate and act as noncompetitive β-antagonists rather than readily reversible β-AR antagonists. These effects may differ from those of propranolol and pindolol, although propranolol did not recover to control levels after washout with buffer.

This study was designed to examine the importance of interaction in the bindings of nonselective β-blockers to β1-adrenoceptors (β1-ARs) as compared with β2-ARs, using molecular modeling. The β-blockers used in this study were bopindolol [4-(benzoyloxy-3-t- butylaminopropyl)-2-methylindol hydrogen malomate], its two metabolites [18-502 – hydrolyzed bopindolol or 4-(3-t-butylamino-2-hydroxypropoxy)-2-methyl indole – and 20-785 – 4-(3-t-butylaminopropoxy)-2-carboxyl indole], and propranolol. Molecular modeling was performed on an Indigo2 workstation (Silicon Graphic) using Discover/Insight II (Molecular Simulations) software. Through molecular modeling, possible binding sites for these drugs were suggested to lie between helices 3, 4, 5, and 6 of the β1-AR. The amine, benzoic acid, indole methyl, t-butyl, phenyl, and indole functional groups of bopindolol possibly interact with Asp138 (transmembrane – TM – 3), Ser190 (TM 4), Ala343 (TM 6), Val137 (TM 3), Pro339 (TM6), Cys336 (TM 4), Leu237 (TM 5), and Pro236 (TM 5) of β1-AR, respectively, by either hydrogen bonding or hydrophobic interactions. In addition, 18-502, 20-785, and propranolol also interacted with sites at the same positions as those of β2-ARs. Thus, the results of the present study suggested that although Ala343 and Val137 of β1-AR among these amino acids were different from those of β2-AR, the interactions at the same sites between ligands and amino acids of β1-AR as those of β2-ARs may occur because these drugs are nonselective.

Yohimbine (10–8 to 10–4 M) produced a concentration-dependent contraction of the isolated dog basilar artery. The maximal amplitude of the contraction induced by yohimbine was approximately 50% of that induced by 5-hydroxytryptamine (5-HT) and twice as large as that of noradrenaline. This response to yohimbine (3 × 10–7 M) was attenuated by 5-HT receptor antagonists such as 5-methoxygramine, methysergide and ketanserin. Phentolamine also antagonized the yohimbine response but prazosin and DG-5128 did not. The 5-HT antagonists and phentolamine produced similar inhibition of the response to 5-HT. Propranolol, atropine and diphenhydramine did not modify the contractile responses to yohimbine and 5-HT. These results suggest that the contractile response to yohimbine is mediated through 5-HT receptors. In addition to the contractile action, yohimbine at concentrations over 10–6 M inhibited the contractile response to 5-HT. Thus, the possibility that yohimbine is a partial agonist on 5-HT receptors in the dog basilar artery has to be considered.

The effects of cinepazide, 1-[(1-pyrrolidinylcarbonyl)methyl]-4-(3,4,5-trimethoxycinnamoyl )piperazine hydrogen maleate, were studied in isolated dog cerebral arteries. Cinepazide in concentrations ranging from 10(-6) to 10(-5) M augmented the relaxing responses to ATP, adenosine and cAMP. However, this agent did not affect the relaxations induced by isoproterenol and papaverine and the contractions induced by 5-HT, prostaglandin F2 alpha and ATP. In the basilar artery preloaded with 3H-norepinephrine or 3H-adenosine, electrical transmural stimulation resulted in a marked increase in 3H-efflux. This efflux accompanied an initial transient contraction followed by a relaxation. Cinepazide slightly reduced the 3H-efflux evoked by electrical stimulation. However, the relaxing response was mostly augmented by the treatment with cinepazide. The relaxing responses to ATP, adenosine, cAMP and electrical transmural stimulation were attenuated by theophylline. These results suggest that cinepazide selectively potentiates the relaxing response mediated through purinergic P1-receptors.

Post- and presynaptic α-adrenoceptors in the canine mesenteric artery show distinct features of α1 and α2 types, respectively. The effects of a newly synthesized pressor agent, 2-[(5-chloro-2-methoxyphenyl)azo]-1H-imidazole (M6434), on the pre- and postsynaptic α-adrenoceptors of the canine mesenteric artery were examined pharmacologically. M6434 produced a contractile response in the mesenteric artery. The EC5o value of M6434 was 1.1 × 10–7M, that was almost the same as that of norepinephrine but significantly lower than the value seen with methoxamine. The maximum amplitude of contraction developed by M6434 was approximately 81 % of that induced by norepinephrine. The contractile response to M6434 was competitively inhibited by prazosin (pA2 = 9.23 ± 0.08) and yohimbine (pA2 = 7.85 ± 0.09). M6434 also reduced the 3H-efflux evoked by electrical transmural stimulation in the arterial tissues preincubated with 3H-norepinephrine, and the maximum inhibition corresponded to about 60 % of the case of norepinephrine. This presynaptic effect was inhibited by yohimbine but not by prazosin. The results show that M6434 is almost a full agonist for α1-adrenoceptor with partial α2-effect.

ATP (10(-8)-10(-5) M) produced phasic contraction in the canine basilar artery and relaxation in the middle cerebral artery. At high concentrations (10(-5) and 10(-4) M) a contractile response in the middle cerebral artery occasionally preceded relaxation. ADP, AMP, c-AMP and adenosine (10(-8)-10(-4) M) caused relaxation in all preparations except that ADP produced contraction in both cerebral arteries at high concentrations (10(-5) and 10(-4) M). These effects were observed in the absence or presence of active tension induced by 5-HT. The potency of ADP in producing vasodilation in the middle cerebral artery was greater than in the basilar artery, whereas that of the other agents was reversed. c-AMP showed the lowest potency among the agents tested on both arteries. The relaxing responses, but not contractile responses, to agents used were partially inhibited by theophylline. Phentolamine, propranolol, atropine, tetrodotoxin, ouabain or quinidine had little effect on both the contractile and relaxing responses. From these results, it is concluded that the canine cerebral arteries respond well to adenine nucleotides and adenosine and that the relaxing response is in part caused through theophylline-sensitive receptors.

Tetraethylammonium chloride (TEA) at concentrations over 10(-3) mol/l produced a concentration-dependent contraction in the isolated canine coronary artery. We investigated mechanisms of the contractile response and the effects of various vasodilators on the contraction. While phentolamine, propranolol, guanethidine, atropine and tetrodotoxin did not affect the TEA response, the response was attenuated by a reduction in the extracellular potassium concentration and was augmented by an increase in the concentration of potassium. The response was also augmented by other potassium conductance blockers (CsCl and 4-AP). On the other hand, the contractile response to phenylephrine used as a control drug was not affected by the extracellular potassium concentration or by CsCl. Verapamil or removal of extracellular calcium abolished the TEA, but not the phenylephrine response. Nicorandil, isoproterenol, adenosine or glyceroltrinitrate produced an equipotent relaxation in the coronary arteries contracted by TEA and by phenylephrine, whereas acetylcholine relaxed to a lesser extent the arteries contracted by TEA than those contracted by phenylephrine. These results suggest that the TEA-induced contractions are strictly dependent on the reduction of potassium conductance and extracellular calcium while the phenylephrine-induced contractions are not and that the relaxing effects of tested vasodilators, except for acetylcholine, may not be affected by such different contractile mechanisms.