Specific but noncompetitive inhibition by 2-alkylthio analogues of adenosine 5'-monophosphate and adenosine 5'-triphosphate of human platelet aggregation induced by adenosine 5'-diphosphate.

Some 2-alkylthio derivatives of adenosine 5'-monophosphate (AMP), adenosine 5'-monophosphorothioate (AMPS) and adenosine 5'-triphosphate (ATP) were examined as inhibitors of human platelet aggregation. 2-Methylthio-AMP, 2-ethylthio-AMP, 2-(pentan-l-yl)thio-AMP, 2-ethylthio-AMPS, 2-methylthio-ATP and 2-ethylthio-ATP (100 microM) each inhibited aggregation induced by adenosine 5'-diphosphate (ADP) but not by 11 alpha, 9 alpha-epoxymethano prostaglandin H2, a stable endoperoxide analogue. Log dose-response curves to ADP in the absence and presence of each inhibitor were not parallel and the inhibition could not be overcome by high concentrations of ADP. The ATP analogues achieved greater inhibition of aggregation induced by ADP (5 microM) than did the AMP analogues. The order of potency of the AMP analogues was 2-ethylthio-AMPS greater than 2-ethylthio-AMP greater than 2-(pentan-l-yl)thio-AMP greater than 2-methylthio-AMP, and 2-methylthio-ATP was more potent than 2-ethylthio-ATP. These 2-alkylthio substituted analogues of AMP and ATP are specific but noncompetitive inhibitors of ADP-induced human platelet aggregation.     

Competitive inhibition by adenosine 5'-triphosphate of the actions on human platelets of 2-chloroadenosine 5'-diphosphate, 2-azidoadenosine 5'-diphosphate and 2-methylthioadenosine 5'-diphosphate.

1 Adenosine 5'-diphosphate (ADP) induces human platelet aggregation and noncompetitively inhibits stimulated human platelet adenylate cyclase; these two effects are mediated by the same ADP receptor, at which adenosine 5'-triphosphate (ATP) is a competitive antagonist. 2 Two ADP analogues, 2-azidoadenosine 5'-diphosphate (2-azido-ADP) and 2-methylthioadenosine 5'-diphosphate (2-methylthio-ADP) have been reported to be more potent as inhibitors of adenylate cyclase than they are as aggregating agents, but no evidence has been presented that these actions are mediated solely by the ADP receptor. 3 We therefore tested the ability of ATP to inhibit the actions of these compounds and of another ADP analogue, 2-chloroadenosine 5'-diphosphate (2-chloro-ADP). 4 2-Chloro-ADP, 2-azido-ADP and 2-methylthio-ADP each induced aggregation and inhibited stimulated adenylate cyclase. Both of these actions were competitively inhibited by ATP (50 microM) with pA2 values similar to those previously found for inhibition by ATP of these effects of ADP. 5 The reported greater potency of 2-azido-ADP and of 2-methylthio-ADP as inhibitors of adenylate cyclase than as aggregating agents is therefore due only to their greater efficacy for this effect, not to some extra actions elsewhere.     

P2-purinoceptor antagonists: III. Blockade of P2-purinoceptor subtypes and ecto-nucleotidases by compounds related to suramin

Effects of suramin and five analogs or fragments of suramin were studied on contractions of the rat vas deferens elicited by ,-methylene ATP (,-McATP; mediated by P_2X-purinoceptors), relaxations of the carbachol-precontracted guinea-pig taenia coli elicited by adenosine 5-O-(2-thiodiphosphate) (ADPS; mediated by P_2Y-purinoceptors), and the degradation of ATP by rat vas deferens tissue. One compound, NF023, differed from suramin by removal of two p-methylbenzamido groups, whereas another, BSt101, differed from NF023 by additional removal of the three sulphonate residues from one of the terminal naphthalene rings.The compounds all shifted the concentration-response curve of ,-MeATP in the rat vas deferens to the right and simultaneously increased the maximum of the curve. Where three concentrations were tested, the Arunlakshana-Schild regression was linear, and the slope did not differ from 1. The apparent K _d values were between 1 and 3672 M. In the guinea-pig taenia coli, the compounds shifted the concentration-response curve of ADPS to the right in a parallel manner, but in the one case where three concentrations were tested, the slope of the Arunlakshana-Schild regression was lower than 1. Apparent K _d values were between 10 and 786 M. The removal of ATP from the medium by vas deferens tissue was decreased only by suramin, NF023 and BSt101, with IC_25% values between 170 and 590 M.The results indicate that P_2X-purinoceptor affinity, P_2Y-purinoceptor affinity and the ecto-nucleotidase effect all increase with the size of the molecule. BSt101 resembled NF023 in potency at all three sites, indicating that the possession of a second naphthalene-trisulphonate group is not a prerequisite for relatively high affinity. NF023 is interesting because it is P_2X- versus P_2Y-selective and, in addition, the compound with the highest P_2X- versus ecto-nucleotidase-selectivity presently available.     

Effects of adenosine derivatives on human and rabbit platelet aggregation

Summary The inhibitory effects of several adenosine analogues, including the new A2-selective agonists 2-[p-(2-carboxyethyl)phenethylamino]-5-N-ethylcarboxamido-adenosine (CGS 21680) and 2-hexynyl-5-N-ethylcarbox-amidoadenosine (2-hexynyl-NECA), were investigated in vitro on human and rabbit platelet aggregation. The compounds examined inhibited ADP-induced platelet aggregation over a wide range of potency. The rank order of activity was similar between the two species thus showing that the rabbit is a useful animal model for studying the effects of adenosine derivatives on platelet aggregation. 2-Hexynyl-NECA was found to be the most potent adenosine compound of those currently available, having IC₅₀ values of 0.10 and 0.07 M in human and rabbit platelets, respectively. Conversely, the A1 agonists R(–)-N-⁶-(2-phenylisopropyl) adenosine (R-PIA), S(+)-N⁶-(2phenylisopropyl) adenosine (S-PIA) and 2-chloro-N⁶-cyclopentyl-adenosine (CCPA) were the least potent compounds with IC₅₀ values in the micromolar range. The potency of the compounds in inhibiting platelet aggregation was found to be highly correlated with their affinity for A2 receptors as measured using 3H-CGS 21680 binding in rat brain striatum.Correspondence to S. Dionisotti at the above address     

Effects of suramin on contractions of the guinea-pig vas deferens induced by analogues of adenosine 5'-triphosphate.

1. Adenosine 5''-triphosphate (ATP) and some of its analogues contract the guinea-pig vas deferens, acting via receptors which have been classified as P2X-purinoceptors. We have recently shown, however, that the effects of ATP are enhanced, rather than inhibited, by the non-selective P2 antagonist, suramin, and that this enhancement could not easily be explained in terms of inhibition by suramin of the breakdown of ATP. We therefore investigated the effects of suramin on contractions induced by ATP analogues, to define the structure-activity relationships of the suramin-resistant response. 2. In the absence of suramin, the order of potency for ATP analogues was adenosine 5''-(alpha,beta-methylene)triphosphonate (AMPCPP) = P1,P5-diadenosine pentaphosphate (Ap5A) = adenosine 5''-tetraphosphate (Ap4) > adenosine 5''-O-(3-thiotriphosphate) (ATP gamma S) = adenylyl 5''-(beta,gamma-methylene) diphosphonate (AMPPCP) > P1,P5-diadenosine tetraphosphate (Ap4A) > adenosine 5''-O-(2- thiodiphosphate) (ADP beta S) > 2-methylthioadenosine 5''-triphosphate (MeSATP) > or = ATP > adenosine 5''-diphosphate (ADP). This is generally in agreement with previously reported structure-activity relationships in this tissue. 3. In the presence of suramin (1 mM), responses to Ap5A, Ap4A, AMPPCP, ADP beta S and ADP were abolished or greatly reduced, and contractions induced by AMPCPP, Ap4 and ATP gamma S were inhibited. Contractions induced by MeSATP however, like those induced by ATP itself, were not reduced, but at concentrations above 100 microM were enhanced.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential inhibition by adenosine or by prostaglandin E1 of human platelet aggregation induced by adenosine 5'-O-(1-thiodiphosphate) and adenosine 5'-O-(2-thiodiphosphate).

Adenosine 5'-diphosphate (ADP) induces human platelet aggregation and inhibits stimulated adenylate cyclase. Adenosine 5'-O-(1-thiodiphosphate) (ADP-alpha-S) and adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-S) act at the ADP receptor and achieve the same maximal rate of human platelet aggregation as each other. Adenosine and prostaglandin E1, which noncompetitively inhibit ADP-induced aggregation by stimulating adenylate cyclase, inhibit aggregation induced by ADP-x-S more than aggregation induced by ADP-beta-S. ADP-x-S, unlike ADP-beta-S and ADP itself, does not inhibit stimulated adenylate cyclase. This suggests that the inhibition of stimulated adenylate cyclase by ADP, although not a cause of aggregation, may be of physiological importance in reducing the effects of endogenous agents such as adenosine and prostaglandins (for example, prostacyclin) to which the platelet may be exposed.     

The P(2)-purinoceptor antagonist suramin is a competitive antagonist at vasoactive intestinal peptide receptors in the rat gastric fundus

The P(2)-purinoceptor antagonist, suramin, was used to investigate the possible involvement of adenosine 5'-triphosphate (ATP) in the inhibitory non-adrenergic non-cholinergic (NANC) innervation of the rat gastric fundus. ATP (1-30 microM) produced biphasic responses consisting of concentration-dependent relaxations followed by concentration-dependent contractions. Suramin (200 microM) significantly reduced relaxations and abolished contractions to ATP. Under NANC conditions, electrical field stimulation (EFS) induced frequency-dependent relaxations. Suramin (200 microM) and the peptidase alpha-chymotrypsin (1 u ml(-1)) had the same effects on EFS-induced relaxations: their duration was reduced, but their magnitude was unaffected. Cumulative relaxations to vasoactive intestinal peptide (VIP; 0.1-100 nM), and to the VIP analogue pituitary adenylate cyclase activating peptide 1-27 (PACAP; 0.2-100 nM), were almost completely abolished by alpha-chymotrypsin (1 u ml(-1)), and were inhibited by suramin (3-200 microM) in an apparently competitive manner. Schild plot analysis indicated that suramin had pA(2) values of 5.1+/-0.2 (Hill slope=0.9+/-0.2) and 5.6+/-0.1 (Hill slope=1.0+/-0.1), against VIP and PACAP, respectively. Concentration-dependent relaxations to nitric oxide (1-30 microM) and cumulative relaxations to isoprenaline (0.1-300 nM) were not affected by suramin (200 microM). No conclusions can be made regarding the possible involvement of ATP in EFS-induced NANC relaxations. The results suggest that suramin acts as a competitive antagonist at VIP receptors in the rat gastric fundus.     

Effects of adenosine analogs and adenine nucleotides on adenosine 5'-diphosphate-induced rat platelet aggregation

Various adenosine analogs and adenine nucleotides have been tested as inhibitors of ADP-induced aggregation of rat platelets. The potent inhibitors of human platelet aggregation, adenosine, 2-fluoroadenosine, 2-chloroadenosine, carbocyclic adenosine and N⁶-phenyl adenosine, had little effect on rat platelet aggregation (0–30 per cent inhibition). The effects of adenosine or its analogs on ADP-induced aggregation of cross-species platelet-rich plasmas (PRPs) (human platelets suspended in rat plasma or rat platelets in human plasma) were similar to those with the native PRPs, indicating that these species differences were due to intrinsic factors in the platelets and not in the plasma. When these analogs were tested in the presence of the cyclic AMP phosphodiesterase inhibitor papaverine, strong inhibiton of rat platelet ADP-induced aggregation was seen. 2′-Deoxyadenosine and 3′-deoxyadenosine were not inhibitory to ADP-induced aggregation of rat PRP even in the presence of papaverine. Adenosine 5′-tetraphosphate strongly inhibited both human and rat platelet aggregation. AMP, like adenosine, did not inhibit rat platelet aggregation but became strongly inhibitory in the presence of papaverine. This inhibitory effect was abolished by preincubating rat PRP with an adenylate cyclase inhibitor, 2′, 5′-dideoxyadenosine or adenosine deaminase. In the later case, however, if the adenosine deaminase inhibitor 2′-deoxycoformycin was included in the incubation mixture, the inhibition by AMP plus papaverine was similar to adenosine plus papaverine. About 50 per cent of [¹⁴C]AMP was converted to [¹⁴ C]adenosine in rat platelet-free plasma or PRP after a 10-min incubation. α,β-Methylene-ADP and β,γ-methylene-ATP (200 μM) inhibited rat platelet aggregation by 50 and 64 per cent, respectively. Cyclic AMP phosphodiesterase of rat and human platelets gave comparable K_m, and V_max values (K_m 0.53 and 0.21μM and V_max 6.0 and 6.7 pmoles/min/10⁷ platelets, respectively).     

体外细胞色素P450同工酶对银杏叶水提取物抗血小板聚集作用的影响

目的应用体外肝细胞模型研究中草药银杏叶提取物的代谢途径,即细胞色素P450酶(CYP450)药物代谢酶系对银杏叶拮抗血小板聚集效应的影响。方法制备人超低温冷冻肝细胞,通过与银杏叶提取物预孵育,评估肝脏CYP450药物代谢酶系(CYP1A2、CYP286、CYP2C19、CYP2E1、CYP3A4)对银杏叶水提取物拮抗血小板活化因子(PAF)激活的血小板聚集作用。富含血小板血清(PRP)和少含血小板血清(PPP)与不同质量浓度(25、50、100、200和1000μg·L~(-1))PAF孵育,建立PAF激活血小板聚集的体外模型。银杏叶水提物与人超低温冷冻复苏肝细胞预孵育后,与PRP和PAF培养,观察银杏叶水提物的体外效应(抗PFA血小板聚集激活作用)是否受人肝细胞代谢的影响。CYP450药物代谢酶的特定抑制物伊曲康唑(CYP3A4)、α-萘黄酮(CYP1A2)、奥芬那君(CYP286)、奥美拉唑(CYP2C19)、4-甲基吡唑(CYP2E1)与人肝细胞预孵育后、与银杏叶水提物和PRP和PAF孵育,评估银杏叶水提物的体外效应与何种CYP450药物代谢同工酶有关。结果PAF激活血小板聚集作用遵循米-曼氏动力学方程,其K_m为98μ·L~(-1)。银杏叶水提物抑制PAF的血小板聚集激活作用,其半数抑制剂量为33μg·L~(-1)。人肝细胞与银杏叶水提物预孵育后,其体外效应(抗PAF血小板聚集激活作用)增强30%,差异有统计学意义(P<0.05)。人肝细胞与细胞色素CYP450同工酶CYP286、CYP2C19、CYP2E1、CYP3A4抑制剂预孵育后,银杏叶水提物的体外效应不受影响。人肝细胞与CYP1A2抑制剂预孵育后,人肝细胞对银杏叶水提物体外效应的增强作用基本消失,差异有统计学意义(P<0.05)。结论银杏叶水提物在体外能抑制PAF的血小板聚集作用,人肝细胞能显著增强这一体外效应,CYP450药物代谢酶CYP1A2可能参与银杏叶的这一体外效应的代谢活化。     

Effects of nebivolol on human platelet aggregation.

It has been documented that beta-adrenergic antagonists can influence platelet aggregation by a mechanism independent of their ability to antagonize beta-adrenoceptors. Nebivolol, a selective beta1-adrenergic receptor antagonist with additional hemodynamic effects, is able to vasodilate human forearm vasculature by acting on the L-arginine/nitric oxide pathway. Constitutive nitric oxide synthase is present also in human platelets, resulting in the formation of nitric oxide, an endogenous inhibitor of platelet aggregation. The aim of this study was to investigate the effects of nebivolol on platelet aggregation and in particular to determine the involvement of the platelet L-arginine/nitric oxide pathway. Propranolol, a nonselective beta-adrenergic antagonist, and carvedilol, a beta-blocker with vasodilating properties, were compared with nebivolol on platelet activity. Plasma from healthy male subjects was used. Platelet aggregation was achieved with adenosine diphosphate (ADP) (3 microM) and collagen (1 microg/ml), using the Born turbidimetric method to measure platelet aggregation. Our results showed that nebivolol, propranolol, and carvedilol all had an inhibitory effect on both ADP- and collagen-induced platelet aggregation. Nebivolol exhibited the greatest inhibition effect on platelet aggregation. The mechanism responsible for the inhibitory effect of nebivolol appeared to involve a nitric oxide-dependent pathway. Indeed, L-arginine augmented the inhibitory effects of nebivolol on platelet aggregation induced by collagen and ADP. Furthermore, the inhibitory effect of nebivolol on platelet aggregation was reduced in the presence of the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA). In conclusion, we have demonstrated in this study that nebivolol's mechanism of platelet aggregation inhibition differs from that of other beta-adrenergic antagonists by being partially dependent on nitric oxide production.     

Suramin: a reversible P2-purinoceptor antagonist in the mouse vas deferens.

The trypanocide Suramin was tested as a possible antagonist at the P2-purinoceptor of the mouse vas deferens. At a concentration of 100 microM, Suramin antagonized the response to alpha,beta-methylene ATP, while responses to carbachol and noradrenaline were unaffected. These results suggest that Suramin may provide a starting point for the development of specific antagonists for P2-purinoceptors.     

Studies on the stereoselectivity of the P2-purinoceptor.

ATP, 2-chloro-ATP, 2-methylthio-ATP, and their unnatural L-enantiomers, were synthesized and their effects tested on the guinea-pig taenia coli and urinary bladder, and the stimulated frog ventricle. The potent P2-purinoceptor agonists, 2-chloro-ATP and 2-methylthio-ATP were, respectively, 30 and 200 times more effective than ATP in relaxing the guinea-pig taenia, but approximately as effective as ATP in contracting the guinea-pig bladder and augmenting the force of contraction of the frog ventricle. A high degree of stereoselectivity was observed for relaxations of the guinea-pig taenia coli produced by the P2-purinoceptoragonists, and 2-methylthio-ATP was over 700 times more effective than its L-enantiomer. In contrast, stereoselectivity for contraction of the guinea-pig bladder was observed only at low concentrations with each pair of enantiomers, and a similar low stereoselectivity was displayed by the frog ventricle. These results show that P2-purinoceptors mediating inhibitory responses in the guinea-pig taenia coli can show a high degree of stereoselectivity, while P2-purinoceptors mediating excitatory responses in the guinea-pig bladder and in the frog ventricle show little stereoselectivity. The partial stereoselectivity of the P2-purinoceptor in smooth muscle contrasts with the absolute stereospecificity of P1-purinoceptors for adenosine on smooth muscle and autonomic nerve terminals and the absolute stereospecificity of the receptor for ADP on the human platelet.     

P2-purinoceptor antagonists: I. Blockade of P2-purinoceptor subtypes and ecto-nucleotidases by small aromatic isothiocyanato-sulphonates

Effects of eight small aromatic isothiocyanatosulphonates, of the aliphatic 2-isothiocyanatoethene-1-sulphonate (IES), and of the parent amines were studied on contractions of the rat vas deferens elicited by ,-methy-lene ATP (,-MeATP; mediated by P_2X-purinoceptors), relaxations of the carbachol-precontracted guinea-pig taenia coli elicited by adenosine 5-O-(2-thiodiphosphate) (ADPS; mediated by P_2Y-purinoceptors), and the degradation of ATP by rat vas deferens tissue.The aromatic isothiocyanato-sulphonates all reduced contractions of the rat vas deferens elicited by ,-methy-lene ATP. The antagonism was non-competitive, with depression of the maximum of the concentration-response curve of ,-MeATP and incomplete reversibility. The IC₅₀ values were between 11 and 54 M. In the guineapig taenia coli, the aromatic compounds shifted the concentration-response curve of ADPS to the right in a surmountable manner (one exception), and where three concentrations were tested, the Arunlakshana-Schild regression was linear and its slope did not differ from 1. The apparent K _d values were between 10 and 214 M. The removal of ATP from the medium by vas deferens tissue was decreased by the aromatic isothiocyanates with IC_25% values between 25 and 464 M. IES and the parent amines were inactive or almost inactive (parent amines not tested on ATP breakdown).The results indicate that the isothiocyanato residue as well as the aromatic core are essential for P₂-purinoceptor blockade. At the P_2X-purinoceptor, potency increases with the size of the molecules but is independent of the position of the isothiocyanato and sulphonate substituents. No simple structure-activity relationship for the P_2Y-purinoceptor and the ATP-degrading ecto-nucleotidases can be derived beyond the apparent lack of a major influence of the position of the substituents. 2-Isothiocyanatonaphthalene1-sulphonate (-INS) seems to be interesting because of relatively high P_2X-selectivity versus both the P_2Y-purinoceptor and ecto-nucleotidases.     

Effects of phosphate-modified analogs of adenosine 5′-diphosphate and adenosine 5′-triphosphate at P2T-purinoceptors mediating human platelet activation by ADP

Adenosine 5′-diphosphate (ADP) induces human platelet aggregation and inhibits stimulated adenylate cyclase by an action at P_2T-purinoceptors. Both of these effects of ADP are inhibited competitively by ATP. Structure-activity relationships for phosphate-modified analogs of ADP and adenosine 5′-triphosphate (ATP) were studied by testing their effects on human platelet activation. Of the ADP analogs, only β,γ-imido-ADP (AMPNHP) induced platelet aggregation, but was a weak partial agonist (pA₅₀ 4.53). ADP-induced platelet aggregation was antagonized noncompetitively by β,γ-methylene-ADP (AMPCP) (pA₂ 3.2), β,γ-ethylene-ADP (AMPCCP) (pA₂ 4.42), and β,γ-difluoromethylene-ADP (AMPCF₂P) (pA₂ 4.77), and competitively by β,γ-dichloromethylene-ADP (AMPCCl₂P) (pA₂ 4.68). None of the ADP analogs inhibited prostaglandin E₁ (PGE₁)-stimulated adenylate cyclase, and ADP-induced inhibition of PGE₁-stimulated adenylate cyclase was unaffected by AMPNHP, AMPCP, or AMPCCP (100 μM), but was antagonized by AMPCF₂P (pA₂ 4.36) and AMPCCl₂P (4.24). ADP-induced platelet aggregation was antagonized competitively by the ATP analogs β,γ-difluoromethylene-ATP (AMP-PCF₂P) (pA₂ 4.55), β,γ-dichloromethylene-ATP (AMP-PCCl₂P) (pA₂ 4.42), and β,γ-imido-ATP (AMP-PNHP) (pA₂ 4.32) and non-competitively by 2-methylthio-β,γ-methylene-ATP (2-MeS-AMP-PCP), 2-methylthio-β,γ-difluoromethylene-ATP (2-MeS-AMP-PCF₂P), and 2-methylthio-β,γ-dichloromethylene-ATP (2-MeS-AMP-PCCl₂P). In summary, agonist activity at the human platelet P_2T-purinoceptor was extremely sensitive to alterations to the diphosphate chain of ADP, and only AMPNHP induced platelet aggregation. Increasing the electronegativity of the methylene group by halogen substitution increased the antagonist potency of the ADP analog AMPCP but resulted in little or no change in the antagonist potencies of the ATP analogs AMP-PCP and 2-MeS-AMP-PCP. © 1996 Wiley-Liss, Inc.     

API_（0134）对四种诱聚剂致血小板聚集的影响

采用比浊法观察到ＡＰＩ０１３４显著抑制二磷酸腺苷（ＡＤＰ）、肾上腺素（Ａｄｒ）、花生四烯酸（ＡＡ）和胶原（Ｃｏｌｌ）诱导的人和大鼠血小板聚集。ＡＰＩ０１３４体外给药（２８．８～９１０ｍｇ·Ｌ－１）呈剂量依赖性的明显抑制ＡＤＰ、Ａｄｒ和ＡＡ诱导的人血小板聚集，其中对ＡＤＰ诱导的血小板聚集抑制作用最强，１ｍｉｎ和５ｍｉｎ时的半抑制浓度（ＩＣ５０）分别为１３４ｍｇ·Ｌ－１和７６．６ｍｇ·Ｌ－１，对Ａｄｒ诱导的血小板聚集之ＩＣ５０分别为１９４和１３７．６ｍｇ·Ｌ－１，对ＡＡ诱导的血小板聚集抑制作用较弱，５ｍｉｎ时的ＩＣ５０为２０３ｍｇ·Ｌ－１，ｉｖＡＰＩ０１３４７０和１００ｍｇ·ｋｇ－１，对ＡＤＰ和Ｃｏｌｌ诱导的大鼠血小板聚集也呈显著抑制作用，抑制率分别为２７．８％～３９．５％和２４．３％～３５．０％。     

Effects of combination of 5-hydroxytryptamine receptor antagonists on 5-HT-induced human platelet aggregation

We have examined the effects of fourteen 5-HT-receptor antagonists on 5-HT-induced platelet aggregation in whole blood. Two different types of inhibitory profile were obtained. The inhibitory effects of seven of the antagonists (designated type 1) could be surmounted by increasing the concentration of 5-HT; the inhibitory effects of the other antagonists (type 2) were insurmountable by 5-HT. The effects of combinations of pairs of different antagonists were investigated. The inhibitory effects of pairs of type 1 antagonists and of pairs of type 2 antagonists were additive. However, a type 1 antagonist interfered with the inhibitory effects of a type 2 antagonist. The two types of antagonist differed in the rate at which they inhibited 5-HT-induced aggregation, a type 2 antagonist exerting its effect more slowly than a type 1 antagonist. Two possible explanations of these results are considered. It is possible that there are two different types of receptors on the surface of platelets, one causing stimulation and the other causing allosteric inhibition of platelet aggregation. Alternatively, the results may stem from different rates of association and dissociation of the agents at a single 5-HT receptor.     

The inhibitory action of suramin on the P2-purinoceptor response in smooth muscle cells of guinea-pig taenia caeci

The effect of suramin on the smooth muscle cell response of guinea-pig taenia caeci to P2-purinoceptor and alpha 1-adrenoceptor stimulation was measured. The ATP-induced relaxation in potassium (20 mM) pre-contracted taenia caeci was inhibited by suramin (3 x 10(-4) M). The P2-purinoceptor-induced hyperpolarization elicited by ATP both in the presence and absence of calcium was also reduced by suramin. The alpha 1-adrenoceptor-mediated relaxation evoked by phenylephrine was only affected by suramin at low concentrations of the agonist. The results indicate that suramin inhibits the ATP response by interacting with P2y-purinoceptors.