Relative activities on and uptake by human blood platelets of 5-hydroxytryptamine and several analogues

1. The specificity of platelet receptor sites for 5-HT uptake and for the rapid morphological change and aggregation was investigated with 5-hydroxy-tryptamine (5-HT) and seventeen analogues as well as with some antagonists of 5-HT.2. The analogues, with the exception of 5-hydroxy-N'N'-dibutyltryptamine, caused the rapid morphological change in platelets. In concentrations below those needed to produce the agonistic action (viz. 0.05-2.0 muM), these analogues themselves inhibited competitively the shape change caused by 5-HT.3. The velocity of change in shape caused by 5-HT was reduced in low Na media.4. Ten analogues produced platelet aggregation; three of these, viz. 5-methoxy-alpha-methyltryptamine, 5-hydroxy-alpha-methyltryptamine and 5-hydroxy-N'N'-diisopropyltryptamine), were approximately equipotent with 5-HT. Six analogues did not induce platelet aggregation.5. All the analogues which prevented the initial change in shape of platelets caused by 5-HT also inhibited its aggregating effect, apparently competitively with low K(i) values (0.02-1.6 muM).6. As with the inhibition of shape change, the inhibition of aggregation shows relatively low structural specificity of the receptor site.7. Methysergide was a potent inhibitor of shape change and aggregation (K(i) approximately 0.03 muM); imipramine was much less inhibitory (K(i) approximately 5-10 muM).8. Only one analogue (5-hydroxy-alpha-methyltryptamine) was taken up like 5-HT by platelets. All the other analogues inhibited the uptake of 5-HT by platelets (K(i)=0.2-2.7 muM).9. Methysergide was a weak inhibitor of 5-HT uptake (K(i) approximately 125 muM) whereas imipramine was very effective (K(i) approximately 0.3 muM).10. Our results show that the initial change in shape of platelets is required for and precedes aggregation. The structural specificity of the platelet receptor concerned with shape change and aggregation caused by 5-HT appears low whereas the uptake mechanism is a highly specific one. The uptake probably proceeds through more than one step, the relationship between the steps is not yet clear.     

THE EFFECT OF A CHROMATOGRAPHICALLY PURE SAMPLE OF ADENOSINE 5'-TETRAPHOSPHATE ON SHEEP AND RABBIT BLOOD PLATELETS

1. The commercially available trisodium salt of adenosine S'-tetraphosphate (ATetraP) (Sigma) was found to be contaminated with ATP, ADP and AMP, and therefore unsuitable for use in platelet studies. 2. The more stable barium salt of ATetraP was converted to the ammonium salt and found to be chromatographically homogeneous. This sample was tested for its influence on sheep blood platelets in citrated platelet-rich plasma by the photometric method. 3. The ammonium salt of ATetraP (5–105 μmol.1^?1) induced platelet aggregation which showed no tendency towards disaggregation. 4. The log dose-response lines for ATetraP and for adenosine diphosphate were parallel. On a molar basis, the tetraphosphate had only 1.5% of the aggregating activity of ADP. 5. The initial rate of aggregation induced by the tetraphosphate was inhibited by adenosine 5′-monophosphate analogues which are selective ADP-antagonists. These compounds also dispersed aggregates produced by ATetraP. 6. Platelets made refractory to ADP were also refractory to ATetraP. 7. Like ADP, ATetraP induced the change in shape of rabbit platelets and in this respect had only 3.4% the activity of ADP. 8. It is concluded that ATetraP per se can induce platelet aggregation and platelet shape change, and appears to exert its effect at the same site on the platelet surface as does ADP.     

5—HT增强家兔ADP介导的血小板聚集反应

AIM: To study the enhanced effects of 5-hydroxytryptamine (5-HT) on ADP-induced aggregation. METHODS: Platelet aggregation was quantified by the light transmission, the cytosolic-free calcium ([Ca2+]i) was measured by digital fluorescent microscopy, and inositol 1,4,5-triphosphate (IP3) was determined by receptor binding assay. RESULTS: In rabbit platelet-rich plasma (PRP), 5-HT 0.03-3 mumol.L-1 induced a decrease in light transmission (DLT) in a concentration-dependent manner with centralization of granules, as revealed by electron microscopy. The DLT was accompanied with neither platelet aggregation nor a release reaction. In single washed platelets loaded with Fura-2, 5-HT caused a concentration-dependent elevation of [Ca2+]i, and IP3 level was also transiently increased in washed platelets at 15 s after stimulation by 5-HT. Adenosine diphosphate (ADP) also caused DLT transiently in PRP before its own aggregation without a release reaction. Pretreatment of PRP or washed platelets with 5-HT, the DLT by ADP was reduced concentration-dependently and ADP-induced aggregation and [Ca2+]i mobilization were enhanced. CONCLUSION: The enhancement of ADP-induced aggregation was attributed to the superimposition of the calcium release from the storage sites and calcium influx induced by ADP over the calcium release from the storage sites by 5-HT.     

In-vitro and Ex-vivo Inhibition of Blood Platelet Aggregation by Naftazone

Because of the considerable interest in the role of platelets and antiplatelet therapy in cardiovascular disease, including the aggregation of platelets to each other during arterial thrombosis and atherogenesis, we have studied the effect of naftazone (Etioven), an original vasculotropic drug on platelet aggregation. Rat and human platelets were prepared and incubated in-vitro with different concentrations of naftazone. We found that naftazone inhibited both platelet secretion and aggregation in platelet-rich plasma (PRP) and washed platelets after stimulation with thrombin or ADP. Rats were also treated intraperitoneally for five days with various naftazone doses (0.125-10 mg kg^?1) and ex-vivo platelet aggregation compared, at various times after the last injection, with that of control animals. Inhibition by naftazone was dose-dependent in both PRP and isolated platelets. The inhibition was transient, a maximum value (～ 50%) being obtained about 3–6 h after the last injection, with a return to near-control values after 24 h. Naftazone also facilitated platelet deaggregation after in-vitro stimulation with thrombin or ADP. In another series of experiments, rats were treated intraperitoneally for five days with 10 mg kg^?1 of aspirin, ticlopidine, dipyridamole or naftazone. Platelets were prepared and tested for aggregation 90 min after the last injection. Thrombin-induced aggregation in PRP and washed platelets was significantly reduced after in-vivo treatment with ticlopidine and naftazone. Except for dipyridamole, all the drugs inhibited ex-vivo ADP-induced aggregation in PRP. In isolated platelet preparation, only naftazone induced a significant inhibition of ADP-or thrombin-stimulated aggregation. We conclude that naftazone inhibits platelet aggregation in-vitro and ex-vivo.     

Effects of ketanserin, a selective 5-HT2 serotonergic antagonist, on the secondary recruitment of human platelets in vitro

Ketanserin, a selective 5-HT2 serotonergic receptor antagonist, reduces in vitro the release-associated human platelet aggregation induced by threshold concentrations of collagen and curtails the second wave of aggregation/release induced by critical concentrations of ADP in particular and, to a lesser extent, of 1-epinephrine. Its inhibitory effect on the second waves becomes more pronounced when the reaction is already attenuated by moderate cyclo-oxygenase inhibition with esculetin, by yohimbine or by propranolol. The first wave of aggregation induced by ADP or 1-epinephrine is not affected. Such an inhibition of secondary platelet recruitment by ketanserin in vitro may be due to an inhibition of the 5-HT2 receptor-mediated amplifying effects of platelet-released 5-HT or to a non-specific interference with the platelet membrane, reducing the release of mediators from the platelets. Reduction of the increased plasma BTG levels in patients after ketanserin may result from such release-inhibiting mechanisms.     

Irreversible aggregation of pig platelets and release of intracellular constituents induced by 5-hydroxytryptamine

Platelet aggregation and the release of intracellular constituents induced by 5-hydroxytryptamine (5-HT) and by ADP were measured at 37° in stirred pig platelet-rich plasma (PRP) anticoagulated with citrate or heparin. In citrated PRP (with calcium partly chelated) aggregation responses to 0·1–10 μM ADP were reversible, but 50 μM ADP or more caused apparently irreversible aggregation. Aggregation induced by 5-HT was always small and reversible. In heparinised PRP, ADP was around 10 times more potent than in citrated PRP, but the pattern of responses was similar. 5-HT-induced aggregation in heparinised PRP was reversible below 1 μM, but higher concentrations caused irreversible aggregation and released adenine nucleotides from the platelets. 5-HT was more effective than was ADP at releasing platelet constituents. Responses in heparinised PRP were not due to the presence of heparin, but to the absence of a chelating anticoagulant: adding calcium to citrated PRP potentiated responses to 5-HT more than those to ADP.     

Action of immobilized neuraminidase on the accumulation of 5-hydroxytryptamine by human platelets

The accumulation of 5-HT was studied in human platelets following graded desialylation with neuraminidase immobilized on macrobeads. Platelets were removed from the soluble sialylglyco-proteins of the plasma by gel filtration on Sepharose 2B; the uptake of 5-HT by gel filtered platelets over four hours was similar to that found using platelet rich plasma. Platelet aggregation induced by 10^?5M 5-HT and 10^?6M ADP was decreased following gel filtration and the use of Sepharose 2B insolubilized apyrase did not ameliorate this effect. Removal of up to 30 per cent of the platelet sialic acid, which results in changes in the glycoprotein pattern as demonstrated by two dimensional gel electrophoresis, appears to accelerate the rate of uptake of 5-HT. This is followed by a significantly decreased rate once 50% desialylation has been achieved. Native platelets and those from which 30 per cent of the sialic acid had been removed were refractory to resialylation with exogenous rat liver sialyltransferase though this enzyme preparation catalyses the resialylation of soluble asialoglycoprotein acceptor.     

Inhibition of 5-hydroxytryptamine-induced and -amplified human platelet aggregation by ketanserin (R 41 468), a selective 5-HT2-receptor antagonist

Ketanserin, a selective 5-HT2-receptor antagonist, inhibits the reversible aggregation induced by 5-hydroxytryptamine (5-HT) in human platelet-rich plasma (PRP). In this respect, the compound is equipotent to cyproheptadine and more active than methysergide (IC50: 1.66 x 10(-8) M, 1.44 x 10(-8) M and 5.62 x 10(-8) M respectively). Ketanserin is active against 5-HT-induced platelet aggregation after both in vitro and oral administration to human volunteers. At concentrations up to 500 times in excess of the IC50 for 5-HT-induced platelet reactions, ketanserin does not affect the aggregation induced by ADP, epinephrine, collagen or Thrombofax, the prostaglandin biosynthesis of thrombin-stimulated platelets, nor the active uptake of 14C-5-HT by platelets. 5-Hydroxytryptamine amplifies the human platelet aggregation induced by threshold concentrations of ADP, collagen, epinephrine, norepinephrine and induced irreversible aggregation of platelets pre-sensitized with Thrombofax. This amplification by 5-hydroxytryptamine results in a platelet response typical for the potentiated agonist; for the combination of the monoamine with collagen, the serotonergic amplification results in enhanced aggregation, release of beta-TG and PF4 and excessive formation of TXB2. Ketanserin, after both in vitro and oral administration to man reduces the amplified response to the level of the potentiated agonist. The present evidence suggests the presence of functional 5-HT2 receptors on the human platelet, different from those involved in the uptake of the monoamine.     

Inhibitory mechanism of ifenprodil tartrate on rabbit platelet aggregation

The effects of dl-erythro-4-benzyl-alpha-(4-hydroxyphenyl)-beta-methyl-l-piperidine-eth anol tartrate (ifenprodil tartrate) on rabbit platelet aggregation in vitro and ex vivo were studied. Ifenprodil tartrate inhibited platelet aggregation in vitro induced by ADP, collagen and epinephrine. It also inhibited 5-hydroxytryptamine (5-HT) uptake into platelets and 5-HT release from platelets. Since these inhibitory effects of ifenprodil tartrate on the functions of rabbit platelets were similar to the effects of imipramine, the effects of ifenprodil tartrate may be due to the stabilizing action of ifenprodil tartrate on the platelet membrane. The platelet aggregation by ADP was significantly inhibited in rabbits after oral administration of ifenprodil tartrate, the maximal plasma level of ifenprodil being reached at 20 ng/ml ex vivo, while the maximal level was only 1/40 of the minimal concentration of ifenprodil tartrate necessary to inhibit platelet aggregation in vitro. These results indicate that factors other than ifenprodil tartrate acting directly on the platelets (e.g., PGI2 which is an endogenous inhibitor of platelet aggregation) are involved in inducing the inhibitory effects of ifenprodil tartrate on platelet aggregation ex vivo. The effects of ifenprodil tartrate on both PGI2 release from the aorta and the inhibitory effects of PGI2 on platelet aggregation in vitro were investigated: PGI2 was found to intensify the inhibitory effects of ifenprodil tartrate on platelet aggregation in vitro, but there was little effect, if any, on PGI2 release. Therefore, it is considered that the ex vivo effects of ifenprodil tartrate might be due to its interaction with endogenous PGI2 in the blood.     

MN-9202对兔血小板聚集、5-HT释放、TXB_2合成及Ca~(2 )转运的影响(英文)

目的:研究新二氢吡啶类钙拮抗剂MN-9202对兔血小板激活的影响,并探讨其作用机制。方法:以Fu-ra-2 AM为荧光探针,采用时间扫描方式记录血小板内Ca~(2 )的变化;分别用HPLC/ECD和放射免疫测定法检测5-HT及TXB_2。结果:MN-9202剂量依赖地抑制ADP或凝血酶诱导的血小板聚集,抑制TXA_2的释放并且能有效阻滞激活血小板胞内Ca~(2 )水平的增加。MN-9202 1μmol·L~(-1)能抑制胶原15mg·L~(-1)诱导的5-HT释放反应,但对胶原45mg·L~(-1)诱导的反应无抑制作用。结论:MN-9202阻滞血小板Ca~(2 )内流并抑制血小板花生四烯酸代谢及激活反应。     

The potentiation of phorbol ester-induced aggregation of human platelets by the prostaglandin endoperoxide analogue, U46619

It was not possible to desensitize human blood platelets to 12-deoxyphorbol-phenylacetate (DOPP) stimulation in a manner analogous with that to platelet aggregating factor (PAF), prostaglandin-endoperoxide analogue (U46619) or adenosine diphosphate (ADP). Platelets previously desensitized to U46619, when challenged with DOPP and ADP, showed an increased aggregation and release of 5-HT. Sub-threshold aggregating doses of U46619 also caused a potentiation of the platelet response and release reaction to DOPP. The concentration of U46619 used to pretreat platelets affected the extent of potentiation of platelet stimulation induced by DOPP. The degree of potentiation was also affected by the time interval between addition of U46619 and DOPP. U46619 did not potentiate the aggregating effects of PAF, or ionophore A23187. The stimulus potentiation of DOPP by U46619 was abolished by prostacyclin (PGI2) and an antibody to U46619, but was unaffected by indomethacin and CP/CPK.     

Investigation on the antiplatelet activity of pyrrolo[3,2-c]pyridine-containing compounds

A series of 4,5,6,7-tetrahydro-1H-pyrrolo[3,2-c]pyridines (THPPs), mostly C(2)-substituted derivatives, and some 2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indoles (THPIs) were synthesized and tested in-vitro for their ability to inhibit aggregation of human platelet-rich plasma (PRP) induced by adenosine 5'-diphosphate (ADP) and adrenaline (epinephrine). 5-Benzyl THPP (3), 2-(benzylamino)methyl THPP (5f) and 2-ethyl THPI (6) moderately and dose-dependently inhibited platelet aggregation induced by adrenaline and, to a lesser extent, by ADP. These compounds inhibited the second phase of the PRP aggregation triggered by adrenaline, which largely depends upon thromboxane A(2) production and ADP release. In the adrenaline stimulated aggregation, the THPI derivative 6 was found to be nearly equipotent with aspirin, their IC50 values (concentration effecting 50% inhibition of aggregation) being 90 and 60 microM, respectively. A relation between activity and calculated octanol-water partition coefficient suggested that a log P value around 2.5 should be the optimal lipophilicity value for the activity of THPP-containing compounds.     

Inhibition of aggregation of rabbit and human platelets induced by adrenaline and 5-hydroxytryptamine by KB-R7943, a Na(+)/Ca(2+) exchange inhibitor

1. We investigated the effect of KB-R7943, a Na(+)/Ca(2+) exchange inhibitor, on the aggregation response induced by adrenaline and 5-hydroxytryptamine (5-HT), alone or in combination in human and rabbit platelets in the presence or absence of ouabain. 2. KB-R7943 inhibited aggregation induced by the combination of adrenaline and 5-HT in a concentration-dependent manner. The IC(50) values of KB-R7943 were 4.2+/-2.0 or 3.0+/-0.7 microM with washed rabbit platelets with or without ouabain pretreatment, respectively. 3. In platelet-rich human plasma, the aggregation was biphasic. The IC(50) value of KB-R7943 was 17.2+/-4.4 microM for the first phase aggregation. 4. KB-R7943 did not inhibit the first phase of aggregation induced by adrenaline alone, or the monophasic aggregation induced by 5-HT alone. 5. The aggregation of rabbit platelets depended on the presence of K(+) in the medium, and K(+)-dependent and K(+)-independent Ca(2+) influx were observed in resting platelets. Ouabain treatment increased only the K(+)-dependent but not the K(+)-independent Ca(2+) influx. 6. KB-R7943 inhibited K(+)-dependent Ca(2+) influx with or without ouabain pretreatment, but not K(+)-independent Ca(2+) influx. 7. From these results, we conclude that KB-R7943 inhibits the adrenaline plus 5-HT induced aggregation of rabbit and human platelets by inhibiting K(+)-dependent Na(+)/Ca(2+) exchange (NCKX). Our results suggest that NCKX plays an important role in platelet aggregation.     

A comparison of the effects of an extract of feverfew and parthenolide, a component of feverfew, on human platelet activity in-vitro

Extracts of the herb feverfew inhibit human blood platelet aggregation and secretion induced by a number of agents in-vitro and this may relate to the beneficial effects of feverfew in migraine. We previously identified several compounds with antisecretory activity in human blood platelets using adrenaline as the stimulant. In the present study, we have compared the inhibitory activity of one of these compounds, parthenolide, with that of crude feverfew extract. The effects of both on [14C]5-HT secretion from platelets and on platelet aggregation induced by a number of different stimulants were determined. The activating agents studied included the phorbol ester PMA, ADP, arachidonic acid, collagen, the thromboxane mimetic U46619, the calcium ionophore A23187, the diacylglycerol analogue OAG and adrenaline. The results show that there are marked similarities between the effects of feverfew extract and of parthenolide on both [14C]5-HT secretion and platelet aggregation, which is consistent with the effects of feverfew extract on platelets being brought about by parthenolide or similar compounds in the extract. Only in one case, when A23187 was used as the stimulatory agent, was there any discrepancy, which may have been due to materials in the extract other than parthenolide. Both feverfew extract and parthenolide were more effective as inhibitors of the [14C]5-HT secretion and aggregation induced by some agents and not others, and were most effective as inhibitors of the [14C]5-HT secretion (but not the aggregation) induced by PMA. This suggests that the effects of feverfew/parthenolide on the protein kinase C pathway warrants further study.     

The effects of dopexamine, a new dopamine analogue, on platelet function in stress.

1. Dopexamine is a novel analogue of dopamine which is free of alpha-adrenoceptor activity and is of therapeutic value in chronic heart failure. The effects of dopexamine on the in vitro function of platelets from 10 healthy subjects at rest, after exercise and after in vitro addition of adrenaline and noradrenaline were investigated. 2. Dopexamine in a wide range of concentrations (10(-9)M-10(-3)M) did not appear to function as an agonist on platelets either in whole blood or in PRP preparations. 3. Dopexamine caused a dose-dependent inhibition of agonist-induced platelet aggregation in both whole blood and PRP. The inhibitory effect of dopexamine was significantly greater in PRP than in whole blood, and significantly greater to adrenaline than to collagen or ADP as agonists in whole blood. 4. After exercise or after in vitro addition of adrenaline and noradrenaline at concentrations commonly seen in myocardial infarction, dopexamine produced similar levels of inhibition seen with platelets from resting subjects. 5. Dopexamine did not affect plasma catecholamine levels but caused an increase in intraplatelet noradrenaline levels. 6. This study suggests that dopexamine is unlikely adversely to affect the hyperaggregable state found in patients with cardiogenic shock after myocardial infarction.     

Synthesis and Antithrombotic Effect of Xanthone Derivatives

A series of xanthone derivatives was synthesized and tested in-vitro for their ability to inhibit aggregation of rabbit washed platelets and human platelet-rich plasma (PRP) induced by various inducers. 2-Prenyloxyxanthone showed the most potent inhibition of rabbit washed platelet aggregation induced by arachidonic acid (IC50 = 10.2 μM). Of the compounds tested in human PRP, 2-[3 (propylamino)-2-hydroxypropoxy]xanthone (4) hydrochloride salt exhibited the most potent inhibition of platelet aggregation induced by adrenaline (IC50 = 4.4 μM), whereas in evaluation of mouse antithrombotic activity, compound 4 exhibited the most potent protection of mice from thrombotic challenge. Compound 4, 2-[3-(isopropylamino)-2-hydroxypropoxylxanthone hydrochloride salt and 2,5 dihydroxyxanthone suppressed the secondary aggregation induced by adrenaline in human PRP. We conclude that the antiplatelet effects of these compounds are mainly due to an inhibitory effect on thromboxane formation.     

Gender differences and endothelium- and platelet-derived factors in the coronary circulation

1. Experiments were designed to determine whether or not interactions of platelets with coronary arteries are affected by gender or oestrogen-status. 2. Platelets and right coronary arteries were isolated from sexually mature male, female and ovariectomized pigs. Arteries were suspended in organ chambers for the measurement of isometric force. Responses of rings, with and without endothelium, were evaluated to aggregating platelets and the platelet products 5-hydroxytryptamine (5-HT) and adenosine diphosphate (ADP). 3. Release of 5-HT, thromboxane A2 (TXA2) and prostacyclin were measured from platelets. 4. Platelets caused relaxations of rings with endothelium from all pigs. However, in rings without endothelium, consistent contractions were observed only in rings from male pigs. 5. The release of 5-HT and prostacyclin was greatest from platelets of ovariectomized pigs compared with male and female pigs. Release of TXA2 was greatest from platelets of male pigs. 6. Endothelium-dependent relaxations to ADP and contractions to 5-HT were similar among the three groups. 7. These results suggest that there may be gender-specific differences in vasomotor responses to autogenous platelets but not to the platelet-derived products 5-HT and ADP. Furthermore, there are gender differences in platelets in the release of cyclo-oxygenase metabolites of arachidonic acid and 5-HT. These products could contribute to gender differences in response to injury in the coronary circulation.     

Potentiation of ADP-induced aggregation in human platelet-rich plasma by 5-hydroxytryptamine and adrenaline.

1. We have used dose-response curves to quantitate the potentiation of adenosine 5'-diphosphate (ADP)-induced aggregation and thromboxane (TXA2) generation by 5-hydroxytryptamine (5-HT) and adrenaline in human citrated platelet-rich plasma. We have also quantitated the inhibition of these responses by aspirin, ketanserin and yohimbine, singly and in pairs. 2. Ketanserin (5 microM) inhibited TXA2 production and the second wave of platelet aggregation induced by a range of concentrations of ADP alone. This indicates that endogenous 5-HT, released from the platelet dense granules, contributes significantly to responses induced by ADP. 3. When 5-HT (10 microM) was added before ADP, a lower concentration of ADP was required to cause 50% aggregation and TXA2 generation. The ratio of ADP concentrations (CR) to cause 50% aggregation in the presence and absence of 5-HT was 2.1 when only added 5-HT was considered, and 5.0 when endogenous 5-HT was also taken into account. 4. Potentiation of ADP-induced aggregation by 5-HT also occurred in the presence of aspirin, resulting in a CR of 2.3. As expected, ketanserin inhibited potentiation by 5-HT in the presence and absence of aspirin. Although aspirin caused substantial inhibition of aggregation induced by ADP and 5-HT (CR 3.4), further inhibition occurred when ketanserin was also present (CR 6.5). 5. A subthreshold concentration of adrenaline (0.25 microM) caused substantial potentiation of ADP-induced aggregation in the absence (CR 4.0) and presence (CR 2.0) of aspirin. As expected, yohimbine (9 microM) inhibited this potentiation.(ABSTRACT TRUNCATED AT 250 WORDS)     

肾上腺素诱导兔血小板聚集的实践与理论探讨

目的创建以Ａｄｒ诱导兔血小板聚集的方法，并对受体分子特性作初步探讨。方法以高Ｋ＋缓冲液等取代兔ＰＲＰ中血浆，以Ａｄｒ诱导聚集，以Ａｐｙｒ证实结果。结果兔血小板悬浮于高Ｋ＋缓冲液时Ａｄｒ能单独诱导真正的聚集。结论由此推测血小板膜上α２肾上腺素受体分子可能为由两种亚单位组成：促聚亚单位和辅助亚单位。人辅助亚单位当Ａｄｒ浓度高达聚集阈值以上时可被激活，与促聚亚单位结合为活性的二聚体，与Ａｄｒ进一步结合发生聚集作用。兔辅助亚单位则还需要高Ｋ＋方能被激活     

Effects of methysergide on platelets incubated with reserpine

1. Platelets were incubated with methysergide and related compounds (2-bromo lysergic acid (BOL), ergotamine and methyl ergotamine) together with reserpine.2. Methysergide inhibited the normal aggregation response of platelets to 5-hydroxytryptamine (5HT) but did not affect the reduction in the 5HT content caused by reserpine, or the uptake of 5HT by the platelets.3. BOL, ergotamine and methyl ergotamine behaved similarly. Methysergide had greater anti5HT potency than BOL, and methyl ergotamine had greater potency than ergotamine.4. The use of platelets as a model for synaptic preparations is discussed.5. The role of 5HT receptor sites on the platelet membrane and the significance of the results for migraine patients treated with methysergide are discussed.