The involvement of 5-HT2-receptor sites in the activation of cat platelets

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Effects of imidazoline and nonimidazoline α-adrenoceptor agonists and antagonists, including xylazine, medetomidine, dexmedetomidine, yohimbine, and atipamezole, on aggregation of feline platelets

OBJECTIVE

To examine the effects of imidazoline and nonimidazoline α-adrenergic agents on aggregation of feline platelets.

SAMPLE

Blood samples from 12 healthy adult cats.

PROCEDURES

In 7 experiments, the effects of 23 imidazoline and nonimidazoline α-adrenoceptor agonists or antagonists on aggregation and antiaggregation of feline platelets were determined via a turbidimetric method. Collagen and ADP were used to initiate aggregation.

RESULTS

Platelet aggregation was not induced by α-adrenoceptor agonists alone. Adrenaline and noradrenaline induced a dose-dependent potentiation of ADP- or collagen-induced aggregation. Oxymetazoline and xylometazoline also induced a small potentiation of ADP-stimulated aggregation, but other α-adrenoceptor agonists did not induce potentiation. The α₂-adrenoceptor antagonists and certain imidazoline α-adrenergic agents including phentolamine, yohimbine, atipamezole, clonidine, medetomidine, and dexmedetomidine inhibited adrenaline-potentiated aggregation induced by ADP or collagen in a dose-dependent manner. The imidazoline compound antazoline inhibited adrenaline-potentiated aggregation in a dose-dependent manner. Conversely, α₁-adrenoceptor antagonists and nonimidazoline α-adrenergic agents including xylazine and prazosin were ineffective or less effective for inhibiting adrenaline-potentiated aggregation. Moxonidine also was ineffective for inhibiting adrenaline-potentiated aggregation induced by collagen. Medetomidine and xylazine did not reverse the inhibitory effect of atipamezole and yohimbine on adrenaline-potentiated aggregation.

CONCLUSIONS AND CLINICAL RELEVANCE

Adrenaline-potentiated aggregation of feline platelets may be mediated by α₂-adrenoceptors, whereas imidazoline agents may inhibit in vitro platelet aggregation via imidazoline receptors. Imidazoline α-adrenergic agents may have clinical use for conditions in which there is platelet reactivity to adrenaline. Xylazine, medetomidine, and dexmedetomidine may be used clinically in cats with minimal concerns for adverse effects on platelet function.

Studies on the localization of 5-hydroxytryptamine and its receptors in human placenta

The localization of 5-hydroxytryptamine (5-HT) and its receptors in human placenta was studied under light and electron microscopy using immunohistochemistry and in situ hybridization. The syncytiotrophoblast, cytotrophoblast, stromal cells and decidual cells in human placentae all appeared to be 5-HT immunoreactive. The 5-HT immunoreactive material was distributed in cytoplasm with negative nuclei. The 5-HT immunoreactive material was also found in capillary endothelium. Trophoblast cells cultured in serum-free medium also showed 5-HT immunoreactivity in the cytoplasm. Fetal white blood cells and both syncytiotrophoblast and cytotrophoblast in human placenta showed 5-HT receptor immunoreactivity and 5-HT1A receptor mRNA hybridized signal was also detected in cytoplasm. The stromal cells and capillary endothelium in placental villi and maternal decidual cells all showed 5-HT receptor immunoreactivity in cytoplasm. The small flattened vesicles and large dense cored vesicles within trophoblast cells showed electron-dense 5-HT receptor immunoreactivity using immunoelectron microscopy. These results suggest that human placenta may not only produce 5-HT but also be a 5-HT target organ, and that 5-HT may not only play roles in placental development and pregnancy maintenance by paracrine and autocrine interactions but may also take part in regulating fetal development.

Decreased serotonergic responsivity in platelets of drug-free patients with schizophrenia

We have recently developed a simplified and time-saving method to measure the magnitude of serotonin (5-hydroxytryptamine, 5HT)-amplified platelet aggregation and dense granule secretion (DGS) responses. To study the effects of neuroleptics on peripheral serotonergic function, we measured physiologic responsivity of the platelet 5HT2 receptor complex in schizophrenic patients (n = 27), both before and after haloperidol withdrawal, and also in normal volunteers (n = 18). In human platelets, 5HT amplifies the adenosine diphosphate (ADP)-induced platelet aggregation and DGS. Such an amplification was significantly enhanced in platelets from both normal volunteers and haloperidol-stabilized patients. Following haloperidol withdrawal, however, the magnitude of 5HT-amplified DGS response was no longer significant in drug-free patients, demonstrating a decreased serotonergic responsivity in schizophrenia. Moreover, in drug-free patients, the net changes of ADP-induced DGS, with and without the presence of 5HT, were correlated significantly and negatively with both Bunney-Hamburg psychosis ratings and Brief Psychiatric Rating Scale (total) scores, but not with scores on the Scale for the Assessment of Negative Symptoms. In the drug-free group, no significant difference of 5HT amplification was demonstrated between relapsed and nonrelapsed patients. The present finding thus suggests that drug-free schizophrenic patients may have a reduced physiologic responsivity mediated through the platelet 5HT2 receptor complex, which can be modified by haloperidol treatment. The pharmacologic action of haloperidol may derive in part from serotonergic mechanisms. The magnitude of 5HT-amplified DGS may be useful in the prediction of therapeutic outcome after haloperidol treatment.

Effects of Antidepressants and antipsychotics on the 5HT2 receptor-mediated signal transducing system in human platelets

The IC50 values of the antidepressants amoxapine, mianserin, desipramine, clomipramine and imipramine, and the antipsychotics spiroperidol, chlorpromazine, levomepromazine and haloperidol for 3H-ketanserin binding, 5HT-induced intracellular Ca2+ increase, 5HT-induced shape change and 3H-paroxetine binding in human platelets were measured and the correlations of each parameter were examined. Results were as follows. Both the antidepressants and the antipsychotics had inhibitory effects on 3H-ketanserin binding, 5HT-induced intracellular Ca2+ increase and 5HT-induced shape change and there were significant positive correlations between the IC50 values of these three parameters. On the other hand, there were no significant correlations between the IC50 values in 3H-paroxetine binding and those in the other three parameters. These results suggest that 3H-ketanserin binding, 5HT-induced intracellular Ca2+ increase and 5HT-induced shape change are useful and reliable tools for the assessment of the effects of the antidepressants and the antipsychotics on the 5HT2 receptor-mediated signal transducing system in human platelets.

Effect of encainide, ODE, MODE, and flecainide on ADP/5-HT induced platelet aggregation and in the anesthetized dog coronary artery stenosis-occlusion model of intravascular thrombosis

Encainide is a class 1C antiarrhythmic agent that is indicated for the treatment of life-threatening arrhythmias, such as sustained ventricular tachycardia. Furthermore, encainide possesses a moderate degree of antiserotonin activity, which was quantitated in this present study by determining displacement of [3H]spiperone binding from rat cortical 5-HT2 binding sites. The Ki for encainide in this model was 66.1 nM, compared to 2.6 nM for ketanserin. Two encainide metabolites, ODE and MODE, were also active, but were weaker than encainide. Additionally, these agents were found to inhibit platelet aggregation induced in vitro in human platelet-rich plasma by the combination of ADP and serotonin. In view of the fact that serotonin is one of a variety of humoral factors capable of activating blood platelets and has been recently implicated as playing a role in certain thrombotic syndromes, encainide, along with its two principal human metabolites, ODE and MODE, and another class 1C antiarrhythmic, flecainide, were evaluated in an in vivo model of intravascular thrombosis. Intraduodenal doses of 1 mg/kg of either encainide, ODE, or MODE significantly inhibited thrombosis in a canine model of coronary artery stenosis-occlusion.

Enhancement of antithrombotic effect of aspirin by metoclopramide in rats

The potential antithrombotic effect of aspirin and metoclopramide were studied in a model of arterial thrombosis in rats. Thrombosis was produced in the abdominal aorta by the combination of local partial obstruction and intravenous administration of hypotonic saline containing 5-HT. The resulting aortic occlusion and the effects of drugs were quantified by measuring rectal temperature. Metoclopramide as well as ketanserine effectively reversed while zacopride failed to alter thrombotic effect. Metoclopramide or ketanserine when combined with aspirin enhanced the antithrombotic effect of the latter. On the other hand, co-administration of metoclopramide with ketanserine failed to show any synergistic effect. As with ketanserine, the antithrombotic effect of metoclopramide appeared to be mediated through the blockade of 5-HT2 receptors in the platelets.

5-HT2 receptor antagonists and migraine therapy

5-Hydroxytryptamine (5-HT; serotonin) has been implicated in the pathophysiology of migraine, and several drugs with potent 5-HT2 receptor blocking activity (methysergide, pizotifen, cyproheptadine and mianserin) have been recognized as being clinically effective in migraine prophylaxis, although the selective 5-HT2 receptor antagonist ketanserin (the principal agent used to identify 5-HT2 receptor-mediated actions) seems to be ineffective in migraine. Pizotifen is the most widely used 5-HT2 receptor antagonist in migraine prophylaxis, because of its superior efficacy compared with cyproheptadine, and because the incidence and severity of adverse effects with pizotifen is lower compared with methysergide and mianserin. These agents have additional antagonistic effects at histamine H1, muscarinic cholinergic, alpha 1-adrenergic, alpha 2-adrenergic and dopamine receptors, but drugs which are selective for these non-5-HT receptors appear to be of no benefit in migraine. Actions mediated by 5-HT2 receptors which could be of relevance to migraine comprise cranial vasoconstriction, increased cranial capillary permeability and platelet aggregation, and some central nervous system effects and neuroendocrine functions. Although pizotifen, cyproheptadine and mianserin are considered to be relatively specific for 5-HT2 receptors, these agents and methysergide all share a high affinity for 5-HT1C binding sites; ketanserin, however, has little affinity for these sites, thus activation of 5-HT1C receptors may be an important step in the pathogenesis of migraine. It is not yet known which 5-HT1C receptor-mediated actions of 5-HT are relevant to migraine, but some behavioural actions and cranial vasodilatation via release of endothelium-derived relaxing factor may be involved.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual antagonism by L-636,499 of serotonin and thromboxane A2 induced aggregation of canine platelets

We evaluated the ability of L-636,499 (3-carboxyl-dibenzo-[b,f] thiepen-5,5-dioxide), a compound structurally similar to cyproheptadine, to antagonize U46619 (a TXA2/PGH2 mimetic) and serotonin (5-hydroxytryptamine; 5HT)-induced aggregation of canine platelets in vitro. L-636,499 antagonized competitively and dose-dependently aggregation induced by both 5HT and U46619, with pA2 values of 5.8 +/- 0.6 and 4.8 +/- 0.2, respectively. L-670,596, a potent TXA2/PGH2 receptor antagonist, and ketanser in, a potent 5HT2 receptor antagonist, yielded pA2 values of 7.0 +/- 0.3 and 9.0 +/- 0.2 vs. their respective agonists. These results show that despite its low potency vs. TXA2- and 5HT2-induced aggregation, L-636,499 antagonizes both physiologic mediators comparably.

5-Hydroxytryptamine and the metabolism of arachidonic acid by the lipoxygenase and cyclooxygenase of washed human platelets

During secondary aggregation, platelets release 5-hydroxytryptamine (5-HT) from their dense granule stores concurrent with arachidonic acid (AA) metabolism. To examine the hypothesis that released 5-HT has a modulatory effect on the metabolism of AA by platelets, we incubated nonaggregating washed human platelets with 5-HT in the presence of [3H]AA. Stimulation with 10(-4) M 5-HT, followed by incubation with 3 microM AA and 1 microCi [3H]AA for 5 min, resulted in a decrease in the formation of thromboxane B2 (TxB2) and 12-hydroxyheptadecatrienoic acid (HHT, P less than 0.05). The same treatment conditions and stimulation with 10(-7) to 10(-4) M 5-HT resulted in an elevation of 12-hydroxyeicosatetraenoic acid (12-HETE) formation (P less than 0.05). Treatment with the monoamine uptake inhibitor imipramine (20 microM) further increased the stimulation of 12-HETE formation observed in the presence of 10(-4) M 5-HT, suggesting that 5-HT may act at the platelet surface. A 5-HT1A receptor agonist, 8-hydroxy-dipropylaminotetralin (DPAT, 10(-6) to 10(-4) M) stimulated the formation of platelet cyclooxygenase (CO) products, whereas (+/-)1-(2,5-dimethoxy-4-iodo phenyl)-amino propane hydrochloride (DOI, 10(-6) to 10(-4) M), a 5-HT2 receptor agonist, had no significant effect on CO product formation. In addition, the 5-HT2 receptor antagonist ketanserin (10(-7) M) did not block the changes in CO or lipoxygenase metabolism induced by 5-HT. Since both DOI and DPAT stimulated 12-HETE formation whereas ketanserin was unable to reverse the 5-HT-enhanced 12-HETE formation, it seems unlikely that the stimulation of a 5-HT2 receptor is responsible for this action of 5-HT on platelets. We conclude that 5-HT depresses CO product formation while increasing 12-HETE formation through interaction with a platelet serotonergic binding site other than the 5-HT2 receptor.

5-Hydroxytryptamine and arachidonic acid metabolites modulate extensive platelet activation induced by collagen in cats in vivo

1. The pathways contributing to the platelet adhesion/aggregation reaction elicited by collagen microfibrils, administered to cats in vivo, were analysed. 2. The intra-aortic infusion of collagen (100 micrograms kg-1 in 1 min) caused an extensive activation of platelets, as evidenced by the time-dependent drop of free platelet numbers in whole blood, and the increases of 5-hydroxyindoles (5-HI), 5-hydroxytryptamine (5-HT) and thromboxane B2 (TXB2) levels in plasma, prepared from effluent venous blood sampled from the inferior caval vein. 3. 5-HT2 receptor blockade with ketanserin (0.63 mg kg-1 i.v., 10 min) and cyclo-oxygenase inhibition with aspirin (10 mg kg-1 i.v., 10 min) slightly attenuated the peak reduction of free platelets in whole blood in response to collagen without affecting changes in plasma 5-HI. Aspirin, but not ketanserin, reduced the collagen-induced changes in plasma TXB2, prostaglandin E2 (PGE2) and 6K-PGF1 alpha. 4. Dual TXA2 synthetase inhibition/TXA2-prostaglandin endoperoxide receptor antagonism with ridogrel (5 mg kg-1 i.v., 10 min) halved the drop in free platelets, reduced the release of platelet 5-HI, inhibited the increase in plasma TXB2 and elevated that of 6K-PGF1 alpha and PGE2 in response to collagen. 5. Combined treatment with ketanserin and aspirin reduced the collagen-induced drop of free platelets and the release of platelet 5-HI to a similar extent as ridogrel alone; plasma prostanoids were affected as with aspirin alone. 6. Combined administration of ketanserin and ridogrel virtually eliminated the collagen-induced platelet adhesion/aggregation response and release of 5-HI; prostanoids were affected as with ridogrel alone. 6. Combined administration of ketanserin and ridogrel virtually eliminated the collagen-induced platelet adhesion/aggregation response and release of 5-HI; prostanoids were affected as with ridogrel alone. 7. The results indicate that the interplay between 5-HT and arachidonic acid metabolites is causally involved in the platelet reaction to activation induced by collagen in cats in vivo.

Characterization of the normal bovine platelet aggregation response

1. Bovine platelets are more sensitive to stimulation by platelet activating factor (PAF) than adenosine-di-phosphate (ADP) or thrombin. 2. While epinephrine, arachidonic acid and serotonin are ineffective by themselves as aggregatory stimulants of bovine platelets they enhance the aggregation response of other platelet agonists. 3. There is no correlation between thromboxane A2 production and release and the extent of platelet aggregation in bovine platelets. 4. The dependence of bovine platelet aggregation on a phospholipid pathway and calcium mobilization is indicated.

Multiple mechanisms of serotonergic signal transduction

In this article we review serotonergic signal transduction mechanisms in the central and peripheral nervous systems and in a variety of target organs. The various classes of pharmacologically defined serotonergic receptors are coupled to three major effector systems: (1) adenylate cyclase; (2) phospholipase C mediated phosphoinositide (PI) hydrolysis and (3) ion channels (K+ and Ca++). Long term occupancy of serotonergic receptors also appears to induce alterations in mRNA and protein synthesis. For all three types of signal transduction there is evidence accumulating which suggests the involvement of guanine nucleotide regulatory proteins. Recent findings suggest that the distinct types of pharmacologically defined serotonergic receptors (5HT1A, 5HT1B, 5HT1c, 5HT2) may be coupled to one or more signal transduction systems. Thus, 5HT1 receptors may both activate and inhibit adenylate cyclase and increase K+-ion conductance in the hippocampus. 5HT2 receptors which activate PI hydrolysis in the brain, both open voltage-gated calcium channels and activate PI metabolism in certain smooth muscle preparations. Thus, each class of serotonergic receptor may be linked to one or more distinct biochemical transduction mechanisms. The possibility is raised that selective agonists and antagonists might be developed which have specific effects on a particular receptor-linked effector system.

Effect of 5-HT2-selective agonists on cat platelet aggregation

The effects of the 5-HT2-selective agonists 1-(4-bromo-2,5-dimethoxyphenyl)-2-aminopropane (DOB) and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on cat platelet aggregation were investigated and compared with those produced by serotonin (5-HT) and a positional isomer of DOB (i.e., isoDOB). Serotonin, DOB, and DOI enhanced the aggregation of platelets induced by a suboptimal concentration of ADP. This effect was completely inhibited by pre-incubation of the platelet suspension with the 5-HT2-selective antagonist ketanserin. IsoDOB, an isomer of DOB with a very low affinity for central 5-HT2 binding sites, was inactive in the platelet aggregation assay. The present results are consistent with the proposed role of 5-HT2 receptors in serotonin-induced platelet aggregation.

Central serotonin receptors: effector systems, physiological roles and regulation

Radioligand binding studies have revealed four distinct serotonin (5HT) binding sites in rat brain that are thought to function as 5HT receptors. These include the 5HT-1a, 5HT-1b, 5HT-1c, and 5HT-2 binding sites. Studies have shown that the 5HT-2 binding site mediates a number of effects of 5HT agonists and serves as a 5HT receptor in neuronal and non-neuronal tissues. The 5HT-2 site employs phosphoinositide hydrolysis for signal transduction. The 5HT-1c binding site is also a functional receptor that is linked to phosphoinositide hydrolysis. However, the physiological role of the 5HT-1c receptor is not yet known. Lack of appropriate pharmacological tools for probing the 5HT-1a and 5HT-1b binding sites has made it difficult to definitively determine whether these binding sites are coupled to biochemical effector systems or mediate any of the physiological responses to 5HT agonists. However, there is some evidence that the 5HT-1a site is coupled to adenylate cyclase, and a number of functional roles for the 5HT-1a and 5HT-1b sites have been proposed.

Effects of combinations 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.

Proposals for the classification and nomenclature of functional receptors for 5-hydroxytryptamine

As a result of controversy in the literature regarding the classification and nomenclature of functional receptors for 5-hydroxytryptamine (5-HT), a framework for classification is proposed. The formulation of these proposals has only been made possible by the recent advent of new drug tools. It is considered that there are three main types of 5-HT receptor, two of which have been well characterised pharmacologically, using selective antagonists, and which it is proposed to name 5-HT2 and 5-HT3. These two groups broadly encompass the "D" and "M" receptors, respectively, which Gaddum identified in the guinea-pig ileum (Gaddum and Picarelli, 1957). The 5-HT2 receptor, which mediates a variety of actions of 5-HT, has been definitively shown to correlate with the 5-HT2 binding site in the brain. No binding studies in brain tissue have yet been published with radiolabelled ligands specific for 5-HT3 receptors. A number of other actions of 5-HT appear to be mediated via receptors distinct from 5-HT2 or 5-HT3 receptors. Since selective antagonists are not yet available, these receptors cannot be definitively characterised, although in many cases they do have some similarities with 5-HT1 binding sites, which are a heterogeneous entity. Criteria are proposed for tentatively classifying these receptors as "5-HT1-like" (Table 1). Definitive characterisation of these receptors will await the identification of specific antagonists. This classification of 5-HT receptors into three main groups (Table 1) is based largely, but not exclusively, on data from studies in isolated peripheral tissues where definitive classification is possible. However, it is believed that this working classification will be relevant to functional responses to 5-HT in the central nervous system.

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.

Dependence of the antagonism at human platelet 5-HT2 receptors by ketanserin on the reaction pH

The potency for the inhibition of 5-hydroxytryptamine (5-HT)-induced human platelet aggregation by ketanserin, pipamperone, spiperone, cyproheptadine and BW 501 in vitro decreased as the reaction pH increased progressively from 7.4 to 8.6, the largest shift (X 1125) in IC50 value (pH 7.4: 4 X 10(-9) M; pH 8.6: 4.5 X 10(-6) M) being found for ketanserin. With such an alkaline pH-shift, the fraction of the ionized form of the drugs decreased, reduction of the inhibitory capacity and of the ionized fraction being strongly correlated. Ketanserin (40 mg orally, - 15 h) in human volunteers, completely inhibited the 5-HT-induced platelet aggregation measured ex vivo, when tested at a reaction pH of 7.4; without gassing with CO2 5% -O2 95%, the plasma pH became alkaline and the inhibitory potency of the drug was strongly reduced (-60%). This study demonstrates the importance of the reaction pH for the aggregation of human platelets induced by 5-HT and its inhibition by ketanserin.

Biochemical mechanisms in 5-hydroxytryptamine-induced human platelet aggregation

The activation of human platelets by 5-hydroxytryptamine (5-HT) is not accompanied by detectable release of ATP or TXB2. The process is unaffected by cyclooxygenase, thromboxane synthetase or combined cyclooxygenase/lipoxygenase inhibition (suprofen, indomethacin, R 19091, dazoxiben, N.D.G.A, BW755C, esculetin), indicating the absence of involvement of arachidonic acid metabolites. Transmembrane Ca2+-entry blockers (flunarizine, nifedipine, nimodipine) have no effect either, indicating that the activator calcium released by 5-HT comes from intracellular stores. The 5-HT-induced platelet activation is inhibited by stimulators of adenylate cyclase (PGE1, PGE2, isoprenaline, adenosine) and inhibitors of cAMP phosphodiesterase (papaverine, anagrelide, RA233), indicating that also for this type of platelet activation cAMP behaves as a unidirectional, inhibitory regulator.

Serotonergic agonists stimulate inositol lipid metabolism in rabbit platelets

The metabolism of inositol phospholipids in response to serotonergic agonists was investigated in rabbit platelets. In platelets prelabelled with [3H]-inositol, in a medium containing 10 mM LiCl which blocks the enzyme inositol-1-phosphatase, 5-hydroxytryptamine (5-HT) caused a dose-dependent accumulation of inositol phosphates (IP). This suggests a phospholipase-C-mediated breakdown of phosphoinositides. Ketanserin, a selective 5-HT2 antagonist, was a potent inhibitor of the 5-HT response, with a Ki of 28 nM, indicating that 5-HT is activating receptors of the 5-HT2 type in the platelet. Lysergic acid diethylamide (LSD) and quipazine also caused dose-related increases in inositol phosphate levels, though these were considerably less than those produced by 5-HT. These results show that relatively small changes in phosphoinositide metabolism induced by serotonergic agonists can be investigated in the rabbit platelet, and this cell may therefore be a useful model for the study of some 5-HT receptors.

Solubilized ketanserin binding sites from pig platelet membranes

Plasma membranes of pig platelets were isolated and subjected to solubilization with digitonin. Binding of [3H]ketanserin to the solubilized membrane fraction, optimal at pH 7.5, was time and protein dependent and was greatly reduced by increasing ionic strength of the incubation buffer. Saturation analysis and inhibition of binding with various 5-HT2 antagonists showed the presence of two binding sites with high and low affinity, respectively. High affinity binding to solubilized membrane proteins correlated well with that found in native membranes with regard to the dissociation constant KD (1.6 and 1.2 nM, respectively) and the displacement by 5-HT2 antagonists. Binding to this site was not inhibited by the dopamine antagonist haloperidol which, however, displaced [3H]ketanserin bound to the low affinity site. At present the nature of the low affinity binding site (KD = 71 nM) is not well understood. Centrifugation on sucrose gradients revealed only one peak of [3H]ketanserin binding corresponding to the molecular weight of catalase (232,000 Da), thus probably reflecting a close association of the two binding sites.

Characterization of 5-hydroxytryptamine binding sites in the plasma membrane of pig blood platelets

Plasma membranes were isolated from pig platelets after glycerol facilitated lysis by sucrose density gradient centrifugation. The purity of the membrane fraction was followed by electron microscopy, gel electrophoresis and analysis of acid phosphatase (EC 3.1.3.2) and phosphodiesterases (EC 3.1.4.1). (3H)5-Hydroxytryptamine ([3H]5-HT) was bound to two saturable binding sites of the membranes. The KD value for the high affinity sites was 0.85 nM and for the low affinity sites 0.48 microM. With the exception of tryptamines little or no (3H)5-HT was displaced by serotonin antagonists and uptake inhibitors suggesting another type of binding than that of 5-HT1. Apparently, enhancement of binding in the presence of Na+ was due to stimulation of an uptake process. Binding of (3H)ketanserin and (3H)LSD to pig platelet membranes showed the characteristics of 5-HT2 binding sites previously identified in rat brain. Since ketanserin inhibited 5-HT induced aggregation of pig platelets (IC50 = 14.2 nM), the ketanserin binding sites can be classified as 5-HT2 receptors. The functional properties of these binding sites and their density in pig platelets as compared with brain membranes may motivate studies on 5-HT2 receptors in pig platelets as models for those in nerve endings.

The effect of ketanserin, a 5-HT2-receptor antagonist, on 5-hydroxytryptamine-induced irreversible platelet aggregation in patients with cardiovascular diseases

Platelet aggregation in response to 5-hydroxytryptamine was investigated in 40 normal subjects, in 45 patients with acute myocardial infarction, and in 65 patients with peripheral arterial obstructive disease. It was found that of the 110 patients with cardiovascular disease, 40% had a biphasic irreversible platelet aggregation, whereas this phenomenon occurred in only 7.5% of the normal population. A double-blind placebo-controlled study further showed that a subacute treatment with ketanserin, a selective 5-HT2-receptor antagonist both on platelets and on vascular tissue, efficiently abolished the irreversible platelet aggregation in patients hyperreactive to 5-hydroxytryptamine. In an additional open study, including 10 patients with peripheral arterial obstructive disease, a chronic treatment with ketanserin 40 mg t.i.d. for a period of 3 months significantly suppressed the primary platelet aggregation to 5-HT at 2 X 10(-5) M and at 2 X 10(-6) M and significantly lowered the plasma beta thromboglobulin levels. Since 5-HT is a potent mediator of vasospasm, treatment with ketanserin might be of therapeutic value in atherosclerotic diseases, where platelet activation is thought to be involved.

Platelet-vessel wall interactions: implication of 5-hydroxytryptamine. A review

The evidence for an impact of platelet-derived 5-hydroxytryptamine (5-HT) on local tissue perfusion is reviewed. By interacting with 5-HT2 serotonergic receptors, 5-HT, directly or through amplification, activates platelets, endothelial and vascular smooth muscle cells producing platelet aggregation, vascular permeability increase and large vessel constriction. Pharmacodissection in experimental animals with selective serotonergic 5-HT2 receptor antagonists, e.g. ketanserin, shows that 5-HT largely contributes to the platelet-mediated increase in vascular permeability, to platelet-vessel wall interaction during hemostasis, to cardiopulmonary dysfunction provoked by thromboembolism and to the platelet-mediated inhibition of peripheral collateral circulation. Clinical results obtained with ketanserin further substantiate an involvement of platelet-derived 5-HT in the pathogenesis of impaired tissue perfusion in some cardiovascular conditions.

Platelet-vessel wall interactions in hemostasis: Implication of 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is implicated in the platelet-vessel wall interactions during hemostasis. Indeed, the tail bleeding time and the initial blood loss in rats are significantly increased by platelet amine depletion with reserpine/parachlorophenylalanine, by the specific 5-HT2 receptor antagonists ketanserin, R 55 667 and R 56 433 and by the ergot derivatives metergoline and methysergide. In the dose range affecting hemostasis, the serotonergic receptor antagonists also inhibit the 5-HT-amplified aggregation of ADP-sensitized platelets in rat whole blood. Bleeding times are also prolonged by alpha 1-adrenergic receptor antagonism with prazosin or phentolamine, by specific thromboxane A2-synthetase inhibition with R 59 655 or dazoxiben, and by anticoagulant treatment. The latter three mechanisms of action are not explanatory for the effect of the tested serotonergic antagonists, since bleeding times are prolonged by specific 5-HT2 receptor antagonists without an effect on alpha-adrenergic receptors, on platelet prostaglandin biosynthesis or on coagulation and fibrinolysis. The present study shows the interplay between the platelet-derived mediators 5-HT and TXA2 at the level of the platelet or the blood vessel to be of major importance in the platelet-vessel wall interactions during hemostasis.

Evidence for functional 5-HT2 receptor sites on human blood platelets

The aggregation of normal human platelets by 5-hydroxytryptamine (5-HT) is the result of a specific interaction of the monoamine with a platelet receptor since it is not influenced by adrenergic receptor blockade, inhibition of fatty acid cyclo-oxygenase or ADP-scavenging. The 5-HT induced platelet reaction is inhibited in a concentration-dependent way by various serotonergic antagonists; the potency of these compounds in this respect correlates strongly with their potential to inhibit the specific binding of [3H] ketanserin, a selective label for 5-HT2 binding sites, to rat prefrontal cortex and striatum and to cat platelet membranes. This study thus provides evidence for a functional role as true receptor initiating a physiological response of the 5-HT2 receptor on human platelets.