Endosulfan induced inhibition of 3H-5-hydroxytryptamine uptake in platelets

Endosulfan, a chlorinated pesticide, is widely used to control various insect pests. Rats exposed to 1 mg and 3 mg endosulfan/kg for periods of 10, 30, and 60 days showed significant (P less than 0.05) inhibition of [3H]5-hydroxytryptamine (5-HT) uptake by platelet-rich plasma (PRP) in an ex vivo study. Rats treated with endosulfan (1 and 3 mg/kg) up to 60 days elucidated a marked inhibition of ADP-induced aggregatory responses of the platelets. Incubation of PRP with 10 microM and 100 microM endosulfan for 15 min at 37 degrees C also resulted in significant (P less than 0.05) inhibition of platelet aggregation in vitro. The paper discusses the use of rat blood platelets as a model for the study of neuro- and cardiovascular toxicity of endosulfan.     

Verapamil inhibition of 5-hydroxytryptamine (5HT) uptake in rat platelets

The effect of verapamil on 5-hydroxytryptamine (5HT) uptake into rat blood platelets was investigated in vitro. Verapamil produced a non-competitive inhibition (IC50 = 8.2 +/- 0.5 microM) of 5HT uptake. The administration of verapamil to rats also produced inhibition of 5HT uptake and reduced the concentration of this amine in the platelets. The present data indicate that verapamil has an influence on the uptake processes for 5HT in blood platelets in rats.     

Inhibition by tetrahydroharmane compounds of 5-hydroxytryptamine and histamine uptake in rabbit blood platelets

Summary The effect of 5-hydroxytryptamine (5-HT), tryptamine (T), 6-hydroxy-1,2,3,4-tetrahydroharmane (6-HTH), and 1,2,3,4-tetrahydroharmane (TH) on the accumulation of radioactive 5-HT and histamine, was studied in rabbit blood platelets. 6-HTH and TH inhibited the uptake of the labelled amine, but they were weaker inhibitors than 5-HT or T. All four compounds inhibited the accumulation of radioactive histamine in the granules as well as in the whole cells, but the latter effect is not the consequence of inhibition of granular accumulation.It is concluded that the methylene bridge, which is the most important structural difference between the harmanes and the tryptamines, does not abolish the affinity of the molecule to the uptake receptor at the platelet membrane. However, since the inhibitory potency was lower than that of the parent amines, one cannot be certain as to whether or not the conformation present in the harmane derivatives offers the best fit for the receptor.     

Uptake of 5-hydroxytryptamine by rat blood platelets and its inhibition by adenosine 5'-diphosphate.

1 The uptake of 5-hydroxytryptamine (5-HT) by rat blood platelets in citrated plasma was linear for at most 10 s and was substantially complete within 3 minutes. 2 Adenosine 5'-diphosphate (ADP) was a potent inhibitor of 5-HT uptake (Ki=0.38 muM) and kinetic analysis revealed that the inhibition was not competitive. 3 Inhibition of 5-HT uptake by ADP was abolished in the presence of prostaglandin E1 and 2-n-amylthio-AMP, which also inhibit the stimulant actions of ADP on blood platelets. 4 It is concluded that ADP could inhibit 5-HT uptake by changing the Na+/K+ distribution across the cell membrane, and the biological significance of this is discussed.     

Effect of fenfluramine on 5-hydroxytryptamine uptake and release by rat blood platelets.

1. (+)-Flenfluramine reduces the central stores of 5-hydroxytryptamine (5-HT) by a poorly understood mechanism. 2. Rat blood platelets have been used in this study as a simple model for serotoninergic nerve endings. 3. (+)-Fenfluramine shows a dual effect: it inhibits the uptake of (14C)-5-HT by platelets and it releases newly absorbed (14C)-5-HT from platelets. 4. The inhibition of (14C)-5-HT uptake induced by (+)-fenfluramine appears very rapidly, is concentration-dependent and seems not to be competitive. (+)-Fenfluramine is ten times less effective than chloroimipramine but tem times more effective than (+)-amphetamine; (+)-fenfluramine is more active than its (-)isomer or its metabolite norfenfluramine ((+)- or (-)-form). 5. The release of (14C)-5-HT from platelets induced by (+)-fenfluramine is concentration-dependent but increases wtih increased incubation time. Both chloroimipramine and (+)-amphetamine are in comparison very poor release inducers; (+)-fenfluramine is more active than its (-)-isomer or its metabolites. 6. The effect on (14C)-5-HT uptake exerted by (+)-fenfluramine and chloroimipramine in vitro could not be observed in vivo. 7. The observed effect on fenfluramine on the uptake and release of 5-HT may explain the lowering action of fenfluramine on the brain 5-HT level, an effect considered of importance for the anoretic effect on this drug.     

Inhibitory effect of ethanol on 5-hydroxytryptamine (serotonin) uptake in human blood platelets in vitro

Ethanol in concentrations higher than 10(-2) M (or about 0.5% W/W) inhibited the uptake of 14C-5-hydroxytryptamine (serotonin) in human blood platelets in an artificial, protein-free medium, by a non-competitive mechanism. The inhibition was not influenced by the alcohol dehydrogenase inhibitor methylpyrazole (10(-3) M). In concentrations up to 0.1 M, ethanol had no effect on 5-HT efflux from the platelets. At higher concentrations, ethanol increased efflux. Inhibition of 5-HT uptake was found to increase progressively in the sequence methanol - ethanol - propanol - butanol.     

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.     

Sodium-dependent accumulation of 5-hydroxytryptamine by rat blood platelets

1. The influence of sodium and potassium on the accumulation of 5-hydroxytryptamine (5-HT) by rat blood platelets was investigated.2. An absolute dependence of 5-HT uptake on the sodium concentration in the medium was found.3. Removal of potassium reduced the uptake by about 60%. High concentrations of potassium inhibited sodium-dependent accumulation.4. The observations have been discussed in terms of a carrier-mediated transport process for 5-HT operating in the platelet membrane.     

Urapidil inhibits 5-hydroxytryptamine induced platelet aggregation and 14C-5-hydroxytryptamine uptake in platelets

The effect of urapidil, an a1-antagonist with additional antihypertensive action via central 5HT1A agonism, was determined on platelet aggregation induced by 5HT and adrenaline. The 5HT uptake in human platelets was determined as well. Urapidil inhibited both the 5HT and adrenaline-induced human platelet aggregation and the 5HT uptake by platelets in vitro. The 5HT-induced aggregation was inhibited with a K1 value of 8.8 microM, the adrenaline-induced aggregation with a K1 value of 21.6 microM, and the 5HT uptake non-competitively with a K11 = 11.5 microM and a K12 = 13 microM.     

Dextromethorphan inhibits 5-hydroxytryptamine uptake by human blood platelets and decreases 5-hydroxyindoleacetic acid content in rat brain.

The effect of dextromethorphan on the uptake and metabolism of 5-hydroxytryptamine (5-HT) was studied in human blood platelets and in rat brain. In the concentration of 120 nM dextromethorphan inhibited the uptake of 5-HT (1 mu-M) into platelets by 50%. The corresponding concentrations of imipramine and methadone under similar conditions were 22 and 590 nM, respectively. Dextromethorphan (20 to 40 mg kg-1) decreased the concentration of brain 5-hydroxyindoleacetic acid (5-HIAA) and the probenecid-induced accumulation of 5-HIAA time- and dose-dependently. However, dextromethorphan did not alter the pargyline-induced changes in brain 5-HT metabolism. Dextromethorphan-induced changes in brain 5-HT metabolism could arise from the inhibition of the re-uptake of 5-HT into neurons.     

Lack of a role for blood platelets in the uptake of 5-hydroxytryptamine by the rabbit heart

Perfused rabbit hearts removed 5-hydroxytryptamine (5-HT) from a perfusion solution of 1.0 × 10^?8 g ml^?1 and at the same time their endogenous 5-HT concentrations increased. Pretreatment of animals with heparin did not affect either the removal of 5-HT from the perfusion fluid or its accumulation in the heart. The proportion of 5-HT removed from the perfusion fluid which was subsequently found in the heart was small in both the heparinised and the non-heparinised groups. The results suggest that platelets trapped within heart tissue are unimportant for cardiac 5-HT uptake.     

The inactivation of 5-hydroxytryptamine by blood platelets in mental deficiency with elevated serum 5-hydroxytryptamine

Summary The mentally defective patients were divided into two groups according to the 5-hydroxytryptamine (5HT) content in serum. In the first (normal) group were those with values <200 ng/ml (6 mongolisms, 4 cerebral palsies) and in the second those with values >200 ng/ml (7 cerebral palsies, 5 encephalopathies).The content of 5HT per platelet was twice as high in the second than in the first group. When the platelet-rich plasma was incubated with tetrabenazine in vitro equal amounts of 5HT were liberated from platelets in both groups. During the liberation slightly more of the 5 HT (in percent from released) was inactivated in the first group than in the second. The ability of platelets to inactivate 5 HT under these experimental conditions does not explain the higher serum and platelet values found in some patients.     

Effects of concanavalin A on 5-hydroxytryptamine uptake by rabbit blood platelets and on their ultrastructure.

1. Effects of concanavalin A (Con A) and other lectins on 5-hydroxytryptamine (5-HT) uptake by rabbit blood platelets and on their ultrastructure were studied. 2. Uptake of [3H]-5-HT by platelets was decreased by application of Con A, E-PHA (lectin from Phaseolus vulgaris) and lentil-PHA (lectin from Lens culinaris), but not by wheat germ agglutinin (WGA). Con A induced specific changes in the ultrastructure of platelets, causing (i) a change in external appearance from a discoid to an irregularly spherical shape, (ii) re-arrangement of the canalicular system and formation of a concentric structure. These effects of Con A on platelets were antagonized by pretreatment with alpha-methyl-D-mannoside (alpha-MM), a specific inhibitor of Con A binding to glycoprotein. 3. The inhibition of 5-HT uptake by Con A was antagonized by colchicine, vinblastine and sodium nitroprusside (SNP), but not by cytochalasin B. 4. Theophylline, papaverine and dibutyryl cyclic adenosine 3',5'-monophosphate (db cyclic AMP) antagonized the effect of Con A on 5-HT uptake, but dibutyryl cyclic guanosine 3',5'-monophosphate had no effect. Theophylline and db cyclic AMP did not influence the effect of Con A on the ultrastructure of platelets. 5. It is suggested that binding of Con A to specific receptor glycoproteins can inhibit the 5-HT uptake system of platelets. Microtubules, contractile protein and the membrane adenylate cyclase system of platelets may also be regulatory factors in this mechanism.     

Inhibition of 5-hydroxytryptamine uptake in human platelets by antidepressant agents in vivo

The accumulation of ¹⁴C-5-hydroxytryptamine (¹⁴C-5-HT) in platelet-rich plasma (PRP) and the concentration of 5-hydroxytryptamine (5-HT) in whole blood of patients treated with the antidepressant agents zimelidine (2×100 mg. daily), desipramine (2×75 mg daily), and clomipramine (2×75 mg daily) were examined before and during the treatment. Clomipramine and zimelidine markedly reduced the accumulation of ¹⁴C-5-HT and the concentration of 5-HT in the blood. Desipramine had a weaker, but significant effect. Added to the PRP in vitro clomipramine was ten-times more potent than norzimelidine, the active metabolite of zimelidine, and 60- and 300-times more active than desipramine and zimelidine, respectively in inhibiting the accumulation of ¹⁴C-5-HT. Analysis of plasma concentrations of zimelidine and norzimelidine showed that the decreased blood 5-HT and the inhibition of ¹⁴C-5-HT accumulation in platelets was mainly produced by norzimelidine. The inhibition of the ¹⁴C-5-HT accumulation and the decrease in blood 5-HT by desipramine were significantly correlated to the log plasma concentration of desipramine. It is concluded that the decrease in blood 5-HT caused by these agents is due to the inhibition of 5-HT uptake in platelets. The half-life of the decrease in blood 5-HT after clompramine and zimelidine was about 5 days. The return to normal 5-HT level after withdrawal of the drugs was 14 days or longer. These observations might indicate that only the newly formed platelets can accumulate 5-HT.     

The actions of flupenthixol upon 5-hydroxytryptamine-induced aggregation and the uptake of 5-hydroxytryptamine and dopamine by human blood platelets.

The effects of the alpha- and beta-isomers of flupenthixol on 5-hydroxytryptamine (5-HT)-induced platelet aggregation and on 5-HT and dopamine uptake were investigated. Alpha-Flupenthixol was 185 times more potent than the beta-isomer as an inhibitor of platelet aggregation. In contrast both isomers were equipotent as inhibitors of uptake of 5-HT and dopamine. The data suggest that 5-HT-induced aggregation and uptake are separate processes.