Search

1 Blood platelets from normal human subjects were isolated and aggregated in vitro with adenosine diphosphate (ADP) or 5-hydroxytryptamine (5-HT). 2 The effects of chlorpromazine and 7 major metabolites upon 5-HT-induced aggregation were investigated. 3 All the phenothiazines inhibited 5-HT-induced aggregation when added to platelet rich plasma 3 min prior to 5-HT. 4 There were no qualitative differences in the inhibitory effects, but inhibitory potency varied over a wide range. The decreasing order of potency was monodesmethylchlorpromazine, chlorpromazine, 7-hydroxychlorpromazine, didesmethylchlorpromazine, 3,7 -dimethoxy-chlorpromazine, didesmethylchlorpromazine sulphoxide, chlorpromazine sulphoxide, chlorpromazine nitroxide.

1 The binding of [3H]-5-hydroxytryptamine ([3H]-5-HT), [3H]-haloperidol and [3H]-dopamine to human blood platelets was investigated and the effects of unlabelled haloperidol on [3H]-5-HT binding and (+)- and (-)-butaclamol on [3H]-haloperidol were studied. 2 Scatchard analysis did not show any specific binding of [3H]-haloperidol or [3H]-dopamine to platelets, but two binding sites were identified for binding of [3H]-5-HT. 3 Unlabelled haloperidol reduced the binding of 5-HT in concentrations comparable to those inhibigint 5-HT-induced platelet aggregation; (+)- and (-)-butaclamol did not affect the binding of [3H]-haloperidol. 4 It is concluded that haloperidol binding represents saturation of the platelet membrane, and that the platelet is not a suitable model for investigation of dopamine-drug interactions.

1 Because normal human blood platelets contain higher concentrations of taurine than any other amino acid, and have a platelet: plasma concentration gradient exceeding 400: 1, we isolated the cells in vitro and incubated them with radioactively labelled taurine in order to investigate the existence of a metabolically-dependent accumulation process.2 Platelets incubated with taurine (1 to 100 nmol/ml) in autologous plasma or Krebs solution accumulated [(14)C]-taurine against the concentration gradient.3 The transport process was saturable at high concentrations, showed a requirement for sodium ions, and was temperature-dependent.4 The kinetics of transport fulfilled the criteria of Michaelis & Menten for saturable enzyme/substrate interactions, but the kinetic constants were influenced by the incubation medium.5 The metabolic inhibitors 2,4-dinitrophenol and iodoacetic acid in combination inhibited taurine transport in Krebs solution, but stimulated transport in autologous plasma. The latter result suggested the involvement of a sodium-dependent ATPase in taurine transport.6 We conclude that platelets actively transport taurine in vitro under experimental conditions closely resembling those likely to occur in vivo, and that this taurine transport process may be involved in the maintenance of the platelet: plasma concentration gradient.

1 Blood platelets from normal human volunteers were isolated and aggregated in vitro with 5-hydroxytryptamine (5-HT), noradrenaline (NA), dopamine and N-dimethyldopamine (DMDA). 2 The effects of 5-HT antagonists, alpha- and beta-adrenoceptor blocking agents, butyrophenones and chlorpromazine upon aggregation induced by the four amines were investigated. 3 Only the the 5-HT antagonists, chlorpromazine and spiroperidol were potent inhibitors of 5-HT-induced aggregation, and only phentolamine was a potent inhibitor of the catecholamine-induced aggregation. 4 Evidence was obtained for two populations of receptors, one for 5-HT and one for the three catecholamines.

1 The pharmacological similarity between human basilar artery and a number of isolated tissues (rat colon, anococcygeus, stomach fundus and aorta and guinea-pig ileum and colon) has been assessed during investigations of the aetiology of cerebral arterial spasm. 2 The responses of each of the six tissues to human normal and abnormal cerebrospinal fluid (CSF) and to human serum were compared with those of the human isolated basilar artery. 3 These studies revealed the presence of a vasodilator factor in CSF from subarachnoid haemorrhage patients and several tissues may be of use for further work in identifying the vasoactive substances in CSF. 4 No tissue displayed exactly the same spectrum of biological reactivity as the human basilar artery. The rat stomach fundus showed the closest similarity and was further studied for similarities in drug-induced responses. 5 The rat stomach fundus, like the human basilar artery, was contracted by 5-hydroxytryptamine, prostaglandin F2alpha and histamine and relaxed by dopamine. However, noradrenaline relaxed the fundus but contracted the basilar artery.

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.

1. We have examined the accumulation by human blood platelets of amino acids that are believed to be involved in neurohumoral transmission in the central nervous system.2. Platelets were incubated in Ca(++)-free Krebs solution at 37 degrees C with radio-active amino acids for various times and then the platelets were analysed for the radioactive substance and its metabolites.3. L-Phenylalanine, L-DOPA, L-tryptophan and L-tyrosine were rapidly accumulated, the equilibrium tissue/medium concentration ratio (C(i)/C(0)) being greater than 10:1 when the concentration of amino acid in the medium was 10(-7)M or lower. Glycine and gamma-aminobutyric acid (GABA) accumulation was less, C(i)/C(0) being lower than 3:1 when C(0) was 10(-7)M.4. Uptake of L-phenylalanine, L-DOPA and L-tryptophan were all decreased or abolished by incubation at 4 degrees C, or with metabolic inhibitors or by disruption of the platelet membrane prior to incubation, while L-tyrosine accumulation was not affected.5. It is considered that L-phenylalanine, L-DOPA and L-tryptophan are accumulated by saturable, energy-dependent processes; that glycine and GABA diffuse into the platelet, and L-tyrosine accumulates as a result of diffusion and intracellular binding.6. None of the amino acids examined showed any significant metabolism during a 20 min incubation. However, some evidence for tyrosine binding to soluble protein was obtained.7. Results are compared to reports of accumulation of these amino acids by the central nervous system.

Circulating platelets of cats infused with (14)C-dopamine take up the amine against a concentration gradient, with maximal concentration occurring 10 min after infusion. 58%, of the platelet-bound radioactive dopamine was localized in a subcellular fraction rich in 5-hydroxytryptamine (5-HT) storage organelles. The results support earlier work with human platelets showing in vitro dopamine uptake by normal platelets, and in vivo accumulation in parkinsonian patients treated with L-dopa.

1. When human blood platelets were incubated aerobically in plasma containing 2 x 10(-7) to 10(-3)M radioactive guanethidine for 10 min to 6 hr, the drug was accumulated against a concentration gradient until concentration ratios (platelet/plasma) of up to 80:1 were obtained.2. The decline in rate of uptake after 3 hr appeared to result from a decrease in platelet viability, because accumulation was reduced by prolonged incubation before addition of guanethidine.3. Uptake was energy-dependent because it was inhibited by cold and ouabain.4. Sodium ions were essential for guanethidine uptake and retention of 5-hydroxytryptamine (5-HT).5. Accumulation was inhibited by 5-HT, desipramine, cocaine, dexamphetamine, bretylium, tyramine and noradrenaline; bethanidine, p-chlorophenylalanine and (-)-alpha-methyldopa were inactive.6. Guanethidine was tightly bound to platelets, only 10% being lost from labelled cells during 60 min incubation in drug-free plasma; but efflux was increased by addition of amphetamine.7. The binding sites for guanethidine seemed to be different from those for 5-HT since guanethidine accumulation was independent of 5-HT levels, and neither guanethidine uptake or release were affected by reserpine.8. Guanethidine was not metabolized by platelets or plasma in vitro.9. We consider that, if our results regarding uptake, binding and release of guanethidine are confirmed in vivo, and also found to apply to other pharmacologically active agents, then the eventual loss of a platelet-bound substance may increase pharmacological action by raising plasma levels.

1. We have studied the mechanism of quinidine uptake by normal human blood platelets.2. Platelets in plasma or Krebs solution took up quinidine extremely rapidly, the maximal drug concentration ratio of platelet to medium being 24 to 1 after 1 minute.3. The effects of metabolic inhibitors on uptake were equivocal. Ouabain had no effect but iodoacetate and dinitrophenol were effective. However, as accumulation was not inhibited by low temperature, it was probably not energy dependent.4. Interactions between quinidine and 5-HT were also investigated.5. Quinidine blocked 5-HT uptake, but conversely, neither 5-HT itself nor 5-HT uptake inhibitors, such as cocaine, dexamphetamine, and desipramine interfered with quinidine accumulation.6. Electronmicrographs of platelets incubated with 10(-5), 10(-4) or 2 x 10(-3)M quinidine in Krebs solution for 2 h showed no changes at the lower concentrations, but gross ultastructural damage, including disintegration of the plasma membrane, at 2 x 10(-3)M.7. Further evidence for intracellular penetration was obtained because quinidine released endogenous 5-HT and ATP, and also (14)C-5-HT from loaded cells.8. We conclude that quinidine is taken up by the platelet by a passive process, unrelated to the 5-HT transport mechanism. It is probably accumulated largely in the plasma membrane and outer protein layer, but intracellular penetration and ultrastructural damage may occur.9. Although the effects on 5-HT and ATP were not due to platelet damage, this may occur in vivo and be the cause of quinidine induced thrombocytopenic purpura.

1. The uptake, efflux and pharmacological actions of procaine hydrochloride were studied on isolated hearts of female guinea-pigs. The hearts were perfused with Krebs solution containing (14)C-procaine (0.1-500 mug/ml.) by the Langendorff technique at 37 degrees C, using a constant flow pump. Hearts and cardiac effluent were assayed for procaine by liquid scintillation spectrometry.2. Accumulation of procaine did not appear to involve active transport mechanisms for the following reasons. The rate of procaine uptake was most rapid at the highest perfusion concentration (500 mug/ml.), when it was three times faster than at the lowest concentration (0.1 mug/ml.); it was not affected by lowering the temperature to 3 degrees C. The ratio of the concentration of procaine in the heart to the concentration in the perfusing fluid decreased with increasing concentration in the perfusion fluid.3. When the efflux of (14)C-procaine from hearts previously perfused with procaine-containing Krebs solution for 10 min was compared with the efflux of (14)C-inulin, the patterns of efflux of both compounds were similar, and showed at least two exponential components. At the highest concentration of procaine (500 mug/ml.) the efflux of procaine was more rapid than that of inulin.4. A relationship was found between the pharmacological action of procaine, the rate of uptake and the level of procaine in the heart. When the procaine-containing perfusion fluid was changed to a procaine-free solution, the heart rate increased rapidly, and there was a rapid decline in the levels of procaine in the hearts.5. It is concluded that guinea-pig hearts accumulate procaine by a passive diffusion process, and that the pattern and rate of efflux indicate that the drug is loosely bound. If it is permissible to extrapolate from these findings to the antiarrhythmic effect in man, the short duration of its action may be due to loose binding rather than to rapid metabolic inactivation.

1. Human blood platelets have been shown to take up dopamine by an energy-dependent, saturable process that is inhibited by 5-hydroxytryptamine (5-HT), desipramine and other drugs.2. Platelets from parkinsonian subjects receiving oral L-DOPA also took up dopamine.3. When the responses of normal and parkinsonian platelets were compared, the parkinsonian cells showed the following differences: increased affinity for the dopamine transport process; decreased equilibrium concentrations of dopamine after incubation for 90 min, and greater efflux of dopamine from loaded platelets during a 10 min incubation.4. There were no differences in the uptake of 5-HT by parkinsonian platelets, but endogenous 5-HT was significantly reduced; ATP was normal.5. In two out of three samples of platelets from parkinsonian subjects, traces of a dopamine-like substance were detected, but this finding requires confirmation.6. If the platelet is a valid model for dopaminergic brain neurones, then the results described would suggest that dopamine uptake and storage may be abnormal in brain neurones in Parkinson's disease.