Ion mechanism of isoproterenol on delayed afterdepolarization and triggered activity in the infarcted ventricle

Objectives This study aimed at investigating the cellular mechanism of isoproterenol （ISO） on delayed afterdepolarizations （DADs） and triggered activity （TA） of the noninfarcted myocardium in the myocardial infarcted rabbit model.Methods Rabbits with the left anterior descending coronary artery occlusion were prepared and recovered for 8 wk （healed myocardial infarction, HMI）. Myocytes were isolated from regions of the noninfarcted left ventricular free wall. ISO was added to cellular surface by perfusion way. Action potentials and ion currents were recorded with whole-cell patch clamp. Results The results showed that treatment with ISO induced more DADs and TA events in HMI myocytes. Iti and IC,_L of myocytes treated with ISO were increased significantly compared with HMI cells, which contributed to DADs-related triggered arrhythmia. Conclusions The results suggested that more arrhythmia events of DADs and TA developed in myocytes with ISO treatment. The underlying mechanism was associated with the augment of I6 and calcium influxing     

Ion mechanism of isoproterenol on delayed afterdepolarization and triggered activity in the infarcted ventricle

Objectives This study aimed at investigating the cellular mechanism of isoproterenol (ISO) on delayed afterdepolarizations (DADs) and triggered activity (TA) of the noninfarcted myocardium in the myocardial infarcted rabbit model.Methods Rabbits with the left anterior descending coronary artery occlusion were prepared and recovered for 8 wk (healed myocardial infarction, HMI). Myocytes were isolated from regions of the noninfarcted left ventricular free wall. ISO was added to cellular surface by perfusion way. Action potentials and ion currents were recorded with whole-cell patch clamp. Results The results showed that treatment with ISO induced more DADs and TA events in HMI myocytes. Iti and ICa-L of myocytes treated with ISO were increased significantly compared with HMI cells, which contributed to DADs-related triggered arrhythmia. Conclusions The results suggested that more arrhythmia events of DADs and TA developed in myocytes with ISO treatment. The underlying mechanism was associated with the augment of Iti and calcium influxing (J Geriatr Cardiol 2010; 7:180-183).     

大蒜素对大鼠心力衰竭模型心室肌细胞迟后除极和触发活动的作用

目的观察大蒜素对大鼠心力衰竭模型细胞迟后除极(DAD)、触发活动(TA)和离子流的作用及机制。方法选取20只SD大鼠利用腹主动脉结扎法制备大鼠心力衰竭模型(模型组),另选取10只SD大鼠制作假手术组作为对照。采用酶解法急性分离2组大鼠心室肌细胞。根据实验设计对模型组是否给以大蒜素干预分为心力衰竭组(未给药)及给药组(200.0μmol/L大蒜素)。膜片钳技术观察心力衰竭组、给药组及对照组细胞DAD和TA的发生,并记录相关离子流(I_(ti),I_(Ca,L),I_(NCX))的改变,以Flou-4AM荧光技术检测细胞内钙离子浓度变化。采用SPSS 17.0软件进行统计学分析。组间比较采用ANOVA分析及SNK-q检验。结果 (1)对照组、心力衰竭组及给药组细胞上DAD和TA发生率依次为[13.3%(2/15),0.0%(0/15)]、[60.0%(9/15),26.7%(4/15)]和[40.0%(6/15),13.3%(2/15)]。与对照组比较,心力衰竭组细胞DAD和TA发生率显著增加;与心力衰竭组比较,给药组细胞上DAD和TA发生率显著下降,下降幅度分别为20.0%和13.4%,差异均有统计学意义(n=15,P0.01)。(2)与对照组比较,心力衰竭组心室肌细胞上I_(Ca,L)、I_(ti)和I_(NCX)电流密度均增加,而与心力衰竭组比较,给药组上述电流均减少,其中I_(ti)内向峰电流密度从(-1.05±0.06)pA/pF降至(-0.53±0.05)pA/pF(n=10,P0.01),I_(NCX)内向电流密度从(-5.8±0.7)pA/pF降至(-4.2±0.4)pA/pF(n=10,P0.01)。同时,给药组通过大蒜素加速通道电流的稳态失活过程,降低I_(Ca,L),I_(Ca,L)的峰电流密度由(-17.2±0.9)pA/pF降为(-13.5±1.0)pA/pF(n=15,P0.01)。(3)与对照组比较,心力衰竭组细胞静息态钙浓度、钙瞬变幅度和胞内钙最高50%的时间等参数显著升高;与心力衰竭组比较,上述参数在给药组细胞中均显著降低(n=15,P0.01)。心力衰竭组细胞内钙瞬变衰减率与对照组比较显著下降,而应用大蒜素后,得到显著恢复(P0.05)。结论大蒜素可能通过减少心力衰竭大鼠心室肌细胞内的钙离子,阻滞I_(ti)和I_(NCX)电流,从而降低心力衰竭后DAD和TA的发生。     

Arrhythmic activity in reoxygenated guinea pig papillary muscles and ventricular cells

Aftercontractions, delayed afterdepolarizations, and automaticity occurred in guinea pig papillary muscles that were reoxygenated after hypoxic conditioning. The emergence of dysfunction was dependent on the severity of hypoxic conditioning and on stimulation during reoxygenation. After 60 minutes of substrate-free hypoxia, reoxygenation induced automaticity in a high proportion of stimulated muscles; the automaticity appeared within 1 minute and lasted for 10-20 minutes. After similar conditioning, muscles reoxygenated for 7-15 minutes were stimulated at various cycle lengths. The incidence of automaticity and the amplitudes of delayed events had W-shaped dependencies on cycle length (200-1,000 msec), whereas coupling intervals had M-shaped dependencies. In ventricular myocytes that displayed automaticity after reoxygenation, extrasystolic upstrokes arose smoothly from delayed afterdepolarizations that reached threshold. In tissue, extrasystolic upstrokes usually rose sharply from delayed afterdepolarizations that were distinctly subthreshold. Thus, threshold was reached elsewhere in the tissue. Further evidence of electrical heterogeneity was obtained from surface mapping of delayed-afterdepolarization amplitude in reoxygenated muscle. There were no detectable aftercontractions, delayed afterdepolarizations, or signs of automaticity in quiescent reoxygenated muscles or in stimulated reoxygenated muscles that were treated with 1 microM ryanodine. We conclude that the dysfunction precipitated by reoxygenation is due to synchronized spontaneous releases of calcium from overloaded sarcoplasmic reticulum.     

Early afterdepolarizations and triggered activity induced by cocaine. A possible mechanism of cocaine arrhythmogenesis.

BACKGROUND. Cocaine may produce life-threatening cardiac arrhythmias, but it is not clear whether this is an indirect effect of coronary vasoconstriction and ischemia or a direct myocardial effect of the substance. Except for its effects on the Na+ current as a local anesthetic, little is known about the direct electrophysiological actions on cardiac cells. Therefore, we studied the effects of cocaine on action potentials and membrane currents in isolated feline ventricular myocytes to test the hypothesis that cocaine-induced arrhythmogenesis may be based on cellular and ionic mechanisms. METHODS AND RESULTS. Action potentials and membrane currents were recorded using the patch clamp technique. Single cells were isolated from feline left ventricles by enzymatic digestion. Exposure to cocaine (10 or 50 microM) depressed the plateau phase of the action potential and prolonged action potential duration. Action potential duration measured at 90% repolarization (APD90) was increased from 280 +/- 12 msec to 325 +/- 17 msec (p less than 0.01) by 5-minute exposure to 10 mumol cocaine, when the cells were stimulated at 1 Hz. During exposure to 50 mumol cocaine, APD90 was markedly increased from 298 +/- 13 msec to 437 +/- 35 msec (p less than 0.01) in seven of 16 cells, and early afterdepolarizations (EADs) developed in these cells. The take-off potential and the amplitude of EADs were -28.3 +/- 2.3 mV and 16.8 +/- 1.2 mV, respectively. Triggered activity arising from EADs was induced in four of the seven cells. Addition of 1 nmol isoproterenol augmented EADs and induced sustained triggered activity, whereas they were suppressed by exposure to 2 microM verapamil. Whole-cell voltage clamp experiments revealed that cocaine (50 microM) reduced the peak L-type Ca2+ current from 1.03 +/- 0.13 nA to 0.79 +/- 0.11 nA (23% reduction, p less than 0.05). Cocaine also reduced the peak delayed rectifier K+ current from 362 +/- 51 pA to 113 +/- 32 pA (69% reduction, p less than 0.01). However, cocaine did not affect activation and inactivation kinetics of these channels. Cocaine had no effect on the inward rectifier K+ current. CONCLUSIONS. We conclude that cocaine can prolong action potential duration and induce EADs and triggered activity by blocking the delayed rectifier K+ current, and that cocaine-induced abnormalities of repolarization, modulated by its inhibitory effects on catecholamine reuptake, may play a role in the potential of cocaine for induction of acute fatal arrhythmias.     

Induction of delayed afterdepolarizations and triggered activity in canine Purkinje fibers by lysophosphoglycerides

Lysophosphoglycerides accumulate in ischemic myocardium and induce electrophysiologic alterations in normoxic tissue in vitro closely resembling those seen with ischemia in vivo. Delayed afterdepolarizations and triggered activity may be particularly important in the pathogenesis of arrhythmias in the ischemic heart. The present study was performed to determine whether lysophosphatidylcholine (LPC), at concentrations comparable to those present in ischemic myocardium, can induce delayed afterdepolarizations and/or triggered activity in normoxic canine Purkinje fibers. In the present study, as little as 75 microM LPC was found to induce delayed afterdepolarizations and as little as 100 microM LPC was found to induce delayed afterdepolarizations and triggered activity even at low cycle lengths. The amplitude of the induced delayed afterdepolarizations was enhanced by augmentation of the extracellular concentration of calcium (7 mM) or by exogenous epinephrine (10(-9) to 10(-6) M). The amplitude was decreased by verapamil (1 mg/l) or Mn++ (2.5 mM). Epinephrine at a concentration of 10(-6) M also initiated triggered activity in Purkinje fibers exposed to LPC (75 microM), a response blocked by l-propranolol (2 X 10(-7) M and 10(-6) M) but not by the alpha 1-adrenergic blocking agent BE-2254 (10(-6) M). Delayed afterdepolarizations induced by LPC (75 microM) and epinephrine (10(-6) M) persisted even in the presence of acidosis (pH 6.7) and hyperkalemia ([K+]o = 7 mM). Thus, delayed afterdepolarizations and triggered activity induced by LPC may contribute to the induction and/or maintenance of arrhythmias early after the onset of myocardial ischemia. However, because of the reversal of these effects after superfusion with media devoid of LPC, they may occur with ischemia in vivo but not be seen in tissue isolated from ischemic regions and evaluated in vitro.     

Histamine and phenyldiguanide induced tachypnea in hypercapnia and hypoxia.

The rapid shallow breathing of pulmonary vagal origin following administration of histamine (H) and phenyldiguanide (PDG) was studied at different levels of hypercapnic and hypoxic stimulation. At all levels of chemical drive H and PDG caused an excitatory effect on timing of breathing and an inhibitory effect on the respiratory output. The latter was evaluated from the mean inspiratory flow rate and the mean rate of change of pressure developed in the lungs during an inspiratory effort against closed airways. The timing effect was greater at low than at high PaCO2 while the opposite was true for the output effect. At all PaCO2, H and PDG decreased the volume-threshold for termination of inspiration (leftward displacement of the VT vs TI relationship) of the VT Vs TI relationship). Hypoxia increased the respiratory output in control as much as with drug stimulation. Moreover, hypoxia did not affect the volume-threshold curve both in control and with H and PDG. We concluded that vagal afferents stimulated by H and PDG (irritant and/or J receptors) interfere with the timing and output response to central chemoreceptors stimulation (CO2 sensitivity) without affecting the response to peripheral chemoreceptors stimulation (mainly hypoxic chemosensitivity).     

Role of nitric oxide and platelet-activating factor in cardiac alterations induced by tumor necrosis factor-alpha in the guinea-pig papillary muscle.

OBJECTIVE: Tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine with negative inotropic properties, is implicated in several pathophysiological events. To clarify the mechanism of action of TNF-alpha on myocardium, we investigated the possible role of platelet-activating factor (PAF) and nitric oxide (NO) as secondary mediators of the depressant effect of this cytokine. METHODS: Isometric twitches and intracellular action potentials were recorded from guinea pig papillary muscles. The effects of TNF-alpha (1-10 ng/ml) were studied in controlled conditions and after treatment with 0.5% Triton X-100, to destroy the endocardial endothelium NG-nitro-L-arginine methyl ester (L-NAME), D-NAME (1 mM) and the two different PAF-receptor antagonists WEB 2170 (3 microM) and CV 3988 (5 microM) were used to study the role of NO and PAF in cardiac depression induced by TNF-alpha. To study the role of NO in cardiac alterations induced by PAF, papillary muscles were pretreated with L-NAME or D-NAME and then challenged with PAF (0.1-1 microM). Nitrite production by papillary muscles challenged with TNF-alpha alone. TNF-alpha in the presence of WEB 2170 or CV 3988, or PAF was studied with the Greiss reagent method. PAF production by papillary muscles stimulated by TNF-alpha was studied by a bioassay method. RESULTS: TNF-alpha induced an initial, transient positive inotropic effect, then reduced the contractility and the action potential duration in a concentration-dependent manner. Treatment of papillary muscle with Triton X-100 did not modify the response to TNF-alpha, suggesting that the effect of TNF-alpha is not mediated by endocardial endothelial cells. Pretreatment with indomethacin reduced the negative effect of TNF-alpha, while propranolol abolished the initial increase of contractility. The role of PAF and NO as mediators of TNF-alpha was suggested by: (1) the protective effect of L-NAME, but not of D-NAME, on electrical and mechanical alterations; (2) the stimulatory effect of TNF-alpha on nitrite production; (3) the inhibitory effect of WEB 2170 and CV 3988, on both the electromechanical alterations and the nitrite production; (4) the synthesis of PAF induced by TNF-alpha. L-NAME blocked the negative effect of PAF and PAF enhanced nitrite production by papillary muscle. CONCLUSIONS: The present results suggest that in cardiac muscle: (1) the release of PAF triggered by TNF-alpha may account for the stimulation of NO production; (2) both PAF and NO contribute to the development of the electrical and mechanical alterations induced by TNF-alpha; (3) NO production was down-stream to the synthesis of PAF.     

Shear wave elastography of the scalene muscles in healthy adults: A preliminary study

The aim of the study is to evaluate the reliability of shear wave elastography to assess the anterior and middle scalene muscles in healthy adult subjects.The study included 60 scalene muscles in 15 healthy subjects. High-resolution ultrasound and shear wave elastography were used to evaluate the anterior scalene and the middle scalene muscles. Stiffness values were measured.The mean shear elastic modulus showed the following values, right anterior scalene muscle 18.83 ± 5.32 kPa, left anterior scalene muscle 21.71 ± 4.8 kPa, right middle scalene muscle 12.84 ± 5.2 kPa, left middle scalene muscle 19.76 ± 5.30 kPa. Positive correlation was noted between the left middle scalene muscle and body mass index (P = .004). No difference in elasticity was noted between the right and left anterior scalene muscles; however, significant difference was noted between the right and left middle scalene muscles (P = .002).The results obtained in our study could be a reference point for future research considering different scalene muscle pathologies.     

Alpha 1-adrenoceptor regulation of delayed afterdepolarizations and triggered activity in subendocardial Purkinje fibers surviving 1 day of myocardial infarction.

Alpha 1-adrenoceptor agonists were shown to induce delayed afterdepolarizations (DADs) and triggered activity in the presence of elevated extracellular Ca2+. We investigated the effects of alpha 1-adrenoceptor stimulation on DADs and triggered activity in canine Purkinje fibers that survived 1-day of myocardial infarction. Endocardial preparations were studied using standard microelectrode techniques. In quiescent preparations showing no DADs and in presence of propranolol (2 x 10(-7) M), phenylephrine (10(-6) M), an alpha 1-adrenoceptor agonist induced DADs (n = 6) and differentially induced triggered activity in ischemic but not in normal Purkinje fibers (n = 4). In 8 preparations that showed subthreshold DADs, phenylephrine increased the DAD amplitude from 4.0 +/- 2.5 mV to 8.0 +/- 3.3 mV (P less than 0.03) and from 3.2 +/- 1.5 mV to 6.5 +/- 3.7 mV (P less than 0.05) at paced cycle lengths of 800 and 400 ms, respectively. Phenylephrine caused subthreshold DADs to reach threshold and result in triggered activity (n = 6). The effects of phenylephrine were abolished by 10(-6) M prazosin, an alpha 1-adrenoceptor blocker. Our results suggest that alpha 1-adrenoceptor stimulation regulates DADs and triggered activity seen in subendocardial Purkinje fibers surviving 1 day of myocardial infarction and may contribute to the spontaneous ventricular tachycardia seen in vivo at this stage.     

Triggered activity induced by combined mild hypoxia and acidosis in guinea-pig Purkinje fibers

The effects of prolonged exposure to combined mild hypoxia (Po2 230 +/- 20 mmHg) and acidosis (pH: 6.8 +/- 0.05) were studied in guinea-pig left ventricular myocardium superfused in vitro. Only Purkinje fibers were impaled by microelectrodes. Triggered activity developed in depolarized Purkinje fibers after 48 +/- 9 min of exposure to hypoxic and acid conditions and was initiated either by short periods of rapid electrical driving or by the background slow Purkinje automaticity. Triggered activity occurred when a delayed afterdepolarization attained its threshold potential and terminated after a subthreshold afterdepolarization. Interaction between triggered activity and slow background automaticity was observed until 90 to 180 min of exposure to hypoxic and acid conditions. These effects were reversed by replacement in standard conditions (Po2 510 +/- 20 mmHg; pH 7.35 +/- 0.05). Norepinephrine (1 X 10(-6)M) significantly accelerated the rate of discharge of triggered foci and led to a stable sustained triggered activity. Increasing extracellular Ca2+ concentration aggravated the effects of combined mild hypoxia and acidosis and led to the occurrence of early afterdepolarizations initiating triggered activity. In addition abnormal automaticity developed in quiescent fibers without any triggering action potential. Lidocaine and verapamil suppressed the triggered activity following a subthreshold afterdepolarization. Their effects were reversed on wash-out. It is concluded that prolonged exposure to combined mild hypoxia and acidosis induces triggered activity by a basic mechanism common to other situations leading to a calcium overload and showing such behaviour.     

黄芩苷诱导Burkitt淋巴瘤细胞凋亡的初步研究

目的:探讨黄芩苷(baicalin)对Burkitt淋巴瘤细胞系Daudi细胞凋亡的诱导作用。方法:用3-(4,5-二甲基噻唑)-2,5-二苯基四氮唑溴盐(MTT)法测定细胞生长抑制率;细胞涂片观察细胞形态学改变;流式细胞术检测细胞凋亡变化;Westernblot法检测凋亡相关蛋白天冬氨酸特异性半胱氨酸蛋白酶(caspase-3)、caspase-9。结果:黄芩苷对Daudi细胞作用72h的半数抑制浓度(IC50)为(10.1±0.5)μg/mL,对其生长抑制作用呈剂量依赖性。与对照组相比,黄芩苷组细胞呈现明显的凋亡状态。48h流式检测细胞凋亡率40μg/mL药物组为61.5%±6.3%,20μg/mL药物组为44.2%±4.7%,明显高于对照组(8.2%±0.9%)(P<0.01)。黄芩苷作用Daudi细胞24、36h后,caspase家族的蛋白前体(procaspase)-3、9表达均下降,而激活的活性片段caspase-3则表达上调,且呈时间依赖性。结论:黄芩苷可有效抑制Burkitt淋巴瘤细胞生长,诱导细胞凋亡,上述凋亡蛋白可能参与了黄芩苷诱导Daudi细胞凋亡的过程。     

Abnormal post-heparin lipolytic activity in obesity. A preliminary note.

Published data have suggested that hypertriglyceridemia in obesity may result from the combination of hepatic overproduction and diminished removal of triglyceride-rich lipoproteins. Diminished catabolism might be expected if tissue lipoprotein lipase activity were decreased, a finding which has been reported in biopsies of adipose tissue from obese subjects. Abnormalities in heparin-released triglyceride lipase activity (PHLA) in obesity have not been reported, however. We have examined the possibility that methods for the measurement of PHLA might have failed to reveal such a defect because of the disproportionality between plasma volume and increasing body mass in obesity. Since it is usual to administer heparin on the basis of body weight, higher plasma heparin levels would be achieved in obese individuals. We performed standard PHLA assays in lean and obese volunteers. In the obese, heparin levels were consistently higher than in lean individuals although PHLA values were similar in both. Thus, PHLA in obesity appeared to be inappropriate for the heparin levels attained in plasma. Pharmacokinetic studies suggest that a decrease in PHLA available for release by heparin rather than heparin insensitivity underlies this phenomenon.     

Modulation of triggered activity by uncoupling in the ischemic border. A model study with phase 1b-like conditions

OBJECTIVE: Triggered beats during regional ischemia may depend upon the electrical source and sink charge interactions between adjacent regions of normal and ischemic cardiac tissue that are partly controlled by electrical coupling. METHODS: To study these relationships, we modified parameters in the Luo-Rudy dynamic membrane equations to reflect physiologic conditions associated with phase 1b arrhythmias. Superthreshold delayed afterdepolarizations (DADs) formed after pacing. Coupling contributions were then examined using: (i) a single phase 1b myocyte connected via a variable resistance to a single normal myocyte, and (ii) a multicellular fiber with a 1-cm segment of phase 1b myocytes connected to a 1-cm normal segment having resistance changes that were confined to the ischemic segment. Integration of ionic, capacitive and coupling currents during DAD initiation allowed charge quantification. RESULTS: In cell pairs, phase 1b myocyte DADs were suppressed at resistances where normal myocyte pacing resulted in phase 1b myocyte excitation. Coupling charge requirements limited capacitive charging in the phase 1b myocyte, which occurred in combination with diastolic hyperpolarization that shifted transmembrane potential from threshold. In multicellular fiber simulations, DADs were suppressed with strong coupling in the phase 1b segment. Moderate uncoupling of that segment allowed superthreshold DAD formation away from the border that initiated action potential propagation in the normal segment. With severe uncoupling, propagation failed at the border. CONCLUSIONS: These findings support the clinical and experimental observation that intermediate uncoupling is an important contributor to phase 1b arrhythmogenesis.     

Adenosine-sensitive afterdepolarizations and triggered activity in guinea pig ventricular myocytes.

This study examines the cellular basis and specificity of the effects of adenosine on early afterdepolarizations (EADs), delayed afterdepolarizations (DADs), and triggered activity (TA) induced by various drugs with different mechanisms of action. Membrane potential and currents were measured in isolated guinea pig ventricular myocytes. Adenosine (10-100 microM) significantly (p less than 0.05) reduced the amplitude of DADs and suppressed TA induced by isoproterenol (10-50 nM) and forskolin (1 microM) but not those induced by dibutyryl cAMP (1 microM), ouabain (1-5 microM), and 7.2 mM [Ca2+]o. Adenosine also abolished EADs and TA induced by isoproterenol. In contrast, adenosine failed to abolish EADs and TA induced by quinidine (3 microM) or those that occurred spontaneously (i.e., in the absence of drugs). Transient inward current (ITi) was induced on repolarization after 2-second-long single depolarizing voltage steps or after 12-second-long trains of 300-msec depolarizing pulses. Concomitant with the attenuation of DADs, adenosine suppressed ITi caused by isoproterenol and forskolin but not those induced by ouabain, dibutyryl cAMP, and elevated [Ca2+]o. The amplitude of ITi was dependent on the magnitude of the activating voltage step, but the suppression of ITi by adenosine was not. The selective A1-adenosine receptor antagonist N-0861 (9-methyladenine derivative) antagonized the effects of adenosine on afterdepolarizations, ITi, and TA. In myocytes from guinea pigs treated with pertussis toxin, adenosine failed to attenuate DADs and ITi or abolish TA induced by isoproterenol or forskolin. In parallel experiments, isoproterenol (10 nM) raised cellular cAMP from 5.7 +/- 0.2 to 8.1 +/- 0.1 pmol and the selective A1 receptor agonist cyclopentyladenosine (5 microM) reduced it to 6.5 +/- 0.2 pmol (p less than 0.05). Thus, adenosine specifically attenuates afterdepolarizations and abolishes TA by suppressing ITiS that are associated with stimulation of adenylate cyclase via a pertussis toxin-sensitive A1 receptor-mediated action. In conclusion, the response of TA to adenosine may identify a mechanism of afterdepolarization related to stimulation of adenylate cyclase.     

Ticlopidine activity on platelet function in patients with enhanced platelet aggregation. A short-term crossover study

In a single-blind crossover study, the effect of ticlopidine (250 mg t.i.d.) on platelet function was investigated in 16 patients, with enhanced platelet aggregation, before treatment, on the third and seventh day of treatment with the drug or the placebo. Bleeding time was significantly lengthened by ticlopidine administration. Platelet aggregation, both in vivo and in vitro, was significantly inhibited during the week on ticlopidine. Beta-thromboglobulin concentration was on an average significantly decreased. The inhibition by PGI2 of platelet aggregation by PGD2 was slightly but not significantly increased. Platelet TxB2 production after stimulation with thrombin was unchanged during ticlopidine administration, whereas conversion of exogenous arachidonic acid into TxB2 was slightly but significantly reduced. The present results confirm that ticlopidine acts in vivo as a powerful antiaggregating agent. The antiaggregating activity seems to be due to an interference of ticlopidine with platelet membrane and, as a consequence, with various platelet membrane receptors and activities.