Recombinant thrombopoietin has been reported to stimulate megakaryocytopoiesis and thrombopoiesis and it may be quite useful to treat patients with low platelet counts after chemotherapy. As little is known regarding the possible activation of platelets by thrombopoietin, we examined the effects of thrombopoietin on platelet aggregation induced by shear stress and various agonists in native plasma. Using hirudin as an anticoagulant, thrombopoietin (1 to 100 ng/mL) enhanced platelet aggregation induced by 2 micromol/L adenosine- diphosphate (ADP) in a dose dependent fashion. The enhancement was not affected by treatment of platelets with 1 mmol/L aspirin plus SQ-29548 (a thromboxane antagonist, 1 micromol/L) but was inhibited by a soluble form of the thrombopoietin receptor, suggesting that the enhancement was mediated by the specific receptors and does not require thromboxane production. Epinephrine (1 micromol/L), which does not induce platelet aggregation in hirudin platelet rich plasma (PRP), did so in the presence of thrombopoietin (10 ng/mL). Thrombopoietin (10 ng/mL) also enhanced or primed platelet aggregation induced by collagen (0.5 micron.mL),. thrombin, serotonin, and vasopressin. Thrombopoietin does not induce any rise in cytosolic ionized calcium concentration nor activation of protein kinase C, as estimated by phosphorylation of preckstrin, indicating that the priming effects of thrombopoietin does not require those processes. The ADP- or thrombin-induced rise in cytosolic ionized calcium concentration was not enhanced by thrombopoietin (100 ng/mL). Further, shear (ca. 90 dyn/cm2)-induced platelet aggregation was also potentiated by thrombopoietin. The priming effect on epinephrine-induced platelet aggregation in hirudin PRP was unique to thrombopoietin, with no effects seen using interleukin-6 (IL-6), IL-11, IL-3, erythropoietin, granulocyte-colony stimulating factor, granulocyte macrophage-colony stimulating factor, or c-kit ligand. These data indicate that monitoring of platelet functions may be necessary in the clinical trials of thrombopoietin.
SummaryParticle-counting methods that employ light scattering (LS) quantify changes in the number of platelet aggregates of different sizes after the application of an aggregating stimulus. Using the LS method, we studied the effects of aggregant concentration, aspirin administration, and ticlopidine administration on aggregate formation and compared the results with those obtained using the conventional optical density (OD) method. Subjects were 47 controls, 31 patients treated with aspirin (330 mg/day), and 37 patients treated with ticlopidine (200 mg/day). Platelet aggregation after stimulation by 0.5, 1.0 or 5.0 μM ADP, or 0.5, 1.0 or 2.0 μg/ml collagen was determined using both methods. Using the LS method, small (9-25 μm), medium (25-50 μm), and large (50-70 μm) aggregates were counted. In patients untreated with antiplatelet medication, greater concentrations of ADP or collagen generated larger aggregates. Generation of small and medium-sized aggregates showed a significant positive correlation with OD levels after stimulation with 0.5 or 1.0 μM ADP, or 0.5 or 1.0 μg/ml collagen. In patients treated with aspirin, the development of small aggregates into large aggregates was inhibited. Thus, the number of small aggregates increased. Inhibition induced by aspirin was more effective against aggregation after stimulation with collagen than with ADP. In patients treated with ticlopidine, small and medium-sized aggregate formation was inhibited after stimulation with low concentrations of ADP or collagen, but was promoted after stimulation with high aggregant concentrations. The capability of the LS method to quantify different sizes of aggregates after stimulation with low concentration agonists may facilitate investigation of the aggregation process, and of how this process is affected by antiplatelet agents.
SUMMARYThis study was performed to elucidate mechanisms responsible for the contraction of isolated canine cerebral arteries induced by uridine 5'-triphosphate (UTP) and to ascertain whether UTP given intracisternally causes cerebral arterial constriction. The latter was proven arteriographically to be the case. In vitro, UTP (lO^M) and UDP were similar in potency, produced sustained contractions, and were more effective than other pyrimidine nucleotides or uridine. Unlike serotonin (5-HT), UTP was not antagonized by cinanserin and failed to cause constriction of mesenteric arteries. Adenosine similarly antagonized 5-HTand UTP. The Ca 2+ antagonist nimodipine abolished contractions caused by high K + but only incompletely antagonized 5-HT or UTP. On the other hand, procedures that hyperpolarize the cell membrane (low K + followed by K + ) abolished tonic contractions induced by UTP. Hyperpolarization prior to UTP (with or without nimodipine) did not, however, prevent the occurrence of a phasic contraction. Papaverine or lanthanum antagonized this phasic response. It was concluded that UTP selectively affects cerebral arteries, may initiate contraction by releasing membrane bound Ca 2+ , depolarizes the cell membrane to open receptor operated and potential sensitive calcium channels, but does not inhibit the electrogenie Na-pump nor specifically antagonize the vasodilator adenosine.
A novel system has been developed to evaluate biochemically induced blood platelet aggregation by means of a particle counting technique that uses light scattering. Variations in the particle size and concentration were measured by detecting the scattered light intensity and the number of particles, respectively, passing through a laser beam. The system is capable of performing sensitive, in situ continuous measurements of platelet aggregation by counting and sizing of aggregates.-
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