The state of platelet actin was determined using a fluorescent method of DNAse I inhibition. Activation of human platelets resulted in mobilization of DNAse-available actin. When pletelets were activated with ADP the change in the state of actin was gradual and preceded the secretion. When thrombin was used as an activator, a sharp and rapid decrease in the DNAse-available actin was observed which paralleled the secretion. Inhibition of ADP-induced aggregation (and secretion) by EDTA resulted in a decrease in the rate of change on actin. Inhibition of the thrombininduced aggregation (but not secretion) by EDTA did not affect the change in the state of actin.Blood platelets undergo dramatic changes during activation by various agents such as thrombin, ADP, epinenephrine and collagen. These changes include a rapid shape change from disc to sphere, formation of pseudopods, disassembly of the peripheral microtubules ring, migration of the dense bodies towards the center of the cell and, later on, secretion and aggregation [l].Actin accounts for 20-30% of total platelet proteins [2]. Early electron microscopic studies showed the presence of actin microfilaments in platelets and these filaments predominate in the pseudopodia, with a clearly defined polarity when the cells are activated. These filaments are also observed in the central area of the activated platelet. Cytochalasin B, known to interfere with microfilament integrity, inhibits several of the processes related to platelet activation [3 -51.These reports indicate that actin mobilization might be involved in the various processes occurring during platelet activation. However, the exact role of actin in platelet activation is not clearly understood. Recently, Carlsson et al. [6] reported that stimulation of platelets by thrombin resulted in a rapid decrease in DNAse-available actin. However, no correlation with serotonin release or aggregation was studied. It is, therefore, of great interest to study changes in the state of actin in human platelets and correlate them with the secretion and aggregation process, in the hope of finding some evidence for the contribution of the state of actin to these processes. We report here the results of a study where human platelets either as platelet-rich plasma or as washed platelets were activated by thrombin or ADP and the degree of actin mobilization was measured. The state of actin was measured at four different points along the aggregation curve and correlated with aggregation, secretion, and other cellular processes preceding the release reaction. EXPERIMENTAL PROCEDURES Preparation of Platelet-Rich PlasmuFreshly collected human blood anticoagulated with 3.2 sodium citrate (1 ml for 9 ml blood) was centrifuged at 120 x g for 13 min at room temperature. The platelet-rich plasma was no}-ethanesulfonic acid.separated from the red blood cells using a siliconized pasteur pipette and used for ADP activation. Preparation of Washed PlateletsPlatelet-rich plasma obtained from 60 ml blood as described above was incubated with...
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