Differential Effects of Cytochalasin B on Platelet Release, Aggregation and Contractility: Evidence against a Contractile Mechanism for the Release of Platelet Granular Contents

Abstract

SummaryCytochalasin B alters the structure and functional properties of filamentous actin. Platelet-mediated clot retraction in dilute platelet-rich plasma (PRP) is inhibited progressively at cytochalasin B concentrations of 0.01 mg/ml, 0.05 mg/ml and 0.1 mg/ml. Dynamic rheological measurements of recalcified PRP in a Weissenberg Rheogoniometer indicate that platelet contractility (as reflected in measurements of elastic moduli) is reduced by 33%, 57% and 63% at cytochalasin B concentrations of 0.01, 0.05 and 0.1 mg/ml, respectively.In contrast, pre-incubation of human platelet-rich plasma (PRP) with 0.01 mg/ml or 0.05 mg/ml cytochalasin B does not inhibit collagen-induced [14C-]serotonin release or collagen-induced-platelet aggregation, which is dependent on the release of ADP from platelet dense granules. Even at a cytochalasin B concentration of 0.1 mg/ml, collagen-induced [14C-]serotonin release and aggregation are impaired only moderately. Cytochalasin B does not interfere with the uptake by platelets of [14C-]-serotonin, or with the kinetics and extent of clot formation in platelet-free plasma.Thus, concentrations of cytochalasin B which impair platelet contractility do not inhibit the release of platelet dense granule contents. It is concluded that neither the platelet release reaction nor platelet aggregation is dependent on platelet contractile mechanisms.

Bruton's tyrosine kinase associates with the actin‐based cytoskeleton in activated platelets

“…Preincubation of platelets with cytochalasin B (20 mM) prevented thrombin-induced actin polymerization but had no effect on platelet aggregation, as earlier reported [Kirkpatrick et al, 1980]. Cytochalasin B treatment completely inhibited the cytoskeletal association of Btk, indicating that actin polymerization is a prerequisite for this process (Fig.…”

Loss of pleckstrin defines a novel pathway for PKC-mediated exocytosis

“…For example, blocking actin polymerization with cytochalasin enhances the release of plateletdense granules, [33][34][35] suggesting that actin contributes to the regulation of platelet exocytosis, which is analogous to the contribution of actin to neuronal cell exocytosis. 47 Other indications that the cytoskeleton has a role in platelet exocytosis include the observation that preventing myosin light chain phosphorylation impairs platelet secretion 48,49 and that by adding recombinant scinderin (a calcium-dependent F-actin severing protein) to permeabilized platelets enhances dense granule exocytosis.…”

“…[33][34][35] Heterologous expression studies show that pleckstrin can reorganize the actin cytoskeleton. 27,28,36 Because the available evidence suggests that disassembly of F-actin facilitates exocytosis in platelets, it is possible that pleckstrin indirectly links PKC with exocytosis by facilitating F-actin reorganization.…”

The actin cytoskeleton differentially regulates platelet α-granule and dense-granule secretion

“…9 Subsequent studies using similar conditions, however, demonstrated little effect of cytochalasin on collagen-induced dense granule secretion. 10 Studies using other agonists such as thrombin, phorbol esters, calcium ionophore, or adenosine diphosphate (ADP) to study the effect of cytochalasins on dense granule secretion have demonstrated either inhibition or augmentation. [11][12][13] A study evaluating ␣-granule secretion showed no effect of cytochalasin E on thrombin-induced ␣-granule secretion.…”