Arachidonic acid and stable analogue of prostaglandin endoperoxides (U46619) induce platelet spreading and thrombi-like aggregate formation on a collagen substrate. Effect of fluid dynamics

Platelet apoptosis in stored platelet concentrates and other models

Apoptosis or programmed cell death was discovered in nucleate cells 30 years ago and has been well documented. In contrast, apoptosis in anucleate platelets has only a five-year research history and as yet but few publications related to it. In this review, we will present the data on platelet apoptosis in several models. These include in vitro models where platelet apoptosis was induced by calcium ionophores, natural platelet agonists, storage in capped tubes at 37 degrees C and storage at room temperature under standard blood banking conditions, and in vivo models where apoptosis was provoked by suppression of thrombopoiesis, malaria infection and injection of tumor necrosis factor or anti-platelet antibodies. Understanding of platelet apoptosis and its role in the platelet storage lesion is an exciting challenge; future research is likely to provide us with further insight into this field.

Growth cone collapse induced by semaphorin 3A requires 12/15-lipoxygenase

Detection of a repellent factor, such as a semaphorin (Sema), causes localized collapse of the growth cone and directs the neurite away from the repellent. Growth cone collapse results from concomitant cytoskeletal rearrangements and detachment of adhesion sites from the extracellular matrix, via mostly unknown signaling mechanisms. In cultures of dorsal root ganglion neurons, we found that Sema3A treatment stimulates the synthesis of the eicosanoid, 12(S)-hydroxyeicosatetraenoic acid (HETE), whereas Sema3A-induced growth cone collapse is prevented when 12(S)-HETE synthesis is blocked with an inhibitor of 12/15-lipoxygenase (LO). Exogenously applied product of 12/15-LO, 12(S)-HETE, mimics Sema3A-induced collapse. As observed by interference reflection and confocal microscopy, 12(S)-HETE causes the loss of growth cone adhesion sites. The adhesion site effect seems partially independent of the actin cytoskeleton because growth cones treated with Sema3A and 12/15-LO inhibitor remain spread despite actin cytoskeleton loss. These studies demonstrate that 12/15-LO activity is a necessary step in Sema3A collapse signaling in growth cones and suggest a mechanism for its action.

Effects of prostacyclin analogue (carbacyclin) on the interaction of platelets with different collagen substrates. Inhibition of cAMP increase by collagens type I, III, and IV

We investigated the effects of a stable prostacyclin analogue, carbacyclin, on the interaction of platelets with collagen substrates differing in their ability to activate platelets: human collagens type I, III, IV and V (CI, CIII, CIV and CV), and commercial calf skin collagen type I (CSC). The total adhesion was measured using 51Cr-labelled platelets, and quantitative morphometry of adherent platelets was performed by scanning electron microscopy (SEM). Carbacyclin in the concentrations inducing a 10-fold rise in platelet cAMP did not affect the adhesion of platelets to weak substrates, CV and CSC, but reduced the adhesion to strong substrates, CIV (by 49%) and CI/CIII (by 78%), which stimulated massive spreading and formation of surface-bound aggregates respectively. Carbacyclin inhibited all morphological manifestations of platelet activation associated with adhesion: conversion of native discoid platelets to spherical ones on CSC; massive spreading on CIV; and aggregate formation on CI/CIII. Massive spreading and aggregation on a weak substrate (CSC) stimulated by arachidonic acid and thrombin was also inhibited by carbacyclin. Under the same concentration of agonists aggregation of platelets was more sensitive to the action of carbacyclin, than spreading. Strong collagen substrates CI, CIII and CIV, but not CV and gelatin, inhibited the carbacyclin-induced rise in platelet cAMP.

Immunotargeting of erythrocyte-bound streptokinase provides local lysis of a fibrin clot

The creation of an anticollagen antibody-erythrocyte-streptokinase complex has been described. Immobilization of both proteins on erythrocyte membrane has been performed using an avidin-biotin interaction. Modification of streptokinase with (6-biotinylamido)hexanoic acid N-hydroxysuccinimide ester at the concentration of 1.1 mM (20% modification of protein amino groups) provides effective (up to 90%) attachment of streptokinase to an avidin-carrying erythrocyte surface. The loss of streptokinase activity due to modification under these conditions is not significant. The maximal attachment of streptokinase was equal to about 50 ng per 10(6) erythrocytes, i.e., about 5 X 10(5) molecules of streptokinase per erythrocyte. The presence of streptokinase in the incubation mixture inhibited the attachment of antibodies by about 50%. Nevertheless, co-immobilization of anticollagen antibody (1.0 X 10(5) molecules per cell) and streptokinase (2.8 X 10(5) molecules per cell) on the erythrocyte surface provided firm and specific binding of such erythrocytes to a collagen-coated surface (1.6 X 10(6) bound cells per 1 cm2 on a collagen-coated surface against 0.006 X 10(6) bound cells on a bovine serum albumin-coated surface). Targeting of such erythrocytes led to local lysis of a fibrin clot in the target zone. The properties described offer in principle the possibility of the application of this or a similar system of fibrinolytic agent targeting for the preventive therapy of rethrombosis during surgical manipulations on vessels.

Decrease of platelet aggregation and spreading via inhibition of the cAMP phosphodiesterase by trapidil

Trapidil (N,N-diethyl-5-methyl[1,2,4]triazolo[1,5-alpha]pyrimidine-7-amine ) inhibits platelet spreading and aggregation induced by arachidonic acid (AA), a stable analogue of prostaglandin (PG) endoperoxides (U46619), ADP, and low concentrations of thrombin, but not by A23187 and high concentrations of thrombin. Trapidil does not affect platelet adenylate cyclase but inhibits the cAMP PDE by approx. 50%. PDE inhibition proceeds via a competitive mechanism (Ki = 0.52 mM) and is not mediated by calmodulin inhibition. Trapidil does not change the platelet basal cAMP level but potentiates an increase of cAMP induced by the stable prostacyclin analogue (6 beta-PGI1). These results suggest that trapidil antiplatelet effects may be due to the inhibition of platelet PDE.