Subcellular localization of platelet factor 3 and platelet factor 4

Influence of platelet factor 4 on the neutralization of heparin by protamine

In vitro, PF4 is comparable to protamine sulfate in the neutralization of heparin, but the complexes formed with heparin are different. Even with an excess of PF4, no large PF4-heparin complexes are formed and none of the complexes are able to activate ATIII, nor do these complexes dissociate on incubation in plasma at 37 degrees C. The action of PF4 and protamine is complementary. However, excess protamine displaces PF4 or prevents its complexing with heparin. When excess protamine is used to neutralize heparin in the presence of PF4, large heparin-protamine complexes are formed incorporating PF4. In contrast to the heparin-protamine complexes formed without PF4, these do not activate ATIII nor do they dissociate on incubation. Since PF4 is liberated during ECB procedures, its contribution to the stability of heparin-protamine complexes in vivo may influence the amount of protamine needed to neutralize heparin as well as affect the reactions which have been reported on injection of protamine after ECB.

Immunological localisation of beta-thromboglobulin and platelet factor 4 in human megakaryocytes and platelets

Beta-thromboglobulin (beta TG) and platelet factor 4 (PF4) have been localised in megakaryocytes and platelets using immunofluorescence and immunoperoxidase techniques. These studies support the concept of synthesis of the proteins by parent megakaryocytes. By immunoelectron microscopy both proteins have been visualised in the alpha granule of the platelet and megakaryocyte, supporting functional studies of the cytoplasmic localisation of these proteins. The light microscopic techniques may allow elucidation of the distribution and role of the megakaryocyte in the pulmonary circulation and, on a practical level, permit its identification and distinction from other multinucleate cells in extramedullary tissue.

Activation of platelets by platelet activating factor (PAF) derived from IgE-sensitized basophils. IV. PAF does not activate platelet factor 3 (PF3)

Platelet activating factor (PAF) derived from antigen-stimulated, IgE-sensitized rabbit basophils acts on platelets to induce aggregation and secretion of their content of granule-bound vasoactive amines. Despite this, PAF did not activate platelet factor 3. In contrast, collagen induced aggregation, secretion and PF3 activation in the washed platelets. Other stimuli (ADP, C3b, thrombin) also initiated both secretion and PF3 activation. A wide dose range of PAC, including those giving maximal secretion and aggregation, were ineffictive in making PF3 available and the possibility that PAF inhibited PF3, or its generation, was also excluded. It is concluded that PAF is a unique stimulus for platelets and that secretion and aggregation are not necessarily accompanied by PF3 generation.

Subcellular localization of the heparin-neutralizing factor in blood platelets

1. The distribution of the heparin-neutralizing factor (platelet factor 4, PF4) in subcellular organelles of blood platelets of rabbits and man was investigated. 2. In both species the organelles storing 5-hydroxytryptamine (5-HT storage organelles) contained only trivial amounts of PF4. 3. In contrast, the content of PF4 was highest in the subcellular fractions rich in alpha-granules. 4. In conclusion, PF4 is probably localized in the alpha-granules and therefore the platelets contain at least two types of organelles (5-HT organelles and alpha-granules) capable of releasing their contents in response to the same stimuli, such as exposure to collagen, thrombin, etc.

Distribution of fibrinogen, and platelet factors 4 and XIII in subcellular fractions of human platelets

The distribution of fibrinogen, platelet factor 4 (heparin-neutralizing factor) and factor XIII amongst subcellular fractions of human platelets was determined. It was found that fibrinogen and platelet factor 4 peaked sharply in the region of the density gradient previously shown to be heavily enriched with alpha-granules. By contrast, factor XIII, fibrin-stabilizing factor, was found exclusively in the supernatant.