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The effects of Lys-plasminoge on platelet α-granule secretion were studied. The level of P-selectin exposed on the surface of plasma membranes of washed human platelets was measured by flow cytometry as a market of α-granule secretion. It was shown that Lys-plasminogen facilitates partial release of α-granules, but impedes thrombin-induced platelet exocytosis. It is suggested that Lys-plasminogen may affect platelet secretion rather through interaction of its non-catalytic (kringle) domains with membrane receptors than due to contaminating plasmin activity. In contrast to Lys-form, native proenzyme (Glu-plasminogen) had no effects on α-granule releasing. Here, we provide the first experimental demonstration that Lys-form of plasminogen is able to modulate platelet α-granule secretion, and this effect can be considered as one of the plausible mechanisms of its anti-aggregating activity.

The effects of several forms of plasminogen on the state of actin cytoskeleton of human platelets were studied. A ratio between various actin pools, which were detected by immunoblotting, was taken as indicator of platelet cytoskeleton reorganization. It was revealed that intact platelets contain globular (G) actin and membrane cortex (MC) actin in amounts that are 56 and 40% of filamentous (F) actin level, respectively. In both thrombin- and collagen-activated platelets, actin is almost entirely presented in F-form. Incubation of resting platelets with Lys-plasminogen causes elevation of MC-actin level up to 79% in respect to F-form content. In addition, Lys-plasminogen inhibits reorganization of actin cytoskeleton typical for activated platelets. In contrast to Lys-form, Glu-plasminogen affects neither platelet aggregation nor redistribution of actin pools. Thus, these data indicate that cytoskeletal structures of platelets are involved in realization of anti-aggregating effects of Lys-plasminogen.