Immunocytochemical study on the association between alpha-granule membrane proteins and adhesive proteins during platelet activation.

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We employed a double-staining method to immunocytochemically study the role of the alpha-granule membrane GPIIb/IIIa complex in the expression of intragranular fibrinogen (Fbg) onto the surface of thrombin-activated human platelets. In the resting platelets, GPIIb/IIIa was distributed evenly on the alpha-granule membrane as well as on the surface membrane including the open canalicular system (OCS), while Fbg was exclusively detected in the granule. At 30 sec after activation by 0.1 U/ml of thrombin under the non-stirring condition, Fbg was redistributed in association with the granule membrane GPIIb/IIIa. At 1 to 5 minutes, the Fbg associating with GPIIb/IIIa became increasingly expressed on the surface membrane. Nevertheless, abundant Fbg was still remaining in the lumen of the swollen OCS, the membrane of which had been already depleted of GPIIb/IIIa. Unlike Fbg, most of the abundant albumin in the alpha-granule was released into the surrounding medium without association to the membrane of either the alpha-granule or the surface. These results suggest that a small portion of the intragranular Fbg binds to Fbg receptors expressed on the activated GPIIb/IIIa on the alpha-granule membrane, resulting in the formation of the GPIIb/IIIa-Fbg complex, which is then moved through the OCS membrane and expressed on the surface membrane. Thus, alpha-granule membrane GPIIb/IIIa acts as a carrier of intragranular Fbg.