The fate of the open canalicular system in surface and suspension-activated platelets

We have examined the movement of fibrinogen-gold (fgn-Au) complexes in platelets activated in suspension and by surface contact. Fgn-Au probes did not react with resting cells but were bound to the external membrane of platelets in suspension 5 seconds after addition of 1 U/mL of thrombin. At intervals over a period of 5 to 20 minutes, fgn-Au probes moved from the cell surface to peripheral and then deep channels of the open canalicular system (OCS). When platelets were surface activated by exposure to carbon-stabilized, formvar-coated grids for 5 to 20 minutes and then exposed to fgn-Au complexes for 5 minutes, probes were also observed in the OCS. At 5 minutes, over 40% of the platelets had concentrated fgn-Au in their OCS. Results after 10 minutes revealed 25% with gold-filled channels, 16% after 15 minutes, and 5% after 20 minutes. The decrease in frequency of OCS staining correlated with the increasing frequency of spread platelets, suggesting that tension produced by spreading may cause collapse of the OCS or that the OCS may evaginate onto the platelet during spreading. To evaluate the latter hypothesis, platelets were initially exposed to grids for 5 minutes and then incubated with fgn-Au for intervals of 5 to 20 minutes. The frequency of platelets with fgn-Au concentrated in the OCS was greatest at 5 minutes (44%) and decreased at the same rate as the frequency of spread platelets increased. Only 14.7% of the cells contained fgn-Au in the OCS after 20 minutes. These were primarily dendritic in form, while fully spread platelets rarely contained an OCS filled with the probe. The study indicates that fgn-Au particles are cleared to channels of the OCS independent of the mechanism of platelet activation. Fgn-Au that has been concentrated in the OCS at early stages of surface activation can be externalized during platelet spreading but remain internalized in suspension-activated cells. The OCS represents a membrane reservoir that can be evaginated onto the platelet surface during interaction with surfaces.