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

Abstract

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.

Actin filaments and microtubule dual‐granule transport in human adhered platelets: the role of α‐dystrobrevins

“…Platelet granules are the most prominent structural features, and upon activation, they coalesce in the centre of the platelet and fuse with the open canalicular system (OCS). The OCS represents a membrane reservoir that is evaginated onto the platelet surface during interaction with surfaces (Stenberg et al, 1984;Escolar et al, 1989), fusing with the plasma membrane (Ginsberg et al, 1980). The release of the granule contents into the OCS and their diffusion into the extracellular environment exert a paracrine role to activate other platelets in the immediate area that are critical to the formation of the haemostatic thrombus .…”

Induction of patching and its reversal on surface‐activated human platelets

“…However, early dendritic platelets did. Fgn/Au particles covered the bodies and extended out on pseudopods of the transformed cells (Escolar et al, 1989). Transformation to late dendritic, early and late spread platelets resulted in movement of the ligand-receptor complexes toward cell centres (Figs 1 and 2).…”

“…Rather, the central zone, like the cell margin, was nearly devoid of the dense probes which were concentrated in an intermediate belt ( Fig 1) and in channels of the open canalicular system (Fig 2) (Leistikow et al, 1988). Fgn/Au-filled OCS channels were present in nearly half of the small number of dendritic platelets on grids incubated 20 min, and in 5-1 5% of the large number of fully spread cells (Escolar et al 1989;Leistikow et al, 1988Leistikow et al, , 1990b.…”

“…Ligand-receptor complexes fill channels of the open canalicular system (OCS) in 5-15% of the surface-activated cells (Escolar et ul, 1989: Leistikow et ul, 1988). Absence of Fgn/Au filled OCS in the majority of spread cell is due to evagination of channels onto the surface and to collapse of OCS openings as tension develops during spreading (Escolar et al, 1989). As a result, Fgn/Au-receptor complexes are concentrated in an intermediate belt and in central zones of most spread platelets (Escolar et al, 1989;Leistikow et al, 1988Leistikow et al, , 1990a.…”

“…A variety of experimental procedures was introduced at this point as described below. The routine procedure, however, was to coat the grid with 10 pl of Fgn/Au solution (Escolar et al, 1989;Leistikow et al, 1988Leistikow et al, , 1990b. After incubation with the adherent platelets for 5 min at 37OC, unattached Fgn/Au particles were removed by rinsing.…”