The blood platelet open canalicular system: a two-way street

Channels of the surface-connected, open canalicular system (OCS) of human platelets serve as the pathway for transport of substances into the cells and as conduits for the discharge of alpha granule products secreted during the platelet release reaction. The purpose of the present study was to determine if both functions of the OCS can take place simultaneously. Suspensions of washed platelets were incubated with 18 to 20-nm colloidal gold particles coupled to fibrinogen molecules (Fgn/Au) for 5 min, then exposed to 0.2, 1 or 5 U of thrombin/ml for 5, 10, 15, 30, 45, or 300 s. The samples were fixed in glutaraldehyde and osmic acid containing tannic acid under conditions that stain alpha granule fibrinogen during secretion from thrombin-stimulated cells. Thrombin caused Fgn/Au particles to bind to platelets and be cleared to channels of the OCS at about the same rate, regardless of the thrombin concentration. Discharge of tannic acid-osmium stained fibrinogen from alpha granules to channels of the OCS and platelet exterior was, on the other hand, time and thrombin concentration dependent. Fgn/Au receptor complexes were observed in the same OCS channels as the tannic acid-osmium stained alpha granule secretion products. Thus, the platelet OCS appears to be a two-way street with different speed limits for incoming and outgoing traffic.