Platelet secretion during clot retraction.

The precise route serving as the pathway for extrusion of products contained in platelet storage organelles following cell activation has been controversial for many years. Some workers believe that the granules and dense bodies fuse together to form secretion vacuoles that move to the exposed cell surface, fuse with it and extrude contained products to the outside. Others suggest that the organelles fuse with channels of the open canalicular system (OCS) and deposit their contents which are subsequently secreted to the exterior. The controversy has been difficult to resolve because nearly all workers have used platelets in suspension to arrive at diverse conclusions. The present study has used ethylenediamine tetracetic acid (EDTA)-treated platelets and isometric clot retraction as a model system, and either polylysine or tannic acid to define the pathway used for secretion. Results indicate that secretion takes place in EDTA platelets under isometric tension by fusion of granules to OCS channels and passage of products through these conduits to the cell exterior. There is no evidence to suggest the organelles form secretion vacuoles or extrude their products by fusion with the inside of the exposed surface membrane.