THE DENSE TUBULAR SYSTEM IS PRESENT IN BOVINE PLATELETS

Abstract

Human platelets contain two channel systems. The open canalicularsystem (OCS) consists of tortuous channels, continuous with the cellsurface and each other, and spreadthroughout the platelet cytoplasm.The dense tubular system (DTS) derives from rough endoplastic reticulum of the parent megakaryocyte.It is also spread randomly through the platelet and forms specialized membrane complexes with elements of the OCS in one or two areasof the cytoplasm. The OCS serves as the conduit for discharge of granule products during the platelet release reaction. Elements of the DTS contain enzymes involved in prostaglandin synthesis and serve asreservoirs for calcium involved inplatelet physiological activities.Recent studies have shown that bovine platelets lack the OCS and usethe surface membrane as a conduit for the release of granule associated products.The existence of a DTS and its functional role in bovine platelets has not been examined previously. Bovine platelets converted 14C-arachidonic acid to thromboxane B2 as efficiently as human cells. Fura 2 loaded bovine platelets stimulated by thrombin in the presence of EDTA developed cytoplasmic levels of calcium flux comparable to human cells, suggesting that bovine platelets must have a DTS. Ultrastructural procedures for cyto-chemical demonstration of platelet peroxidase and glucose 6 phosphatase resulted in selective deposition ofreaction product in channels of a DTS in bovine cells. Membrane complexes formed by interaction of theOCS and DTS in human platelets were absent. However, some elements of the DTS in cattle platelets are located very close to the cell surface. Results of these studies demonstrate that bovine platelets possess a functional DTS involved in prostaglandin synthesis and calcium flux.