Rapid ultrastructural changes in the dense tubular system following platelet activation

The dense tubular system (DTS) functions to regulate platelet activation by sequestering or releasing calcium, similar to the sarcotubules of skeletal muscle. In resting platelets, the DTS exists as thin elongated membranes. Within 10 seconds of the addition of thrombin, platelets show a major ultrastructural change in their DTS: from the thin elongated form to a rounded vesicular form. These morphologic changes were demonstrated with two different stains and two different fixation methods. Platelets exposed to the calcium ionophore A23187 showed the same ultrastructural changes in the DTS. In contrast, the DTS remains in a thin elongated form when platelets are stimulated by the protein kinase C activators phorbol 12-myristate 13-acetate (PMA) and oleoylacetylglycerol (OAG). These morphologic changes may be related to the discharge of calcium from the DTS because this is stimulated by thrombin and A23187, but not by PMA. Preincubation of the platelets with the intracellular calcium chelator 5,5'-dimethyl-bis-(0-aminophenoxy)-ethane-N,N,N',N tetra acetic acid (BAPTA) largely prevented both the thrombin-induced rise in intracellular calcium and the changes in DTS morphology, suggesting that the changes in DTS morphology are secondary to the increase in cytosolic calcium. The results provide a morphologic correlate to existing biochemical evidence showing that the DTS is involved early during paltelet activation.