Possible involvement of MRP4 in platelet transmitter storage and release. Platelets contain the ATP-dependent export pump MRP4 in the membrane of dense (δ)-granules and in the plasma membrane depending on platelet activation. MRP4 may be involved in the active transport of ADP into the dense granules and in the ADP release from the platelets. MTS indicates microtubular system; DTS, dense tubular system.