Effect of Thrombin on Phosphorylation of Platelet Membrane Proteins

A study of platelet protein phosphorylation in Duchenne muscular dystrophy. Further evidence against the generalised membrane defect theory

As a test of the generalised defect theory for Duchenne muscular dystrophy (DMD), basal and calcium-dependent platelet protein phosphorylation was examined in order to determine if the increased concentration of calcium in DMD skeletal muscle is reflected in DMD platelets. Protein phosphorylation was quantitated by gradient slab gel electrophoresis and autoradiography. The number of phosphoproteins in each phosphoprotein peak was determined by comparison with two-dimensional gel electrophoresis. Many phosphoprotein peaks were present in unstimulated platelet preparations both in whole platelet homogenates and in intact platelets. Two of these phosphoprotein peaks were calcium-dependent, one was a single phosphoprotein, the other consisted of 4 phosphoproteins. No disease-related differences were observed in either basal or calcium-stimulated phosphoproteins. These results do not support previous reports of platelet abnormalities in DMD, and provide further evidence that the biochemical defect in Duchenne muscular dystrophy is neither generalised nor a membrane defect. The biochemical defect in DMD should be regarded as a skeletal muscle abnormality until proved otherwise.

Are polyphosphoinositides involved in platelet activation?

In human platelets, the amounts of triphosphoinositides (TPI) and diphosphoinositides (DPI) increase after 30 sec and level off after 120 sec of thrombin stimulation. After 180 sec of thrombin challenge, TPI and DPI increase accounts for 66 and 80%, respectively. Polyphosphoinositide changes roughly parallel the release of N-acetyl-beta-D-glucosaminidase and appear as a later event compared to aggregation and serotonin secretion. It is concluded that an increased phosphorylation of polyphosphoinositides might participate in platelets to the process of stimulus-activation coupling and might be linked to thrombin receptor occupancy. A role of DPI in platelet activation is suggested by the observation that DPI promote platelet aggregation, the mechanism of which is discussed.