Platelet hyperactivity and abnormal Ca2+ homeostasis in diabetes mellitus

We sought to determine the mechanisms for hyperactivity and abnormal platel et Ca2+ homeostasis in diabetes. The glycosylated Hb (HbA(1c)) level was us ed as an index of glycemic control. Human platelets were loaded with Ca-gre en- fura red, and cytosolic Ca2+ ([Ca2+](i)) and aggregation were simultane ously measured. In the first series of experiments, the platelets from diab etic and normal subjects were compared for the ability to release Ca2+ or t o promote Ca2+ influx. A potent and relatively specific inhibitor of Na+/Ca 2+ exchange, 5-(4- chlorobenzyl)-2',4-dimethylbenzamil (CB-DMB), increased the second phase of thrombin-induced Ca2+ response, suggesting that the Na/Ca2+ exchanger works in the forward mode to mediate Ca2+ efflux. In contra st, in the platelets from diabetics, CB-DMB decreased the Ca2+ response, in dicating that the Na+/Ca2+ exchanger works in the reverse mode to mediate C a2+ influx. In the second series of experiments we evaluated the direct eff ect of hyperglycemia on platelets in vitro. We found that thrombin- and col lagen-induced increases in [Ca2+](i) and aggregation were not acutely affec ted by high glucose concentrations of 45 mM. However, when the platelet- ri ch plasma was incubated with a high glucose concentration at 37 degrees C f or 24 h, the second phase after thrombin activation was inhibited by CB- DM B. In addition, collagen-stimulated [Ca2+](i) response and aggregation were also increased. Thus in diabetes the direction and activity of the Na+/Ca2 + exchanger is changed, which may be one of the mechanisms for the increase d platelet [Ca2+](i) and hyperactivity. Prolonged hyperglycemia in vitro ca n induce similar changes, suggesting hyperglycemia per se may be the factor responsible for the platelet hyperactivity in diabetes.