Platelet hyperactivity and abnormal Ca(2+) homeostasis in diabetes mellitus.
We sought to determine the mechanisms for hyperactivity and abnormal platelet Ca(2+) homeostasis in diabetes. The glycosylated Hb (HbA(1c)) level was used as an index of glycemic control. Human platelets were loaded with Ca- green-fura red, and cytosolic Ca(2+) ([Ca(2+)](i)) and aggregation were simultaneously measured. In the first series of experiments, the platelets from diabetic and normal subjects were compared for the ability to release Ca(2+) or to promote Ca(2+) 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 Ca(2+) response, suggesting that the Na(+)/Ca(2+) exchanger works in the forward mode to mediate Ca(2+) efflux. In contrast, in the platelets from diabetics, CB-DMB decreased the Ca(2+) response, indicating that the Na(+)/Ca(2+) exchanger works in the reverse mode to mediate Ca(2+) influx. In the second series of experiments we evaluated the direct effect of hyperglycemia on platelets in vitro. We found that thrombin- and collagen-induced increases in [Ca(2+)](i) and aggregation were not acutely affected by high glucose concentrations of 45 mM. However, when the platelet-rich plasma was incubated with a high glucose concentration at 37 degrees C for 24 h, the second phase after thrombin activation was inhibited by CB-DMB. In addition, collagen-stimulated [Ca(2+)](i) response and aggregation were also increased. Thus in diabetes the direction and activity of the Na(+)/Ca(2+) exchanger is changed, which may be one of the mechanisms for the increased platelet [Ca(2+)](i) and hyperactivity. Prolonged hyperglycemia in vitro can induce similar changes, suggesting hyperglycemia per se may be the factor responsible for the platelet hyperactivity in diabetes.
Department of Pharmacology and Therapeutics, University of Manitoba, Winnipeg R3E OW3, Canada.
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D000584:Amiloride / Q000031:analogs & derivatives* / Q000494:pharmacology; D001786:Blood Glucose / Q000378:metabolism; D001792:Blood Platelets / Q000187:drug effects / Q000502:physiology*; D002118:Calcium / Q000097:blood*; D003094:Collagen / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism; D003920:Diabetes Mellitus / Q000097:blood / Q000503:physiopathology*; D003922:Diabetes Mellitus, Type 1 / Q000097:blood / Q000503:physiopathology; D003924:Diabetes Mellitus, Type 2 / Q000097:blood / Q000503:physiopathology; D006706:Homeostasis; D006801:Humans; D066298:In Vitro Techniques; D015759:Ionomycin / Q000494:pharmacology; D010974:Platelet Aggregation; D012016:Reference Values; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors / Q000097:blood; D013917:Thrombin / Q000494:pharmacology
D001786:Blood Glucose; D019831:Sodium-Calcium Exchanger; C054391:5-(N-4-chlorobenzyl)-N-(2',4'-dimethyl)benzamil; D015759:Ionomycin; D000584:Amiloride; D003094:Collagen; D013917:Thrombin; D002118:Calcium