We studied a patient (V.R.) with a mild hemorrhagic disorder and prolonged bleeding time. Platelet aggregation induced by ADP (2-50 uM) in V.R.'s citrated PRP or washed platelets (WP) was greatly impaired; in contrast ADP (0.1-1 uM) induced normal platelet shape change of V.R. platelets. Collagen-induced aggregation was absent at low (1-2 ug/ml) but not at high concentration (10 ug/ml). Thrombin at high (1 U/ml) but not at low concentration (0.05 U/ml) induced normal aggregation of V.R.'s WP.

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... yrase added to normal PRP or WP caused defective platelet aggregation similar to that observed for V.R.'s platelets. Chymotrypsin-treated WP from V.R. and from a normal control aggregated to the same extent upon addition of fibrinogen. CIE and two-dimensional non-reduced/reduced SDS-PAGE showed no abnormalities of platelet membrane glycoproteins. 125I-fibrinogen (125I-Fg) binding induced by ADP (10 uM) to V.R.'s platelets was absent. Thrombin(0.05-1 U/ml)-induced binding of 125I-Fg to was 20-30% of normal. Chymotrypsin-treated V.R's platelets bound normal amounts of 125I-Fg. Addition of apyrase to thrombin—stimulated normal platelets lowered 125I-Fg binding to that of thrombin-stimulated V.R.'s platelets. However, 125I-Fg binding to thrombin—stimulated V.R.'s platelets was not affected by apyrase. Unstimulated V.R.'s WP bound the 125I-labeled monoclonal antibody 7E3 (anti Ilb/IIIa) at the same rate and to the same extent as normal WP. Thrombin (1 U/ml) increased the rate of binding of 125I-7E3 to both V.R.'s and control platelets. In contrast, ADP (10 uM) increased the rate of 125I-7E3 binding to control but not to V.R.'s platelets. C-ADP binding to V.R.'s platelets was normal. Therefore, ADP normally interacts with V.R.'s platelets, but does not induce fibrinogen receptors exposure. Selective defects in the ADP pathway of platelet aggregation have not been previously described.