Desensitization of human platelet aggregation. (A) Washed platelets were preincubated under nonstirred conditions with vehicle (no pretreatment), 100 nM ADP, 1 μM ADP, or 10 μM ADP for 2 minutes, as indicated, and another minute with 1 mg/mL fibrinogen to allow aggregation. Platelets were then stimulated with 10 μM ADP, and the resultant aggregation response was monitored by optical aggregometry. Shown are traces representative of at least 3 experiments. (B) Washed platelets were preincubated under nonstirred conditions with vehicle or with 1 μM ADP for 2 minutes and for another minute with 1 mg/mL fibrinogen to allow aggregation. Platelets were then stimulated with ADP (10 μM), collagen (30 μg/mL), thrombin (0.1 U/mL), or 5-HT (20 μM) plus adrenaline (20 μM), and the resultant aggregation response was monitored by optical aggregometry. The maximal extent of aggregation 3 minutes after stimulation was measured, and data were expressed as a percentage of the control aggregation response (ie, in the absence of a desensitizing dose of ADP) and represent the mean ± SEM (n = 3).