NK₁functions in a positive-feedback pathway. (A) Platelets preincubated for 2 minutes with increasing concentrations of anti-NK₁ antibody or control antibody were stimulated with collagen (5 μg/mL) for 90 seconds. Aggregation was monitored by turbidometric aggregometry. Traces are representative of 3 separate experiments. (B) Platelets from NK₁-deficient mice and litter-matched controls were stimulated for 90 seconds with thrombin at the concentrations shown. The percentage of single platelets remaining in suspension was determined. Values shown are mean percentage of single platelets cf basal ± SEM (n = 6).