SP enhances the aggregation response of platelets to other agonists. Washed platelets were stimulated for 90 seconds with SP (1.25 μM) and (A) collagen (1 μg/mL) or (B) thrombin (0.025 U/mL) either alone or in combination (arrow indicates addition of agonist). Aggregation was monitored by turbidometric aggregometry at 37° C under stirring conditions. Traces are representative of 3 separate experiments.