ADP-dependent recruitment following collagen stimulation.

(A) Tracing of parallel recruitment experiments showing dependence of recruitment on ADP release following collagen stimulation. Aggregation was induced by 6 μg/mL collagen fibrils. At 7 minutes following stimulation, additional platelets were added simultaneously with 1 U/mL apyrase or the same volume of vehicle. Apyrase, which cleaves ADP, inhibited recruitment of further platelets (graph representative of 9 similar experiments). (B) Enhancement of ADP-dependent platelet aggregation by reactive oxygen species. Tracing of a parallel experiment showing enhanced aggregation of a subthreshold concentration of ADP (1 μM) in the presence of exogenously created O$2−$ using xanthine (300 μM, X) and xanthine oxidase (2 mU/mL, XO, graph representative of 9 similar experiments). (C) ADP in platelet supernatants. ADP measured in the supernatants of collagen-stimulated platelets is decreased in the presence of SOD (500 U/mL) and increased in the presence of X/XO (concentrations as above). In supernatants of unstimulated platelets, there was little release of ADP, which was not significantly increased by treatment with X/XO alone (n = 11). **Significantly different versus control atP < .01; #, P < .05 versus collagen.