Influence of antioxidants on recruitment following collagen stimulation.

(A) Representative tracings of platelet recruitment experiments. Recruitment induced by various agonists showed different reactions to scavenging of O$2−$ and inhibition of NAD(P)H oxidase. Seven minutes after platelets had been stimulated (white arrow) in the presence of either vehicle or SOD (500 U/mL), an equal amount of untreated platelets was added (black arrow) and recruitment of additional platelets to the preformed thrombus measured for another 5 minutes. Aggregation of recruited platelets resembled that of ADP-induced aggregation, indicating that ADP release from platelets during aggregation is the main stimulus acting on recruitable platelets. (B) Left: Aggregation of recruited platelets at 5 minutes after addition to a preformed thrombus as percent of control (recruitment following collagen stimulation). SOD (500 U/mL) and DPI (100 μM) significantly decreased recruitment, whereas L-NA (30 μM) enhanced it (n = 10 each). Right: SOD only decreased recruitment when stimulation was due to an agonist that induces platelet O$2−$ production like collagen, but not when it was due to thrombin or ADP (n = 10 each). Boiled SOD was without effect (not shown). **Significantly different versus control atP < .01.