Rescue of thrombin-induced PDI release from HPS6^−/− platelets using substimulatory ADP, and reduction of PDI release from WT platelets with apyrase. PDI antigen in releasate of thrombin-stimulated (0.36 nM) WT and HPS6^−/− platelets with and without substimulatory concentrations of ADP (1 μM) or with and without apyrase (Apy; 0.1 U/mL). Samples were obtained 10 minutes after thrombin activation. (A) PDI antigen in releasates of thrombin-stimulated WT and HPS6^−/− platelets were detected by SDS-PAGE, followed by immunoblotting with anti-PDI antibodies (DL-11; 1 mg/mL). (B) Bar graph comparing PDI release in WT and HPS6^−/− platelets in the presence (+) or absence (-) of ADP. Data represent mean ± SD; n = 2; **P < .01. (C) Bar graph comparing PDI release in WT and HPS6^−/− platelets in the presence (+) or absence (-) of apyrase. Data represent mean ± SD (n = 2; *P < .01). (D) Effect of high levels of ADP on HPS6^−/− release of PDI. WT and HPS6^−/− platelets were incubated with 10 μM (-) or 100 μM (+) of ADP, and releasate was collected and resolved on SDS-PAGE. PDI was quantitated by immunoblotting with rabbit polyclonal anti-PDI antibody (n = 2; mean ± SD; **P = .02). WT, closed bars; HPS6^−/−, open bars.