Inhibition of urokinase (uPA) catalytic activity by released platelet PN-1. uPA catalytic activity was measured after incubation with the supernatants from resting platelets (■) or activated platelets () as described in “Thrombin activity assay.” Products of human platelet secretion from (A) control donors or (B) GPS patients were incubated with uPA in the presence or absence of an anti–PN-1 IgG and/or an anti–PAI-1 IgG. (C) Secretion products of platelets from mice deficient for PN-1 or PAI-1 or their WT littermates. Data are presented as mean ± SD of 3 independent experiments, each performed in triplicate. *P < .05, significantly different from resting platelet supernatant. †P < .05 significantly different from activated platelet supernatant in the presence of an anti–PN-1 IgG or an anti–PAI-1 IgG.