Effect of potassium channel inhibitors on collagen-plus-thrombin–induced platelet procoagulant activity. Platelets at 10⁷ mL^–1 were preincubated with various inhibitors for 30 minutes followed by activation with collagen (10 μg/mL) plus thrombin (4 nM) for 10 minutes. Samples from these incubations were diluted 10-fold in HEPES buffer containing 3 mM CaCl₂ and subsequently analyzed for prothrombinase activity (A) or percentage of annexin-positive cells (B) as described in “Materials and methods.” Valinomycin was added 5 minutes prior to activation of the platelets (to avoid long-term effects of this ionophore). ▪ indicates absence of valinomycin; and ▦, presence of valinomycin. Concentrations used: clotrimazol, 10 μM; charybdotoxin, 20 nM; quinine, 0.4 mM; apamin, 500 nM; iberiotoxin, 200 nM; and valinomycin, 3 μM. Data are expressed as percentage of control (ie, platelets in absence of inhibitor [mean ± 1 SD] panel A, n > 6; panel B, n > 3]). Student t test was used to compare data in the presence and absence of valinomycin.