CK2β is a critical regulator of platelet IP₃-triggered Ca²⁺release from internal stores with subsequent extracellular Ca²⁺influx, resulting in impaired granule secretion, integrin α_IIbβ₃activation, and aggregation in ck2β-null platelets. (A) Representative tracings of Fura-2-fluorescence reflecting the cytosolic Ca²⁺ concentration [Ca²⁺]_i (right) and arithmetic means of maximal ∆[Ca²⁺]_i ± standard deviations (n = 6; left) of ck2β^fl/fl (blue line) and ck2β^−/− (gray line) platelets before and after stimulation with CRP (5 µg/ml) and thrombin (20 mU/ml) in the absence (0.5 mM EGTA) of extracellular Ca²⁺. (B) Representative tracings of Fura-2-fluorescence reflecting the cytosolic Ca²⁺ concentration [Ca²⁺]_i (right) and arithmetic means of maximal ∆[Ca²⁺]_i ± standard deviations (n = 6; left) of ck2β^fl/fl (blue line) and ck2β^−/− (gray line) platelets before and after stimulation with CRP (5 µg/ml) and thrombin (20 mU/ml) in the presence (1 mM Ca²⁺) of extracellular Ca²⁺. (C) Representative tracings of Fura-2-fluorescence reflecting cytosolic Ca²⁺ concentration [Ca²⁺]_i (lower) and arithmetic means of maximal ∆[Ca²⁺]_i ± standard deviations (n = 6; upper) of ck2β^fl/fl (blue line) and ck2β^−/− (gray line) platelets before and after stimulation with IP₃ (2.5 µM). The cytosolic Ca²⁺ concentration before and after flash photolysis (365 nm) in the absence of IP₃ was measured as a control (tracing not shown). (D) Flow cytometric analysis of P-selectin exposure reflecting α-granule release in ck2β^fl/fl (blue bars) and ck2β^−/− (gray bars) platelets in response to increasing concentrations of CRP (in micrograms per milliliter) or thrombin (in microunits per milliliter). Arithmetic means ± standard errors of the means (n = 6) are shown. (E) Luminescence analysis of ATP release reflecting the secretion of dense granules in response to increasing concentrations of CRP (in micrograms per milliliter) or thrombin (in microunits per milliliter). Representative ATP release tracings of ck2β^fl/fl (blue lines) and ck2β^−/− (gray lines) mice are shown (n = 6). (F) Flow cytometric analysis of α_IIbβ₃ integrin activation in ck2β^fl/fl (blue bars) and ck2β^−/− (gray bars) platelets in response to increasing concentrations of CRP (in micrograms per milliliter) or thrombin (in microunits per milliliter). Arithmetic means ± standard errors of the means (n = 6) are shown. (G) Light transmission aggregometry after stimulation with increasing concentrations of CRP (in micrograms per milliliter) or thrombin (in microunits per milliliter). Representative aggregation tracings of ck2β^fl/fl (blue lines) and ck2β^−/− (gray lines) platelets are shown (n = 4). Unpaired Student t test in panels A-D. *P < .05; **P < .01.