α-granule but not dense granule secretion is defective in ILK-deficient mice. The exposure of P-selectin at the platelet surface was measured by flow cytometry in resting (basal) and stimulated (collagen 100 μg/mL) platelets (A; mean ± SEM, n = 4; ***P ≤ .005). Western analysis of total cellular P-selectin (140 kDa) is shown in panel B. Densitometric analysis shows the protein levels of P-selectin are unchanged in the absence of ILK (n = 3, KO = 85.77% ± 5.54%). Equivalent levels of protein loading were demonstrated by reprobing blots for GAPDH (B, bottom panel) (35 kDa). Release of platelet factor 4 (PF4) was measured in washed platelets stimulated with collagen (10 μg/mL) and thrombin (5 units/mL; C; mean ± SEM, n = 4; *P ≤ .05). Washed platelets loaded with [³H] 5-HT were stimulated with collagen (10 μg/mL) (n = 3, P = .64) and thrombin (5 units/mL; P = .69; D) as a measure of dense granule secretion. TEM images of control (Ctrl; E) and KO (F) washed mouse platelets show normal formation of mitochondria (letter M), dense granules (letter D), α-granule (letter A), and glycerol stores (letter G) in the KO platelets.