Decreased calcium mobilization in GPIb-V-IX–deficient platelets but normal responses in the absence of the GPIbα N-terminal domain. (A) Washed platelets from GPIbβ^+/+ (β^+/+) or GPIbβ^−/− (β^−/−) mice and GPIbβ^+/+ platelets treated with Nk protease (WT + Nk) were loaded with Fura-2. Intracellular calcium levels were quantified by spectrofluorometry after stimulation with ADP (5μM), thrombin (10nM), or a mixture of CRP (35 μg/mL) and the PAR4 agonist peptide AYPGKF (0.5mM). (B) Intracellular calcium levels were quantified in platelets from GPIbβ^+/+ or GPIbβ^−/− mice after stimulation with U46619 (1μM), CRP (35 μg/mL), or a mixture of CRP (35 μg/mL) and U46619 (1μM). Use of a different batch of CRP likely explains the increased calcium levels compared with panel A. Results are expressed as the mean calcium concentration in nanomolars (± SEM) and are from 3 to 6 independent experiments (*P < .05 vs WT).