Fibrinogen binding to the integrin alpha(IIb)beta(3) modulates store-mediated calcium entry in human platelets

Effects of the occupation of integrin alpha (IIb)beta (3) by fibrinogen on Ca++ signaling in fura-2-loaded human platelets were investigated. Adding f ibrinogen to washed platelet suspensions inhibited increases in cytosolic [ Ca++] concentrations ([Ca++](i)) evoked by adenosine diphosphate (ADP) and thrombin in a concentration-dependent manner in the presence of external Ca ++ but not in the absence of external Ca++ or in the presence of the nonsel ective cation channel blocker SKF96365, indicating selective inhibition of Ca++ entry. Fibrinogen also inhibited store-mediated Ca++ entry (SMCE) acti vated after Ca++ store depletion using thapsigargin. The inhibitory effect of fibrinogen was reversed if fibrinogen binding to alpha (IIb)beta (3) was blocked using RDGS or abciximab and was absent in platelets from patients homozygous for Glanzmann thrombasthenia, Fibrinogen was without effect on S MCE once activated. Activation of SMCE in platelets occurs through conforma tional coupling between the intracellular stores and the plasma membrane an d requires remodeling of the actin cytoskeleton. Fibrinogen inhibited actin polymerization evoked by ADP or thapsigargin in control cells and in cells loaded with the Ca++ chelator dimethyl BAPTA. It also inhibited the transl ocation of the tyrosine kinase p60(src) to the cytoskeleton. These results indicate that the binding of fibrinogen to integrin alpha (IIb)beta (3) inh ibits the activation of SMCE in platelets by a mechanism that may involve m odulation of the reorganization of the actin cytoskeleton and the cytoskele tal association of p60(src). This action may be important in intrinsic nega tive feedback to prevent the further activation of platelets subjected to l ow-level stimuli in vivo. (Blood.2001;97:2648-2656) (C) 2001 by The America n Society of Hematology.