Two pathways for store-mediated calcium entry differentially dependent on the actin cytoskeleton in human platelets

A major pathway for stimulated Ca2+ entry in non-excitable cells is activated following depletion of intracellular Ca2+ stores. Secretion-like coupling between elements in the plasma membrane (PM) and Ca2+ stores has been proposed as the most likely mechanism to activate this store-mediated Ca2+ entry (SMCE) in several cell types. Here we identify two mechanisms for SMCE in human platelets activated by depletion of two independent Ca2+ pools, which are differentially modulated by the actin cytoskeleton. Ca2+ entry induced by depletion of a 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ)sensitive pool is increased by disassembly of the actin cytoskeleton and that induced by a TBHQ-insensitive pool is reduced. Stabilization of the actin cytoskeleton prevented Ca2+ entry by both mechanisms. We propose that the membrane-associated actin network prevents constitutive Ca2+ entry via both pathways. Reorganization of the actin cytoskeleton permits the activation of Ca2+ entry via both mechanisms, but only SMCE activated by the TBHQ-insensitive pool requires new actin polymerization, which may support membrane trafficking toward the PM.