Lectin-induced calcium mobilization in human platelets : use of fluorescent probes

The cytosolic calcium concentration, (Ca(2+))(c), represents in many types of cells a versatile regulatory system involved in several signal transduction pathways of cell activation (1), and is tightly regulated in human platelets. Under basal conditions, platelets maintain the (Ca(2+))(c) at about 80 nM by actively sequestering calcium into the intracellular stores or by extruding it into the extracellular medium. The binding of specific agonists to their receptors induces a rapid increase of the (Ca(2+))(c), which can reach the micromolar range, depending on the type and the dose of agonist used. This effect is owing to Ca(2+) influx from the extracellular medium to the cytosol and to the release of Ca(2+) from internal stores (1,2) This latter effect is mediated by a phospholipase C-dependent mechanism; phospholipase C hydrolyzes plasma membrane phosphati-dylinositol 4,5-bisphosphate and produces diacylglycerol and inositol 1,4,5-trisphosphate (3). Inositol 1,4,5-trisphosphate causes the release of Ca(2+) from the dense tubular system by interacting with a specific receptor that behaves as a Ca(2+) channel by Itself. Much less is known about the Ca(2) influx across the plasma membrane Several studies suggested a role for membrane glycoproteins, such as glycoprotein IIb-IIIa complex (GPIIb-IIIa) in controlling calcium homeostasis in platelets. GPIIb-IIIa is the platelet receptor for fibrinogen and other adhesive proteins and belongs to the integrin superfamily (4). It represents a high affinity binding site for Ca(2+) in the plasma membrane (5) and may regulate the Ca(2+) homeostasis in human platelets (6), by acting as a channel by itself (7) or by interacting with a closely adjacent channel (8). Moreover, some authors reported a role for GPIIb-IIIa occupation in the regulation of phospholipase C activation (9).