Ca2+ transport across the platelet plasma membrane. A role for membrane glycoproteins IIB and IIIA.

Human platelets maintain a low cytosolic free Ca2+ concentration in part by controlling plasma membrane Ca2+ transport. The present studies examine the role in this process of two well-characterized membrane proteins: glycoproteins IIb and IIIa. These glycoproteins form a Ca2+-dependent complex which serves as both the platelet fibrinogen receptor and the principle site for high affinity Ca2+ binding on the platelet surface. The kinetics of plasma membrane Ca2+ exchange were compared in normal platelets and in thrombasthenic platelets, which lack the IIb X IIIa complex. Under steady-state conditions, the maximum rate of plasma membrane Ca2+ exchange in the thrombasthenic platelets was half the rate observed in normal platelets. The size of the cytosolic exchangeable Ca2+ pool and the cytosolic free Ca2+ concentration, however, were normal. A quantitatively similar decrease in plasma membrane Ca2+ exchange was seen in normal platelets after incubation with ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) at 37 degrees C, conditions that dissociate the IIb X IIIa complex. This decrease in the Ca2+ exchange rate in normal platelets could be prevented by preincubating platelets with a complex-specific anti-IIb X IIIa monoclonal antibody, but not by preincubating platelets with an anti-IIIa monoclonal antibody. In order to determine whether loss of the IIb X IIIa complex primarily affects Ca2+ influx or Ca2+ efflux, both processes were also examined under nonsteady-state conditions. An immediate decrease in the 45Ca2+ influx rate was seen when Ca2+ was added back to platelets preincubated with EGTA at 37 degrees C. The 45Ca2+ efflux rate, on the other hand, was not immediately affected. These data suggest, therefore, that an intact IIb X IIIa complex is necessary for normal Ca2+ homeostasis in platelets.