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.
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D000911:Antibodies, Monoclonal; D001791:Blood Platelet Disorders / Q000097:blood; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000097:blood*; D002462:Cell Membrane / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D006023:Glycoproteins / Q000097:blood* / Q000172:deficiency / Q000276:immunology; D006801:Humans; D007158:Immunologic Techniques; D007700:Kinetics; D008565:Membrane Proteins / Q000097:blood* / Q000172:deficiency / Q000276:immunology; D010980:Platelet Membrane Glycoproteins
D000911:Antibodies, Monoclonal; D006023:Glycoproteins; D008565:Membrane Proteins; D010980:Platelet Membrane Glycoproteins; D004533:Egtazic Acid; D002118:Calcium