Role of the glycoprotein IIb-IIIa complex in plasma membrane Ca2+ transport: a comparison of results obtained with platelets and human erythroleukemia cells

Several studies have suggested that the glycoprotein (GP) IIb-IIIa complex, which serves as the platelet fibrinogen receptor, also plays a role in the regulation of Ca2+ influx across the platelet plasma membrane. To examine this possibility further, we have compared Ca2+ transport in platelets and human erythroleukemia (HEL) cells, a megakaryoblastic cell line which synthesizes GP IIb-IIIa complexes that appear to be identical to those found on platelets. As with platelets, the results show the presence in unstimulated HEL cells of a rapidly exchangeable cytosolic Ca2+ pool that is in equilibrium with an intracellular sequestered Ca2+ pool and with extracellular Ca2+. Allowing for differences in cell size, the rate constants for Ca2+ exchange in HEL cells were similar to those in platelets. As in platelets, thrombin caused an increase in cytosolic Ca2+ that was due partly to enhanced Ca2+ influx and partly to the mobilization of internal Ca2+ stores. Incubation of the HEL cells with EDTA at 37 degrees C irreversibly altered the GP IIb-IIIa complex as evidenced by decreased binding of a complex-specific monoclonal antibody. In platelets this was accompanied by a 40% decrease in the rate of Ca2+ influx. However, in HEL cells there was neither a diminution in Ca2+ influx nor a reduction in the magnitude of the increase in cytosolic Ca2+ caused by thrombin. These results show that the parameters of Ca2+ distribution and movement are similar in HEL cells and platelets and that in HEL cells, as in platelets, the GP IIb-IIIa complex can be altered by removing Ca2+. However, unlike platelets, dissociation of the HEL cell IIb-IIIa complex has no discernible effect on plasma membrane Ca2+ transport. This suggests that earlier observations in platelets correlating changes in the rate of Ca2+ influx with changes in the number of intact IIb-IIIa complexes reflect an indirect, rather than a direct, role of these proteins in Ca2+ transport.