Affinity Modulation of the Platelet Integrin aIIbP 3 by a - Chymotrypsin : A Possible Role for Na + / Ca 2 + Exchanger

In the present study, we have investigated the mechanism of affinity modulation of mll& by chymotrypsin. We first confirmed that mu-chymotrypsin could activate a143 (-7,000 molecules per platelet) without major intracellular signaling. However, we unexpectedly found that high concentrations of amiloride dose-dependently inhibited '%fibrinogen binding to the chymotrypsin-treated platelets, as well as the platelet aggregation (IC5, 150% inhibitory concentration] for fibrinogen binding, 530 pmol/L). In contrast, amiloride did not inhibit ml&J activation induced by anti-mll& monoclonal antibody PT25-2 or AP5. To identify the pathway involved, the effects of alteration of Na' gradient in platelets were examined. Lowering Na+ gradient by replacing extracellular Na' with tetramethylammonium (TMA) increased the number of activated mtlbpj by twofold, as assessed by fibrinogenbinding assay. The incubation of platelets with ouabain, a Na+/K'-adenosine triphosphatase (ATPase) inhibitor, further