In the present study, we have investigated the mechanism of affinity modulation of alpha IIb beta 3 by chymotrypsin. We first confirmed that alpha-chymotrypsin could activate alpha IIb beta 3 (approximately 7,000 molecules per platelet) without major intracellular signaling. However, we unexpectedly found that high concentrations of amiloride dose-dependently inhibited 125I-fibrinogen binding to the chymotrypsin-treated platelets, as well as the platelet aggregation (IC50 [50% inhibitory concentration] for fibrinogen binding, 530 mumol/L). In contrast, amiloride did not inhibit alpha IIb beta 3 activation induced by anti-alpha IIb beta 3 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 alpha IIb beta 3 by twofold, as assessed by fibrinogen-binding assay. The incubation of platelets with ouabain, a Na+/K(+)- adenosine triphosphatase (ATPase) inhibitor, further augmented alpha IIb beta 3 activation. These data suggested that a likely candidate for the pathway was Na+/Ca2+ exchanger. At 140 mmol/L [Na+]o, 45Ca2+ influx to the chymotrypsin-treated platelets was twofold greater than that to non-treated platelets. Replacement of Na+ with TMA further increased the Ca2+ influx, and the increase was inhibited by amiloride dose-dependently. 3′,4′-Dichlorobenzamil (DCB) and bepridil, relatively specific inhibitors of Na+/Ca2+ exchanger, also inhibited the chymotrypsin-induced alpha IIb beta 3 activation, and the IC50 values of these inhibitors for fibrinogen binding were 25 mumol/L and 52 mumol/L, respectively. Moreover, platelet aggregation induced by various physiologic agonists was inhibited by DCB or bepridil, while platelet agglutination by ristocetin was not. Our data newly suggest that Na+/Ca2+ exchanger operating in reverse mode may be directly involved in inside-out signaling that activates alpha IIb beta 3.