Inhibition of aggregation of rabbit and human platelets induced by adrenaline and 5-hydroxytryptamine by KB-R7943, a Na+/Ca2+ exchange inhibitor

1 We investigated the effect of KB-R7943, a Na+/Ca2+ exchange inhibitor, on the aggregation response induced by adrenaline and 5-hydroxytryptamine (5-HT), alone or in combination in human and rabbit platelets in the presence or absence of ouabain. 2 KB-R7943 inhibited aggregation induced by the combination of adrenaline and 5-HT in a concentration-dependent manner. The ICS, values of KB-R7943 were 4.2 +/-2.0 or 3.0 +/-0.7 muM with washed rabbit platelets with or without ouabain pretreatment, respectively. 3 In platelet-rich human plasma, the aggregation was biphasic. The IC50 value of KB-R7943 was 17.2 +/-4.4 muM for the first phase aggregation. 4 KB-R7943 did not inhibit the first phase of aggregation induced by adrenaline alone, or the monophasic aggregation induced by 5-HT alone. 5 The aggregation of rabbit platelets depended on the presence of K+ in the medium, and K+-dependent and K+-independent Ca2+ influx were observed in resting platelets. Ouabain treatment increased only the Ki-dependent but not the K+-independent Ca2+ influx. 6 KB-R7943 inhibited K+-dependent Ca2+ influx with or without ouabain pretreatment, but not K+-independent Ca2+ influx. 7 From these results, we conclude that KB-R7943 inhibits the adrenaline plus 5-HT induced aggregation of rabbit and human platelets by inhibiting K+-dependent Na+/Ca2+ (NCKX). Our results suggest that NCKX plays an important role in platelet aggregation.