Aequorin-detected calcium changes in stimulated thrombasthenic platelets. Aggregation-dependent calcium movement in response to ADP.

Calcium changes in normal and thrombasthenic platelets were recorded using the PICA-apparatus. Aequorin was loaded in the presence of DMSO, EGTA and PGE1. Platelets of three patients with type I thrombasthenia stimulated with A-23, 187, thrombin, PMA in the presence 1 mM Ca++ and 1 mM Mg++ were able to normally raise their calcium concentrations. The maximal values could be found below the normal range with collagen, ADP and PAF-acether. Calcium mobilization from internal stores in response to thrombin was normal. There were two calcium peaks in normal platelets stimulated with ADP. The second one was suppressed by omitting fibrinogen, stirring, or by adding aspirin, and was absent in thrombasthenic platelets. Thus the GP IIb-IIIa complex is not a prerequisite for calcium fluxes but is involved, when weak agonists such ADP are used, through an aggregation-dependent reinforcement of platelet activation.