Decreased Association of 45Calcium with Platelets Unable to Aggregate due to Thrombasthenia or Prolonged Calcium Deprivation

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Abstract
45Calcium uptake was studied with aspirin-treated platelets that were gel-filtered through a column of Sepharose 2B equilibrated with divalent cation-free modified Tyrode's solution to remove readily exchangeable surface-associated calcium. These platelets aggregated almost immediately when exposed to ADP, fibrinogen and at least 30 μm CaCl2. At this calcium ion concentration, 108 platelets took up 36.6 ± SEM 2.7 pmol of 45calcium within 1–2 min. The presence of ADP and fibrinogen did not affect the amount of calcium bound. Over 90% of this platelet-associated calcium was removed by EDTA in 5 min suggesting that it was surface-bound. Calcium uptake increased rapidly for 10 min, then more slowly for up to 2 h. At 60 min, maximal uptake was approached at CaCl2 concentrations between 250 and 300 μm when an average of 276 ± SEM 18 pmol of calcium was associated with 108 platelets. Only 50–60% of this calcium could be removed by EDTA in 5 min, and about 70% in 20 min, suggesting that some of it had been internalized. Platelets from two patients with thrombasthenia that were unable to aggregate took up 50% less calcium than platelets from normal volunteers. Similarly, platelets that had been incubated with EDTA at 37°C, pH 7.8 for 8 min lost the ability to aggregate despite recalcification and took up 40–60% less calcium than CaEDTA-treated controls. Platelets from a patient with the Bernard-Soulier syndrome aggregated and bound calcium normally. Thus the platelets’ ability to take up calcium after removal of surface-associated calcium correlates with their ability to aggregate. Since thrombasthenic platelets and platelets rendered incapable of aggregating after prolonged calcium deprivation with EDTA do not bind fibrinogen, we postulate that some of the surface-associated calcium normally binds to the fibrinogen receptors.