The involvement of calcium in epinephrine or ADP potentiation of human platelet aggregation

The mechanism by which low doses of epinephrine or ADP potentiate primary platelet aggregation was investigated. Aspirin (lmg/ml)-treated human blood platelets were isolated by albumin density gradient centrifugation. Platelet ⁴⁵Ca uptake associated with epinephrine or ADP addition was determined over a 240 sec time course. Pretreatment of the platelets with ADP (0.5μM) significantly increased aggregation in response to epinephrine (0.1μM). This increased aggregation was associated with a substantially greater ⁴⁵Ca uptake than that which occurred in the presence of epinephrine (0.1μM) alone. The potentiated epinephrine response was inhibited by the Ca²⁺ antagonist verapamil (25μM). This inhibition could in turn be reduced by Ca²⁺ (1mM) addition. Pretreatment of platelets with epinephrine (0.1μM) also increased aggregation in response to ADP (0.5μM). Although this potentiated response was not associated with measurable ⁴⁵Ca uptake, it was nevertheless completely abolished by verapamil (25μM) treatment. These findings suggest that low doses of ADP promote the ability of epinephrine to stimulate an increase in membrane permeability to Ca²⁺.