Platelet Aggregation and Secretion: Effects of Anticoagulants and Divalent Cation Chelators
The aggregation and secretion responses of human platelets to ADP and epinephrine have been studied in citrated platelet-rich plasma (cPRP) and in heparinized platelet-rich plasma (hPRP) containing varying concentrations of divalent cation chelating agents. ADP induced a single wave of aggregation in hPRP while E induced biphasic aggregation in both hPRP and cPRP. Secretion of 14C-serotonin (5HT*) occurred in both hPRP and cPRP in response to ADP and E, but generally to a lesser extent in hPRP. The extent of 5HT* secretion in hPRP, relative to that in cPRP, decreased with increasing ADP concentration but remained essentially constant oyer the same range of E concentration. Addition of citrate or EDTA to hPRP produced a clear maximum in ADP-induced 5HT* secretion at levels of 5-20mM citrate or 2mM EDTA and, in some cases, was associated with the appearance of a biphasic aggregation response. With citrate, but not with EDTA, the chelator concentration corresponding to maximum secretion varied directly with the concentration of ADP. These results indicate that ADP-induced secretion is a function of divalent cation concentration but does not occur only at the low levels of divalent cation present in cPRP and that divalent cation may be more directly associated with ADP-platelet interactions than in interactions of platelets and E.