AGGREGATION FAILS TO INCREASE CYTOSOLIC CA2-LOADED HUMAN PLATELETS(!IN AEQUORIN)

Studies were performed to determine whether formation of platelet aggr egates itself, could cause an increase in cytosolic Ca2+!(Ca2+!i) Wh ich is independent of that resulting from the addition of agonists whi ch induce aggregation. An increase in Ca2+!i did not coincide with ag gregate formation when this response was dissociated from the addition of ADP or thrombin by delay either in initiating stirring or, for ADP , in adding fibrinogen. No increase in Ca2+!i occurred when aggregati on was induced by addition of 1,2-dioctanoylglycerol or of ristocetin, or for chymotrypsin-treated platelets by addition of fibrinogen. The results demonstrate clearly that aggregate formation does not cause an increase in Ca2+!i, and therefore exclude this possibility as an exp lanation for the discrepancies observed when Ca2+!i is measured, usin g aequorin and Fura2 as probes and as an underlying mechanism to accou nt for contact-induced responses.