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.