ADP-INDUCED CYTOPLASMIC CALCIUM MOBILIZATION AND SHAPE CHANGE IN PLATELETS ARE MEDIATED BY DIFFERENT BINDING SITES

Platelet stimulation with ADP results in a number of responses including increase in cytoplasmic ionized calcium concentration [Ca2+]i, shape change, aggregation, secretion, and inhibition of cAMP accumulation caused by PGI2.5'-Fluorosulphonylbenzoyladenosine (FSBA), which covalently labels ADP binding site on platelets, blocks platelet shape change but not inhibition of cyclic AMP levels by ADP, while p-chloromercuribenzenesulfonate (pCMBS), a non-penetrating thiol reagent, blocks ADP-induced inhibition of adenylate cyclase but not shape change. We examined the effect of FSBA and pCMBS on ADP-induced increase in [Ca2+]i to determine whether it is linked to the binding site mediating shape change or that for inhibition of adenylate cyclase. In platelets loaded with Ca2+ indicators, quin 2 or fura 2, and in presence of adenosine deaminase (AD), FSBA (50-200 μM) induced a dose-dependent, rapid rise in [Ca2+]i. from basal levels of 70-90 nM to peak levels of 300-500 nM in the presence of 1 mM external Ca2+ providing direct evidence that FSBA is a platelet agonist. The [Ca2+ ]i. returned to near basal levels over 30 min. The effect of FSBA on [Ca2+]i. was inhibited by ZK 36,374 (40 nM), a stable PGI2 analog. AdP concentrations eliciting similar responses were about 10-fold less than those for FSBA. Platelet incubation with FSBA (50-100 μM) in the presence of AD for 30 min (to ensure optimal covalent labelling of the ADP binding sites) abolished shape change but jjid not inhibit ADP (5, 25 μM)-induced increase in [Ca2+]i. or block the inhibitory effect of ADP on cAMP accumulation in1platelets exposed to ZK 36,374 (50 nM) in.presence of theophylline (7 mM). Incubation with pCMBS (5-100 pM, 2 min) abolished the effect of ADP on [Ca2+]. and on the inhibition of cAMP levels; shape change was not 1 inhabited even at 1 mM. pCMBS (0.5-1 mM) inhibited the rise in [Ca2+ ]. by FSBA alone. These observations suggest that ADP-induced Ca mobilization is mediated by platelet binding sites which are distinct from those mediating shape change but probably the same as those modulating adenylate cyclase.