INHIBITION OF AGONIST-INDUCED RISE IN PLATELET CA2+ BY ANTITHROMBOTICNIPECOTAMIDES

The effects of three structural types of nipecotamides and their stere oisomers on collagen-induced aggregation and intraplatelet ionized cal cium ([Ca2+]i) rise in human platelets were evaluated using aequorin a s the [Ca2+]i indicator. The orders of potencies of racemic nipecotami des were different when collagen was the agonist compared with those o btained using ADP. It is suggested that in addition to their earlier h ypothesized interactions with platelet anionic phospholipids of the pl asma and organelle membranes, nipecotamides may, in addition, act at o ther receptor sites. In general, the inhibition of collagen-induced ag gregation paralleled their inhibitory effects on the rise of [Ca2+]i. The compounds were stereoselective in inhibiting aggregation as well a s [Ca2+]i rise. The meso diastereomers of I and II were more potent th an the corresponding enantiomeric pairs. A single [Ca2+]i peak was not iced when the incubate contained 1.0 mM extracellular calcium [Ca2+]o. On the other hand a biphasic [Ca2+]i rise was noticed when the nomina lly Ca2+-free buffer contained 75 muM ethylene glycol tetraacetate (EG TA). The first peak corresponded with platelet shape change, suggestin g Ca2+ discharge from internal stores, and the second, with aggregatio n. The second peak may reflect either Ca2+ flux across the plasma memb rane or aequorin leak from internal cellular locations or from the can icular system. Inhibition of the rise in intraplatelet Ca2+ appears to be associated with the platelet aggregation-inhibitory actions of nip ecotamides.