PLATELET-RELEASED ADP STABILIZES PAF-INDUCED RABBIT PLATELET-AGGREGATION BY STABILIZING INTRACELLULAR CALCIUM

AIM: To examine whether platelet-released adenosine diphosphate (ADP) would contribute to the stabilization of rabbit platelet aggregation i nduced by platelet activating factor (PAF). METHODS: Rabbit platelet a ggregation induced by PAF was measured turbimetrically. ADP release fr om rabbit platelets stimulated by PAF was determined by HPLC. Intracel lular Ca2+ was measured using Ca2+-sensitive fluorescent indicator Fur a 2-AM. RESULTS: PAF greater than or equal to 1 nmol.L-1 induced full platelet aggregation, which did not deaggregate over 5 min after aggre gation reached peak. Platelet aggregation was deaggregated in a concen tration-dependent manner by subsequent addition of ADP scavenger ATP-d iphosphohydrolase (apyrase) at 5 - 100 mg.L-1 PAF 3 nmol.L-1 stimulate d release of.ADP (29 % vs 6 % of control), and elicited a sapid rise i n intracellular calcium ([Ca2+](i)) which peaked at approximately 15 s . Then the [Ca2+](i) gradually decayed from 585 +/- 80 nmol.L-1 within 100 s to a low level (364 +/- 82 nmol.L-1). Apyrase 100 mg.L-1, added 2 min after PAF, reduced [Ca2+](i) to a lower level (171 +/- 29 nmol. L-1). CONCLUSION: Platelet-released ADP stabilizes PAF-induced rabbit platelet aggregation by stabilizing [Ca2+](i) at elevated level.