Adenosine diphosphate (ADP) induced aggregation of rabbit platelets, without the release reaction, causes a significant decrease (7%) in the amount of phosphatidylinositol-4,5-bisphosphate (PIP2) at 10 sec and at 60 sec (11%). In platelets prelabeled with 32P-phosphate, this decrease in PIP2 is associated with a decrease in PIP2 radioactivity, which is significant at 50 sec. The decrease in PIP2 is sufficient to mobilize about 0.18 nmole Ca2+/10(9) platelets. In view of the key role played by Ca2+ in ADP-induced platelet shape change and aggregation, this evidence is compatible with the hypothesis that changes in PIP2 can be a source of calcium for cellular responses to agonists.

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