Inositol phospholipid metabolism in human platelets stimulated by ADP
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abstract
ADP‐induced changes in inositol phospholipids, phosphatidic acid and inositol phosphates of human platelets have been studied in detail, using not only 32P labelling, but also by examining changes in amounts of the phospholipids, their labelling with [3H]glycerol and their specific radioactivities; changes in the labelling of inositol phosphates in platelets prelabelled with [3H]inositol were also measured. During the early (10 s) stage of reversible ADP‐induced primary aggregation in a medium containing fibrinogen and with a concentration of Ca2+ in the physiological range (2 mM), the amounts of phosphatidylinositol 4,5‐bisphosphate (PtdInsP2) and phosphatidylinositol 4‐phosphate (PtdInsP) decreased (by 11.2 ± 4.9% and 11.3 ± 5.3%, respectively) while the labelling, but not the amount, of phosphatidic acid increased. The decreases do not appear to be attributable to the action of phospholipase C because the specific radioactivity of phosphatidic acid labelling with [3H]glycerol was not significantly increased at 10 s (although the initial specific radioactivities of the inositol phospholipids and PtdCho were more than double that of phosphatidic acid), and no increases in the labelling of inositol trisphosphate (InsP3), inositol bisphosphate (InsP2) or inositol phosphate (InsP) were detectable at 10 s. Shifts in the interconversions between PtdInsP2 and PtdInsP, and PtdInsP and PtdIns may occur. By 30 to 60 s, when deaggregation was beginning, the amounts of PtdInsP2, PtdInsP and phosphatidic acid were not different from those in unstimulated platelets, but large increases in the 32P‐labelling and [3H]glycerol labelling of phosphatidic acid were observed. Formation of [3H]inositol‐labelled InsP3 was not detectable at any time in association with ADP‐induced primary aggregation, indicating that degradation of PtdInsP2 by phospholipase C is not appreciably stimulated by ADP. These findings were compared with those obtained when platelets were aggregated by ADP in a medium without added of Ca2+ in which secondary aggregation associated with thromboxane A2 (TXA2) formation and release of granule contents occurs. At 10 s (during primary aggregation) the changes were similar in the two media. At 30 s and 60 s (during secondary aggregation in the low‐Ca2+ medium), the increases in PtdInsP2, PtdInsP and phosphatidic acid in platelets suspended in the absence of added Ca2+ were larger than those in platelets suspended in the presence of 2 mM Ca2+. In the absence of added Ca2+. ADP‐induced increases in the labelling of InsP3. InsP2 and InsP which were probably due to the effects of TXA2 since they were abolished by aspirin. Thus, ADP‐induced primary aggregation differs from that induced by agonists such as thombin and TXA2 and from the secondary phase of ADP‐induced aggregation that occurs in a medium containing an unphysiological, low concentration of Ca2+, in that aggregation occurs without evidence of degradation of PtdInsP2 by phospholipase C.
