Influx of extracellular calcium and agonist-coupling appear essential for the activation of thromboxane A2-dependent phospholipase A2 in human platelets.

The present study demonstrates the existence of a unique mechanism for arachidonic acid (AA)-specific phospholipase A2 (PLA2) activation, which requires both sustained elevation of cytosolic Ca2+ coupled to the influx of extracellular Ca2+ and agonist interaction in platelets. The activation of PLA2 in platelets exposed to thapsigargin was abolished by the inhibition of cyclooxygenase (COX), thus suggesting a requirement of endogenously produced COX metabolite(s) for the activation of this enzyme. A thromboxane A2 (TXA2) analog, U46619, restored the activation of this AA-specific PLA2 activation supporting the requirements of COX metabolite(s) especially TXA2. Our subsequent studies demonstrated that both the effects of TXA2, and U46619 could be mimicked by collagen. Neither the transient cytosolic Ca2+ rise nor the agonists such as U46619 or collagen alone were sufficient to prime the activation of this PLA2 in the absence of thapsigargin. Since collagen behaves very similarly to TXA2, we suggest that this PLA2, is not only responsive to TXA2, but also to other agonists such as collagen, as shown in this study. We suggest that the activation of this distinct TXA2- and collagen-sensitive PLA2 involves two steps: (a) sustained elevation of cytosolic Ca2+ coupled to the influx of extracellular Ca2+; and (b) interaction with agonists such as TXA2 and collagen.