Aggregation and/or oxygenated products of arachidonic acid are not required for collagen-induced deacylation of phosphatidylcholine in human platelets

Effect of berberine on arachidonic acid metabolism in rabbit platelets and endothelial cells

The antiplatelet effect of berberine has been demonstrated in both laboratory research and clinical trials. In the present study, we show ex vivo that berberine significantly inhibited rabbit platelet aggregation induced by adenosine diphosphate, arachidonic acid, collagen or calcium ionophore A23187. The most potent inhibition was observed in collagen-induced platelet aggregation. Using radioimmunoassay, we show in vitro that berberine significantly inhibited synthesis of thromboxane A(2) in rabbit platelets induced by adenosine diphosphate, arachidonic acid or collagen in which collagen-induced thromboxane A(2) synthesis was also most potently inhibited. In our in vivo study using radioimmunoassay, the plasma prostacyclin level was reduced by 34.6% during a 30-min period after intravenous administration of 50 mg/kg of berberine. These results suggest that berberine might inhibit arachidonic acid metabolism in rabbit platelets and endothelial cells at two or more sites: cyclooxygenase in the arachidonic acid cascade and possibly the enzyme(s) for arachidonic acid liberation from membrane phospholipid(s).

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.