Potentiation of Adrenaline-Induced Platelet Aggregation by Angiotensin II

The effects of angiotensin II (ANG II) alone and in combination with other agonists on human platelet aggregation, thromboxane B2 (TxB2) and cytosolic Ca2+i were investigated. ANG II (10(-11) - 10(-7) M) alone had no direct effect on aggregation, TxB2 production or Ca2+i after short- (less than 2 min) or long-term (30 min) incubation. In contrast, low concentrations of ANG II (10(-11) M) enhanced adrenaline-induced platelet aggregation but high concentrations (10(-7) M) had an inhibitory effect. Moreover, ANG II (10(-11) - 10(-7) M) augmented platelet responses to the TxA2 mimetic, U44069. Pretreatment of platelets with flurbiprofen abolished this facilitatory effect of ANG II on adrenaline- but not on U44069-induced platelet aggregation. These results suggest that ANG II stimulation of agonist-induced platelet activation may be due to potentiation of the effects rather than the synthesis of TxA2.