Angiotensin II potentiates agonist-induced platelet activation, likely by enhancing the effects rather than the synthesis of thromboxane A2.
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.
Ding et al. (1985) studied this question.