LPA induces platelet aggregation via ADP-mediated activation of the P2Y₁ and P2Y₁₂ receptors and is insensitive to inhibition by aspirin. (A) Washed human platelets were preincubated with buffer, MRS2179 (100 μM), AR-C69931MX (1 μM), or a combination of both before addition of 10 μM 1-acyl-LPA (16:0). Serotonin secretion (percent of total) is indicated in brackets. (B) Human blood anticoagulated with hirudin was preincubated with the P2Y₁₂ receptor antagonist AR-C69931MX (1 μM), the P2Y₁ receptor antagonist MRS2179 (100 μM), or aspirin (ASS; 1 mM) before addition of 10 μM 1-acyl-LPA (16:0) (mean ± SD, n = 3).