PGE₁ inhibits Ca²⁺ and RhoA/ROCK-dependent phosphorylation of MLC. Washed platelets (3 × 10⁸ platelets per milliliter) were stimulated with thrombin (0.05 U/mL) for 1 minute in the presence or absence of Y27632 (10 µM), BAPTA-AM (20 µM), or PGE₁ (100 nM), and phospho-MLC^ser19 was assessed by immunoblotting. (A) Representative immunoblots from 5 independent experiments. (B) Densitometry of MLC^ser19 phosphorylation of 5 different experiments. ns, not significant. *P < .05 compared with thrombin only; **P < .01 compared with thrombin only.