Effects of magnesium lithospermate B on aggregation and 5-HT release in rabbit washed platelets

AIM: To study the effects and mechanism of magnesium lithospermate B(MLB) o n rabbit platelet aggregation and 5-HT release. METHODS: The platelet aggre gation was determined by Born's method. Release of serotonin (5-HT) and for mation of thromboxane A(2) (TXA(2)) were measured by fluorophotometry and r adioimmunoassay (RIA) respectively. Cytoplasmic free Ca2+ concentration ([C a2+](i)) in platelets was measured by Fura 2-AM fluorescence technique. RES ULTS: In washed platelets, thrombin (200 U/L) or arachidonic acid (AA) (30 mu mol/l)-induced aggregation was inhibited by MLB 50 - 800 mg/L in a conce ntration-dependent manner. In addition, MLB had more inhibitory effects on platelet aggregation in the absence of extracellular calcium with IC50 of 1 02 mg/L than in the presence of CaCl2 1 mmol/L with IC50 of 194 mg/L. MLB c oncentration-dependently decreased the thrombin-activated release of 5-HT, whereas it did not affect the formation of TXA(2) in platelets. Furthermore , MLB not only inhibited the rise of [Ca2+](i) in thrombin stimulated plate lets, but decreased the [Ca2+](i) in resting platelets. CONCLUSION: MLB inh ibited the aggregation and 5-HT release in rabbit platelets and it is proba bly by attenuating intracellular calcium concentration.