On the inhibitory effect of chloroquine on blood platelet aggregation.

Chloroquine inhibited aggregation of rat blood platelets in a dose-dependent way. The inhibitory effect decreased, depending on the aggregation stimulus used, in the rank order of potency: ADP > thrombin > A23187. Effect of 1 millimolar chloroquine on A23187-stimulated aggregation was significantly decreased or completely abolished by addition of Ca2+ ions in the concentration of 0.1 or 1 mmol/l, respectively. For thrombin-stimulated aggregation the effect of chloroquine was partially decreased by corresponding isomolar calcium; platelet aggregation induced by ADP was not changed in the presence of extracellular Ca2+ ions. Addition of chloroquine during aggregation resulted in disaggregation of ADP-stimulated platelets and inhibition of thrombin- and A23187-induced aggregation.