Anti-aggregatory activity of PGI2 in whole blood measured by platelet counter

Anti-aggregatory effect of PGI2 was evaluated by two methods. First, by measurement of the number of free platelets in a sample of whole citrated blood after addition of ADP, and second, by monitoring changes in light transmission induced by ADP in platelet-rich plasma. Platelet aggregation in response to ADP, as assessed by the first method, was dose-dependent, reproducible, and stable during 10-60 min storage of blood samples. EC50's for ADP were (microM): 0.7 (first method) and 2.9 (second method). IC50's for anti-aggregatory effect of PGI2-tested with 1 microM of ADP were (nM): 0.48 (first method) and 2.32 (second method), the difference suggesting higher sensitivity of the first method. Measurement by the first methods can be performed 1 min after blood collection. It is concluded that evaluation of the anti-aggregatory effect of PGI2 by monitoring the free platelet number in ADP-treated whole blood is more sensitive than the conventional turbidimetric technique since it allows detection of blood PGI2 levels more than 100 times smaller than the turbidimetric method and seems suitable for monitoring PGI2 therapy in clinical studies.