Formation of prostacyclin-sensitive platelet aggregates in human whole blood in vitro. Part I. The occurrence of the phenomenon in healthy male volunteers.

When unstirred citrated blood of young males was left to stand at 21 degrees C, the number of free platelets decreased as measured with a whole blood platelet counter. This decrease indicated spontaneous platelet aggregation (SPA) since it was accompanied by the time-dependent increase in platelet micro-aggregates and plasma beta-thromboglobulin while lactate dehydrogenase level did not change significantly. Addition of PGI2 to whole blood increased free platelet count and decreased the number of platelet micro-aggregates. The de-aggregatory effect of PGI2 (measured as a percentage increase in free platelet number) was correlated with the initial amount of platelet micro-aggregates. Blood storage enhanced SPA and de-aggregatory effect of prostacyclin. Immediately after blood collection de-aggregatory effect of prostacyclin was absent. Our results favour an assumption that prostacyclin-sensitive platelet aggregates in blood of young volunteers are formed in vitro rather than in vivo.

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.

Effect of PGI2 on platelet aggregates in myocardial infarction evaluated in vitro in whole blood. The method for quantification of platelet aggregates.

PGI2 was found to increase the number of free platelets (FP) in citrated whole blood (WB) as a result of incomplete dispersion of platelet aggregates of various magnitudes. In WB from young, male volunteers the effect of PGI2 on FP number was absent immediately after blood collection, increased with time of blood storage, and was dose-dependent. In WB from 14 males with myocardial infarction (MI) effect of PGI2 on FP number was higher than in 11 male patient controls (PC). On 14th day after MI effect of PGI2 on FP in WB was reduced. It is concluded that measurement of PGI2-sensitive platelet aggregates may be of some clinical significance. However, presented results did not allow for differentiation between circulating platelet aggregates and those formed in vitro during spontaneous aggregation of platelets.