Inactivation of prostacyclin (PGI2) by erythrocytes

Different roles of endothelium-derived substances on inhibiting platelet aggregation in whole blood

1. The inhibitory effects of adenosine, nitroprusside (a nitric oxide donor) and prostacyclin on collagen-induced rabbit platelet aggregation were studied under two different conditions: in whole blood with an impedance method and in platelet-rich plasma (PRP) with a turbidimetric method. 2. All substances tested were less potent in whole blood than in PRP, and the differences in IC50 value between whole blood and PRP were not of the same order of magnitude; adenosine (669-fold), nitroprusside (54-fold) and prostacyclin (2-fold). 3. These results imply that (a) some other, as yet unknown, factors in blood modulate the platelet aggregation; (b) adenosine and nitric oxide act close to the endothelium, and (c) prostacyclin acts as a relatively long lasting circulating hormone.

Mechanism of ethanol-induced aggregation in whole blood

Effects of ethanol on blood clotting and platelet aggregation have been reported in many models, but its in vitro actions in whole blood, impedance aggregometry have not been reported. We investigated the effect of ethanol in vitro in whole blood and platelet rich plasma of humans and rats, as measured by impedance aggregometry. Ethanol (34 to 170 mM) induced concentration-dependent aggregation in whole blood but not platelet rich plasma. In further studies in rats, aggregation was inhibited by pretreatment of whole blood with the prostacyclin analog iloprost or the enzyme apyrase, which degrades ADP to AMP. Levels of ethanol which produced aggregation in whole blood were also associated with concentration-dependent hemolysis. Based on the requirement for whole blood for ethanol-induced aggregation, the inhibitory effect of apyrase and our observation of hemolysis, and previous studies which have demonstrated the potential contribution of ADP from lysed red blood cells to platelet aggregation, we conclude that ethanol-induced aggregation in whole blood is mediated by erythrocyte lysis and the ADP released from these cells.

Modification of membrane properties of erythrocytes by PGI2

The addition of prostaglandin I2 (PGI2) enhanced the adhesion of red cells from normals and patients with diabetes mellitus or sickle cell anemia (SCA) to human cultured endothelial cells (p less than 0.025). The maximal effect was reached with 10(-11) M PGI2 after 30 min incubation. Red cell adhesion was also increased by PGD2 but PGE1 and 6-keto-PGF1 alpha had no significant effect. Since enhanced adhesion of red cell to endothelium and increased red cell calcium content have been proposed to be related in SCA, we have investigated the calcium binding to human resealed normal erythrocyte membrane by using (45Ca) calcium in presence of the different PG which alter red cell adhesion or not. Calcium binding was time-dependent and potentiated in presence of PGI2 (p less than 0.01) but not of PGD2. The fact that erythrocyte adhesion is enhanced by both PGI2 and PGD2 while calcium binding is increased only by PGI2 suggests that the two phenomenon can be dissociated.