Binding and inactivation of prostacyclin (PGI2) by human erythrocytes

Summary. Prostacyclin (PGI2), the most potent inhibitor of platelet aggregation known, is rapidly hydrolysed in aqueous solution at neutral pH to its inactive derivative 6‐keto‐PGF1α. In previous studies (Willems et al, 1979), we found that PGI2 is stabilized by plasma. Yet, PGI2 is rapidly inactivated in vivo. These findings prompted us to study the fate of PGI2 when incubated in whole human blood. After 20 min of incubation (at 37°C, pH 7·8), 29 ± 10% of the amount of PGI2, added to whole blood, remained in the supernatant plasma whereas, under the same conditions, 80 ± 3% and 86 ± 2% were recovered when PGI2 was added to platelet‐rich plasma or cell‐free plasma, respectively. When PGI2 was incubated with washed erythrocytes resuspended in plasma, 20 ± 15% of the added PGI2 remained in the supernatant after 10 min of incubation. These findings indicate that PGI2, when incubated with whole blood, is preferentially bound to erythrocytes. To study the kinetics of binding in more detail, [3H]PGI2 was incubated with washed erythrocytes resuspended in plasma. The binding was time‐ and concentration‐dependent. The observed binding of label represented binding of [3H]PGI2, because (1) acid‐treated label did not bind to erythrocytes; (2) no substantial binding of [3H]6‐keto‐PGF1α occurred, and (3) changes in the specific activity of the PGI2 preparation did not alter the binding percentage of labelled PGI2. The binding of [3H]PGI2 was not influenced by PGE1 or 6‐keto‐PGF1α. Repeated incubations of erythrocytes with PGI2 revealed that PGI2 was rapidly degraded into a biologically inactive non‐binding substance, presumably 6‐keto‐PGF1α. Binding and metabolism of PGI2 by erythrocytes may explain the apparent instability of PGI2 in whole blood and provides an explanation for the rapid loss of the biological effects of PGI2 on termination of the infusion.