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-PGF_1α. 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-PGF_1α 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-PGF_1α. Repeated incubations of erythrocytes with PGI₂ revealed that PGI₂ was rapidly degraded into a biologically inactive non-binding substance, presumably 6-keto-PGF_1α. 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.