Binding and inactivation of prostacyclin (PGI₂) by human erythrocytes

Abstract

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