The impact of nebulized epoprostenol and iloprost on hemoglobin oxygen affinity: an ex vivo experiment

Inhalational prostacyclins act as strong vasodilators, potentially improving oxygenation by reducing shunt fraction and ventilation-perfusion mismatch. As prostacyclin receptors are known to be present on human erythrocytes, possible direct effects on hemoglobin oxygen transport were further explored by examining the sole in vitro influence of prostacyclins on hemoglobin oxygen (Hb-O₂) affinity. Venous blood samples from 20 healthy volunteers were exposed in vitro to supramaximal doses of epoprostenol, iloprost, and compared with control. By high-throughput measurements, hemoglobin oxygen dissociation curves (ODCs) were derived. Hb-O₂ affinity, expressed by P50 and Hill coefficient, was determined and analyzed for three subgroups: males (n = 10), females not taking oral contraceptives (n = 4), and females taking oral contraceptives (n = 6). Epoprostenol significantly decreased P50 in all (males, females without contraceptives, and females taking oral contraceptives) [27.5 (26.4-28.6) mmHg (control) vs. 24.2 (22.7-25.3) mmHg; P < 0.001. median (interquartile range, IQR)] thereby increasing Hb-O₂ affinity. Inversely, iloprost only showed significant effects in females taking oral contraceptives where P50 was markedly increased and therefore Hb-O₂ affinity decreased [28.4 (27.9-28.9) mmHg (control) vs. 34.4 (32.2-36.0) mmHg; P < 0.001]. Prostacyclin-receptor stimulation and subsequent cAMP-mediated ATP release from erythrocytes are discussed as a possible underlying mechanism for the effect of epoprostenol on Hb-O₂ affinity. The reason for the sex hormone-modified iloprost effect remains unclear. Being aware of potentially differing effects on Hb-O₂ affinity might help select the right prostacyclin (epoprostenol vs. iloprost) depending on the patient and the underlying disease (e.g., acute respiratory distress syndrome vs. peripheral arterial disease).