Prostaglandin E₂ stimulates a Ca²⁺-dependent K⁺ channel in human erythrocytes and alters cell volume and filterability

To understand the mechanism by which human red blood cells (RBCs) contribute to hemostasis and thrombosis, we have examined the effects of metabolites released by activated platelets on intact RBCs. Prostaglandin E₂ (PGE₂), a signal molecule produced by activated platelets, was observed to lower the filterability of human erythrocytes by ~30% at 10^-10 M. PGE₂ also caused a reduction in mean cell volume of ~10%. The shrinkage of red cells after PGE₂ treatment was confirmed by documenting a decrease in osmotic fragility and an increase in cell density following exposure to the hormone. Careful analysis, however, revealed that only ~15% of the erythrocytes responded to stimulation with PGE₂. Examination of the cause of cell shrinkage showed that induction of a PGE₂-stimulated K⁺ efflux pathway leading to rapid loss of cellular K⁺ was responsible. The PGE₂-stimulated K⁺ loss was also observed to be Ca²⁺-dependent, suggesting the possible involvement of the Gardos channel. Gardos channel participation was supported by the observation that two Gardos channel inhibitors, charybdotoxin and clotrimazole, independently blocked the PGE₂-stimulated K⁺ efflux. Further evidence for Gardos channel activation came from experiments aimed at characterizing the efflux pathway followed by the obligatory counterion. Thus, K⁺ efflux was readily stimulated even when NO₃/^- was substituted for Cl^-, suggesting that neither KCl cotransport nor Na/K/2Cl cotransport plays a prominent role in the PGE₂-induced cell shrinkage. Further, the anion transporter band 3 was implicated as the counterion efflux route, since DIDS inhibited the PGE₂-stimulated cell volume change without blocking the change in membrane potential. Taken together, we propose that release of PGE₂ by activated platelets constitutes part of a mechanism by which activated platelets may recruit adjacent erythrocytes to assist in clot formation.