Endothelium-derived nitric oxide production is increased by ATP released from red blood cells incubated with hydroxyurea

•Hydroxyurea increases flow-induced ATP release from human red blood cells.•Red blood cell ATP can diffuse to cultured endothelial cells to induce NO production.•We establish that the mechanism of HU-mediated NO release from bPAECs is ATP-mediated.•We also establish that this mechanism is dependent on red blood cell NO production in the presence of HU.

Red blood cells (RBCs) release adenosine triphosphate (ATP) in response to a variety of stimuli, including flow-induced deformation. Hydroxyurea (HU), a proven therapy for individuals with sickle cell disease (SCD), is known to improve blood flow. However, the exact mechanism leading to the improved blood flow is incomplete. Here, we report that the incubation of human RBCs with HU enhances ATP release from these cells and that this ATP is capable of stimulating nitric oxide (NO) production in an endothelium. RBCs incubated with HU were pumped through micron-size flow channels in a microfluidic device. The release of ATP from the RBCs was measured using the luciferin–luciferase assay in detection wells on the device that were separated from the flow channels by a porous polycarbonate membrane. NO released from a layer of bovine artery endothelial cells (bPAECs) cultured on the polycarbonate membrane was also measured using the extracellular NO probe DAF-FM. ATP release from human RBCs incubated with 100 μM HU was observed to be 2.06 ± 0.37-fold larger than control samples without HU (p < 0.05, N ⩾ 3). When HU-incubated RBCs were flowed under a layer of bPAECs, NO released from the bPAEC layer was measured to be 1.34 ± 0.10-fold higher than controls. An antagonist of the P2Y receptor established that this extra 30% increase in NO release is ATP mediated. Furthermore, when RBCs were incubated with L-NAME, a significant decrease in endothelium-derived NO production was observed. Control experiments suggest that RBC-generated NO indirectly affects endothelial NO production via its effects on RBC-derived ATP release.