Novel role of human RBCs in vascular tonus: RBCs contribute to adrenergic vasoconstriction via generation of thromboxane A2

• RBCs can effect vasoconstriction via adrenergic receptor-mediated COX activation leading to TxA2 synthesis and release. • RBC-derived TxA2 may contribute to physiologic adrenergic responses and pathological conditions related to TxA2 increase. Red blood cells (RBCs) are increasingly recognized as regulators of vascular tone, capable of releasing vasodilator substances dependent upon local oxygenation status. RBCs can also induce vasoconstriction, although the identity of the mediator(s) is not clear. We examined whether prostanoids could account for this effect using human RBCs and isolated rat aortic segments. Freshly prepared RBCs were characterized by flow cytometry and biochemical assays. Thoracic aortas were procured from anesthetized rats and segments with intact endothelium were suspended between wire myographs in Krebs buffer (95% oxygen/5% carbon dioxide; pH, 7.4; 37°C). Constrictor responses to phenylephrine (Phe) were assessed in the presence of increasing RBC suspensions (pseudo-hematocrit 10–55%) and in combination with adrenergic or prostanoid-active agents. RBCs were found to express the α1-adrenergic receptors, contain active COX enzyme, and release TxA2 after exposure to Phe. In addition, the RBC suspensions were free of hemoglobin and platelets. In the tissue baths, Phe-mediated vasoconstriction was augmented by RBCs in a hematocrit-dependent manner. This response was significantly reduced by Indomethacin (nonselective-COX inhibitor) and Carvedilol (Adrenergic receptor inhibitor). The RBC response was also attenuated by Dazoxiben (TxA2 synthesis inhibitor) and GR32191B (TxA2 receptor antagonist) but not by AL-8810 (prostaglandin F2α receptor antagonist. Finally, Tx metabolites were elevated in the bath solutions following RBC enhanced vasoconstriction. These findings identify RBC-derived TxA2, generated via α1 adrenergic receptor activation, as a mediator of adrenergic vasoconstriction under oxygenated conditions. By demonstrating a prostanoid-dependent mechanism, our work provides a potential mechanism for how RBCs may regulate vasoconstriction in vivo .