It has been previously demonstrated that both externally generated and internally synthesized nitric oxide (NO) can affect red blood cell (RBC) deformability. Further studies have shown that the RBC has active NO synthesizing mechanisms and that these mechanisms may play role in maintaining normal RBC mechanical properties. However, hemoglobin within the RBC is known to be a potent scavenger of NO; oxy-hemoglobin scavenges NO faster than deoxy-hemoglobin via the dioxygenation reaction to nitrate. The present study aimed at investigating the role of hemoglobin oxygenation in the modulation of RBC rheologic behavior by NO. Human blood was obtained from healthy volunteers, anticoagulated with sodium heparin (15 IU/mL), and the hematocrit was adjusted to 0.4 L/L by adding or removing autologous plasma. Several two mL aliquots of blood were equilibrated at room temperature (22 +/- 2 degrees C) with moisturized air or 100% nitrogen by a membrane gas exchanger, The NO donor sodium nitroprusside (SNP), at a concentration range of 10(-7)-10(-4) M, was added to the equilibrated aliquots which were maintained under the same conditions for an additional 60 min. The effect of the non-specific NOS inhibitor L-NAME was also tested at a concentration of 10(-3) M. RBC deformability was measured using an ektacytometer with an environment corresponding to that used for the prior incubation (i.e., oxygenated or deoxygenated). Our results indicate an improvement of RBC deformability with the NO donor SNP that was much more pronounced in the deoxygenated aliquots. SNP also had a more pronounced effect on RBC aggregation for deoxygenated RBC. Conversely, L-NAME had no effect on deoxygenated blood but resulted in impaired deformability, with no change in aggregation for oxygenated blood. These findings can be explained by a differential behavior of hemoglobin under oxygenated and deoxygenated conditions; the influence of oxygen partial pressure on NOS activity may also play a role. It is therefore critical to consider the oxygenation state of intracellular hemoglobin while studying the role of NO as a regulator of RBC mechanical properties. (C) 2009 Elsevier Inc. All rights reserved.