LOW CONCENTRATIONS OF NITRIC-OXIDE INCREASE OXYGEN-AFFINITY OF SICKLEERYTHROCYTES IN-VITRO AND IN-VIVO

The hallmark of sickle cell disease (SCD) is the polymerization of deo xygenated sickle hemoglobin (HbS). In SCD patients, one strategy to re duce red blood cell (RBC) sickling is to increase HbS oxygen affinity. Our objective was to determine if low concentrations of nitric oxide (NO) gas would augment the oxygen affinity of RBCs containing homozygo us HbS (SS), Blood containing normal adult hemoglobin (AA) or SS RBCs was incubated in vitro in the presence of varying concentrations of NO up to 80 ppm, and oxygen dissociation curves (ODCs) were measured. In addition, blood was obtained from three AA and nine SS volunteers, be fore and after breathing 80 ppm NO in air for 45 min, and the ODCs wer e measured. Exposure of SS RBCs to 80 ppm NO in vitro for 5 min or lon ger decreased the partial pressure of oxygen at which hemoglobin is 50 % saturated with oxygen (P-50), an average of 15% (4.8+/-1.7 mmHg mean +/-SE; Pt 0.001). The increase in SS RBC oxygen affinity correlated wi th the NO concentration. The P-50 of AA RBCs was unchanged (P > 0.1) b y 80 ppm NO. In SS volunteers breathing 80 ppm NO for 45 min, the P-50 decreased (P < 0.001) by 4.6+/-2.0 mmHg. 60 min after NO breathing wa s discontinued, the RBC P-50 remained decreased in five of seven volun teers in whom the ODC was measured. There was no RBC P-50 change (P > 0.1) in AA volunteers breathing NO. Methemoglobin (Mhb) remained low i n all subjects breathing NO (SS Mhb 1.4+/-0.5%), and there was no corr elation (r = 0.02) between the reduction in P-50 and the change in Mhb . Thus, low concentrations of NO augment the oxygen affinity of sickle erythrocytes in vitro and in vivo without significant Mhb production. These results suggest that low concentrations of NO gas may offer an attractive new therapeutic model for the treatment of SCD.