Low concentrations of nitric oxide increase oxygen affinity of sickle erythrocytes in vitro and in vivo.

Abstract

The hallmark of sickle cell disease (SCD) is the polymerization of deoxygenated sickle hemoglobin (HbS). In SCD patients, one strategy to reduce 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 homozygous 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, before and after breathing 80 ppm NO in air for 45 min, and the ODCs were measured. Exposure of SS RBCs to 80 ppm NO in vitro for 5 min or longer decreased the partial pressure of oxygen at which hemoglobin is 50% saturated with oxygen (P 50 ), an average of 15% (4.8 Ϯ 1.7 mmHg mean Ϯ SE; P Ͻ 0.001). The increase in SS RBC oxygen affinity correlated with the NO concentration. The P 50 of AA RBCs was unchanged ( P Ͼ 0.1) by 80 ppm NO. In SS volunteers breathing 80 ppm NO for 45 min, the P 50 decreased ( P Ͻ 0.001) by 4.6 Ϯ 2.0 mmHg. 60 min after NO breathing was discontinued, the RBC P 50 remained decreased in five of seven volunteers in whom the ODC was measured. There was no RBC P 50 change ( P Ͼ 0.1) in AA volunteers breathing NO. Methemoglobin (Mhb) remained low in all subjects breathing NO (SS Mhb 1.4 Ϯ 0.5%), and there was no correlation ( r ϭ 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. ( J. Clin. Invest. 1997. 100: 1193-1198.)

Inhaled nitric oxide augments nitric oxide transport on sickle cell hemoglobin without affecting oxygen affinity

“…Proteolytic digestion increases the signal/noise ratio, but loss of NO occurs before complete digestion, again limiting sensitivity. These results, however, are inconsistent with most of the mechanisms postulated to explain the reported increase in oxygen affinity with NO exposure (9).…”

“…The analyzer measured the oxygen tension with a standard Clark O2 electrode (Model 5331 Oxygen Probe; Yellow Springs Instrument Co., Yellow Springs, Ohio, USA) and simultaneously calculated the hemoglobin saturation using dual-wavelength spectrophotometry (9,10). The ODCs were recorded during both deoxygenation and reoxygenation.…”

Hydroxyurea downregulates endothelin-1 gene expression and upregulates ICAM-1 gene expression in cultured human endothelial cells

“…HU is believed to be a chemical donor of NO, 29,30 a potent vasodilator, and NO seems to provide a variety of beneficial effects in SCA: (i) low-dose inhalation of NO increases the oxygen affinity of sickle red cells both in vivo and in vitro, 31 (ii) NO inhalation at 20 ppm seems to provide a rapid protection against severe hypoxic stress in a transgenic mice model for SCD, 32 (iii) in SCA patients with ACS, inhaled NO improves ventilation-perfusion matching 33 and (iv) SCA patients with elevated levels of NO metabolites report lower pain scores. 34 NOS tightly regulates the NO expression in endothelial cells.…”

The potential of Angeli's salt to decrease nitric oxide scavenging by plasma hemoglobin

“…These results support the approach of oxidizing the cell-free Hb to diminish NO scavenging. Indeed, NO inhalation therapy in sickle cell disease and other hemolytic conditions has been gaining increased attention [17,68,72,80]. Although use of NO inhalation therapy holds promise for treatment of hemolytic conditions, its use is not practical in a variety of settings, particularly where chronic treatment is desired.…”