NITROSYL HEMOGLOBIN IN BLOOD OF NORMOXIC AND HYPOXIC SHEEP DURING NITRIC-OXIDE INHALATION

During nitric oxide (NO) inhalation therapy, NO combines with deoxyhem oglobin to form nitrosyl hemoglobin (HbNO). We used electron spin reso nance (ESR) spectroscopy to measure HbNO in arterial and mixed venous blood of normoxic and hypoxic sheep during NO inhalation. Our aim was to quantitatively measure HbNO levels in the blood during NO inhalatio n, because large amounts of HbNO reduce the oxygen capacity of blood, particularly in hypoxia. Another aim was to investigate the transfer o f exogenous NO to the alpha-heme iron of hemoglobin. Thirteen sheep we re anesthetized with pentobarbital sodium, and 60 parts per million (p pm) NO were administered for 1 h in the presence of normoxia and hypox ia. Two-way analysis of variance revealed that the HbNO level was depe ndent on the oxygen level (normoxia vs. hypoxia) and NO inhalation, an d there was a significant negative correlation between the HbNO level and arterial O-2 saturation (Sa(O2)) Although the HbNO level increased during NO inhalation in hypoxia, the HbNO level at Sa(O2) >60% was <1 1 mu mol/l monomer hemoglobin (0.11% of total 10 mmol/l monomer hemogl obin). The peak of the HbNO ESR spectrum in arterial blood is located in almost the same position in mixed venous blood with an asymmetric H bNO signal, indicating that the NO in beta-heme HbNO molecules had bee n transferred to alpha-heme molecules. The three-line hyperfine struct ure of HbNO on ESR spectra was distinct in venous blood in hypoxia dur ing NO inhalation, indicating pentacoordinate alpha-NO heme formation in hypoxic blood. In conclusion, the amount of HbNO during 60 ppm NO i nhalation did not considerably reduce the oxygen capacity of the blood even in the presence of hypoxia, and the NO of HbNO was transferred t o the alpha-heme iron of hemoglobin, forming pentacoordinate alpha-NO heme in mixed venous blood in hypoxia.