*3.4.3 NOHb*

Nakajima and Oda have shown that the NOHb concentration is 0.13% in the blood during 20 min of 10 ppm NO inhalation [56]. This low concentration suggests the rapid turnover of NOHb, in which NOHb is presumably an intermediate in the conversion from NO to NO2 <sup>−</sup> and NO3 −.

#### *3.4.4 Conversion of NO2 <sup>−</sup> to NO3 −*

When 5 mM NO2 <sup>−</sup> is added to human blood, NO3 <sup>−</sup> changes are detected within 10 min [57]. The intravenous injection of sodium nitrite to rabbits results in the rapid disappearance of NO2 <sup>−</sup>. After the intratracheal injection of 13NO2 <sup>−</sup>, 70% of 13NO2 <sup>−</sup> changed to 13NO3 <sup>−</sup>, and 26% remained as 13NO2 <sup>−</sup> [58]. These observations suggest that NO2 <sup>−</sup> is converted to NO3 <sup>−</sup> in red blood cells [59]. NO2 <sup>−</sup> and NO3 <sup>−</sup> are stable and unchanged in plasma without red blood cells.

#### **Figure 6.**

*Reaction of NO with Hb. NO reacts with Fe2+ of oxy-Hb making MetHb and NO3 <sup>−</sup> and combines with deoxy-Hb making NOHb. NOHb reacts with O2 making MetHb and NO3 −.*

**75**

**Figure 7.**

*Metabolic fate of NO. Almost all inhaled NO is converted to NO3*

*urine; 10% is changed to nitrogen compound except NO3*

*and discharged outside of the body.*

*Endogenous and Inhaled Nitric Oxide for the Treatment of Pulmonary Hypertension*

A clear understanding of the fate of iNO is critical for its clinical use in humans.

<sup>−</sup> → NO2

<sup>−</sup> → N2 by stomach flora.

*<sup>−</sup>. Forty-five percent of NO3*

*<sup>−</sup> and excreted in feces; 10% is changed to* 

*<sup>−</sup> and NO2*

*urea through the digestive tract and liver and excreted in urine. The rest will be changed to N2 in the stomach* 

*<sup>−</sup> is excreted in* 

As previously mentioned, the metabolic fate of iNO was examined in the early 1980s, and it was found that retention of iNO in the body was lacking (**Figure 7**). An inhalation study of 15NO in rats investigated the metabolism of iNO. In the carcasses, 1.6% of total inhaled 15N was detected, similar to the level of natural 15N. This result suggests that iNO largely does not remain in the body. About 55% of total inhaled 15N was recovered in urine [41, 42], including 45% as nitrates and 10% as urea (**Figure 7**). About 10% of total inhaled 15N was recovered as undetermined nitrogen compounds in feces. The remaining 35% was not recovered but is assumed

During cardiopulmonary bypass(CPB), hemolysis causes an increase in Hb plasma concentration due to the destruction of red blood cells. Hb includes oxy-Hb and deoxy-Hb. Oxy-Hb causes vasoconstriction, which is partly due to the depletion of NO available to induce vascular smooth muscle relaxation. NO is produced in and released from endothelial cells, some of which reaches adjacent vascular smooth muscles, causing vasorelaxation, and some of which diffuses into the plasma. If the amount of oxy-Hb in the plasma increases, the binding of NO to oxy-Hb increases, resulting in less NO reaching adjacent smooth muscle cells. Thus, the presence of large amounts of oxy-Hb might decrease NO availability in vascular smooth muscle

*DOI: http://dx.doi.org/10.5772/intechopen.89381*

to be N2 produced from the reduction of NO3

**3.5 iNO effects in remote organs**

cells (**Figure 8**).

*3.4.5 No retention of iNO in the body*

*Endogenous and Inhaled Nitric Oxide for the Treatment of Pulmonary Hypertension DOI: http://dx.doi.org/10.5772/intechopen.89381*

#### *3.4.5 No retention of iNO in the body*

A clear understanding of the fate of iNO is critical for its clinical use in humans. As previously mentioned, the metabolic fate of iNO was examined in the early 1980s, and it was found that retention of iNO in the body was lacking (**Figure 7**).

An inhalation study of 15NO in rats investigated the metabolism of iNO. In the carcasses, 1.6% of total inhaled 15N was detected, similar to the level of natural 15N. This result suggests that iNO largely does not remain in the body. About 55% of total inhaled 15N was recovered in urine [41, 42], including 45% as nitrates and 10% as urea (**Figure 7**). About 10% of total inhaled 15N was recovered as undetermined nitrogen compounds in feces. The remaining 35% was not recovered but is assumed to be N2 produced from the reduction of NO3 <sup>−</sup> → NO2 <sup>−</sup> → N2 by stomach flora.

#### **3.5 iNO effects in remote organs**

During cardiopulmonary bypass(CPB), hemolysis causes an increase in Hb plasma concentration due to the destruction of red blood cells. Hb includes oxy-Hb and deoxy-Hb. Oxy-Hb causes vasoconstriction, which is partly due to the depletion of NO available to induce vascular smooth muscle relaxation. NO is produced in and released from endothelial cells, some of which reaches adjacent vascular smooth muscles, causing vasorelaxation, and some of which diffuses into the plasma. If the amount of oxy-Hb in the plasma increases, the binding of NO to oxy-Hb increases, resulting in less NO reaching adjacent smooth muscle cells. Thus, the presence of large amounts of oxy-Hb might decrease NO availability in vascular smooth muscle cells (**Figure 8**).

#### **Figure 7.**

*Metabolic fate of NO. Almost all inhaled NO is converted to NO3 <sup>−</sup>. Forty-five percent of NO3 <sup>−</sup> is excreted in urine; 10% is changed to nitrogen compound except NO3 <sup>−</sup> and NO2 <sup>−</sup> and excreted in feces; 10% is changed to urea through the digestive tract and liver and excreted in urine. The rest will be changed to N2 in the stomach and discharged outside of the body.*

#### **Figure 8.**

*NO formed in the endothelium is scavenged by plasma oxygenated Hb. (A) NO produced in the endothelium diffuses into adjacent smooth muscle cells and binds with guanylate cyclase. (B) If O2Hb (Fe2+) increases, NO produced in the endothelium diffuses into the blood and scavenged by O2Hb (Fe2+), decreasing the amount of NO diffused into smooth muscle cells. (C) If O2 Hb(Fe2+) is converted to MetHb(Fe3+) by NO inhalation, NO is not scavenged, which recovers the amount of NO diffusing to the smooth muscle side.*

Acute kidney injury is a common complication after cardiac surgery with prolonged CPB. Because oxygen tension is high during CPB, plasma oxy-Hb exhibits substantial hemolysis, causing vasoconstriction in the kidney. Recently, NO was demonstrated to decrease the occurrence of acute kidney injury and chronic kidney disease 1 year postoperatively [47]. NO inhalation at 80 ppm was started at the onset of CPB via a CPB machine and was continued after CPB via a mechanical ventilator for 24 h or less if patients were ready to be extubated early. Under NO inhalation, oxy-Hb was converted to MetHb, which recovered NO availability to vascular smooth muscle cells due to the decrease in oxy-Hb. Thus, exogenous NO inhalation might increase endogenous NO availability to counteract renal vasoconstriction during CPB.

In brief, the reduction of nitrite and nitrate produces NO (**Figure 9**). Nitrite is reduced by deoxy-Hb, respiratory chain enzymes, xanthine oxidoreductase, deoxygenated myoglobin, and protons, facilitating the transfer of protons to NO2 <sup>−</sup> and thereby producing NO. These reactions are intensified under acidic and hypoxic states. After iNO is converted to NO2 <sup>−</sup> and NO3 <sup>−</sup>, NO can be recycled from nitrite and used to protect organs from ischemia-reperfusion injury [48, 51].

In liver transplantation, the inhalation of 80 ppm NO until reperfusion ameliorates apoptosis, attenuated increases of liver enzymes, and enhanced the recovery of coagulation factors [48].

In orthopedic knee surgery, NO inhalation prevented increases in the adhesion molecule expression on granulocytes, plasma selectin levels, and NF-κB expression in quadriceps muscles [51]. NO inhalation was started before the tourniquet application and was continued during reperfusion until the completion of surgery.

**77**

*Endogenous and Inhaled Nitric Oxide for the Treatment of Pulmonary Hypertension*

**4. Update on the clinical application of iNO in pediatric patients**

*relation. Please count the number of O before and after the reduction responses.*

PAH or operability for children with congenital heart disease.

**premature newborns**

*), and cytochrome enzymes (Fe2+). NO2*

**Figure 9.**

*(H+*

**4.1 Role of inhaled NO in the prevention of bronchopulmonary dysplasia in** 

Bronchopulmonary dysplasia (BPD), which is characterized by impaired pulmonary development resulting from insults affecting the immature lung, including inflammation, hyperoxia, and mechanical ventilation, is associated with high mortality and adverse long-term neurological and respiratory outcomes in infants born very preterm. Although the effectiveness of iNO for the treatment of PPHN is largely due to its function as a selective pulmonary vasodilator, laboratory observations also suggest other important biological effects of NO, such as roles in decreasing lung inflammation (e.g., lung vascular protein leak; pulmonary neutrophil accumulation) [61], reducing oxidant stress [62], decreasing pulmonary vascular cell proliferation [63], and enhancing alveolarization and lung growth [64–66]. These observations have led to investigations into the use of iNO to prevent the development of BPD in premature newborns. In an initial randomized,

After NO was identified as an endothelial cell-derived relaxation factor and following preclinical studies, iNO therapy has been studied extensively in multicenter randomized trials as well as in early pilot studies of infants with severe hypoxemia associated with PH or infants with congenital diaphragmatic hernia (CDH) [60]. These studies have demonstrated improved oxygenation and reduction in the need for extracorporeal membrane oxygenation (ECMO) therapy, leading to the approval of iNO therapy by the Food and Drug Administration for use in patients at >34-week gestation with hypoxemic respiratory failure and persistent PH of the newborn (PPHN). Over the last two decades, the discussion of its application has been extended to premature infants and acute pulmonary vascular response testing to assess indications for specific pulmonary vasodilator therapy for patients with

*Recycle of NO from nitrite and nitrate. Nitrate is reduced to nitrite, and subsequent reduction of nitrite forms NO. Reducing substances are deoxy-Hb [Hb(Fe2+)], myoglobin [Mb(Fe2+)], xanthine oxidase, hydrogen ion* 

*<sup>−</sup> is reduced by deoxy-Hb(Fe2+) to NO showing stoichiometric* 

*DOI: http://dx.doi.org/10.5772/intechopen.89381*

*Endogenous and Inhaled Nitric Oxide for the Treatment of Pulmonary Hypertension DOI: http://dx.doi.org/10.5772/intechopen.89381*

#### **Figure 9.**

*Recycle of NO from nitrite and nitrate. Nitrate is reduced to nitrite, and subsequent reduction of nitrite forms NO. Reducing substances are deoxy-Hb [Hb(Fe2+)], myoglobin [Mb(Fe2+)], xanthine oxidase, hydrogen ion (H+ ), and cytochrome enzymes (Fe2+). NO2 <sup>−</sup> is reduced by deoxy-Hb(Fe2+) to NO showing stoichiometric relation. Please count the number of O before and after the reduction responses.*
