A Breakthrough in Respiratory Physiology: Inhaled Nitric Oxide Transported as SNO-Hb

Key Points

• Inhaled nitric oxide (NO) increases circulating levels of SNO-Hb, a bioactive form of NO

• Inhaled NO also increases circulating levels of nitrite, another NO metabolite

• The lungs might act as a reservoir of SNO-Hb, releasing it into circulation as needed

The Breathing Diabetic Summary

Inhaled nitric oxide (NO) has many systemic impacts.  An overview of these effects can be found here and here.  However, it has remained unclear how inhaled NO exerts these effects.  In general, inhaled NO is believed to react and become inactive after reaching the lungs.  Thus, conventional thinking is that the systemic effects of NO are due to improved gas exchange in the lungs, which then has positive downstream impacts.

Interestingly, despite its widespread clinical use, there have been very few studies testing this hypothesis to truly discover how inhaled NO exerts its systemic effects.  This paper sought to fill that gap.

To do this, they recruited 15 healthy volunteers.  They had them inhale NO at concentrations of 40 ppm (the maximum produced in the paranasal sinuses is on the order of ~20 ppm, but typically much less).  They inhaled the added NO for 15 minutes.  Blood samples were collected before inhalation, at the end of the 15 minutes of inhalation, and then at 5, 15, and 30 minutes post-inhalation. 

A Breakthrough in Cardio-Respiratory Physiology

The results were striking.  They found that NO inhalation significantly increased circulating levels of SNO-Hb and nitrite.  This is important because SNO-Hb plays a significant role in whole-body oxygenation.  A 2015 PNAS study discovered that SNO-Hb “senses” areas of low oxygen, and then releases bioactive NO to increase blood flow and oxygenation.  This discovery led to breathing be considered as a three-gas system involving oxygen, carbon dioxide, and NO.  Thus, if inhaling NO increases SNO-Hb, it could be playing a critical role in whole-body oxygenation.  This gets even more intriguing (see next two sections), but first, let’s cover their nitrite finding.

They also observed increases in circulating nitrite.  This is important because, like SNO-Hb, nitrite can also release bioactive NO in regions of hypoxia. However, nitrite can do this independent of the hemoglobin, thus providing a “back-up mechanism” for increasing blood flow in regions of low oxygen.

The Lungs as a Reservoir of SNO-Hb

An interesting finding from this study was that nitrite levels were most significant at the 5-min post inhalation mark.  In contrast, SNO-Hb continued rising throughout the 30 minutes.  This led the authors to believe that the lungs might be acting as an SNO-Hb reservoir, releasing it "as needed." 

Why These Findings Matter

When we breathe through our nose, we carry NO into the lungs (although not at concentrations as high as those studied here).  Based on these findings, we can now be reasonably confident this NO enters the bloodstream and is carried as SNO-Hb and nitrite.  Thus, breathing through your nose might not just improve gas exchange in the lungs.  It might also help make sure oxygen gets delivered where it is needed most throughout the body. 

Additionally, their finding that SNO-Hb levels continued increasing after NO inhalation is intriguing.  It might support the idea that nose breathing provides a baseline level of NO that keeps SNO-Hb in its normal range.  Then, when excess NO is inhaled, the body stores that "just in case."  This is speculative, but interesting to contemplate.

Finally, this is one study, and it’s relatively new.  We’ll need more to confirm/deny that NO inhalation consistently increases SNO-Hb and nitrite across different populations.  In the meantime, let’s keep breathing through our noses.  It may just be the key to whole-body oxygenation.

Abstract

Rationale: Inhaled nitric oxide (NO) exerts a variety of effects through metabolites and these play an important role in regulation of hemodynamics in the body. A detailed investigation into the generation of these metabolites has been overlooked. 

Objectives: We investigated the kinetics of nitrite and S-nitrosothiol-hemoglobin (SNO-Hb) in plasma derived from inhaled NO subjects and how this modifies the cutaneous microvascular response.

Findings: We enrolled 15 healthy volunteers. Plasma nitrite levels at baseline and during NO inhalation (15 minutes at 40 ppm) were 102 (86-118) and 114 (87-129) nM, respectively. The nitrite peak occurred at 5 minutes of discontinuing NO (131 (104-170) nM). Plasma nitrate levels were not significantly different during the study. SNO-Hb molar ratio levels at baseline and during NO inhalation were 4.7E-3 (2.5E-3-5.8E-3) and 7.8E-3 (4.1E-3-13.0E-3), respectively. Levels of SNO-Hb continued to climb up to the last study time point (30 min: 10.6E-3 (5.3E-3-15.5E-3)). The response to acetylcholine iontophoresis both before and during NO inhalation was inversely associated with the SNO-Hb level (r: -0.57, p = 0.03, and r: -0.54, p = 0.04, respectively).

Conclusions: Both nitrite and SNO-Hb increase during NO inhalation. Nitrite increases first, followed by a more sustained increase in Hb-SNO. Nitrite and Hb-SNO could be a mobile reservoir of NO with potential implications on the systemic microvasculature.

Journal Reference:

Tonelli AR, Aulak KS, Ahmed MK, Hausladen A, Abuhalimeh B, Casa CJ, Rogers SC, Timm D, Doctor A, Gaston B, Dweik RA. A pilot study on the kinetics of metabolites and microvascular cutaneous effects of nitric oxide inhalation in healthy volunteers. PLoS One. 2019 Aug 30;14(8):e0221777. doi: 10.1371/journal.pone.0221777. PMID: 31469867; PMCID: PMC6716644.

A Concise Review of Inhaled Nitric Oxide’s Systemic Impacts

Key Points

• The classical viewpoint that inhaled nitric oxide (NO) only has local effects cannot explain observations.

• For example, inhaled NO has many systemic effects, including the ability to selectively increase blood flow where it is needed most.

• SNO-Hb might be the likely candidate for how inhaled NO is transferred into the blood and transported throughout the body while retaining its bioactivity.

The Breathing Diabetic Summary

This paper presented a concise review of inhaled NO’s systemic effects.  So, I’ll keep the summary brief as well. 

The classical view that inhaled NO only has local effects in the airways and lungs is not supported by observations.  It turns out that inhaled NO has many systemic effects.  Notably, inhaled NO selectively increases blood flow where it is needed most.  Thus, our bodies have a way of using inhaled NO other than just in the airways and lungs.  It can also be transported to distant regions where blood flow is restricted, resulting in vasodilation and increased blood flow.  This was also shown in the Cannon et al. (2001) study.   

Here, as in that study and others, the precise mechanism for how this is done is unknown.  However, there is one pathway that has been brought up repeatedly, which is SNO-Hb.  As we learned in a 2015 PNAS study, SNO-Hb is critical to blood flow regulation and oxygen delivery.  It “senses” regions of hypoxia, releases bioactive NO, and improves blood flow to get more oxygen to the tissues.

The authors suspect that this is also the mechanism by which inhaled NO is selectively improving blood flow, stating that this pathway “likely represents an important mechanism by which inhaled NO can cause systemic effects.”  The difficulty is that SNO-Hb is hard to measure; therefore, there have been no conclusive studies to show that this is the mechanism by which inhaled NO works. 

Altogether, this paper shows that the traditional view of inhaled NO is not adequate to explain its systemic effects.  It’s selective vasodilating effects suggest that SNO-Hb is the mechanism by which inhaled NO is transported throughout the body.  Still, more studies are needed to support this hypothesis.

Abstract

Many effects of inhaled nitric oxide (NO) are not explained by the convention that NO activates pulmonary guanylate cyclase or is inactivated by ferrous deoxy- or oxyheme. Inhaled NO can affect blood flow to a variety of systemic vascular beds, particularly under conditions of ischemia/reperfusion. It affects leukocyte adhesion and rolling in the systemic periphery. Inhaled NO therapy can overcome the systemic effects of NO synthase inhibition. In many cases, these systemic-NO synthase-mimetic effects of inhaled NO seem to involve reactions of NO with circulating proteins followed by transport of NO equivalents from the lung to the systemic periphery. The NO transfer biology associated with inhaled NO therapy is rich with therapeutic possibilities. In this article, many of the whole-animal studies regarding the systemic effects of inhaled NO are reviewed in the context of this emerging understanding of the complexities of NO biochemistry.

Journal Reference:

Gaston B. Summary: systemic effects of inhaled nitric oxide. Proc Am Thorac Soc. 2006 Apr;3(2):170-2. doi: 10.1513/pats.200506-049BG. PMID: 16565427.