Estimation of Nitric Oxide Concentration in Blood for Different Rates of Generation: EVIDENCE THAT INTRAVASCULAR NITRIC OXIDE LEVELS ARE TOO LOW TO EXERT PHYSIOLOGICAL EFFECTS

Xiaoping Liu,Qingtao Yan,Kim L Baskerville,Jay L Zweier

doi:10.1074/jbc.m611684200

Xiaoping Liu, Qingtao Yan + Show 2 more

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https://doi.org/10.1074/jbc.m611684200

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Abstract

Endothelium-derived nitric oxide (NO) is a potent vasodilator in the cardiovascular system. Several lines of experimental evidence suggest that NO or NO equivalents may also be generated in the blood. However, blood contains a large amount of hemoglobin (Hb) in red blood cells (RBCs). The RBC-encapsulated Hb can react very quickly with NO, which is only limited by the rate of NO diffusion into the RBCs. It is unclear what the possible NO concentration levels in blood are and how the NO diffusion coefficient (D) and the permeability (Pm) of RBC membrane to NO affect the level of NO concentration. In this study, a steady-state concentration experimental method combined with a spherical diffusion model are presented for determining D and Pm and examining the effect of NO generation rate (V0) and hematocrit (Hct) on NO concentration. It was determined that Pm is 4.5 +/- 1.5 cm/s and D is 3410 +/- 50 microm2/s at 37 degrees C. Simulations based on experimental parameters show that, when the rate of NO formation is as high as 100 nm/s, the maximal NO concentration in blood is below 0.012 nM at Pm = 4.5 cm/s and Hct = 45%. Thus, it is unlikely that NO is directly exported or generated from the RBC as an intravascular signaling molecule, because its concentration would be too low to exert a physiological role. Furthermore, our results suggest that, if RBCs export NO bioactivity, this would be through NO-derived species that can release or form NO rather than NO itself.

Highlights

Endothelium-derived nitric oxide (NO) is a potent vasodilator and plays an important role in maintaining vascular tone
It has been generally accepted that the relatively slower NO consumption rate by red blood cells (RBCs) is caused by the resistance to NO diffusion from the solution into RBCs, but there is debate on whether the RBC membrane or the extracellular unstirred solution layer is the main source of resistance [2,3,4,5,6,7]
Formation of Steady-state NO Concentration in the Presence of RBCs—To form a steady-state NO concentration, NO was first continuously injected into the phosphate-buffered saline solution at a constant rate by a Hamilton syringe driven with a syringe pump

Summary

Introduction

Endothelium-derived NO is a potent vasodilator and plays an important role in maintaining vascular tone. In addition to the relatively slower NO consumption by RBCs, it has been reported that the cell-free layer adjacent to the endothelium further reduces the NO consumption rate by RBCs [8, 9] These theoretical and experimental results show that, the large amount of Hb in the blood is a sink of the endothelium-derived NO, the diffusion resistance from the endothelial surface to the inside of the RBCs forms a physical barrier to prevent NO from being rapidly consumed by the RBC-encapsulated Hb. Two other mechanisms for protecting NO bioactivity were proposed (10 –13). The other mechanism assumes that the relatively stable nitrite is a source of NO in the blood and tissues, which is converted to the bioactive NO or NO equivalents during physiological hypoxia to dilate blood vessels [12] It is uncertain whether the RBC-exported or -generated NO bioactivity is the diatomic molecule NO or other NO congeners/equivalents. Computer simulations based on experimental data were performed to predict the possible NO concentration in blood at different generation rates

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