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J. Biol. Chem., Vol. 282, Issue 12, 8831-8836, March 23, 2007

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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^*

From the Davis Heart and Lung Research Institute and Division of Cardiovascular Medicine, Department of Internal Medicine, Ohio State University, Columbus, Ohio 43210

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 (P_m) 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 P_m and examining the effect of NO generation rate (V₀) and hematocrit (Hct) on NO concentration. It was determined that P_m is 4.5 ± 1.5 cm/s and D is 3410 ± 50 µm²/s at 37 °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 P_m = 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.

Received for publication, December 21, 2006 , and in revised form, January 29, 2007.

^* This study was partially supported by an American Heart Association Scientist Development Grant (to X. L.) and by NHLBI, National Institutes of Health Grants HL-38324, HL-63744, and HL-65608 (to J. L. Z.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Davis Heart and Lung Research Institute, Ohio State University, 473 W. 12th Ave., Columbus, OH 43210. Tel.: 614-292-1305; Fax: 614-292-8778; E-mail: Xiaoping.liu{at}osumc.edu.

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