MBI Publications

MBI Publications for Rate Constant (2)

  • X. Liu, P. Srinivasan, E. Collard, P. Grajdeanu, J. Zweier and A. Friedman
    Nitric Oxide Diffusion Rate is Reduced in the Aortic Wall
    Biophysical JournalVol. 94 No. 5 (2008) pp. 1880-1889q

    Abstract

    Endogenous nitric oxide (NO) plays important physiological roles in the body. As a small diatomic molecule, NO has been assumed to freely diffuse in tissues with a diffusion rate similar to that in water. However, this assumption has not been tested experimentally. In this study, a modified Clark-type NO electrode attached with a customized aorta holder was used to directly measure the flux of NO diffusion across the aortic wall at 37°C. Experiments were carefully designed for accurate measurements of the apparent NO diffusion coefficient D and the partition coefficient α in the aortic wall. A mathematical model was presented for analyzing experimental data. It was determined that α = 1.15 ± 0.11 and D = 848 ± 45 μm2/s (n = 12). The NO diffusion coefficient in the aortic wall is nearly fourfold smaller than the reported diffusion coefficient in solution at 37°C, indicating that NO diffusion in the vascular wall is no longer free, but markedly dependent on the environment in the tissue where these NO molecules are. These results imply that the NO diffusion rate in the vascular wall may be upregulated and downregulated by certain physiological and/or pathophysiological processes affecting the composition of tissues.
  • X. Liu, P. Srinivasan, E. Collard, P. Grajdeanu, K. Lok, S. Boyle, A. Friedman and J. Zweier
    Oxygen regulates the effective diffusion distance of nitric oxide in the aortic wall
    Free Radic Biol MedVol. 48 No. 4 (2010) pp. 554-559

    Abstract

    Endothelium-derived nitric oxide (NO) is critical in maintaining vascular tone. Accumulating evidence shows that NO bioavailability is regulated by oxygen concentration. However, it is unclear to what extent the oxygen concentration regulates NO bioavailability in the vascular wall. In this study, a recently developed experimental setup was used to measure the NO diffusion flux across the aortic wall at various oxygen concentrations. It was observed that for a constant NO concentration at the endothelial surface, the measured NO diffusion flux out of the adventitial surface at [O2] = 0 μM is around fivefold greater than at [O2] = 150 μM, indicating that NO is consumed in the aortic wall in an oxygen-dependent manner. Analysis of experimental data shows that the rate of NO consumption in the aortic wall is first order with respect to [NO] and first order with respect to [O2], and the rate constant k1 was determined as (4.0 ± 0.3) — 103 Mˆ’1 sˆ’1. Computer simulations demonstrate that NO concentration distribution significantly changes with oxygen concentration and the effective NO diffusion distance at low oxygen level ([O2] ‰ 25 μM) is significantly longer than that at high oxygen level ([O2] = 200 μM). These results suggest that oxygen-dependent NO consumption may play an important role in dilating blood vessels during hypoxia by increasing the effective NO diffusion distance.

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