Abstract 238: Oxygen Regulates the Flux of Nitric Oxide Diffusion across the Vascular Wall

Abstract

Endothelium-derived nitric oxide (NO) plays an important role in maintaining vascular tone. It is known that NO may be consumed by heme proteins, superoxide and oxygen during diffusion from the endothelium to smooth muscle cells in the vascular wall. Due to the limitation of available techniques, it is unclear to what extent these consumptions can affect the diffusion distance of NO, and if the vascular NO consumption could serve as a “sensor” of oxygen concentrations in the blood vessels. In this study, rat aortas were used as an experimental model for studying NO diffusion process in the vascular system. A Clark-type NO electrode was used to directly measure the flux of NO diffusion across the vascular wall at 37 °C. A segment of aorta was isolated from a 12-week old WKY rat. After the aorta was cleaned and surrounding tissue was removed, it was longitudinally opened. A specifically-designed aorta holder was attached on the tip of the Clark-type NO electrode. The aorta holder surface and the electrode tip surface were aligned in the same plane so that the opened aorta segment could be placed flat on the electrode tip surface and pinned to the aorta holder. Using this technique, we measured the flux of NO diffusion across the aortic wall at different oxygen concentration. It was observed that the NO flux increased 6 to 10 fold when oxygen concentrations dropped from 200 μM to zero. A mathematical model describing the steady-state diffusion-reaction was used in analyzing the experimental data. It was found that the rate of NO decay is first order with respect to [O 2 ] and first order with respect to [NO], and hence of the form k[O 2 ][NO]. The rate constant k was determined as (3.8±0.4)x10 −3 μM −1 s −1 (n=6). With this rate constant, the half-life of NO in the aortic wall in the presence of 200 μM O 2 (equilibrium with room air) will be 0.9 seconds. Our results show that the flux and diffusion distance of NO in the aortic wall is largely regulated by oxygen concentration. When oxygen concentrations drop, NO diffusion distance will significantly increase. As a result, the blood vessel will dilate to a larger extent to allow more blood to be delivered to the hypoxic tissues. Therefore this vascular NO consumption appears to play the role of an oxygen sensor in the regulation of blood flow in the body.