Effective diffusion distance of nitric oxide in the microcirculation.

Despite its well-documented importance, the mechanism for nitric oxide (NO) transport in vivo is still unclear. In particular, the effect of hemoglobin-NO interaction and the range of NO action have not been characterized in the microcirculation, where blood flow is optimally regulated. Using a mathematical model and experimental data on NO production and degradation rates, we investigated factors that determine the effective diffusion distance of NO in the microcirculation. This distance is defined as the distance within which NO concentration is greater than the equilibrium dissociation constant (0.25 μM) of soluble guanylyl cyclase, the target enzyme for NO action. We found that the size of the vessel is an important factor in determining the effective diffusion distance of NO. In ∼30- to 100-μm-ID microvessels the luminal NO concentrations and the abluminal effective diffusion distance are maximal. Furthermore, the model suggests that if the NO-erythrocyte reaction rate is as fast as the rate reported for the in vitro NO-hemoglobin reaction, the NO concentration in the vascular smooth muscle will be insufficient to stimulate smooth muscle guanylyl cyclase effectively. In addition, the existence of an erythrocyte-free layer near the vascular wall is important in determining the effective NO diffusion distance. These results suggest that 1) the range of NO action may exhibit significant spatial heterogeneity in vivo, depending on the size of the vessel and the local chemistry of NO degradation, 2) the NO binding/reaction constant with hemoglobin in the red blood cell may be much smaller than that with free hemoglobin, and 3) the microcirculation is the optimal site for NO to exert its regulatory function. Because NO exhibits vasodilatory function and antiatherogenic activity, the high NO concentration and its long effective range in the microcirculation may serve as intrinsic factors to prevent the development of systemic hypertension and atherosclerotic pathology in microvessels.