Computational Modeling of Nitric Oxide Bioavailability in Hemolytic Anemias.

Intravascular hemoglobin (Hb) scavenges nitric oxide (NO), which decreases the availability of endothelial-derived NO, thereby reducing smooth muscle relaxation. Under normal conditions, there are various mechanisms that limit consumption of NO by Hb in vivo including

1. a physical barrier to NO diffusion across the red blood cell (RBC) membrane,

2. an unstirred layer creating a concentration gradient in NO outside the RBC, and

3. and a cell-free zone between the endothelium where NO is made and the location of the RBCs.

However, in hemolytic conditions, these mechanisms can be compromised, leading to reduced levels of NO reaching the smooth muscle cells to cause vasodilation. Although it is generally accepted that cell-free Hb scavenges NO more effectively than Hb encapsulated in the red blood cell (RBC), some do not believe that the low concentrations of cell-free Hb that are present in hemolytic conditions, such as sickle cell disease, can impact NO bioavailability in the presence of the large amounts of RBC encapsulated Hb (about 10 mM in heme) present in vivo. We have performed computational modeling looking at the effects of cell-free Hb on NO bioavailability within blood vessels. We have found that concentrations of cell-free Hb as low as one micromolar significantly reduces the availability of NO. Thus, 0.01% hemolysis has a significant affect on steady state NO levels. Extravasation of cell-free Hb into the interstitial space of as low as one micromolar further reduces the bioavailability of NO. At low hematocrit values, cell-free Hb scavenging of NO was found to be more efficient than at high hematocrit values. We have also found that the effect of RBC membrane permeability diminishes at cell-free Hb concentrations of only four micromolar and above. These theoretical results support experimental evidence indicating that cell-free Hb contributes to the pathology of hemolytic diseases. Supported by NIH grant RG0489.