Nitric oxide modelling and its bioavailability influenced by red blood cells

Nitric oxide (NO) is an important vasodilator responsible for maintaining vascular tone in the human body. Its production in endothelial cells (ECs) is regulated by the rise of cytoplasmic Ca²⁺ concentration and shear stress perceived by blood flow. The increase in cytoplasmic Ca²⁺ concentration is mainly activated by adenosine triphosphate (ATP) released from red blood cells (RBCs) and ECs. However, RBCs, which act as NO scavengers, can affect the bioavailability of NO in blood vessels. In this study, we developed a model that incorporates ATP and shear stress-dependent NO production, integrating various biochemical pathways. The model results are qualitatively consistent with the experimental findings. Given that ATP concentration and shear stress vary spatially within blood circulation, influenced by factors such as vessel width, flow strength and RBC concentration, these variations can significantly affect NO bioavailability. Here, we study RBC flow, ATP release from RBCs and ECs, and [Formula: see text] and NO dynamics in a two-dimensional channel using the immersed boundary lattice Boltzmann method. The main findings from the study include: (i) an increase in RBC concentration leads to a rise in ATP and cytoplasmic Ca²⁺ concentrations for all variation in channel widths, while NO concentration exhibits a decrease; (ii) NO bioavailability is significantly influenced by RBC distribution, particularly in strongly confined channels; and (iii) two phases of NO bioavailability are observed in different regions of the blood vessels: one with a significant concentration change at low RBC concentration and another with a minimal concentration change at high RBC concentration, across all confinements. The outcomes of this study may provide valuable insights into the mechanisms of NO-dependent vasodilation and the transport of oxygen by RBCs within microvascular networks for future studies.