Modulation of NO Bioavailability by Temporal Variation of the Cell-Free Layer Width in Small Arterioles PENG KAI ONG,SWATI JAIN, and SANGHO KIM Division of Bioengineering and Department of Surgery, Faculty of Engineering, National University of Singapore, 9 Engineering Drive 1, Block EA #03-12, Singapore 117576, Singapore (Received 3 September 2010; accepted 19 November 2010; published online 1 December 2010) Associate Editor Kerry Hourigan oversaw the review of this article. Abstract—The cell-free layer exhibits dynamic characteristics in the time domain that may be capable of altering nitric oxide (NO) bioavailability in small arterioles. However, this effect has not been fully elucidated. This study utilized a compu- tational model on NO transport to predict how temporal variations in the layer width could modulate NO bioavail- ability in the arterioles. Data on the layer width was acquired from high-speed video recordings in arterioles (ID = 48.4 ± 1.8 lm) of the rat cremaster muscle. We found that when wall shear stress response was not considered, the layer variability could lead to a slight decrease (1.6–6.6%) in NO bioavail- ability that was independent of transient changes in NO scavenging rate. Conversely, the transient response in wall shear stress and NO production rate played a dominant role in reversing this decline such that a significant augmentation (5.3–21.0%) in NO bioavailability was found with increasing layer variability from 24.6 to 63.8%. This study highlighted the importance of the temporal changes in wall shear stress and NO production rate caused by the layer width variations in prediction of NO bioavailability in arterioles. Keywords—Nitric oxide, Microcirculation, Wall shear stress, Vasodilation. INTRODUCTION The functional role of nitric oxide (NO) in the reg- ulation of vascular tone is especially important in the arteriolar network of the microcirculation 11 and is mainly attributed to the specialized structure of the arterioles that enables active diameter changes in response to blood flow-induced signals. These small blood vessels are distinctively characterized by the presence of a NO-responsive smooth muscle layer that surrounds the endothelial cells (ECs) which forms part of the vascular wall. The ECs serve as essential sites for NO production in the arterioles 9,34 by responding to hemodynamic signals acting on the luminal surface of these cells exposed to blood flow. Due to its high dif- fusivity in interstitial fluids, the newly synthesized NO can readily diffuse from the endothelium in opposite radial directions into either the blood lumen (LU) or the smooth muscle layer. The bioavailability of NO in the smooth muscle layer is a critical determinant of vascular tone since NO can activate soluble guanylate cyclase (sGC), an enzyme that stimulates the release of cyclic GMP (cGMP). This signaling molecule is responsible for relaxing the smooth muscle cells, causing vasodila- tion. 2,33 Thus, physiological concentrations of NO required for eliciting smooth muscle relaxation often refer to the NO level required for eliciting half-maxi- mum activity of sGC which ranges from 23 7,49 to 250 nM. 40 Due to the close proximity of the NO pro- duction source to red blood cells and the high reac- tivity of NO with hemoglobin (Hb), 9,26 it is unclear how sufficient NO can be maintained in the smooth muscle layer to elicit physiological response on vascu- lar tone in the presence of the red blood cells 26 flowing in the LU. NO preservation is known to be a conse- quence of the attenuation of NO interaction with the Hb by some forms of NO diffusion barrier. While an initial study based on the ‘‘competitive experi- ment’’ 43,44 as well as subsequent experimental and theoretical work 10 are in favor of intracellular diffu- sion limitations such as red blood cell membrane and associated cytoskeleton NO-inert proteins, other studies 28,42 are supportive of extracellular diffu- sion factors such as the unstirred boundary layer Address correspondence to Sangho Kim, Division of Bioengi- neering and Department of Surgery, Faculty of Engineering, National University of Singapore, 9 Engineering Drive 1, Block EA #03-12, Singapore 117576, Singapore. Electronic mail: bieks@nus. edu.sg Annals of Biomedical Engineering, Vol. 39, No. 3, March 2011 (Ó 2010) pp. 1012–1023 DOI: 10.1007/s10439-010-0216-y 0090-6964/11/0300-1012/0 Ó 2010 Biomedical Engineering Society 1012