Dynamics of Regulating Oxygen Supply by Erythrocytes

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Arguably the most important function of the vasculature is the delivery of adequate oxygen (O2) to every cell of the body. Since the distance O2 can diffuse from red blood cells (RBCs) in a vessel to where it is consumed in the cell to produce energy is <0.1-mm, the vasculature must properly distribute O2 supply down to the level of a single terminal arteriole (very small artery) supplying a small number of capillaries supporting these cells. This level of accuracy in distributing O2 cannot be built into the vascular structure during development, it must be dynamically regulated to respond to varying needs of the tissue being served (e.g. in skeletal or cardiac muscle depending on the level of physical activity). We were the first to propose that the red blood cell, which is the primary carrier of O2, is also the sensor which enables this fine level of control of the distribution of O2. RBCs sense the balance between the O2 supply and O2 consumption by the change in the amount of O2 carried by hemoglobin in the RBC. As O2 binds to hemoglobin the shape of the molecule changes which both activates enzymes within the RBC to produce adenosine triphosphate (ATP) and triggers a signalling pathway in the RBC membrane for ATP to be released from the RBC. The ATP activates specialized receptors (P2y) on the inner surface of endothelial cells lining the vessel which result in an increase in arteriolar diameter, either locally in arterioles by the release of nitric oxide or prostaglandins or remotely from capillaries and venules via a electrical signal conducted through endothelial cells to the arterioles. There are unresolved issues about the O2 dependent release of ATP which existing technologies cannot answers. The primary question is how long does it take for ATP to be released from the RBC following a change in hemoglobin O2 saturation? The answer is important since the release time determines the spatial accuracy with which O2 saturation levels can be reported to the vascular endothelium. Existing techniques only provide the how much ATP is released for a change in O2 saturation and cannot be used to determine fast ATP is released. Recent advances by my Ph.D. student, Richard Sové, make it possible for us to answer this question and, in doing so; gain new insights into the mechanisms regulating ATP release. Sové developed and published a computational model for the potential design of a microfluidic device which could be used to measure the dynamics of ATP release. He added a dynamic signaling pathway model for ATP release into his microfluidic model and has demonstrated that this microfluidic approach is able to reveal how the speed of each stage in the signaling pathway. Sové’s approach can also answer the question is the release rate of ATP from the RBC governed by the level of O2 saturation or by the rate at which O2 saturation is changing? Another model predicts that O2 distribution can only be regulated by RBCs if ATP release depends on the rate of O2 saturation change; thus the answer is critical to our understanding of the RBC’s role in regulating O2 supply. This proposal focuses on developing the microfluidic system proposed by Sové for fully characterizing the dynamics of O2 dependent ATP release from RBCs. All research will be carried out on rat blood to maintain consistency with previous research. This technology creates the opportunity for other graduate student projects investigating the impact of conditions the RBC would encounter in the microvasculature in vivo and forms the basis for a future collaborative health research projects to translate this research into a screening to tool for patients with cardiovascular disease.