<p>One of the key mechanisms that mediate renal autoregulation is the tubuloglomerular feedback (TGF) system, which is a negative feedback loop in the kidney that balances glomerular filtration with tubular reabsorptive capacity. In this dissertation, we develop several mathematical models of the TGF system to study TGF-mediated model dynamics. </p><p>First, we develop a mathematical model of compliant thick ascending limb (TAL) of a short loop of Henle in the rat kidney, called TAL model, to investigate the effects of spatial inhomogeneous properties in TAL on TGF-mediated dynamics. We derive a characteristic equation that corresponds to a linearized TAL model, and conduct a bifurcation analysis by finding roots of that equation. Results of the bifurcation analysis are also validated via numerical simulations of the full model equations. </p><p>We then extend the TAL model to explicitly represent an entire short-looped nephron including the descending segments and having compliant tubular walls, developing a short-looped nephron model. A bifurcation analysis for the TGF loop-model equations is similarly performed by computing parameter boundaries, as functions of TGF gain and delay, that separate differing model behaviors. We also use the loop model to better understand the effects of transient as well as sustained flow perturbations on the TGF system and on distal NaCl delivery.</p><p>To understand the impacts of internephron coupling on TGF dynamics, we further develop a mathematical model of a coupled-TGF system that includes any finite number of nephrons coupled through their TGF systems, coupled-nephron model. Each model nephron represents a short loop of Henle having compliant tubular walls, based on the short-looped nephron model, and is assumed to interact with nearby nephrons through electrotonic signaling along the pre-glomerular vasculature. The characteristic equation is obtained via linearization of the loop-model equations as in TAL model. To better understand the impacts of parameter variability on TGF-mediated dynamics, we consider special cases where the relation between TGF delays and gains among two coupled nephrons is specifically chosen. By solving the characteristic equation, we determine parameter regions that correspond to qualitatively differing model behaviors. </p><p>TGF delays play an essential role in determining qualitatively and quantitatively different TGF-mediated dynamic behaviors. In particular, when noise arising from external sources of system is introduced, the dynamics may become significantly rich and complex, revealing a variety of model behaviors owing to the interaction with delays. In our next study, we consider the effect of the interactions between time delays and noise, by developing a stochastic model. We begin with a simple time-delayed transport equation to represent the dynamics of chloride concentration in the rigid-TAL fluid. Guided by a proof for the existence and uniqueness of the steady-state solution to the deterministic Dirichlet problem, obtained via bifurcation analysis and the contraction mapping theorem, an analogous proof for stochastic system with random boundary conditions is presented. Finally we conduct multiscale analysis to study the effect of the noise, specifically when the system is in subcritical region, but close enough to the critical delay. To analyze the solution behaviors in long time scales, reduced equations for the amplitude of solutions are derived using multiscale method.</p> / Dissertation

Endothelial cells (EC) exposed to oscillatory shear stress (OS) experience oxidative stress as a signature of atherosclerosis. Conversely, unidirectional laminar shear stress (LS) reduces reactive oxygen species (ROS) levels and inflammatory responses. Peroxiredoxins (PRX) are antioxidant enzymes that reduce hydrogen peroxide, but have yet to be investigated in response to shear stress. We hypothesized that LS, compared to OS, promotes increased expression of PRX, which in turn influences the balance of ROS in EC. In this study, we identified all six PRX family members in bovine aortic endothelial cells (BAEC). Furthermore, we revealed that PRX are regulated by shear stress in EC. When compared to OS and static culture (ST), exposure to chronic LS upregulated PRX1 levels intracellularly. LS also upregulated PRX5 relative to ST, but not OS. In addition, PRX exhibited broad subcellular localization in BAEC, but these patterns did not change in response to shear stress. To establish the functional importance of PRX1 in shear stress-dependent redox balance, we next examined the role of PRX1 in LS-mediated hydrogen peroxide regulation. Here, Amplex Red assay was used to measure ROS levels in BAEC. Depletion of PRX1 using siRNA resulted in significantly higher ROS levels following LS, OS, and ST, while PRX5 depletion did not. These findings indicated that chronic exposure to LS upregulates PRX1 expression to keep ROS levels low in EC. To identify the pathway by which atheroprotective LS stimulates PRX1 protein production, we also undertook gene expression studies. We discovered that LS upregulates Prdx1 gene in a time-dependent manner compared to OS or ST. However, this increase in expression was not due to stabilization of Prdx1 mRNA. In addition, Prdx1 promoter analysis revealed a Nrf2 transcription factor binding site 160bp upstream of the gene. Nrf2 overexpression promoted basal PRX1 protein production, while Nrf2 depletion reduced Prdx1 mRNA following exposure to LS. Collectively, our work illustrated that LS affects PRX1 by inducing the Prdx1 gene, in part via the transcription factor Nrf2. Moreover, this discovery of PRX1 as a mechanosensitive antioxidant may contribute important insights into endothelial cell biology and provide a novel therapeutic target for vascular diseases.

Diss. (sammanfattning) Stockholm : Karol. inst., 2002. / Härtill 7 uppsatser.

Diss. (sammanfattning) Stockholm : Karol. inst., 2002. / Härtill 5 uppsatser.

Diss. (sammanfattning) Stockholm : Karol. inst., 2002. / Härtill 6 uppsatser.

Diss. (sammanfattning) Linköping : Univ., 2003. / Härtill 4 uppsatser.

Thesis (M. S.)--Chemical and Biomolecular Engineering, Georgia Institute of Technology, 2005. / Dr. Athanassios Sambanis, Committee Member ; Dr. Timothy M. Wick, Committee Member ; Dr. Ajit P.Yoganathan, Committee Chair. Includes bibliographical references.

Thesis (Ph. D.)--Biomedical Engineering, Georgia Institute of Technology, 2005. / Giddens, P. Don, Committee Chair ; Vito, P. Raymond, Committee Member ; Taylor, Robert, W., Committee Member ; Oshinski, John, Committee Member ; Bao, Gang, Committee Member. Includes bibliographical references.

Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2005. / Guldberg, Robert E., Committee Member ; Levenston, Marc E., Committee Member; Vito, Raymond P., Committee Chair ; Taylor, W. Robert, Committee Member ; Giddens, Don P., Committee Member. Vita. Includes bibliographical references.

Thesis (M.S.)--University of Michigan, 1988.