Renal potassium handling in rats with subtotal nephrectomy: modeling and analysis

We sought to decipher the mechanisms underlying the kidney's response to changes in K+ load and intake, under physiological and pathophysiological conditions. To accomplish that goal, we applied a published computational model of epithelial transport along rat nephrons in a sham rat, an uninephrectomized (UNX) rat, and a 5/6-nephrectomized (5/6-NX) rat that also considers adaptations in glomerular filtration rate and tubular growth. Model simulations of an acute K+ load indicate that elevated expression levels and activities of Na+/K+-ATPase, epithelial sodium channels, large-conductance Ca2+-activated K+ channels, and renal outer medullary K+ channels, together with downregulation of sodium-chloride cotransporters (NCC), increase K+ secretion along the connecting tubule, resulting in a >6-fold increase in urinary K+ excretion in sham rats, which substantially exceeds the filtered K+ load. In the UNX and 5/6-NX models, the acute K+ load is predicted to increase K+ excretion, but at significantly reduced levels compared with sham. Acute K+ load is accompanied by natriuresis in sham rats. Model simulations suggest that the lesser natriuretic effect observed in the nephrectomized groups may be explained by impaired NCC downregulation in these kidneys. At a single-nephron level, a high K+ intake raises K+ secretion along the connecting tubule and reabsorption along the collecting duct in sham, and even more in UNX and 5/6-NX. However, the increased K+ secretion per tubule fails to sufficiently compensate for the reduction in nephron number, such that nephrectomized rats have an impaired ability to excrete an acute or chronic K+ load.This research was supported by the Department of Veterans Affairs (V. Vallon), National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) Grants R01-DK-112042 (V. Vallon) and R01-DK-106102 (A. T. Layton and V. Vallon), and University of Alabama at Birmingham-University of California San Diego O'Brien Center for Acute Kidney Injury Research (NIDDK Grant P30-DK-079337; V. Vallon). (Department of Veterans Affairs; R01-DK-112042 - National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK); R01-DK-106102 - National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK); P30-DK-079337 - University of Alabama at Birmingham-University of California San Diego O'Brien Center for Acute Kidney Injury Research (NIDDK Grant))Accepted manuscrip

Mathematical Modeling of Kidney Function During Pregnancy

During pregnancy major adaptations must occur in the maternal body to be able to support the rapidly developing fetus and placenta. Adaptations occur in almost all tissues and organs. These changes are dynamic, complex, and not fully understood. In particular, due to the altered requirements of an expanded plasma volume and increased electrolyte needs of the fetus and placenta, major adaptations must occur in the kidneys. The goal for this thesis is to investigate the functional implications of the pregnancy-induced adaptations that happen in the kidneys by developing and analyzing computational models. In particular, we first developed pregnancy-specific epithelial transport models of a single nephron in a mid- and late pregnant rat to quantify how individual renal adaptations in morphology, hemodynamics, and transporter activity affect handling of electrolytes and volume along the nephron. Our results predict which transport adaptations are essential for a healthy pregnancy as well as predict transport adaptations that have not been investigated experimentally. We then developed full pregnancy-specific kidney models that include heterogeneity in the nephron populations for a more accurate accounting of whole kidney function during pregnancy. Additionally, we developed models for renal function in a hypertensive female rat to analyze the effects of hypertension as well as hypertension with pregnancy on nephron transport. Our results suggest that increased Na+ transporter expression along the distal segments in female rats when compared to males may better prepare females for the demands of pregnancy. During pregnancy, it appears that significantly increased activity of distal segment Na+ transporters with increased proximal tubule size are key in ensuring Na+ retention occurs. K+ retention is likely achieved through decreased distal segment K+ secretion as well as increased activity of the K+ pump. Together these changes allow the maternal body to retain sufficient Na+ and K+ for fetal cell function. During hypertension, similar relative changes in transporter activity in males and females results in similar transport properties. Specifically, Na+ load is shifted to the distal segments requiring altered transporter expression to avoid excess natriuresis

Pulsatile Flow Through Idealized Renal Tubules: Fluid-structure Interaction and Dynamic Pathologies

Kidney tubules are lined with flow-sensing structures, yet information about the flow itself is not easily obtained. We aim to generate a multiscale biomechanical model for analyzing fluid flow and fluid-structure interactions within an elastic kidney tubule when the driving pressure is pulsatile. We developed a two-dimensional macroscopic mathematical model of a single fluid-filled tubule corresponding to a distal nephron segment and determined both flow dynamics and wall strains over a range of driving frequencies and wall compliances using finite-element analysis. The results presented here demonstrate good agreement with available analytical solutions and form a foundation for future inclusion of elastohydrodynamic coupling by neighboring tubules. Overall, we are interested in exploring the idea of dynamic pathology to better understand the progression of chronic kidney diseases such as Polycystic Kidney Disease

Renal pericytes: regulators of medullary blood flow

Regulation of medullary blood flow (MBF) is essential in maintaining normal kidney function. Blood flow to the medulla is supplied by the descending vasa recta (DVR), which arise from the efferent arterioles of juxtamedullary glomeruli. DVR are composed of a continuous endothelium, intercalated with smooth muscle-like cells called pericytes. Pericytes have been shown to alter the diameter of isolated and in situ DVR in response to vasoactive stimuli that are transmitted via a network of autocrine and paracrine signalling pathways. Vasoactive stimuli can be released by neighbouring tubular epithelial, endothelial, red blood cells and neuronal cells in response to changes in NaCl transport and oxygen tension. The experimentally described sensitivity of pericytes to these stimuli strongly suggests their leading role in the phenomenon of MBF autoregulation. Because the debate on autoregulation of MBF fervently continues, we discuss the evidence favouring a physiological role for pericytes in the regulation of MBF and describe their potential role in tubulo-vascular cross-talk in this region of the kidney. Our review also considers current methods used to explore pericyte activity and function in the renal medulla

Mathematical Model of Ammonia Handling in the Rat Renal Medulla

The kidney is one of the main organs that produces ammonia and release it into the circulation. Under normal conditions, between 30 and 50% of the ammonia produced in the kidney is excreted in the urine, the rest being absorbed into the systemic circulation via the renal vein. In acidosis and in some pathological conditions, the proportion of urinary excretion can increase to 70% of the ammonia produced in the kidney. Mechanisms regulating the balance between urinary excretion and renal vein release are not fully understood. We developed a mathematical model that reflects current thinking about renal ammonia handling in order to investigate the role of each tubular segment and identify some of the components which might control this balance. The model treats the movements of water, sodium chloride, urea, NH3 and [Formula: see text], and non-reabsorbable solute in an idealized renal medulla of the rat at steady state. A parameter study was performed to identify the transport parameters and microenvironmental conditions that most affect the rate of urinary ammonia excretion. Our results suggest that urinary ammonia excretion is mainly determined by those parameters that affect ammonia recycling in the loops of Henle. In particular, our results suggest a critical role for interstitial pH in the outer medulla and for luminal pH along the inner medullary collecting ducts

Calcium Transport and Regulation in Male and Female Rat Kidney

Calcium is an essential mineral that plays a critical role in many biological processes, such as muscle contraction, nerve function, and bone health. However, maintaining the right balance of calcium in the body is crucial since both high and low levels can have harmful effects. To ensure this balance, the kidney plays a crucial role in regulating calcium homeostasis by reabsorbing calcium in specific segments of the nephron, including the proximal tubule, thick ascending limb, and the late part of the distal convoluted tubule and connecting tubule, and controlling the amount of calcium that is excreted in the urine. This thesis focuses on the computational modelling of renal epithelial calcium handling and sex differences of this process in male and female rat kidneys. Renal epithelial isolated cell models with emphasis on calcium transport are discussed and expanded to a nephron model for both male and female models. Sex differences were observed in the permeability of the proximal tubule to calcium and the expression of calcium channels and pumps in the late part of the distal convoluted tubule and connecting tubule. Our results revealed lower calcium reabsorption along the proximal tubule and thick ascending limb, and higher reabsorption along the late part of the distal convoluted tubule and connecting tubule in the female rat kidney model. Furthermore, the male rat kidney model showed higher urinary calcium excretion compared to the female rat kidney model, consistent with animal experiments. We investigated the effects of perturbations in calcium-specific channels and transporters and sodium-specific transporters on renal calcium handling in male and female rat kidneys. Our findings revealed that inhibiting sodium-specific transporters had a significant impact on renal calcium transport, whereas inhibitions of calcium-specific channels and transporters had minimal effects on sodium transport. We observed that inhibiting Na+/H+ exchanger 3, Na+-K+-2Cl− co-transporter, and Na+-Cl− co-transporter resulted in increased urinary calcium excretion in both male and female models, while epithelial Na+ channel inhibition led to decreased urinary calcium excretion in both male and female models

The zebrafish cationic amino acid transporter/glycoprotein-associated family: sequence and spatiotemporal distribution during development of the transport system b 0,+ (slc3a1/slc7a9)

System b0,+ absorbs lysine, arginine, ornithine, and cystine, as well as some (large) neutral amino acids in the mammalian kidney and intestine. It is a heteromeric amino acid transporter made of the heavy subunit SLC3A1/rBAT and the light subunit SLC7A9/b0,+AT. Mutations in these two genes can cause cystinuria in mammals. To extend information on this transport system to teleost fish, we focused on the slc3a1 and slc7a9 genes by performing comparative and phylogenetic sequence analysis, investigating gene conservation during evolution (synteny), and defining early expression patterns during zebrafish (Danio rerio) development. Notably, we found that slc3a1 and slc7a9 are non-duplicated in the zebrafish genome. Whole-mount in situ hybridization detected co-localized expression of slc3a1 and slc7a9 in pronephric ducts at 24 h post-fertilization and in the proximal convoluted tubule at 3 days post-fertilization (dpf). Notably, both the genes showed co-localized expression in epithelial cells in the gut primordium at 3 dpf and in the intestine at 5 dpf (onset of exogenous feeding). Taken together, these results highlight the value of slc3a1 and slc7a9 as markers of zebrafish kidney and intestine development and show promise for establishing new zebrafish tools that can aid in the rapid screening(s) of substrates. Importantly, such studies will help clarify the complex interplay between the absorption of dibasic amino acids, cystine, and (large) neutral amino acids and the effect(s) of such nutrients on organismal growth.publishedVersio

Mathematical Modelling of the Intrarenal Renin Angiotensin System in Hypertension

Hypertension is the leading cause of cardiovascular disease and premature death world-wide. It is a highly multi-factorial disease associated with multiple risk factors and patho-physiological changes, including impaired kidney function and an over-active renin angiotensin system (RAS). Many hypertensive actions of angiotensin II (Ang II), the primary bio-active product of the RAS, are mediated within the kidney; an organ that also expresses and independently regulates all RAS constituents. The interconnected nature of the systems involved makes it difficult, and in many cases impossible, to identify their individual contributions to the observed pathology in vivo. Thus, the goal of this thesis is to investigate the role of the local intrarenal RAS in the pathogenesis and progression of hypertension in silico. In particular, we first developed a computational model of the intrarenal and systemic RASs in isolation to unravel the mechanisms that mediate the former's over-activity in Ang II infused hypertensive rats (an experimental model of hypertension). Then, by extending the model to include a pharmacokinetic representation of an angiotensin receptor blocker (ARB), a common RAS-modulating anti-hypertensive therapy, we examined the impact of this class of medication on the kidney. Lastly, by coupling our model to one of whole-body blood pressure regulation in the rat and creating the first model of long-term blood pressure regulation that considers a intrarenal RAS, we zoomed back out to determine how the aforementioned effects actually contribute to blood pressure dis-regulation. Our results suggest that Ang II accumulates in the kidney during the development of Ang II-induced hypertension because of enhanced angiotensin type 1 receptor (AT1R)-mediated uptake of circulating Ang II, which is facilitated by positive feedback on intrarenal AT1R expression. By inhibiting this feedback loop, and others inherent to the intrarenal RAS, ARBs effectively prevent intrarenal Ang II levels from increasing. However, it is rather by restricting Ang II to extracellular regions of the kidney that ARBs effectively restore normotension. In the absence of treatment, rising concentrations of cell-associated Ang II act to increase blood pressure by stimulating sodium reabsorption along the nephron. The timing of this response also affects blood pressure dynamics. Indeed, slow-pressor hypertension is a consequence of systemic and intrarenal RAS decoupling: The progressive accumulation of Ang II in the kidney permits the sequential activation of sodium reabsorption by aldosterone, then Ang II. Our results shed light on the functional importance of the intrarenal RAS in hypertension induced by Ang II infusion, and thus clinical hypertension associated with an over-active RAS