Determinants of basal nitric oxide concentration in the renal medullary microcirculation

Evaluation of donor kidneys prior to transplantation: an update of current and emerging methods

The lack of suitable kidney donor organs has led to rising numbers of patients with end stage renal disease waiting for kidney transplantation. Despite decades of clinical experience and research, no evaluation process that can reliably predict the outcome of an organ has yet been established. This review is an overview of current methods and emerging techniques in the field of donor kidney evaluation prior to transplantation. Established techniques like histological evaluation, clinical scores, and machine perfusion systems offer relatively reliable predictions of delayed graft function but are unable to consistently predict graft survival. Emerging techniques including molecular biomarkers, new imaging technologies, and normothermic machine perfusion offer innovative approaches toward a more global evaluation of an organ with better outcome prediction and possibly even identification of targets for therapeutic interventions prior to transplantation. These techniques should be studied in randomized controlled trials to determine whether they can be safely used in routine clinical practice to ultimately reduce the discard rate and improve graft outcomes.

Impacts of nitric oxide and superoxide on renal medullary oxygen transport and urine concentration

The goal of this study was to investigate the reciprocal interactions among oxygen (O2), nitric oxide (NO), and superoxide (O2 (-)) and their effects on medullary oxygenation and urinary output. To accomplish that goal, we developed a detailed mathematical model of solute transport in the renal medulla of the rat kidney. The model represents the radial organization of the renal tubules and vessels, which centers around the vascular bundles in the outer medulla and around clusters of collecting ducts in the inner medulla. Model simulations yield significant radial gradients in interstitial fluid oxygen tension (Po2) and NO and O2 (-) concentration in the OM and upper IM. In the deep inner medulla, interstitial fluid concentrations become much more homogeneous, as the radial organization of tubules and vessels is not distinguishable. The model further predicts that due to the nonlinear interactions among O2, NO, and O2 (-), the effects of NO and O2 (-) on sodium transport, osmolality, and medullary oxygenation cannot be gleaned by considering each solute's effect in isolation. An additional simulation suggests that a sufficiently large reduction in tubular transport efficiency may be the key contributing factor, more so than oxidative stress alone, to hypertension-induced medullary hypoxia. Moreover, model predictions suggest that urine Po2 could serve as a biomarker for medullary hypoxia and a predictor of the risk for hospital-acquired acute kidney injury.

DCD pigs' kidneys analyzed by MRI to assess ex vivo their viability

BACKGROUND

Magnetic resonance imaging (MRI) gadolinium-perfusion was applied in simulated Donation after Cardiac Death (DCD) in porcine kidneys to measure intrarenal perfusion. Adenosine triphosphate (ATP) resynthesis during oxygenated hypothermic perfusion was compared to evaluate the "ex vivo organ viability". Adenine nucleotide (AN) was measured by P nuclear magnetic resonance (NMR) spectroscopy. Whereas this latter technique requires sophisticated hardware, gadolinium-perfusion can be realized using any standard proton-MRI scanner. The aim of this work was to establish a correlation between the two methods.

METHODS

Twenty-two porcine kidneys presenting up to 90 min warm ischemia were perfused with oxygenation at 4 °C using our magnetic resonance-compatible machine. During the perfusion, P NMR spectroscopy and gadolinium-perfusion sequences were performed. Measures obtained from the gadolinium-perfusion were the speed of elimination of the cortical gadolinium and the presence or absence of a corticomedullar shunt. For ATP resynthesis analysis, P chemical shift imaging was acquired and analyzed. All the kidneys have been submitted to histologic examination.

RESULTS

ATP resynthesis was observed in all organs presenting a cortical gadolinium elimination slope of (-) 23° or greater. In organs with lower gadolinium elimination, no AN or only precursors were detected. This study reveals a link between the two methods and demonstrates ex vivo viability in 93% of the analyzed kidneys. Benefits and side effects of both methods are discussed.

CONCLUSION

Oxygenated hypothermic perfusion enables the evaluation of kidneys in DCD simulated situation; gadolinium-perfusion can be introduced into any center equipped with a proton-MRI scanner allowing results superposable with ATP measurement.

Myofibroblasts in Fibrotic Kidneys

Fibrosis of the kidney glomerulus and interstitium are characteristic features of almost all chronic kidney diseases. Fibrosis is tightly associated with destruction of capillaries, inflammation, and epithelial injury which progresses to loss of nephrons, and replacement of kidney parenchyma with scar tissue. Understanding the origins and nature of the cells known as myofibroblasts that make scar tissue is central to development of new therapeutics for kidney disease. Whereas many cell lineages in the body have become defined by well-established markers, myofibroblasts have been much harder to identify with certainty. Recent insights from genetic fate mapping and the use of dynamic reporting of cells that make fibrillar collagen in mice have identified with greater clarity the major population of myofibroblasts and their precursors in the kidney. This review will explore the nature of these cells in health and disease of the kidney to underst and their central role in the pathogenesis of kidney disease.

Cellular mechanisms of tissue fibrosis. 3. Novel mechanisms of kidney fibrosis

Chronic kidney disease, defined as loss of kidney function for more than three months, is characterized pathologically by glomerulosclerosis, interstitial fibrosis, tubular atrophy, peritubular capillary rarefaction, and inflammation. Recent studies have identified a previously poorly appreciated, yet extensive population of mesenchymal cells, called either pericytes when attached to peritubular capillaries or resident fibroblasts when embedded in matrix, as the progenitors of scar-forming cells known as myofibroblasts. In response to sustained kidney injury, pericytes detach from the vasculature and differentiate into myofibroblasts, a process not only causing fibrosis, but also directly contributing to capillary rarefaction and inflammation. The interrelationship of these three detrimental processes makes myofibroblasts and their pericyte progenitors an attractive target in chronic kidney disease. In this review, we describe current understanding of the mechanisms of pericyte-to-myofibroblast differentiation during chronic kidney disease, draw parallels with disease processes in the glomerulus, and highlight promising new therapeutic strategies that target pericytes or myofibroblasts. In addition, we describe the critical paracrine roles of epithelial, endothelial, and innate immune cells in the fibrogenic process.

Cellular mechanisms underlying nitric oxide-induced vasodilation of descending vasa recta

It has been observed that vasoactivity of explanted descending vasa recta (DVR) is modulated by intrinsic nitric oxide (NO) and superoxide (O(2)(-)) production (Cao C, Edwards A, Sendeski M, Lee-Kwon W, Cui L, Cai CY, Patzak A, Pallone TL. Am J Physiol Renal Physiol 299: F1056-F1064, 2010). To elucidate the cellular mechanisms by which NO, O(2)(-) and hydrogen peroxide (H(2)O(2)) modulate DVR pericyte cytosolic Ca(2+) concentration ([Ca](cyt)) and vasoactivity, we expanded our mathematical model of Ca(2+) signaling in pericytes. We incorporated simulations of the pathways that translate an increase in [Ca](cyt) to the activation of myosin light chain (MLC) kinase and cell contraction, as well as the kinetics of NO and reactive oxygen species formation and their effects on [Ca](cyt) and MLC phosphorylation. The model reproduced experimentally observed trends of DVR vasoactivity that accompany exposure to N(ω)-nitro-L-arginine methyl ester, 8-Br-cGMP, Tempol, and H(2)O(2). Our results suggest that under resting conditions, NO-induced activation of cGMP maintains low levels of [Ca](cyt) and MLC phosphorylation to minimize basal tone. This results from stimulation of Ca(2+) uptake from the cytosol into the SR via SERCA pumps, Ca(2+) efflux into the extracellular space via plasma membrane Ca(2+) pumps, and MLC phosphatase (MLCP) activity. We predict that basal concentrations of O(2)(-) and H(2)O(2) have negligible effects on Ca(2+) signaling and MLC phosphorylation. At concentrations above 1 nM, O(2)(-) is predicted to modulate [Ca(cyt)] and MCLP activity mostly by reducing NO bioavailability. The DVR vasoconstriction that is induced by high concentrations of H(2)O(2) can be explained by H(2)O(2)-mediated downregulation of MLCP and SERCA activity. We conclude that intrinsic generation of NO by the DVR wall may be sufficient to inhibit vasoconstriction by maintaining suppression of MLC phosphorylation.

Nitric oxide and superoxide transport in a cross section of the rat outer medulla. I. Effects of low medullary oxygen tension

To examine the impact of the complex radial organization of the rat outer medulla (OM) on the distribution of nitric oxide (NO), superoxide (O(2)(-)) and total peroxynitrite (ONOO), we developed a mathematical model that simulates the transport of those species in a cross section of the rat OM. To simulate the preferential interactions among tubules and vessels that arise from their relative radial positions in the OM, we adopted the region-based approach developed by Layton and Layton (Am J Physiol Renal Physiol 289: F1346-F1366, 2005). In that approach, the structural organization of the OM is represented by means of four concentric regions centered on a vascular bundle. The model predicts the concentrations of NO, O(2)(-), and ONOO in the tubular and vascular lumen, epithelial and endothelial cells, red blood cells (RBCs), and interstitial fluid. Model results suggest that the large gradients in Po(2) from the core of the vascular bundle toward its periphery, which stem from the segregation of O(2)-supplying descending vasa recta (DVR) within the vascular bundles, in turn generate steep radial NO and O(2)(-) concentration gradients, since the synthesis of both solutes is O(2) dependent. Without the rate-limiting effects of O(2), NO concentration would be lowest in the vascular bundle core, that is, the region with the highest density of RBCs, which act as a sink for NO. Our results also suggest that, under basal conditions, the difference in NO concentrations between DVR that reach into the inner medulla and those that turn within the OM should lead to differences in vasodilation and preferentially increase blood flow to the inner medulla.

Nitric oxide and superoxide transport in a cross section of the rat outer medulla. II. Reciprocal interactions and tubulovascular cross talk

In a companion study (Edwards A and Layton AT. Am J Physiol Renal Physiol. doi:10.1152/ajprenal.00680.2009), we developed a mathematical model of nitric oxide (NO), superoxide (O(2)(-)), and total peroxynitrite (ONOO) transport in mid-outer stripe and mid-inner stripe cross sections of the rat outer medulla (OM). We examined how the three-dimensional architecture of the rat OM, together with low medullary oxygen tension (Po(2)), affects the distribution of NO, O(2)(-), and ONOO in the rat OM. In the current study, we sought to determine generation rate and permeability values that are compatible with measurements of medullary NO concentrations and to assess the importance of tubulovascular cross talk and NO-O(2)(-) interactions under physiological conditions. Our results suggest that the main determinants of NO concentrations in the rat OM are the rate of vascular and tubular NO synthesis under hypoxic conditions, and the red blood cell (RBC) permeability to NO (P(NO)(RBC)). The lower the P(NO)(RBC), the lower the amount of NO that is scavenged by hemoglobin species, and the higher the extra-erythrocyte NO concentrations. In addition, our results indicate that basal endothelial NO production acts to significantly limit NaCl reabsorption across medullary thick ascending limbs and to sustain medullary perfusion, whereas basal epithelial NO production has a smaller impact on NaCl transport and a negligible effect on vascular tone. Our model also predicts that O(2)(-) consumption by NO significantly reduces medullary O(2)(-) concentrations, but that O(2)(-) , when present at subnanomolar concentrations, has a small impact on medullary NO bioavailability.

Modeling transport in the kidney: investigating function and dysfunction

Mathematical models of water and solute transport in the kidney have significantly expanded our understanding of renal function in both health and disease. This review describes recent theoretical developments and emphasizes the relevance of model findings to major unresolved questions and controversies. These include the fundamental processes by which urine is concentrated in the inner medulla, the ultrastructural basis of proteinuria, irregular flow oscillation patterns in spontaneously hypertensive rats, and the mechanisms underlying the hypotensive effects of thiazides. Macroscopic models of water, NaCl, and urea transport in populations of nephrons have served to test, confirm, or refute a number of hypotheses related to the urine concentrating mechanism. Other macroscopic models focus on the mechanisms, role, and irregularities of renal hemodynamic control and on the regulation of renal oxygenation. At the mesoscale, models of glomerular filtration have yielded significant insight into the ultrastructural basis underlying a number of disorders. At the cellular scale, models of epithelial solute transport and pericyte Ca2+ signaling are being used to elucidate transport pathways and the effects of hormones and drugs. Areas where further theoretical progress is conditional on experimental advances are also identified.

Chronic ouabain treatment induces vasa recta endothelial dysfunction in the rat

Descending vasa recta (DVR) are 15-microm vessels that perfuse the renal medulla. Ouabain has been shown to augment DVR endothelial cytoplasmic Ca(2+) ([Ca(2+)](CYT)) signaling. In this study, we examined the expression of the ouabain-sensitive Na-K-ATPase alpha2 subunit in the rat renal vasculature and tested effects of acute ouabain exposure and chronic ouabain treatment on DVR. Immunostaining with antibodies directed against the alpha2 subunit verified its expression in both DVR pericytes and endothelium. Acute application of ouabain (100 or 500 nM) augmented the DVR nitric oxide generation stimulated by acetylcholine (ACh; 10 microM). At a concentration of 1 mM, ouabain constricted microperfused DVR, whereas at 100 nM, it was without effect. Acute ouabain (100 nM) did not augment constriction by angiotensin II (0.5 or 10 nM), whereas l-nitroarginine methyl ester-induced contraction of DVR was slightly enhanced. Ouabain-hypertensive (OH) rats were generated by chronic ouabain treatment (30 microg.kg(-1).day(-1), 5 wk). The acute endothelial [Ca(2+)](CYT) elevation by ouabain (100 nM) was absent in DVR endothelia of OH rats. The [Ca(2+)](CYT) response to 10 nM ACh was also eliminated, whereas the response to 10 microM ACh was not. The endothelial [Ca(2+)](CYT) response to bradykinin (100 nM) was significantly attenuated. We conclude that endothelial responses may offset the ability of acute ouabain exposure to enhance DVR vasoconstriction. Chronic exposure to ouabain, in vivo, leads to hypertension and DVR endothelial dysfunction, manifested as reduced [Ca(2+)](CYT) responses to both ouabain- and endothelium-dependent vasodilators.

Renal oxygen delivery: matching delivery to metabolic demand

The kidneys are second only to the heart in terms of O2 consumption; however, relative to other organs, the kidneys receive a very high blood flow and oxygen extraction in the healthy kidney is low. Despite low arterial-venous O2 extraction, the kidneys are particularly susceptible to hypoxic injury and much interest surrounds the role of renal hypoxia in the development and progression of both acute and chronic renal disease. Numerous regulatory mechanisms have been identified that act to maintain renal parenchymal oxygenation within homeostatic limits in the in vivo kidney. However, the processes by which many of these mechanisms act to modulate renal oxygenation and the factors that influence these processes remain poorly understood. A number of such mechanisms specific to the kidney are reviewed herein, including the relationship between renal blood flow and O2 consumption, pre- and post-glomerular arterial-venous O2 shunting, tubulovascular cross-talk, the differential control of regional kidney blood flow and the tubuloglomerular feedback mechanism. The roles of these mechanisms in the control of renal oxygenation, as well as how dysfunction of these mechanisms may lead to renal hypoxia, are discussed.

Computation of plasma hemoglobin nitric oxide scavenging in hemolytic anemias

Intravascular hemoglobin limits the amount of endothelial-derived nitric oxide (NO) available for vasodilation. Cell-free hemoglobin scavenges NO more efficiently than red blood cell-encapsulated hemoglobin. Hemolysis has recently been suggested to contribute to endothelial dysfunction based on a mechanism of NO scavenging by cell-free hemoglobin. Although experimental evidence for this phenomenon has been presented, support from a theoretical approach has, until now, been missing. Indeed, due to the low amounts of cell-free hemoglobin present in these pathological conditions, the role of cell-free hemoglobin scavenging of NO in disease has been questioned. In this study, we model the effects of cell-free hemoglobin on NO bioavailability, focusing on conditions that closely mimic those under known pathological conditions. We find that as little as 1 microM cell-free intraluminal hemoglobin (heme concentration) can significantly reduce NO bioavailability. In addition, extravasation of hemoglobin out of the lumen has an even greater effect. We also find that low hematocrit associated with anemia increases NO bioavailability but also leads to increased susceptibility to NO scavenging by cell-free hemoglobin. These results support the paradigm that cell-free hemoglobin released into plasma during intravascular hemolysis in human disease contributes to the experimentally observed reduction in NO bioavailability and endothelial dysfunction.

A model of nitric oxide tubulovascular cross talk in a renal outer medullary cross section

We developed a two-dimensional model of NO transport in a cross section of the inner stripe (IS) of the rat outer medulla to determine whether tubular and vascular generation of NO result in significant NO concentration (C(NO)) differences between the periphery and the center of vascular bundles and thereby affect medullary blood flow distribution. Following the approach of Layton and Layton (Layton AT, Layton HE. Am J Physiol Renal Physiol 289: F1346-F1366, 2006), the structural heterogeneity of the IS was incorporated in a representative unit consisting of four concentric regions centered on a vascular bundle. Our model suggests that the diffusion distance of NO in the interstitium is limited to a few micrometers. We predict that, under basal conditions, epithelial NO generation raises the average C(NO) in pericytes surrounding peripheral descending vasa recta (DVR) by a few nanomoles relative to that in pericytes surrounding central DVR. The short descending limbs and long ascending limbs are found to exert the greatest effect on C(NO) in pericytes; long descending limbs and short ascending limbs only have a moderate effect, whereas outer medullary collecting ducts, which are situated far from the vascular bundle center, do not affect pericyte C(NO). Our results suggest that selective stimulation of epithelial NO production should significantly raise the periphery-to-center DVR diameter ratio, thereby increasing the outer medulla-to-inner medulla blood flow ratio. However, concomitant increases in epithelial superoxide (O(2)(-)) production would counteract this effect. This model confirms the importance of NO and O(2)(-) interactions in mediating tubulovascular cross talk.

Mathematical model of nitric oxide convection and diffusion in a renal medullary vas rectum

In this study, the generation, convection, diffusion, and consumption of nitric oxide (NO) in and around a single renal medullary descending or ascending vas rectum in rat were modeled using CFD. The vascular lumen (with a core RBC-rich layer and a parietal layer), the endothelium, the pericytes and the interstitium were represented as concentric cylinders. We accounted for the generation of NO by vascular endothelial cells, and that by the epithelial cells of medullary thick ascending limbs (mTALs) and inner medullary collecting ducts (IMCDs), the latter via interstitial boundary conditions. Luminal velocity profiles were obtained by modeling blood flow dynamics. Our results suggest that convection (i.e., blood flow per se) does not significantly affect NO concentrations along the cortico-medullary axis, because the latter are mostly determined by the rate of NO production and that of NO consumption by hemoglobin. However, the shear stress-mediated effects of blood flow on NO generation rates, and therefore NO concentrations, were predicted to be important. Finally, we found that unless epithelial NO generation rates (per unit tubular surface area) are at least 10 times lower than endothelium NO generation rates, NO production by mTALs and IMCDs affects vascular NO concentrations, with possible consequences for medullary blood flow distribution.

A model of glucose transport and conversion to lactate in the renal medullary microcirculation

In this study, we modeled mathematically the transport of glucose across renal medullary vasa recta and its conversion to lactate by anaerobic glycolysis. Uncertain parameter values were determined by seeking good agreement between predictions and experimental measurements of lactate generation rates, as well as glucose and lactate concentration ratios between the papilla and the corticomedullary junction; plausible kinetic rate constant and permeability values are summarized in tabular form. Our simulations indicate that countercurrent exchange of glucose from descending (DVR) to ascending vasa recta (AVR) in the outer medulla (OM) and upper inner medulla (IM) severely limits delivery to the deep inner medulla, thereby limiting medullary lactate generation. If the permeability to glucose of OMDVR and IMDVR is taken to be the same and equal to 4 x 10(-4) cm/s, the fraction of glucose that bypasses the IM is calculated as 54%; it is predicted as 37% if the presence of pericytes in OMDVR reduces the glucose permeability of these vessels by a factor of 2 relative to that of IMDVR. Our results also suggest that red blood cells (RBCs) act as a reservoir that reduces the bypass of glucose from DVR to AVR. The rate of lactate generation by anaerobic glycolysis of glucose supplied by blood from glomerular efferent arterioles is predicted to range from 2 to 8 nmol/s, in good agreement with lower estimates obtained from the literature (Bernanke D and Epstein FH. Am J Physiol 208: 541-545, 1965; Bartlett S, Espinal J, Janssens P, and Ross BD. Biochem J 219: 73-78, 1984).