Modulation of outer medullary NaCl transport and oxygenation by nitric oxide and superoxide

Renal oxygenation: From data to insight

Computational models have made a major contribution to the field of physiology. As the complexity of our understanding of biological systems expands, the need for computational methods only increases. But collaboration between experimental physiologists and computational modellers (ie theoretical physiologists) is not easy. One of the major challenges is to break down the barriers created by differences in vocabulary and approach between the two disciplines. In this review, we have two major aims. Firstly, we wish to contribute to the effort to break down these barriers and so encourage more interdisciplinary collaboration. So, we begin with a "primer" on the ways in which computational models can help us understand physiology and pathophysiology. Second, we aim to provide an update of recent efforts in one specific area of physiology, renal oxygenation. This work is shedding new light on the causes and consequences of renal hypoxia. But as importantly, computational modelling is providing direction for experimental physiologists working in the field of renal oxygenation by: (a) generating new hypotheses that can be tested in experimental studies, (b) allowing experiments that are technically unfeasible to be simulated in silico, or variables that cannot be measured experimentally to be estimated, and (c) providing a means by which the quality of experimental data can be assessed. Critically, based on our experience, we strongly believe that experimental and theoretical physiology should not be seen as separate exercises. Rather, they should be integrated to permit an iterative process between modelling and experimentation.

Superoxide increases surface NKCC2 in the rat thick ascending limbs via PKC

The apical Na⁺-K⁺-2Cl^- cotransporter (NKCC2) mediates NaCl reabsorption by the thick ascending limb (TAL). The free radical superoxide ( O2- ) stimulates TAL NaCl absorption by enhancing NKCC2 activity. In contrast, nitric oxide (NO) scavenges O2- and inhibits NKCC2. NKCC2 activity depends on the number of NKCC2 transporters in the TAL apical membrane and its phosphorylation. We hypothesized that O2- stimulates NKCC2 activity by enhancing apical surface NKCC2 expression. We measured surface NKCC2 expression in rat TALs by surface biotinylation and Western blot analysis. Treatment of TALs with O2- produced by exogenous xanthine oxidase (1 mU/ml) and hypoxanthine (500 µM) stimulated surface NKCC2 expression by ~18 ± 5% (P < 0.05). O2- -stimulated surface NKCC2 expression was blocked by the O2- scavenger tempol (50 µM). Scavenging H₂O₂ with 100 U/ml catalase did not block the stimulatory effect of xanthine oxidase-hypoxanthine (22 ± 8% increase from control, P < 0.05). Inhibition of endogenous NO production with N^ω-nitro-l-arginine methyl ester enhanced surface NKCC2 expression by 21 ± 6% and, when added together with xanthine oxidase-hypoxanthine, increased surface NKCC2 by 41 ± 10% (P < 0.05). Scavenging O2- with superoxide dismutase (300 U/ml) decreased this stimulatory effect by 60% (39 ± 4% to 15 ± 10%, P < 0.05). Protein kinase C inhibition with Gö-6976 (100 nM) blocked O2- -stimulated surface NKCC2 expression (P < 0.05). O2- did not affect NKCC2 phosphorylation at Thr^96/101 or its upstream kinases STE20/SPS1-related proline/alanine-rich kinase-oxidative stress-responsive kinase 1. We conclude that O2- increases surface NKCC2 expression by stimulating protein kinase C and that this effect is blunted by endogenous NO. O2- -stimulated apical trafficking of NKCC2 may be involved in the enhanced surface NKCC2 expression observed in Dahl salt-sensitive rats.

Adaptive responses of rat descending vasa recta to ischemia

tested whether rat descending vasa recta (DVR) undergo regulatory adaptations after the kidney is exposed to ischemia. Left kidneys (LK) were subjected to 30-min renal artery cross clamp. After 48 h, the postischemic LK and contralateral right kidney (RK) were harvested for study. When compared with DVR isolated from either sham-operated LK or the contralateral RK, postischemic LK DVR markedly increased their NO generation. The selective inducible NOS (iNOS) inhibitor 1400W blocked the NO response. Immunoblots from outer medullary homogenates showed a parallel 2.6-fold increase in iNOS expression ( P = 0.01). Microperfused postischemic LK DVR exposed to angiotensin II (ANG II, 10 nM), constricted less than those from the contralateral RK, and constricted more when exposed to 1400W (10 µM). Resting membrane potentials of pericytes from postischemic LK DVR pericytes were hyperpolarized relative to contralateral RK pericytes (62.0 ± 1.6 vs. 51.8 ± 2.2 mV, respectively, P < 0.05) or those from sham-operated LK (54.9 ± 2.1 mV, P < 0.05). Blockade of NO generation with 1400W did not repolarize postischemic pericytes (62.5 ± 1.4 vs. 61.1 ± 3.4 mV); however, control pericytes were hyperpolarized by exposure to NO donation from S-nitroso- N-acetyl- dl-penicillamine (51.5 ± 2.9 to 62.1 ± 1.4 mV, P < 0.05). We conclude that postischemic adaptations intrinsic to the DVR wall occur after ischemia. A rise in 1400W sensitive NO generation and iNOS expression occurs that is associated with diminished contractile responses to ANG II. Pericyte hyperpolarization occurs that is not explained by the rise in ambient NO generation within the DVR wall.

Modeling glucose metabolism and lactate production in the kidney

The metabolism of glucose provides most of the ATP required for energy-dependent transport processes. In the inner medulla of the mammalian kidney, limited blood flow and O2 supply yield low oxygen tension; therefore, a substantial fraction of the glucose metabolism in that region is anaerobic. Lactate is considered to be a waste product of anaerobic glycolysis, which yields two lactate molecules for each glucose molecule consumed, thereby likely leading to the production and accumulation of a significant amount of lactate in the inner medulla. To gain insights into the transport and metabolic processes in the kidney, we have developed a detailed mathematical model of 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 and glucose and lactate concentrations in the outer medulla and upper inner medulla. In the deep inner medulla, interstitial fluid concentrations become much more homogeneous, as the radial organization of tubules and vessels is not distinguishable. Using this model, we have identified parameters concerning glucose transport and basal metabolism, as well as lactate production via anaerobic glycolysis, that yield predicted blood glucose and lactate concentrations consistent with experimental measurements in the papillary tip. In addition, simulations indicate that the radial organization of the rat kidney may affect lactate buildup in the inner medulla.

Sex-specific computational models of the spontaneously hypertensive rat kidneys: factors affecting nitric oxide bioavailability

The goals of this study were to 1) develop a computational model of solute transport and oxygenation in the kidney of the female spontaneously hypertensive rat (SHR), and 2) apply that model to investigate sex differences in nitric oxide (NO) levels in SHR and their effects on medullary oxygenation and oxidative stress. To accomplish these goals, we first measured NO synthase (NOS) 1 and NOS3 protein expression levels in total renal microvessels of male and female SHR. We found that the expression of both NOS1 and NOS3 is higher in the renal vasculature of females compared with males. To predict the implications of that finding on medullary oxygenation and oxidative stress levels, we developed a detailed computational model of the female SHR kidney. The model was based on a published male kidney model and represents solute transport and the biochemical reactions among O₂, NO, and superoxide ([Formula: see text]) in the renal medulla. Model simulations conducted using both male and female SHR kidney models predicted significant radial gradients in interstitial fluid oxygen tension (Po₂) and NO and [Formula: see text] concentration in the outer medulla and upper inner medulla. The models also predicted that increases in endothelial NO-generating capacity, even when limited to specific vascular segments, may substantially raise medullary NO and Po₂ levels. Other potential sex differences in SHR, including [Formula: see text] production rate, are predicted to significantly impact oxidative stress levels, but effects on NO concentration and Po₂ are limited.

How do stones form? Is unification of theories on stone formation possible?

There are two basic pathways for formation of calcium based kidney stones. Most idiopathic calcium oxalate (CaOx) stones are formed in association with sub-epithelial plaques of calcium phosphate (CaP), known as Randall's plaques, on renal papillary surfaces. Crystal formation and retention within the terminal collecting ducts, the ducts of Bellini, leading to the formation of Randall's plugs, is the other pathway. Both pathways require supersaturation leading to crystallization, regulated by various crystallization modulators produced in response to changing urinary conditions. High supersaturation, as a result of a variety of genetic and environmental factors, leads to crystallization in the terminal collecting ducts, eventually plugging their openings into the renal pelvis. Stasis behind the plugs may lead to the formation of attached or unattached stones in the tubular lumen. Deposition of crystals on the plug surface facing the pelvic or tubular urine may result in stone formation on the Randall's plugs. Kidneys of idiopathic stone formers may be subjected to oxidative stress as a result of increased urinary excretion of calcium/oxalate/phosphate and/or decrease in the production of functional crystallization inhibitors or in relation to co-morbidities such as hypertension, atherosclerosis, or acute kidney injury. We have proposed that production of reactive oxygen species (ROS) causes dedifferentiation of epithelial/endothelial cells into osteoblast type cells and deposition of CaP in the basement membrane of renal tubules or vessels. Growth, aggregation and melding of CaP crystals leads to the formation of plaque which grows by further calcification of interstitial collagen and membranous vesicles. Plaque becomes exposed to pelvic urine once the covering papillary epithelium is breached. Surface layers of CaP are replaced by CaOx through direct transformation or demineralization of CaP and mineralization of CaOx. Alternatively, or in addition, CaOx crystals nucleate directly on the plaque surface. Stone growth may also depend upon supersaturation in the pelvic urine, triggering further nucleation, growth and aggregation.

Diffusive shunting of gases and other molecules in the renal vasculature: physiological and evolutionary significance

Countercurrent systems have evolved in a variety of biological systems that allow transfer of heat, gases, and solutes. For example, in the renal medulla, the countercurrent arrangement of vascular and tubular elements facilitates the trapping of urea and other solutes in the inner medulla, which in turn enables the formation of concentrated urine. Arteries and veins in the cortex are also arranged in a countercurrent fashion, as are descending and ascending vasa recta in the medulla. For countercurrent diffusion to occur, barriers to diffusion must be small. This appears to be characteristic of larger vessels in the renal cortex. There must also be gradients in the concentration of molecules between afferent and efferent vessels, with the transport of molecules possible in either direction. Such gradients exist for oxygen in both the cortex and medulla, but there is little evidence that large gradients exist for other molecules such as carbon dioxide, nitric oxide, superoxide, hydrogen sulfide, and ammonia. There is some experimental evidence for arterial-to-venous (AV) oxygen shunting. Mathematical models also provide evidence for oxygen shunting in both the cortex and medulla. However, the quantitative significance of AV oxygen shunting remains a matter of controversy. Thus, whereas the countercurrent arrangement of vasa recta in the medulla appears to have evolved as a consequence of the evolution of Henle’s loop, the evolutionary significance of the intimate countercurrent arrangement of blood vessels in the renal cortex remains an enigma.

Modeling Glucose Metabolism in the Kidney

The mammalian kidney consumes a large amount of energy to support the reabsorptive work it needs to excrete metabolic wastes and to maintain homeostasis. Part of that energy is supplied via the metabolism of glucose. To gain insights into the transport and metabolic processes in the kidney, we have developed a detailed model of the renal medulla of the rat kidney. The model represents water and solute flows, transmural fluxes, and biochemical reactions in the luminal fluid of the nephrons and vessels. In particular, the model simulates the metabolism of oxygen and glucose. Using that model, we have identified parameters concerning glucose transport and basal metabolism that yield predicted blood glucose concentrations that are consistent with experimental measurements. The model predicts substantial axial gradients in blood glucose levels along various medullary structures. Furthermore, the model predicts that in the inner medulla, owing to the relatively limited blood flow and low tissue oxygen tension, anaerobic metabolism of glucose dominates.

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.

Oxygen transport in a cross section of the rat inner medulla: impact of heterogeneous distribution of nephrons and vessels

We have developed a highly detailed mathematical model of oxygen transport in a cross section of the upper inner medulla of the rat kidney. The model is used to study the impact of the structured organization of nephrons and vessels revealed in anatomic studies, in which descending vasa recta are found to lie distant from clusters of collecting ducts. Specifically, we formulated a two-dimensional oxygen transport model, in which the positions and physical dimensions of renal tubules and vessels are based on an image obtained by immunochemical techniques (T. Pannabecker and W. Dantzler, Three-dimensional architecture of inner medullary vasa recta, Am. J. Physiol. Renal Physiol. 290 (2006) F1355-F1366). The model represents oxygen diffusion through interstitium and other renal structures, oxygen consumption by the Na(+)/K(+)-ATPase activities of the collecting ducts, and basal metabolic consumption. Model simulations yield marked variations in interstitial PO2, which can be attributed, in large part, to the heterogeneities in the position and physical dimensions of the collecting ducts. Further, results of a sensitivity study suggest that medullary oxygenation is highly sensitive to medullary blood flow, and that, at high active consumption rates, localized patches of tissue may be vulnerable to hypoxic injury.

Chronic activation of vasopressin V2 receptor signalling lowers renal medullary oxygen levels in rats

AIM

In the present study, we aimed to elucidate the effects of chronic vasopressin administration on renal medullary oxygen levels.

METHODS

Adult Sprague Dawley or vasopressin-deficient Brattleboro rats were treated with the vasopressin V2 receptor agonist, desmopressin (5 ng/h; 3d), or its vehicle via osmotic minipumps. Immunostaining for pimonidazole and the transcription factor HIF-1α (hypoxia-inducible factor-1α) were used to identify hypoxic areas. Activation of HIF-target gene expression following desmopressin treatment was studied by microarray analysis.

RESULTS

Pimonidazole staining was detected in the outer and inner medulla of desmopressin-treated rats, whereas staining in control animals was weak or absent. HIF-1α immunostaining demonstrated nuclear accumulation in the papilla of desmopressin-treated animals, whereas no staining was observed in the controls. Gene expression analysis revealed significant enrichment of HIF-target genes in the group of desmopressin-regulated gene products (P = 2.6*10(-21) ). Regulated products included insulin-like growth factor binding proteins 1 and 3, angiopoietin 2, fibronectin, cathepsin D, hexokinase 2 and cyclooxygenase 2.

CONCLUSION

Our results demonstrate that an activation of the renal urine concentrating mechanism by desmopressin causes renal medullary hypoxia and an upregulation of hypoxia-inducible gene expression.

Impact of nitric oxide-mediated vasodilation on outer medullary NaCl transport and oxygenation

The present study aimed to elucidate the reciprocal interactions between oxygen (O(2)), nitric oxide (NO), and superoxide (O(2)(-)) and their effects on vascular and tubular function in the outer medulla. We expanded our region-based model of transport in the rat outer medulla (Edwards A, Layton AT. Am J Physiol Renal Physiol 301: F979-F996, 2011) to incorporate the effects of NO on descending vasa recta (DVR) diameter and blood flow. Our model predicts that the segregation of long DVR in the center of vascular bundles, away from tubular segments, gives rise to large radial NO concentration gradients that in turn result in differential regulation of vasoactivity in short and long DVR. The relative isolation of long DVR shields them from changes in the rate of NaCl reabsorption, and hence from changes in O(2) requirements, by medullary thick ascending limbs (mTALs), thereby preserving O(2) delivery to the inner medulla. The model also predicts that O(2)(-) can sufficiently decrease the bioavailability of NO in the interbundle region to affect the diameter of short DVR, suggesting that the experimentally observed effects of O(2)(-) on medullary blood flow may be at least partly mediated by NO. In addition, our results indicate that the tubulovascular cross talk of NO, that is, the diffusion of NO produced by mTAL epithelia toward adjacent DVR, helps to maintain blood flow and O(2) supply to the interbundle region even under basal conditions. NO also acts to preserve local O(2) availability by inhibiting the rate of active Na(+) transport, thereby reducing the O(2) requirements of mTALs. The dual regulation by NO of oxygen supply and demand is predicted to significantly attenuate the hypoxic effects of angiotensin II.