Renal PO2 Research Articles

Differential responses of cerebral and renal oxygenation to altered perfusion conditions during experimental cardiopulmonary bypass in sheep.

We tested whether the brain and kidney respond differently to cardiopulmonary bypass (CPB) and to changes in perfusion conditions during CPB. Therefore, in ovine CPB, we assessed regional cerebral oxygen saturation (rSO2 ) by near-infrared spectroscopy and renal cortical and medullary tissue oxygen tension (PO2 ), and, in some protocols, brain tissue PO2 , by phosphorescence lifetime oximetry. During CPB, rSO2 correlated with mixed venous SO2 (r = 0.78) and brain tissue PO2 (r = 0.49) when arterial PO2 was varied. During the first 30 min of CPB, brain tissue PO2 , rSO2 and renal cortical tissue PO2 did not fall, but renal medullary tissue PO2 did. Nevertheless, compared with stable anaesthesia, during stable CPB, rSO2 (66.8 decreasing to 61.3%) and both renal cortical (90.8 decreasing to 43.5 mm Hg) and medullary (44.3 decreasing to 19.2 mm Hg) tissue PO2 were lower. Both rSO2 and renal PO2 increased when pump flow was increased from 60 to 100 mL kg-1 min-1 at a target arterial pressure of 70 mm Hg. They also both increased when pump flow and arterial pressure were increased simultaneously. Neither was significantly altered by partially pulsatile flow. The vasopressor, metaraminol, dose-dependently decreased rSO2 , but increased renal cortical and medullary PO2 . Increasing blood haemoglobin concentration increased rSO2 , but not renal PO2 . We conclude that both the brain and kidney are susceptible to hypoxia during CPB, which can be alleviated by increasing pump flow, even without increasing arterial pressure. However, increasing blood haemoglobin concentration increases brain, but not kidney oxygenation, whereas vasopressor support with metaraminol increases kidney, but not brain oxygenation.

Peripheral chemoreflex modulation of renal hemodynamics and renal tissue PO2 in chronic heart failure with reduced ejection fraction.

Aberrant carotid body chemoreceptor (CBC) function contributes to increased sympathetic nerve activity (SNA) and reduced renal blood flow (RBF) in chronic heart failure (CHF). Intermittent asphyxia (IA) mimicking sleep apnea is associated with additional increases in SNA and may worsen reductions in RBF and renal PO2 (RPO2) in CHF. The combined effects of decreased RBF and RPO2 may contribute to biochemical changes precipitating renal injury. This study sought to determine the role of CBC activity on glomerular filtration rate (GFR), RBF and RPO2 in CHF, and to assess the additive effects of IA. Furthermore, we sought to identify changes in gene expression that might contribute to renal injury. We hypothesized that GFR, RBF, and RPO2 would be reduced in CHF, that decreases in RBF and RPO2 would be worsened by IA, and that these changes would be ameliorated by CBC ablation (CBD). Finally, we hypothesized that CHF would be associated with pro-oxidative pro-fibrotic changes in renal gene expression that would be ameliorated by CBD. CHF was induced in adult male Sprague Dawley rats using coronary artery ligation (CAL). Carotid body denervation was performed by cryogenic ablation. GFR was assessed in conscious animals at the beginning and end of the experimental period. At 8-weeks post-CAL, cardiac function was assessed via echocardiography, and GFR, baseline and IA RBF and RPO2 were measured. Renal gene expression was measured using qRT-PCR. GFR was lower in CHF compared to sham (p < 0.05) but CBD had no salutary effect. RBF and RPO2 were decreased in CHF compared to sham (p < 0.05), and this effect was attenuated by CBD (p < 0.05). RBF and RPO2 were reduced to a greater extent in CHF vs. sham during exposure to IA (p < 0.05), and this effect was attenuated by CBD for RBF (p < 0.05). Downregulation of antioxidant defense and fibrosis-suppressing genes was observed in CHF vs. sham however CBD had no salutary effect. These results suggest that aberrant CBC function in CHF has a clear modulatory effect on RBF during normoxia and during IA simulating central sleep apnea.

Renal Oxygen Sensing Mechanisms May Contribute to Maintaining Cerebral Perfusion During Acute Anemia

Introduction The integrative physiological response to anemia is complex and incompletely understood. We utilized data from studies of acute anemia in rodents to determine the relationship between changes in blood oxygen content (CaO2) and the heterogenous response of the kidney, heart and brain. We hypothesize that renal hypoxia sensing mechanisms contribute to adaptive physiological responses to maintain cerebral oxygen delivery (DO2) during acute anemia. Methods With animal care committee approval, we synthesized novel and published data from 5 previously published studies. Outcomes included: assessment of the relationship between CaO2 and microvascular renal and brain PO2 (phosphorescence quenching of oxyphor G4); cardiac output (CO) and renal and cerebral blood flow (ultrasound Doppler); hypoxia induced cellular responses (brain and kidney erythropoietin (EPO) mRNA and serum protein levels (ELISA)). Statistical analysis (SigmaPlot 14) was performed by ANOVA, Holm-Sidak and Mann-Whitney rank sum test when appropriate. Significance was assigned at p<0.05. Results In two models of anemia (hemodilution and RBC antibody mediated), acute reductions in blood CaO2 were associated with larger decreases in renal microvascular PO2, relative to brain microvascular PO2 (p<0.05). After acute hemodilution, there was a strong relationship between CaO2 and renal microvascular PO2 (r2=0.75). The magnitude of reduction in renal microvascular PO2 correlated with the degree of renal EPO mRNA expression and serum EPO protein levels. The magnitude of the increase in EPO mRNA was much larger in the kidney than in the brain (p<0.03). While no change in renal blood flow was observed in either model, a significant increase in common carotid and internal carotid blood flow was observed in both models (p<0.012). When DO2 was assessed, the kidney DO2 was reduced at all levels of anemia (p<0.01) whereas brain tissue DO2 was maintained in mild and moderate (Hb 90 and 70 g/L) (p=0.44) anemia but reduced in severe anemia (Hb 50 g/L) (p<0.02). The role of active cardiovascular increases in brain blood flow and maintained DO2 during anemia was impaired by systemic beta blockade, suggesting that active cardiovascular mechanisms are required to maintain optimal brain DO2 during anemia. Discussion Our analysis demonstrated: evidence of quantitative renal PO2 sensing of changes in CaO2; the clamping of renal blood flow (reduced DO2) during anemia may be a central mechanism allowing for sensing of changes in CaO2; reduced arterial CaO2 resulted in a local renal hypoxia response (increased serum EPO) and may have initiated the cardiovascular response to increase cerebral blood flow and maintain cerebral DO2. Inhibition of the adrenergic system impaired these responses and resulted in reduced brain DO2. Understanding the heterogeneous adaptive responses to acute anemia may inform clinical practice and optimize management of acutely anemia patients.

Effect of Polyethylene-glycolated Carboxyhemoglobin on Renal Microcirculation in a Rat Model of Hemorrhagic Shock.

Primary resuscitation fluid to treat hemorrhagic shock remains controversial. Use of hydroxyethyl starches raised concerns of acute kidney injury. Polyethylene-glycolated carboxyhemoglobin, which has carbon monoxide-releasing molecules and oxygen-carrying properties, was hypothesized to sustain cortical renal microcirculatory PO2 after hemorrhagic shock and reduce kidney injury. Anesthetized and ventilated rats (n = 42) were subjected to pressure-controlled hemorrhagic shock for 1 h. Renal cortical PO2 was measured in exposed kidneys using a phosphorescence quenching method. Rats were randomly assigned to six groups: polyethylene-glycolated carboxyhemoglobin 320 mg · kg, 6% hydroxyethyl starch (130/0.4) in Ringer's acetate, blood retransfusion, diluted blood retransfusion (~4 g · dl), nonresuscitated animals, and time control. Nitric oxide and heme oxygenase 1 levels were determined in plasma. Kidney immunohistochemistry (histologic scores of neutrophil gelatinase-associated lipocalin and tumor necrosis factor-α) and tubular histologic damages analyses were performed. Blood and diluted blood restored renal PO2 to 51 ± 5 mmHg (mean difference, -18; 95% CI, -26 to -11; P < 0.0001) and 47 ± 5 mmHg (mean difference, -23; 95% CI, -31 to -15; P < 0.0001), respectively, compared with 29 ± 8 mmHg for hydroxyethyl starch. No differences between polyethylene-glycolated carboxyhemoglobin and hydroxyethyl starch were observed (33 ± 7 mmHg vs. 29 ± 8 mmHg; mean difference, -5; 95% CI, -12 to 3; P = 0.387), but significantly less volume was administered (4.5 [3.3-6.2] vs. 8.5[7.7-11.4] ml; mean rank difference, 11.98; P = 0.387). Blood and diluted blood increased the plasma bioavailability of nitric oxide compared with hydroxyethyl starch (mean rank difference, -20.97; P = 0.004; and -17.13; P = 0.029, respectively). No changes in heme oxygenase 1 levels were observed. Polyethylene-glycolated carboxyhemoglobin limited tubular histologic damages compared with hydroxyethyl starch (mean rank difference, 60.12; P = 0.0012) with reduced neutrophil gelatinase-associated lipocalin (mean rank difference, 84.43; P < 0.0001) and tumor necrosis factor-α (mean rank difference, 49.67; P = 0.026) histologic scores. Polyethylene-glycolated carboxyhemoglobin resuscitation did not improve renal PO2 but limited tubular histologic damages and neutrophil gelatinase-associated lipocalin upregulation after hemorrhage compared with hydroxyethyl starch, whereas a lower volume was required to sustain macrocirculation.

Renal perfusion, oxygenation, and sympathetic nerve activity during volatile or intravenous general anaesthesia in sheep

BackgroundGlobal and intra-renal perfusion and oxygenation may be affected by the choice of anaesthetic. We compared the effects of isoflurane with those of propofol and fentanyl on renal blood flow (RBF) and intra-renal perfusion and oxygenation, and assessed how these were associated with renal sympathetic nerve activity (RSNA). MethodsA renal artery flow probe and laser Doppler and oxygen-sensing probes were surgically implanted in the renal medulla and cortex in 20 Merino ewes. RSNA was measured in 12 additional ewes. We compared the effects of volatile or i.v. anaesthesia on global RBF, renal oxygen delivery (RDO2), intra-renal perfusion, and RSNA with the non-anaesthetised state on postoperative day 3 as control reference. ResultsCompared with a non-anaesthetised state, volatile anaesthesia reduced global RBF [–76 (82–68)%], RDO2 [−76 (83–71)%], and cortical [–68 (74–54)%] and medullary [–76 (84–72)%] perfusion. I.V. anaesthesia reduced RBF [–55 (67–38)%], RDO2 [–55 (65–44)%], and cortical [–27 (45–6)%] and medullary [–35 (48–30)%] perfusion, but to a lesser extent than volatile anaesthesia. Renal PO2 was not influenced by anaesthesia, whilst RSNA was elevated during volatile, but not during i.v. anaesthesia. ConclusionsVolatile and i.v. general anaesthesia markedly reduced global RBF, RDO2, and regional kidney perfusion. These effects were greater with volatile anaesthesia, and were paralleled by an increase in RSNA. Our findings suggest a neurogenic modulatory effect of anaesthetics on renal perfusion and oxygenation.

BOLD quantified renal pO2 is sensitive to pharmacological challenges in rats

Blood oxygen level-dependent (BOLD) MRI has been effectively used to monitor changes in renal oxygenation. However, R2* (or T2*) is not specific to blood oxygenation and is dependent on other factors. This study investigates the use of a statistical model that takes these factors into account and maps BOLD MRI measurements to blood pO2. Spin echo and gradient echo images were obtained in six Sprague-Dawley rats and R2 and R2* maps were computed. Measurements were made at baseline, post-nitric oxide synthase inhibitor (L-NAME), and post-furosemide administration. A simulation of each region was performed to map R2' (computed as R2*-R2) to blood pO2. At baseline, blood pO2 in the outer medulla was 30.5 ± 1.2 mmHg and 51.9 ± 5.2 mmHg in the cortex, in agreement with previous invasive studies. Blood pO2 was found to decrease within the outer medulla following L-NAME (P < 0.05) and increase after furosemide (P < 0.05). Blood pO2 in the cortex increased following furosemide (P < 0.05). Model-derived blood pO2 is sensitive to pharmacological challenges, and baseline pO2 is comparable to literature values. Reporting pO2 instead of R2* could lead to a greater clinical impact of renal BOLD MRI and facilitate the identification of hypoxic regions. Magn Reson Med 78:297-302, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Chronic Renal Hypoxia Leads to Injury and Dysfunction: Role of HIF1alpha

Renal hypoxia precedes renal injury and impaired GFR in multiple renal disease models and in hypertension. Further, in imaging studies in humans, renal hypoxia correlates with hypertension and reduced renal function. We propose that renal injury that accompanies hypertension is mediated by events stimulated by hypoxia and responses linked to HIF1a. To test these effects, we developed a pilot study to generate reproducible renal hypoxia in rats by treatment with 2,4‐dinitrophenol (DNP, 30 mg/kg/day). DNP is a mitochondrial uncoupling agent that acts as a proton ionophore, which reduces production of ATP, yet still consumes O2. Renal cortical and medullary pO2 levels were lower in DNP‐treated rats compared to vehicle (Cortex pO2: 34±4 vs 44±4 mmHg; Medulla pO2: 22±3 ve 31±3 mmHg, p&lt;0.001, n=6). Blood pO2 was unchanged by DNP treatment. Expressions of HIF1a and the pro‐oxidant enzyme, NADPH oxidase‐2 (NOX‐2) were increased in DNP‐treated rats. Conversely the antioxidant enzyme, superoxide dismutase‐2 (SOD‐2), was not changed. Further, the renal injury markers TGFb and NGAL were higher in DNP‐treated rats. MAP, basal GFR and RBF were unchanged in response to DNP‐generated renal hypoxia. Separate DNP‐fed rats were treated with direct left renal injections of siRNA directed to HIF1a, 48 hours prior to measuring renal pO2. Expression of HIF1a was reduced by 66±3% in the left kidneys treated with siRNA in DNP‐treated rats and was 32% lower than HIF1a in Veh‐treated rats. Expression of HIF1a in the right kidney was not altered by siRNA injection in the left kidney. Knockdown of HIF1a further reduced pO2 compared to DNP‐treated rats (Cort pO2 = −4±2 mmHg; Med pO2 = −8±1 mmHg, p&lt;0.01), suggesting HIF1a protects the kidney from greater renal hypoxia. Further, expression of renal injury markers was higher in HIF1a reduced kidneys. We conclude that chronic renal hypoxia induces renal injury via oxidant pathways and stimulates HIF1a, which is only partially protective. DNP‐induced renal hypoxia is a viable model to test the long‐term effects of hypertension on renal oxygen metabolism.Support or Funding InformationHL089583; HL068686

Renal epithelium regulates erythropoiesis via HIF-dependent suppression of erythropoietin.

The adult kidney plays a central role in erythropoiesis and is the main source of erythropoietin (EPO), an oxygen-sensitive glycoprotein that is essential for red blood cell production. Decreases of renal pO2 promote hypoxia-inducible factor 2-mediated (HIF-2-mediated) induction of EPO in peritubular interstitial fibroblast-like cells, which serve as the cellular site of EPO synthesis in the kidney. It is not clear whether HIF signaling in other renal cell types also contributes to the regulation of EPO production. Here, we used a genetic approach in mice to investigate the role of renal epithelial HIF in erythropoiesis. Specifically, we found that HIF activation in the proximal nephron via induced inactivation of the von Hippel-Lindau tumor suppressor, which targets the HIF-α subunit for proteasomal degradation, led to rapid development of hypoproliferative anemia that was associated with a reduction in the number of EPO-producing renal interstitial cells. Moreover, suppression of renal EPO production was associated with increased glucose uptake, enhanced glycolysis, reduced mitochondrial mass, diminished O2 consumption, and elevated renal tissue pO2. Our genetic analysis suggests that tubulointerstitial cellular crosstalk modulates renal EPO production under conditions of epithelial HIF activation in the kidney.

Hyperpolarized 13C urea relaxation mechanism reveals renal changes in diabetic nephropathy

PurposeOur aim was to assess a novel 13C radial fast spin echo golden ratio single shot method for interrogating early renal changes in the diabetic kidney, using hyperpolarized (HP) [13C,15N2]urea as a T2 relaxation based contrast bio‐probe.MethodsA novel HP 13C MR contrast experiment was conducted in a group of streptozotocin type‐1 diabetic rat model and age matched controls.ResultsA significantly different relaxation time (P = 0.004) was found in the diabetic kidney (0.49 ± 0.03 s) compared with the controls (0.64 ± 0.02 s) and secondly, a strong correlation between the blood oxygen saturation level and the relaxation times were observed in the healthy controls.ConclusionHP [13C,15N2]urea apparent T2 mapping may be a useful for interrogating local renal pO2 status and renal tissue alterations. Magn Reson Med, 2015. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made. Magn Reson Med 75:515–518, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.

Telemetry‐based Recording of Renal Cortex Oxygenation During Endogenous RAS Activation: Preliminary Observations

Upon feeding of the food additive indole‐3‐carbinol (I3C) the liver of Cyp1A1Ren2 transgenic rats produces high levels of renin which leads to hypertension and tissue damage in the kidney. We hypothesized that endogenous RAS activation in this model would be associated with renal hypoxia. Using a new technology that enables the telemetric recording of intra‐renal tissue oxygen pressure (PO2) in conscious, free roaming animals, we here report preliminary observations on renal cortical PO2 changes during endogenous RAS activation. 1 week after implantation of the electrodes the relative change in renal cortex PO2 during bouts of 3 days of endogenous activation of RAS by feeding an 0.3% or 0.6% I3C containing diet (n=5) were recorded. 3±2 days after start of the 0.6% I3C diet, s.c. bolus infusions of valsartan/losartan (20/30 mg/kg) were administered. During 0.3% and 0.6% I3C feeding, renal PO2 levels fell to 75±7 and 63±13% from baseline values (mean±SD), with a nadir after 25±5 hours. Exogenous blockade of the angiotensin‐II receptor increased oxygen levels within 14±3 min to 98±38% of baseline values (mean±SD). These data suggest that oxygen levels in the renal cortex decrease rapidly during the onset of endogenous RAS hyperactivation. The effect is mediated by AT1‐receptor activation and may precede the known development of systemic hypertension and renal injury in this model.

Regulation and function of the FGF23/klotho endocrine pathways.

Calcium (Ca(2+)) and phosphate (PO(4)(3-)) homeostasis are coordinated by systemic and local factors that regulate intestinal absorption, influx and efflux from bone, and kidney excretion and reabsorption of these ions through a complex hormonal network. Traditionally, the parathyroid hormone (PTH)/vitamin D axis provided the conceptual framework to understand mineral metabolism. PTH secreted by the parathyroid gland in response to hypocalcemia functions to maintain serum Ca(2+) levels by increasing Ca(2+) reabsorption and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] production by the kidney, enhancing Ca(2+) and PO(4)(3-) intestinal absorption and increasing Ca(2+) and PO(4)(3-) efflux from bone, while maintaining neutral phosphate balance through phosphaturic effects. FGF23 is a recently discovered hormone, predominately produced by osteoblasts/osteocytes, whose major functions are to inhibit renal tubular phosphate reabsorption and suppress circulating 1,25(OH)(2)D levels by decreasing Cyp27b1-mediated formation and stimulating Cyp24-mediated catabolism of 1,25(OH)(2)D. FGF23 participates in a new bone/kidney axis that protects the organism from excess vitamin D and coordinates renal PO(4)(3-) handling with bone mineralization/turnover. Abnormalities of FGF23 production underlie many inherited and acquired disorders of phosphate homeostasis. This review discusses the known and emerging functions of FGF23, its regulation in response to systemic and local signals, as well as the implications of FGF23 in different pathological and physiological contexts.

Effect of Pneumoperitoneum on Renal Tissue Oxygenation and Blood Flow in a Rat Model

To determine the correlation between the renal blood flow (RBF) and tissue oxygenation (PO(2)) at varying intra-abdominal pressures (IAPs) and to compare the effects on renal blood flow from carbon dioxide-induced pneumoperitoneum. Carbon dioxide pneumoperitoneum was established in Sprague-Dawley rats (n = 6). Licox oxygen/temperature tissue probes were laparoscopically inserted into the renal parenchyma, with the renal PO(2) and RBF recorded every 30 seconds while the IAP was gradually increased. Microprobes measuring the RBF, mean arterial pressures and serum pH were placed into the parenchyma to compare the effects of carbon dioxide pneumoperitoneum (n = 7) with that of open surgery (n = 6) and medical air pneumoperitoneum (n = 6). Renal PO(2) was inversely related to the IAP (P < .001). Despite the reduction in IAP, the renal PO(2) in the recovery phase was lower than at baseline (P = .045). The renal PO(2) and RBF changed in a virtually identical pattern at varying levels of IAP (P > .05). The RBF significantly declined with a pneumoperitoneal pressure of 15 and 20 mm Hg (P = .022), regardless of the gas used to create the pneumoperitoneum. A partial reversal of the RBF occurred with a decrease of the IAP. The RBF in the open surgical arm remained unchanged. Although both the serum pH and the mean arterial pressure were inversely proportional to the IAP (P < .001), the mean arterial pressure was depressed to the greatest extent in the medical air group (P = .02). These results have demonstrated that elevated IAP secondary to pneumoperitoneum causes significant renal hypoxia and decreased RBF. Additionally, this experiment has demonstrated the use of the Licox probes in monitoring the renal PO(2) and established a novel method for evaluating the effects of IAP on the kidney.

NADPH oxidase inhibition reduces tubular Na+ transport and improves kidney oxygenation in diabetic rats

Sustained hyperglycemia is associated with increased oxidative stress and decreased renal oxygen tension (pO2) secondary to increased oxygen consumption. The NADPH oxidase is a major source of oxygen radicals in the kidney and we therefore studied the effects of NADPH oxidase inhibition (apocynin; APO) on tubular Na+ transport and kidney pO2 in diabetic rats. Glomerular filtration rate (GFR; inulin clearance), renal blood flow (RBF; transonic flow probe), proximal tubular electrolyte transport (Li+ clearance) and tissue pO2 (microelectrodes) were measured in Inactin-anesthetized control and streptozotocin-diabetic rats during baseline and after acute APO administration (10 mg/kg bolus). Diabetic rats had increased GFR (2.2±0.3 vs. 1.4±0.1 ml/min/kidney), reduced pO2 in cortex and medulla (34.6±0.9 & 15.7±1.2 vs. 41.6±1.9 & 26.1±2.3 mmHg), increased Na+ excretion (24.0±4.7 vs. 9.0±2.0 μm/h/kidney) and reduced Li+ excretion compared to controls (0.23±0.02 vs. 0.38±0.06). RBF was similar in both groups. In diabetics, APO increased Na+ (156%) and Li+ excretion (48%) and improved renal pO2 (21–35%), whereas GFR and RBF were unchanged. All parameters were unaffected by APO administered to control animals. Type-1 diabetic patients have increased risk to develop salt-sensitive hypertension, and our findings may provide a mechanistic explanation for this phenomenon.

Multiple mechanisms act to maintain kidney oxygenation during renal ischemia in anesthetized rabbits

We examined the mechanisms that maintain stable renal tissue PO(2) during moderate renal ischemia, when changes in renal oxygen delivery (DO(2)) and consumption (VO(2)) are mismatched. When renal artery pressure (RAP) was reduced progressively from 80 to 40 mmHg, VO(2) (-38 ± 7%) was reduced more than DO(2) (-26 ± 4%). Electrical stimulation of the renal nerves (RNS) reduced DO(2) (-49 ± 4% at 2 Hz) more than VO(2) (-30 ± 7% at 2 Hz). Renal arterial infusion of angiotensin II reduced DO(2) (-38 ± 3%) but not VO(2) (+10 ± 10%). Despite mismatched changes in DO(2) and VO(2), renal tissue PO(2) remained remarkably stable at ≥40 mmHg RAP, during RNS at ≤2 Hz, and during angiotensin II infusion. The ratio of sodium reabsorption to VO(2) was reduced by all three ischemic stimuli. None of the stimuli significantly altered the gradients in PCO(2) or pH across the kidney. Fractional oxygen extraction increased and renal venous PO(2) fell during 2-Hz RNS and angiotensin II infusion, but not when RAP was reduced to 40 mmHg. Thus reduced renal VO(2) can help prevent tissue hypoxia during mild renal ischemia, but when renal VO(2) is reduced less than DO(2), other mechanisms prevent a fall in renal PO(2). These mechanisms do not include increased efficiency of renal oxygen utilization for sodium reabsorption or reduced washout of carbon dioxide from the kidney, leading to increased oxygen extraction. However, increased oxygen extraction could be driven by altered countercurrent exchange of carbon dioxide and/or oxygen between renal arteries and veins.

Impact of Arterial and Arteriovenous Renal Clamping with and without Intrarenal Cooling on Renal Oxygenation and Temperature in a Porcine Model

As laparoscopic partial nephrectomy increases in prominence, more needs to be understood about the combined effect of the pneumoperitoneum and renal ischemia during tumor resection. The purpose of this study is to investigate the effect of combined renal hilar clamping (arterial only versus arteriovenous) and retrograde intrarenal cooling on renal temperature and oxygenation in a porcine laparoscopic partial nephrectomy model. Under general anesthesia, laparoscopic access with intra-abdominal pressure of 15 mm Hg to the left renal hilum was obtained. Licox tissue oxygenation and temperature probes were placed into the kidney transcutaneously; measurements were taken every 30 seconds. After establishing baseline readings, either the artery alone (n=18) or the artery and vein (n=18) were clamped for 30, 60 or 90 minutes (n=12 each). During vascular clamping, retrograde, intrarenal cooling was performed with ice cold saline infused via a percutaneously placed ureteric catheter in 18 pigs. Changes in renal pO2 and temperature were analyzed with repeated measures ANCOVA in SPSS 16. Retrograde cooling decreased renal parenchyma to 75.8% of baseline temperature (27.9 degrees C) within 15 minutes. There were no differences in cooling whether arterial or arteriovenous clamping was used (p=0.79). In uncooled animals, there was no significant difference in the decrease in renal pO2 during the clamp phase (p=0.18) or during the recovery phase (p=0.52). During the recovery phase, renal pO2 in uncooled animals was significantly higher than in those who received cooling (p=0.01). Animals who underwent hilar clamping for extended periods (60 and 90 min) had a slower recovery of renal pO2 to baseline than those with hilar clamping for 30 minutes (p=0.04) Retrograde intrarenal cooling can reliably cool the porcine kidney to 28 degrees C, regardless of whether arterial or arteriovenous clamping is used. Renal pO2 is not significantly different between animals that undergo artery only versus en bloc hilar clamping. Pigs that were provided with retrograde cooling had a slower return of pO2 to baseline following release of hilar clamps, possibly due to hypothermic vasospasm. Clamp durations greater than 60 minutes were also associated with slower return of renal oxygenation to baseline.

INTRARENAL OXYGENATION IN CHRONIC RENAL FAILURE

In chronic renal failure (CRF), renal impairment correlates with tubulointerstitial fibrosis characterized by inflammation, interstitial expansion with accumulation of extracellular matrix (ECM), tubular atrophy and vascular obliteration. Tubulointerstitial injury subsequent to glomerular sclerosis may be induced by proteinuria, leakage of glomerular filtrate or injury to the post-glomerular peritubular capillaries (hypoxia). In vivo data in animal models suggest that CRF is associated with hypoxia, with the decline in renal Po2 preceding ECM accumulation. Chronic renal failure is characterized by loss of microvascular profiles but, in the absence of microvascular obliteration, hypoxia can occur by a variety of complementary mechanisms, including anaemia, decreased capillary flow, increased vasoconstriction, increased metabolic demand and increased diffusion distances due to ECM deposition. Hypoxia regulates a wide array of genes, including many fibrogenic factors. Hypoxia-inducible factors (HIF) are the major, but not the sole, transcriptional regulators in the hypoxic response. In CRF, hypoxia may play a role in the sustained inflammatory response. In vitro studies in tubulointerstitial cells suggest that hypoxia can induce profibrogenic changes in proximal tubular epithelial cells and interstitial fibroblasts consistent with changes observed in CRF in vivo. The effect of hypoxia on renal microvascular cells warrants investigation. Hypoxia may play a role in the recruitment, retention and differentiation of circulating progenitor cells to the kidney contributing to the disease process and may also affect intrinsic stem cell populations. Chronic hypoxia in CRF fails to induce a sustained angiogenic response. Therapeutic manipulation of the hypoxic response may be of benefit in slowing progression of CRF. Potential therapies include correction of anaemia, inhibition of the renin-angiotensin system, administration of exogenous pro-angiogenic factors to protect the microvasculature, activation of HIF and hypoxia-mediated targeting of engineered progenitor cells.

Growth hormone corrects acidosis-induced renal nitrogen wasting and renal phosphate depletion and attenuates renal magnesium wasting in humans

We have shown previously that chronic hyperchloremic metabolic acidosis (CMA) induces severe negative nitrogen balance and renal phosphate depletion and decreases serum insulin-like growth factor-1 (IGF-1) in association with growth hormone (GH) insensitivity in humans. The present study investigated whether acidosis-induced renal nitrogen wasting and renal phosphate depletion are mediated by GH insensitivity/low IGF-1 and thereby responsive to GH treatment. The effects of GH on acidosis-induced changes in divalent cation metabolism and acidosis-induced hypothyroidism were also investigated. CMA (Δ[HCO 3], −10.5 mmol/L) was induced in six healthy male subjects ingesting 4.2 mmol NH 4Cl/kg body weight [BW]/d for 7 days. Recombinant human GH (0.1 U/kg BW/12 h subcutaneously) was administered for 7 days while acid feeding was continued. GH increased serum IGF-1 from 22.1 ± 1.4 to 87 ± 8.4 nmol/L (control level, 36.4 ± 2.2). GH decreased urinary nitrogen excretion, resulting in a cumulative nitrogen retention of 2,404 mmol, thereby correcting the acidosis-induced cumulative increase in nitrogen excretion (2,506 mmol) despite continued acid feeding. GH attenuated the acidosis-induced hyperphosphaturia (cumulative phosphate retention, 91 mmol) and corrected the hypophosphatemia. GH did not affect acidosis-induced ionized hypercalcemia, but further exacerbated acidosis-induced hypercalciuria (cumulative loss, 27.3 mmol). GH significantly further increased serum 1,25-dihydroxyvitamin D (1,25(OH) 2D) and further decreased intact PTH (from 10 ± 1 to 6 ± 1 pg/mL). Acidosis also induced hypomagnesemia and hypermagnesuria (cumulative loss, 9.4 mmol, ie, renal magnesium wasting), a novel finding, which was significantly attenuated by GH (cumulative retention, 5.0 mmol). In conclusion, GH corrected acidosis-induced renal nitrogen wasting, which may be caused, at least in part, by decreased IGF-1 levels. GH further increased serum 1,25(OH) 2D and the systemic calcium load, which account for the suppression of parathyroid hormone (PTH) despite renal PO 4 retention and correction of hypophosphatemia. GH attenuated acidosis-induced renal magnesium wasting.

Gloxy: an oxygen-sensitive coal for accurate measurement of low oxygen tensions in biological systems.

This paper describes the characteristics of a new oxygen sensitive, paramagnetic material that has some significant advantages for measurements of tissue pO2 by in vivo EPR. This paramagnetic component of Welsh coal, termed "gloxy" was found to have valuable EPR features that allow accurate measurement of low oxygen tensions in vivo; these include large oxygen-dependent changes in linewidth, a high number of paramagnetic spin centers (resulting in high signal amplitude), and stability in tissue allowing repeated pO2 measurements to be made in vivo with high precision. Renal pO2 was measured deep in the medulla region of isolated perfused kidneys and found to be lower than that in the cortex (1.7 +/- 0.05 and 7.1 +/- 0.3 mm Hg, respectively). The quality of the EPR signal obtained from the renal outer medulla and also from tumors in mice was such that the pO2 measurements were obtained with a precision of +/-3% of the measured pO2 (Kidney: 1.7 +/- 0.05 mmHg; Tumor: 1.37 +/- 0.04 mmHg). In vitro tests on the viability of cells and in vivo studies using Gloxy demonstrate the stability and inertness of this oxygen-sensitive material.

Endotoxin-induced changes in intrarenal pO 2, measured by in vivo electron paramagnetic resonance oximetry and magnetic resonance imaging

Electron Paramagnetic Resonance (EPR) oximetry was used to measure tissue oxygen tension (pO 2—partial pressure of oxygen) simultaneously in the kidney cortex and outer medulla in vivo in mice. pO 2 in the cortex region was higher compared to that in the outer medulla. An intravenous injection of endotoxin resulted in a sharp drop in pO 2 in the cortex and an increase in the medulla region, resulting in a transient period of equal pO 2 in both regions. In control kidneys, functional Magnetic Resonance (MR) images showed the cortex region to have high signal intensity (T 2 ∗-weighted images), indicating that this region was well supplied with oxygenated hemoglobin, whereas the outer medulla showed low signal intensity. After administration of endotoxin, we observed an immediate increase in signal intensity in the outer medulla region, reflecting an increased level of oxygenated blood in this region. Pretreatment of mice with N O-monomethyl- l-arginine prevented both the changes in tissue pO 2 and distribution of oxygenated hemoglobin, suggesting that localized production of nitric oxide has a critical role to play in renal medullary hemodynamics. In combining in vivo EPR with MR images of kidneys, we demonstrate the usefulness of these techniques for monitoring renal pO 2 and changes in the distribution of oxygen.

Effect of cortisone acetate on renal PO 4 transport mechanism in

Effect of cortisone acetate on renal PO 4 transport mechanism in