Absence of renal hypoxia in the subacute phase of severe renal ischemia reperfusion injury

 This is the author accepted manuscript. The final version is available from American Physiological Society via the DOI in this recordTissue hypoxia has been proposed as an important event in renal ischemia reperfusion injury (IRI) particularly during the period of ischemia and in the immediate hours following reperfusion. However, little is known about renal oxygenation during the subacute phase of IRI. We employed four different methods to assess the temporal and spatial changes in tissue oxygenation during the subacute phase (24 h and 5 days after reperfusion) of a severe form of renal IRI in rats. We hypothesized that the kidney is hypoxic 24 h and 5 days after an hour of bilateral renal ischemia, driven by a disturbed balance between renal oxygen delivery (DO2) and oxygen consumption (VO2). Renal DO2 was not significantly reduced in the subacute phase of IRI. In contrast, renal VO2 was 55% less 24 h, and 49% less 5 days after reperfusion than after sham-ischemia. Inner medullary tissue PO2, measured by radiotelemetry was 25 {plus minus} 12% greater 24 h after ischemia than after sham-ischemia. By 5 days after reperfusion, tissue PO2 was similar to that in rats subjected to sham-ischemia. Tissue PO2 measured by Clark electrode was consistently greater 24 h, but not 5 days, after ischemia than after sham-ischemia. Cellular hypoxia, assessed by pimonidazole adduct immunohistochemistry, was largely absent at both time-points and tissue levels of hypoxia inducible factors were downregulated following renal ischemia. Thus, in this model of severe IRI, tissue hypoxia does not appear to be an obligatory event during the subacute phase, likely due to the markedly reduced oxygen consumption.British Heart FoundationBritish Heart FoundationNational Health and Medical Research Council of AustraliaEuropean Union, Seventh Framework Programm

Quantitative assessment of renal perfusion and oxygenation by invasive probes: basic concepts

Renal tissue hypoperfusion and hypoxia are early key elements in the pathophysiology of acute kidney injury of various origins, and may also promote progression from acute injury to chronic kidney disease. Here we describe basic principles of methodology to quantify renal hemodynamics and tissue oxygenation by means of invasive probes in experimental animals. Advantages and disadvantages of the various methods are discussed in the context of the heterogeneity of renal tissue perfusion and oxygenation.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by a separate chapter describing the experimental procedure and data analysis

Effects of furosemide, acetazolamide and amiloride on renal cortical and medullary tissue oxygenation in non-anaesthetised healthy sheep.

It has been proposed that diuretics can improve renal tissue oxygenation through inhibition of tubular sodium reabsorption and reduced metabolic demand. However, the impact of clinically used diuretic drugs on the renal cortical and medullary microcirculation is unclear. Therefore, we examined the effects of three commonly used diuretics, at clinically relevant doses, on renal cortical and medullary perfusion and oxygenation in non-anaesthetised healthy sheep. Merino ewes received acetazolamide (250 mg; n = 9), furosemide (20 mg; n = 10) or amiloride (10 mg; n = 7) intravenously. Systemic and renal haemodynamics, renal cortical and medullary tissue perfusion and P O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ , and renal function were then monitored for up to 8 h post-treatment. The peak diuretic response occurred 2 h (99.4 ± 14.8 mL/h) after acetazolamide, at which stage cortical and medullary tissue perfusion and P O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ were not significantly different from their baseline levels. The peak diuretic response to furosemide occurred at 1 h (196.5 ± 12.3 mL/h) post-treatment but there were no significant changes in cortical and medullary tissue oxygenation during this period. However, cortical tissue P O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ fell from 40.1 ± 3.8 mmHg at baseline to 17.2 ± 4.4 mmHg at 3 h and to 20.5 ± 5.3 mmHg at 6 h after furosemide administration. Amiloride did not produce a diuretic response and was not associated with significant changes in cortical or medullary tissue oxygenation. In conclusion, clinically relevant doses of diuretic agents did not improve regional renal tissue oxygenation in healthy animals during the 8 h experimentation period. On the contrary, rebound renal cortical hypoxia may develop after dissipation of furosemide-induced diuresis

Effects of sodium-glucose transporter-2 inhibition on systemic hemodynamics, renal function, and intra-renal oxygenation in sepsis-associated acute kidney injury

BACKGROUND: People with type 2 diabetes mellitus treated with sodium-glucose transporter-2 inhibitors (SGLT2i) have lower rates of acute kidney injury (AKI). Sepsis is responsible for the majority of AKI in critically ill patients. This study investigated whether SGLT2i is renoprotective in an ovine model of Gram-negative septic AKI. METHODS: Sixteen healthy merino ewes were surgically instrumented to enable measurement of mean arterial pressure, cardiac output, renal blood flow, renal cortical and medullary perfusion, and oxygenation. After a 5-day recovery period, sepsis was induced via slow and continuous intravenous infusion of live Escherichia coli. Twenty-three hours later, sheep were randomized to receive an intravenous bolus of 0.2 mg/kg empagliflozin (n = 8) or a fluid-matched vehicle (n = 8). RESULTS: Empagliflozin treatment did not significantly reduce renal medullary hypoperfusion or hypoxia, improve kidney function, or induce histological changes. Renal cortical oxygenation during the intervention period was 47.6 ± 5.9 mmHg in the empagliflozin group compared with 40.6 ± 8.2 mmHg in the placebo group (P = 0.16). Renal medullary oxygenation was 28.0 ± 18.5 mmHg in the empagliflozin compared with 25.7 ± 16.3 mmHg (P = 0.82). Empagliflozin treatment did not result in significant between-group differences in renal blood flow, kidney function, or renal histopathological changes. CONCLUSION: In a large mammalian model of septic AKI, a single dose of empagliflozin did not improve renal microcirculatory perfusion, oxygenation, kidney function, or histopathology

Renal arterial infusion of tempol prevents medullary hypoperfusion, hypoxia, and acute kidney injury in ovine Gram-negative sepsis

AIM: Renal medullary hypoperfusion and hypoxia precede acute kidney injury (AKI) in ovine sepsis. Oxidative/nitrosative stress, inflammation, and impaired nitric oxide generation may contribute to such pathophysiology. We tested whether the antioxidant and anti-inflammatory drug, tempol, may modify these responses. METHODS: Following unilateral nephrectomy, we inserted renal arterial catheters and laser-Doppler/oxygen-sensing probes in the renal cortex and medulla. Noanesthetized sheep were administered intravenous (IV) Escherichia coli and, at sepsis onset, IV tempol (IVT; 30 mg kg-1  h-1 ), renal arterial tempol (RAT; 3 mg kg-1  h-1 ), or vehicle. RESULTS: Septic sheep receiving vehicle developed renal medullary hypoperfusion (76 ± 16% decrease in perfusion), hypoxia (70 ± 13% decrease in oxygenation), and AKI (87 ± 8% decrease in creatinine clearance) with similar changes during IVT. However, RAT preserved medullary perfusion (1072 ± 307 to 1005 ± 271 units), oxygenation (46 ± 8 to 43 ± 6 mmHg), and creatinine clearance (61 ± 10 to 66 ± 20 mL min-1 ). Plasma, renal medullary, and cortical tissue malonaldehyde and medullary 3-nitrotyrosine decreased significantly with sepsis but were unaffected by IVT or RAT. Consistent with decreased oxidative/nitrosative stress markers, cortical and medullary nuclear factor-erythroid-related factor-2 increased significantly and were unaffected by IVT or RAT. However, RAT prevented sepsis-induced overexpression of cortical tissue tumor necrosis factor alpha (TNF-α; 51 ± 16% decrease; p = 0.003) and medullary Thr-495 phosphorylation of endothelial nitric oxide synthase (eNOS; 63 ± 18% decrease; p = 0.015). CONCLUSIONS: In ovine Gram-negative sepsis, renal arterial infusion of tempol prevented renal medullary hypoperfusion and hypoxia and AKI and decreased TNF-α expression and uncoupling of eNOS. However, it did not affect markers of oxidative/nitrosative stress, which were significantly decreased by Gram-negative sepsis

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

Persistent Renal Hypoxia and Histologic Changes at 4 Weeks after Cardiopulmonary Bypass in Sheep

BACKGROUND: The sustained renal effects of exposure to cardiopulmonary bypass are unknown. This study aimed to test whether cardiopulmonary bypass (CPB) is associated with sustained renal tissue hypoxia and whether such hypoxia is associated with histologic injury. METHODS: The study included 12 adult female sheep undergoing CPB with a 2-h aortic cross-clamp. Systemic and renal hemodynamics and oxygen delivery, kidney function, and renal tissue oxygenation were measured before and during CPB, in the 48 h after CPB, and weekly for 4 weeks. The sheep were euthanized at 4 weeks and obtained renal tissue to perform histopathologic assessments for comparison with an independent cohort of five healthy animals that were euthanized without undergoing surgical or experimental interventions. These histologic assessments were performed by an independent, treatment-blinded pathologist. RESULTS: Compared with baseline, renal blood flow and renal medullary tissue oxygenation decreased significantly during CPB. In the first 48 h after CPB, there was a continuing significant decrease in medullary tissue oxygenation (from 39.2 ± 13.8 mmHg at baseline to 21.7 ± 16.2 mmHg at 48 h; Ptime = 0.006) with stage 1 acute kidney injury in 42% of the animals. Moreover, in the following 4 weeks, medullary (16.1 ± 12.9 mmHg at 4 weeks; Ptime = 0.005) and cortical (17.2 ± 6.5 mmHg at 4 weeks; Ptime = 0.005) tissue oxygenation remained significantly lower than baseline. Finally, compared with healthy sheep, at 4 weeks after CPB, sheep kidneys had significantly more peritubular inflammation (8 of 8 vs . 1 of 5; P = 0.007), interstitial fibrosis (6 of 8 vs . 0 of 5; P = 0.021), and tubular casts (8 of 8 vs . 1 of 5; P = 0.007). CONCLUSIONS: Exposure to CPB triggers sustained medullary and cortical tissue hypoxia and is associated with histopathologic renal injury. These findings suggest that the renal effect of exposure to CPB may be more profound and longer lasting than currently appreciated