Renal Medullary Blood Flow Research Articles

Anemia increases the risk of renal cortical and medullary hypoxia during cardiopulmonary bypass

Anemia is an independent predictor of acute kidney injury (AKI) following cardiopulmonary bypass (CPB), possibly due to inadequate renal oxygen delivery. The objective of this study was to investigate the effects of CPB and anemia on tissue oxygen tension (pO2) and blood flow in the renal cortex and medulla. Rats (n=6/group) underwent 1 hr of normothermic cardiopulmonary bypass (CPB), with target hemoglobin concentrations (Hb) of 10 g/dL (CPB) or 6.5 g/dL (anemia-CPB). Renal blood flow (RBF) and tissue PO2 were measured before, during and after 1 hr of CPB. To confirm the observed differences in renal cortical and medullary PO2, HIF-1α (ODD) luciferase mice were exposed to 8% O2 (hypoxia) and HIF-1α dependent luminescence was measured in the renal cortex and medulla (n=5). Renal tissue PO2 values decreased initially and returned towards baseline, however, values at the end of CPB. Anemia-CPB resulted in a significant increase in both renal cortical and medullary blood flow, PO2 remained significantly reduced throughout anemia-CPB. Renal medullary HIF-1α-dependent luminescence confirmed a greater degree of hypoxia in the renal medulla. During CPB, renal O2 delivery was transiently jeopardized, but recovered after 1 hr. Anemia-CPB resulted in a dramatic and sustained reduction in renal cortical and medullary PO2, which suggests an increased risk of renal hypoxic injury with anemia. The clear difference in the degree of hypoxia in the renal cortex and medulla may be useful in understanding the progress of medullary hypoxia during CPB with anemia and the potential development of AKI. Further studies should aim at identifying early markers of medullary hypoxia and potential agents that may decrease the work and O2 consumption in the renal medulla to reduce the risk of hypoxic damage during CPB and anemia.

Regulation of Renal Sodium Excretion by Medullary NLRP3 Inflammasome Activation beyond Turning on Inflammation

NLRP3 inflammasomes were found abundant in the renal medulla and their activation altered the renal function and increased the salt sensitivity of arterial blood pressure (AP). The present study further explored the mechanisms mediating the effects of NLRP3 inflammasomes on renal excretory function, which may be independent of typical inflammatory response. In anesthetized mice, intramedullary infusion of NLRP3 inflammasome activators, monosodium urate (MSU) or ATP induced significant decreases in sodium excretion by more than 50% and medullary blood flow (MBF) by 15% without changes in mean AP and renal cortical blood flow (CBF). Pre‐infusion of caspase‐1 inhibitor, Ac‐YVAD‐CMK abolished MSU or ATP‐induced decreases in renal sodium excretion and MBF. By co‐immunoprecipitation analysis, inflammasome component proteins ASC, caspase‐1 and NLRP3 were found to interact more actively in the renal medulla with infusion of MSU or ATP compared to control mice, suggesting enhanced inflammasome formation. Moreover, the caspase‐1 activity was shown significantly increased by 1.5 folds in the renal medulla receiving MSU or ATP, which was accompanied by a 20% increase in local IL‐1β level. Interestingly, SOD mimetic TEMPOL was found to not only block increases in O2− levels induced by MSU or ATP, but also prevent caspase‐1 activation and IL‐1β production. Correspondingly, MSU or ATP‐induced reduction of sodium excretion and renal MBF were reversed by TEMPOL. In additional group of mice, direct infusion of IL‐1β (10 ng/kg/min) into the renal medulla mimicked the effects of MSU or ATP, reducing sodium excretion by 35% and MBF by 10%. Taken together, our results support the view that beyond turning on inflammation, activation of NLRP3 inflammasomes in the renal medulla has direct effects on renal vessels and tubules to induce sodium retention, ultimately resulting in hypertension (Supported by NIH grants HL057244, HL‐75316 and DK54927).

Saline loading does not change renal medullary blood flow in essential hypertension

In essential hypertension (EH), the natriuresis during saline loading is exaggerated. It was hypothesized that this phenomenon is associated with an increase in renal medullary blood flow. Patients (n=6) with mild, monosymptomatic EH were investigated after an 18‐d drug washout period including 4 d on standardized diet. Regional, renal blood flows were determined before and after saline loading (0.72 mmol/kg/h for 4 h) by Positron Emission Tomography and Computer Tomography (PET/CT) using H215O tracer (Am J Physiol Regul Integr Comp Physiol 299:R1449, 2010). Renal medullary blood flows (ml/(g tissue)/min) were 2.0 ±0.4 and 1.7 ±0.3* before and after saline loading, respectively. During saline loading BP (mmHg) changed from 147.6/86.1 ±6.5/3.1 to 157.1*/92.1† ±4.4/3.3. Renal sodium excretion (μmol/min) increased from 39.2 ±7.0 to 145.6 ±38.5†. It is concluded that the increase in renal sodium excretion during saline loading is not associated with an increase in renal medullary blood flow. *(P = ns) †(P < 0.05)Funded by The Danish Heart Foundation and The University of Southern Denmark.

Serine proteases affect in situ vasa recta capillary diameter: mechanism for kidney failure associated with pancreatitis?

Serine proteases are proteolytic enzymes involved in the regulation of renal function and hemodynamics. Serum trypsin (the archetypal serine protease) levels are elevated in a number of pathophysiological conditions, notably pancreatitis, which is associated with acute renal failure.Here we use the kidney slice model to investigate the effect of trypsin on vasa recta diameter in situ. Kidney slices (200 μm thick) were obtained from adult male rats as previously described [1]. Kidney slices were superfused with trypsin and DIC video imaging techniques were used to record real‐time changes in vasa recta diameter.Application of trypsin (2 U/ml) caused a significant constriction of vasa recta capillaries, specifically at smooth muscle‐like pericyte cell sites (15.5±1.4% p<0.05).Here we demonstrate that acute application of trypsin (at levels similar to those detected in patients with severe acute pancreatitis) significantly decreases vasa recta diameter and thus renal medullary blood flow, specifically via its action at pericytes. Data presented here identifies a novel potential role for pericytes in the acute renal failure observed in patients with acute pancreatitis.Research supported by BBSRC, Wellcome Trust and the MRC

Acute Renal Medullary Interstitial Infusion of either saline or 2% albumin indistinctly rises RIHP and RMBF in the rat

The effects of an acute renal medullary interstitial infusion (RMII) of 100 μl of either 0.9% saline (Sal) or 2% albumin in saline (Alb) on the time course (30 min) of renal interstitial hydrostatic pressure (RIHP, subcapsular), renal medullary blood flow (RMBF, L‐D), urinary nitrogen oxides excretion (UNOxV, Griess) and urinary flow rate (V) were assessed in the left denervated kidney of anesthetized male Wistar rats under control of renal perfusion pressure at 100 mmHg. After a 10 min control period (CP) and a 15s RMII period, three 10 min post‐infusion periods (P‐IP) of urinary collections were generated. The results are mean ± SEM; ^ p<0.05 vs. CP. Group CP RMII P‐IP 10min 20 min 30 min RIHP (mmmHg) Sal 3.0 3.8 3.9 5.3 6.8 n=17 ± 0.4 ± 0.5 ±0.5^ ± 0.7^ ± 0.8^ Alb 3.0 3.2 3.5 4.0 4.8 n=17 ± 0.3 ± 0.3^ ± 0.3^ ± 0.5^ ± 0.7^ RMBF (%Δ) Sal 0.0 2.0 10.7 12.5 8.2 n=17 ± 3.4 ± 3.3^ ± 4.0 ± 4.3 Alb 0.0 2.2 9.1 6.6 8.7 n=17 ± 5.0 ± 4.5^ ± 4.7 ± 5.2 There were no differences between groups in UNOxV nor in V through time.ConclusionsAcute RMII of either 0.9% saline or 2% albumin indistinctly rises RIHP and RMBF in the anesthetized rat, without affecting UNOxV nor urinary flow rate at all. Supported by IPL's financial resources.

Evaluation of the role of the NAD(P)H oxidase subunit p67phox in the regulation of medullary blood flow in Dahl S rats

Studies were performed in male Dahl salt‐sensitive rats (SS; SS/JrHsdMcwi) and SS rats with a null mutation in the p67phox gene (SSp67phox null mutant; SS‐Ncf2em1Mcwi+/+) aged 10–12 weeks (n=5/group) and maintained from birth on a 0.4% NaCl diet (LS). Optical fibers were implanted in the renal medulla and medullary blood flow (MBF) measured in the conscious animal by laser‐Doppler flowmetry. Mean arterial pressure (MAP) was measured from indwelling carotid catheters by telemetry. Following a 7 day recovery from surgery, baseline measurements of MAP and MBF were made daily to obtain 3 stable control days on LS. Rats were then switched to 4% NaCl diet (HS) and MAP and MBF measured over 7 days. During the LS period, MAP was comparable between SS (127±2mmHg) and p67phox null (121±3mmHg) rats. HS caused a rapid and sustained decrease in MBF in SS rats which was significantly different from LS by day 2 (reduction of 23±4%, p<0.05). Reduced MBF was associated with a progressive increase in MAP which averaged 149±4mmHg at day 7. HS had no significant effect on MBF in SSp67phoxnull rats and the pressor response was blunted, MAP averaged 133±8mmHg at day 7. In summary, when p67phox is not functional in the SS rat HS does not affect MBF and salt‐sensitive hypertension is attenuated. This suggests that p67phox contributes to the increased oxidative stress in the renal medulla associated with salt induced hypertension.

Hemodynamic Effects of Furosemide on Renal Perfusion as Evaluated by ASL-MRI

The aim of this study was to investigate the short-term effects of furosemide on renal perfusion by using arterial spin labeling (ASL) magnetic resonance imaging. Eleven healthy human subjects were enrolled in the study. The measurement of renal blood flow (RBF) was performed by applying an ASL technique with flow-sensitive alternating inversion recovery spin preparation and a single-shot fast spin-echo imaging strategy on a 3.0-T magnetic resonance scanner. For all subjects, the ASL magnetic resonance images were obtained before agent injection as a baseline scan. Then 20 mg of furosemide was injected intravenously. Postfurosemide ASL images were acquired following administration to evaluate the renal hemodynamic response. Postinjection scans showed that cortical RBF decreased from 366.59 ± 41.19 mL/100 g/min at baseline to 314.33 ± 48.83 mL/100 g/min at 10 minutes after the administration of furosemide (paired t test, P = .04 vs baseline), and medullary RBF decreased from 118.59 ± 24.69 mL/100 g/min at baseline to 97.38 ± 18.40 mL/100 g/min at 10 minutes after the administration of furosemide (paired t test, P = .01 vs baseline). There was a negative correlation between the furosemide-induced diuretic effect and the reduction of RBF (Spearman's r = -0.61). The dominant hemodynamic effect of furosemide on the kidney is associated with a decrease in both cortical and medullary blood perfusion. Furthermore, the quantitative ASL technique may provide an alternative way to noninvasively monitor the change in renal function due to furosemide administration.

Abstract 54: Tissue Oxygenation Using Fractional Hypoxia Correlates Inversely With Blood Flow in Human Renovascular Disease (RVD) and Essential Hypertension (EH)

Determinants of Intra-renal oxygenation in human RVD are poorly understood. Blood oxygen level-dependent (BOLD) MRI measures deoxyhemoglobin (R2*) to determine cortical and medullary R2* which have focal variations and gradients. We hypothesized that tissue fractional hypoxia (% of axial kidney area with R2*>30s -1 ) would correlate better with tissue perfusion, renal blood flow (RBF), and O2 consumption than using regions of interest (ROI). METHODS: Inpatient studies were performed in EH (n=68 kidneys), and high-grade RVD (n=18 kidneys, 71±5.5% occlusion) before and 3 months after stents. All were treated with ACE/ARB Rx and thiazides during 150 mEq/d Na+ intake. BOLD MR images were obtained at 3T before and after furosemide (20mg IV), and axial images analyzed to estimate R2* levels above 30s -1 (fractional hypoxia) over the entire kidney section. Maximal R2* levels using regions of interest (ROIs) also were determined. Single-kidney cortical and medullary RBF (ml/min) perfusion (ml/min/cc) were determined by MDCT. Results: RBF was lower in kidneys with untreated RVD than EH (285.6±141 vs. 399±159 ml/min p<0.05), as was cortical perfusion (2.58±.63 vs. 3.50±1.05, p<.05 ). Cortical and medullary R2* did not differ, but fractional hypoxia was greater in RVD as compared to EH (18.3±12 vs. 11.5±8.5%, p<.05). Fractional hypoxia was inversely correlated with blood flow (R=-.29, p<.01) and perfusion (R=-.26, p<.01). R2* fell after furosemide (p<0.01) indicating a role for O2 consumption in determining tissue hypoxia. RBF and cortical perfusion increased after stenting (285±141 vs. 318±178 ml/min and 2.58±0.6 vs. 2.9±0.7ml/min/cc, p<0.05) respectively. Fractional hypoxia, but not cortical or medullary R2* based on ROI-methods, decreased in patients 3 months after renal revascularization (18.3 ±12 vs. 10.52%, p<0.05) Conclusions: These data demonstrate for the first time that fractional hypoxia in the kidney is directly related to chronically reduced blood flow and cortical perfusion in human RVD, whereas selective ROI-based measures identified only minor changes in peak R2* gradients between cortex and medulla. The reversibility of fractional hypoxia after stenting supports the use of this new method to identify critical renovascular lesions.

Production and Actions of the Anandamide Metabolite Prostamide E2 in the Renal Medulla

Medullipin has been proposed to be an antihypertensive lipid hormone released from the renal medulla in response to increased arterial pressure and renal medullary blood flow. Because anandamide (AEA) possesses characteristics of this purported hormone, the present study tested the hypothesis that AEA or one of its metabolites represents medullipin. AEA was demonstrated to be enriched in the kidney medulla compared with cortex. Western blotting and enzymatic analyses of renal cortical and medullary microsomes revealed opposite patterns of enrichment of two AEA-metabolizing enzymes, with fatty acid amide hydrolase higher in the renal cortex and cyclooxygenase-2 (COX-2) higher in the renal medulla. In COX-2 reactions with renal medullary microsomes, prostamide E2, the ethanolamide of prostaglandin E₂, was the major product detected. Intramedullarily infused AEA dose-dependently increased urine volume and sodium and potassium excretion (15-60 nmol/kg/min) but had little effect on mean arterial pressure (MAP). The renal excretory effects of AEA were blocked by intravenous infusion of celecoxib (0.1 μg/kg/min), a selective COX-2 inhibitor, suggesting the involvement of a prostamide intermediate. Plasma kinetic analysis revealed longer elimination half-lives for AEA and prostamide E2 compared with prostaglandin E₂. Intravenous prostamide E2 reduced MAP and increased renal blood flow (RBF), actions opposite to those of angiotensin II. Coinfusion of prostamide E2 inhibited angiotensin II effects on MAP and RBF. These results suggest that AEA and/or its prostamide metabolites in the renal medulla may represent medullipin and function as a regulator of body fluid and MAP.

Direct infusion of fenoldopam into the renal arteries to protect against contrast‐induced nephropathy in patients at increased risk

1. Fenoldopam is a dopamine-like receptor agonist that selectively increases renal medullary blood flow. Despite promising pathophysiology, randomized controlled trials have failed to show a benefit with intravenous infusion of fenoldopam in preventing contrast-induced nephropathy (CIN). 2. The aims of the present study were to examine the efficacy of intrarenal (i.r.) fenoldopam in preventing CIN in high-risk patients undergoing coronary angiograms and percutaneous revascularization. 3. The present study was a single centre, retrospective observational matched cohort study that included 52 patients at risk of developing CIN to whom i.r. fenoldopam was given during catheter-based coronary procedures. For each patient treated, a control patient was identified on basis of an identical Mehran score. Observed rates of CIN and in-hospital dialysis were compared between the i.r. fenoldopam and control groups. 4. Direct comparison of the 52 patients treated with i.r. fenoldopam and the 52 matched controls revealed that the two groups had similar baseline clinical characteristics and creatinine levels (2.4 ± 0.7 vs. 2.2 ± 0.6 mg/dL; P = 0.12). In the i.r. fenoldopam group, six patients (11.5%) developed CIN, compared with 16 patients in the control group (30.8%; risk ratio 0.38; 95% confidence interval 0.16-0.88; P = 0.012). 5. None of the patients in the fenoldopam group needed in-hospital dialysis, compared with three patients in the control group. 6. A larger prospective randomized study is needed to prove the effectiveness of this protective approach.

The serial effect of iodinated contrast media on renal hemodynamics and oxygenation as evaluated by ASL and BOLD MRI

Contrast-induced nephropathy is a prevalent cause of renal failure, and the mechanisms underlying this injury are not fully understood. We utilized noninvasive functional MRI in order to determine the serial effect of a single administration of iodinated contrast media (CM) on renal hemodynamics and oxygenation. Fifteen rabbits were randomized to receive an intravenous injection of CM (i.e. iopamidol-370; 6 ml kg(-1) body weight) or an equivalent amount of 0.9% saline. Both arterial spin-labeling and blood oxygen level-dependent imaging sequences were performed at 24 h before and at intervals of 1, 24, 48 and 72 h after injection to obtain serial renal blood flow (RBF) and relative spin-spin relaxation rate (R(2)*). Results showed that, in the iopamidol group, the mean cortical RBF decreased at 1 h (p = 0.04 vs baseline), reached its minimum at 24 h (p = 0.01) and gradually returned to baseline by 48 h (p = nonsignificant, NS). The outer medullary RBF decreased to its minimum by 24 h (p = 0.00) and remained less than baseline until 72 h. R(2)* in inner stripes was dramatically increased at 1 h (p = 0.00), remained elevated at 24 h (p = 0.05), but returned to baseline by 48 h (p = NS). R(2)* values within the cortex and outer stripes and inner medulla were slightly increased, but the changes did not reach a statistical significance (p = NS). Saline did not produce positive change in either RBF or R(2)* within different compartments of the kidney. We conclude that iopamidol is associated with a relatively longer-term hypoperfusion in whole kidney and decreased oxygen level in the inner stripes of the outer medulla.

Renal cortical and medullary blood flow during modest saline loading in humans

Renal medullary blood flow (RMBF) is considered an important element of sodium homeostasis, but the experimental evidence is incongruent. Studies in anaesthetized animals generally support the concept in contrast to measurements in conscious animals. We hypothesized that saline-induced natriuresis is associated with changes in RMBF in humans. After 4 days of low-sodium diet, healthy men were subjected to slow intravenous saline loading (12 μmol kg(-1) min(-1)) for 4 h. Renal medullary and cortical blood flow was determined by positron emission tomography with H(2)(15)O before and after saline infusion using two independent imaging processing methods. One based on a previously published algorithm (voxel peeling) and a novel method based on contrast-enhanced computed tomography (CT). Blood pressure was measured oscillometrically every 10 min. Cardiac output, heart rate and total peripheral resistance were recorded continuously. Saline loading increased the urinary sodium excretion by 3.6-fold (21-76 μmol min(-1) , P < 0.01). The RMBF was 2.6 ± 0.2 mL g(-1) tissue min(-1) before and 2.7 ± 0.1 mL g(-1) tissue min(-1) after saline (n.s.). Cortical blood flow was 3.6 ± 0.1 before and 3.4 ± 0.2 after saline (n.s.). Mean arterial blood pressure did not change measurably (90 vs. 90 mmHg). Bland-Altman analysis suggested agreement between results obtained with voxel peeling (2.6 ± 0.2 mL g(-1) tissue min(-1)) and contrast-enhanced CT (2.0 ± 0.1 mL g(-1) tissue min(-1)). In normal humans, changes in RMBF are not necessarily involved in the natriuretic response to modest saline loading. This result is in line with data from conscious rodents.

Experimental selective elevation of renal medullary blood flow in hypertensive rats: evidence against short‐term hypotensive effect

Renal medullary blood flow (MBF) can be selectively increased by intrarenal or systemic infusion of bradykinin (Bk) in anaesthetized normotensive rats. We reproduced this effect in a number of rat models of arterial hypertension and examined whether increased perfusion of the renal medulla can cause a short-term decrease in blood pressure (BP) that is not mediated by increased renal excretion and depletion of body fluids. In uninephrectomized Sprague-Dawley rats, BP was elevated to approx. 145 mmHg by acute i.v. infusion of noradrenaline (NA) or angiotensin II (Ang II) (groups 1, 2), 2-week exposure to high-salt diet (3), high-salt diet + chronic low-dose infusion of Ang II using osmotic minipumps (4) or chronic high-dose Ang II infusion on normal diet (5). Uninephrectomized spontaneous hypertensive rats (SHR) were also examined (6,7). To selectively increase medullary perfusion, in anaesthetized rats, bradykinin was infused during 30-75 min into the renal medullary interstitium or intravenously. Bradykinin increased outer- and inner-medullary blood flow (laser-Doppler fluxes) by 10-20% in groups (1, 2), by 30-50% in groups (3, 4, 5) and approx. 20% in SHR (6, 7). The concurrent increase in total renal blood flow (Transonic probe) was < 3%. A minor (<3%) decrease in BP was seen only in rats acutely rendered hypertensive by NA or Ang II infusions; however, the decreases in BP and increases in medullary perfusion were not correlated. Thus, there was no evidence that in hypertensive rats, substantial selective increases in medullary perfusion can cause a short-term decrease in BP.

Effect of Phosphodiesterase Inhibitor on Diabetic Nephropathy

Effect of Phosphodiesterase Inhibitor on Diabetic Nephropathy

Role of medullary blood flow in the pathogenesis of renal ischemia–reperfusion injury

Renal ischemia-reperfusion injury (IRI) is a common cause of acute kidney injury (AKI). Alterations in renal medullary blood flow (MBF) contribute to the pathogenesis of renal IRI. Here we review recent insights into the mechanisms of altered MBF in the pathogenesis of IRI. Although cortical blood flow fully recovers following 30-45 min of bilateral IRI, recent studies have indicated that there is a prolonged secondary fall in MBF that is associated with a long-term decline in renal function. Recent findings indicate that angiopoietin-1, atrial natriuretic peptide, heme oxygenase-1, and the gasotransmitters CO and H(2)S, may limit the severity of IRI by preserving MBF. Additional studies have also suggested a role for cytochrome P450-derived 20-HETE in the postischemic fall in MBF. Impaired MBF contributes to the pathogenesis of renal IRI. Measurement of renal MBF provides valuable insight into the underlying mechanisms of many renoprotective pathways. Identification of molecules that preserve renal MBF in IRI may lead to new therapies for AKI.

Differential Effects of Angiotensin II on Intra-Renal Hemodynamics in Rats; Contribution of Prostanoids, NO and K&amp;lt;sup&amp;gt;+&amp;lt;/sup&amp;gt; Channels

Many agents are known to cause qualitative and quantitative differences in intrarenal blood flow. This study tested the hypothesis that angiotensin II (AII) evokes a differential effect on cortical (CBF) and medullary blood flow (MBF) and that AT2 receptor mediates AII-induced increase in renal MBF by mechanisms related to nitric oxide (NO) and prostanoids. AII (100, 300 and 1000 ng/kg/min) increased mean arterial blood pressure (MABP) by 24% ± 7% (p G nitro-L-arginine (L-NNA), an inhibitor of NO synthase (100 mg/L in drinking water) enhanced AII effects on MABP (169 ± 75, p ATP channel blocker, did not affect intra-renal hemodynamics elicited by AII. Blockade of AT2 receptors with PD123319 (50 μg/kg/min) did not change basal or AII-induced changes MABP or CBF but blunted AII-induced increase in MBF by 60% ± 11 % (p 2 receptor agonist, elicited a reduction in MABP and increases in CBF and MBF that were abolished or attenuated by PD123319. These findings demonstrate that AII elicited differential changes in intrarenal blood flow; an AT1-mediated reduction in CBF but an AT2-mediated increase in MBF. The AT2 receptor-mediated increase in MBF involves guanylase cyclase, NO and dilator prostanoids but not KATP channels.

Arginase-2 mediates diabetic renal injury.

OBJECTIVETo determine 1) whether renal arginase activity or expression is increased in diabetes and 2) whether arginase plays a role in development of diabetic nephropathy (DN).RESEARCH DESIGN AND METHODSThe impact of arginase activity and expression on renal damage was evaluated in spontaneously diabetic Ins2Akita mice and in streptozotocin (STZ)-induced diabetic Dilute Brown Agouti (DBA) and arginase-2–deficient mice (Arg2−/−).RESULTSPharmacological blockade or genetic deficiency of arginase-2 conferred kidney protection in Ins2Akita mice or STZ-induced diabetic renal injury. Blocking arginases using S-(2-boronoethyl)-l-cysteine for 9 weeks in Ins2Akita mice or 6 weeks in STZ-induced diabetic DBA mice significantly attenuated albuminuria, the increase in blood urea nitrogen, histopathological changes, and kidney macrophage recruitment compared with vehicle-treated Ins2Akita mice. Furthermore, kidney arginase-2 expression increased in Ins2Akita mice compared with control. In contrast, arginase-1 expression was undetectable in kidneys under normal or diabetes conditions. Arg2−/− mice mimicked arginase blockade by reducing albuminuria after 6 and 18 weeks of STZ-induced diabetes. In wild-type mice, kidney arginase activity increased significantly after 6 and 18 weeks of STZ-induced diabetes but remained very low in STZ-diabetic Arg2−/− mice. The increase in kidney arginase activity was associated with a reduction in renal medullary blood flow in wild-type mice after 6 weeks of STZ-induced diabetes, an effect significantly attenuated in diabetic Arg2−/− mice.CONCLUSIONSThese findings indicate that arginase-2 plays a major role in induction of diabetic renal injury and that blocking arginase-2 activity or expression could be a novel therapeutic approach for treatment of DN.

Short-term effects of tolvaptan on renal function and volume in patients with autosomal dominant polycystic kidney disease

Tolvaptan and related V(2)-specific vasopressin receptor antagonists have been shown to delay disease progression in animal models of polycystic kidney disease. Slight elevations in serum creatinine, rapidly reversible after drug cessation, have been found in clinical trials involving tolvaptan. Here, we sought to clarify the potential renal mechanisms to see whether the antagonist effects were dependent on underlying renal function in 20 patients with autosomal dominant polycystic kidney disease (ADPKD) before and after 1 week of daily split-dose treatment. Tolvaptan induced aquaresis (excretion of solute-free water) and a significant reduction in glomerular filtration rate (GFR). The serum uric acid increased because of a decreased uric acid clearance, and the serum potassium fell, but there was no significant change in renal blood flow as measured by para-aminohippurate clearance or magnetic resonance imaging (MRI). No correlation was found between baseline GFR, measured by iothalmate clearance, and percent change in GFR induced by tolvaptan. Blinded post hoc analysis of renal MRIs showed that tolvaptan significantly reduced total kidney volume by 3.1% and individual cyst volume by 1.6%. Preliminary analysis of this small cohort suggested that these effects were more noticeable in patients with preserved renal function and with larger cysts. No correlation was found between changes of total kidney volume and body weight or estimated body water. Thus, functional and structural effects of tolvaptan on patients with ADPKD are likely due to inhibition of V(2)-driven adenosine cyclic 3',5'-monophosphate generation and its aquaretic, hemodynamic, and anti-secretory actions.

Endothelin Receptor A Antagonism Attenuates Renal Medullary Blood Flow Impairment in Endotoxemic Pigs

BackgroundEndothelin-1 is a potent endogenous vasoconstrictor that contributes to renal microcirculatory impairment during endotoxemia and sepsis. Here we investigated if the renal circulatory and metabolic effects of endothelin during endotoxemia are mediated through activation of endothelin-A receptors.Methods and FindingsA randomized experimental study was performed with anesthetized and mechanically ventilated pigs subjected to Escherichia coli endotoxin infusion for five hours. After two hours the animals were treated with the selective endothelin receptor type A antagonist TBC 3711 (2 mg⋅kg−1, n = 8) or served as endotoxin-treated controls (n = 8). Renal artery blood flow, diuresis and creatinine clearance decreased in response to endotoxemia. Perfusion in the cortex, as measured by laser doppler flowmetry, was reduced in both groups, but TBC 3711 attenuated the decrease in the medulla (p = 0.002). Compared to control, TBC 3711 reduced renal oxygen extraction as well as cortical and medullary lactate/pyruvate ratios (p<0.05) measured by microdialysis. Furthermore, TBC 3711 attenuated the decline in renal cortical interstitial glucose levels (p = 0.02) and increased medullary pyruvate levels (p = 0.03). Decreased creatinine clearance and oliguria were present in both groups without any significant difference.ConclusionsThese results suggest that endothelin released during endotoxemia acts via endothelin A receptors to impair renal medullary blood flow causing ischemia. Reduced renal oxygen extraction and cortical levels of lactate by TBC 3711, without effects on cortical blood flow, further suggest additional metabolic effects of endothelin type A receptor activation in this model of endotoxin induced acute kidney injury.

Extracellular nucleotides affect pericyte‐mediated regulation of rat in situ vasa recta diameter

We hypothesized that extracellular nucleotides, established as being released from renal tubular epithelial cells, act at pericytes to regulate vasa recta capillary diameter. A rat live kidney slice model and video imaging techniques were used to investigate the effects of extracellular nucleotides on in situ (subsurface) vasa recta diameter at pericyte and non-pericyte sites. In addition, RT-qPCR was used to quantify P2 receptor mRNA expression in isolated vasa recta. Extracellular ATP, UTP, benzylbenzyl ATP (BzATP) or 2-methylthioATP (2meSATP) evoked a significantly greater vasoconstriction of subsurface vasa recta at pericytes than at non-pericyte sites. The rank order of agonist potency was BzATP = 2meSATP > ATP = UTP. The vasoconstriction evoked at pericyte sites by ATP was significantly attenuated by the P2 receptor antagonists suramin, pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS) or Reactive Blue-2 (RB-2). UTP-evoked vasoconstriction at pericytes was attenuated by suramin or RB-2 but not PPADS. Interestingly, suramin or PPADS, when applied in the absence of a P2 receptor agonist, evoked a weak but significant vasoconstriction of vasa recta at pericyte sites, suggesting tonic vasodilation by nucleotides. Significant levels of P2X(1, 3 and 7) and P2Y(4 and 6) receptor mRNA were detected in vasa recta. Extracellular nucleotides act at pericytes to cause vasoconstriction of in situ vasa recta. Pharmacological characterization, supported by RT-qPCR data, suggests that P2X(1 and 7) and P2Y(4) receptors mediate nucleotide-evoked vasoconstriction of vasa recta by pericytes. We propose that nucleotides released from renal tubular epithelial cells, in close proximity to vasa recta capillaries, are key in regulating renal medullary blood flow.