AT1 receptors mediate angiotensin II–induced release of nitric oxide in afferent arterioles

Nitric Oxide Signalling in Descending Vasa Recta after Hypoxia/Re-Oxygenation

Reduced renal medullary oxygen supply is a key factor in the pathogenesis of acute kidney injury (AKI). As the medulla exclusively receives blood through descending vasa recta (DVR), dilating these microvessels after AKI may help in renoprotection by restoring renal medullary blood flow. We stimulated the NO-sGC-cGMP signalling pathway in DVR at three different levels before and after hypoxia/re-oxygenation (H/R). Rat DVR were isolated and perfused under isobaric conditions. The phosphodiesterase 5 (PDE5) inhibitor sildenafil (10⁻⁶ mol/L) impaired cGMP degradation and dilated DVR pre-constricted with angiotensin II (Ang II, 10⁻⁶ mol/L). Dilations by the soluble guanylyl cyclase (sGC) activator BAY 60-2770 as well as the nitric oxide donor sodium nitroprusside (SNP, 10⁻³ mol/L) were equally effective. Hypoxia (0.1% O₂) augmented DVR constriction by Ang II, thus potentially aggravating tissue hypoxia. H/R left DVR unresponsive to sildenafil, yet sGC activation by BAY 60-2770 effectively dilated DVR. Dilation to SNP under H/R is delayed. In conclusion, H/R renders PDE5 inhibition ineffective in dilating the crucial vessels supplying the area at risk for hypoxic damage. Stimulating sGC appears to be the most effective in restoring renal medullary blood flow after H/R and may prove to be the best target for maintaining oxygenation to this vulnerable area of the kidney.

A critical role of the podocyte cytoskeleton in the pathogenesis of glomerular proteinuria and autoimmune podocytopathies

Selective glomerular filtration relies on the membrane separating the glomerular arterioles from the Bowman space. As a major component of the glomerular filtration barrier, podocytes form foot processes by the actin cytoskeleton, which dynamically adjusts in response to environmental changes to maintain filtration barrier integrity. The slit diaphragms bridge the filtration slits between neighboring foot processes and act as signaling hubs interacting with the actin cytoskeleton. Focal adhesions relay signals to regulate actin dynamics while allowing podocyte adherence to the basement membrane. Mutations in actin regulatory and signaling proteins may disrupt the actin cytoskeleton, resulting in foot process retraction, effacement, and proteinuria. Large-scale gene expression profiling platforms, transgenic animal models, and other in vivo gene delivery methods now enhance our understanding of the interactions among podocyte focal adhesions, slit diaphragms, and actin dynamics. In addition, our team found that at least 66% of idiopathic nephrotic syndrome (INS) children have podocyte autoantibodies, which was defined as a new disease subgroup-, autoimmune podocytopathies. This review outlines the pathophysiological mechanisms of podocyte cytoskeleton protein interactions in proteinuria and glomerular podocytopathy.

Novel soluble guanylyl cyclase activators increase glomerular cGMP, induce vasodilation and improve blood flow in the murine kidney

BACKGROUND AND PURPOSE

Generation of cGMP via NO-sensitive soluble guanylyl cyclase (sGC) has been implicated in the regulation of renal functions. Chronic kidney disease (CKD) is associated with decreased NO bioavailability, increased oxidative stress and oxidation of sGC to its haem-free form, apo-sGC. Apo-sGC cannot be activated by NO, resulting in impaired cGMP signalling that is associated with chronic kidney disease progression. We hypothesised that sGC activators, which activate apo-sGC independently of NO, increase renal cGMP production under conditions of oxidative stress, thereby improving renal blood flow (RBF) and kidney function.

EXPERIMENTAL APPROACH

Two novel sGC activators, runcaciguat and BAY-543, were tested on murine kidney. We measured cGMP levels in real time in kidney slices of cGMP sensor mice, vasodilation of pre-constricted glomerular arterioles and RBF in isolated perfused kidneys. Experiments were performed at baseline conditions, under L-NAME-induced NO deficiency, and in the presence of oxidative stress induced by ODQ.

KEY RESULTS

Mouse glomeruli showed NO-induced cGMP increases. Under baseline conditions, sGC activator did not alter glomerular cGMP concentration or NO-induced cGMP generation. In the presence of ODQ, NO-induced glomerular cGMP signals were markedly reduced, whereas sGC activator induced strong cGMP increases. L-NAME and ODQ pretreated isolated glomerular arterioles were strongly dilated by sGC activator. sGC activator also increased cGMP and RBF in ODQ-perfused kidneys.

CONCLUSION AND IMPLICATION

sGC activators increase glomerular cGMP, dilate glomerular arterioles and improve RBF under disease-relevant oxidative stress conditions. Therefore, sGC activators represent a promising class of drugs for chronic kidney disease treatment.

High phosphate impairs arterial endothelial function through AMPK-related pathways in mouse resistance arteries

AIMS

In patients with renal disease, high serum phosphate shows a relationship with cardiovascular risk. We speculate that high phosphate (HP) impairs arterial vasodilation via the endothelium and explore potential underlying mechanisms.

METHODS

Isolated vessel relaxation, endothelial function, glomerular filtration rate (GFR), oxidative stress status and protein expression were assessed in HP diet mice. Mitochondrial function and protein expression were assessed in HP-treated human umbilical vein endothelial cells (HUVECs).

RESULTS

High phosphate (1.3%) diet for 12 weeks impaired endothelium-dependent relaxation in mesenteric arteries, kidney interlobar arteries and afferent arterioles; reduced GFR and the blood pressure responses to acute administration of acetylcholine. The PPARα/LKB1/AMPK/eNOS pathway was attenuated in the endothelium of mesenteric arteries from HP diet mice. The observed vasodilatory impairment of mesenteric arteries was ameliorated by PPARα agonist WY-14643. The phosphate transporter PiT-1 knockdown prevented HP-mediated suppression of eNOS activity by impeding phosphorus influx in HUVECs. Endothelium cytoplasmic and mitochondrial reactive oxygen species (ROS) were increased in HP diet mice. Moreover HP decreased the expression of mitochondrial-related antioxidant genes. Finally, mitochondrial membrane potential and PGC-1α expression were reduced by HP treatment in HUVECs, which was partly restored by AMPKα agonist.

CONCLUSIONS

HP impairs endothelial function by reducing NO bioavailability via decreasing eNOS activity and increasing mitochondrial ROS, in which the AMPK-related signalling pathways may play a key role.

Role of soluble guanylyl cyclase in renal afferent and efferent arterioles

Renal arteriolar tone depends considerably on the dilatory action of nitric oxide (NO) via activation of soluble guanylyl cyclase (sGC) and cGMP action. NO deficiency and hypoxia/reoxygenation are important pathophysiological factors in the development of acute kidney injury. It was hypothesized that the NO-sGC-cGMP system functions differently in renal afferent arterioles (AA) compared with efferent arterioles (EA) and that the sGC activator cinaciguat differentially dilates these arterioles. Experiments were performed in isolated, perfused mouse glomerular arterioles. Hypoxia (0.1% oxygen) was achieved by using a hypoxia chamber. Phosphodiesterase 5 (PDE5) and sGC subunits were considerably expressed on the mRNA level in AA. PDE5 inhibition with sildenafil, which blocks cGMP degradation, diminished the responses to ANG II bolus application in AA, but not significantly in EA. Vasodilation induced by sildenafil in ANG II-preconstricted vessels was stronger in EA than AA. Cinaciguat, an NO- and heme-independent sGC activator, dilated EA more strongly than AA after N^G-nitro-l-arginine methyl ester (l-NAME; NO synthase inhibitor) treatment and preconstriction with ANG II. Cinaciguat-induced dilatation of l-NAME-pretreated and ANG II-preconstricted arterioles was similar to controls without l-NAME treatment. Cinaciguat also induced dilatation in iodinated contrast medium treated AA. Furthermore, it dilated EA, but not AA, after hypoxia/reoxygenation. The results reveal an important role of the NO-sGC-cGMP system for renal dilatation and that EA have a more potent sGC activated dilatory system. Furthermore, AA seem to be more sensitive to hypoxia/reoxygenation than EA under these experimental conditions.

Neurohumoral interactions contributing to renal vasoconstriction and decreased renal blood flow in heart failure

In heart failure (HF), increases in renal sympathetic nerve activity (RSNA), renal norepinephrine spillover, and renin release cause renal vasoconstriction, which may contribute to the cardiorenal syndrome. To increase our understanding of the mechanisms causing renal vasoconstriction in HF, we investigated the interactions between the increased activity of the renal nerves and the renal release of norepinephrine and renin in an ovine pacing-induced model of HF compared with healthy sheep. In addition, we determined the level of renal angiotensin type-1 receptors and the renal vascular responsiveness to stimulation of the renal nerves and α1-adrenoceptors. In conscious sheep with mild HF (ejection fraction 35%–40%), renal blood flow (276 ± 13 to 185 ± 18 mL/min) and renal vascular conductance (3.8 ± 0.2 to 3.1 ± 0.2 mL·min−1·mmHg−1) were decreased compared with healthy sheep. There were increases in the burst frequency of RSNA (27%), renal norepinephrine spillover (377%), and plasma renin activity (141%), whereas the density of renal medullary angiotensin type-1 receptors decreased. In anesthetized sheep with HF, the renal vasoconstrictor responses to electrical stimulation of the renal nerves or to phenylephrine were attenuated. Irbesartan improved the responses to nerve stimulation, but not to phenylephrine, in HF and reduced the responses in normal sheep. In summary, in HF, the increases in renal norepinephrine spillover and plasma renin activity are augmented compared with the increase in RSNA. The vasoconstrictor effect of the increased renal norepinephrine and angiotensin II is offset by reduced levels of renal angiotensin type-1 receptors and reduced renal vasoconstrictor responsiveness to α1-adrenoceptor stimulation.

ADAMTS13 protects mice against renal ischemia-reperfusion injury by reducing inflammation and improving endothelial function

Acute kidney injury (AKI) is a serious condition without efficient therapeutic options. Recent studies have indicated that recombinant human a disintegrin and metalloprotease with thrombospondin motifs 13 (rhADAMTS13) provides protection against inflammation. Therefore, we hypothesized that ADAMTS13 might protect against AKI by reducing inflammation. Bilateral renal ischemia-reperfusion injury (I/R) was used as AKI models in this study. Prophylactic infusion of rhADAMTS13 was employed to investigate potential mechanisms of renal protection. Renal function, inflammation, and microvascular endothelial function were assessed after 24 h of reperfusion. Our results showed that I/R mice increased plasma von Willebrand factor levels but decreased ADAMTS13 expression. Administration of rhADAMTS13 to I/R mice recovered renal function, histological injury, and apoptosis. Renal inflammation was reduced by rhADAMTS13, accompanied with the downregulation of p38/extracellular signal-regulated protein kinase phosphorylation and cyclooxygenase-2 expression. rhADAMTS13 restored vasodilation in afferent arterioles in I/R mice. Furthermore, rhADAMTS13 treatment enhanced phosphorylation of Akt at Ser473 and eNOS at Ser1177. Administration of the Akt pathway inhibitor wortmannin reduced the protective effect of rhADAMTS13. Our conclusions are that treatment with rhADAMTS13 ameliorates renal I/R injury by reducing inflammation, tubular cell apoptosis, and improving microvascular endothelial dysfunction. rhADAMTS13 could be a promising strategy to treat AKI in clinical settings.

Enhanced Renal Afferent Arteriolar Reactive Oxygen Species and Contractility to Endothelin-1 Are Associated with Canonical Wnt Signaling in Diabetic Mice

BACKGROUND/AIMS

Canonical Wnt signaling is involved in oxidative stress, vasculopathy and diabetes mellitus but its role in diabetic renal microvascular dysfunction is unclear. We tested the hypothesis that enhanced canonical Wnt signaling in renal afferent arterioles from diabetic mice increases reactive oxygen species (ROS) and contractions to endothelin-1 (ET-1).

METHODS

Streptozotocin-induced diabetes or control C57Bl/6 mice received vehicle or sulindac (40 mg·kg-1·day-1) to block Wnt signaling for 4 weeks. ET-1 contractions were measured by changes of afferent arteriolar diameter. Arteriolar H2O2, O2 -, protein expression and enzymatic activity were assessed using sensitive fluorescence probes, immunoblotting and colorimetric assay separately.

RESULTS

Compared to control, diabetic mouse afferent arteriole had increased O2- (+ 84%) and H2O2 (+ 91%) and enhanced responses to ET-1 at 10-8 mol·l-1 (-72±4% of versus -43±4%, P< 0.05) accompanied by reduced protein expressions and activities for catalase and superoxide dismutase 2 (SOD2). Arteriolar O2 - was increased further by ET-1 and contractions to ET-1 reduced by PEG-SOD in both groups whereas H2O2 unchanged by ET-1 and contractions were reduced by PEG-catalase selectively in diabetic mice. The Wnt signaling protein β-catenin was upregulated (3.3-fold decrease in p-β-catenin/β-catenin) while the glycogen synthase kinase-3β (GSK-3β) was downregulated (2.6-fold increase in p-GSK-3β/ GSK-3β) in preglomerular vessels of diabetic mice. Sulindac normalized the Wnt signaling proteins, arteriolar O2 -, H2O2 and ET-1 contractions while doubling microvascular catalase and SOD2 expression in diabetic mice.

CONCLUSION

Increased ROS, notably H2O2 contributes to enhanced afferent arteriolar responses to ET-1 in diabetes, which is closely associated with Wnt signaling. Antioxidant pharmacological strategies targeting Wnt signaling may improve vascular function in diabetic nephropathy.

Effects of GIP on regional blood flow during normoglycemia and hyperglycemia in anesthetized rats

The incretin hormone glucose-dependent insulinotropic polypeptide (GIP) potentiates glucose-stimulated insulin secretion, and affects β-cell turnover. This study aimed at evaluating if some of the beneficial effects of GIP on glucose homeostasis can be explained by modulation of islet blood flow. Anesthetized Sprague-Dawley rats were infused intravenously with different doses of GIP (10, 20, or 60 ng/kg*min) for 30 min. Subsequent organ blood flow measurements were performed with microspheres. In separate animals, islets were perfused ex vivo with GIP (10-6 -10-12  mol/L) during normo- and hyperglycemia and arteriolar responsiveness was recorded. The highest dose of GIP potentiated insulin secretion during hyperglycemia, but had no effect in normoglycemic rats. The highest GIP concentration decreased blood perfusion of whole pancreas, pancreatic islets, duodenum, colon, liver and kidneys. The decrease in blood flow was unaffected by ganglion blockade or adenosine receptor inhibition. In contrast to this, in single perfused islets GIP induced a dose-dependent arteriolar dilation. Thus, high doses of GIP exert a direct dilatory effect on islet arterioles in isolated islets, but induce a generalized vasoconstriction in splanchnic organs, including the whole pancreas and islets, in vivo. The latter effect is unlikely to be mediated by adenosine, the autonomic nervous system, or endothelial mediators.

Glucose dilates renal afferent arterioles via glucose transporter-1

Glomerular hyperfiltration occurs during the early stage of diabetes. An acute glucose infusion increases glomerular filtration rate. The involvement of tubuloglomerular feedback response and direct effect of glucose on the afferent arterioles (Af-Arts) have been suggested. However, the signaling pathways to trigger Af-Art dilatation have not been fully identified. Therefore, in the present study we tested our hypothesis that an increase in glucose concentration enhances endothelial nitric oxide synthesis activity and dilates the Af-Arts via glucose transporter-1 (GLUT1) using isolated mouse Af-Arts with perfusion. We isolated and microperfused the Af-Arts from nondiabetic C57BL/6 mice. The Af-Arts were preconstricted with norepinephrine (1 µM). When we switched the d-glucose concentration from low (5 mM) to high (30 mM) in the perfusate, the preconstricted Af-Arts significantly dilated by 37.8 ± 7.1%, but L-glucose did not trigger the dilation. GLUT1 mRNA was identified in microdisserted Af-Arts measured by RT-PCR. Changes in nitric oxide (NO) production in Af-Art were also measured using fluorescent probe when ambient glucose concentration was increased. When the d-glucose concentration was switched from 5 to 30 mM, NO generation in Af-Art was significantly increased by 19.2 ± 6.2% (84.7 ± 4.1 to 101.0 ± 9.3 U/min). l-Glucose had no effect on the NO generation. The GLUT1-selective antagonist 4-[({[4-(1,1-Dimethylethyl)phenyl]sulfonyl}amino)methyl]- N-3-pyridinylbenzamide and the nitric oxide synthase inhibitor N^G-nitro-l-arginine methyl ester blocked the high glucose-induced NO generation and vasodilation. In conclusion, we demonstrated that an increase in glucose concentration dilates the Af-Art by stimulation of the endothelium-derived NO production mediated by GLUT1.

Extravasal albumin concentration modulates contractile responses of renal afferent arterioles

AIM

Afferent arterioles (AA) hold a key position in the regulation of renal blood flow and glomerular filtration rate. Being the effector site of tubuloglomerular feedback, the afferent arteriole contributes to the renal handling of sodium and fluid. Dehydration goes along with increased renal interstitial protein concentration. Here, the hypothesis was tested that extravasal protein concentration directly modulates afferent arteriolar tone, a mechanism which may contribute to body fluid volume control.

METHOD

The effect of increased extravasal albumin concentration on the vascular reactivity was investigated in renal AA and interlobar arteries of mice, in rat renal AA and in pancreatic islet arterioles.

RESULTS

Albumin (2 and 4% in the bath solution) significantly potentiated the contractile response of renal afferent arterioles induced by angiotensin II and adenosine, as well as their combination, compared to the control situation (0.1% albumin). Albumin did not influence the contractility of larger renal vessels or pancreatic islet arterioles. Mimicking the increase in the osmolality induced by 4% albumin by applying mannitol to the bath solution also increased the response of renal arterioles to Ang II. However, the effect was smaller compared to that of albumin. The nitric oxide bioavailability, measured by DAF-FM fluorescence, was reduced in afferent arterioles exposed to 4% albumin.

CONCLUSION

The protein-induced modulation of AA tone is mediated by the increased osmolality as well as by NO scavenging. The results suggest a possible contribution of these mechanisms to the control of extracellular fluid volume via adjustment of renal blood flow and glomerular filtration rate.

Nitric oxide production by glomerular podocytes

Nitric Oxide (NO), a potent vasodilator and vital signaling molecule, has been shown to contribute to the regulation of glomerular ultrafiltration. However, whether changes in NO occur in podocytes during the pathogenesis of salt-sensitive hypertension has not yet been thoroughly examined. We showed here that podocytes produce NO, and further hypothesized that hypertensive animals would exhibit reduced NO production in these cells in response to various paracrine factors, which might contribute to the damage of glomeruli filtration barrier and development of proteinuria. To test this, we isolated glomeruli from the kidneys of Dahl salt-sensitive (SS) rats fed a low salt (LS; 0.4% NaCl) or high salt (HS; 4% NaCl, 3 weeks) diets and loaded podocytes with either a combination of NO and Ca²⁺ fluorophores (DAF-FM and Fura Red, respectively) or DAF-FM alone. Changes in fluorescence were observed with confocal microscopy in response to adenosine triphosphate (ATP), angiotensin II (Ang II), and hydrogen peroxide (H₂O₂). Application of Ang II resulted in activation of both NO and intracellular calcium ([Ca²⁺]_i) transients. In contrast, ATP promoted [Ca²⁺]_i transients, but did not have any effects on NO production. SS rats fed a HS diet for 3 weeks demonstrated impaired NO production: the response to Ang II or H₂O₂ in podocytes of glomeruli isolated from SS rats fed a HS diet was significantly reduced compared to rats fed a LS diet. Therefore, glomerular podocytes from hypertensive rats showed a diminished NO release in response to Ang II or oxidative stress, suggesting that podocytic NO signaling is dysfunctional in this condition and likely contributes to the development of kidney injury.

Hypoxia-reoxygenation enhances murine afferent arteriolar vasoconstriction by angiotensin II

We tested the hypothesis that hypoxia-reoxygenation (H/R) augments vasoreactivity to angiotensin II (ANG II). In particular, we compared an in situ live kidney slice model with isolated afferent arterioles (C57Bl6 mice) to assess the impact of tubules on microvessel response. Hematoxylin and eosin staining was used to estimate slice viability. Arterioles in the slices were located by differential interference contrast microscopy, and responses to vasoactive substances were assessed. Cytosolic calcium transients and NADPH oxidase (NOX) mRNA expression were studied in isolated afferent arterioles. SOD activity was measured in live slices. Both experimental models were subjected to control and H/R treatment (60 min). Slices were further analyzed after 30-, 60-, and 90-min hypoxia followed by 10- or 20-min reoxygenation (H/R). H/R resulted in enhanced necrotic tissue damage compared with control conditions. To characterize the slice model, we applied ANG II (10^-7 M), norepinephrine (NE; 10^-5 M), endothelin-1 (ET-1; 10^-7 M), and ATP (10^-4 M), reducing the initial diameter to 44.5 ± 2.8, 50.0 ± 2.2, 45.3 ± 2.6, and 74.1 ± 1.8%, respectively. H/R significantly increased the ANG II response compared with control in live slices and in isolated afferent arterioles, although calcium transients remained similar. TEMPOL incubation prevented the H/R effect on ANG II responses. H/R significantly increased NOX2 mRNA expression in isolated arterioles. SOD activity was significantly decreased after H/R. Enhanced arteriolar responses after H/R occurred independently from the surrounding tissue, indicating no influence of tubules on vascular function in this model. The mechanism of increased ANG II response after H/R might be increased oxidative stress and increased calcium sensitivity of the contractile apparatus.

Glomerular and tubular effects of nitric oxide (NO) are regulated by angiotensin II (Ang II) in an age-dependent manner through activation of both angiotensin receptors (AT1Rs and AT2Rs) in conscious lambs

Renin-angiotensin (RAS) and nitric oxide (NO) systems and the balance and interaction between them are considered of primary importance in maintaining fluid and electrolyte homeostasis. It has been suggested that the effects of NO may be modulated at least in part by the angiotensin (Ang) II, yet the roles of angiotensin receptor type 1 (AT1R) and type 2 (AT2R) are not well understood. Even though both Ang II and NO are elevated at birth and during the newborn period, their contribution to the adaptation of the newborn to life after birth as well as their physiological roles during development are poorly understood. The aim of this study was to determine if NO regulation of renal function during postnatal maturation is modulated by Ang II through activation of AT1R or AT2R or both receptors. Glomerular and tubular effects of either AT1R selective antagonist ZD 7155, AT2R selective antagonist PD 123319, and both antagonists ZD 7155 plus PD 123319, were measured in 1- (N = 9) and 6-week-old (N = 13) conscious, chronically instrumented lambs before and after removal of endogenous NO with L-arginine analogue, L-NAME. Two-way analysis of variance (ANOVA) procedures for repeated measures over time with factors age and treatment were used to compare the effects of the treatments on several glomerular and tubular variables in both groups. This study showed that L-NAME infusion after pre-treatment with ATR antagonists did not alter glomerular function in 1- or 6-week-old lambs. NO effects on electrolytes handling along the nephron during postnatal development were modulated by Ang II through AT1R and AT2R in an age-dependent manner. Selective inhibition of AT1R and AT2R increased excretion of Na⁺, K⁺, and Cl^- in 6- but not in 1-week-old lambs. In 6-week-old lambs, urinary flow rate increased by 200%, free water clearance increased by 50%, and urine osmolality decreased by 40% after L-NAME was added to the pre-treatment with ZD 7155 plus PD 123319. When L-NAME was added either to ZD 7155 or PD 123319, the same trend in the alterations of these variables was observed, albeit to a lower degree. In conclusion, in conscious animals, during postnatal maturation, Ang II modulates the effects of NO on glomerular function, fluid, and electrolyte homeostasis through AT1Rs and AT2Rs in an age-dependent manner. Under physiological conditions, AT2Rs may potentiate the effects of AT1R, providing evidence of a crosstalk between ATRs in modulating NO effects on fluid and electrolyte homeostasis during postnatal maturation. This study provides new insights on the regulation of renal function during early postnatal development showing that, compared with later in life, newborns have impaired capacity to regulate glomerular function, water, and electrolyte balance.

Myoglobin facilitates angiotensin II-induced constriction of renal afferent arterioles

Vasoconstriction plays an important role in the development of acute kidney injury in rhabdomyolysis. We hypothesized that myoglobin enhances the angiotensin II (ANG II) response in afferent arterioles by increasing superoxide and reducing nitric oxide (NO) bioavailability. Afferent arterioles of C57Bl6 mice were isolated perfused, and vasoreactivity was analyzed using video microscopy. NO bioavailability, superoxide concentration in the vessel wall, and changes in cytosolic calcium were measured using fluorescence techniques. Myoglobin treatment (10−5 M) did not change the basal arteriolar diameter during a 20-min period compared with control conditions. NG-nitro-l-arginine methyl ester (l-NAME, 10−4 M) and l-NAME + myoglobin reduced diameters to 94.7 and 97.9% of the initial diameter, respectively. Myoglobin or l-NAME enhanced the ANG II-induced constriction of arterioles compared with control (36.6 and 34.2%, respectively, vs. 65.9%). Norepinephrine responses were not influenced by myoglobin. Combined application of myoglobin and l-NAME further facilitated the ANG II response (7.0%). Myoglobin or l-NAME decreased the NO-related fluorescence in arterioles similarly. Myoglobin enhanced the superoxide-related fluorescence, and tempol prevented this enhancement. Tempol also partly prevented the myoglobin effect on the ANG II response. Myoglobin increased the fura 2 fluorescence ratio (cytosolic calcium) during ANG II application (10−12 to 10−6 M). The results suggest that the enhanced afferent arteriolar reactivity to ANG II is mainly due to a myoglobin-induced increase in superoxide and associated reduction in the NO bioavailability. Signaling pathways for the augmented ANG II response include enhanced cytosolic calcium transients. In conclusion, myoglobin may contribute to the afferent arteriolar vasoconstriction in this rhabdomyolysis model.

Detection of Nitric Oxide Induced by Angiotensin II Receptor Type 1 Using Soluble Guanylate Cyclase beta1 Subunit Fused to a Yellow Fluorescent Protein, Venus

Nitric oxide (NO) is an important gaseous molecule involved in many physiological and pathophysiological processes, including the regulation of G protein-coupled receptors (GPCRs). Here, we report the development of a high-affinity method to detect NO using soluble guanylate cyclase beta1 subunit fused to Venus, a variant of yellow fluorescent protein (sGC-Venus). We measured the fluorescence intensity of sGC-Venus with and without an NO donor using purified probes. At 560 nm emission, the fluorescence intensity of sGC-Venus at 405 nm excitation was increased by approximately 2.5-fold by the NO donor, but the fluorescence intensities of sGC-Venus excited by other wavelengths showed much less of an increase or no significant increase. To measure NO in living cells, the fluorescence intensity of sGC-Venus at 405 nm excitation was normalized to that at 488 nm excitation because it showed no significant difference with or without the NO donor. In HEK293 cells overexpressing the angiotensin II receptor type 1 (AT1 receptor), the production of NO induced by activation of the AT1 receptor was detected using sGC-Venus. These data indicate that sGC-Venus will be a useful tool for visualizing intracellular NO in living cells and that NO might be a common tool to regulate GPCRs.

Increased hydrogen peroxide impairs angiotensin II contractions of afferent arterioles in mice after renal ischaemia-reperfusion injury

AIM

Renal ischaemia-reperfusion injury (IRI) increases angiotensin II (Ang II) and reactive oxygen species (ROS) that are potent modulators of vascular function. However, the roles of individual ROS and their interaction with Ang II are not clear. Here we tested the hypothesis that IRI modulates renal afferent arteriolar responses to Ang II via increasing superoxide (O2-) or hydrogen peroxide (H2 O2 ).

METHODS

Renal afferent arterioles were isolated and perfused from C57BL/6 mice 24 h after IRI or sham surgery. Responses to Ang II or noradrenaline were assessed by measuring arteriolar diameter. Production of H2 O2 and O2- was assessed in afferent arterioles and renal cortex. Activity of SOD and catalase, and mRNA expressions of Ang II receptors were assessed in pre-glomerular arterioles and renal cortex.

RESULTS

Afferent arterioles from mice after IRI had a reduced maximal contraction to Ang II (-27±2 vs. -42±1%, P < 0.001), but retained a normal contraction to noradrenaline. Arterioles after IRI had a 38% increase in H2 O2 (P < 0.001) and a 45% decrease in catalase activity (P < 0.01). Contractions were reduced in normal arterioles after incubation with H2 O2 (-22±2 vs. -42±1%, P < 0.05) similar to the effects of IRI. However, the impaired contractions were normalized by incubation with PEG catalase despite a reduced AT1 R expression.

CONCLUSIONS

Renal IRI in mice selectively impairs afferent arteriolar responses to Ang II because of H2 O2 accumulation that is caused by a reduced catalase activity. This could serve to buffer the effect of Ang II after IRI and may be a protective mechanism.

Noradrenaline enhances angiotensin II responses via p38 MAPK activation after hypoxia/re-oxygenation in renal interlobar arteries

AIM

Hypoxia and sympathetic activation are main factors in the pathogenesis of acute kidney injury (AKI). We tested the hypothesis that noradrenaline (NE) in combination with hypoxia aggravates the vasoreactivity of renal arteries after hypoxia/re-oxygenation (H/R). We tested the role of adrenergic receptors and p38 MAPK using an in vitro H/R protocol.

METHODS

Mouse interlobar arteries (ILA) and afferent arterioles (AA) were investigated under isometric and isotonic conditions respectively. The in vitro protocol consisted of 60-min hypoxia and control condition, respectively, 10-min re-oxygenation followed by concentration-response curves for Ang II or endothelin.

RESULTS

Hypoxia reduced the response to Ang II. Hypoxia and NE (10(-9)  mol L(-1) ) together increased it in ILA and AA. In ILA, NE alone influenced neither Ang II responses under control conditions nor endothelin responses after hypoxia. Prazosin or yohimbine treatment did not significantly influence the NE+hypoxia effect. The combination of prazosin and yohimbine or propranolol alone inhibited the effect of NE+hypoxia. BRL37344 (β3 receptor agonist) mimicked the NE effect. In contrast, the incubation with β3 receptor blocker did not influence the mentioned effect. Phosphorylation of p38 MAPK and MLC(20) was increased after H/R with NE and Ang II treatment. The selective p38 MAPK inhibitor SB202190 blocked the NE+hypoxia effect on the Ang II response.

CONCLUSION

The results suggest an interaction of NE and hypoxia in enhancing vasoreactivity, which may be important for the pathogenesis of AKI. The effect of NE+hypoxia in ILA is mediated by several adrenergic receptors and requires the p38 MAPK activation.

NADPH Oxidase in the Renal Microvasculature Is a Primary Target for Blood Pressure–Lowering Effects by Inorganic Nitrate and Nitrite

Renal oxidative stress and nitric oxide (NO) deficiency are key events in hypertension. Stimulation of a nitrate–nitrite–NO pathway with dietary nitrate reduces blood pressure, but the mechanisms or target organ are not clear. We investigated the hypothesis that inorganic nitrate and nitrite attenuate reactivity of renal microcirculation and blood pressure responses to angiotensin II (ANG II) by modulating nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and NO bioavailability. Nitrite in the physiological range (10 −7 –10 −5 mol/L) dilated isolated perfused renal afferent arterioles, which were associated with increased NO. Contractions to ANG II (34%) and simultaneous NO synthase inhibition (56%) were attenuated by nitrite (18% and 26%). In a model of oxidative stress (superoxide dismutase-1 knockouts), abnormal ANG II–mediated arteriolar contractions (90%) were normalized by nitrite (44%). Mechanistically, effects of nitrite were abolished by NO scavenger and xanthine oxidase inhibitor, but only partially attenuated by inhibiting soluble guanylyl cyclase. Inhibition of NADPH oxidase with apocynin attenuated ANG II–induced contractility (35%) similar to that of nitrite. In the presence of nitrite, no further effect of apocynin was observed, suggesting NADPH oxidase as a possible target. In preglomerular vascular smooth muscle cells and kidney cortex, nitrite reduced both basal and ANG II–induced NADPH oxidase activity. These effects of nitrite were also abolished by xanthine oxidase inhibition. Moreover, supplementation with dietary nitrate (10 −2 mol/L) reduced renal NADPH oxidase activity and attenuated ANG II–mediated arteriolar contractions and hypertension (99±2–146±2 mm Hg) compared with placebo (100±3–168±3 mm Hg). In conclusion, these novel findings position NADPH oxidase in the renal microvasculature as a prime target for blood pressure–lowering effects of inorganic nitrate and nitrite.

Blood lipids affect rat islet blood flow regulation through β₃-adrenoceptors

Pancreatic islet blood perfusion varies according to the needs for insulin secretion. We examined the effects of blood lipids on pancreatic islet blood flow in anesthetized rats. Acute administration of Intralipid to anesthetized rats increased both triglycerides and free fatty acids, associated with a simultaneous increase in total pancreatic and islet blood flow. A preceding abdominal vagotomy markedly potentiated this and led acutely to a 10-fold increase in islet blood flow associated with a similar increase in serum insulin concentrations. The islet blood flow and serum insulin response could be largely prevented by pretreatment with propranolol and the selective β₃-adrenergic inhibitor SR-59230A. The nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester prevented the blood flow increase but was less effective in reducing serum insulin. Increased islet blood flow after Intralipid administration was also seen in islet and whole pancreas transplanted rats, i.e., models with different degrees of chronic islet denervation, but the effect was not as pronounced. In isolated vascularly perfused single islets Intralipid dilated islet arterioles, but this was not affected by SR-59230A. Both the sympathetic and parasympathetic nervous system are important for the coordination of islet blood flow and insulin release during hyperlipidemia, with a previously unknown role for β₃-adrenoceptors.

Effects of Mn-DPDP and manganese chloride on hemodynamics and glucose tolerance in anesthetized rats

BACKGROUND

Previous studies have demonstrated that magnetic resonance imaging may be a method of choice to visualize transplanted pancreatic islets. However, contrast agents may interfere with microcirculation and affect graft function.

PURPOSE

To evaluate the effects manganese-containing contrast media on regional blood flow and glucose tolerance.

MATERIAL AND METHODS

Anesthetized rats were injected intravenously with MnCl2 (10 µM/kg body weight) or Mn-DPDP (Teslascan™; 5 µM/kg body weight). Blood flow measurements were made with a microsphere technique 10 min later. In separate animals vascular arteriolar reactivity in isolated, perfused islets was examined. Furthermore, an intraperitoneal glucose tolerance test was performed in separate rats.

RESULTS

Glucose tolerance was unaffected by both agents. No changes in regional blood flow were seen after administration of Mn-DPDP, except for an increase in arterial liver blood flow. MnCl2 increased all blood flow values except that of the kidney. MnCl2, but not Mn-DPDP, caused a vasoconstriction in isolated rat islet arterioles but only at very high doses.

CONCLUSION

Mn-DPDP administration does not affect glucose tolerance or regional blood flow, besides an increase in arterial hepatic blood flow, and may therefore be suitable for visualization of islets.

Renal vasoconstriction by vasopressin V1a receptors is modulated by nitric oxide, prostanoids, and superoxide but not the ADP ribosyl cyclase CD38

Renal blood flow (RBF) responses to arginine vasopressin (AVP) were tested in anesthetized wild-type (WT) and CD38(-/-) mice that lack the major calcium-mobilizing second messenger cyclic ADP ribose. AVP (3-25 ng) injected intravenously produced dose-dependent decreases in RBF, reaching a maximum of 25 ± 2% below basal RBF in WT and 27 ± 2% in CD38(-/-) mice with 25 ng of AVP. Renal vascular resistance (RVR) increased 75 ± 6% and 78 ± 6% in WT and CD38(-/-) mice. Inhibition of nitric oxide (NO) synthase with nitro-L-arginine methyl ester (L-NAME) increased the maximum RVR response to AVP to 308 ± 76% in WT and 388 ± 81% in CD38(-/-) (P < 0.001 for both). Cyclooxygenase inhibition with indomethacin increased the maximum RVR response to 125 ± 15% in WT and 120 ± 14% in CD38(-/-) mice (P < 0.001, <0.05). Superoxide suppression with tempol inhibited the maximum RVR response to AVP by 38% in both strains (P < 0.005) but was ineffective when administered after L-NAME. The rate of RBF recovery (relaxation) after AVP was slowed by L-NAME and indomethacin (P < 0.001, <0.005) but was unchanged by tempol. All vascular responses to AVP were abolished by an AVP V1a receptor antagonist. A V2 receptor agonist or antagonist had no effect on AVP-induced renal vasoconstriction. Taken together, the results indicate that renal vasoconstriction by AVP in the mouse is strongly buffered by vasodilatory actions of NO and prostanoids. The vasoconstriction depends on V1a receptor activation without involvement of CD38 or concomitant vasodilatation by V2 receptors. The role of superoxide is to enhance the contractile response to AVP, most likely by reducing the availability of NO rather than directly stimulating intracellular contraction signaling pathways.

Nonvasoconstrictive hemoglobin particles as oxygen carriers

Artificial oxygen carriers, favorably hemoglobin-based oxygen carriers (HBOCs), are being investigated intensively during the last 30 years with the aim to develop a universal blood substitute. However, serious side effects mainly caused by vasoconstriction triggered by nitric oxide (NO) scavenging due to penetration of nanosized HBOCs through the endothelial gaps of the capillary walls and/or oxygen oversupply in the precapillary arterioles due to their low oxygen affinity led to failure of clinical trials and FDA disapproval. To avoid these effects, HBOCs with a size between 100 and 1000 nm and high oxygen affinity are needed. Here we present for the first time unique hemoglobin particles (HbPs) of around 700 nm with high oxygen affinity and low immunogenicity using a novel, highly effective, and simple technique. The fabrication procedure provides particles with a narrow size distribution and nearly uniform morphology. The content of hemoglobin (Hb) in the particles corresponded to 80% of the Hb content in native erythrocytes. Furthermore, we demonstrate a successful perfusion of isolated mouse glomeruli with concentrated HbP suspensions in vitro. A normal, nonvasoconstrictive behavior of the afferent arterioles is observed, suggesting no oxygen oversupply and limited NO scavenging by these particles, making them a highly promising blood substitute.

Short-term glucosamine infusion increases islet blood flow in anesthetized rats

Impaired glucose tolerance and type 2 diabetes in rodents are associated with increased islet blood flow. If this is important for modulation of the endocrine function is at present unknown. We evaluated if glucosamine infusion, which induces peripheral insulin resistance and glucose intolerance, could be used to acutely increase islet blood flow. We infused anaesthetized Sprague-Dawley rats for 2 h with glucosamine (6 mg/kg body weight), in some cases followed by glucose administration. The former induced a 2-fold increase in serum insulin concentrations while plasma glucose remained unchanged. In vitro an augmented insulin response to hyperglycemia and decreased insulin content in batch type islet incubations with glucosamine for 24 h were seen. After 2 h glucosamine exposure in vitro, insulin release was decreased. In vivo glucosamine infusion increased islet blood flow, without affecting other regional blood flow values. Glucose increased islet blood flow to the same extent in control and glucosamine-infused rats. When exposed to 10 mmol/L glucosamine arterioles of isolated perfused islets showed a 10% dilation of their vascular smooth muscle. Thus, application of this model leads to acute hyperinsulinemia in vivo but a decreased insulin release in vitro, which suggests that effects not located to β cells are responsible for the effects seen in vivo. An increased islet blood flow in previously healthy animals was also seen after glucose administration, which can be used to further dissect the importance of blood flow changes in islet function.

Interactions between adenosine, angiotensin II and nitric oxide on the afferent arteriole influence sensitivity of the tubuloglomerular feedback

Adenosine, via activation of A1 receptors on the afferent arteriole (AA), mediates the tubuloglomerular feedback (TGF) mechanism. Angiotensin II and nitric oxide (NO) can modulate the sensitivity of the TGF mechanism. However, the interaction among these substances in regulating the TGF resetting phenomenon has been debated. Studies in isolated perfused AA have shown a biphasic response to accumulating doses of adenosine alone. In the nanomolar range adenosine has a weak contractile effect (7%), whereas vasodilatation is observed at high concentrations. However, a synergistic interaction between the contractile response by adenosine and that of angiotensin II has been demonstrated. Adenosine in low concentrations strongly enhances the response to angiotensin II. At the same time, angiotensin II in physiological concentrations increases significantly the contractile response to adenosine. Moreover, addition of a NO donor (spermine NONOate) to increase NO bioavailability abolished the contractile response from combined application of angiotensin II and adenosine. These mutual modulating effects of adenosine and angiotensin II, and the effect of NO on the response of AA can contribute to the resetting of the TGF sensitivity.

Angiotensin II-mediated vascular changes in aged offspring rats exposed to perinatal nicotine

This study determined the long-term influence of prenatal nicotine exposure (PN) on blood pressure and vascular functions in the aged offspring rats. PN did not affect body weight and plasma adrenocorticotropic hormone level; however, it significantly reduced plasma angiotensin I and angiotensin II in both sexes. Systolic pressure in the male aged PN offspring was significantly higher. Angiotensin II-increased mean arterial pressure was higher in the aged PN offspring than that in the control regardless of sex. AT1 receptor blocker losartan, not AT2 receptor antagonist PD123319, reduced blood pressure in the aged PN rats more than that in the control. In the aged PN offspring, angiotensin II-increased vessel contraction and intracellular calcium level were higher in small mesenteric arteries. Acetylcholine-mediated vascular relaxation was weaker, and nitric oxide-related endothelial functions were damaged in aortic rings of PN offspring. Thickness of the wall of mesenteric arteries was increased in the male aged PN offspring. Ratio of AT1/AT2 receptors was significantly increased in the vessel of the PN group regardless of sex. These data provide new information on the very long term influence of PN on vascular structures and functions in the aged offspring, demonstrate that the aged PN female rats were not free of vascular risks after menopause, and suggest that multiple pathways may be involved in the detrimental alterations of the cardiovascular system of the PN rats.

Nitric oxide in afferent arterioles after uninephrectomy depends on extracellular l-arginine

Uninephrectomy (UNX) causes hyperperfusion of the contralateral remaining kidney via increased nitric oxide (NO) synthesis. Although the exact mechanism remains largely unknown, we hypothesize that this would be localized to the afferent arteriole and that it depends on cellular uptake of l-arginine. The experiments were performed in rats 2 days (early) or 6 wk (late) after UNX and compared with controls (Sham) to study acute and chronic effects on NO metabolism. Renal blood flow was increased after UNX (21 ± 2 ml·min−1·kg−1 in sham, 30 ± 3 in early, and 26 ± 1 in late, P < 0.05). NO inhibition with Nω-nitro-l-arginine methyl ester hydrochloride (l-NAME) caused a greater increase in renal vascular resistance in early UNX compared with Sham and late UNX (138 ± 24 vs. 88 ± 10, and 84 ± 7%, P < 0.01). The lower limit of autoregulation was increased both in early and late UNX compared with Sham ( P < 0.05). l-NAME did not affect the ANG II-induced contraction of isolated afferent arterioles (AA) from Sham. AA from early UNX displayed a more pronounced contraction in response to l-NAME (−57 ± 7 vs. −16 ± 7%, P < 0.05) and in the absence of l-arginine (−41 ± 4%, P < 0.05) compared with both late UNX and Sham. mRNA expression of endothelial NO synthase was reduced, whereas protein expression was unchanged. Cationic amino acid transporter-1 and -2 mRNA was increased, while protein was unaffected in isolated preglomerular resistance vessels. In conclusion, NO-dependent hyperperfusion of the remaining kidney in early UNX is associated with increased NO release from the afferent arteriole, which is highly dependent on extracellular l-arginine availability.

Angiotensin II type 2 receptor mediates sex differences in mice renal interlobar arteries response to angiotensin II

OBJECTIVE

Functional sex differences are described in several vascular beds. In the case of renal vessels, sex differences could influence processes like regulation of blood pressure and ion balance. Angiotensin II and nitric oxide are important regulators of renal vascular tone. Females have higher nitric oxide synthase expression, nitric oxide bioavailability and ratio of angiotensin II type 2/type 1 receptors. Thus, our objective was to examine whether renal interlobar arteries present sex differences in their response to angiotensin II, and whether angiotensin II type 2 receptors play a role in such differences.

METHODS

We investigated the isometric contraction and relaxation of interlobar arteries from female and male mice under blockade of nitric oxide synthases and angiotensin II type 2 receptors. We also investigated the expression of angiotensin II receptors (type 1 and 2) and endothelial nitric oxide synthase.

RESULTS

Significantly less intense contraction to angiotensin II were seen in arteries from females in comparison to male mice. Inhibition of nitric oxide synthases and endothelial removal abolished this difference. Angiotensin II type 2 receptors blockade enhanced contraction to angiotensin II in females, but not in males. Endothelial-dependent vasodilation was more dependent on nitric oxide in females than in males. Expression of angiotensin II type 1 and type 2 receptors was similar between sexes. Expression of endothelial nitric oxide synthase was higher in females.

CONCLUSION

A sex-specific, nitric oxide-mediated effect via angiotensin II type 2 receptors underlies the sex differences in the response of interlobar arteries to angiotensin II. Our findings may help understanding sex differences in renal hemodynamics and blood pressure control.

Renal afferent arteriolar and tubuloglomerular feedback reactivity in mice with conditional deletions of adenosine 1 receptors

Adenosine 1 receptors (A1AR) have been shown in previous experiments to play a major role in the tubuloglomerular feedback (TGF) constrictor response of afferent arterioles (AA) to increased loop of Henle flow. Overexpression studies have pointed to a critical role of vascular A1AR, but it has remained unclear whether selective deletion of A1AR from smooth muscle cells is sufficient to abolish TGF responsiveness. To address this question, we have determined TGF response magnitude in mice in which vascular A1AR deletion was achieved using the loxP recombination approach with cre recombinase being controlled by a smooth muscle actin promoter (SmCre/A1ARff). Effective vascular deletion of A1AR was affirmed by absence of vasoconstrictor responses to adenosine or cyclohexyl adenosine (CHA) in microperfused AA. Elevation of loop of Henle flow from 0 to 30 nl/min caused a 22.1 ± 3.1% reduction of stop flow pressure in control mice and of 7.2 ± 1.5% in SmCre/A1ARff mice (P < 0.001). Maintenance of residual TGF activity despite absence of A1AR-mediated responses in AA suggests participation of extravascular A1AR in TGF. Support for this notion comes from the observation that deletion of A1ARff by nestin-driven cre causes an identical TGF response reduction (7.3 ± 2.4% in NestinCre/A1ARff vs. 20.3 ± 2.7% in controls), whereas AA responsiveness was reduced but not abolished. A1AR on AA smooth muscle cells are primarily responsible for TGF activation, but A1AR on extravascular cells, perhaps mesangial cells, appear to contribute to the TGF response.

An intact kidney slice model to investigate vasa recta properties and function in situ

Background

Medullary blood flow is via vasa recta capillaries, which possess contractile pericytes. In vitro studies using isolated descending vasa recta show that pericytes can constrict/dilate descending vasa recta when vasoactive substances are present. We describe a live kidney slice model in which pericyte-mediated vasa recta constriction/dilation can be visualized in situ.

Methods

Confocal microscopy was used to image calcein, propidium iodide and Hoechst labelling in ‘live’ kidney slices, to determine tubular and vascular cell viability and morphology. DIC video-imaging of live kidney slices was employed to investigate pericyte-mediated real-time changes in vasa recta diameter.

Results

Pericytes were identified on vasa recta and their morphology and density were characterized in the medulla. Pericyte-mediated changes in vasa recta diameter (10–30%) were evoked in response to bath application of vasoactive agents (norepinephrine, endothelin-1, angiotensin-II and prostaglandin E₂) or by manipulating endogenous vasoactive signalling pathways (using tyramine, L-NAME, a cyclo-oxygenase (COX-1) inhibitor indomethacin, and ATP release).

Conclusions

The live kidney slice model is a valid complementary technique for investigating vasa recta function in situ and the role of pericytes as regulators of vasa recta diameter. This technique may also be useful in exploring the role of tubulovascular crosstalk in regulation of medullary blood flow.

Adenosine A1-receptor deficiency diminishes afferent arteriolar and blood pressure responses during nitric oxide inhibition and angiotensin II treatment

Adenosine mediates tubuloglomerular feedback responses via activation of A1-receptors on the renal afferent arteriole. Increased preglomerular reactivity, due to reduced nitric oxide (NO) production or increased levels of ANG II and reactive oxygen species (ROS), has been linked to hypertension. Using A1-receptor knockout (A1−/−) and wild-type (A1+/+) mice we investigated the hypothesis that A1-receptors modulate arteriolar and blood pressure responses during NO synthase (NOS) inhibition or ANG II treatment. Blood pressure and renal afferent arteriolar responses were measured in nontreated mice and in mice with prolonged Nω-nitro-l-arginine methyl ester hydrochloride (l-NAME) or ANG II treatment. The hypertensive responses to l-NAME and ANG II were clearly attenuated in A1−/− mice. Arteriolar contractions to l-NAME (10−4 mol/l; 15 min) and cumulative ANG II application (10−12 to 10−6 mol/l) were lower in A1−/− mice. Simultaneous treatment with tempol (10−4 mol/l; 15 min) attenuated arteriolar responses in A1+/+ but not in A1−/− mice, suggesting differences in ROS formation. Chronic treatment with l-NAME or ANG II did not alter arteriolar responses in A1−/− mice, but enhanced maximal contractions in A1+/+ mice. In addition, chronic treatments were associated with higher plasma levels of dimethylarginines (asymmetrical and symmetrical) and oxidative stress marker malondialdehyde in A1+/+ mice, and gene expression analysis showed reduced upregulation of NOS-isoforms and greater upregulation of NADPH oxidases. In conclusion, adenosine A1-receptors enhance preglomerular responses during NO inhibition and ANG II treatment. Interruption of A1-receptor signaling blunts l-NAME and ANG II-induced hypertension and oxidative stress and is linked to reduced responsiveness of afferent arterioles.

High-salt diet increases glomerular ACE/ACE2 ratio leading to oxidative stress and kidney damage

BACKGROUND

Angiotensin II (AngII) contributes to salt-driven kidney damage. In this study, we aimed at investigating whether and how the renal damage associated with a high-salt diet could result from changes in the ratio between angiotensin-converting enzyme (ACE) and angiotensin-converting enzyme 2 (ACE2).

METHODS

Forty-eight rats randomly allocated to three different dietary contents of salt were studied for 4 weeks after undergoing a left uninephrectomy. We focussed on kidney functional, structural and molecular changes. At the same time, we studied kidney molecular changes in 20 weeks old Ace2-knockout mice (Ace2KO), with and without ACE inhibition.

RESULTS

A high salt content diet significantly increased the glomerular ACE/ACE2 ratio. This was associated with increased oxidative stress. To assess whether these events were related, we measured renal oxidative stress in Ace2KO, and found that the absence of ACE2 promoted oxidative stress, which could be prevented by ACE inhibition.

CONCLUSION

One of the mechanisms by which a high-salt diet leads to renal damage seems to be the modulation of the ACE/ACE2 ratio which in turn is critical for the cause of oxidative stress, through AngII.

Superoxide dismutase 1 limits renal microvascular remodeling and attenuates arteriole and blood pressure responses to angiotensin II via modulation of nitric oxide bioavailability

Oxidative stress is associated with vascular remodeling and increased preglomerular resistance that are both implicated in the pathogenesis of renal and cardiovascular disease. Angiotensin II induces superoxide production, which is metabolized by superoxide dismutase (SOD) or scavenged by NO. We investigated the hypothesis that SOD1 regulates renal microvascular remodeling, blood pressure, and arteriolar responsiveness and sensitivity to angiotensin II using SOD1-transgenic (SOD1-tg) and SOD1-knockout (SOD1-ko) mice. Blood pressure, measured telemetrically, rose more abruptly during prolonged angiotensin II infusion in SOD1-ko mice. The afferent arteriole media:lumen ratios were reduced in SOD1-tg and increased in SOD1-ko mice. Afferent arterioles from nontreated wild types had graded contraction to angiotensin II (sensitivity: 10(-9) mol/L; responsiveness: 40%). Angiotensin II contractions were less sensitive (10(-8) mol/L) and responsive (14%) in SOD1-tg but more sensitive (10(-13) mol/L) and responsive (89%) in SOD1-ko mice. Arterioles from SOD1-ko had 4-fold increased superoxide formation with angiotensin II at 10(-9) mol/L. N(G)-nitro-l-arginine methyl ester reduced arteriole diameter of SOD1-tg and enhanced angiotensin II sensitivity and responsiveness of wild-type and SOD1-tg mice to the level of SOD1-ko mice. SOD mimetic treatment with Tempol increased arteriole diameter and normalized the enhanced sensitivity and responsiveness to angiotensin II of SOD1-ko mice but did not affect wild-type or SOD1-tg mice. Neither SOD1 deficiency nor overexpression was associated with changes in nitrate/nitrite excretion or renal mRNA expression of NO synthase, NADPH oxidase, or SOD2/SOD3 isoforms and angiotensin II receptors. In conclusion, SOD1 limits afferent arteriole remodeling and reduces sensitivity and responsiveness to angiotensin II by reducing superoxide and maintaining NO bioavailability. This may prevent an early and exaggerated blood pressure response to angiotensin II.

Endothelin type A and B receptors in the control of afferent and efferent arterioles in mice

BACKGROUND

Endothelin 1 contributes to renal blood flow control and pathogenesis of kidney diseases. The differential effects, however, of endothelin 1 (ET-1) on afferent (AA) and efferent arterioles (EA) remain to be established.

METHODS

We investigated endothelin type A and B receptor (ETA-R, ETB-R) functions in the control of AA and EA. Arterioles of ETB-R deficient, rescued mice [ETB(-/-)] and wild types [ETB(+/+)] were microperfused.

RESULTS

ET-1 constricted AA stronger than EA in ETB(-/-) and ETB(+/+) mice. Results in AA: ET-1 induced similar constrictions in ETB(-/-) and ETB(+/+) mice. BQ-123 (ETA-R antagonist) inhibited this response in both groups. ALA-ET-1 and IRL1620 (ETB-R agonists) had no effect on arteriolar diameter. L-NAME did neither affect basal diameters nor ET-1 responses. Results in EA: ET-1 constricted EA stronger in ETB(+/+) compared to ETB(-/-). BQ-123 inhibited the constriction completely only in ETB(-/-). ALA-ET-1 and IRL1620 constricted only arterioles of ETB(+/+) mice. L-NAME decreased basal diameter in ETB(+/+), but not in ETB(-/-) mice and increased the ET-1 response similarly in both groups. The L-NAME actions indicate a contribution of ETB-R in basal nitric oxide (NO) release in EA and suggest dilatory action of ETA-R in EA.

CONCLUSIONS

ETA-R mediates vasoconstriction in AA and contributes to vasoconstriction in EA in this mouse model. ETB-R has no effect in AA but mediates basal NO release and constriction in EA. The stronger effect of ET-1 on AA supports observations of decreased glomerular filtration rate to ET-1 and indicates a potential contribution of ET-1 to the pathogenesis of kidney diseases.

Angiotensin II regulates NOS expression in afferent arterioles of the developing porcine kidney

NO protection is crucial against angiotensin II (ANG II) mediated vasoconstriction in postnatal preglomerular resistance vessels. Although whole kidney NOS is developmentally regulated, NOS regulation in developing renal resistance vessels is unknown. The hypothesis was NOS expression and function in developing afferent arterioles are regulated by ANG II through AT1 and AT2 receptors. Afferent arterioles from porcine kidneys, ages newborn, 7, 21 d, and adult, were dissected using a polybead perfusion technique. Dissected afferent arterioles were treated with ANG II and with either the AT1 receptor inhibitor candesartan or the AT2 receptor inhibitor PD 123319 and evaluated for NOS isoform expression and NOS enzymatic activity. Although NOS activity and neuronal NOS (nNOS) expression were greater in the newborn than in the adult, endothelial NOS (eNOS) expression was greater in the adult. ANG II increased NOS activity and eNOS expression at all ages, but nNOS expression only in developing afferents. AT1 and AT2 receptor blockade significantly attenuated NOS activity and eNOS expression at all ages, but nNOS expression only in developing afferents. ANG II regulates nNOS and eNOS expression and NOS activity in afferent arterioles of the developing kidney via AT1 and AT2 receptors.

Losartan increases NO release in afferent arterioles during regression of l-NAME-induced renal damage

Inhibition of nitric oxide synthesis (NOS) induces hypertension and heavy proteinuria. Renal structure and function have shown striking improvement after interventions targeting ANG II or endothelin (ET) receptors in rats recovering after long-term NOS inhibition. To search for mechanisms underlying losartan-assisted regression of renal disease in rodents, we measured NO release and contractility to ET in afferent arterioles (AAs) from Sprague-Dawley rats recovering for 2 wk after 4 wk of NG-nitro-l-arginine methyl ester treatment. Losartan administration during the recovery period decreased blood pressure (113 ± 4 vs. 146 ± 5 mmHg, P < 0.01), reduced protein/creatinine ratio more (proteinuria decrease: Δ1,836 ± 214 vs. Δ1,024 ± 180 mg/mmol, P < 0.01), and normalized microvascular hypertrophy (AA media/lumen ratio: 1.74 ± 0.05 vs. 2.09 ± 0.08, P < 0.05) compared with no treatment. In diaminofluorescein-FM-loaded AAs from losartan-treated animals, NO release (% of baseline) was increased compared with untreated animals after stimulation with 10−7 M ACh (118 ± 4 vs. 90 ± 7%, t = 560 s, P < 0.001) and 10−9 M ET (123 ± 4 vs. 101 ± 5%, t = 560 s, P < 0.001). There was also a blunted contractile response to 10−7 M ET in AAs from losartan-treated animals compared with untreated animals (Δ4.01 ± 2.9 vs. Δ14.6 ± 1.7 μm, P < 0.01), which disappeared after acute NOS inhibition (Δ10.7 ± 3.7 vs. Δ12.5 ± 2.9 μm, not significant). Contractile dose responses to ET (10−9, 10−8, 10−7 M) were enhanced by NOS inhibition and blunted by exogenous NO (10−2 mM S-nitroso- N-acetyl-penicillamine) in losartan-treated but not in untreated vessels. Reducing blood pressure similar to losartan with hydralazine did not improve AA hypertrophy, ET-induced contractility, ET-induced NO release, and NO sensitivity. In conclusion, blockade of the local action of ANG II improved endothelial function in AAs, a mechanism that is likely to contribute to the beneficial effects of AT1aR antagonism during the recovery of renal function after long-term NOS inhibition in rats.

Myogenic responses of mouse isolated perfused renal afferent arterioles: effects of salt intake and reduced renal mass

Because defects in renal autoregulation may contribute to renal barotrauma in chronic kidney disease, we tested the hypothesis that the myogenic response is diminished by reduced renal mass. Kidneys from 5/6 nephrectomized mice had only a minor increase in the glomerular sclerosis index. The telemetric mean arterial pressure (108+/-10 mm Hg) was unaffected after 3 months of high-salt intake (6% salt in chow) or reduced renal mass. Afferent arterioles from 5/6 nephrectomized mice and sham-operated controls were perfused ex vivo during step changes in pressure from 40 to 134 mm Hg. Afferent arterioles developed a constriction and a linear increase in active wall tension above a perfusion pressure of 36+/-6 mm Hg, without a plateau. The slope of active wall tension versus perfusion pressure defined the myogenic response, which was similar in sham mice fed normal or high-salt diets for 3 months (2.90+/-0.22 versus 3.22+/-0.40 dynes x cm(-1)/mm Hg; P value not significant). The myogenic response was unaffected after 3 days of reduced renal mass on either salt diet (3.39+/-0.61 versus 4.04+/-0.47 dynes x cm(-1)/mm Hg) but was reduced (P<0.05) in afferent arterioles from reduced renal mass groups fed normal and high salt at 3 months (2.10+/-0.28 and 1.35+/-0.21 dynes x cm(-1)/mm Hg). In conclusion, mouse renal afferent arterioles develop a linear increase in myogenic tone around the range of ambient perfusion pressures. This myogenic response is impaired substantially in the mouse model of prolonged reduced renal mass, especially during high salt intake.

Enhanced tubuloglomerular feedback in mice with vascular overexpression of A1 adenosine receptors

Adenosine 1 receptors (A1AR) in the kidney are expressed in the vasculature and the tubular system. Pharmacological inhibition or global genetic deletion of A1AR causes marked reductions or abolishment of tubuloglomerular feedback (TGF) responses. To assess the function of vascular A1AR in TGF, we generated transgenic mouse lines in which A1AR expression in smooth muscle was augmented by placing A1AR under the control of a 5.38-kb fragment of the rat smooth muscle alpha-actin promoter and first intron (12). Two founder lines with highest expression in the kidney [353 +/- 42 and 575 +/- 43% compared with the wild type (WT)] were used in the experiments. Enhanced expression of A1AR at the expected site in these lines was confirmed by augmented constrictor responses of isolated afferent arterioles to administration of the A1AR agonist N6-cyclohexyladenosine. Maximum TGF responses (0-30 nl/min flow step) were increased from 8.4 +/- 0.9 mmHg in WT (n = 21) to 14.2 +/- 0.7 mmHg in A1AR-transgene (tg) 4 (n = 22; P < 0.0001), and to 12.6 +/- 1.2 mmHg in A1AR-tg7 (n = 12; P < 0.02). Stepwise changes in perfusion flow caused greater numerical TGF responses in A1AR-tg than WT in all flow ranges with differences reaching levels of significance in the intermediate flow ranges of 7.5-10 and 10-15 nl/min. Proximal-distal single-nephron glomerular filtration rate (SNGFR) differences (free-flow micropuncture) were also increased in A1AR-tg, averaging 6.25 +/- 1.5 nl/min compared with 2.6 +/- 0.51 nl/min in WT (P = 0.034). Basal plasma renin concentrations as well as the suppression of renin secretion after volume expansion were similar in A1AR-tg and WT mice, suggesting lack of transgene expression in juxtaglomerular cells. These data indicate that A1AR expression in vascular smooth muscle cells is a critical component for TGF signaling and that changes in renal vascular A1AR expression may determine the magnitude of TGF responses.

Angiotensin II enhances the afferent arteriolar response to adenosine through increases in cytosolic calcium

AIMS

Angiotensin II (Ang II) is a strong renal vasoconstrictor and modulates the tubuloglomerular feedback (TGF). We hypothesized that Ang II at low concentrations enhances the vasoconstrictor effect of adenosine (Ado), the mediator of TGF.

METHODS

Afferent arterioles of mice were isolated and perfused, and both isotonic contractions and cytosolic calcium transients were measured.

RESULTS

Bolus application of Ang II (10(-12) and 10(-10) M) induced negligible vasoconstrictions, while Ang II at 10(-8) m reduced diameters by 35%. Ang II at 10(-12), 10(-10) and 10(-8) m clearly enhanced the arteriolar response to cumulative applications of Ado (10(-11) to 10(-4) M). Ado application increased the cytosolic calcium concentrations in the vascular smooth muscle, which were higher at 10(-5) M than at 10(-8) M. Ang II (10(-11) to 10(-6) M) also induced concentration-dependent calcium transients, which were attenuated by AT(1) receptor inhibition. Simultaneously applied Ang II (10(-10) M) additively enhanced the calcium transients induced by 10(-8) and 10(-5) M Ado. The transients were partly inhibited by AT(1) or A(1) receptor antagonists, but not significantly by A(2) receptor antagonists.

CONCLUSION

A low dose of Ang II enhances Ado-induced constrictions, partly via AT(1) receptor-mediated calcium increase. Ado increases intracellular calcium by acting on A(1) but not A(2) receptors. The potentiating effect of Ang II on Ado-induced arteriolar vasoconstrictions may involve calcium sensitization of the contractile machinery, as Ang II only additively increased cytosolic calcium concentrations, while its effect on the arteriolar constriction was more than additive. The potentiating effect of Ang II might contribute to the resetting of TGF.

Role of NOX2 in the regulation of afferent arteriole responsiveness

NADPH oxidases (NOX) are the major source of reactive oxygen species (ROS) in the vasculature and contribute to the control of renal perfusion. The role of NOX2 in the regulation of blood pressure and afferent arteriole responsiveness was investigated in NOX2(-/-) and wild-type mice. Arteriole constrictions to ANG II (10(-14)-10(-6) mol/l) were weaker in NOX2(-/-) compared with wild types. N(omega)-nitro-l-arginine methyl ester (l-NAME; 10(-4) mol/l) treatment reduced basal diameters significantly more in NOX2(-/-) (-18%) than in wild types (-6%) and augmented ANG II responses. Adenosine (10(-11)-10(-4) mol/l) constricted arterioles of wild types but not of NOX2(-/-). However, simultaneous inhibition of adenosine type-2 receptors induced vasoconstriction, which was stronger in NOX2(-/-). Adenosine (10(-8) mol/l) enhanced the ANG II response in wild type, but not in NOX2(-/-). This sensitizing effect by adenosine was abolished by apocynin. Chronic ANG II pretreatment (14 days) did not change the ANG II responses in NOX2(-/-), but strengthened the response in wild types. ANG II pretreatment augmented the l-NAME response in NOX2(-/-) (-33%), but not in wild types. Simultaneous application of l-NAME and ANG II caused a stronger constriction in the NOX2(-/-) (-64%) than in wild types (-46%). Basal blood pressures were similar in both genotypes, however, chronic ANG II infusion elevated blood pressure to a greater extent in wild-type (15 +/- 1%) than in NOX2(-/-) (8 +/- 1%) mice. In conclusion, NOX2 plays an important role in the control of afferent arteriole tone and is involved in the contractile responses to ANG II and/or adenosine. NOX2 can be activated by elevated ANG II and may play an important role in ANG II-induced hypertension. NOX2-derived ROS scavenges nitric oxide, causing subsequent nitric oxide-deficiency.

Pivotal role of angiotensin II receptor subtype 1A in the development of two-kidney, one-clip hypertension: study in angiotensin II receptor subtype 1A knockout mice

OBJECTIVE

The present study was performed to examine in two-kidney, one-clip (2K1C) Goldblatt hypertensive mice: first, the relative contribution of angiotensin II receptor subtypes 1A (AT(1A)) and 1B (AT(1B)); second, the role of angiotensin II type 2 (AT(2)) receptors in the development of hypertension in wild-type (AT(1A)+/+) and AT(1A) receptor knockout (AT(1A)-/-) mice; and third, the role of increased nitric oxide synthase activity in counteracting the hypertensinogenic action of angiotensin II in this model.

METHODS

AT(1A)+/+ and AT(1A)-/- mice underwent clipping of one renal artery and were infused with either saline vehicle or selective AT(2) receptor agonist CGP-42112A (CGP). Blood pressure was monitored by radiotelemetry. Blood pressure responses to the nitric oxide synthase inhibitor nitro-L-arginine-methyl-ester were evaluated.

RESULTS

AT(1A)+/+ mice responded to clipping by a rise in blood pressure that was not modified by CGP infusion. Clip placement caused a slight increase in blood pressure in AT(1A)-/- mice that remained significantly lower than in AT(1A)+/+ mice. Acute nitric oxide synthase inhibition caused greater increase in blood pressure in 2K1C/AT(1A)+/+ than in AT(1A)+/+ mice.

CONCLUSION

The present data support the critical role of AT(1A) receptors in the development of 2K1C hypertension, whereas AT(1B) receptors play only a minor role in blood pressure regulation in this model of angiotensin II-dependent hypertension. Activation of AT(2) receptors does not play an antagonistic role in the AT(1) receptor-mediated hypertensinogenic actions of angiotensin II in this model. Finally, enhanced nitric oxide synthase activity plays a protective role by counteracting the vasoconstrictor influences of angiotensin II in 2K1C hypertensive mice.

Angiotensin II-induced contraction is attenuated by nitric oxide in afferent arterioles from the nonclipped kidney in 2K1C

Two-kidney, one-clip (2K1C) is a model of renovascular hypertension where we previously found an exaggerated intracellular calcium (Ca(i)(2+)) response to ANG II in isolated afferent arterioles (AAs) from the clipped kidney (Helle F, Vagnes OB, Iversen BM. Am J Physiol Renal Physiol 291: F140-F147, 2006). To test whether nitric oxide (NO) ameliorates the exaggerated ANG II response in 2K1C, we studied ANG II (10(-7) mol/l)-induced calcium signaling and contractility with or without the NO synthase (NOS) inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME). In AAs from the nonclipped kidney, l-NAME increased the ANG II-induced Ca(i)(2+) response from 0.28 +/- 0.05 to 0.55 +/- 0.09 (fura 2, 340 nm/380 nm ratio) and increased contraction from 80 +/- 6 to 60 +/- 6% of baseline (P < 0.05). In vessels from sham and clipped kidneys, l-NAME had no effect. In diaminofluorescein-FM diacetate-loaded AAs from the nonclipped kidney, ANG II increased NO-derived fluorescence to 145 +/- 34% of baseline (P < 0.05 vs. sham), but not in vessels from the sham or clipped kidney. Endothelial NOS (eNOS) mRNA and ser-1177 phosphorylation were unchanged in both kidneys from 2K1C, while eNOS protein was reduced in the clipped kidney compared with sham. Cationic amino acid transferase-1 and 2 mRNAs were increased in 2K1C, indicating increased availability of l-arginine for NO synthesis, but counteracted by decreased scavenging of the eNOS inhibitor asymmetric dimethylarginine by dimethylarginine dimethylaminohydrolase 2. In conclusion, the Ca(i)(2+) and contractile responses to ANG II are blunted by NO release in the nonclipped kidney. This may protect the nonclipped kidney from the hypertension and elevated ANG II levels in 2K1C.