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

Role of Alström syndrome 1 in the regulation of glomerular hemodynamics

Inactivating mutations in the ALMS1 gene in humans cause Alström syndrome, characterized by the early onset of obesity, insulin resistance, and renal dysfunction. However, the role of ALMS1 in renal function and hemodynamics is unclear. We previously found that ALMS1 is expressed in thick ascending limbs, where it binds and decreases Na+-K+-2Cl- cotransporter activity. We hypothesized that ALMS1 is expressed in macula densa cells and that its deletion enhances tubuloglomerular feedback (TGF) and reduces glomerular filtration rate (GFR) in rats. To test this, homozygous ALMS1 knockout (KO) and littermate wild-type Dahl salt-sensitive rats were studied. TGF sensitivity was higher in ALMS1 KO rats as measured by in vivo renal micropuncture. Using confocal microscopy, we confirmed immunolabeling of ALMS1 in macula densa cells (nitric oxide synthase 1 positive), supporting a role for ALMS1 in TGF regulation. Baseline glomerular capillary pressure was higher in ALMS1 KO rats, as was mean arterial pressure. Renal interstitial hydrostatic pressure was lower in ALMS1 KO rats, which is linked to increased Na+ reabsorption and hypertension. GFR was reduced in ALMS1 KO rats. Seven-week-old ALMS1 KO rats were not proteinuric, but proteinuria was present in 18- to 22-wk-old ALMS1 KO rats. The glomerulosclerosis index was higher in 18-wk-old ALMS1 KO rats. In conclusion, ALMS1 is involved in the control of glomerular hemodynamics in part by enhancing TGF sensitivity, and this may contribute to decreased GFR. Increased TGF sensitivity, enhanced glomerular capillary pressure, and hypertension may lead to glomerular damage in ALMS1 KO rats. These are the first data supporting the role of ALMS1 in TGF and glomerular hemodynamics.NEW & NOTEWORTHY ALMS1 is a novel protein involved in regulating tubuloglomerular feedback (TGF) sensitivity, glomerular capillary pressure, and blood pressure, and its dysfunction may reduce renal function and cause glomerular damage.

High Na+ Salt Diet and Remodeling of Vascular Smooth Muscle and Endothelial Cells

Our knowledge on essential hypertension is vast, and its treatment is well known. Not all hypertensives are salt-sensitive. The available evidence suggests that even normotensive individuals are at high cardiovascular risk and lower survival rate, as blood pressure eventually rises later in life with a high salt diet. In addition, little is known about high sodium (Na⁺) salt diet-sensitive hypertension. There is no doubt that direct and indirect Na⁺ transporters, such as the Na/Ca exchanger and the Na/H exchanger, and the Na/K pump could be implicated in the development of high salt-induced hypertension in humans. These mechanisms could be involved following the destruction of the cell membrane glycocalyx and changes in vascular endothelial and smooth muscle cells membranes’ permeability and osmolarity. Thus, it is vital to determine the membrane and intracellular mechanisms implicated in this type of hypertension and its treatment.

Reactive oxygen species in renal vascular function

Reactive oxygen species (ROS) are produced by the aerobic metabolism. The imbalance between production of ROS and antioxidant defence in any cell compartment is associated with cell damage and may play an important role in the pathogenesis of renal disease. NADPH oxidase (NOX) family is the major ROS source in the vasculature and modulates renal perfusion. Upregulation of Ang II and adenosine activates NOX via AT1R and A1R in renal microvessels, leading to superoxide production. Oxidative stress in the kidney prompts renal vascular remodelling and increases preglomerular resistance. These are key elements in hypertension, acute and chronic kidney injury, as well as diabetic nephropathy. Renal afferent arterioles (Af), the primary resistance vessel in the kidney, fine tune renal hemodynamics and impact on blood pressure. Vice versa, ROS increase hypertension and diabetes, resulting in upregulation of Af vasoconstriction, enhancement of myogenic responses and change of tubuloglomerular feedback (TGF), which further promotes hypertension and diabetic nephropathy. In the following, we highlight oxidative stress in the function and dysfunction of renal hemodynamics. The renal microcirculatory alterations brought about by ROS importantly contribute to the pathophysiology of kidney injury, hypertension and diabetes.

Purinoceptors, renal microvascular function and hypertension

Proper renal blood flow (RBF) and glomerular filtration rate (GFR) are critical for maintaining normal blood pressure, kidney function and water and electrolyte homeostasis. The renal microvasculature expresses a multitude of receptors mediating vasodilation and vasoconstriction, which can influence glomerular blood flow and capillary pressure. Despite this, RBF and GFR remain quite stable when arterial pressure fluctuates because of the autoregulatory mechanism. ATP and adenosine participate in autoregulatory control of RBF and GFR via activation of two different purinoceptor families (P1 and P2). Purinoceptors are widely expressed in renal microvasculature and tubules. Emerging data show altered purinoceptor signaling in hypertension-associated kidney injury, diabetic nephropathy, sepsis, ischemia-reperfusion induced acute kidney injury and polycystic kidney disease. In this brief review, we highlight recent studies and new insights on purinoceptors regulating renal microvascular function and renal hemodynamics. We also address the mechanisms underlying renal microvascular injury and impaired renal autoregulation, focusing on purinoceptor signaling and hypertension-induced renal microvascular dysfunction. Interested readers are directed to several excellent and comprehensive reviews that recently covered the topics of renal autoregulation, and nucleotides in kidney function under physiological and pathophysiological conditions (Inscho 2009, Navar et al. 2008, Carlstrom et al. 2015, Vallon et al. 2020).

Antagonism of major histocompatibility complex class II invariant chain peptide during chronic lipopolysaccharide treatment rescues autoregulatory behavior

Toll-like receptor 4 (TLR4) activation contributes to vascular dysfunction in pathological conditions such as hypertension and diabetes, but the role of chronic TLR4 activation on renal autoregulatory behavior is unknown. We hypothesized that subclinical TLR4 stimulation with low-dose lipopolysaccharide (LPS) infusion increases TLR4 activation and blunts renal autoregulatory behavior. We assessed afferent arteriolar autoregulatory behavior in male Sprague-Dawley rats after prolonged LPS (0.1 mg·kg^-1·day^-1 sq) infusion via osmotic minipump for 8 or 14 days. Some rats also received daily cotreatment with either anti-TLR4 antibody (1 μg ip), competitive antagonist peptide (CAP; 3 mg/kg ip) or tempol (2 mmol/l, drinking water) throughout the 8-day LPS treatment period. Autoregulatory behavior was assessed using the in vitro blood-perfused juxtamedullary nephron preparation. Selected physiological measures, systolic blood pressure and baseline diameters were normal and similar across groups. Pressure-dependent vasoconstriction averaged 72 ± 2% of baseline in sham rats, indicating intact autoregulatory behavior. Eight-day LPS-treated rats exhibited significantly impaired pressure-mediated vasoconstriction (96 ± 1% of baseline), whereas it was preserved in rats that received anti-TLR4 antibody (75 ± 3%), CAP (84 ± 2%), or tempol (82 ± 2%). Using a 14-day LPS (0.1 mg·kg^-1·day^-1 sq) intervention protocol, CAP treatment started on day 7, where autoregulatory behavior is already impaired. Systolic blood pressures were normal across all treatment groups. Fourteen-day LPS treatment retained the autoregulatory impairment (95 ± 2% of baseline). CAP intervention starting on day 7 rescued pressure-mediated vasoconstriction with diameters decreasing to 85 ± 1% of baseline. These data demonstrate that chronic subclinical TLR4 activation impairs afferent arteriolar autoregulatory behavior through mechanisms involving reactive oxygen species and major histocompatibility complex class II activation.

High Salt Enhances Reactive Oxygen Species and Angiotensin II Contractions of Glomerular Afferent Arterioles From Mice With Reduced Renal Mass

High salt, Ang II (angiotensin II), and reactive oxygen species enhance progression of chronic kidney disease. We tested the hypothesis that a high salt intake generates specific reactive oxygen species to enhance Ang II contractions of afferent arterioles from mice with reduced renal mass (RRM). C57BL/6 mice were subjected to surgical RRM or sham operations and received 6% or 0.4% NaCl salt diet for 3 months. Ang II contractions were measured in perfused afferent arterioles and superoxide (O2-) and hydrogen peroxide (H2O2) by fluorescence microscopy. RRM enhanced the afferent arteriolar gene expression for p47phox and neutrophil oxidase (NOX) 2 and high salt intake in RRM mice enhanced gene expression for angiotensin type 1 receptors, POLDIP2 and NOX4 and reduced catalase. High salt in mice with RRM enhanced arteriolar O2- and H2O2 generation and maximal contractions to Ang II (10-6 mol/L) that were dependent on O2- because they were prevented by gene deletion of p47phox and on H2O2 because they were prevented by transgenic smooth muscle cell expression of catalase (tgCAT-SMC) and POLDIP2 gene deletion. Three months of tempol normalized arteriolar reactive oxygen species and Ang II contractions. However, arteriolar contractions to lower concentrations of Ang II (10-8 to 10-11 mol/L) were paradoxically inhibited by H2O2 and POLDIP2. In conclusion, both O2- from p47phox/NOX2 and H2O2 from NOX4/POLDIP2 enhance maximal arteriolar Ang II contractions from RRM mice during high salt, but H2O2 and NOX4/POLDIP2 reduce the sensitivity to lower concentrations of Ang II by >100-fold. Tempol prevents all of these changes in function.

Novel hemodynamic structures in the human glomerulus

To investigate human glomerular structure under conditions of physiological perfusion, we have analyzed fresh and perfusion-fixed normal human glomeruli at physiological hydrostatic and oncotic pressures using serial resin section reconstruction, confocal, multiphoton, and electron microscope imaging. Afferent and efferent arterioles (21.5 ± 1.2 µm and 15.9 ± 1.2 µm diameter), recognized from vascular origins, lead into previously undescribed wider regions (43.2 ± 2.8 µm and 38.4 ± 4.9 µm diameter) we have termed vascular chambers (VCs) embedded in the mesangium of the vascular pole. Afferent VC (AVC) volume was 1.6-fold greater than efferent VC (EVC) volume. From the AVC, long nonbranching high-capacity conduit vessels ( n = 7) (Con; 15.9 ± 0.7 µm diameter) led to the glomerular edge, where branching was more frequent. Conduit vessels have fewer podocytes than filtration capillaries. VCs were confirmed in fixed and unfixed specimens with a layer of banded collagen identified in AVC walls by multiphoton and electron microscopy. Thirteen highly branched efferent first-order vessels (E1; 9.9 ± 0.4 µm diameter) converge on the EVC, draining into the efferent arteriole (15.9 ± 1.2 µm diameter). Banded collagen was scarce around EVCs. This previously undescribed branching topology does not conform to the branching of minimum energy expenditure (Murray's law), suggesting that even distribution of pressure/flow to the filtration capillaries is more important than maintaining the minimum work required for blood flow. We propose that AVCs act as plenum manifolds possibly aided by vortical flow in distributing and balancing blood flow/pressure to conduit vessels supplying glomerular lobules. These major adaptations to glomerular capillary structure could regulate hemodynamic pressure and flow in human glomerular capillaries.

Acute toll-like receptor 4 activation impairs rat renal microvascular autoregulatory behaviour

AIM

Little is known about how toll-like receptor 4 (TLR4) influences the renal microvasculature. We hypothesized that acute TLR4 stimulation with lipopolysaccharide (LPS) impairs afferent arteriole autoregulatory behaviour, partially through reactive oxygen species (ROS).

METHODS

We assessed afferent arteriole autoregulatory behaviour after LPS treatment (1 mg kg^-1 ; i.p.) using the in vitro blood-perfused juxtamedullary nephron preparation. Autoregulatory behaviour was assessed by measuring diameter responses to stepwise changes in renal perfusion pressure. TLR4 expression was assessed by immunofluorescence, immunohistochemistry and Western blot analysis in the renal cortex and vasculature.

RESULTS

Baseline arteriole diameter at 100 mmHg averaged 15.2 ± 1.2 μm and 12.2 ± 1.0 μm for control and LPS groups (P < 0.05) respectively. When perfusion pressure was increased in 15 mmHg increments from 65 to 170 mmHg, arteriole diameter in control kidneys decreased significantly to 69 ± 6% of baseline diameter. In the LPS-treated group, arteriole diameter remained essentially unchanged (103 ± 9% of baseline), indicating impaired autoregulatory behaviour. Pre-treatment with anti-TLR4 antibody or the TLR4 antagonist, LPS-RS, preserved autoregulatory behaviour during LPS treatment. P2 receptor reactivity was normal in control and LPS-treated rats. Pre-treatment with Losartan (angiotensin type 1 receptor blocker; (AT₁ ) 2 mg kg^-1 ; i.p.) increased baseline afferent arteriole diameter but did not preserve autoregulatory behaviour in LPS-treated rats. Acute exposure to Tempol (10^-3 mol L^-1 ), a superoxide dismutase mimetic, restored pressure-mediated vasoconstriction in kidneys from LPS-treated rats.

CONCLUSION

These data demonstrate that TLR4 activation impairs afferent arteriole autoregulatory behaviour, partially through ROS, but independently of P2 and AT₁ receptor activation.

Superoxide and hydrogen peroxide counterregulate myogenic contractions in renal afferent arterioles from a mouse model of chronic kidney disease

Myogenic contractions protect kidneys from barotrauma but are impaired in chronic kidney disease (CKD). Since myogenic contractions are enhanced by superoxide but impaired by hydrogen peroxide, we tested the hypothesis that they are counterregulated by superoxide and H2O2 from NOX2/p47phox and/or NOX4/POLDIP2 in CKD. Myogenic contraction in isolated perfused afferent arterioles from mice with surgical 5/6 nephrectomy or sham operations fed a 6% sodium chloride diet was measured directly while superoxide and H2O2 were measured by fluorescence microscopy. Compared to sham-operated animals, an increase in perfusion pressure of arterioles from CKD mice doubled superoxide (21 versus 11%), increased H2O2 seven-fold (29 versus 4%), and reduced myogenic contractions profoundly (-1 versus -14%). Myogenic contractions were impaired further by PEG-superoxide dismutase or in arterioles from p47phox-/- (versus wild type) mice but became supra-normal by PEG-catalase or in mice with transgenic expression of catalase in vascular smooth muscle cells (-11 versus -1%). Single arterioles from mice with CKD expressed over 40% more mRNA and protein for NOX4 and POLDIP2. Myogenic responses in arterioles from POLDIP2 +/- (versus wild type) mice with CKD had over an 85% reduction in H2O2, but preserved superoxide and a normal myogenic response. Tempol administration to CKD mice for 3 months decreased afferent arteriolar superoxide and H2O2 and maintained myogenic contractions. Thus, afferent arteriolar superoxide generated by NOX2/p47phox opposes H2O2 generated by NOX4/POLDIP2 whose upregulation in afferent arterioles from mice with CKD accounts for impaired myogenic contractions.

RACK1 regulates angiotensin II-induced contractions of SHR preglomerular vascular smooth muscle cells

The preglomerular microcirculation of spontaneously hypertensive rats (SHR) is hypersensitive to angiotensin (ANG) II, and studies have shown that this is likely due to enhanced coincident signaling between G protein subunits αq (Gαq; released by ANG II) and βγ (Gβγ; released by Gi-coupled receptors) to active phospholipase C (PLC). Here we investigated the molecular basis for the enhanced coincident signaling between Gβγ and Gαq in SHR preglomerular vascular smooth muscle cells (PGVSMCs). Because receptor for activated C kinase 1 (RACK1; a scaffolding protein) organizes interactions between Gβγ, Gαq, and PLC, we included RACK1 in this investigation. Cell fractionation studies demonstrated increased levels of membrane (but not cytosolic) Gβ, Gαq, PLCβ3, and RACK1 in SHR PGVSMCs compared with Wistar-Kyoto rat PGVSMCs. In SHR PGVSMCs, coimmunoprecipitation demonstrated RACK1 binding to Gβ and PLCβ3, but only at cell membranes. Pertussis toxin (which blocks Gβγ) and U73122 (which blocks PLC) reduced membrane RACK1; however, RACK1 knockdown (shRNA) did not affect membrane levels of Gβ, Gαq, or PLCβ3 In a novel gel contraction assay, RACK1 knockdown in SHR PGVSMCs attenuated contractions to ANG II and abrogated the ability of neuropeptide Y (which signals via Gβγ) to enhance ANG II-induced contractions. We conclude that in SHR PGVSMCs the enlarged pool of Gβγ and PLCβ3 recruits RACK1 to membranes and RACK1 then organizes signaling. Consequently, knockdown of RACK1 prevents coincident signaling between ANG II and the Gi pathway. This is the first study to implicate RACK1 in vascular smooth muscle cell contraction and suggests that RACK1 inhibitors could be effective cardiovascular drugs.

High-salt diet induces outward remodelling of efferent arterioles in mice with reduced renal mass

AIM

The glomerular filtration rate (GFR) falls progressively in chronic kidney disease (CKD) which is caused by a reduction in the number of functional nephrons. The dysfunctional nephron exhibits a lower glomerular capillary pressure that is induced by an unbalance between afferent and efferent arteriole. Therefore, we tested the hypothesis that oxidative stress induced by CKD differentially impairs the structure or function of efferent vs. afferent arterioles.

METHODS

C57BL/6 mice received sham operations (sham) or 5/6 nephrectomy (RRM) and three months of normal- or high-salt diet or tempol. GFR was assessed from the plasma inulin clearance, arteriolar remodelling from media/lumen area ratio, myogenic responses from changes in luminal diameter with increases in perfusion pressure and passive wall compliance from the wall stress/strain relationships.

RESULTS

Mice with RRM fed a high salt (vs. sham) had a lower GFR (553 ± 25 vs. 758 ± 36 μL min^-1  g^-1 kidney, P < 0.01) and a larger efferent arteriolar diameter (9.6 ± 0.8 vs. 7.4 ± 0.7 μm, P < 0.05) resulting in a lower media/lumen area ratio (1.4 ± 0.1 vs. 2.4 ± 0.2, P < 0.01). These alterations were corrected by tempol. The myogenic responses of efferent arterioles were about one-half that of afferent arterioles and were unaffected by RRM or salt. Passive wall compliance was reduced by high salt in both afferent and efferent arterioles.

CONCLUSION

A reduction in renal mass with a high-salt diet induces oxidative stress that leads to an outward eutrophic remodelling in efferent arterioles and reduced wall compliance in both afferent and efferent arterioles. This may contribute to the lower GFR in this model of CKD.

RGS2 squelches vascular Gi/o and Gq signaling to modulate myogenic tone and promote uterine blood flow

Uterine artery blood flow (UABF) is critical to maintaining uterine perfusion in nonpregnant states and for uteroplacental delivery of nutrients and oxygen to the fetus during pregnancy. Impaired UABF is implicated in infertility and several pregnancy complications including fetal growth restriction, small for gestational age, and preeclampsia. The etiology of abnormal UABF is not known. Here, we determined whether deficiency or loss of RGS2, a GTPase-activating protein for Gq/11 and Gi/o class G proteins, affects UABF in nonpregnant mice. We used Doppler ultrasonography to assess UABF in wild type (WT), Rgs2 heterozygous (Rgs2+/-), and homozygous knockout (Rgs2-/-) mice. Video microscopy was used for ex vivo examination of uterine artery myogenic tone and fura-2 imaging for in vitro assessment of internal stores Ca(2+) release. We found that baseline UABF velocity was markedly decreased while impedance measured as resistive index (WT = 0.58 ± 0.04 vs. Rgs2-/- = 0.71 ± 0.03, P < 0.01) and pulsatile index (WT = 0.90 ± 0.06 vs. Rgs2-/- = 1.25 ± 0.11, P < 0.01) was increased in Rgs2-/- mice. Uterine artery tone was augmented in Rgs2+/- and Rgs2-/- mice, which was normalized to WT levels following Gi/o and Gq inactivation. Conversely, blockade of ryanodine receptors increased WT myogenic tone to RGS2 mutant levels. The data together indicate that RGS2 deficiency decreases UABF by increasing myogenic tone at least partly through prolonged G protein activation. Mutations that decrease vascular RGS2 expression may be a predisposition to decreased uterine blood flow. Targeting G protein signaling therefore might improve uterine and uteroplacental underperfusion disorders.

Attenuated flow-induced dilatation of middle cerebral arteries is related to increased vascular oxidative stress in rats on a short-term high salt diet

KEY POINTS

Recent studies have shown that high salt (HS) intake leads to endothelial dysfunction and impaired vascular reactivity in different vascular beds in both animal and human models, due to increased oxidative stress. The objective of this study was to assess vascular response to flow-induced dilatation (FID) and to elucidate the role of vascular oxidative stress/antioxidative capacity in middle cerebral arteries (MCAs) of HS-fed rats in vitro. The novelty of this study is in demonstrating impaired flow-induced dilatation of MCAs and down-regulation of vascular antioxidant genes with HS intake, leading to increased levels of oxidative stress in blood vessels and peripheral lymph organs, which together contribute to impaired FID. In addition, results show increased oxidative stress in leukocytes of peripheral lymph organs, suggesting the occurrence of inflammatory processes due to HS intake. Recirculation of leukocytes might additionally increase vascular oxidative stress in vivo.

ABSTRACT

The aim of this study was to determine flow-induced dilatation (FID) and the role of oxidative stress/antioxidative capacity in isolated, pressurized middle cerebral arteries (MCAs) of high salt (HS)-fed rats. Healthy male Sprague-Dawley rats (11 weeks old) were fed low salt (0.4% NaCl; LS group) or high salt (4% NaCl; HS group) diets for 1 week. Reactivity of MCAs in response to stepwise increases in pressure gradient (Δ10-Δ100 mmHg) was determined in the absence or presence of the superoxide dismutase (SOD) mimetic TEMPOL and/or the nitric oxide synthases (NOS) inhibitor N(ω) -nitro-l-arginine methyl ester (l-NAME). mRNA levels of antioxidative enzymes, NAPDH-oxidase components, inducible (iNOS) and endothelial nitric oxide synthases (eNOS) were determined by quantitative real-time PCR. Blood pressure (BP), antioxidant enzymes activity, oxidative stress in peripheral leukocytes, lipid peroxidation products and the antioxidant capacity of plasma were measured for both groups. FID was reduced in the HS group compared to the LS group. The presence of TEMPOL restored dilatation in the HS group, with no effect in the LS group. Expression of glutathione peroxidase 4 (GPx4) and iNOS in the HS group was significantly decreased; oxidative stress was significantly higher in the HS group compared to the LS group. HS intake significantly induced basal reactive oxygen species production in the leukocytes of mesenteric lymph nodes and splenocytes, and intracellular production after stimulation in peripheral lymph nodes. Antioxidant enzyme activity and BP were not affected by HS diet. Low GPx4 expression, increased superoxide production in leukocytes, and decreased iNOS expression are likely to underlie increased oxidative stress and reduced nitric oxide bioavailability, leading to impairment of FID in the HS group without changes in BP values.

Differential effects of superoxide and hydrogen peroxide on myogenic signaling, membrane potential, and contractions of mouse renal afferent arterioles

Myogenic contraction is the principal component of renal autoregulation that protects the kidney from hypertensive barotrauma. Contractions are initiated by a rise in perfusion pressure that signals a reduction in membrane potential (Em) of vascular smooth muscle cells to activate voltage-operated Ca(2+) channels. Since ROS have variable effects on myogenic tone, we investigated the hypothesis that superoxide (O2 (·-)) and H2O2 differentially impact myogenic contractions. The myogenic contractions of mouse isolated and perfused single afferent arterioles were assessed from changes in luminal diameter with increasing perfusion pressure (40-80 mmHg). O2 (·-), H2O2, and Em were assessed by fluorescence microscopy during incubation with paraquat to increase O2 (·-) or with H2O2 Paraquat enhanced O2 (·-) generation and myogenic contractions (-42 ± 4% vs. -19 ± 4%, P < 0.005) that were blocked by SOD but not by catalase and signaled via PKC. In contrast, H2O2 inhibited the effects of paraquat and reduced myogenic contractions (-10 ± 1% vs. -19 ± 2%, P < 0.005) and signaled via PKG. O2 (·-) activated Ca(2+)-activated Cl(-) channels that reduced Em, whereas H2O2 activated Ca(2+)-activated and voltage-gated K(+) channels that increased Em Blockade of voltage-operated Ca(2+) channels prevented the enhanced myogenic contractions with paraquat without preventing the reduction in Em Myogenic contractions were independent of the endothelium and largely independent of nitric oxide. We conclude that O2 (·-) and H2O2 activate different signaling pathways in vascular smooth muscle cells linked to discreet membrane channels with opposite effects on Em and voltage-operated Ca(2+) channels and therefore have opposite effects on myogenic contractions.

Remodeling of Afferent Arterioles From Mice With Oxidative Stress Does Not Account for Increased Contractility but Does Limit Excessive Wall Stress

Because superoxide dismutase (SOD) knockout enhances arteriolar remodeling and contractility, we hypothesized that remodeling enhances contractility. In the isolated and perfused renal afferent arterioles from SOD wild type (+/+) and gene-deleted mice, contractility was assessed from reductions in luminal diameter with perfusion pressure from 40 to 80 mm Hg (myogenic responses) or angiotensin II (10(-6) mol/L), remodeling from media:lumen area ratio, superoxide (O2 (·-)) and hydrogen peroxide (H2O2) from fluorescence microscopy, and wall stress from wall tension/wall thickness. Compared with +/+ strains, arterioles from SOD1-/-, SOD2+/-, and SOD3-/- mice developed significantly (P<0.05) more O2 (·-) with perfusion pressure and angiotensin II and significantly increased myogenic responses (SOD1-/-: -20.7±2.2% versus -12.7±1.6%; SOD2+/-: -7.4±1.3% versus -12.6±1.4%; and SOD3-/-: -9.1±1.9% versus -15.8±2.2%) and angiotensin II contractions and ≈2-fold increased media:lumen ratios. Media:lumen ratios correlated with myogenic responses (r(2) =0.23; P<0.01), angiotensin II contractions (r(2)=0.57; P<0.0001), and active wall tension (r(2) =0.19; P<0.01), but not with active wall stress (r(2)=0.08; NS). Differences in myogenic responses among SOD3 mice were abolished by bath addition of SOD and were increased 3 days after inducing SOD3 knockout (-26.9±1.7% versus -20.1±0.7%; P<0.05), despite unchanged media:lumen ratios (2.01±0.09 versus 2.02±0.03; NS). We conclude that cytosolic, mitochondrial, or extracellular O2 (·-) enhance afferent arteriolar contractility and remodeling. Although remodeling does not enhance contractility, it does prevent the potentially damaging effects of increased wall stress.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Thromboxane prostanoid receptors enhance contractions, endothelin-1, and oxidative stress in microvessels from mice with chronic kidney disease

Cardiovascular disease is frequent in chronic kidney disease and has been related to angiotensin II, endothelin-1 (ET-1), thromboxane A2, and reactive oxygen species (ROS). Because activation of thromboxane prostanoid receptors (TP-Rs) can generate ROS, which can generate ET-1, we tested the hypothesis that chronic kidney disease induces cyclooxygenase-2 whose products activate TP-Rs to enhance ET-1 and ROS generation and contractions. Mesenteric resistance arterioles were isolated from C57/BL6 or TP-R+/+ and TP-R-/- mice 3 months after SHAM-operation (SHAM) or surgical reduced renal mass (RRM, n=6/group). Microvascular contractions were studied on a wire myograph. Cellular (ethidium: dihydroethidium) and mitochondrial (mitoSOX) ROS were measured by fluorescence microscopy. Mice with RRM had increased excretion of markers of oxidative stress, thromboxane, and microalbumin; increased plasma ET-1; and increased microvascular expression of p22(phox), cyclooxygenase-2, TP-Rs, preproendothelin and endothelin-A receptors, and increased arteriolar remodeling. They had increased contractions to U-46,619 (118 ± 3 versus 87 ± 6, P<0.05) and ET-1 (108 ± 5 versus 89 ± 4, P<0.05), which were dependent on cellular and mitochondrial ROS, cyclooxygenase-2, and TP-Rs. RRM doubled the ET-1-induced cellular and mitochondrial ROS generation (P<0.05). TP-R-/- mice with RRM lacked these abnormal structural and functional microvascular responses and lacked the increased systemic and the increased microvascular oxidative stress and circulating ET-1. In conclusion, RRM leads to microvascular remodeling and enhanced ET-1-induced cellular and mitochondrial ROS and contractions that are mediated by cyclooxygenase-2 products activating TP-Rs. Thus, TP-Rs can be upstream from enhanced ROS, ET-1, microvascular remodeling, and contractility and may thereby coordinate vascular dysfunction in chronic kidney disease.

High-salt diet blunts renal autoregulation by a reactive oxygen species-dependent mechanism

High dietary salt is common in Western countries and is an important contributor to increased cardiovascular disease. Autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR) is an essential function of the renal microcirculation that could be affected by excessive dietary salt. High salt (HS) increases renal ROS generation partly by the enzyme NADPH oxidase. We hypothesized that a HS diet would impair autoregulation via NADPH oxidase-dependent ROS generation. The role of NADPH-dependent ROS production on the blunted autoregulatory response with a HS diet was assessed in vitro and in vivo using the blood-perfused juxtamedullary nephron preparation and anesthetized rats, respectively. The increase in renal lipid peroxidation and p67(phox) expression induced by HS was prevented by apocynin treatment. Control afferent arterioles exhibited normal autoregulatory behavior in response to acute increases in renal perfusion pressure, whereas arterioles from HS rats exhibited a blunted response. Autoregulatory behavior in HS rats was restored in vitro by acute exposure to the NADPH oxidase inhibitor apocynin. At the whole kidney level, in vivo experiments showed that both RBF and GFR declined in HS rats when left kidney renal perfusion pressure was reduced from ambient to 95 mmHg, whereas control rats maintained stable GFR and RBF consistent with efficient autoregulatory behavior. Apocynin treatment improved in vivo autoregulatory behavior in HS rats and had no detectable effect in normal salt diet-fed rats. These data support the hypothesis that impaired renal autoregulatory behavior in rats fed a HS diet is mediated by NADPH oxidase-derived ROS.

L-carnosine alters some hemorheologic and lipid peroxidation parameters in nephrectomized rats

Background

Chronic kidney disease (CKD) is a major health problem worldwide. Oxidative stress is one of the mediators of this disease. Systemic complications of oxidative stress are involved in the pathogenesis of hypertension, endothelial dysfunction, shortened erythrocyte lifespan, deformability, and nitric oxide (NO) dysfunction. L-carnosine is known as an antioxidant. In this study, our aim was to investigate the effect of carnosine on hemorheologic and cardiovascular parameters in CKD-induced rats.

Material/Methods

We used 4-month-old male Sprague-Dawley rats divided into 4 groups of 6 rats each. Three days after subtotal nephrectomy and sham operations, the surviving rats were divided into the 4 groups; 1) Sham (S), 2) Sham+Carnosine (S-C), 3) Subtotal nephrectomy (Nx), and 4) Subtotal nephrectomy + Carnosine (N-C). Carnosine was injected intraperitoneally (i.p.) (50 mg/kg) for 15 days. The control group received the same volume of physiological saline.

Results

In CKD rats, malondialdehyde (MDA) levels were increased, and NO and RBC deformability were decreased compared to Sham. Carnosine treatment decreased MDA levels, improved RBC (red blood cell) ability to deform, and increased NO levels. However, carnosine did not affect blood pressure levels in these rats.

Conclusions

We found that carnosine has beneficial effects on CKD in terms of lipid peroxidation and RBC deformability. Carnosine may have a healing effect in microcirculation level, but may not have any effect on systemic blood pressure in CKD-induced rats.

Oxidative stress in hypertension: role of the kidney

SIGNIFICANCE

Renal oxidative stress can be a cause, a consequence, or more often a potentiating factor for hypertension. Increased reactive oxygen species (ROS) in the kidney have been reported in multiple models of hypertension and related to renal vasoconstriction and alterations of renal function. Nicotinamide adenine dinucleotide phosphate oxidase is the central source of ROS in the hypertensive kidney, but a defective antioxidant system also can contribute.

RECENT ADVANCES

Superoxide has been identified as the principal ROS implicated for vascular and tubular dysfunction, but hydrogen peroxide (H2O2) has been implicated in diminishing preglomerular vascular reactivity, and promoting medullary blood flow and pressure natriuresis in hypertensive animals.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Increased renal ROS have been implicated in renal vasoconstriction, renin release, activation of renal afferent nerves, augmented contraction, and myogenic responses of afferent arterioles, enhanced tubuloglomerular feedback, dysfunction of glomerular cells, and proteinuria. Inhibition of ROS with antioxidants, superoxide dismutase mimetics, or blockers of the renin-angiotensin-aldosterone system or genetic deletion of one of the components of the signaling cascade often attenuates or delays the onset of hypertension and preserves the renal structure and function. Novel approaches are required to dampen the renal oxidative stress pathways to reduced O2(-•) rather than H2O2 selectivity and/or to enhance the endogenous antioxidant pathways to susceptible subjects to prevent the development and renal-damaging effects of hypertension.

Body weight telemetry is useful to reduce interdialytic weight gain in patients with end-stage renal failure on hemodialysis

Lacking compliance with liquid intake restrictions is one of the major problems in patients on hemodialysis and causes an increased mortality. In 120 patients on hemodialysis with an average interdialytic weight gain (IWG) exceeding 1.5 kg on at least 2 days during the 4 weeks preceding the intervention, the effect of telemetric body weight measurement (TBWM) on IWG, ultrafiltration rate, and blood pressure was evaluated over a period of 3 months. Patients of the telemetric group (TG) were supplied with automatic scales, which transferred the weight via telemetry on a daily basis. In the case of IWG of more than 0.75 kg/24 h, a telephonic contact was made as required, and in the case of an IWG of more than 1.5 kg, telephonic contacting was obligatory along with the advice of a liquid intake restriction to 0.5 L/day until the next dialysis. The patients of the control group (CG) received standard treatment without telemetric monitoring. We examined specific data of the second interdialytic interval (IDI2) and the average within 1 week. The average difference of IWG between TG and CG was not significant before the start of the study but 0.2 kg (p=0.027) (IDI2)/0.27kg (p=0.001) (WP) at the end of the study, respectively. The average difference in the ultrafiltration rate within 1 week was 19.0 mL/h (p=0.282) (IDI2)/8.2 mL/h (p=0.409) before the start of the study but 28.4 mL/h (p=0.122) (IDI2)/30.9 mL/h (p=0.004) at the end of the study, respectively. Thus, TBWM is a feasible method for optimizing the IWG and reducing the ultrafiltration rate.

Autoregulation and mechanotransduction control the arteriolar response to small changes in hematocrit

Here, we present an analytic model of arteriolar mechanics that accounts for key autoregulation mechanisms, including the myogenic response and the vasodilatory effects of nitric oxide (NO) in the vasculature. It couples the fluid mechanics of blood flow in arterioles with solid mechanics of the vessel wall and includes the effects of wall shear stress- and stretch-induced endothelial NO production. The model can be used to describe the regulation of blood flow and NO transport under small changes in hematocrit and to analyze the regulatory response of arterioles to small changes in hematocrit. Our analysis revealed that the experimentally observed paradoxical increase in cardiac output with small increases in hematocrit results from the combination of increased NO production and the effects of a strong myogenic response modulated by elevated levels of WSS. Our findings support the hypothesis that vascular resistance varies inversely with blood viscosity for small changes in hematocrit in a healthy circulation that responds to shear stress stimuli. They also suggest beneficial effects independent of changes in O(2) carrying capacity associated with the postsurgical transfusion of one or two units of blood.

Effects of the antioxidant drug tempol on renal oxygenation in mice with reduced renal mass

We tested the hypothesis that reactive oxygen species (ROS) contributed to renal hypoxia in C57BL/6 mice with &frac56; surgical reduction of renal mass (RRM). ROS can activate the mitochondrial uncoupling protein 2 (UCP-2) and increase O(2) usage. However, UCP-2 can be inactivated by glutathionylation. Mice were fed normal (NS)- or high-salt (HS) diets, and HS mice received the antioxidant drug tempol or vehicle for 3 mo. Since salt intake did not affect the tubular Na(+) transport per O(2) consumed (T(Na/)Q(O2)), further studies were confined to HS mice. RRM mice had increased excretion of 8-isoprostane F(2α) and H(2)O(2), renal expression of UCP-2 and renal O(2) extraction, and reduced T(Na/)Q(O2) (sham: 20 ± 2 vs. RRM: 10 ± 1 μmol/μmol; P < 0.05) and cortical Po(2) (sham: 43 ± 2, RRM: 29 ± 2 mmHg; P < 0.02). Tempol normalized all these parameters while further increasing compensatory renal growth and glomerular volume. RRM mice had preserved blood pressure, glomeruli, and patchy tubulointerstitial fibrosis. The patterns of protein expression in the renal cortex suggested that RRM kidneys had increased ROS from upregulated p22(phox), NOX-2, and -4 and that ROS-dependent increases in UCP-2 led to hypoxia that activated transforming growth factor-β whereas erythroid-related factor 2 (Nrf-2), glutathione peroxidase-1, and glutathione-S-transferase mu-1 were upregulated independently of ROS. We conclude that RRM activated distinct processes: a ROS-dependent activation of UCP-2 leading to inefficient renal O(2) usage and cortical hypoxia that was offset by Nrf-2-dependent glutathionylation. Thus hypoxia in RRM may be the outcome of NADPH oxidase-initiated ROS generation, leading to mitochondrial uncoupling counteracted by defense pathways coordinated by Nrf-2.

p47(phox) is required for afferent arteriolar contractile responses to angiotensin II and perfusion pressure in mice

Myogenic and angiotensin contractions of afferent arterioles generate reactive oxygen species. Resistance vessels express neutrophil oxidase-2 and -4. Angiotensin II activates p47(phox)/neutrophil oxidase-2, whereas it downregulates NOX-4. Therefore, we tested the hypothesis that p47(phox) enhances afferent arteriolar angiotensin contractions. Angiotensin II infusion in p47(phox) +/+ but not -/- mice increased renal cortical NADPH oxidase activity (7±1-12±1 [P<0.01] versus 5±1-7±1 10(3) · RLU · min(-1) · μg protein(-1) [P value not significant]), mean arterial pressure (77±2-91±2 [P<0.005] versus 74±2-77±1 mm Hg [P value not significant]), and renal vascular resistance (7.5±0.4-10.1±0.7 [P<0.01] versus 7.9±0.4-8.3±0.4 mm Hg/mL · min(-1) · gram kidney weight(-1) [P value not significant]). Afferent arterioles from p47(phox) -/- mice had a lesser myogenic response (3.1±0.4 versus 1.4±0.2 dynes · cm(-1) · mm Hg(-1); P<0.02) and a lesser (P<0.05) contraction to 10(-6) M angiotensin II (diameter change +/+: 9.3±0.2-3.4±0.6 μm versus -/-: 9.9±0.6-7.5±0.4 μm). Angiotensin and increased perfusion pressure generated significantly (P<0.05) more reactive oxygen species in p47(phox) +/+ than -/- arterioles. Angiotensin II infusion increased the maximum responsiveness of afferent arterioles from p47(phox) +/+ mice to 10(-6) M angiotensin II yet decreased the response in p47(phox) -/- mice. The angiotensin infusion increased the sensitivity to angiotensin II only in p47(phox) +/+ mice. We conclude that p47(phox) is required to enhance renal NADPH oxidase activity and basal afferent arteriolar myogenic and angiotensin II contractions and to switch afferent arteriolar tachyphylaxis to sensitization to angiotensin during a prolonged angiotensin infusion. These effects likely contribute to hypertension and renal vasoconstriction during infusion of angiotensin II.

Superoxide modulates myogenic contractions of mouse afferent arterioles

Reactive oxygen species enhance or impair autoregulation. Because superoxide is a vasoconstrictor, we tested the hypothesis that stretch generates superoxide that mediates myogenic responses. Increasing perfusion pressure of mouse isolated perfused renal afferent arterioles from 40 to 80 mm Hg reduced their diameter by 13.3±1.8% (P<0.001) and increased reactive oxygen species (ethidium: dihydroethidium fluorescence) by 9.8±2.3% (P<0.05). Stretch-induced fluorescence was reduced significantly (P<0.05) by incubation with Tempol (3.7±0.8%), pegylated superoxide dismutase (3.2±1.0%), or apocynin (3.5±0.9%) but not by pegylated catalase, L-nitroarginine methylester, or Ca(2+)-free medium, relating it to Ca(2+)-independent vascular superoxide. Compared with vehicle, basal tone and myogenic contractions were reduced significantly (P<0.05) by pegylated superoxide dismutase (5.4±0.8), Tempol (4.1±1.0%), apocynin (1.0±1.3%), and diphenyleneiodinium (3.9±0.9%) but not by pegylated catalase (10.1±1.6%). L-Nitroarginine methylester enhanced basal tone, but neither it (15.8±3.3%) nor endothelial NO synthase knockout (10.2±1.8%) significantly changed myogenic contractions. Tempol had no further effect after superoxide dismutase but remained effective after catalase. H(2)O(2) >50 μmol/L caused contractions but at 25 μmol/L inhibited myogenic responses (7.4±0.8%; P<0.01). In conclusion, increasing the pressure within afferent arterioles led to Ca(2+)-independent increased vascular superoxide production from nicotinamide adenine dinucleotide phosphate oxidase, which enhanced myogenic contractions largely independent of NO, whereas H(2)O(2) impaired pressure-induced contractions but was not implicated in the normal myogenic response.

P2X1 receptor-mediated vasoconstriction of afferent arterioles in angiotensin II-infused hypertensive rats fed a high-salt diet

Experiments tested the hypothesis that P2 receptor reactivity is impaired in angiotensin (Ang) II hypertensive rats fed an 8%NaCl diet (Ang II+HS). Juxtamedullary afferent arteriolar autoregulatory behavior was determined over a pressure range of 65 to 200 mm Hg. Arteriolar responsiveness to P2X1 (β,γ-methylene ATP) or P2Y2 receptor (uridine triphosphate) activation was determined in vitro. Systolic blood pressure averaged 126±3 and 225±4 mm Hg in control and Ang II+HS rats, respectively (P<0.05). In control kidneys, β,γ-methylene ATP (10(-8) to 10(-4) mol/L) reduced arteriolar diameter by 8±3%, 13±5%, 19±5%, 22±6%, and 24±9%, respectively, whereas uridine triphosphate reduced diameter by 2±1%, 2±2%, 9±3%, 37±7%, and 58±7%. Autoregulation was markedly blunted in Ang II+HS kidneys, with arteriolar diameter remaining essentially unchanged when perfusion pressure increased to 200 mm Hg compared with a 40±2% decline in diameter observed in normal kidneys over the same pressure range (P<0.05). P2X1 receptor-mediated vasoconstriction was significantly attenuated in Ang II+HS kidneys. β,γ-Methylene ATP reduced arteriolar diameter by 1±1%, 3±2%, 6±1%, 9±3%, and 7±1%, respectively (P<0.05), versus control rats. Similar patterns were noted when hypertensive perfusion pressures were used. Uridine triphosphate-mediated responses were unchanged in Ang II+HS rats compared with control, indicating preservation of P2Y2 receptor function. Ang II+HS blunted P2X1-mediated increases in intracellular Ca2+ concentration in preglomerular smooth muscle cells. Therefore, Ang II+HS rats exhibit attenuated afferent arteriolar responses to P2X1 receptor stimulation. These data support the hypothesis that P2X1 receptors are important for pressure-mediated autoregulatory responses. Impairment of P2X1 receptor function may explain the hypertension-induced decline in renal autoregulatory capability.