Impaired autoregulation of renal blood flow in the fawn-hooded rat

Changes in Proximal Tubular Reabsorption Modulate Microvascular Regulation via the TGF System

This review paper considers the consequences of modulating tubular reabsorption proximal to the macula densa by sodium–glucose co-transporter 2 (SGLT2) inhibitors, acetazolamide, and furosemide in states of glomerular hyperfiltration. SGLT2 inhibitors improve renal function in early and advanced diabetic nephropathy by decreasing the glomerular filtration rate (GFR), presumably by activating the tubuloglomerular feedback (TGF) mechanism. Central in this paper is that the renoprotective effects of SGLT2 inhibitors in diabetic nephropathy can only be partially explained by TGF activation, and there are alternative explanations. The sustained activation of TGF leans on two prerequisites: no or only partial adaptation should occur in reabsorption proximal to macula densa, and no or only partial adaptation should occur in the TGF response. The main proximal tubular and loop of Henle sodium transporters are sodium–hydrogen exchanger 3 (NHE3), SGLT2, and the Na-K-2Cl co-transporter (NKCC2). SGLT2 inhibitors, acetazolamide, and furosemide are the most important compounds; inhibiting these transporters would decrease sodium reabsorption upstream of the macula densa and increase TGF activity. This could directly or indirectly affect TGF responsiveness, which could oppose sustained TGF activation. Only SGLT2 inhibitors can sustainably activate the TGF as there is only partial compensation in tubular reabsorption and TGF response. SGLT2 inhibitors have been shown to preserve GFR in both early and advanced diabetic nephropathy. Other than for early diabetic nephropathy, a solid physiological basis for these effects in advanced nephropathy is lacking. In addition, TGF has hardly been studied in humans, and therefore this role of TGF remains elusive. This review also considers alternative explanations for the renoprotective effects of SGLT2 inhibitors in diabetic patients such as the enhancement of microvascular network function. Furthermore, combination use of SGLT2 inhibitors and angiotensin-converting enzyme inhibitors (ACEi) or angiotensin receptor blockers (ARBs). in diabetes can decrease inflammatory pathways, improve renal oxygenation, and delay the progression of diabetic nephropathy.

The adducin saga: pleiotropic genomic targets for precision medicine in human hypertension-vascular, renal, and cognitive diseases

Hypertension is a leading risk factor for stroke, heart disease, chronic kidney disease, vascular cognitive impairment, and Alzheimer's disease. Previous genetic studies have nominated hundreds of genes linked to hypertension, and renal and cognitive diseases. Some have been advanced as candidate genes by showing that they can alter blood pressure or renal and cerebral vascular function in knockout animals; however, final validation of the causal variants and underlying mechanisms has remained elusive. This review chronicles 40 years of work, from the initial identification of adducin (ADD) as an ACTIN-binding protein suggested to increase blood pressure in Milan hypertensive rats, to the discovery of a mutation in ADD1 as a candidate gene for hypertension in rats that were subsequently linked to hypertension in man. More recently, a recessive K572Q mutation in ADD3 was identified in Fawn-Hooded Hypertensive (FHH) and Milan Normotensive (MNS) rats that develop renal disease, which is absent in resistant strains. ADD3 dimerizes with ADD1 to form functional ADD protein. The mutation in ADD3 disrupts a critical ACTIN-binding site necessary for its interactions with actin and spectrin to regulate the cytoskeleton. Studies using Add3 KO and transgenic strains, as well as a genetic complementation study in FHH and MNS rats, confirmed that the K572Q mutation in ADD3 plays a causal role in altering the myogenic response and autoregulation of renal and cerebral blood flow, resulting in increased susceptibility to hypertension-induced renal disease and cerebral vascular and cognitive dysfunction.

What Evolutionary Evidence Implies About the Identity of the Mechanoelectrical Couplers in Vascular Smooth Muscle Cells

Loss of pressure-induced vasoconstriction increases susceptibility to renal and cerebral vascular injury. Favored paradigms underlying initiation of the response include transient receptor potential channels coupled to G protein-coupled receptors or integrins as transducers. Degenerin channels may also mediate the response. This review addresses the 1) evolutionary role of these molecules in mechanosensing, 2) limitations to identifying mechanosensitive molecules, and 3) paradigm shifting molecular model for a VSMC mechanosensor.

Knockout of γ-Adducin Promotes NG-Nitro-L-Arginine-Methyl-Ester-Induced Hypertensive Renal Injury

Previous studies identified a region on chromosome 1 associated with NG-nitro-L-arginine methyl ester (L-NAME) hypertension-induced renal disease in fawn-hooded hypertensive (FHH) rats. This region contains a mutant γ-adducin (Add3) gene that impairs renal blood flow (RBF) autoregulation, but its contribution to renal injury is unknown. The present study evaluated the hypothesis that knockout (KO) of Add3 impairs the renal vasoconstrictor response to the blockade of nitric oxide synthase and enhances hypertension-induced renal injury after chronic administration of L-NAME plus a high-salt diet. The acute hemodynamic effect of L-NAME and its chronic effects on hypertension and renal injury were compared in FHH 1Brown Norway (FHH 1BN) congenic rats (WT) expressing wild-type Add3 gene versus FHH 1BN Add3 KO rats. RBF was well autoregulated in WT rats but impaired in Add3 KO rats. Acute administration of L-NAME (10 mg/kg) raised mean arterial pressure (MAP) similarly in both strains, but RBF and glomerular filtration rate (GFR) fell by 38% in WT versus 15% in Add3 KO rats. MAP increased similarly in both strains after chronic administration of L-NAME and a high-salt diet; however, proteinuria and renal injury were greater in Add3 KO rats than in WT rats. Surprisingly, RBF, GFR, and glomerular capillary pressure were 41%, 82%, and 13% higher in L-NAME-treated Add3 KO rats than in WT rats. Hypertensive Add3 KO rats exhibited greater loss of podocytes and glomerular nephrin expression and increased interstitial fibrosis than in WT rats. These findings indicate that loss of ADD3 promotes L-NAME-induced renal injury by altering renal hemodynamics and enhancing the transmission of pressure to glomeruli. SIGNIFICANCE STATEMENT: A mutation in the γ-adducin (Add3) gene in fawn-hooded hypertensive rats that impairs autoregulation of renal blood flow is in a region of rat chromosome 1 homologous to a locus on human chromosome 10 associated with diabetic nephropathy. The present results indicate that loss of ADD3 enhanced NG-nitro-L-arginine methyl ester-induced hypertensive renal injury by altering the transmission of pressure to the glomerulus.

Aging Impairs Renal Autoregulation in Mice

Impaired renal autoregulation permits more transmission of disturbance in systemic blood pressure, which initiates barotrauma in intrarenal microvasculatures such as glomerular and tubulointerstitial capillaries, contributing to the development of kidney damage and deterioration in renal function, especially under the conditions with high blood pressure. Although it has been postulated that autoregulatory efficiency is attenuated in the aging kidney, direct evidence remains lacking. In the present study, we measured the autoregulation of renal blood flow, myogenic response of afferent arteriole (Af-Art), tubuloglomerular feedback in vivo with micropuncture, as well as tubuloglomerular feedback in vitro in isolated perfused juxtaglomerular apparatus in young and aged C57BL/6 mice. We found that renal blood flow was not significantly changed in response to a defined elevation of renal arterial pressure in young mice but significantly increased in aged mice. Additionally, myogenic response of Af-Art measured by microperfusion with a stepwise increase in perfusion pressure was significantly blunted in the aging kidney, which is associated with the attenuation of intraluminal pressure-induced intracellular calcium increases, as well as the reduced expression of integrin α5 (Itga5) in Af-Art. Moreover, both tubuloglomerular feedback in vivo and in vitro were nearly inactive in the aging kidney, which is associated with the significantly reduced expression of adenosine A1 receptor (A1AR) and suppressed vasoconstrictor response to adenosine in Af-Art. In conclusion, this study demonstrates that aging impairs renal autoregulation with blunted myogenic response and inhibited tubuloglomerular feedback response. The underlying mechanisms involve the downregulations of integrin α5 and A1AR in the Af-Art.

Impaired myogenic response of the afferent arteriole contributes to the increased susceptibility to renal disease in Milan normotensive rats

Milan normotensive (MNS) rats are more susceptible to the development of renal disease than Milan hypertensive (MHS) rats, but the genes and pathways involved are unknown. This study compared the myogenic response of isolated perfused afferent arterioles (Af‐Art) and autoregulation of renal blood flow (RBF) and glomerular capillary pressure (Pgc) in 6–9‐week‐old MNS and MHS rats. The diameter of the Af‐Art of MHS rats decreased significantly from 14.3 ± 0.5 to 11.5 ± 0.6 μm when perfusion pressure was elevated from 60 to 120 mmHg. In contrast, the diameter of Af‐Art of MNS rats did not decrease. RBF was well autoregulated in MHS rats, but it increased by 26% in MNS rats. Pgc rose by 11 mmHg when renal perfusion pressure (RPP) was increased from 100 to 140 mmHg in MNS but not in MHS rats. Protein excretion increased from 10 ± 1 to 245 ± 36 mg/day in MNS rats as they aged from 3 to 11 months but it did not increase in MHS rats. We also compared the development of proteinuria in MNS and MHS rats following the induction of diabetes with streptozotocin. Protein excretion rose from 16 ± 3 to 234 ± 43 mg/day in MNS rats, but it remained unaltered in MHS rats. These data indicate that the myogenic response of the Af‐art is impaired in MNS rats and increased transmission of pressure to the glomerulus may contribute to renal injury in MNS rats similar to what is seen in fawn‐hooded hypertensive and Dahl salt‐sensitive rats.

Prevention of the progression of renal injury in diabetic rodent models with preexisting renal disease with chronic endothelin A receptor blockade

The endothelin (ET) system has emerged as a therapeutic target for the treatment of diabetic nephropathy (DN). The present study examined whether chronic endothelin A (ET_A) receptor blockade with atrasentan prevents the progression of renal injury in two models of DN with preexisting renal disease that exhibit an increased renal ET-1 system compared with nondiabetic rats: streptozotocin-treated Dahl salt-sensitive (STZ-SS) and type 2 diabetic nephropathy (T2DN) rats. Nine week-old SS rats were treated with (STZ; 50 mg/kg ip) to induce diabetes. After 3 wk of diabetes, proteinuria increased to 353 ± 34 mg/day. The rats were then separated into two groups: 1) vehicle and 2) atrasentan (5 mg·kg^-1·day^-1) via drinking water. After 6 wk of treatment with atrasentan, mean arterial pressure (MAP) and proteinuria decreased by 12 and 40%, respectively, in STZ-SS rats. The degree of glomerulosclerosis and renal fibrosis was significantly reduced in the kidneys of atrasentan-treated STZ-SS rats compared with vehicle STZ-SS rats. Interestingly, treatment with atrasentan did not affect GFR but significantly increased renal blood flow by 33% and prevented the elevations in filtration fraction and renal vascular resistance by 23 and 20%, respectively, in STZ-SS rats. In contrast to the STZ-SS study, atrasentan had no effect on MAP or proteinuria in T2DN rats. However, treatment with atrasentan significantly decreased glomerular injury and renal fibrosis and prevented the decline in renal function in T2DN rats. These data indicate that chronic ET_A blockade produces advantageous changes in renal hemodynamics that slow the progression of renal disease and also reduces renal histopathology in the absence of reducing arterial pressure and proteinuria.

Detection of TRPV4 channel current-like activity in Fawn Hooded hypertensive (FHH) rat cerebral arterial muscle cells

The transient receptor potential vallinoid type 4 (TRPV4) is a calcium entry channel known to modulate vascular function by mediating endothelium–dependent vasodilation. The present study investigated if isolated cerebral arterial myocytes of the Fawn Hooded hypertensive (FHH) rat, known to display exaggerated K_Ca channel current activity and impaired myogenic tone, express TRPV4 channels at the transcript and protein level and exhibit TRPV4-like single-channel cationic current activity. Reverse transcription polymerase chain reaction (RT-PCR), Western blot, and immunostaining analysis detected the expression of mRNA transcript and translated protein of TRPV4 channel in FHH rat cerebral arterial myocytes. Patch clamp recording of single-channel current activity identified the presence of a single-channel cationic current with unitary conductance of ~85 pS and ~96 pS at hyperpolarizing and depolarizing potentials, respectively, that was inhibited by the TRPV4 channel antagonist RN 1734 or HC 067074 and activated by the potent TRPV4 channel agonist GSK1016790A. Application of negative pressure via the interior of the patch pipette increased the NPo of the TRPV4-like single-channel cationic current recorded in cell-attached patches at a patch potential of 60 mV that was inhibited by prior application of the TRPV4 channel antagonist RN 1734 or HC 067047. Treatment with the TRPV4 channel agonist GSK1016790A caused concentration-dependent increase in the NPo of K_Ca single-channel current recorded in cell-attached patches of cerebral arterial myocytes at a patch potential of 40 mV, which was not influenced by pretreatment with the voltage-gated L-type Ca²⁺ channel blocker nifedipine or the T-type Ca²⁺ channel blocker Ni²⁺. These findings demonstrate that FHH rat cerebral arterial myocytes express mRNA transcript and translated protein for TRPV4 channel and display TRPV4-like single-channel cationic current activity that was stretch-sensitive and activation of which increased the open state probability of K_Ca single-channel current in these arterial myocytes.

Mechanical challenges to the glomerular filtration barrier: adaptations and pathway to sclerosis

Podocytes are lost as viable cells by detachment from the glomerular basement membrane (GBM), possibly due to factors such as pressure and filtrate flow. Distension of glomerular capillaries in response to increased pressure is limited by the elastic resistance of the GBM. The endothelium and podocytes adapt to changes in GBM area. The slit diaphragm (SD) seems to adjust by shuttling SD components between the SD and the adjacent foot processes (FPs), resulting in changes in SD area that parallel those in perfusion pressure.Filtrate flow tends to drag podocytes towards the urinary orifice by shear forces, which are highest within the filtration slits. The SD represents an atypical adherens junction, mechanically interconnecting the cytoskeleton of opposing FPs and tending to balance the shear forces.If under pathological conditions, increased filtrate flows locally overtax the attachment of FPs, the SDs are replaced by occluding junctions that seal the slits and the attachment of podocytes to the GBM is reinforced by FP effacement. Failure of these temporary adaptive mechanisms results in a steady process of podocyte detachment due to uncontrolled filtrate flows through bare areas of the GBM and, subsequently, the labyrinthine subpodocyte spaces, presenting as pseudocysts. In our view, shear stress due to filtrate flow-not capillary hydrostatic pressure-is the major challenge to the attachment of podocytes to the GBM.

Knockdown of Add3 impairs the myogenic response of renal afferent arterioles and middle cerebral arteries

We have reported that the myogenic response of the renal afferent arteriole (Af-art) and middle cerebral artery (MCA) and autoregulation of renal and cerebral blood flow are impaired in Fawn-Hooded Hypertensive (FHH) rats. Transfer of a region of chromosome 1 containing γ-adducin (Add3) from the Brown Norway rat rescued the vascular dysfunction and the development of renal disease. To examine whether Add3 is a viable candidate gene altering renal and cerebral hemodynamics in FHH rats, we knocked down the expression of Add3 in rat Af-arts and MCAs cultured for 36-h using a 27-mer Dicer-substrate short interfering RNA (DsiRNA). Control Af-arts constricted by 10 ± 1% in response to an elevation in pressure from 60 to 120 mmHg but dilated by 4 ± 3% when treated with Add3 DsiRNA. Add3 DsiRNA had no effect on the vasoconstrictor response of the Af-art to norepinephrine (10^-7 M). Add3 DsiRNA had a similar effect on the attenuation of the myogenic response in the MCA. Peak potassium currents were threefold higher in smooth muscle cells isolated from Af-arts or MCAs transfected with Add3 DsiRNA than in nontransfected cells isolated from the same vessels. This is the first study demonstrating that Add3 plays a role in the regulation of potassium channel function and vascular reactivity. It supports the hypothesis that sequence variants in Add3, which we previously identified in FHH rats, may play a causal role in the impaired myogenic response and autoregulation in the renal and cerebral circulation.

A gap junction inhibitor, carbenoxolone, induces spatiotemporal dispersion of renal cortical perfusion and impairs autoregulation

Renal autoregulation dynamics originating from the myogenic response (MR) and tubuloglomerular feedback (TGF) can synchronize over large regions of the kidney surface, likely through gap junction-mediated electrotonic conduction and reflecting distributed operation of autoregulation. We tested the hypotheses that inhibition of gap junctions reduces spatial synchronization of autoregulation dynamics, abrogates spatial and temporal smoothing of renal perfusion, and impairs renal autoregulation. In male Long-Evans rats, we infused the gap junction inhibitor carbenoxolone (CBX) or the related glycyrrhizic acid (GZA) that does not block gap junctions into the renal artery and monitored renal blood flow (RBF) and surface perfusion by laser speckle contrast imaging. Neither CBX nor GZA altered RBF or mean surface perfusion. CBX preferentially increased spatial and temporal variation in the distribution of surface perfusion, increased spatial variation in the operating frequencies of the MR and TGF, and reduced phase coherence of TGF and increased its dispersion. CBX, but not GZA, impaired dynamic and steady-state autoregulation. Separately, infusion of the Rho kinase inhibitor Y-27632 paralyzed smooth muscle, grossly impaired dynamic autoregulation, and monotonically increased spatial variation of surface perfusion. These data suggest CBX inhibited gap junction communication, which in turn reduced the ability of TGF to synchronize among groups of nephrons. The results indicate that impaired autoregulation resulted from degraded synchronization, rather than the reverse. We show that network behavior in the renal vasculature is necessary for effective RBF autoregulation.

Molecular mechanisms of renal blood flow autoregulation

Diabetes and hypertension are the leading causes of chronic kidney disease and their incidence is increasing at an alarming rate. Both are associated with impairments in the autoregulation of renal blood flow (RBF) and greater transmission of fluctuations in arterial pressure to the glomerular capillaries. The ability of the kidney to maintain relatively constant blood flow, glomerular filtration rate (GFR) and glomerular capillary pressure is mediated by the myogenic response of afferent arterioles working in concert with tubuloglomerular feedback that adjusts the tone of the afferent arteriole in response to changes in the delivery of sodium chloride to the macula densa. Despite intensive investigation, the factors initiating the myogenic response and the signaling pathways involved in the myogenic response and tubuloglomerular feedback remain uncertain. This review focuses on current thought regarding the molecular mechanisms underlying myogenic control of renal vascular tone, the interrelationships between the myogenic response and tubuloglomerular feedback, the evidence that alterations in autoregulation of RBF contributes to hypertension and diabetes-induced nephropathy and the identification of vascular therapeutic targets for improved renoprotection in hypertensive and diabetic patients.

P2 receptors in renal autoregulation

Autoregulation of renal blood flow and glomerular filtration rate is an essential function of the renal microcirculation. While the existence of this phenomenon has been known for many years, the exact mechanisms that underlie this regulatory system remain poorly understood. The work of many investigators has provided insights into many aspects of the autoregulatory mechanism, but many critical components remain elusive. This review is intended to update the reader on the role of P2 purinoceptors as a postulated mechanism responsible for renal autoregulatory resistance adjustments. It will summarize recent advances in normal function and it will touch on more recent ideas regarding autoregulatory insufficiency in hypertension and inflammation. Current thoughts on the nature of the mechanosensor responsible for myogenic behavior will be also be discussed as well as current thoughts on the mechanisms involved in ATP release to the extracellular fluid space.

Bleeding Diathesis in Fawn Hooded Rats-Possible Implications for Invasive Procedures and Refinement Strategies

Simple Summary

The Fawn Hooded (FH) rat population is commonly used in biomedical research. It is widely acknowledged that the FH rat has a bleeding disorder, which may lead to abundant bleeding. However, the clinical consequences of this bleeding disorder have not been described in current literature. During our study, surgical procedures on FH rats led to an unanticipated loss of animals due to abundant bleeding. Adjustments made to minimize the impact of this disorder in animals undergoing invasive procedures are described. It is strongly recommended to take the bleeding diathesis into account when performing invasive procedures in FH rats and to apply the described refinements.

Abstract

The Fawn hooded (FH) rat is commonly used in biomedical research. It is widely acknowledged that the FH rat has a bleeding disorder; leading to abundant bleedings. Although this bleeding disorder is investigated to model the storage pool defect; its impact on commonly performed invasive laboratory procedures has not yet been described. Our research group experienced clinically significant consequences of this bleeding disorder following invasive procedures (including intraperitoneal injections and neurocranial surgery) in the Rjlbm: FH stock. The clinical consequences of the surgical and anesthetic protocols applied; are described including the subsequent procedural refinements applied to minimize the impact of this disorder. It is strongly recommended to take the bleeding diathesis into account when performing invasive procedures in FH rats and to apply the suggested refinement of procedures.

Genetic susceptibility and loss of Nr4a1 enhances macrophage-mediated renal injury in CKD

Nuclear hormone receptors of the NR4A subgroup have been implicated in cancer, atherosclerosis, and metabolic disease. However, little is known about the role of these receptors in kidney health or disease. Nr4a1-deficient rats (Nr4a1(-/-)) developed on a genetic background susceptible to kidney injury (fawn-hooded hypertensive rat [FHH]) were evaluated for BP, proteinuria, renal function, and metabolic parameters from 4 to 24 weeks-of-age. By week 24, Nr4a1(-/-) rats exhibited significantly higher proteinuria (approximately 4-fold) and decreased GFR compared with FHH controls. The severity of tubular atrophy, tubular casts, and interstitial fibrosis increased significantly in Nr4a1(-/-) rats and was accompanied by a large increase in immune cell infiltration, predominantly macrophages and to a lesser extent T cells and B cells. Global transcriptome and network analyses at weeks 8, 16, and 24 identified several proinflammatory genes and pathways differentially regulated between strains. Bone marrow crosstransplantation studies demonstrated that kidney injury in Nr4a1(-/-) rats was almost completely rescued by bone marrow transplanted from FHH controls. In vitro, macrophages isolated from Nr4a1(-/-) rats demonstrated increased immune activation compared with FHH-derived macrophages. In summary, the loss of Nr4a1 in immune cells appears to cause the increased kidney injury and reduced renal function observed in the Nr4a1(-/-) model.

High-NaCl diet impairs dynamic renal blood flow autoregulation in rats with adenine-induced chronic renal failure

This study examined the effects of 2 wk of high-NaCl diet on kidney function and dynamic renal blood flow autoregulation (RBFA) in rats with adenine-induced chronic renal failure (ACRF). Male Sprague-Dawley rats received either chow containing adenine or were pair-fed an identical diet without adenine (controls). After 10 wk, rats were randomized to either remain on the same diet (0.6% NaCl) or to be switched to high 4% NaCl chow. Two weeks after randomization, renal clearance experiments were performed under isoflurane anesthesia and dynamic RBFA, baroreflex sensitivity (BRS), systolic arterial pressure variability (SAPV), and heart rate variability were assessed by spectral analytical techniques. Rats with ACRF showed marked reductions in glomerular filtration rate and renal blood flow (RBF), whereas mean arterial pressure and SAPV were significantly elevated. In addition, spontaneous BRS was reduced by ∼50% in ACRF animals. High-NaCl diet significantly increased transfer function fractional gain values between arterial pressure and RBF in the frequency range of the myogenic response (0.06–0.09 Hz) only in ACRF animals (0.3 ± 4.0 vs. −4.4 ± 3.8 dB; P < 0.05). Similarly, a high-NaCl diet significantly increased SAPV in the low-frequency range only in ACRF animals. To conclude, a 2-wk period of a high-NaCl diet in ACRF rats significantly impaired dynamic RBFA in the frequency range of the myogenic response and increased SAPV in the low-frequency range. These abnormalities may increase the susceptibility to hypertensive end-organ injury and progressive renal failure by facilitating pressure transmission to the microvasculature.

βENaC is required for whole cell mechanically gated currents in renal vascular smooth muscle cells

Myogenic constrictor responses in small renal arteries and afferent arterioles are suppressed in mice with reduced levels of β-epithelial Na⁺ channel (βENaC(m/m)). The underlying mechanism is unclear. Decreased activity of voltage-gated calcium channels (VGCC) or mechanically gated ion channels and increased activity of large conductance calcium-activated potassium (BK) channels are a few possible mechanisms. The purpose of this study was to determine if VGCC, BK, or mechanically gated ion channel activity was altered in renal vascular smooth muscle cell (VSMC) from βENaC(m/m) mice. To address this, we used whole cell patch-clamp electrophysiological approaches in freshly isolated renal VSMCs. Compared with βENaC(+/+) controls, the current-voltage relationships for VGCC and BK activity are similar in βENaC(m/m) mice. These findings suggest neither VGCC nor BK channel dysfunction accounts for reduced myogenic constriction in βENaC(m/m) mice. We then examined mechanically gated currents using a novel in vitro assay where VSMCs are mechanically activated by stretching an underlying elastomer. We found the mechanically gated currents, predominantly carried by Na⁺, are observed with less frequency (87 vs. 43%) and have smaller magnitude (-54.1 ± 12.5 vs. -20.9 ± 4.9 pA) in renal VSMCs from βENaC(m/m) mice. Residual currents are expected in this model since VSMC βENaC expression is reduced by 50%. These findings suggest βENaC is required for normal mechanically gated currents in renal VSMCs and their disruption may account for the reduced myogenic constriction in the βENaC(m/m) model. Our findings are consistent with the role of βENaC as a VSMC mechanosensor and function of evolutionarily related nematode degenerin proteins.

Comparison of the Development Diabetic Induced Renal Disease in Strains of Goto-Kakizaki Rats

This study compared temporal changes in renal hemodynamics, proteinuria and the development of renal disease in Goto-Kakizaki (GK) type II diabetic rats that are resistant to the development of diabetic nephropathy and a genetically modified GK substrain (T2DN) carrying the mitochondrial genome and other alleles from Fawn hooded-hypertensive (FHH) rats is more susceptible to the development of renal injury. Both GK and T2DN rats were diabetic (>250 mg/ dL) and blood glucose levels were not significantly different at 3, 6 and 18 months of age. Blood pressure was also similar in both strains at all 3 ages. Renal blood flow (RBF) was 45% higher in 3 month old T2DN rats than GK rats but glomerular filtration rate (GFR) was similar. T2DN rats exhibited a progressive increase in proteinuria from 41 ± 2 to 524 ± 50 mg/day and 57% fall in GFR as they aged from 3 to 18 months of age. In contrast, proteinuria only increased to 162 ± 31 mg/day in GK rats and GFR remained unaltered. The kidneys from 18 month old T2DN rats exhibited severe glomerulosclerosis, interstitial fibrosis and tubular necrosis while kidneys from GK rats did not. Plasma creatinine levels were 2.4 fold higher in 18 month old T2DN than in GK rats. These data demonstrate that T2DN rats develop most of the features of diabetic nephropathy including progressive proteinuria and chronic kidney disease whereas the closely related GK strain does not, even though blood pressure and the level of hyperglycemia are similar.

Genetic basis of the impaired renal myogenic response in FHH rats

This study examined the effect of substitution of a 2.4-megabase pair (Mbp) region of Brown Norway (BN) rat chromosome 1 (RNO1) between 258.8 and 261.2 Mbp onto the genetic background of fawn-hooded hypertensive (FHH) rats on autoregulation of renal blood flow (RBF), myogenic response of renal afferent arterioles (AF-art), K(+) channel activity in renal vascular smooth muscle cells (VSMCs), and development of proteinuria and renal injury. FHH rats exhibited poor autoregulation of RBF, while FHH.1BN congenic strains with the 2.4-Mbp BN region exhibited nearly perfect autoregulation of RBF. The diameter of AF-art from FHH rats increased in response to pressure but decreased in congenic strains containing the 2.4-Mbp BN region. Protein excretion and glomerular and interstitial damage were significantly higher in FHH rats than in congenic strains containing the 2.4-Mbp BN region. K(+) channel current was fivefold greater in VSMCs from renal arterioles of FHH rats than cells obtained from congenic strains containing the 2.4-Mbp region. Sequence analysis of the known and predicted genes in the 2.4-Mbp region of FHH rats revealed amino acid-altering variants in the exons of three genes: Add3, Rbm20, and Soc-2. Quantitative PCR studies indicated that Mxi1 and Rbm20 were differentially expressed in the renal vasculature of FHH and FHH.1BN congenic strain F. These data indicate that transfer of this 2.4-Mbp region from BN to FHH rats restores the myogenic response of AF-art and autoregulation of RBF, decreases K(+) current, and slows the progression of proteinuria and renal injury.

Identification of a region of rat chromosome 1 that impairs the myogenic response and autoregulation of cerebral blood flow in fawn-hooded hypertensive rats

This study examined the effects of transfer of a 2.4-Mbp region of rat chromosome 1 (RNO1) from Brown Norway (BN) into fawn-hooded hypertensive (FHH) rats on autoregulation (AR) of cerebral blood flow (CBF) and the myogenic response of middle cerebral arteries (MCAs). AR of CBF was poor in FHH and FHH.1(BN) AR(-) congenic strains that excluded the critical 2.4-Mbp region. In contrast, AR was restored in FHH.1(BN) AR(+) congenic strains that included this region. The diameter of MCAs of FHH rats increased from 140 ± 14 to 157 ± 18 μm when transmural pressure was increased from 40 to 140 mmHg, but it decreased from 137 ± 5 to 94 ± 7 μm in FHH.1(BN) AR(+) congenic strains. Transient occlusion of MCAs reduced CBF by 80% in all strains. However, the hyperemic response following ischemia was significantly greater in FHH and AR(-) rats than that seen in AR(+) congenic strains (AR(-), 173 ± 11% vs. AR(+), 124 ± 5%). Infarct size and edema formation were also significantly greater in an AR(-) strain (38.6 ± 2.6 and 12.1 ± 2%) than in AR(+) congenic strains (27.6 ± 1.8 and 6.5 ± 0.9%). These results indicate that there is a gene in the 2.4-Mbp region of RNO1 that alters the development of myogenic tone in cerebral arteries. Transfer of this region from BN to FHH rats restores AR of CBF and vascular reactivity and reduces cerebral injury after transient occlusion and reperfusion of the MCA.

Refined mapping of the renal failure RF-3 quantitative trait locus

Rf-3, a quantitative trait locus (QTL) on rat chromosome 3, affects the development of CKD in Fawn-Hooded Hypertensive (FHH) rats. This QTL spans 110 Mb and approximately 1400 genes; therefore, narrowing the position of this locus is necessary to elucidate potential candidate genes. Here, we used congenic models and comparative genomics to refine the Rf-3 candidate region. We generated congenic lines carrying smaller intervals (subcongenics) of the Rf-3 region and used these lines to reduce the Rf-3 candidate region by 94% (to 7.1 Mb). We used comparative genomics to identify QTL for both nephropathy and albuminuria in the syntenic region of this interval for both human and mouse. We also used the overlapping homologous regions to reduce the number of likely positional candidate genes to 13 known or predicted genes. By combining congenic models and cross-species studies, we narrowed the list of candidate genes to a level that we could sequence the whole interval to further identify the causative gene in future studies.

High-NaCl intake impairs dynamic autoregulation of renal blood flow in ANG II-infused rats

The aim of this study was to investigate dynamic autoregulation of renal blood flow (RBF) in ANG II-infused rats and the influence of high-NaCl intake. Sprague-Dawley rats received ANG II (250 ng·kg−1·min−1 sc) or saline vehicle (sham) for 14 days after which acute renal clearance experiments were performed during thiobutabarbital anesthesia. Rats ( n = 8–10 per group) were either on a normal (NNa; 0.4% NaCl)- or high (HNa; 8% NaCl)-NaCl diet. Separate groups were treated with 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (tempol; 1 M in drinking water). Transfer function analysis from arterial pressure to RBF in the frequency domain was used to examine the myogenic response (MR; 0.06–0.09 Hz) and the tubuloglomerular feedback mechanism (TGF; 0.03–0.06 Hz). MAP was elevated in ANG II-infused rats compared with sham groups ( P < 0.05). RBF in ANG II HNa was reduced vs. sham NNa and sham HNa (6.0 ± 0.3 vs. 7.9 ± 0.3 and 9.1 ± 0.3 ml·min−1·g kidney wt−1, P < 0.05). transfer function gain in ANG II HNa was significantly elevated in the frequency range of the MR (1.26 ± 0.50 dB, P < 0.05 vs. all other groups) and in the frequency range of the TGF (−0.02 ± 0.50 dB, P < 0.05 vs. sham NNa and sham HNa). Gain values in the frequency range of the MR and TGF were significantly reduced by tempol in ANG II-infused rats on HNa diet. In summary, the MR and TGF components of RBF autoregulation were impaired in ANG II HNa, and these abnormalities were attenuated by tempol, suggesting a pathogenetic role for superoxide in the impaired RBF autoregulatory response.

Vascular responses in aortic rings of a consomic rat panel derived from the Fawn Hooded Hypertensive strain

The present experiments, utilizing the high-throughput vascular protocol of PhysGen (Program for Genomic Applications) characterized the responses of aortic rings to vasoconstrictor (phenylephrine) and vasodilator (acetylcholine, sodium nitroprusside, and reduced tissue bath Po(2)) stimuli in consomic rat strains derived from a cross between the Fawn Hooded Hypertensive rat (FHH/EurMcwi) and the Brown Norway normotensive (BN/NHsdMcwi) rat. The effects of substituting individual BN chromosomes into the FHH genetic background were determined in animals that were maintained on a low-salt (0.4% NaCl) diet or switched to a high-salt (4% NaCl) diet for 3 wk. Sex-specific differences were evaluated in male and female consomic rats on similar dietary salt intake. Multiple chromosomes affected various vascular reactivity phenotypes in the FHH × BN consomic panel, and substantial salt-dependent changes in vascular reactivity and sex-specific differences in aortic reactivity were observed in individual consomic strains. However, compared with earlier studies of consomic rats derived from a cross between the BN rat and the Dahl salt-sensitive (SS) rat, only 3-7% of the vascular phenotypes were affected in a similar manner by substituting specific BN chromosomeschromosomes into the FHH genetic background versus the SS genetic background. The findings of the present study stress the potential value of consomic rat panels in gaining insight into genetic factors influencing vascular reactivity and suggest that the chromosomes that appear to be involved in the determination of aortic ring reactivity in different rodent models of hypertension are highly strain- and sex specific.

Renal vascular dysfunction precedes the development of renal damage in the hypertensive Fawn-Hooded rat

It is unknown whether generalized vascular dysfunction precedes the development of kidney disease. Therefore, we studied myogenic constriction and endothelium-mediated dilatory responses in two inbred Fawn-Hooded (FH) rat strains, one of which spontaneously develops hypertension, proteinuria, and glomerulosclerosis (FHH), whereas the other (FHL) does not. Small renal, mesenteric resistance arteries and thoracic aorta isolated from FH rats before (7 wk old) and after the development of mild proteinuria (12 wks old) were mounted in perfused and isometric set-ups, respectively. Myogenic response, endothelium-dependent relaxation, and the contribution of endothelium-mediated dilatory compounds were studied using their respective inhibitors. Myogenic reactivity was assessed constructing pressure-diameter curves in the presence and absence of calcium. At the age of 7 wk, renal arteries isolated from kidneys of FHH rats developed significantly lower myogenic tone compared with FHL, most likely because of excessive cyclo-oxygenase 1-mediated production of constrictive prostaglandins. Consequently, young FHH demonstrated reduced maximal myogenic tone (22 ± 4.8 vs. 10.8 ± 2.0%, P = 0.03) and the peak myogenic index (−6.9 ± 4.8 vs. 0.6 ± 0.8%/mmHg, P = 0.07 for FHL vs. FHH, respectively). Active myogenic curves obtained in mesenteric arteries isolated from 7-wk-old rats did not differ between either strain, demonstrating a similar level of systemic myogenic tone in FHL and FHH rats. Therefore, before any renal end-organ damage is present, myogenic response seems selectively impaired in renal vasculature of FHH rats. Aortic reactivity did not differ between FHL and FHH at the time points studied. The present study shows that vascular dysfunction in both small renal and systemic arteries precedes renal end-organ damage in a spontaneous model of hypertension-associated renal damage. These early vascular changes might be potentially involved in the increased susceptibility of FHH rats to renal injury.

Role of endogenous CYP450 metabolites of arachidonic acid in maintaining the glomerular protein permeability barrier

This study examined the metabolism of arachidonic acid (AA) by cytochrome P-450 enzymes in isolated glomeruli and the effects of selective inhibitors of the synthesis of 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatetraenoic acids (EETs) on glomerular permeability to albumin (P(alb)). Glomeruli avidly produced 20-HETE, EETs, dihydroxyeicosatetraenoic acids (diHETEs), and HETEs when incubated with exogenous AA. N-hydroxy-N'-(4-butyl-2-methylphenyl)formamidine (HET0016; 10 microM) selectively inhibited the formation of 20-HETE by 95% and increased P(alb) from 0.00 +/- 0.08 to 0.73 +/- 0.10 (n = 43 glomeruli, 4 rats). Addition of a 20-HETE mimetic, 20-hydroxyeicosa-5(Z),14(Z)-dienoic acid (20-5,14-HEDE; 1 microM) opposed the effects of HET0016 (10 microM) to increase P(alb) (0.21 +/- 0.10, n = 36 glomeruli, 4 rats). Preincubation of glomeruli with exogenous AA to increase basal production of 20-HETE had a similar effect. We also examined the effect of an epoxygenase inhibitor, N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MSPPOH; 5 microM), on P(alb). MSPPOH (5 microM) significantly increased P(alb) but had no effect on the synthesis of EETs in glomeruli incubated with AA. However, MSPPOH (5 microM) selectively reduced epoxygenase activity by 50% in glomeruli incubated without added AA. Pretreatment with 8,9-EET (100 nM) attenuated the effects of MSPPOH (5 microM) on P(alb). These results indicate that glomeruli produce 20-HETE, EETs, diHETEs, and HETEs and that endogenously formed 20-HETE and EETs play an essential role in the maintenance of the glomerular permeability barrier to albumin.

Assessment of renal autoregulation

The kidney displays highly efficient autoregulation so that under steady-state conditions renal blood flow (RBF) is independent of blood pressure over a wide range of pressure. Autoregulation occurs in the preglomerular microcirculation and is mediated by two, perhaps three, mechanisms. The faster myogenic mechanism and the slower tubuloglomerular feedback contribute both directly and interactively to autoregulation of RBF and of glomerular capillary pressure. Multiple experiments have been used to study autoregulation and can be considered as variants of two basic designs. The first measures RBF after multiple stepwise changes in renal perfusion pressure to assess how a biological condition or experimental maneuver affects the overall pressure-flow relationship. The second uses time-series analysis to better understand the operation of multiple controllers operating in parallel on the same vascular smooth muscle. There are conceptual and experimental limitations to all current experimental designs so that no one design adequately describes autoregulation. In particular, it is clear that the efficiency of autoregulation varies with time and that most current techniques do not adequately address this issue. Also, the time-varying and nonadditive interaction between the myogenic mechanism and tubuloglomerular feedback underscores the difficulty of dissecting their contributions to autoregulation. We consider the modulation of autoregulation by nitric oxide and use it to illustrate the necessity for multiple experimental designs, often applied iteratively.

Mechanisms of renal blood flow autoregulation: dynamics and contributions

Autoregulation of renal blood flow (RBF) is caused by the myogenic response (MR), tubuloglomerular feedback (TGF), and a third regulatory mechanism that is independent of TGF but slower than MR. The underlying cause of the third regulatory mechanism remains unclear; possibilities include ATP, ANG II, or a slow component of MR. Other mechanisms, which, however, exert their action through modulation of MR and TGF are pressure-dependent change of proximal tubular reabsorption, resetting of RBF and TGF, as well as modulating influences of ANG II and nitric oxide (NO). MR requires < 10 s for completion in the kidney and normally follows first-order kinetics without rate-sensitive components. TGF takes 30-60 s and shows spontaneous oscillations at 0.025-0.033 Hz. The third regulatory component requires 30-60 s; changes in proximal tubular reabsorption develop over 5 min and more slowly for up to 30 min, while RBF and TGF resetting stretch out over 20-60 min. Due to these kinetic differences, the relative contribution of the autoregulatory mechanisms determines the amount and spectrum of pressure fluctuations reaching glomerular and postglomerular capillaries and thereby potentially impinge on filtration, reabsorption, medullary perfusion, and hypertensive renal damage. Under resting conditions, MR contributes approximately 50% to overall RBF autoregulation, TGF 35-50%, and the third mechanism < 15%. NO attenuates the strength, speed, and contribution of MR, whereas ANG II does not modify the balance of the autoregulatory mechanisms.

Identification of a QTL on chromosome 1 for impaired autoregulation of RBF in fawn-hooded hypertensive rats

The present study evaluated whether the impairment in autoregulation of renal blood flow (RBF) in the fawn-hooded Hypertensive (FHH) rat colocalizes with the Rf-1 region on chromosome 1 that has been previously linked to the development of proteinuria in this strain. Autoregulation of RBF was measured in FHH and a consomic strain (FHH.1BN) in which chromosome 1 from the Brown-Norway (BN) rat was introgressed into the FHH genetic background. The autoregulation indexes (AI) averaged 0.80 ± 0.08 in the FHH and 0.19 ± 0.05 in the FHH.1BNrats. We next performed a genetic linkage analysis for autoregulation of RBF in 85 F2 rats generated from a backcross of FHH.1BNconsomic and FHH rats. The results revealed a significant quantitative trait locus (QTL) with a peak logarithm of the odds score of 6.3 near marker D1Rat376. To confirm the existence of this QTL, five overlapping congenic strains were created that spanned the region from markers D1Rat234 to D1Mit14. Transfer of a region of BN chromosome 1 from markers D1Mgh13 to D1Rat89 into the FHH genetic background improved autoregulation of RBF (AI = 0.23 ± 0.04) and reduced protein excretion. In contrast, RBF was poorly autoregulated and the rats were not protected from proteinuria in congenic strains in which other regions of chromosome 1 that exclude the D1Rat376 marker were transferred. These results indicate that there is a gene(s) that influences autoregulation of RBF and proteinuria between markers D1Mgh13 and D1Rat89 on chromosome 1 that lies within the confidence interval of the Rf-1 QTL previously linked to the development of proteinuria in FHH rats.

Systolic blood pressure as the trigger for the renal myogenic response: protective or autoregulatory?

PURPOSE OF REVIEW

The ability of the kidney to autoregulate renal blood flow and glomerular filtration rate has long been viewed as existing to prevent fluctuations in blood pressure from causing parallel fluctuations in renal function and distal delivery of filtrate. This review, however, points out that the primary consequence of the loss of this autoregulatory capacity is not a disturbance in volume regulation, but rather an increased susceptibility to hypertensive injury. Moreover, the kinetic requirements for renal protection indicate that current views of dynamic autoregulation cannot explain how the kidney is normally protected against acute elevations in systolic blood pressure.

RECENT FINDINGS

Recent findings suggest that the kinetics of the myogenic mechanism of the afferent arteriole are uniquely suited to protect against acute elevations in the systolic blood pressure, in that this vessel not only senses this rapidly oscillating blood pressure component, but that its response is exclusively dependent on this signal.

SUMMARY

These new findings are consistent with recent data indicating that it is the systolic blood pressure elevations that most closely correlate with target organ damage. The fact that the myogenic mechanism is also a necessary component of renal autoregulation may explain the strong linkage between autoregulatory impairment and increased susceptibility to hypertensive injury.

Substitution of chromosome 1 ameliorates L-NAME hypertension and renal disease in the fawn-hooded hypertensive rat

Linkage analysis studies previously identified genetic loci associated with proteinuria and hypertension on chromosome 1 of fawn-hooded hypertensive (FHH) rats. The present studies were performed on conscious male and female rats to evaluate the influence of transfer of chromosome 1 from the Brown Norway (BN) rat to the FHH genetic background (FHH-1BN). Rats were maintained for 2 wk on 8.0% NaCl chow with NG-nitro-L-arginine methyl ester (L-NAME) in the drinking water (12.5 mg/l) to induce hypertension and accelerate the onset of renal disease. Mean arterial blood pressure (MAP) was significantly higher in the male FHH (188 +/- 3 mmHg, n = 13) compared with the BN (121 +/- 3 mmHg, n = 8); MAP in the FHH-1(BN) was midway between the two parental strains (167 +/- 5 mmHg, n = 9). Urinary protein and albumin excretion rates in the male FHH-1(BN) (Uprot = 189 +/- 36 mg/day, Ualb = 69 +/- 16 mg/day, n = 10) were also midway between levels observed in the FHH (Uprot = 485 +/- 54 mg/day; Ualb = 206 +/- 25 mg/day, n = 13) and the BN (Uprot = 32 +/- 5 mg/day, Ualb = 5 +/- 1 mg/day, n = 8). Creatinine clearance was elevated, and the degree of glomerular damage was significantly reduced in the FHH-1BN compared with the FHH. Qualitatively similar results were obtained from female FHH, FHH-1BN, and BN rats. The present results indicate that genes contributing to l-NAME-induced hypertension and renal disease are found on chromosome 1 of the FHH rat.

Transfer of the Rf-1 region from FHH onto the ACI background increases susceptibility to renal impairment

The genetically hypertensive fawn-hooded (FHH/Eur) rat is characterized by the early presence of systolic and glomerular hypertension, progressive proteinuria (UPV), and albuminuria (UAV), and focal glomerulosclerosis, resulting in premature death from renal failure. Previous studies showed that at least five genetic loci (Rf-1 to Rf-5) were linked to the development of renal impairment. Of these five, Rf-1 appears to play a major role. To study the impact of Rf-1 in the absence of the other loci, we transferred the Rf-1 region of chromosome 1, between the markers D1Mit34 and D1Rat156, Rf-1B for short, onto the genomic background of the normotensive August x Copenhagen Irish (ACI) rat. In this congenic strain, named ACI.FHH-D1Mit34/Rat156 or ACI.FHH-Rf1B, we challenged the renal hemodynamic function of these animals by studying the effects of unilateral nephrectomy (UNX) alone, or combined with N(G)-nitro-L-arginine methyl ester (L-NAME)-induced hypertension. Following UNX, the congenic strain developed significantly more UPV and UAV than the ACI progenitor. The differences were even more pronounced when UNX was combined with an L-NAME-induced rise in systolic blood pressure to about 150 mmHg, i.e., the level of hypertension present in the parental FHH strain. These findings indicate that the Rf-1B region of the FHH rat contains at least one gene affecting the susceptibility to progressive renal failure, especially in the presence of an increase in blood pressure.

Blood pressure and the susceptibility to renal damage after unilateral nephrectomy and L-NAME-induced hypertension in rats

BACKGROUND

Fawn-hooded hypertensive (FHH) rats carry several genes which determine the susceptibility to develop renal damage, while renal damage resistant August x Copenhagen Irish (ACI) rats do not. Kidneys from heterozygous (FHH x ACI) F(1) rats, appear to be largely, but not completely, protected after blood pressure elevation with N(omega)-nitro-L-arginine methyl ester (L-NAME). We examined the role of an increased haemodynamic burden on the development of renal damage combining unilateral nephrectomy (UNx)- and L-NAME-induced hypertension in F(1) and ACI rats. Additionally, we investigated whether a general toxic effect of L-NAME, independent from a blood pressure elevation, caused renal damage in F(1) rats in animals simultaneously treated with L-NAME and the ACE inhibitor lisinopril.

METHODS

Surgery was performed and L-NAME treatment (50 or 150 mg/l) was started at the age of 15 weeks. Systolic blood pressure (SBP) and urinary albumin excretion (UaV) were measured at 6 and 12 weeks post-UNx, followed by autopsy to determine the incidence of focal glomerulosclerosis (FGS). Using lisinopril (LIS) and L-NAME, another group of rats was evaluated at 12, 18, and 24 weeks after start of treatment.

RESULTS

At similar L-NAME intake, F, rats developed more severe hypertension and more UaV than ACI rats. The increase in UaV per mmHg increase in SBP was fivefold higher in F(1) compared with ACI rats. In F(1) rats, the increase in UaV per percentage incidence increase in FGS was three times higher. In LIS treated F(1) rats, no significant UaV or FGS was measured at low blood pressure levels, indicating that renal damage in hypertensive F(1) rats is not a direct effect of L-NAME, but the result of the high blood pressure or another action of the renin-angiotensin system.

CONCLUSION

We conclude that heterozygosity for the genes influencing the development of renal damage in the FHH strain increases the susceptibility of the kidney to develop damage after UNx combined with systemic hypertension.

Impaired myogenic autoregulation in kidneys of Brown Norway rats

The Brown Norway (BN) rat is normotensive and has an extended lifespan but is extremely sensitive to hypertension-induced renal injury. Relative impairment of autoregulation has been implicated in the progression of renal failure whereas absence of myogenic autoregulation is associated with early renal failure. Therefore, we tested the hypothesis that there is conditional failure of renal autoregulation in BN rats. In isoflurane-anesthetized BN rats, the pressure-flow transfer function was normal when pressure fluctuated spontaneously. External forcing increased pressure fluctuation and exposed weakness of the myogenic component of autoregulation; the component mediated by tubuloglomerular feedback was less affected. In the presence of vasopressin to raise renal perfusion pressure, myogenic autoregulation was further impaired during forcing in BN rats but not in Wistar rats. Compensation by the myogenic system was rapidly restored on cessation of forcing, suggesting a functional limitation rather than a structural failure. Graded forcing in Wistar rats and in spontaneously hypertensive rats revealed that compensation due to the myogenic system was strong and independent of forcing amplitude. In contrast, graded forcing in BN rats showed that compensation was reduced when fluctuation of blood pressure was increased but that the reduction was independent of forcing amplitude. The results demonstrate conditional failure of myogenic autoregulation in BN rats. These acute studies provide a possible explanation for the observed sensitivity to hypertension-induced renal injury in BN rats.