Interactions contributing to kidney blood flow autoregulation

Modeling the interaction between tubuloglomerular feedback and myogenic mechanisms in the control of glomerular mechanics

Introduction: Mechanical stresses and strains exerted on the glomerular cells have emerged as potentially influential factors in the progression of glomerular disease. Renal autoregulation, the feedback process by which the afferent arteriole changes in diameter in response to changes in blood pressure, is assumed to control glomerular mechanical stresses exerted on the glomerular capillaries. However, it is unclear how the two major mechanisms of renal autoregulation, the afferent arteriole myogenic mechanism and tubuloglomerular feedback (TGF), each contribute to the maintenance of glomerular mechanical homeostasis. Methods: In this study, we made a mathematical model of renal autoregulation and combined this model with an anatomically accurate model of glomerular blood flow and filtration, developed previously by us. We parameterized the renal autoregulation model based on data from previous literature, and we found evidence for an increased myogenic mechanism sensitivity when TGF is operant, as has been reported previously. We examined the mechanical effects of each autoregulatory mechanism (the myogenic, TGF and modified myogenic) by simulating blood flow through the glomerular capillary network with and without each mechanism operant. Results: Our model results indicate that the myogenic mechanism plays a central role in maintaining glomerular mechanical homeostasis, by providing the most protection to the glomerular capillaries. However, at higher perfusion pressures, the modulation of the myogenic mechanism sensitivity by TGF is crucial for the maintenance of glomerular mechanical homeostasis. Overall, a loss of renal autoregulation increases mechanical strain by up to twofold in the capillaries branching off the afferent arteriole. This further corroborates our previous simulation studies, that have identified glomerular capillaries nearest to the afferent arteriole as the most prone to mechanical injury in cases of disturbed glomerular hemodynamics. Discussion: Renal autoregulation is a complex process by which multiple feedback mechanisms interact to control blood flow and filtration in the glomerulus. Importantly, our study indicates that another function of renal autoregulation is control of the mechanical stresses on the glomerular cells, which indicates that loss or inhibition of renal autoregulation may have a mechanical effect that may contribute to glomerular injury in diseases such as hypertension or diabetes. This study highlights the utility of mathematical models in integrating data from previous experimental studies, estimating variables that are difficult to measure experimentally (i.e. mechanical stresses in microvascular networks) and testing hypotheses that are historically difficult or impossible to measure.

Immunopathophysiology of trauma-related acute kidney injury

Physical trauma can affect any individual and is globally accountable for more than one in every ten deaths. Although direct severe kidney trauma is relatively infrequent, extrarenal tissue trauma frequently results in the development of acute kidney injury (AKI). Various causes, including haemorrhagic shock, rhabdomyolysis, use of nephrotoxic drugs and infectious complications, can trigger and exacerbate trauma-related AKI (TRAKI), particularly in the presence of pre-existing or trauma-specific risk factors. Injured, hypoxic and ischaemic tissues expose the organism to damage-associated and pathogen-associated molecular patterns, and oxidative stress, all of which initiate a complex immunopathophysiological response that results in macrocirculatory and microcirculatory disturbances in the kidney, and functional impairment. The simultaneous activation of components of innate immunity, including leukocytes, coagulation factors and complement proteins, drives kidney inflammation, glomerular and tubular damage, and breakdown of the blood-urine barrier. This immune response is also an integral part of the intense post-trauma crosstalk between the kidneys, the nervous system and other organs, which aggravates multi-organ dysfunction. Necessary lifesaving procedures used in trauma management might have ambivalent effects as they stabilize injured tissue and organs while simultaneously exacerbating kidney injury. Consequently, only a small number of pathophysiological and immunomodulatory therapeutic targets for TRAKI prevention have been proposed and evaluated.

Effect of continuous intraoperative infusion of methoxamine on renal function in elderly patients undergoing gastrointestinal tumor surgery: a randomized controlled trial

Background

Acute renal injury (AKI) caused by hypotension often occurs in elderly patients after gastrointestinal tumor surgery. Although vasoactive drugs can increase effective filtration pressure, they may increase renal vascular resistance and reduce renal blood flow. The effect of methoxamine on renal function is not clear.

Methods

After obtaining written informed consent, 180 elderly patients undergoing elective gastrointestinal tumor surgery were randomly allocated into two groups: M group (continuous infusion of methoxamine at 2 μg/kg/min) and N group (continuous infusion of normal saline). The patients’ mean arterial pressure was maintained within 20% of baseline by a continuous infusion of methoxamine or normal saline. Maintenance fluid was kept at 5 mL/kg/h. According to Kidney disease improve global outcome (KDIGO) guidelines, creatinine was measured at 1, 2 and 7 days after operation, and urine volume at 6, 12 and 24 h after operation was measured to evaluate the occurrence of AKI. 162 patients were included in the final data analysis.

Results

Significant differences in the incidence of postoperative Acute kidney injury (M group: 7.5%; N group: 18.3%; P < 0.05), the frequency of hypotension (M group: 1 [1–3]; N group: 3 [1–5]; P < 0.05), and the duration of intraoperative hypotension (M group: 2[0–10]; N group: 10 [5–16]; P < 0.05) were identified between the groups. Multivariate logistic regression analyses demonstrated that preoperative creatinine and the frequency of intraoperative hypotension were the common factors leading to the occurrence of postoperative AKI. The results of Cox multivariate analysis showed that age and AKI were independent risk factors for 30-day death.

Conclusion

Compared with the intraoperative continuous infusion of placebo and methoxamine, continuous infusion of 2 μg/kg/min methoxamine reduced the incidence of postoperative AKI and other clinical complications in elderly patients undergoing gastrointestinal surgery by raising blood pressure and improved the prognosis of patients.

Trial registration

Trial registration: Chinese Clinical Trial Registry, ChiCTR1900020536, registered 7 January, 2019,

Renal effects of norepinephrine-induced variations in mean arterial pressure after liver transplantation: A randomized cross-over trial

BACKGROUND

Acute kidney injury is commonly seen after liver transplantation. The optimal perioperative target mean arterial pressure (MAP) for renal filtration, perfusion and oxygenation in liver recipients is not known. The effects of norepinephrine-induced changes in MAP on renal blood flow (RBF), oxygen delivery (RDO₂ ), glomerular filtration rate (GFR) and renal oxygenation (=renal oxygen extraction, RO₂ Ex) were therefore studied early after liver transplantation.

METHODS

Ten patients with an intra- and post-operative vasopressor-dependent systemic vasodilation were studied early after liver transplantation during sedation and mechanical ventilation. To achieve target MAP levels of 60, 75 and 90 mm Hg, the norepinephrine infusion rate was randomly and sequentially titrated. At each target MAP, data on cardiac index (CI), RBF and GFR were obtained by transpulmonary thermodilution (PiCCO), the renal vein thermodilution technique and renal extraction of chromium ethylenediaminetetraaceticacid (⁵¹ Cr-EDTA), respectively. Renal oxygen consumption (RVO₂ ) and extraction (RO₂ Ex) were calculated according to standard formulas.

RESULTS

At a target MAP of 75 mm Hg, CI (13%), RBF (18%), RDO₂ (24%), GFR (31%) and RVO₂ (20%) were higher while RO₂ Ex was unchanged compared to a target MAP of 60 mm Hg. Increasing MAP from 75 up to 90 mm Hg increased RVR by 38% but had no further effects on CI, RBF, RDO₂ or GFR.

CONCLUSIONS

In patients undergoing liver transplantation, RBF and GFR are pressure-dependent at MAP levels below 75 mm Hg. Our results suggest that MAP should probably be targeted to approximately 75 mm Hg for optimal perioperative renal filtration, perfusion and oxygenation in patients undergoing liver transplantation.

Blood pressure deficits in acute kidney injury: not all about the mean arterial pressure?

Acute kidney injury (AKI) is associated with increased morbidity and mortality. Although there are many causes of AKI, it is known that patients undergoing high-risk surgery are known to be at significant risk. Although much effort has centred on the minimum arterial pressure needed to maintain renal perfusion, this tends to be based on relatively crude measures such as the mean arterial pressure (MAP), which is widely used as an index for the optimal blood pressure. The rationale behind maintaining MAP is to provide adequate organ perfusion, although this is difficult to assess other than by applying crude end-points. Recent studies have examined the progression of AKI as defined by the KDIGO criteria in terms of time-weighted average values for premorbid and within-ICU haemodynamic pressure-related parameters. Although principally performed on patients who had undergone cardiovascular surgery and who were on vasopressor support, some interesting results were obtained suggesting that crude MAP may not be an adequate target in AKI. In patients with AKI progression, greater observed deficits in mean perfusion pressure, diastolic arterial perfusion, and diastolic perfusion pressures were observed. This study may highlight potential modifiable risk factors for the prevention of progression of AKI, and hopefully translate into improved outcomes.

Acute Kidney Injury from Excessive Potentiation of Calcium-channel Blocker via Synergistic CYP3A4 Inhibition by Clarithromycin Plus Voriconazole

CYP3A4-inhibitors can potentiate the hypotensive effect of calcium-channel blockers. However, insufficient attention to such drug interactions may result in serious adverse reactions. A 71-year-old hypertensive man prescribed nifedipine was hospitalized for infectious endophthalmitis. Antimicrobial therapy with voriconazole lowered the blood pressure, and then clarithromycin further lowered it through the excessively elevated nifedipine concentration, leading to ischemic acute kidney injury. After the discontinuation of clarithromycin and voriconazole, the blood pressure and renal function were recovered. The combination of CYP3A4-inhibitors such as clarithromycin plus voriconazole can synergistically potentiate calcium-channel blockers. Co-prescription of multiple CYP3A4-inhibitors with calcium-channel blockers increases the risk of hypotension and acute kidney injury.

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.

Detecting Interactions between the Renal Autoregulation Mechanisms in Time and Space

OBJECTIVE

Our objective is to identify localized interactions between the renal autoregulation mechanisms over time.

METHODS

A time-varying phase-randomized wavelet bicoherence detector for quadratic phase coupling between tubuloglomerular feedback and the myogenic response is presented. Through simulations we show its ability to interrogate quadratic phase coupling. The method is applied to kidney blood flow and laser speckle imaging sequences of cortical perfusion from anesthetized rats before and after nonselective inhibition of nitric-oxide synthase.

RESULTS

Quadratic phase coupling in kidney blood flow data was present in four out of nine animals during the control period for 13.0 ± 5.6% (mean ± SD) of time and in five out of nine animals during inhibition of nitric-oxide synthase for 15.8 ± 8.2% of time. Approximately 60% of time-series extracted from laser speckle imaging pixels of the renal cortex showed significant quadratic phase coupling. Pixels with significant coupling had a median coupling length of 10.8 ± 2.2% and 12.1 ± 3.1% of time with the 95th percentile of pixels being coupled for 25.5 ± 4.4% and 30.9 ± 6.4% of time during control and inhibition of nitric-oxide synthase, respectively.

CONCLUSION

These results indicate quadratic phase coupling exists in short time intervals between tubuloglomerular feedback and the myogenic response and is detected more often in local renal perfusion signals than whole kidney blood flow in anesthetized rats.

SIGNIFICANCE

Combining the detector and laser speckle imaging provides identification of coordination between renal autoregulation mechanisms that is localized in time and space.

A pharmacologic activator of endothelial KCa channels increases systemic conductance and reduces arterial pressure in an anesthetized pig model

SKA-31, an activator of endothelial KCa2.3 and KCa3.1 channels, reduces systemic blood pressure in mice and dogs, however, its effects in larger mammals are not well known. We therefore examined the hemodynamic effects of SKA-31, along with sodium nitroprusside (SNP), in anesthetized, juvenile male domestic pigs. Experimentally, continuous measurements of left ventricular (LV), aortic and inferior vena cava (IVC) pressures, along with flows in the ascending aorta, carotid artery, left anterior descending coronary artery and renal artery, were performed during acute administration of SKA-31 (0.1, 0.3, 1.0, 3.0 and 5.0mg/ml/kg) and a single dose of SNP (5.0 μg/ml/kg). SKA-31 dose-dependently reduced mean aortic pressure (mPAO), with the highest dose decreasing mPAO to a similar extent as SNP (-23 ± 3 and -28 ± 4 mmHg, respectively). IVC pressure did not change. Systemic conductance and conductance in coronary and carotid arteries increased in response to SKA-31 and SNP, but renal artery conductance was unaffected. There was no change in either LV stroke volume (SV) or heart rate (versus the preceding control) for any infusion. With no change in SV, drug-evoked decreases in LV stroke work (SW) were attributed to reductions in mPAO (SW vs. mPAO, r(2)=0.82, P<0.001). In summary, SKA-31 dose-dependently reduced mPAO by increasing systemic and arterial conductances. Primary reductions in mPAO by SKA-31 largely account for associated decreases in SW, implying that SKA-31 does not directly impair cardiac contractility.

Segmentation of renal perfusion signals from laser speckle imaging into clusters with phase synchronized dynamics

Renal perfusion signals contain dynamics arising from the renal autoregulation feedback mechanisms as the contraction and dilation of vessels alter flow patterns. We can capture the time-varying dynamics at points across the renal surface using laser speckle imaging. We segment an imaged area of the renal cortex into clusters with phase synchronized dynamics. Our approach first uses phase coherence with a surrogate data derived threshold to identify synchronized pixel pairs. Non-negative matrix factorization is then applied to segment phase coherence estimates into phase synchronized regions. The method is applied to laser speckle imaging of the renal cortex of anaesthetized rats to identify regions on the renal surface with phase synchronized myogenic activity. In three out of six animals imaged after bolus infusion of N(ω)-nitro-l-arginine methyl ester (NAM), the renal surfaces are segmented into clusters with high phase coherence. No more than two clusters were identified during control period for any animal. In the remaining three animals, a strong myogenic signal could not be detected in surface perfusion during control or NAM. This method can be used to identify synchronization in renal autoregulation dynamics across the renal surface.

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.

Hemodynamic and hormonal changes to dual renin-angiotensin system inhibition in experimental hypertension

We examined the antihypertensive effects of valsartan, aliskiren, or both drugs combined on circulating, cardiac, and renal components of the renin-angiotensin system in congenic mRen2.Lewis hypertensive rats assigned to: vehicle (n=9), valsartan (via drinking water, 30 mg/kg per day; n=10), aliskiren (SC by osmotic mini-pumps, 50 mg/kg per day; n=10), or valsartan (30 mg/kg per day) combined with aliskiren (50 mg/kg per day; n=10). Arterial pressure and heart rate were measured by telemetry before and during 2 weeks of treatment; trunk blood, heart, urine, and kidneys were collected for measures of renin-angiotensin system components. Arterial pressure and left-ventricular weight/tibia length ratio were reduced by monotherapy of valsartan, aliskiren, and further reduced by the combination therapy. Urinary protein excretion was reduced by valsartan and further reduced by the combination. The increases in plasma angiotensin (Ang) II induced by valsartan were reversed by the treatment of aliskiren and partially suppressed by the combination. The decreases in plasma Ang-(1-7) induced by aliskiren recovered in the combination group. Kidney Ang-(1-12) was increased by the combination therapy whereas the increases in urinary creatinine mediated by valsartan were reversed by addition of aliskiren. The antihypertensive and antiproteinuric actions of the combined therapy were associated with marked worsening of renal parenchymal disease and increased peritubular fibrosis. The data show that despite improvements in the surrogate end points of blood pressure, ventricular mass, and proteinuria, dual blockade of Ang II receptors and renin activity is accompanied by worsening of renal parenchymal disease reflecting a renal homeostatic stress response attributable to loss of tubuloglomerular feedback by Ang II.

Impact of changes in blood pressure during the treatment of acute decompensated heart failure on renal and clinical outcomes

AIMS

One of the primary determinants of blood flow in regional vascular beds is perfusion pressure. Our aim was to investigate if reduction in blood pressure during the treatment of decompensated heart failure would be associated with worsening renal function (WRF). Our secondary aim was to evaluate the prognostic significance of this potentially treatment-induced form of WRF.

METHODS AND RESULTS

Subjects included in the Evaluation Study of Congestive Heart Failure and Pulmonary Artery Catheterization Effectiveness (ESCAPE) trial limited data were studied (386 patients). Reduction in systolic blood pressure (SBP) was greater in patients experiencing WRF (-10.3 ± 18.5 vs. -2.8 ± 16.0 mmHg, P < 0.001) with larger reductions associated with greater odds for WRF (odds ratio = 1.3 per 10 mmHg reduction, P < 0.001). Systolic blood pressure reduction (relative change > median) was associated with greater doses of in-hospital oral vasodilators (P ≤ 0.017), thiazide diuretic use (P = 0.035), and greater weight reduction (P = 0.023). In patients with SBP-reduction, WRF was not associated with worsened survival [adjusted hazard ratio (HR) = 0.76, P = 0.58]. However, in patients without SBP-reduction, WRF was strongly associated with increased mortality (adjusted HR = 5.3, P < 0.001, P interaction = 0.001).

CONCLUSION

During the treatment of decompensated heart failure, significant blood pressure reduction is strongly associated with WRF. However, WRF that occurs in the setting of SBP-reduction is not associated with an adverse prognosis, whereas WRF in the absence of this provocation is strongly associated with increased mortality. These data suggest that WRF may represent the final common pathway of several mechanistically distinct processes, each with potentially different prognostic implications.

Understanding urine output in critically ill patients

Urine output often is used as a marker of acute kidney injury but also to guide fluid resuscitation in critically ill patients. Although decrease of urine output may be associated to a decrease of glomerular filtration rate due to decrease of renal blood flow or renal perfusion pressure, neurohormonal factors and functional changes may influence diuresis and natriuresis in critically ill patients. The purpose of this review is to discuss the mechanisms of diuresis regulation, which may help to interpret the urine output in critically ill patients and the appropriate treatment to be initiated in case of changes in urine output.

Notch3 is essential for regulation of the renal vascular tone

The Notch3 receptor participates in the development and maturation of vessels. Mutations of Notch3 in humans are associated with defective regulation of cerebral blood flow. To investigate the role of Notch3 in the regulation of renal hemodynamics, we used mice lacking expression of the Notch3 gene (Notch3-/- mice). Bolus injections of norepinephrine and angiotensin II increased renal vascular resistance and decreased renal blood flow in a dose-dependent manner in wild-type mice. In sharp contrast, renal vascular resistance of Notch3-/- mice varied little after boluses of norepinephrine and angiotensin II. Inversely, bradykinin and prostacyclin relaxed renal vasculature in wild-type mice. Both vasodilators had a negligible effect on renal vascular resistance of Notch3-/- mice. Afferent arterioles freshly isolated from Notch3-/- mice displayed decreased thickness of vascular wall compared with wild -type mice and showed a deficient contractile response to angiotensin II. To examine the physiopathological consequences of the above-described deficiency, hypertension was induced by continuous infusion of angiotensin II. Angiotensin II gradually increased blood pressure in both strains, but this increase was lesser in the Notch3-/- mice. Despite this blunted systemic effect, Notch3-/- mice displayed high mortality rates (65%) attributed to heart failure. In the kidney, the surviving Notch3-/- mice showed focal structural alterations characteristic of nephroangiosclerosis. These data show that Notch3 is necessary for the adaptive response of the renal vasculature to vasoactive systems. A deficiency in the expression of Notch3 could have important physiopathological consequences in the adaptation of the cardiac and renal function to chronic increase of blood pressure.

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.

The renin-angiotensin system and the third mechanism of renal blood flow autoregulation

Autoregulation of renal blood flow comprises three mechanisms: the myogenic response (MR), the tubuloglomerular feedback (TGF), and a third mechanism (3M). The nature of 3M is unknown; it may be related to hypotensive resetting of autoregulation that probably relies on pressure-dependent stimulation of the renin-angiotensin system (RAS). Thus we used a normotensive angiotensin II clamp in anesthetized rats and studied autoregulation 1) by slow ramp-shaped reductions in renal perfusion pressure (RPP) followed by ramp-shaped RPP restorations and 2) by means of the step response technique: after 30 s of either total or partial suprarenal aortic occlusion, a step increase in RPP was made and the response of renal vascular conductance analyzed to assess the mechanisms' strength and initial direction (vasodilation or constriction). The angiotensin clamp abolished the resetting of autoregulation during ramp-shaped RPP changes. Under control conditions, the initial TGF response was dilatory after total occlusions but constrictive after partial occlusions. The initial 3M response presented a mirror image to the TGF: it was constrictive after total but dilatory after partial occlusions. The angiotensin clamp suppressed the TGF and turned the initial 3M response following total occlusions into dilation. We conclude that 1) pressure-dependent RAS stimulation is a major cause behind hypotensive resetting of autoregulation, 2) TGF sensitivity strongly depends on pressure-dependent changes in RAS activity, 3) the 3M is modulated, but not mediated, by the RAS, and 4) the 3M acts as a counterbalance to the TGF and might possibly be related to the recently described connecting tubule glomerular feedback.

Strain vessel hypothesis: a viewpoint for linkage of albuminuria and cerebro-cardiovascular risk

Albuminuria is closely associated with stroke and cardiovascular diseases (CVDs) as well as the salt sensitivity of blood pressure (BP). Although albuminuria may reflect generalized endothelial dysfunction, there may be more specific hemodynamic mechanisms underlying these associations. Cerebral hemorrhage and infarction occur most frequently in the area of small perforating arteries that are exposed to high pressure and that have to maintain strong vascular tone in order to provide large pressure gradients from the parent vessels to the capillaries. Analogous to the perforating arteries are the glomerular afferent arterioles of the juxtamedullary nephrons. Hypertensive vascular damage occurs first and more severely in the juxtamedullary glomeruli. Therefore, albuminuria may be an early sign of vascular damages imposed on 'strain vessels' such as perforating arteries and juxtamedullary afferent arterioles. Coronary circulation also occurs under unique hemodynamic conditions, in which the entire epicardial segments are exposed to very high pressure with little flow during systolic phases. From the evolutionary point of view, we speculate that such circulatory systems in the vital organs are mandatory for survival under the danger of hypoperfusion due to difficult access to salt and water as well as high risks of wound injuries. In addition, albuminuria would indicate an impairment of renal medullary circulation, downstream from the juxtamedullary glomeruli, and therefore an impaired pressure natriuresis, which would lead to salt sensitivity of BP. Our 'strain vessel hypothesis' may explain why hypertension and diabetes, unforeseen in the concept of evolution, preferentially affect vital organs such as the brain, heart and kidney.

Gender-specific reference value of urine albumin-creatinine ratio in healthy Chinese adults: results of the Beijing CKD survey

BACKGROUND

The reference value of urine albumin-creatinine ratio (ACR) has racial disparities. The ACR reference value in a healthy Beijing population is reported.

METHODS

A reference Beijing population was sampled via a multistage, clustered complex sampling method. By excluding subjects with self-reported kidney disease, hypertension, diabetes, dyslipidemia, cardiovascular disease, obesity or underweight condition, overt proteinuria, hematuria, or pyuria, as well as those with an estimated glomerular filtration rate (eGFR) > 200ml/min/1.73m2 or < 60ml/min/1.73m2, apparently healthy subjects (1260 males, 2305 females, aged 18-84y) were selected to be included in the current analysis. Urine albumin was measured using the immunoturbidimetic method, creatinine was measured using Jaffe's kinetic method on a morning spot-urine sample, and ACR was calculated. The 95th percentile of ACR was used as the normal upper limit. The association between ACR and each of gender, age, systolic blood pressure, body mass index, serum glucose, lipids, and eGFR was examined.

RESULTS

The normal upper limit of ACR was 14mg/g (1.58mg/mmol) for males and 20mg/g (2.26mg/mmol) for females. Females had higher ACR values than males, and age, systolic blood pressure, and eGFR were positively correlated with ACR.

CONCLUSIONS

The ACR reference value in the healthy Beijing population is lower than that of the Western population. Age, systolic blood pressure, and eGFR were found to correlate with ACR.

Renal blood flow and dynamic autoregulation in conscious mice

Autoregulation of renal blood flow (RBF) occurs via myogenic and tubuloglomerular feedback (TGF) mechanisms that are engaged by pressure changes within preglomerular arteries and by tubular flow and content, respectively. Our understanding of autoregulatory function in the kidney largely stems from experiments in anesthetized animals where renal perfusion pressure is precisely controlled. However, normally occurring variations in blood pressure are sufficient to engage both myogenic and TGF mechanisms, making the assessment of autoregulatory function in conscious animals of significant value. To our knowledge, no studies have evaluated the dynamics of RBF in conscious mice. Therefore, we used spectral analysis of blood pressure and RBF and identified dynamic operational characteristics of the myogenic and TGF mechanisms in conscious, freely moving mice instrumented with ultrasound flow probes and arterial catheters. The myogenic response generates a distinct resonance peak in transfer gain at 0.31 +/- 0.01 Hz. Myogenic-dependent attenuation of RBF oscillations, indicative of active autoregulation, is apparent as a trough in gain below 0.3 Hz (-6.5 +/- 1.3 dB) and a strong positive phase peak (93 +/- 9 deg), which are abolished by amlodipine infusion. Operation of TGF produces a local maximum in gain at 0.05 +/- 0.01 Hz and a positive phase peak (62.3 +/- 12.3 deg), both of which are eliminated by infusion of furosemide. Administration of amlodipine eliminated both myogenic and TGF signature peaks, whereas furosemide shifted the myogenic phase peak to a slower operational frequency. These data indicate that myogenic and TGF dynamics may be used to investigate the effectiveness of renal autoregulatory mechanisms in conscious mice.

Nephroangiosclerosis in cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy: is NOTCH3 mutation the common culprit?

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a systemic arterial disease characterized by impairment of vascular smooth muscle cell structure and function related to NOTCH3 mutations. Pathological findings include pathognomonic granular osmiophilic material (GOM) deposition with nonspecific hyalinization within the artery wall in a variety of tissues. The main clinical presentation is iterative strokes in young adults despite the lack of cardiovascular risk factors, leading to early dementia. Although arteriosclerosis and GOM have been found in kidneys from patients with CADASIL, kidney disease has been described only once up to now, in association with immunoglobulin A nephropathy. We report the case of a 61-year-old patient with a medical history of CADASIL and recent mild hypertension. His mother also showed neuropsychiatric symptoms and end-stage renal disease of unknown cause. The patient had a chronic kidney disease defined by means of estimated glomerular filtration rate using the 4-variable Modification of Diet in Renal Disease Study equation of 58 mL/min/1.73 m(2) associated with mild proteinuria and intermittent microscopic hematuria. Renal histological analysis showed severe arteriosclerosis and mild interstitial fibrosis. Glomeruli did not show mesangial immunoglobulin A deposition or focal segmental proliferation. Electron microscopic analysis showed typical GOM deposition in the vicinity of altered vascular smooth muscle cells in interlobular and juxtaglomerular arteries. The nephroangiosclerosis-like lesions were unusually severe in contrast to the recent mild hypertension. The presence of GOM strongly suggests that renal lesions were related to the NOTCH3 mutation. Here, we describe the first case of familial occurrence of kidney disease with decreased kidney function in the absence of coexisting nephropathy in patients with CADASIL. We discuss the role of NOTCH3 mutation in the pathogenesis of nephroangiosclerosis through functional impairment of renal microcirculation or primary Notch3-related vascular disease.

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.