Single Nephron Glomerular Filtration Rate Research Articles

SO022EVALUATION OF GLOMERULAR HEMODYNAMIC CHANGES BY SGLT2 INHIBITION IN TYPE 2 DIABETIC RATS USING IN VIVO IMAGING TECHNIQUES

Abstract Background and Aims In recent clinical trials, the SGLT2 inhibitor (SGLT2i) slowed the progression of kidney disease compared with the placebo in patients with type 2 diabetes. Improvement of glomerular hyperfiltration via tubuloglomerular feedback (TGF) is considered to be one of the possible pathways for renal protection with SGLT2 inhibition (SGLT2i) in diabetic kidney disease (DKD). We have successfully developed the novel method to measure single-nephron GFR (SNGFR) in mice using multiphoton laser microscopy and demonstrated that the adenosine/adenosine A1 receptor (A1aR) pathway plays a pivotal role in the TGF mechanism in the type 1 diabetic model, Akita mice (Kengo Kidokoro, David Z. I. Cherney et al. Circulation. 2019). It has been suggested that the mechanism of improvement effects in glomerular hyperfiltration by SGLT2i is different in type 1 diabetes and type 2 diabetes. However, the detailed regulatory mechanism of GFR by SGLT2i is not fully understood in type 2 diabetes. This study aims to clarify the effects of SGLT2i on glomerular hemodynamics in type 2 diabetic rats. Method Zucker lean (ZL) rats and Zucker diabetic fatty (ZDF) rats were used. In the first experiment, SNGFR and diameters of glomerular afferent/efferent arterioles were measured in both groups. Next, we examined the change of SNGFR and diameters of glomerular afferent/efferent arterioles, as well as urinary excretions of glucose and sodium in ZDF after a single-dose administration of SGLT2i (luseogliflozin; 10mg/kg, gavage) for 120 minutes, which generated the following three groups: SGLT2i group, SGLT2i + adenosine A1 receptor (A1aR) antagonist (8-cyclopentyl-1,3-dipropylxanthine, 1mg/kg) group, and insulin group. Results SNGFR in the ZDF group was significantly higher than in the ZL group. The diameter of the afferent arteriole and efferent arteriole was also wider in ZDF rats than in ZL rats. The SNGFR and diameter of the afferent arteriole were significantly decreased after a single-dose administration of SGLT2i in ZDF. However, there was no significant diameter change in the efferent arteriole. Moreover, a decrease of SNGFR was not observed in the A1aR antagonist group after SGLT2i administration. Urinary excretions of glucose and sodium showed a similar pattern in the SGLT2i and SGLT2i+ A1aR antagonist groups. Conclusion The adenosine/A1aR pathway plays an important role in the regulation of the tonus of the afferent arteriole and is involved in the suppression of glomerular hyperfiltration by SGLT2 inhibition in type 2 diabetes.

P0089KEAP1/NRF2 PATHWAY REGULATES GFR BY INCREASING THE GLOMERULAR EFFECTIVE AREA WITHOUT AFFECTING THE AFFERENT/EFFERENT ARTERIOLE RATIO

Abstract Background and Aims The Keap1/Nrf2 pathway regulates the expression of a series of cytoprotective, anti-inflammatory and antioxidant genes. The Nrf2 activator, bardoxolone methyl (BARD), has consistently increased estimated GFR (eGFR) in clinical studies in patients with chronic kidney disease. BARD demonstrated improvement of renal function assessed by inulin clearance, the clinical gold standard for measuring GFR, in diabetic kidney disease patients. These findings suggest the Keap1/Nrf2 system is deeply involved in the regulatory mechanisms of GFR. However, the precise mechanisms are not fully elucidated. We pharmacologically and genetically investigated the mechanisms of GFR regulation by Keap1/Nrf2 system using in vivo multiphoton microscope (MPM) imaging techniques. Method C57BL/6 (Cont), Nrf2 knockout (Nrf2-KO), and Nrf2-activated Keap1-knockdown mice (Keap1-KD) were used. The mice were treated the synthetic triterpenoid RTA dh404 (10 mg/kg/day by gavage) which is a Nrf2 activator for rodents, for a week. We successfully developed the technique to evaluate single-nephron GFR (SNGFR) using MPM (Circulation 2019). The glomerular hemodynamics, diameter of the afferent/efferent arterioles and glomerular permeability were also evaluated. The calcium influx into cells in response to ATP and angiotensin II stimulation and the effect on [Ca2+]i by RTAdh404 were evaluated using Fluo 4 and Fura red in cultured mesangial cells and podocytes. Production of reactive oxygen species and nitric oxide (NO) availability were assessed by fluorescent method using CellROX® Deep Red and diaminofluorescein-FM diacetate (DAF-FM DA) upon the exposure to these stimuli. Results SNGFR in Keap1-KD mice was significantly higher than in the control (9.13±0.55 vs 4.40±0.39 nl/min, Figure 1). RTA dh404 increased SNGFR in the control but not in the Nrf2-KO mice (6.00±0.40 vs 4.66±0.35 nl/min, Figure 1). There was no significant change in the ratio of the glomerular afferent/efferent arteriole diameter in all groups. RTA dh404 treatment increased glomerular volume but did not affect the glomerular permeability of albumin and 40kd-dextran. RTA dh404-treatment inhibited calcium influx into cultured podocytes and mesangial cells induced by angiotensin II or ATP, thereby affecting contractile responses. Oxidative stress and NO-bioavailablity were also ameliorated with RTA dh404. Conclusion The Keap1/Nrf2 pathway plays a pivotal role in controlling GFR and presumably underlies the effect of BARD on GFR in patients.

P0754FGF23 AND URINARY PHOSPHATE EXCRETION FOR ESTIMATION OF NEPHRON NUMBER: DIFFERENCES BETWEEN CHRONIC KIDNEY FAILURE AND KIDNEY TRANSPLANTATION

Abstract Background and Aims Assuming that FGF23 levels correlate with phosphate excretion per nephron, nephron number can be estimated by measuring FGF23 levels and urinary phosphate excretion (FEP). Kuro-O proposed that the ratio of FEP to serum FGF23 levels should correlate with nephron number and is defined as the Nephron Index (NI). The aim of the study is calculating NI as nephron number estimation in patients affected by various degree of chronic kidney disease, both transplanted and not. Method In 147 CRF patients kidney function, mineral metabolism biomarkers and NI were evaluated. Nephron Index was calculated following Kuro-o’s equation (kuro-o 2019): NI=FEP·Ps·eGFRFGF23 Observed patients were divided into two groups: patients with CKD (noTX) and transplanted ones (TX). Results noTX group was made up of 67 patients (40 males and 27 females) affected by CKD stages from G1 to G5. TX group was composed by 80 patient (49 males and 31 females) with various degree of CKD (G1T-G4T) showing mean graft age of 83,2 ± 54,8 months (range: 10,3-268,0). The two groups differed for age (mean age 59 ± 15,6 years in CKD, 55± 10,3years in TX). Mean eGFR did not differ between TX and noTX but NI was higher in TX since FEP was higher despite lower FGF23 levels in TX. The difference in FGF23 levels does not appear to depend on Klotho and PTH whose serum levels were no different between Tx and noTx. (Table1). As far as correlations are concerned, NI correlated with eGFR, FGF23, PTH and 1,25D in both group, while NI correlated with FEP only in the TX group. It's interesting that no correlation existed between FGF23 and sP, FEP, eGFR and sKlotho in TX differently from noTX (Table2). Discussion. NI could not be properly defined as nephron number estimation in TX pts. However it may represent higher function of residual nephrons, since higher FEP did not correlate to FGF23 and could be determined by compensatory hyperfiltration (increased single nephron GFR) in transplanted patients. Conclusion After kidney transplantation, high NI value could have a functional meaning rather than represent residual number of nephrons.

Single-Nephron GFR in Patients With Obesity-Related Glomerulopathy

IntroductionObesity-related glomerulopathy (ORG) is a slowly progressive kidney disease occurring in association with obesity. It is characterized histopathologically by glomerulomegaly, likely caused by single-nephron hyperfiltration that has not been demonstrated in humans because of technical difficulty in measuring single-nephron glomerular filtration rate (SNGFR) in the clinical setting.MethodsTotal glomerular number per kidney, with or without global glomerulosclerosis, was estimated by the combination of cortical volume assessment via unenhanced computed tomography and biopsy-based stereology. Mean glomerular volume was calculated from the measured area of glomerular tufts. Both SNGFR and single-nephron urinary protein excretion (SNUPE) were estimated by dividing values for estimated glomerular filtration rate and urinary protein excretion by the number of nonsclerotic glomeruli. Living kidney donors were used as healthy controls.ResultsA total of 48 ORG patients with average nonsclerotic glomerular numbers of 456,000 ± 235,000 per kidney were included. The values for SNGFR in ORG patients with chronic kidney disease (CKD) stages 1 and 2 were higher than for nonobese and obese controls (97 ± 43 vs. 59 ± 21 vs. 64 ± 21 nl/min, respectively, P = 0.001). Nonsclerotic glomerular number decreased with advancing stages of renal functional impairment. The presence of ORG with more advanced CKD stages was associated with lower SNGFR and marked elevation in SNUPE levels, with no difference in the mean glomerular volume between the stages.ConclusionsThese results provide functional evidence for single-nephron hyperfiltration in patients with ORG, and identify compensatory failure to maintain effective SNGFR as a feature of advanced-stage ORG.

Mesangial Cells Regulate the Glomerular Filtration Rate: A Multiphoton Microscopy Study

The intra‐glomerular mesangial cells fill the space between the glomerular capillaries and may support the structural integrity of the glomerulus. Data from cultured mesangial cells suggest a contractile cell phenotype. However, the in vivo relevance of mesangial cells for the function of the glomerular filtration barrier and the regulation of glomerular filtration remains unclear.Here we hypothesized that contractile mesangial cells actively regulate the single nephron glomerular filtration rate (snGFR) and that mesangial cells support the integrity of the glomerular filtration barrier. To test this hypothesis, we assessed the function of mesangial cells in the life animal by intravital multiphoton microscopy. Mesangial cells were depleted in Munich Wistar Froemter rats using the Thy1.1 antibody model. Administration of the Thy1.1 antibody (1 mg/kg BW) caused the loss of approximately 80% of mesangial cells. Immune‐staining of kidneys for Thy1.1 showed an exclusive binding of the antibody to mesangial cells. To assess if mesangial cells contribute to the regulation of snGFR by vasoconstrictor agents, angiotensin II (12.8 ng/kg/min) was infused during intravital imaging. In response to angiotensin II, the systolic arterial blood pressure rose from 100.5 to 125.1 mm Hg mm Hg in Thy1.1‐treated rats (n=17; p<.0001), and, similarly, from 98.8 mm Hg to 125.2 mm Hg in controls (n=19; p<.0001). SnGFR, as determined by flow measurements in the proximal tubule after bolus injections of Lucifer Yellow, decreased after angiotensin II infusion by 35±12% in controls (n=5; p=.004), whereas it remained unchanged in Thy1.1‐treated rats (−12±10%, n=5). In control rats, changes in snGFR after angiotensin II infusion were accompanied by a marked rotating movement of the capillary loops within Bowman’s space. This phenomenon was absent in Thy1.1‐treated rats. In addition, glomerular capillaries at baseline were dilated, and blood flow velocity was reduced in Thy1.1‐treated rats compared with controls, whereas the calculated capillary flow was similar in both groups (5.8±.6 nl/min in controls vs. 4.9±.5 nl/min in Thy1.1‐treated rats; n=6; n.s.). Glomerular capillary flow declined in response to angiotensin II in controls but it remained unchanged in Thy1.1‐treated rats (2.7±.2 nl/min vs. 3.7±.4 nl/min, respectively).Next, to determine the relevance of mesangial cells for the integrity of the glomerular barrier function, we measured the sieving coefficient for albumin (GSC). Albumin GSC was low in control rats averaging 0.00059±.00006. After angiotensin II infusion, the GSC for albumin increased to 0.00112±.00015 in control rats (n=8; p<.0001). In contrast, in Thy1.1‐treated rats, baseline albumin GSC was elevated (0.0033±.00080; n=7; p<.0001) and did not change in response to angiotensin II (0.0047±.001; n.s.).In summary, the depletion of mesangial cells by Thy1.1 antibody application abolished the regulation of the snGFR by angiotensin II and markedly increased the permeability of the filtration barrier for plasma proteins. Or data suggest that mesangial cells contribute to the regulation of the snGFR in vivo. Furthermore, mesangial cells are crucially involved in the maintenance of the integrity of the glomerular filtration barrier.Support or Funding InformationSFB 1350/B2

Chronic sodium bicarbonate may promote metabolic dysregulation in a rodent model of CKD

Chronic kidney disease (CKD) has no cure, and few treatment options targeted at slowing the decline in glomerular filtration rate (GFR). Several small clinical trials have indicated that oral sodium bicarbonate (NaHCO3) targets and slows the decline in GFR in CKD. The reported beneficial effects of NaHCO3 in this setting have led to its now common use clinically. It’s known however, that chronic alkali ingestion can promote potassium (K+) wasting, and that K+ depletion has been linked with metabolic dysregulation. Further, CKD patients who are supplemented chronically with oral NaHCO3 display significantly decreased plasma K+ levels relative to control treated counterparts, although these decreases in plasma K+ remain within normal range. No study has investigated the metabolic impacts of chronic NaHCO3 treatment and K+ wasting in CKD. To this end, we tested the hypothesis that in the setting of high tubular flow, chronic NaHCO3 ingestion would promote K+ wasting and metabolic dysfunction. In order to test our hypothesis, 9 male, 8 wk old Sprague Dawley (SD) rats underwent a 2/3 nephrectomy (Nx) and telemetry implantation. The 2/3 Nx is a model of reduced renal mass that has increased single nephron GFR in the absence of metabolic acidosis, similar to most human patients with CKD. Following 4 weeks of recovery, animals were given either 0.1M NaHCO3 (Bicarb, n=3), 0.1M NaCl (Vehicle, n=3) or 0.2M NH4Cl‐NaHCO3 (AC/B, n=3) in the drinking water for 12 weeks. Following 9 weeks of drinking water treatment, we observed a significantly higher hemoglobin A1c in our bicarb treated group compared to vehicle and AC/B treated groups (Bicarb 4.6 ± 0.1 vs Veh 4.2 ± 0.3 vs AC/B 4.2 ± 0.2; One‐way ANOVA, p=0.05). Following another 3 weeks of drinking water treatment, we performed blood gas measurements to assess plasma K+. In contrast to our hypothesis we observed the highest levels of plasma K+ in our bicarb treated animals compared to our vehicle or AC/B groups (Bicarb 5.18 ± 0.28 vs Veh 4.48 ± 0.66 vs AC/B 4.5 ± 0.21; One‐way ANOVA, p=0.16). Our data show that oral NaHCO3 could worsen blood glucose control in the setting of high tubular flow. Additionally, plasma K+ may not be a reliable measure for assessment of K+ status in CKD patients supplemented with NaHCO3. As the majority of K+ is stored intracellularly, further studies characterizing the effect of chronic alkali ingestion on total body K+ are warranted to assess the impact of NaHCO3 on K+ depletion in this patient population.Support or Funding InformationNIH PO1H134604 and DK099548

Effects of Acute SGLT2 Blockade and Dietary NaCl on Glomerular Hemodynamics in Diabetic Rats

Inhibitors of the main proximal tubular Na‐glucose cotransporter (SGLT2) mitigate diabetic glomerular hyperfiltration and are approved by FDA for slowing the progression of diabetic kidney disease. It has been proposed that SGLT2 inhibitors improve hard renal outcomes by reducing the glomerular capillary pressure gradient (ΔP) through TGF signaling, which begins with a decrease in fluid reabsorption by the proximal tubule (Jprox). However, the effect of SGLT2 inhibition on ΔP has not been measured up to now.We studied effects of acute SGLT2 blockade (ertugliflozin) on Jprox and glomerular hemodynamics by 2‐period micropuncture in STZ‐diabetic rats fed high or low NaCl diets (HS/LS). ΔP was measured by direct capillary puncture in Wistar Froemter rats (MWF), which have surface glomeruli suitable for this purpose. ΔP was computed from tubular stop‐flow pressure in Wistar rats (Wis), which lack surface glomeruli. Hence, TGF was allowed to operate normally while measuring ΔP in MWF but rendered inoperative while measuring ΔP in Wis. SGLT2 blockade was confirmed to reduce Jprox by collecting fluid from late proximal tubules for volume and inulin clearance.Summary and Conclusions about the diabetic kidney under conditions of moderate hyperglycemia: SGLT2 blockade acutely reduces Jprox, which leads to a decrease in glomerular capillary pressure, but only when TGF is permitted to operate normally. Baseline glomerular capillary pressure varies inversely with dietary NaCl and the impact of SGLT2 blockade applies across a wide range of dietary NaCl.Support or Funding InformationNIDDK RO1 DK112042 Merck Group (n) Glucose (mg/dl) GFR(ml/min) ΔP (mmHg) Jprox' (nl/min) control SGLT2 blocked control SGLT2 blocked control SGLT2 blocked MWF LS (6) 344±41 4.2±0.5 3.5±0.5 * 41.2±1.5 33.0±1.9 * MWF HS (8) 408±35 4.1±0.3 3.3±0.4 * 36.6±1.3 ** 32.4±1.6 * Wis LS (6) 355±41 4.8±0.4 3.8±0.5 * 43.6±1.9 43.7±2.4 18.9±1.3 10.2±1.1 * Wis HS (9) 400±52 4.5±0.5 3.8±0.6 * 32.6±2.3 ** 35.7±3.0 18.1±1.3 12.6±1.1 * *P<0.01 vs control period. **P<0.01 vs LS. Jprox’ is Jprox adjusted for single nephron GFR by ANCOVA to obtain the "primary" effect of SGLT2 blockade on the tubule.

A Systematic Review of Renal Functional Reserve in Adult Living Kidney Donors

IntroductionThe kidney’s capacity to increase its glomerular filtration rate (GFR) in response to a higher functional demand is known as the renal functional reserve (RFR). Good short-term outcomes after living kidney donation have led to more acceptance of borderline donors (with hypertension, obesity, older age) due the ongoing shortage of donor organs. Given recent concerns about increased long-term risk in some donor subgroups, better donor stratification is needed. Measurement of RFR could inform assessment of donor risk.MethodsA systematic literature review of studies that assessed RFR in donors pre- and/or post-donation was performed. Given study heterogeneity, descriptive analysis and narrative synthesis was conducted.ResultsSixteen of 3250 identified studies published between 1956 and 2019 met inclusion criteria. Most studies were cross-sectional and conducted before (n = 8) and/or after (n = 16) kidney donation. Methods for measurement of GFR, effective renal plasma flow (ERPF) and RFR were not standardized. Changes in filtration fraction (FF) and ERPF relative to GFR observed after donation varied depending on stimulus used to induce RFR. Overall, RFR fell after donation; however, over the shorter term, RFR was largely preserved in young healthy donors. RFR was more significantly reduced in donors with hypertension, obesity, or older age.ConclusionExisting data suggest possible blunting of RFR post-donation in older, obese, and hypertensive donors, which may represent increased single-nephron GFR at baseline. The long-term implications of these changes deserve further study to determine utility in informing selection of borderline kidney donors.

Effects of C-peptide on the single-nephron glomerular filtration rate in early diabetic rats

Effects of C-peptide on the single-nephron glomerular filtration rate in early diabetic rats

In-vivo techniques for determining nephron number.

Many studies have suggested low nephron endowment at birth contributes to the risk of developing hypertension and chronic kidney disease (CKD) later in life. Loss of nephrons with age and disease is largely a subclinical process. New technologies are needed to count nephrons as glomerular filtration rate (GFR) is a poor surrogate for nephron number. Cortical volume, glomerular density, and percent globally sclerotic glomeruli are imperfect surrogates for nephron number. The disector-fractionator method is the most accurate method to count nephrons but is limited to autopsy settings. Glomerular density combined with kidney imaging and ultrafiltration coefficient-based methods require a kidney biopsy, and have been applied in living humans (kidney donors). Low nephron number predicts a higher postdonation urine albumin. Contrast-enhanced MRI has detected glomeruli without a biopsy, but so far, not in living humans. Currently, there is no accurate and well tolerated method for determining nephron number in living humans. A clinically useful method may allow GFR to be replaced by its more relevant determinants: nephron number and single nephron GFR. This could revolutionize nephrology by separating the measurement of chronic disease (nephron loss) from more reversible hemodynamic effects (nephron hyperfiltration/hypofiltration).

Progressive Nephron Loss in Aging Kidneys: Clinical-Structural Associations Investigated by Two Anatomical Methods.

ABSTRACTTwo major studies of structural changes associated with aging in human kidneys are reviewed and new information presented. The studies are the Monash University stereologically analyzed series of 319 autopsy kidneys from the United States in which 44% were white and the Mayo Clinic CT angiogram/biopsy analysis of 1,388 US kidney donors in which 97% were white. Hypertension rates in the Monash series were 48% and included moderate and severe hypertension. In the Mayo Clinic study, 12% had mild hypertension. The studies showed no relationship between glomerular number and hypertension except for a weak relationship with older white women in the Monash series. An inverse relationship was present between glomerular number and glomerular volume, a reciprocity that tended to enhance glomerular mass and by inference filtration capacity with lower nephron numbers. This relationship seemed to be present whether low nephron numbers were intrinsic or acquired. In the Mayo Clinic studies, pretransplant iothalamate clearances demonstrated that single nephron (SN) glomerular filtration rates (GFR) were similar throughout the range of glomerular number in subjects younger than 70 years, but that increased SNGFR correlated with nephron hypertrophy and increased nephrosclerosis particularly at 70 years of age and over. Hypertension at least through middle age cannot be related to a deficiency of glomeruli, but glomeruli are lost with later aging in association with adaptive nephron hypertrophy that can maintain GFR near normal. These studies help define an age‐related nephropathy that overlaps with hypertension as a potential cause of end‐stage renal disease when glomerulosclerosis is advanced.

Functional implications of the sex differences in transporter abundance along the rat nephron: modeling and analysis.

The goal of the present study was to investigate the functional implications of sexual dimorphism in the pattern of transporters along the rodent nephron as reported by Veiras et al. (J Am Soc Nephrol 28: 3504-3517, 2017). To do so, we developed sex-specific computational models of water and solute transport along the superficial nephrons from male and female rat kidneys. The models account for the sex differences in the abundance of apical and basolateral transporters, single nephron glomerular filtration rate, and tubular dimensions. Model simulations predict that ~70% and 60% of filtered Na+ is reabsorbed by the proximal tubule of male and female rat kidneys, respectively. The lower fractional Na+ reabsorption in female kidneys is due primarily to their smaller transport area, lower Na+/H+ exchanger activity, and lower claudin-2 abundance, culminating in significantly larger fractional delivery of water and Na+ to the downstream nephron segments in female kidneys. Conversely, the female distal nephron exhibits a higher abundance of key Na+ transporters, including Na+-K+-Cl- cotransporters, Na+-Cl- cotransporters, and epithelial Na+ channels. The higher abundance of transporters accounts for the enhanced water and Na+ transport along the female, relative to male, distal nephron, resulting in similar urine excretion between the sexes. Consequently, in response to a saline load, the Na+ load delivered distally is greater in female rats than male rats, overwhelming transport capacity and resulting in higher natriuresis in female rats.

CKD: A Call for an Age-Adapted Definition.

Current criteria for the diagnosis of CKD in adults include persistent signs of kidney damage, such as increased urine albumin-to-creatinine ratio or a GFR below the threshold of 60 ml/min per 1.73 m2 This threshold has important caveats because it does not separate kidney disease from kidney aging, and therefore does not hold for all ages. In an extensive review of the literature, we found that GFR declines with healthy aging without any overt signs of compensation (such as elevated single-nephron GFR) or kidney damage. Older living kidney donors, who are carefully selected based on good health, have a lower predonation GFR compared with younger donors. Furthermore, the results from the large meta-analyses conducted by the CKD Prognosis Consortium and from numerous other studies indicate that the GFR threshold above which the risk of mortality is increased is not consistent across all ages. Among younger persons, mortality is increased at GFR <75 ml/min per 1.73 m2, whereas in elderly people it is increased at levels <45 ml/min per 1.73 m2 Therefore, we suggest that amending the CKD definition to include age-specific thresholds for GFR. The implications of an updated definition are far reaching. Having fewer healthy elderly individuals diagnosed with CKD could help reduce inappropriate care and its associated adverse effects. Global prevalence estimates for CKD would be substantially reduced. Also, using an age-specific threshold for younger persons might lead to earlier identification of CKD onset for such individuals, at a point when progressive kidney damage may still be preventable.

SUN-230 ASSOCIATION BETWEEN ESTIMATED PROTEIN INTAKE AND SINGLE-NEPHRON GLOMERULAR FILTRATION RATE IN HEALTHY ADULTS

High protein intake can increase renal plasma flow and glomerular filtration rate (GFR) in response to excretory overload, which may exacerbate kidney disease progression. Glomerular hypertrophy has been shown to be closely associated with sclerosis at the level of a single nephron in experimental models. However, the mechanisms by which an excessive protein diet causes glomerular hyperfiltration and increases single-nephron GFR (SNGFR) have not been fully elucidated in humans. The present study aimed to calculate SNGFR by estimating the number of nephrons in living kidney donors and to evaluate the clinicopathological findings associated with SNGFR.

How Do Kidneys Adapt to a Deficit or Loss in Nephron Number?

A deficit or loss in the number of nephrons, the functional unit of the kidney, can induce compensatory growth and hyperfunction of remaining nephrons. An increase in single nephron glomerular filtration rate (SNGFR) aims to compensate but may be deleterious in the long term. The increase in SNGFR is determined by the dynamics of nephron loss, total remaining GFR, the body's excretory demand, and the functional capacity to sustain single nephron hyperfunction.

Impaired SIRT1 activity leads to diminution in glomerular endowment without accelerating age-associated GFR decline.

Glomerular filtration rate (GFR) declines with age such that the prevalence of chronic kidney disease is much higher in the elderly. SIRT1 is the leading member of the sirtuin family of NAD +‐dependent lysine deacetylases that mediate the health span extending properties of caloric restriction. Since reduction in energy intake has also been shown to decrease age‐related kidney disease in rodents, we hypothesized that a diminution in SIRT1 activity would accelerate the GFR decline and structural injury with age. To test this hypothesis, we compared changes in the kidney structure and function in control mice and mice that carry a point mutation at a conserved histidine (H355Y) of SIRT1 that renders the enzyme catalytically inactive. Taking advantage of this mouse model along with the disector/fractionator technique for glomerular counting and direct measurements of GFR by inulin clearance, we assessed the impact of SIRT1 inactivity on kidney aging. At 14 months of age, SIRT1 catalytically inactive (Sirt1 Y/Y) mice had lower GFRs and fewer glomeruli than their wild‐type (Sirt1 +/+) counterparts. This was not, however, due to either accelerated GFR decline or increased glomerulosclerosis and loss, but rather to reduced glomerular endowment in Sirt1 Y/Y mice. Moreover, the compensatory glomerular hypertrophy and elevated single nephron GFR that customarily accompany reduction in nephron number were absent in Sirt1 Y/Y mice. These findings suggest a role for SIRT1 not only in determining nephron endowment but also in orchestrating the response to it.

Heavy Metal Intoxication: A Key-Player in Chronic Kidney Disease (A Review)

Heavy metals gain entry into biological systems mainly via inhalation and ingestion, and also via radiation or radio-therapeutic measures. The accumulation of these heavy metals in biological systems overtime may cause several deleterious health challenges such as liver, kidney and brain damages amongst others. Intoxication with heavy metals may either be acute or chronic, and because the kidney has the ability to reabsorb and accumulate divalent metals, it happens to be the primary target organ for heavy metal toxicity, inducing renal damage. The extent of this damage depends on the dose, nature, route and duration of exposure to the metal. Chronic kidney disease (CKD) is characterized by a permanent loss of nephrons accompanied by an eventual decline in glomerular filtration rate (GFR); this (to a greater extent) maybe due to heavy metal intoxication and the renal reabsorption of these heavy metals. Although 70 percent of the heavy metals are reabsorbed in the proximal tubule, all segments of the nephrons are involved in the reabsorption of these metals, where several transporters such as the Divalent Metal Transporter (DMT)-1, Na+/amino acid co-transporter, Zinc Transporter (ZnT)-1 and stretch-activated cation channels (SAC) facilitate the reabsorption. In the nephrons of each kidney, heavy metals are primarily reabsorbed via the apical membrane and accumulate at the basolateral membrane; these heavy metals do not readily exit the basolateral membrane, which overtime may result in chronic inflammation of the nephrons, fibrosis and kidney failure. However, the loss of nephrons and decline in GFR in CKD are compensated by certain changes (glomerular and cellular, enhanced renal blood flow, enhanced single nephron glomerular filtration rate and tubular hypertrophy) in the remaining functional nephrons. These changes help to deliver solutes to the remaining functional nephrons for uptake by the epithelial cells of the renal tubules. Also, because the luminal and basolateral surfaces of tubular epithelial cells of the remaining healthy nephrons are also potentially exposed to higher levels of metabolic wastes, xenobiotics, heavy metals and other nephrotoxicants, renal injury, tubular or glomerulosclerosis, and death of these nephrons occur. These compensatory changes become insufficient once about 75 percent of the nephrons are no longer functional and incapable of maintaining homeostasis and renal function. This results in the accumulation of metabolic wastes in the blood, and induction of metabolic disturbances and/or organ intoxication.

Functional Implications of Sexual Dimorphism of Transporter Patterns along the Rat Nephron

The goal of this study is to investigate the functional implications of the sexual dimorphism in transporter patterns along the rodent nephron as reported by Veiras et al. (J Am Soc Nephrol 2018). To do so, we developed sex‐specific computational models of water and solute transport from Bowman space to the papillary tip of a superficial nephron of the kidneys of the male and female rats. The models account for the sex differences in expression levels of the apical and basolateral transporters, in single‐nephron glomerular filtration rate, and in tubular dimensions. Model simulations predict that ~70 and 40% of the filtered volume is reabsorbed by the proximal tubule of the male and female rat kidneys, respectively. The lower fractional volume reabsorption in the female rat can be attributed to their smaller transport area and lower aquaporin‐1 expression level. The latter also results in a larger contribution of the paracellular pathway to water transport. Similar fractions of the filtered Na+ are reabsorbed by the male and female proximal tubule models, respectively. The lower fractional Na+ reabsorption in female is due primarily to their smaller transport area, lower Na+/H+‐exchanger (NHE3) activity, and lower claudin‐2 abundance. This results in significantly larger delivery of water and Na+ to the downstream nephron segments in female. Unlike in the proximal tubule, the distal nephron of the female rat exhibits higher expression levels in key Na+ transporters, including Na+‐K+‐Cl− cotransporter, Na+‐Cl− cotransporter, and epithelial Na+ channel. Not only do the higher expression levels of these transporters account for the enhanced water and Na+ transport along the female distal nephron, relative to male, but they may also contribute to the female rat's ability to excrete a saline load more rapidly than male.Support or Funding InformationThis research was supported by the Canada 150 Research Chair program and by the National Institutes of Health: National Institute of Diabetes and Digestive and Kidney Diseases, grant R01DK106102.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Glomerular Capillary Hypertrophy in the Diabetic Rat Normalizes Wall Shear Stress: A Modeling Study

Diabetes mellitus (DM) reduces renal hemodynamic autoregulation, leading to increased glomerular pressure (ΔP) and afferent plasma flow (Qa) which is assumed to increase glomerular capillary wall shear stress (SS). However, glomerular capillaries undergo hypertrophy in DM, causing an increase in capillary diameter that would theoretically reduce capillary wall SS (Seyer‐Hansen, Kjartan. Kid. Int. 23.4 (1983): 643–646.). The actual magnitudes of SS in the glomerular capillaries have not been estimated in DM and are of significance because in response to increased SS, endothelial cells increase expression of adhesion molecules and growth factors associated with glomerulopathy.We developed a mathematical model of blood flow through an anatomically‐accurate rat glomerular capillary network to estimate the magnitudes of SS on the glomerular capillary walls in DM and control. Individual filtration rates and wall SS were calculated for each capillary segment of the network based on values of Qa, ΔP and single nephron GFR (SNGFR) obtained from micropuncture studies. To validate mechanical predictions of our model, we compared our calculated SS magnitudes to results from intravital imaging studies of blood flow in rat glomeruli (Ferrell, Nicholas, et al. Am. J. Physiology‐Renal 308.6 (2015): F588‐F593.). To calculate SS magnitudes in the rat glomerulus in control and DM, we performed simulations with parameters obtained from micropuncture studies using normal rats and rats with streptozotocin induced Type I diabetes. In this study, DM increased Qa and ΔP by 74.6% and 33.3%, respectively but reduced the filtration coefficient (Kf) such that filtration fraction (FF) was maintained (Zatz, Roberto, et al. J. clin. Inv. 77.6 (1986): 1925–1930).The model was calibrated using an algorithm previously described (Remuzzi, Andrea, et al. Am. J. Physiology‐Renal 263.3 (1992): F562‐F572). Our model's calculated mean glomerular capillary wall SS was within 10% of the mean SS predicted using intravital imaging, validating our model for use in predicting mechanical aspects of blood flow in the glomerulus. Our simulations demonstrated a wide variance in SS throughout the glomerular capillary network, consistent with previous modeling results. Using parameters from micropuncture experiments and taking into account hypertrophy of the glomerular capillaries, the model predicted mean SS values of 42.1 dynes/cm2 in control and 42.3 dynes/cm2 in DM.We have developed and validated an anatomically‐accurate mathematical model of glomerular filtration for use in predicting functional and mechanical results of varied glomerular pressure, afferent plasma flow and Kf. Using this model, we demonstrated that hypertrophy of the capillaries normalizes capillary wall SS in DM, which may be a protective mechanism limiting the increase in SS and the consequent glomerular injury.Support or Funding InformationThis work was supported by the National Institute of General Medical Sciences IDeA Program (CoBRE P30GM103337) (principal investigator: L. G. Navar) O. Richfield is a graduate student supported by NSF IGERT Grant NSF DGE‐1144646.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

A computational model of epithelial solute and water transport along a human nephron.

We have developed the first computational model of solute and water transport from Bowman space to the papillary tip of the nephron of a human kidney. The nephron is represented as a tubule lined by a layer of epithelial cells, with apical and basolateral transporters that vary according to cell type. The model is formulated for steady state, and consists of a large system of coupled ordinary differential equations and algebraic equations. Model solution describes luminal fluid flow, hydrostatic pressure, luminal fluid solute concentrations, cytosolic solute concentrations, epithelial membrane potential, and transcellular and paracellular fluxes. We found that if we assume that the transporter density and permeabilities are taken to be the same between the human and rat nephrons (with the exception of a glucose transporter along the proximal tubule and the H+-pump along the collecting duct), the model yields segmental deliveries and urinary excretion of volume and key solutes that are consistent with human data. The model predicted that the human nephron exhibits glomerulotubular balance, such that proximal tubular Na+ reabsorption varies proportionally to the single-nephron glomerular filtration rate. To simulate the action of a novel diabetic treatment, we inhibited the Na+-glucose cotransporter 2 (SGLT2) along the proximal convoluted tubule. Simulation results predicted that the segment’s Na+ reabsorption decreased significantly, resulting in natriuresis and osmotic diuresis.