Single Nephron Glomerular Filtration Rate Research Articles

Sparsentan improves glomerular hemodynamics, cell functions, and tissue repair in a mouse model of FSGS.

Dual endothelin-1 (ET-1) and angiotensin II (AngII) receptor antagonism with sparsentan has strong antiproteinuric actions via multiple potential mechanisms that are more pronounced, or additive, compared with current standard of care using angiotensin receptor blockers (ARBs). Considering the many actions of ET-1 and AngII on multiple cell types, this study aimed to determine glomeruloprotective mechanisms of sparsentan compared to the ARB losartan by direct visualization of its effects in the intact kidney in focal segmental glomerulosclerosis (FSGS) using intravital multiphoton microscopy. In both healthy and FSGS models, sparsentan treatment increased afferent/efferent arteriole diameters; increased or preserved blood flow and single-nephron glomerular filtration rate; attenuated acute ET-1 and AngII-induced increases in podocyte calcium; reduced proteinuria; preserved podocyte number; increased both endothelial and renin lineage cells and clones in vasculature, glomeruli, and tubules; restored glomerular endothelial glycocalyx; and attenuated mitochondrial stress and immune cell homing. These effects were either not observed or of smaller magnitude with losartan. The pleiotropic nephroprotective effects of sparsentan included improved hemodynamics, podocyte and endothelial cell functions, and tissue repair. Compared with losartan, sparsentan was more effective in the sustained preservation of kidney structure and function, which underscores the importance of the ET-1 component in FSGS pathogenesis and therapy.

Evaluation of glomerular hemodynamic changes by sodium-glucose-transporter 2 inhibition in type 2 diabetic rats using in vivo imaging

The mechanisms responsible for glomerular hemodynamic regulation with sodium-glucose co-transporter 2 (SGLT2) inhibitors in kidney disease due to type 2 diabetes remain unclear. Therefore, we investigated changes in glomerular hemodynamic function using an animal model of type 2 diabetes, treated with an SGLT2 inhibitor alone or in combination with a renin-angiotensin-aldosterone system inhibitor using male Zucker lean (ZL) and Zucker diabetic fatty (ZDF) rats. Afferent and efferent arteriolar diameter and single-nephron glomerular filtration rate (SNGFR) were evaluated in ZDF rats measured at 0, 30, 60, 90, or 120 minutes after the administration of a SGLT2 inhibitor (luseogliflozin). Additionally, we assessed these changes under the administration of the adenosine A1 receptor (A1aR) antagonist (8-cyclopentyl-1,3-dipropylxanthine), along with coadministration of luseogliflozin and an angiotensin II receptor blocker (ARB), telmisartan. ZDF rats had significantly increased SNGFR, and afferent and efferent arteriolar diameters compared to ZL rats, indicating glomerular hyperfiltration. Administration of luseogliflozin significantly reduced afferent vasodilatation and glomerular hyperfiltration, with no impact on efferent arteriolar diameter. Urinary adenosine levels were increased significantly in the SGLT2 inhibitor group compared to the vehicle group. A1aR antagonism blocked the effect of luseogliflozin on kidney function. Co-administration of the SGLT2 inhibitor and ARB decreased the abnormal expansion of glomerular afferent arterioles, whereas the efferent arteriolar diameter was not affected. Thus, regulation of afferent arteriolar vascular tone via the A1aR pathway is associated with glomerular hyperfiltration in type 2 diabetic kidney disease.

#439 Nephron number determination in whole slide mice kidney specimens using a deep learning assisted disector/fractionator approach

Abstract Background and Aims Nephron number highly varies between different species and within species, ranging in humans from 200 000 to 2 000 000 nephrons per kidney. Low nephron numbers can promote early onset of end-stage kidney disease. Nephron assessment is highly needed for better diagnosis and more targeted treatment. However, its assessment in vivo is not established in clinical practice yet. Pioneer studies suggest nephron number estimation in vivo through a combination of glomerular densities in kidney biopsies together with medical imaging. In contrast, post-mortem nephron assessment either using optical clearing or by exhaustive kidney sectioning is well studied but remains laborious. Here, we propose a new semi-automated nephron counting approach using serial PAS-stained whole slide kidney sections, deep learning segmentation and slide registration. Method Mice were sacrificed, kidneys harvested, measured, halved in the coronary plane and then opposingly embedded in paraffin. Subsequently, the entire kidney was sectioned with 10 µm thickness per slide. Tissue was stained with PAS and slides were digitalized using Aperio GT 450 DX scanner (Leica, Wetzlar, Germany) (Fig. a). Deep learning segmentation of glomeruli was performed in the first and every 10th (reference) section and subsequent (look-up) section using Tensorflow. Obtained segmentation results were manually quality controlled and adjusted, if necessary, in QuPath (Fig. b). Final segmentations of the look-up slides were registered to the reference slides in ImageJ using manual landmarks with the ImageJ BigWarp plugin (Fig. c). Glomeruli being present either in the look-up slide, or in the reference slide were counted as disector particles (Q-). Kidney areas on every examined slide were measured. Total number of glomeruli and total kidney area were determined as previously described (Fig. d). Results A kidney from a 12-week-old mouse suffering chronic kidney disease (mouse line Alb-creERT2 (ki/ki); Glut9 (fl/fl)) was used. Kidney weight was 162 mg, length 11 mm, width 6 mm, depth 4 mm and kidney volume 260 mm³. In total the kidney was sectioned in 95 slices. 20 reference sections and its respective look-up sections were stained and digitalized. In total 5981 glomeruli were segmented in 40 kidney slices using the deep learning algorithm. Glomerular segmentation quality was verified by two independent observers (mean dice score 0.79). Inaccuracies, if any, where manually corrected. For image registration in average 18.1 landmarks per slide were created within the ImageJ BigWarp plugin. Registration took in average 5 minutes per slide. Glomeruli without match (disector particles, Q-) were automatically counted. Number of disector particles (Q-) ranged from 32 to 244 per slide. Total nephron number was 11.085 and kidney area 210 mm³. The differences to the manual measured volume (260 mm³) might be explained by discard of small kidney fractions for the used sectioning technique. Conclusion We report, to our knowledge, the first deep learning assisted disector/fractionator approach to determine nephron number in whole slide kidney specimens. This method requires no physical disector setup, is scalable, cheap, time efficient, and unbiased. The method was so far validated in a mouse model of chronic kidney disease. The method will further be validated on different animal models. Additionally, occurrence of atubular glomeruli will be considered in nephron number determination. Furthermore, the integration of GFR measurements might allow the calculation of mean single nephron GFR and might together with automated glomerular area measurements unravel pathophysiologic processes in kidney ageing and disease.

Glomerular Hemodynamics During Recovery from Renal Ischemia-Reperfusion Injury Addressed with Micropuncture in Rats

Objective: To establish the physical basis for low GFR in ischemic renal injury (IR) and determine whether abnormalities in GFR determinants persist after renal recovery. Method: IR was generated in male and female Wistar Froemter rats by clamping both renal pedicles for 60 minutes. Controls had same surgery without pedicle clamp. Glomerular hemodynamics were assessed by renal micropuncture (MP) after 1 day, 1 week, or 4 weeks. Single nephron GFR (SNGFR) was paired with tubular stop-flow (PSF) and free-flow pressure (PFF) in each nephron. Afferent Starling pressure (EFPa) is PSF-PFF. Glomerular capillary pressure (PGC) was computed from PSF, PFF and plasma oncotic pressure. ANOVA factors included IR, sex, time point and date of experiment (nested). Inter-nephron heterogeneity was extracted from normalized root-square difference between each data point and the mean for that animal. (total n=349 nephrons, 47 rats). Results: [Formula: see text] Summary and conclusion: The physical cause of low GFR in IR was low EFPa, which resulted from low PGC with normal PFF in all animals except 1. The injured kidney manifest elevated inter-nephron variability for all variables. Mean values for SNGFR and intra-renal pressures returned to normal in the “recovered” kidney, but variability between nephrons did not return to normal. Female rats had lower baseline SNGFR and PGC than males but the hemodynamic patterns during IR and renal recovery were not differentiable by sex (excluding one female outlier with high PFF). Despite “normal” GFR and PGC, the “recovered” kidney is not a “normal” kidney. NIH NIDDK R01 132690. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Insights into the Regulation of GFR by the Keap1-Nrf2 Pathway.

Literature data suggest that the activation of the Kelch-like ECH-associated protein 1 (Keap1)-nuclear factor (erythroid-derived 2)-like 2 (Nrf2) pathway increases glomerular filtration rate (GFR) in patients with type 2 diabetes and chronic kidney disease. However, the mechanisms whereby the Keap1-Nrf2 pathway regulates GFR are unknown. Methods: Various renal physiological parameters were assessed in C57BL/6 mice (WT), Nrf2-deficient mice, and Nrf2-activated Keap1-knockdown mice. Also, these parameters were assessed following the administration of RTA dh404 (CDDO-dhTFEA), a Nrf2 activator. Pharmacological and genetic Keap1-Nrf2 activation increased renal blood flow (p <0.05), glomerular volume (p <0.05) and GFR (p <0.05), but did not alter the afferent-to-efferent arteriolar diameter ratio or glomerular permeability. Calcium influx into the podocytes through TRPC channels in response to H2O2 was suppressed by Keap1-Nrf2 activation and TRPCs inhibition. Treatment with a TRPC6 and TRPC5 inhibitors increased single-nephron GFR in WT mice. In conclusion, the Keap1-Nrf2 pathway regulates GFR through changes in ultrafiltration by modulating redox-sensitive intracellular calcium signaling and cellular contractility, mediated through TRPC activity, in glomerular cells, in particular the podocytes.

Single-Nephron GFR in Different Glomerular Basement Membrane Stages of Membranous Nephropathy.

Alterations in single-nephron dynamics have been demonstrated in animal models of membranous nephropathy (MN). This study applied a recently developed technique to estimate single-nephron parameters in human MN. Single-nephron GFR (SNGFR) and single-nephron urinary protein excretion (SNUPE) were calculated by dividing total GFR and UPE by the total estimated number of non-sclerotic glomeruli (NSG). The NSG number per kidney was estimated using cortical volume assessment and biopsy-based stereology. MN staging by electron microscopy was performed using Ehrenreich-Churg (EC) criteria. Single-nephron parameters were analyzed in relation to clinicopathological factors known to associate with disease outcomes. The study included 109 MN patients (mean age 65 years; 73% male; eGFR 62 mL/min, 36% on renin-angiotensin aldosterone system [RAAS] inhibitors pre-biopsy). EC stages were I, 19%;II, 49%; III, 26%; and IV, 6%. There was no difference in glomerular volume among EC stage groups. With advancing EC stage, SNGFR and SNUPE decreased from mean 56 to 42 nL/min and 5.1 to 3.8 μg/day, respectively. In multivariable models, EC stage was associated with SNGFR even after adjustment for key clinicopathological factors, such as reduced GFR, serum albumin, UPE, segmental glomerulosclerosis, chronic tubulointerstitial damage and pre-biopsy use of RAAS inhibitors. In contrast, EC stage was not associated with glomerular volume and SNUPE after multivariable adjustment. These results provide the first clinical evidence of alterations in single-nephron dynamics with advancing EC stage of human MN and support a role for disease-specific GBM structural lesions as determinants of SNGFR.

Proteinuria observed post-uninephrectomy stems from partial suppression of protein endocytosis along the proximal tubule

Living kidney donors are at higher risk of developing proteinuria, yet few studies have addressed the etiology. To investigate whether post-uninephrectomy (UNX) proteinuria was due to increased filtration and/or reduced endocytosis/reabsorption along the proximal tubule (PT), we compared renal function, morphology, and PT endocytic machinery in male Sprague Dawley rats before and after UNX.At 12 wk post-UNX, the weight of the remnant kidney rose by 50%, PT diameters by 30%, and single nephron GFR by 50% (based on 26% fall in creatinine clearance and 50% decrease in the number of nephrons). Urinary albumin excretion increased 10-fold post-UNX. Since the filtered load of proteins decreases in parallel with GFR post-UNX, increased albumin excretion likely reflects decreased endocytosis along the PT, a notion supported by a 50% reduction in abundance of albumin and plasminogen in renal cortex homogenates from remnant versus explant kidneys. We tested the hypothesis that the protein endocytic machinery was reduced following UNX. Immunoblot assays of renal cortex revealed decreased abundance of megalin and Dab2, two key mediators of protein endocytosis along the PT, and no change in cubilin abundance. Immunohistochemistry confirmed a 50% fall in megalin along the PT apical membrane.Mathematical modeling predicted that albumin uptake per nephron increases post-UNX, owing to higher single nephron GFR and PT hypertrophy, but is attenuated by the decrease in megalin abundance; moreover, modeling predicts that if the endocytic machinery were not partly suppressed, overall albumin excretion would remain at pre-UNX levels.In addition to albumin, urinary plasminogen and angiotensinogen were similarly increased post-UNX, while uromodulin (synthesized and secreted distally) was decreased post-UNX. Urinary biomarkers of renal injury, RBP4 and KIM-1, were increased post-UNX likely reflecting the impact of the increased endocytosis of albumin which can carry bound toxic lipids into the PT.Taken together, our findings suggest that the blunting of albumin endocytosis post-UNX, apparent as increased albuminuria, may serve to protect PT cells from albumin-induced cytotoxicity. DK083785; Keck School of Medicine Dean’s Pilot Funding This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

The Role of Macula Densa Nitric Oxide Synthase 1 Beta Splice Variant in Modulating Tubuloglomerular Feedback.

Abnormalities in renal electrolyte and water excretion may result in inappropriate salt and water retention, which facilitates the development and maintenance of hypertension, as well as acid-base and electrolyte disorders. A key mechanism by which the kidney regulates renal hemodynamics and electrolyte excretion is via tubuloglomerular feedback (TGF), an intrarenal negative feedback between tubules and arterioles. TGF is initiated by an increase of NaCl delivery at the macula densa cells. The increased NaCl activates luminal Na-K-2Cl cotransporter (NKCC2) of the macula densa cells, which leads to activation of several intracellular processes followed by the production of paracrine signals that ultimately result in a constriction of the afferent arteriole and a tonic inhibition of single nephron glomerular filtration rate. Neuronal nitric oxide (NOS1) is highly expressed in the macula densa. NOS1β is the major splice variant and accounts for most of NO generation by the macula densa, which inhibits TGF response. Macula densa NOS1β-mediated modulation of TGF responses plays an essential role in control of sodium excretion, volume and electrolyte hemostasis, and blood pressure. In this article, we describe the mechanisms that regulate macula densa-derived NO and their effect on TGF response in physiologic and pathologic conditions. © 2023 American Physiological Society. Compr Physiol 13:4215-4229, 2023.

PS-BPB06-1: GLOMERULAR HEMODYNAMIC CHANGES WITH SGLT2 INHIBITION IN TYPE 2 DIABETIC RATS USING IN VIVO IMAGING

Background and aim: Clinical trials have shown that sodium-glucose co-transporter 2 inhibitor (SGLT2i) not only reduce the risk of heart failure and cardiovascular events in people with type 2 diabetes, but also inhibit DKD progression. Improvement of glomerular hyperfiltration via tubuloglomerular feedback (TGF) is considered to be one of the possible pathways for renal protection with SGLT2i in DKD. However, the mechanisms responsible for kidney protection, especially in the setting of T2D, remain incompletely understood. In previous work, we established a method for measuring single nephron GFR (SNGFR) in mice using in vivo imaging and found that the adenosine/adenosine A1 receptor (A1aR) pathway is central for tubuloglomerular feedback (TGF) signaling and the pathogenesis of glomerular hyperfiltration in animal models of type 1 diabetes (Kidokoro K. et al. Circulation 2019). The mechanism of development of glomerular hyperfiltration, and improvement of glomerular hyperfiltration by SGLT2i may, however, be different in type 1 vs type 2 diabetes and related DKD. However, the detailed regulatory mechanism of GFR have not yet been elucidated in DKD related to T2D. Methods: 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; 10 mg/kg, gavage) for 120 minutes, which generated the following three groups: SGLT2i group, SGLT2i + adenosine A1 receptor (A1aR) antagonist (8-cyclopentyl-1,3-dipropylxanthine, 1 mg/kg) group, and insulin group. Results: SNGFR, afferent and efferent arteriole diameter were significantly increased in ZDF compared to ZL, with greater afferent vs. efferent dilatation. SGLT2i treatment resulted in correction of afferent arteriole vasodilation, with post-SGLT2i diameter that was comparable to baseline levels (p &gt; 0.05). Post-SGLT2i SNGFR was also similar compared to baseline levels. Changes in efferent arteriole diameter were not significant. The inhibitory effect on hyperfiltration by SGLT2i was abolished by the concomitant use of an A1aR antagonist. In contrast, urinary sodium and urinary glucose in SGLT2 inhibitor group increased significantly regardless of adenosine A1 receptor antagonist use. In response to SGLT2i, urinary adenosine and PGE2 increased significantly (p &gt; 0.05), while urine NOx did not change. Conclusion: Regulation of afferent arteriolar tone with SGLT2i is dependent on adenosine/ A1aR signaling an experimental model of T2D related hyperfiltration, regardless of ARB use.

PS-BPB08-6: REGULATORY MECHANISM OF GLOMERULAR FILTRATION RATE BY THE KEAP1/NRF2 PATHWAY

The activation of the Kelch like ECH associated protein 1 (Keap1)/nuclear factor (erythroid derived 2) like 2 (Nrf2) pathway increases glomerular filtration rate (GFR) in patients with type 2 diabetes and chronic kidney disease. However, the mechanisms whereby the Keap1/Nrf2 pathway regulates GFR are unknown. C57BL/6 mice (WT), Nrf2 deficient mice, and Nrf2 activated Keap1 knockdown mice were compared, and RTA dh404 (CDDO dhTFEA) was used as a Nrf2 activator. Glomerular hemodynamics and the permeability of glomeruli to macromolecules were evaluated using laser Doppler techniques and in vivo imaging. Calcium influx and the rearrangement of the actin cytoskeleton in response to angiotensin II, H2O2, the Nrf2 activator dh404, and transient receptor potential cation channel, subfamily C (TRPC) channel inhibitors were evaluated in podocytes. Finally, the effects of a TRPC6 inhibitor on GFR were investigated in vivo. Pharmacological and genetic Keap1/Nrf2 activation increased renal blood flow (p &lt; 0.05), glomerular volume (p &lt; 0.05), and GFR (p &lt; 0.05), but did not alter afferent or efferent arteriolar diameter or glomerular permeability. Calcium influx into podocytes through TRPC channels in response to angiotensin II or H2O2 was suppressed by Keap1/Nrf2 activation. TRPC inhibitors and dh404 reduced the activation of the small GTPase(RhoA)/Rho associated kinase 1/LIM domain kinase 1/cofilin pathway and the subsequent actin rearrangement induced by angiotensin II or H2O2. Treatment with a TRPC6 inhibitor increased single nephron GFR in WT mice. In conclusion, the Keap1/Nrf2 pathway regulates GFR through changes in ultrafiltration via intracellular calcium signaling and cellular contractility, mediated through TRPC activity, in glomerular cells.

Glomerular hyperfiltration: part 1 - defining the threshold - is the sky the limit?

Glomerular hyperfiltration (GHF) is an increase in single-nephron glomerular filtration rate (GFR) that occurs in both physiological states and pathological states. Whole-kidney GHF is often used as a surrogate for single-nephron hyperfiltration since determining single-nephron GFR is impossible in routine clinical care. A clear definition (read threshold) of GHF is lacking. The aim of the first part of this review was to find evidence for defining the threshold for GHF, based on literature review, including systematic reviews and meta-analysis data, with both measured and estimated GFR. The consensus pediatric threshold for GHF as obtained from reviews, measured and estimated GFR studies, can reliably be set to 135mL/min/1.73 m2 for children aged > 2years. Diagnosing GHF from SCr-based estimated GFR is not reliable in subjects with reduced muscle mass. In these cases, it could be of interest to confirm the state of GHF using cystatin C-based eGFR, or preferably, by measured GFR, using methods that are accurate in the high GFR-range.

Responses in Blood Pressure and Kidney Function to Soluble Guanylyl Cyclase Stimulation or Activation in Normal and Diabetic Rats

Introduction: Agonists of soluble guanylate cyclase (sGC) are being developed as treatment for cardiovascular disease. Most effects of nitric oxide (NO) on glomerular and tubular function are mediated through sGC, but whether sGC agonists mimic these effects is unknown. Methods: Renal clearance and micropuncture studies were performed in Wistar-Froemter rats (WF), with or without streptozotocin diabetes (STZ-WF), and in Goto-Kakizaki rats (GK) with mild type 2 diabetes to test for acute effects of the sGC “stimulator” BAY 41-2272, which synergizes with endogenous NO, and the “activator” runcaciguat, which generates cGMP independent of NO. Results: Both sGC agonists reduced arterial blood pressure (MAP). For MAP reductions <10%, the drugs increased GFR in WF and STZ-WF but not in GK. Larger MAP reductions outweighed this effect and GFR declined, with better preserved GFR in STZ-WF. Changes in GFR could not be accounted for by changes in RBF, suggesting parallel changes in ultrafiltration pressure and/or ultrafiltration coefficient. The doses chosen for micropuncture in WF and GK reduced MAP by 2–10%, and the net effect on single nephron GFR and ultrafiltration pressure was neutral. The effects of the drugs on tubular reabsorption were dominated by declining MAP, and no natriuretic effect was observed at any dose. Discussion/Conclusion: sGC agonists impact kidney function directly and because they reduce MAP. The direct tendency to increase GFR is most apparent for MAP reductions <10%. The direct effect is otherwise subtle and overridden when MAP declines more. Effects of sGC agonists on tubular reabsorption are dominated by the effects on MAP.

Intravital imaging of hemodynamic glomerular effects of enalapril or/and empagliflozin in STZ-diabetic mice.

Background: Diabetic kidney disease is the leading cause of end-stage renal disease. Administration of ACE inhibitors or/and SGLT2 inhibitors show renoprotective effects in diabetic and other kidney diseases. The underlying renoprotective mechanisms of SGLT2 inhibition, especially in combination with ACE inhibition, are incompletely understood. We used longitudinal intravital microscopy to directly elucidate glomerular hemodynamics on a single nephron level in response to the ACE inhibitor enalapril or/and the SGLT2 inhibitor empagliflozin. Methods: Five weeks after the induction of diabetes by streptozotocin, male C57BL/6 mice were treated with enalapril, empagliflozin, enalapril/empagliflozin or placebo for 3 days. To identify hemodynamic regulation mechanisms, longitudinal intravital multiphoton microscopy was employed to measure single nephron glomerular filtration rate (snGFR) and afferent/efferent arteriole width. Results: Diabetic mice presented a significant hyperfiltration. Compared to placebo treatment, snGFR was reduced in response to enalapril, empagliflozin, or enalapril/empagliflozin administration under diabetic conditions. While enalapril treatment caused significant dilation of the efferent arteriole (12.55 ± 1.46 µm vs. control 11.92 ± 1.04 µm, p < 0.05), empagliflozin led to a decreased afferent arteriole diameter (11.19 ± 2.55 µm vs. control 12.35 ± 1.32 µm, p < 0.05) in diabetic mice. Unexpectedly under diabetic conditions, the combined treatment with enalapril/empagliflozin had no effects on both afferent and efferent arteriole diameter change. Conclusion: SGLT2 inhibition, besides ACE inhibition, is an essential hemodynamic regulator of glomerular filtration during diabetes mellitus. Nevertheless, additional mechanisms—independent from hemodynamic regulation—are involved in the nephroprotective effects especially of the combination therapy and should be further explored in future studies.

Mapping single-nephron filtration in the isolated, perfused rat kidney using magnetic resonance imaging.

The kidney has an extraordinary ability to maintain glomerular filtration despite natural fluctuations in blood pressure and nephron loss. This is partly due to local coordination between single-nephron filtration and vascular perfusion. An improved understanding of the three-dimensional (3-D) functional coordination between nephrons and the vasculature may provide a new perspective of the heterogeneity of kidney function and could inform targeted therapies and timed interventions to slow or prevent the progression of kidney disease. Here, we developed magnetic resonance imaging (MRI) tools to visualize single-nephron function in 3-D throughout the isolated perfused rat kidney. We used an intravenous slow perfusion of a glomerulus-targeted imaging tracer [cationized ferritin (CF)] to map macromolecular dynamics and to identify glomeruli in 3-D, followed by a bolus of a freely filtered tracer (gadolinium diethylenetriamine penta-acetic acid) to map filtration kinetics. There was a wide intrakidney distribution of CF binding rates and estimated single-nephron glomerular filtration rate (eSNGFR) between nephrons. eSNGFR and CF uptake rates did not vary significantly by distance from the kidney surface. eSNGFR varied from ∼10 to ∼100 nL/min throughout the kidney. Whole single-kidney GFR was similar across all kidneys, despite differences in the distributions eSNGFR of and glomerular number, indicating a robust adaptive regulation of individual nephrons to maintain constant single-kidney GFR in the presence of a natural variation in nephron number. This work provides a framework for future studies of single-nephron function in the whole isolated perfused kidney and experiments of single-nephron function in vivo using MRI.NEW & NOTEWORTHY We report MRI tools to measure and map single-nephron function in the isolated, perfused rat kidney. We used imaging tracers to identify nephrons throughout the kidney and to measure the delivery and filtration of the tracers at the location of the glomeruli. With this technique, we directly measured physiological parameters including estimated single-nephron glomerular filtration rate throughout the kidney. This work provides a foundation for new studies to simultaneously map the function of large numbers of nephrons.

Modeling the Progression of Hypertensive Kidney Disease in African Americans

Chronic kidney disease (CKD) is associated with the progressive functional loss of nephrons and hypertension (HTN). Several clinical studies demonstrate calcium channel blocker (CCB) therapy mitigates the decline in renal function in humans with essential HTN, but there are few long‐term clinical studies that determine the impact of CCBs in patients with hypertensive CKD. African Americans (AA) have a faster progression to end stage renal disease as compared to the white population which may be partially explained by factors such as salt‐sensitive blood pressure (BP) and greater prevalence of uncontrolled HTN. Observational studies demonstrate BP to be a strong independent risk factor for CKD progression, yet intense BP lowering in AA does not diminish the renal function decline any more than standard therapy. The African American Study of Kidney Disease and Hypertension (AASK) trial suggests AA with proteinuric CKD may have detrimental responses to CCB as compared to an angiotensin converting enzyme (ACE) inhibitor. Therefore, using a large physiological model, we set out to replicate the AASK trial findings and predict the roles of the renin‐angiotensin system, salt intake, and intensive antihypertensive therapy in a virtual population during CCB therapy. Our current mathematical model, HumMod, is a large physiological simulator that is comprised of key systems that play an integral role in BP control such as neural, endocrine, circulatory, and renal systems. All of the parameters (n=335) that control these systems were randomly varied and resulted in over 3,000 unique models that we define as a virtual population. We calibrated this virtual population to patient level data from the AASK trial (BP and glomerular filtration rate [GFR] before and after 3 years of amlodipine). After matching the virtual population to individual level data, the virtual population (n=140) was associated with statistically similar BP and GFR before and after CCB therapy (Figure) as well as similar increases in glomerular damage. Baseline factors such as elevated single nephron GFR and increased tubuloglomerular feedback were correlated with greater declines in renal function and increased glomerulosclerosis after 3 years of CCB. Interestingly, when looking at the models with severe CKD (&lt;35 mL/min), glomerulosclerosis and angiotensin II significantly increased with CCB despite significant falls in mean BP (‐7 ± 7 mmHg). Adding an ACE inhibitor decreased angiotensin II levels and abrogated the increase in glomerulosclerosis while further decreasing BP (‐12 ± 9 mmHg). Decreasing salt intake in addition to the CCB+ACE inhibitor therapy further benefited systemic and glomerular pressures but significantly decreased overall GFR independent of any glomerular damage. Finally, all antihypertensive therapies were associated with significant decreases in left ventricular mass. These data suggest the current clinical markers of renal function such as eGFR may not fully reflect the actual progression of CKD, especially with intensive antihypertensive treatment. Our simulation also supports that angiotensin system inhibitors are renoprotective and cardioprotective when combined with CCB therapy in African American CKD patients.

Single nephron glomerular filtration rate measured by linescan multiphoton microscopy compared to conventional micropuncture

Renal micropuncture, which requires the direct access to the renal tubules, has for long time been the technique of choice to measure the single nephron glomerular filtration rate (SNGFR) in animal models. This approach is challenging by virtue of complex animal preparation and numerous technically difficult steps. The introduction of intravital multiphoton microscopy (MPM) offers another approach to the measure of the SNGFR by mean of the high laser-tissue penetration and the optical sectioning capacity. Previous MPM studies measuring SNGFR in vivo relied on fast full-frame acquisition during the filtration process obtainable with high performance resonant scanners. In this study, we describe an innovative linescan–based MPM method. The new method can discriminate SNGFR variations both in conditions of low and high glomerular filtration, and shows results comparable to conventional micropuncture both for rats and mice. Moreover, this novel approach has improved spatial and time resolution and is faster than previous methods, thus enabling the investigation of SNGFR from more tubules and improving options for data-analysis.

Glomerular hyperfiltration.

Circulating blood is filtered across the glomerular barrier to form an ultrafiltrate of plasma in the Bowman's space. The volume of glomerular filtration adjusted by time is defined as the glomerular filtration rate (GFR), and the total GFR is the sum of all single-nephron GFRs. Thus, when the single-nephron GFR is increased in the context of a normal number of functioning nephrons, single glomerular hyperfiltration results in 'absolute' hyperfiltration in the kidney. 'Absolute' hyperfiltration can occur in healthy people after high protein intake, during pregnancy and in patients with diabetes, obesity or autosomal-dominant polycystic kidney disease. When the number of functioning nephrons is reduced, single-nephron glomerular hyperfiltration can result in a GFR that is within or below the normal range. This 'relative' hyperfiltration can occur in patients with a congenitally reduced nephron number or with an acquired reduction in nephron mass consequent to surgery or kidney disease. Improved understanding of the mechanisms that underlie 'absolute' and 'relative' glomerular hyperfiltration in different clinical settings, and of whether and how the single-nephron haemodynamic and related biomechanical forces that underlie glomerular hyperfiltration promote glomerular injury, will pave the way toward the development of novel therapeutic interventions that attenuate glomerular hyperfiltration and potentially prevent or limit consequent progressive kidney injury and loss of function.

Estimating Nephron Number from Biopsies: Impact on Clinical Studies.

Accumulating evidence supports an association between nephron number and susceptibility to kidney disease. However, it is not yet possible to directly measure nephron number in a clinical setting. Recent clinical studies have used glomerular density from a single biopsy and whole kidney cortical volume from imaging to estimate nephron number and single nephron glomerular filtration rate. However, the accuracy of these estimates from individual subjects is unknown. Furthermore, it is not clear how sample size or biopsy location may influence these estimates. These questions are critical to study design, and to the potential translation of these tools to estimate nephron number in individual subjects. We measured the variability in estimated nephron number derived from needle or virtual biopsies and cortical volume in human kidneys declined for transplantation. We performed multiple needle biopsies in the same kidney, and examined the three-dimensional spatial distribution of nephron density by magnetic resonance imaging. We determined the accuracy of a single-kidney biopsy to predict the mean nephron number estimated from multiple biopsies from the same kidney. A single needle biopsy had a 15% chance and virtual biopsy had a 60% chance of being within 20% of the whole-kidney nephron number. Single needle biopsies could be used to detect differences in nephron number between large cohorts of several hundred subjects. The number of subjects required to accurately detect differences in nephron number between populations can be predicted on the basis of natural intrakidney variability in glomerular density. A single biopsy is insufficient to accurately predict nephron number in individual subjects.

Efficacy analysis of retroperitoneal laparoscopic selective renal artery branch occlusion and nephron sparing surgery for ≥ T1b stage renal carcinoma

Objective: To investigate the safety and efficacy of retroperitoneal laparoscopic selective renal artery branch occlusion with nephron sparing surgery in patients with renal carcinoma of stage ≥ T1b. Methods: From July 2016 to September 2020, 35 patients with renal cancer ≥T1b underwent retroperitoneoscopic nephron sparing surgery in the First Affiliated Hospital of Shenzhen University. The surgical methods were retroperitoneoscopic nephron sparing surgery with total renal artery occlusion (group A) or selective renal artery branch occlusion (group B). Operation time, heat ischemia time, blood transfusion rate, positive margin rate, intraoperative blood loss, postoperative complications and length of hospital stay were compared between the two groups, and the total glomerular filtration rate (GFR) and the single-nephron glomerular filtration rate (sGFR) of the offected kidneys were compared between the two groups before, 3 months after and 12 months after surgery. Results: Among the 35 patients, 19 were male and 16 were female, aged (55.7±8.4) years and the body mass index is (24.6±3.1) kg/m2. The tumor diameter was (54.7±10.3) mm. The difference was statistically significant of operative time between group A and B [(103.5±14.3) vs (123.2±14.1) min,P=0.003]. There were no significant differences in thermal ischemia time, blood transfusion rate, positive margin, intraoperative blood loss, incidence of postoperative complications and length of hospital stay between the two groups (all P>0.05). The decrease of renal sGFR in the group A was significantly higher than group B at 3 months and 12 months after surgery [(23.1±3.6) vs (29.1±7.1) ml/min;(25.9±4.7) vs (30.7±7.2),both P<0.05]. Conclusion: Retroperitoneal laparoscopic selective renal artery branch occlusion and neon-sparing surgery for patients with ≥ T1b stage renal carcinoma is a safe and effective surgical method, which can well protect the renal function of patients in the early postoperative stage without increasing intraoperative blood loss and postoperative complications.

Adaptive changes in single-nephron GFR, tubular morphology, and transport in a pregnant rat nephron: modeling and analysis.

Normal pregnancy is characterized by massive increases in plasma volume and electrolyte retention. Given that the kidneys regulate homeostasis of electrolytes and volume, the organ undergoes major adaptations in morphology, hemodynamics, and transport to achieve the volume and electrolyte retention required in pregnancy. These adaptations are complex, sometimes counterintuitive, and not fully understood. In addition, the demands of the developing fetus and placenta change throughout pregnancy. For example, during late pregnancy, K+ retention and thus enhanced renal K+ reabsorption are required despite many kaliuretic factors. The goal of this study was to unravel how known adaptive changes along the nephrons contribute to the ability of the kidney to meet volume and electrolyte requirements in mid and late pregnancy. We developed computational models of solute and water transport in the superficial nephron of the kidney of a rat in mid and late pregnancy. The midpregnant and late-pregnant rat superficial nephron models predicted that morphological adaptations and increased activity of Na+/H+ exchanger 3 (NHE3) and epithelial Na+ channel are essential for the enhanced Na+ reabsorption observed during pregnancy. Model simulations showed that for sufficient K+ reabsorption, increased activity of H+-K+-ATPase and decreased K+ secretion along the distal segments is required in both mid and late pregnancy. The model results also suggested that certain known sex differences in renal transporter pattern (e.g., the higher NHE3 protein abundance but lower activity in the proximal tubules of virgin female rats compared with male rats) may serve to better prepare females for the increased transport demand in pregnancy.NEW & NOTEWORTHY Normal pregnancy in mammals is generally characterized by massive changes in plasma volume and electrolyte retention. This study provides insights into how the volume and electrolyte requirement in different pregnancy stages are met by coordinated adaptive changes in the kidney. The model results also suggested that certain known sex differences in the renal transporter pattern may serve to better prepare females for the increased transport demand in pregnancy.