A study of size-selective renal elimination using a novel human model

The recently discovered selective glomerular hypofiltration syndromes have increased interest in the actual elimination of molecules in the human kidney. In the present study, a novel human model was introduced to directly measure the single-pass renal elimination of molecules of increasing size. Plasma concentrations of urea, creatinine, C-peptide, insulin, pro-BNP, β2-microglobulin, cystatin C, troponin-T, orosomucoid, albumin, and IgG were analysed in arterial and renal venous blood from 45 patients undergoing Transcatheter Aortic Valve Implantation (TAVI). The renal elimination ratio (RER) was calculated as the arteriovenous concentration difference divided by the arterial concentration. Estimated glomerular filtration rate (eGFR) was calculated by the CKD-EPI equations for both creatinine and cystatin C. Creatinine (0.11 kDa) showed the highest RER (21.0 ± 6.3%). With increasing molecular size, the RER gradually decreased, where the RER of cystatin C (13 kDa) was 14.4 ± 5.3% and troponin-T (36 kDa) was 11.3 ± 4.6%. The renal elimination threshold was found between 36 and 44 kDa as the RER of orosomucoid (44 kDa) was -0.2 ± 4.7%. The RER of creatinine and cystatin C showed a significant and moderate positive linear relationship with eGFR (r = 0.48 and 0.40). In conclusion, a novel human model was employed to demonstrate a decline in renal elimination with increasing molecular size. Moreover, RERs of creatinine and cystatin C were found to correlate with eGFR, suggesting the potential of this model to study selective glomerular hypofiltration syndromes.

Quantifying Ultrafiltration in Peritoneal Dialysis Using the Sodium Dip

Key Points

Ultrafiltration (UF) is a key component of clinical peritoneal dialysis prescription, but the traditional method to assess UF is hampered by large inaccuracies. Here we propose a novel method, based on a computational model and on a single dialysate sodium measurement, to accurately estimate UF and osmotic conductance to glucose in patients on peritoneal dialysis.

Background

Volume overload is highly prevalent among patients treated with peritoneal dialysis (PD), contributes to hypertension, and is associated with an increased risk of cardiovascular events and death in this population. As a result, optimizing peritoneal ultrafiltration (UF) is a key component of high-quality dialysis prescription. Osmotic conductance to glucose (OCG) reflects the water transport properties of the peritoneum, but measuring it requires an accurate quantification of UF, which is often difficult to obtain because of variability in catheter patency and peritoneal residual volume.

Methods

In this study, we derived a new mathematical model for estimating UF during PD, on the basis of sodium sieving, using a single measure of dialysate sodium concentration. The model was validated experimentally in a rat model of PD, using dialysis fluid with two different sodium concentrations (125 and 134 mmol/L) and three glucose strengths (1.5%, 2.3%, and 4.25%). Then, the same model was tested in a cohort of PD patients to predict UF.

Results

In experimental and clinical conditions, the sodium-based estimation of UF rate correlated with UF rate measurements on the basis of volumetry and albumin dilution, with a R 2 =0.35 and R 2 =0.76, respectively. UF on the basis of sodium sieving was also successfully used to calculate OCG in the clinical cohort, with a Pearson r of 0.77.

Conclusions

Using the novel mathematical models in this study, the sodium dip can be used to accurately estimate OCG, and therefore, it is a promising measurement method for future clinical use.

Prompt Thrombo-Inflammatory Response to Ischemia-Reperfusion Injury and Kidney Transplant Outcomes

Introduction

In kidney transplantation (KT), the role of the intravascular innate immune system (IIIS) in response to ischemia-reperfusion injury (IRI) is not well-understood. Here, we studied parallel changes in the generation of key activation products of the proteolytic cascade systems of the IIIS following living donor (LD) and deceased donor (DD) transplantation and evaluated potential associations with clinical outcomes.

Methods

In a cohort study, 63 patients undergoing LD (n = 26) and DD (n = 37) transplantation were prospectively included. Fifteen DD kidneys were preserved with hypothermic machine perfusion (HMP), and the remaining were cold stored. Activation products of the kallikrein-kinin, coagulation, and complement systems were measured in blood samples obtained systemically at baseline and locally from the transplant renal vein at 1, 10, and 30 minutes after reperfusion.

Results

DD kidneys exhibited a prompt and interlinked activation of all 3 cascade systems of IIIS postreperfusion, indicating a robust and local thrombo-inflammatory response to IRI. In this initial response, the complement activation product sC5b-9 exhibited a robust correlation with other IIIS activation markers and displayed a strong association with short-term and mid-term (24-month) graft dysfunction. In contrast, LD kidneys did not exhibit this thrombo-inflammatory response. The use of HMP was associated with reduced thromboinflammation and preserved mid-term kidney function.

Conclusion

Kidneys from DD are vulnerable to a prompt thrombo-inflammatory response to IRI, which adversely affects both short-term and long-term allograft function. Strategies aimed at minimizing graft immunogenicity prior to reperfusion are crucial to mitigate the intricate inflammatory response to IRI.

Experimental hemodialysis in diet-induced ketosis and the potential use of dialysis as an adjuvant cancer treatment

Numerous in vivo studies on the ketogenic diet, a diet that can induce metabolic conditions resembling those following extended starvation, demonstrate strong outcomes on cancer survival, particularly when combined with chemo-, radio- or immunological treatments. However, the therapeutic application of ketogenic diets requires strict dietary adherence from well-informed and motivated patients, and it has recently been proposed that hemodialysis might be utilized to boost ketosis and further destabilize the environment for cancer cells. Yet, plasma ketones may be lost in the dialysate-lowering blood ketone levels. Here we performed a single 180-min experimental hemodialysis (HD) session in six anesthetized Sprague-Dawley rats given ketogenic diet for five days. Median blood ketone levels pre-dialysis were 3.5 mmol/L (IQR 2.2 to 5.6) and 3.8 mmol/L (IQR 2.2 to 5.1) after 180 min HD, p = 0.54 (95% CI - 0.6 to 1.2). Plasma glucose levels were reduced by 36% (- 4.5 mmol/L), p < 0.05 (95% CI - 6.7 to - 2.5). Standard base excess was increased from - 3.5 mmol/L (IQR - 4 to - 2) to 0.5 mmol/L (IQR - 1 to 3), p < 0.01 (95% CI 2.0 to 5.0). A theoretical model was applied confirming that intra-dialytic glucose levels decrease, and ketone levels slightly increase since hepatic ketone production far exceeds dialytic removal. Our experimental data and in-silico modeling indicate that elevated blood ketone levels during ketosis are maintained during hemodialysis despite dialytic removal.

The Peritoneal Membrane and its Role in Peritoneal Dialysis

A healthy and functional peritoneal membrane is key to achieve sufficient ultrafiltration and to restore fluid balance, a major component of high-quality prescription in patients treated with peritoneal dialysis (PD). Variability in membrane function at the start of PD or changes over time on treatment influence dialysis prescription and outcomes, and dysfunction of the peritoneal membrane contributes to fluid overload and associated complications. In this review, we summarize the current knowledge about the structure, function and pathophysiology of the peritoneal membrane with a focus on clinical implications for patient-centered care. We also discuss the molecular and genetic mechanisms of solute and water transport across the peritoneal membrane, including the role of aquaporin water channels in crystalloid vs. colloid osmosis; why and how to assess membrane function using peritoneal equilibration tests; the etiologies of membrane dysfunction and their specific management; and the impact of genetic variation on membrane function and outcomes in patients treated with PD. This review also identifies the gaps in current knowledge and perspectives for future research to improve our understanding of the peritoneal membrane and, ultimately, to improve the care of patients treated with PD.

High versus low ultrafiltration rates during experimental peritoneal dialysis in rats: Acute effects on plasma volume and systemic haemodynamics

INTRODUCTION

Intradialytic hypotension is a common complication of haemodialysis, but uncommon in peritoneal dialysis (PD). This may be due to lower ultrafiltration rates in PD compared to haemodialysis, allowing for sufficient refilling of the blood plasma compartment from the interstitial volume, but the underlying mechanisms are unknown. Here we assessed plasma volume and hemodynamic alterations during experimental PD with high versus low ultrafiltration rates.

METHODS

Experiments were conducted in two groups of healthy Sprague-Dawley rats: one group with a high ultrafiltration rate (N = 7) induced by 8.5% glucose and a low UF group (N = 6; 1.5% glucose), with an initial assessment of the extracellular fluid volume, followed by 30 min PD with plasma volume measurements at baseline, 5, 10, 15 and 30 min. Mean arterial pressure, central venous pressure and heart rate were continuously monitored during the experiment.

RESULTS

No significant changes over time in plasma volume, mean arterial pressure or central venous pressure were detected during the course of the experiments, despite an ultrafiltration (UF) rate of 56 mL/h/kg in the high UF group. In the high UF group, a decrease in extracellular fluid volume of -7 mL (-10.7% (95% confidence interval: -13.8% to -7.6%)) was observed, in line with the average UF volume of 8.0 mL (standard deviation: 0.5 mL).

CONCLUSION

Despite high UF rates, we found that plasma volumes were remarkably preserved in the present experiments, indicating effective refilling of the plasma compartment from interstitial tissues. Further studies should clarify which mechanisms preserve the plasma volume during high UF rates in PD.

Phloretin Improves Ultrafiltration and Reduces Glucose Absorption during Peritoneal Dialysis in Rats

BACKGROUND

Harmful glucose exposure and absorption remain major limitations of peritoneal dialysis (PD). We previously showed that inhibition of sodium glucose cotransporter 2 did not affect glucose transport during PD in rats. However, more recently, we found that phlorizin, a dual blocker of sodium glucose cotransporters 1 and 2, reduces glucose diffusion in PD. Therefore, either inhibiting sodium glucose cotransporter 1 or blocking facilitative glucose channels by phlorizin metabolite phloretin would reduce glucose transport in PD.

METHODS

We tested a selective blocker of sodium glucose cotransporter 1, mizagliflozin, as well as phloretin, a nonselective blocker of facilitative glucose channels, in an anesthetized Sprague-Dawley rat model of PD.

RESULTS

Intraperitoneal phloretin treatment reduced glucose absorption by >30% and resulted in a >50% higher ultrafiltration rate compared with control animals. Sodium removal and sodium clearances were similarly improved, whereas the amount of ultrafiltration per millimole of sodium removed did not differ. Mizagliflozin did not influence glucose transport or osmotic water transport.

CONCLUSIONS

Taken together, our results and previous results indicate that blockers of facilitative glucose channels may be a promising target for reducing glucose absorption and improving ultrafiltration efficiency in PD.

Dual SGLT1/SGLT2 inhibitor phlorizin reduces glucose transport in experimental peritoneal dialysis

INTRODUCTION

Glucose absorption during peritoneal dialysis (PD) is commonly assumed to occur via paracellular pathways. We recently showed that SGLT2 inhibition did not reduce glucose absorption in experimental PD, but the potential role of glucose transport into cells is still unclear. Here we sought to elucidate the effects of phlorizin, a non-selective competitive inhibitor of sodium glucose co-transporters 1 and 2 (SGLT1 and SGLT2), in an experimental rat model of PD.

METHODS

A 120-min PD dwell was performed in 12 anesthetised Sprague-Dawley rats using 1.5% glucose fluid with a fill volume of 20 mL with (n = 6) or without (n = 6) intraperitoneal phlorizin (50 mg/L). Several parameters for peritoneal water and solute transport were monitored during the treatment.

RESULTS

Phlorizin markedly increased the urinary excretion of glucose, lowered plasma glucose and increased plasma creatinine after PD. Median glucose diffusion capacity at 60 min was significantly lower (p < 0.05) being 196 µL/min (IQR 178-213) for phlorizin-treated animals compared to 238 µL/min (IQR 233-268) in controls. Median fractional dialysate glucose concentration at 60 min (D/D ₀) was significantly higher (p < 0.05) in phlorizin-treated animals being 0.65 (IQR 0.63-0.67) compared to 0.61 (IQR 0.60-0.62) in controls. At 120 min, there was no difference in solute or water transport across the peritoneal membrane.

CONCLUSION

Our findings indicate that a part of glucose absorption during the initial part of the dwell occurs via transport into peritoneal cells.

Optimised versus standard automated peritoneal dialysis regimens pilot study (OptiStAR): A randomised controlled crossover trial

BACKGROUND

The continuous global rise of end-stage kidney disease creates a growing demand of economically beneficial home-based kidney replacement therapies such as peritoneal dialysis (PD). However, undesirable absorption and exposure of peritoneal tissues to glucose remain major limitations of PD.

METHODS

We compared a reference (standard) automated PD regimen 6 × 2 L 1.36% glucose (76 mmol/L) over 9 h with a novel, theoretically glucose sparing (optimised) prescription consisting of 'ultrafiltration cycles' with high glucose strength (126 mmol/L) and 'clearance cycles' with ultra-low, physiological glucose (5 mmol/L) for approximately 40% of the treatment time. Twenty-one prevalent PD patients underwent the optimised regimen (7 × 2 L 2.27% glucose + 5 × 2 L 0.1% glucose over 8 h) and the standard regimen in a crossover fashion. Six patients were excluded from data analysis.

RESULTS

Median glucose absorption was 43 g (IQR 41-54) and 44 g (40-55) for the standard and optimised intervention, respectively (p = 1). Ultrafiltration volume, weekly Kt/V creatinine and urea were significantly improved during optimised interventions, while no difference in sodium removal was detected. Post hoc analysis showed significantly improved ultrafiltration efficiency (ml ultrafiltration per gram absorbed glucose) during optimised regimens. No adverse events were observed except one incidence of drain pain.

CONCLUSION

Optimised treatments were feasible and well tolerated in this small pilot study. Despite no difference in absorbed glucose, results indicate possible improvements of ultrafiltration efficiency and small solute clearances by optimised regimens. Use of optimised prescriptions as glucose sparing strategy should be evaluated in larger study populations.

Potential relationship between eGFRcystatin C /eGFRcreatinine -ratio and glomerular basement membrane thickness in diabetic kidney disease

Diabetic kidney disease (DKD) is a leading cause of end-stage renal disease and renal replacement therapy worldwide. A pathophysiological hallmark of DKD is glomerular basal membrane (GBM) thickening, whereas this feature is absent in minimal change disease (MCD). According to fundamental transport physiological principles, a thicker GBM will impede the diffusion of middle-molecules such as cystatin C, potentially leading to a lower estimated GFR (eGFR) from cystatin C compared to that of creatinine. Here we test the hypothesis that thickening of the glomerular filter leads to an increased diffusion length, and lower clearance, of cystatin C. Twenty-nine patients with a kidney biopsy diagnosis of either DKD (n = 17) or MCD (n = 12) were retrospectively included in the study. GBM thickness was measured at 20 separate locations in the biopsy specimen and plasma levels of cystatin C and creatinine were retrieved from health records. A modified two-pore model was used to simulate the effects of a thicker GBM on glomerular water and solute transport. The mean age of the patients was 52 years, and 38% were women. The mean eGFRcystatin C /eGFRcreatinine -ratio was 74% in DKD compared to 98% in MCD (p < 0.001). Average GBM thickness was strongly inversely correlated to the eGFRcystatin C /eGFRcreatinine -ratio (Pearson's r = -0.61, p < 0.01). Two-pore modeling predicted a eGFRcystatin C /eGFRcreatinine -ratio of 78% in DKD. We provide clinical and theoretical evidence suggesting that thickening of the glomerular filter, increasing the diffusion length of cystatin C, lowers the eGFRcystatin C /eGFRcreatinine -ratio in DKD.

Clemizole and La3+ salts ameliorate angiotensin II-induced glomerular hyperpermeability in vivo

Angiotensin II (Ang II) induces marked, dynamic increases in the permeability of the glomerular filtration barrier (GFB) in rats. After binding to its receptor, Ang II elicits Ca²⁺ influx into cells, mediated by TRPC5 and TRPC6 (transient receptor potential canonical type 5 and 6). Clemizole and La³⁺ salts have been shown to block TRPC channels in vitro, and we therefore tested their potential effect on Ang II‐induced glomerular hyperpermeability. Anesthetized male Sprague‐Dawley rats were infused with Ang II (80 ng kg^–1 min^–1) alone, or together with clemizole or low‐dose La³⁺ (activates TRPC5, blocks TRPC6) or high‐dose La³⁺ (blocks both TRPC5 and TRPC6). Plasma and urine samples were taken during baseline and at 5 min after the start of the infusions and analyzed by high‐performance size‐exclusion chromatography for determination of glomerular sieving coefficients for Ficoll 10–80 Å (1–8 nm). Ang II infusion evoked glomerular hyperpermeability to large Ficolls (50–80 Å), which was ameliorated by clemizole, having no significant effect on glomerular filtration rate (GFR) or Ang II‐mediated increase in mean arterial pressure (ΔMAP). In contrast, high‐ and low‐dose La³⁺ significantly lowered ΔMAP and reduced Ang II‐induced hyperpermeability. Combined, clemizole and low‐dose La³⁺ were less effective at ameliorating Ang II‐induced glomerular hyperpermeability than low‐dose La³⁺ alone. In conclusion, our data show that both clemizole and La3+ are effective against Ang II‐induced glomerular hyperpermeability, with differential effects on blood pressure. Further research using more specific blockers of TRPC5 and TRPC6 should be performed to reveal the underlying mechanisms.

Optimization of bimodal automated peritoneal dialysis prescription using the three-pore model

BACKGROUND

Previous studies suggested that automated peritoneal dialysis (APD) could be improved in terms of shorter treatment times and lower glucose absorption using bimodal treatment regimens, having 'ultrafiltration (UF) cycles' using a high glucose concentration and 'clearance cycles' using low or no glucose. The purpose of this study is to explore such regimes further using mathematical optimization techniques based on the three-pore model.

METHODS

A linear model with constraints is applied to find the shortest possible treatment time given a set of clinical treatment goals. For bimodal regimes, an exact analytical solution often exists which is herein used to construct optimal regimes giving the same Kt/V urea and/or weekly creatinine clearance and UF as a 6 × 2 L 1.36% glucose regime and an 'adapted' (2 × 1.5 L 1.36% + 3 × 3 L 1.36%) regime.

RESULTS

Compared to the non-optimized (standard and adapted regimes), the optimized regimens demonstrated marked reductions (>40%) in glucose absorption while having an identical weekly creatinine clearance (35 L) and UF (0.5 L). Larger fill volumes of 1200 mL/m² (UF cycles) and 1400 mL/m² (clearance cycles) can be used to shorten the total treatment time.

CONCLUSION

These theoretical results imply substantial improvements in glucose absorption using optimized APD regimens while achieving similar water and solute removal as non-optimized APD regimens. While the current results are based on a well-established theoretical model for peritoneal dialysis, experimental and clinical studies need to be performed to validate the current findings.

SGLT2 inhibition does not reduce glucose absorption during experimental peritoneal dialysis

INTRODUCTION

Unwanted glucose absorption during peritoneal dialysis (PD) remains a clinical challenge, especially in diabetic patients. Recent experimental data indicated that inhibitors of the sodium and glucose co-transporter (SGLT)-2 could act to reduce glucose uptake during PD, which raises the question of whether glucose absorption may also occur via intracellular or trans-cellular pathways.

METHODS

We performed PD in anesthetized Sprague-Dawley rats using a fill volume of 20 mL with either 1.5% glucose fluid or 4.25% glucose fluid for 120 min dwell time to evaluate the effects of SGLT2 inhibition by empagliflozin on peritoneal water and solute transport. To assess the diffusion capacity of glucose, we developed a modified equation to measure small solute diffusion capacity, taking convective- and free water transport into account.

RESULTS

SGLT2 inhibition markedly increased the urinary excretion of glucose and lowered plasma glucose after PD compared to sham groups. Glucose absorption for 1.5% glucose was 165 mg 95% CI (145-178) in sham animals and 157 mg 95% CI (137-172) for empagliflozin-treated animals. For 4.25% glucose, absorption of glucose was 474 mg 95% CI (425-494) and 472 mg 95% CI (420-506) for sham and empagliflozin groups, respectively. No significant changes in the transport of sodium or water across the peritoneal barrier could be detected.

CONCLUSION

We could not confirm recent findings that SGLT2 inhibition reduced glucose absorption and increased osmotic water transport during experimental PD.

Novel Method for Osmotic Conductance to Glucose in Peritoneal Dialysis

Introduction

The osmotic conductance to glucose (OCG) is a crucial determinant of ultrafiltration (UF) in peritoneal dialysis (PD) patients and can be used to monitor membrane integrity in patients on long-term PD. It has been proposed that OCG can be assessed based on drained volumes in 2 consecutive 1-hour glucose dwells, usually 1.5% and 4.25% glucose, in a so-called double mini-peritoneal equilibration test (dm-PET). However, recent data indicated that the dm-PET provides a poor estimate of OCG unless the residual volume (RV) is taken into account. We introduce an easy, robust, and accurate method to measure OCG and compare it with conventional methods.

Methods

In a prospective cohort of 21 PD patients, a modified version of the dm-PET was performed, along with the determination of RV before, between, and after dwells. Based on computer simulations derived from the 3-pore model (TPM) for membrane permeability, we developed and validated a novel single-dwell method to estimate OCG. We next validated the equation in an independent cohort consisting of 32 PD patients.

Results

Single-dwell OCG correlated more closely with actual UF (r = 0.94 vs. r = 0.07 for conventional dm-PET), sodium sieving, and free water transport (FWT) compared with other methods. These findings were replicated in the validation cohort in which OCG calculated using the single-dwell method closely correlated with parameters of osmotic water transport, even when RV was not taken into account, using only drained volumes.

Conclusion

We propose a novel, easy, and robust single-dwell method to determine OCG in individual patients and to monitor membrane integrity over time on PD.

Effect of diabetes mellitus on the recovery of changes in renal functions and glomerular permeability following reversible 24-hour unilateral ureteral obstruction

BACKGROUND

Following reversal of short periods of ureteral obstruction (UO), glomerular and tubular renal dysfunction recovers with time. Diabetes mellitus (DM) affects glomerular function; thus, the ability of diabetic kidneys to recover from UO may be impaired. This study investigated the effects of long-term DM on the recovery of glomerular and tubular function, as well as permeability of the glomerular filtration barrier (GFB), after unilateral UO (UUO) reversal.

METHODS

Diabetes mellitus was induced in Wistar rats by intraperitoneal streptozotocin. All diabetic and age-matched control rats underwent reversible 24-hour left UUO. The renal function of both kidneys was measured using clearance techniques 3 hours and 7 and 30 days after UUO reversal. Glomerular permeability was assessed by measuring the glomerular sieving coefficients for fluorescein isothiocyanate-conjugated Ficoll (molecular radius: 20-90 Å).

RESULTS

Unilateral UO induced transient changes in the size selectivity of GFB small pores. However, the size selectivity function of large pores had not returned to baseline even 30 days after UUO reversal. Diabetes mellitus caused exaggerated early alterations in glomerular hemodynamic and tubular function, as well as size selectivity dysfunction of both small and large pores. At 30 days after UUO reversal, despite glomerular hemodynamic and tubular function and the size selectivity of small pores returning to normal in both diabetic and non-diabetic rats, the residual size selectivity dysfunction of large pores was more severe in diabetic rats.

CONCLUSION

Unilateral UO caused long-term dysfunction in the size selectivity of large pores of the GFB. In addition, DM significantly exaggerated this dysfunction, indicating a more ominous outcome in diabetic kidneys following UUO.

Albumin infusion rate and plasma volume expansion: a randomized clinical trial in postoperative patients after major surgery

Background

Optimal infusion rate of colloids in patients with suspected hypovolemia is unknown, and the primary objective of the present study was to test if plasma volume expansion by 5% albumin is greater if fluid is administered slowly rather than rapidly.

Methods

Patients with signs of hypoperfusion after major abdominal surgery were randomized to intravenous infusion of 5% albumin at a dose of 10 ml/kg (ideal body weight) either rapidly (30 min) or slowly (180 min). Plasma volume was measured using radiolabeled albumin at baseline, at 30 min, and at 180 min after the start of infusion. Primary outcome was change in plasma volume from the start of infusion to 180 min after the start of infusion. Secondary outcomes included the change in the area under the plasma volume curve and transcapillary escape rate (TER) for albumin from 180 to 240 min after the start of albumin infusion.

Results

A total of 33 and 31 patients were included in the analysis in the slow and rapid groups, respectively. The change in plasma volume from the start of infusion to 180 min did not differ between the slow and rapid infusion groups (7.4 ± 2.6 vs. 6.5 ± 4.1 ml/kg; absolute difference, 0.9 ml/kg [95%CI, − 0.8 to 2.6], P = 0.301). Change in the area under the plasma volume curve was smaller in the slow than in the rapid infusion group and was 866 ± 341 and 1226 ± 419 min ml/kg, respectively, P < 0.001. TER for albumin did not differ and was 5.3 ± 3.1%/h and 5.4 ± 3%/h in the slow and in the rapid infusion groups, respectively, P = 0.931.

Conclusions

This study does not support our hypothesis that a slow infusion of colloid results in a greater plasma volume expansion than a rapid infusion. Instead, our result of a smaller change in the area under the plasma volume curve indicates that a slow infusion results in a less efficient plasma volume expansion, but further studies are required to confirm this finding. A rapid infusion has no effect on vascular leak as measured after completion of the infusion.

Trial registration

EudraCT2013-004446-42 registered December 23, 2014.

Electronic supplementary material

The online version of this article (10.1186/s13054-019-2477-7) contains supplementary material, which is available to authorized users.

Sustained, delayed, and small increments in glomerular permeability to macromolecules during systemic ET-1 infusion mediated via the ETA receptor

Emerging evidence indicates that endogenous production of endothelin (ET)-1, a 21-amino acid peptide vasoconstrictor, plays an important role in proteinuric kidney disease. Previous studies in rats have shown that chronic administration of ET-1 leads to increased glomerular albumin leakage. The underlying mechanisms are, however, currently not known. Here, we used size-exclusion chromatography to measure glomerular sieving coefficients for neutral FITC-Ficoll (molecular Stokes-Einstein radius: 15-80 Å, molecular weight: 70 kDa/400 kDa) in anesthetized male Sprague-Dawley rats (n = 12) at baseline and at 5, 15, 30, and 60 min after intravenous administration of ET-1. In separate experiments, ET-1 was given together with the selective ET type A (ET_A) or ET type B (ET_B) receptor antagonists JKC-301 and BQ-788, respectively. At both 15 and 30 min postadministration, the glomerular sieving coefficient for macromolecular Ficoll (70 Å) was significantly increased to 4.4 × 10^-5 ± 0.7 × 10^-5 (P = 0.024) and 4.5 × 10^-5 ± 0.8 × 10^-5 (P = 0.007), respectively, compared with baseline (2.2 × 10^-5 ± 0.4 ×10^-5). Decreased urine production after ET-1 prevented the use of higher doses of ET-1. Data analysis using the two-pore model indicated changes in large-pore permeability after ET-1, with no changes in the small-pore pathway. Administration of ET_A blocker abrogated the permeability changes induced by ET-1 at 30 min, whereas blockade of ET_B receptors was ineffective. Mean arterial pressure was only significantly increased at 60 min, being 123 ± 4 mmHg compared with 111 ± 2 mmHg at baseline (P = 0.02). We conclude that ET-1 evoked small, delayed, and sustained increases in glomerular permeability, mediated via the ET_A receptor.

Computer Simulations of Continuous Flow Peritoneal Dialysis Using the 3-Pore Model-A First Experience

Background:Continuous flow peritoneal dialysis (CFPD) is performed using a continuous flux of dialysis fluid via double or dual-lumen PD catheters, allowing a higher dialysate flow rate (DFR) than conventional treatments. While small clinical studies have revealed greatly improved clearances using CFPD, the inability to predict ultrafiltration (UF) may confer a risk of potentially harmful overfill. Here we performed physiological studies of CFPD in silico using the extended 3-pore model.Method:A 9-h CFPD session was simulated for: slow (dialysate to plasma creatinine [D/P crea] < 0.6), fast (D/P crea > 0.8) and average (0.6 ≤ D/P crea ≤ 0.8) transporters using 1.36%, 2.27%, or 3.86% glucose solutions. To avoid overfill, we applied a practical equation, based on the principle of mass-balance, to predict the UF rate during CFPD treatment.Results:Increasing DFR > 100 mL/min evoked substantial increments in small- and middle-molecule clearances, being 2 - 5 times higher compared with a 4-h continuous ambulatory PD (CAPD) exchange, with improvements typically being smaller for average and slow transporters. Improved UF rates, exceeding 10 mL/min, were achieved for all transport types. The β₂-microglobulin clearance was strongly dependent on the UF rate and increased between 60% and 130% as a function of DFR. Lastly, we tested novel intermittent-continuous regimes as an alternative strategy to prevent overfill, being effective for 1.36% and 2.27%, but not for 3.86% glucose.Conclusion:While we find substantial increments in solute and water clearance with CFPD, previous studies have shown similar improvements using high-volume tidal automated PD (APD). Lastly, the current in silico results need confirmation by studies in vivo.

Mechanisms of Crystalloid versus Colloid Osmosis across the Peritoneal Membrane

Background Osmosis drives transcapillary ultrafiltration and water removal in patients treated with peritoneal dialysis. Crystalloid osmosis, typically induced by glucose, relies on dialysate tonicity and occurs through endothelial aquaporin-1 water channels and interendothelial clefts. In contrast, the mechanisms mediating water flow driven by colloidal agents, such as icodextrin, and combinations of osmotic agents have not been evaluated.Methods We used experimental models of peritoneal dialysis in mouse and biophysical studies combined with mathematical modeling to evaluate the mechanisms of colloid versus crystalloid osmosis across the peritoneal membrane and to investigate the pathways mediating water flow generated by the glucose polymer icodextrin.ResultsIn silico modeling and in vivo studies showed that deletion of aquaporin-1 did not influence osmotic water transport induced by icodextrin but did affect that induced by crystalloid agents. Water flow induced by icodextrin was dependent upon the presence of large, colloidal fractions, with a reflection coefficient close to unity, a low diffusion capacity, and a minimal effect on dialysate osmolality. Combining crystalloid and colloid osmotic agents in the same dialysis solution strikingly enhanced water and sodium transport across the peritoneal membrane, improving ultrafiltration efficiency over that obtained with either type of agent alone.Conclusions These data cast light on the molecular mechanisms involved in colloid versus crystalloid osmosis and characterize novel osmotic agents. Dialysis solutions combining crystalloid and colloid particles may help restore fluid balance in patients treated with peritoneal dialysis.

Inhibition of mammalian target of rapamycin decreases intrarenal oxygen availability and alters glomerular permeability

An increased kidney oxygen consumption causing tissue hypoxia has been suggested to be a common pathway toward chronic kidney disease. The mammalian target of rapamycin (mTOR) regulates cell proliferation and mitochondrial function. mTOR inhibitors (e.g., rapamycin) are used clinically to prevent graft rejection. mTOR has been identified as a key player in diabetes, which has stimulated the use of mTOR inhibitors to counter diabetic nephropathy. However, the effect of mTOR inhibition on kidney oxygen consumption is unknown. Therefore, we investigated the effects of mTOR inhibition on in vivo kidney function, oxygen homeostasis, and glomerular permeability. Control and streptozotocin-induced diabetic rats were chronically treated with rapamycin, and the functional consequences were studied 14 days thereafter. In both groups, mTOR inhibition induced mitochondrial uncoupling, resulting in increased total kidney oxygen consumption and decreased intrarenal oxygen availability. Concomitantly, mTOR inhibition induced tubular injury, as estimated from urinary excretion of kidney injury molecule-1 (KIM-1) and reduced urinary protein excretion. The latter corresponded to reduced sieving coefficient for large molecules. In conclusion, mTOR inhibition induces mitochondrial dysfunction leading to decreased oxygen availability in normal and diabetic kidneys, which translates into increased KIM-1 in the urine. Reduced proteinuria after mTOR inhibition is an effect of reduced glomerular permeability for large molecules. Since hypoxia has been suggested as a common pathway in the development of chronic kidney disease, mTOR inhibition to patients with preexisting nephropathy should be used with caution, since it may accelerate the progression of the disease.

Glomerular hyperpermeability after acute unilateral ureteral obstruction: effects of Tempol, NOS, RhoA, and Rac-1 inhibition

It is well known that proteinuria following urinary tract obstruction is mainly of a tubular nature. However, it is unknown whether there are also changes in glomerular permeability. In this study, we compared glomerular sieving coefficients (θ) of polydisperse fluorescein isothiocyanate (FITC)-Ficoll 70/400 following a 120- or 180-min unilateral ureteral obstruction (UUO) in anesthetized Sprague-Dawley rats. Samples were collected from the obstructed kidney at 5, 15, and 30 min postrelease and analyzed by means of high-pressure size-exclusion chromatography. After 120-min UUO, mean θ for Ficoll_70Å was increased ( P < 0.01) from 2.2 ± 0.5 × 10^-5 (baseline) to 10.6 ± 10 × 10^-5 15 min postrelease (highest value). After 180-min UUO, mean θ for Ficoll_70Å was further increased ( P < 0.001) from 1.4 ± 0.5 × 10^-5 (baseline) to 40 ± 10 × 10^-5 at 5 min postrelease (highest value). Administration of a reactive oxygen species (ROS) scavenger (Tempol; 1 mg·kg^-1·min^-1) partly abrogated the permeability effects following 120-min UUO but not after 180 min. Moreover, administration of the RhoA kinase inhibitor Y-27632, the nitric oxide synthase inhibitor N^G-nitro-l-arginine methyl ester, or Rac-1 inhibition did not ameliorate glomerular hyperpermeability following 180-min UUO. We show, for the first time, that acute UUO results in marked elevations in glomerular permeability. In addition, our data suggest a time-dependent pathophysiology of UUO-induced hyperpermeability, where reactive oxygen species generation may play an important role in the early stages.

Optimizing Automated Peritoneal Dialysis Using an Extended 3-Pore Model

Introduction

In the current study, an extended 3-pore model (TPM) is presented and applied to the problem of optimizing automated peritoneal dialysis (APD) with regard to osmotic water transport (UF), small/middle-molecule clearance, and glucose absorption.

Methods

Simulations were performed for either intermittent APD (IPD) or tidal APD (TPD). IPD was simulated for fill and drain volumes of 2 L, whereas TPD was simulated using a tidal volume of 0.5 L, 1 L, or 1.5 L with full drains and subsequent fills (2 L) occurring after every fifth dwell. A total of 25 cycles for a large number of different dialysate flow rates (DFR) were simulated using 3 different glucose concentrations (1.36%, 2.27%, and 3.86%) and 3 different peritoneal transport types: slow (peritoneal equilibrium test D/P_crea < 0.6), fast (peritoneal equilibrium test D/P_crea > 0.8), and average. Solute clearance and UF were simulated to occur during the entire dwell, including both fill and drain periods.

Results

It is demonstrated that DFRs exceeding ∼ 3 L/h are of little benefit both for UF and small-solute transport, whereas middle-molecule clearance is enhanced at higher DFRs. The simulations predict that large reductions (> 20%) in glucose absorption are possible by using moderately higher DFRs than a standard 6 × 2 L prescription and by using shorter optimized “bi-modal” APD regimens that alternate between a glucose-free solution and a glucose-containing solution.

Discussion

Reductions in glucose absorption appear to be significant with the proposed regimens for APD; however, further research is needed to assess the feasibility and safety of these regimens.

A distributed solute model: an extended two-pore model with application to the glomerular sieving of Ficoll

One of the many unresolved questions regarding the permeability of the glomerular filtration barrier is the reason behind the marked difference in permeability between albumin and polysaccharide probe molecules such as Ficoll and dextran of the same molecular size. Although the differences in permeability have been mainly attributed to charge effects, we have previously shown that this would require a highly charged filtration barrier, having a charge density that is ~10 times more than that on the albumin molecule. In this article, the classic two-pore model was extended by introducing size distributions on the solute molecules, making them conformationally flexible. Experimental sieving data for Ficoll from the rat glomerulus and from precision-made silicon nanopore membranes were analyzed using the model. For the rat glomerulus a small-pore radius of 36.2 Å and a geometric standard deviation (gSD) for the Ficoll size-distribution of 1.16 were obtained. For the nanopore membranes, a gSD of 1.24 and a small-pore radius of 43 Å were found. Interestingly, a variation of only ~16% in the size of the polysaccharide molecule is sufficient to explain the difference in permeability between albumin and Ficoll. Also, in line with previous data, the effects of applying a size distribution on the solute molecule are only evident when the molecular size is close to the pore size. Surely there is at least some variation in the pore radii, and, likely, the gSD obtained in the current study is an overestimation of the "true" variation in the size of the Ficoll molecule.

Differential effects of gaseous versus injectable anesthetics on changes in regional cerebral blood flow and metabolism induced by l-DOPA in a rat model of Parkinson's disease

Preclinical imaging of brain activity requires the use of anesthesia. In this study, we have compared the effects of two widely used anesthetics, inhaled isoflurane and ketamine/xylazine cocktail, on cerebral blood flow and metabolism in a rat model of Parkinson's disease and l-DOPA-induced dyskinesia. Specific tracers were used to estimate regional cerebral blood flow (rCBF - [¹⁴C]-iodoantipyrine) and regional cerebral metabolic rate (rCMR - [¹⁴C]-2-deoxyglucose) with a highly sensitive autoradiographic method. The two types of anesthetics had quite distinct effects on l-DOPA-induced changes in rCBF and rCMR. Isoflurane did not affect either the absolute rCBF values or the increases in rCBF in the basal ganglia after l-DOPA administration. On the contrary, rats anesthetized with ketamine/xylazine showed lower absolute rCBF values, and the rCBF increases induced by l-DOPA were masked. We developed a novel improved model to calculate rCMR, and found lower metabolic activities in rats anesthetized with isoflurane compared to animals anesthetized with ketamine/xylazine. Both anesthetics prevented changes in rCMR upon l-DOPA administration. Pharmacological challenges in isoflurane-anesthetized rats indicated that drugs mimicking the actions of ketamine/xylazine on adrenergic or glutamate receptors reproduced distinct effects of the injectable anesthetics on rCBF and rCMR. Our results highlight the importance of anesthesia in studies of cerebral flow and metabolism, and provide novel insights into mechanisms mediating abnormal neurovascular responses to l-DOPA in Parkinson's disease.

Is Adapted APD Theoretically More Efficient than Conventional APD?

Background

A modified version of automated peritoneal dialysis (APD) using not only variable dwell times but also variable fill volumes has been tested against conventional APD (cAPD) with fixed dwell volumes in a randomized controlled clinical study. The results have indicated that the modified schedule for APD, denoted adapted APD (aAPD), can lead to improved small solute clearances, and, above all, a markedly increased sodium removal (NaR). To theoretically test these results, we have modeled aAPD vs cAPD in computer simulations using the 3-pore model (TPM).

Methods

The TPM, modified by including a transient, initial inflation of small solute mass transfer area coefficients (PS values), was employed. For simulations of osmotic ultrafiltration (UF), the TPM uses a constantly inflated value for PS for glucose and also a reduced value for PS for Na+, setting the peritoneal lymphatic reabsorption term at 0.3 mL/min. The simulations were performed by assuming that increases in intraperitoneal hydrostatic pressure (IPP) are transmitted to the capillary level ( via vein compression) and therefore do not significantly affect the Starling balance. Furthermore, the effective peritoneal surface area (A) was set to be variable as a function of intraperitoneal volume (IPV).

Results

The simulations demonstrated a minor improvement of small solute clearances (∼0.7 – 1.6%) and a very small improvement of UF and NaR in aAPD compared to cAPD.

Conclusions

Due mainly to the increased fill volumes in 3 out of 5 dwells in aAPD, this modality caused minor increases in small solute clearances and marginal effects on UF and NaR. The computer simulations point to a need for accurate sodium determinations in aAPD, considering all the methodological problems and pitfalls relevant to determining dialysate Na+ concentrations and peritoneal sodium mass balance.

The importance of albumin infusion rate for plasma volume expansion following major abdominal surgery - AIR: study protocol for a randomised controlled trial

BACKGROUND

Administration of fluids to restore normovolaemia is one of the most common therapeutic interventions performed peri-operatively and in the critically ill, but no study has evaluated the importance of infusion rate for the plasma volume-expanding effect of a resuscitation fluid. The present study is designed to test the hypothesis that a slow infusion of resuscitation fluid results in better plasma volume expansion than a rapid infusion.

METHODS/DESIGN

The study is a single-centre, assessor-blinded, parallel-group, randomised prospective study. Patients over 40 years of age admitted to the post-operative care unit after a Whipple procedure or major gynaecological surgery and presenting with signs of hypovolaemia are eligible for inclusion. Patients are randomised in a 1:1 fashion with no stratification to either rapid (30 minutes) or slow (180 minutes) infusion of 5% albumin at a dose of 10 ml/kg ideal body weight. Plasma volume is measured using 125I human serum albumin at baseline (prior to albumin infusion) as well as at 30 minutes and 180 minutes after infusion start. The primary endpoint is change in plasma volume from baseline to 180 minutes after the start of 5% albumin infusion. Secondary endpoints include the integral of plasma volume over time from baseline to 180 minutes after the start of the infusion and transcapillary escape rate of albumin (%/h) from 180 minutes to 240 minutes after the start of albumin infusion. In addition, diuresis, change in central venous oxygen saturation, lactate and blood pressure will be evaluated. A total of 70 patients will be included in the study, and the study has 80% power to detect a difference of 4 ml/kg in plasma volume expansion between the two groups.

DISCUSSION

The present study is the first clinical investigation of the importance of infusion rate for the plasma volume-expanding effect of a resuscitation fluid.

TRIAL REGISTRATION

EudraCT identifier: 2013-004446-42 . Registration date: 20 December 2013. ClinicalTrials.gov identifier: NCT02728921 . Registration date: 31 March 2016.

Nitric oxide synthase inhibition causes acute increases in glomerular permeability in vivo, dependent upon reactive oxygen species

There is increasing evidence that the permeability of the glomerular filtration barrier (GFB) is partly regulated by a balance between the bioavailability of nitric oxide (NO) and that of reactive oxygen species (ROS). It has been postulated that normal or moderately elevated NO levels protect the GFB from permeability increases, whereas ROS, through reducing the bioavailability of NO, have the opposite effect. We tested the tentative antagonism between NO and ROS on glomerular permeability in anaesthetized Wistar rats, in which the left ureter was cannulated for urine collection while simultaneously blood access was achieved. Rats were systemically infused with either l-NAME or l-NAME together with the superoxide scavenger Tempol, or together with l-arginine or the NO-donor DEA-NONOate, or the cGMP agonist 8-bromo-cGMP. To measure glomerular sieving coefficients (theta, θ) to Ficoll, rats were infused with FITC-Ficoll 70/400 (mol/radius 10-80 Å). Plasma and urine samples were analyzed by high-performance size-exclusion chromatography (HPSEC) for determination of θ for Ficoll repeatedly during up to 2 h. l-NAME increased θ for Ficoll_70Å from 2.27 ± 1.30 × 10^-5 to 8.46 ± 2.06 × 10^-5 (n = 6, P < 0.001) in 15 min. Tempol abrogated these increases in glomerular permeability and an inhibition was also observed with l-arginine and with 8-bromo-cGMP. In conclusion, acute NO synthase inhibition in vivo by l-NAME caused rapid increases in glomerular permeability, which could be reversed by either an ROS antagonist or by activating the guanylyl cyclase-cGMP pathway. The data strongly suggest a protective effect of NO in maintaining normal glomerular permeability in vivo.

Reduced glomerular size selectivity in late streptozotocin-induced diabetes in rats: application of a distributed two-pore model

Microalbuminuria is an early manifestation of diabetic nephropathy. Potential contributors to this condition are reduced glomerular filtration barrier (GFB) size- and charge selectivity, and impaired tubular reabsorption of filtered proteins. However, it was recently reported that no significant alterations in charge selectivity of the GFB occur in early experimental diabetic nephropathy. We here aimed at investigating the functional changes in the GFB in long-term type-1 diabetes in rats, applying a novel distributed two-pore model. We examined glomerular permeability in 15 male Wistar rats with at least 3 months of streptozotocin (STZ)-induced diabetes (blood glucose ∼20 mmol/L) and in age-matched control rats. The changes in glomerular permeability were assessed by determining the glomerular sieving coefficients (θ) for FITC-Ficoll (molecular radius 20–90 Å) using size exclusion HPLC. The values of θ for FITC-Ficoll of radius >50 Å were significantly increased in STZ-diabetic rats compared to age-matched controls (θ for 50–69 Å = 0.001 vs. 0.0002, and θ for 70–90 Å = 0.0007 vs. 0.00006, P < 0.001), while θ for FITC-Ficoll <50 Å tended to be lower in diabetic rats than in controls (θ for 36–49 Å = 0.013 vs. 0.016, ns). According to the distributed two-pore model, there was primarily an increase in macromolecular transport through large pores in the glomerular filter of diabetic rats associated with a loss of small-pore area. Deterioration in the glomerular size selectivity due to an increase in the number and size-spread of large pores, with no changes in the permeability of the small-pore system, represent the major functional changes observed after 3 months of induced experimental diabetes.

A distributed two-pore model: theoretical implications and practical application to the glomerular sieving of Ficoll

In the present study, an extended two-pore theory is presented where the porous pathways are continuously distributed according to small- and large-pore mean radii and SDs. Experimental glomerular sieving data for Ficoll were analyzed using the model. In addition, several theoretical findings are presented along with analytic solutions to many of the equations used in distributed pore modeling. The results of the data analysis revealed a small-pore population in the glomerular capillary wall with a mean radius of 36.6 Å having a wide arithmetic SD of ∼5 Å and a large-pore radius of 98.6 Å with an even wider SD of ∼44 Å. The small-pore radius obtained in the analysis was close to that of human serum albumin (35.5 Å). By reanalyzing the data and setting the distribution spread of the model constant, we discovered that a narrow distribution is compensated by an increased mean pore radius and a decreased pore area-to-diffusion length ratio. The wide distribution of pore sizes obtained in the present analysis, even when considering electrostatic hindrance due to the negatively charged barrier, is inconsistent with the high selectivity to proteins typically characterizing the glomerular filtration barrier. We therefore hypothesize that a large portion of the variance in the distribution of pore sizes obtained is due to the molecular “flexibility” of Ficoll, implying that the true variance of the pore system is lower than that obtained using flexible probes. This would also, in part, explain the commonly noted discrepancy between the pore area-to-diffusion length ratio and the filtration coefficient.

Quantification of the electrostatic properties of the glomerular filtration barrier modeled as a charged fiber matrix separating anionic from neutral Ficoll

In the current study we explore the electrostatic interactions on the transport of anionic Ficoll (aFicoll) vs. neutral Ficoll (nFicoll) over the glomerular filtration barrier (GFB) modeled as a charged fiber matrix. We first analyze experimental sieving data for the rat glomerulus, and second, we explore some of the basic implications of a theoretical model for the electrostatic interactions between a charged solute and a charged fiber-matrix barrier. To explain the measured difference in glomerular transport between nFicoll and aFicoll (Axelsson J, Sverrisson K, Rippe A, Fissell W, Rippe B. Am J Physiol 301: F708-F712, 2011), the present simulations demonstrate that the surface charge density needed on a charged fiber matrix must lie between -0.005 C/m(2) and -0.019 C/m(2), depending on the surface charge density of the solute. This is in good agreement with known surface charge densities for many proteins in the body. In conclusion, the current results suggest that electrical charge makes a moderate contribution to glomerular permeability, while molecular size and conformation seem to be more important. Yet, the weak electrical charge obtained in this study can be predicted to nearly totally exclude albumin from permeating through "high-selectivity" pathways in a charged-fiber matrix of the GFB.