Novel Method for Osmotic Conductance to Glucose in Peritoneal Dialysis

Transcellular transport of 18F-deoxyglucose via facilitative glucose channels in experimental peritoneal dialysis

BACKGROUND

Local and systemic side effects of glucose remain major limitations of peritoneal dialysis (PD). Glucose transport during PD is thought to occur via inter-endothelial pathways, but recent results show that phloretin, a general blocker of facilitative glucose channels (glucose transporters [GLUTs]), markedly reduced glucose diffusion capacity indicating that some glucose may be transferred via facilitative glucose channels (GLUTs). Whether such transport mainly occurs into (absorption), or across (trans-cellular) peritoneal cells is as yet unresolved.

METHODS

Here we sought to elucidate whether diffusion of radiolabeled 18F-deoxyglucose ([18F]-DG) in the opposite direction (plasma → dialysate) is also affected by GLUT inhibition. During GLUT inhibition, such transport may either be increased or unaltered (favors absorption hypothesis) or decreased (favors transcellular hypothesis). Effects on the transport of solutes other than [18F]-DG (or glucose) during GLUT inhibition indicate effects on paracellular transport (between cells) rather than via GLUTs.

RESULTS

GLUT inhibition using phloretin markedly reduced [18F]-DG diffusion capacity, improved ultrafiltration (UF) rates and enhanced the sodium dip. No other solutes were significantly affected with the exception of urea and bicarbonate.

CONCLUSION

The present results indicate that part of glucose is transported via the transcellular route across cells in the peritoneal membrane. Regardless of the channel(s) involved, inhibitors of facilitative GLUTs may be promising agents to improve UF efficacy in patients treated with PD.

Pulsed peritoneal dialysis in an experimental rat model: A first experience

BACKGROUND

Peritoneal dialysis (PD) is commonly performed using either intermittent or tidal exchanges, whereas other exchange techniques such as continuous flow PD are little used. Previous research indicated that stirring the intra-peritoneal dialysate markedly increases small solute clearances. Here, we tested the hypothesis that stirring of the dialysate increases small solute clearances by using a novel exchange technique where the dialysate is pulsed back and forth during the treatment without addition of fresh fluid.

METHODS

PD was performed in anesthetized Sprague-Dawley rats with either no pulsations (20 mL fill volume), 2 mL (10%) pulses (21 mL fill volume), or 5 mL (25%) pulses (22.5 mL fill volume) utilizing a pulse flow rate of 5 mL/min. The higher fill volume for the pulsed treatments compensates for the fact that the average intra-peritoneal volume would otherwise be lower in pulsed treatments. Water and solute transport were closely monitored during the treatment.

RESULTS

Net ultrafiltration decreased significantly during pulsed PD with the 25% pulse volume. The 60 min sodium dip was unaltered, whereas the fluid absorption rate was increased for the 25% group. Solute clearances did not significantly differ between groups, except for a slightly lower calcium clearance in the 25% group.

CONCLUSION

Our data indicate that stirring the dialysate using pulsed exchanges does not provide any advantage compared to conventional exchange techniques. In contrast, pulsed treatments had slightly lower ultrafiltration and small solute transport. The present findings may have implications regarding the choice of tidal volume in automated PD, favoring smaller tidal volumes.

Determining the residual volume in peritoneal dialysis using low molecular weight markers

BACKGROUND

Variation in residual volume between peritoneal dialysis dwells creates uncertainty in ultrafiltration determination, dialysis efficiency, and poses a risk of overfill if the residual volume is large. Measuring the dilution of a marker molecule during fluid fill offers a convenient approach, however, estimation accuracy depends on the choice of dilution marker. We here evaluate the feasibility of creatinine and urea as dilution markers compared to albumin-based residual volumes and three-pore model estimations.

METHOD

This clinical, retrospective analysis comprises 56 residual volume estimations from 20 individuals, based on the dilution of pre-fill dialysate creatinine, urea and albumin concentrations during the dialysis fluid fill phase. Outcomes were compared individually. Bias induced by ultrafiltration, marker molecule mass-transfer and influence of fluid glucose contents was quantified using the three-pore model. Linear regression established conversion factors enabling conversion between the various marker molecules.

RESULTS

Creatinine-based calculations overestimated residual volumes by 115 mL (IQR 89-149) in 1.5% dwells and 252 mL (IQR 179-313) in 4.25% glucose dwells. In hypertonic dwells, ultrafiltration was 52 mL (IQR 38-66), while intraperitoneal creatinine mass increased by 67% during fluid fill, being the leading cause of overestimation. Albumin-based volumes conformed strongly with three-pore model estimates. Correction factors effectively enabled marker molecule interchangeability.

CONCLUSIONS

Mass-transfer of low molecular weight marker molecules is associated with residual volume overestimation. However, by applying correction factors, creatinine and urea dilution can still provide reasonable estimates, particularly when the purpose is to exclude the presence of a very large residual volume.

The Peritoneal Membrane and Its Role in Peritoneal Dialysis

A healthy and functional peritoneal membrane is key to achieving sufficient ultrafiltration and restoring 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 versus colloid osmosis; why and how to assess membrane function using peritoneal equilibration tests; the etiologies of membrane dysfunction and their specific management; and the effect 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, the care of patients treated with PD.

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.

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.

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.

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.

Peritoneal physicochemical transport mechanisms: Hypotheses, models and controversies

This study answers criticisms by Waniewski et al. of the recent paper by Wolf on peritoneal transport kinetic models. Their criticisms centre on the accuracy of the data used for model fits, the hypothesis presented, which involves changes in glucose membrane parameters at high peritoneal glucose concentration and on the necessary techniques required to achieve accurate model parameter estimation. In response, this article shows that (1) the mean values previously captured from graphical depictions of Heimburger et al. are not different than those captured from the recent Waniewski et al. graphs, (2) a much simpler hypothesis is proposed, which centres on intraperitoneal pressure-induced lymph flow during the dialysis dwell and (3) the finding that the new model predictions, with only two constant parameter values, as estimated by the Powell algorithm, give a closer fit than the Waniewski model, which uses many time-varying parameters. The current findings again bring into question of the validity of their vasodilation hypothesis, leading to transient changes in capillary surface area during the dwell.

SGLT2 Inhibition by Intraperitoneal Dapagliflozin Mitigates Peritoneal Fibrosis and Ultrafiltration Failure in a Mouse Model of Chronic Peritoneal Exposure to High-Glucose Dialysate

Peritoneal dialysis (PD) is limited by glucose-mediated peritoneal membrane (PM) fibrosis, angiogenesis, and ultrafiltration failure. Influencing PM integrity by pharmacologically targeting sodium-dependent glucose transporter (SGLT)-mediated glucose uptake has not been studied. In this study, wildtype C57Bl/6N mice were treated with high-glucose dialysate via an intraperitoneal catheter, with or without addition of selective SGLT2 inhibitor dapagliflozin. PM structural changes, ultrafiltration capacity, and peritoneal equilibration testing (PET) status for glucose, urea, and creatinine were analyzed. Expression of SGLT and facilitative glucose transporters (GLUT) was analyzed by real-time PCR, immunofluorescence, and immunohistochemistry. Peritoneal effluents were analyzed for cellular and cytokine composition. We found that peritoneal SGLT2 was expressed in mesothelial cells and in skeletal muscle. Dapagliflozin significantly reduced effluent transforming growth factor (TGF-β) concentrations, peritoneal thickening, and fibrosis, as well as microvessel density, resulting in improved ultrafiltration, despite the fact that it did not affect development of high-glucose transporter status. In vitro, dapagliflozin reduced monocyte chemoattractant protein-1 release under high-glucose conditions in human and murine peritoneal mesothelial cells. Proinflammatory cytokine release in macrophages was reduced only when cultured in high-glucose conditions with an additional inflammatory stimulus. In summary, dapagliflozin improved structural and functional peritoneal health in the context of high-glucose PD.