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.

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.

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/Pcrea < 0.6), fast (peritoneal equilibrium test D/Pcrea > 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.

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.

Microproteinuria Predicts Organ Failure in Patients Presenting with Acute Pancreatitis.

BACKGROUND AND AIMS: The disease course of acute pancreatitis (AP) ranges from mild and self-limiting to severe inflammation, associated with significant morbidity and mortality. At present, there are no universally accepted and reliable predictors for severity. Microproteinuria has been associated with the presence of systemic inflammatory response syndrome as well as trauma, although its association with AP is not well understood. The aim of this study was to investigate the value of microproteinuria to predict development of organ failure in AP. METHODS: Consecutive AP patients were prospectively enrolled. Urine samples were collected upon admission, 12-24 h after admission, and 3 months post-discharge for calculation of urine α1-microglobulin-, albumin-, IgG-, and IgM/creatinine ratios. Data regarding AP etiology, severity, and development of organ failure were registered. RESULTS: Overall, 92 AP patients were included (14 % with organ failure; 6 % with severe AP). The α1-microglobulin-, albumin-, and IgG/creatinine ratios correlated with high-sensitivity C-reactive protein 48 h after admission (r = 0.47-0.61, p < 0.001 for all). They were also significantly higher in patients with versus without organ failure (p < 0.05 for all). The α1-microglobulin/creatinine ratio upon admission predicted organ failure [adjusted odds ratio 1.286, 95 % confidence interval (CI) 1.024-1.614] with similar accuracy (AUROC 0.81, 95 % CI 0.69-0.94) as the more complex APACHE II score (AUROC 0.86, 95 % CI 0.70-1.00). CONCLUSION: The α1-microglobulin/creatinine ratio upon presentation with AP is related to inflammation and predicts development of organ failure. Further studies are warranted to evaluate its potential usefulness in predicting outcome for AP patients.

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 Ficoll70Å 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.

Albumin Turnover in Peritoneal and Hemodialysis.

The turnover of albumin is increased in both peritoneal dialysis (PD) and hemodialysis (HD) due to increased external losses, normally leading to compensatory increases in the hepatic albumin synthesis. The normal rate of albumin synthesis is on the order of 12 g/day corresponding to an equally large albumin fractional catabolic rate of ~4% daily. Most albumin catabolism is assumed to occur in the endothelium, but there is also renal and hepatic catabolism and leakage into the gastrointestinal tract. In PD the daily losses are on the order of 5 g/day. There are also external albumin losses in HD, particularly when high-performance membranes are used, the losses per session ranging between 1 and 8 g (or more). The dialytic albumin losses cannot be detected by assessing the transcapillary escape rate of albumin from the plasma compartment to the interstitium. In PD, tracer albumin that has been injected into the peritoneal fluid is absorbed to the tissues surrounding the peritoneal cavity without much edema formation, due to the process of "volume recirculation". A small fraction of the dialysate albumin tracer (0.2-0.3 ml/minute) is directly reabsorbed to the plasma via the lymphatics. A significant portion of dialysis patients are affected by chronic inflammation, such as in the malnutrition inflammation and atherosclerosis syndrome, which is also associated with cardiovascular mortality and fluid overload. These patients usually have a reduced ability to compensate for external losses of albumin, which may result in hypoalbuminemia. Reduced plasma albumin levels in dialysis patients may thus be regarded as a sign of chronic inflammation rather than reflecting malnutrition.

Acute reactive oxygen species (ROS)-dependent effects of IL-1ß, TNF-α, and IL-6 on the glomerular filtration barrier (GFB) in vivo.

This study was performed to investigate the immediate actions of the proinflammatory cytokines IL-1ß, TNF-α, and IL-6 on the permeability of the glomerular filtration barrier (GFB) in rats and to test whether these actions are dependent upon the release of reactive oxygen species (ROS). In anesthetized rats, blood access was achieved and the left ureter was cannulated for urine collection. Rats were continuously infused intravenously with either IL-1ß (0.4 and 2 µg·kg(-1)·h(-1)), TNF-α (0.4 and 2 µg·kg(-1)·h(-1)), or IL-6 (4 and 8 µg·kg(-1)·h(-1)), together with polydisperse FITC-Ficoll-70/400 and inulin for 1 h. Plasma and urine samples were analyzed by high performance size exclusion chromatography (HPSEC) for determination of glomerular sieving coefficients (θ). The glomerular filtration rate (GFR) was also assessed (51Cr-EDTA). In separate experiments, the superoxide scavenger tempol (30 mg·kg(-1)·h(-1)) was given before and during cytokine infusions. IL-1ß and TNF-α caused rapid, partly reversible increases in glomerular permeability to large molecules (Ficoll50-80Å), peaking at 5-30 min, while IL-6 caused a more gradual increase in permeability, leveling off at 60 min. Tempol almost completely abrogated the glomerular permeability effects of the cytokines infused. In conclusion IL-1ß, TNF-α, and IL-6, when infused systemically, caused immediate and partly reversible increases in glomerular permeability, which could be inhibited by the superoxide scavenger tempol, suggesting an important role of ROS in acute cytokine-induced permeability changes in the GFB.

Plasma pro-inflammatory cytokines, IgM-uria and cardiovascular events in patients with chest pain: A comparative study.

BACKGROUND: Risk stratification of patients presenting with acute chest pain is crucial for immediate and long-term management. Traditional predictors are suboptimal; therefore inflammatory biomarkers are studied for clinical assessment of patients at risk. Recently, we reported the association of IgM-uria with worse cardiovascular outcome in patients with acute chest pain. In this study, in the same cohort of patients with chest pain, we compared the value of IgM-uria to pro-inflammatory cytokines in predicting the occurrence of subsequent cardiovascular events. METHODS: A total of 178 consecutive patients presenting with acute chest pain to the emergency department at the University Hospital of Lund, were recruited. Twenty-seven of 57 patients with acute coronary syndrome (ACS), and 18 of 118 patients with non-specific chest pain at baseline developed a subsequent major cardiovascular event during the 18 months follow-up. Urinary proteins (IgM-uria and Microalbuminuria) and plasma inflammatory markers (IL-6, Il-8, IL-10, IFN-γ and TNF-α) were measured at time of admission. RESULTS: Using the receiver operating characteristic curves, the area under the curve for predicting cardiovascular events was 0.71 (95%CI 0.61-0.81) for IgM-uria, 0.61 (95%CI 0.51-0.71) for IL-6, 0.63 (95%CI 0.53-0.72) for IL-8, 0.65 (95%CI 0.56-0.74) for IL-10, and 0.64 (95% CI 0.54-0.74) for TNF-α. In multivariate Cox-regression analysis adjusted for age, microalbuminuria, IgM-uria, IL-10, TNF-α, troponin T, hsCRP and ACS at baseline; IgM-uria was the only biomarker that remained an independent predictor of outcome (HR = 4.2, 95%CI 2.2-7.8, p < 0.001). CONCLUSION: In patients with chest pain with or without acute coronary syndrome, IgM-uria could better predict the occurrence of cardiovascular events than plasma pro-inflammatory cytokines.

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.

Reduction in glomerular pore size is not restricted to pregnant women. Evidence for a new syndrome: 'Shrunken pore syndrome'.

The plasma levels of cystatin C, ß2-microglobulin, beta-trace protein, retinol binding protein (RBP) and creatinine were determined in plasma samples from 111 randomly selected patients with eGFRcystatin C ≤ 60% of eGFRcreatinine and from 55 control patients with 0.9eGFRcreatinine ≤ eGFRcystatin C ≤ 1.1eGFRcreatinine (eGFRcystatin C ≈ eGFRcreatinine). The concentration ratios of cystatin C/creatinine, ß2-microglobulin/creatinine, beta-trace protein/creatinine and RBP/creatinine were significantly higher in patients with eGFRcystatin C ≤ 60% of eGFRcreatinine than in patients with eGFRcystatin C ≈ eGFRcreatinine. When the patients were divided into three groups with different estimated GFR intervals (≤ 40, 40-60 and ≥ 60 mL/min/1.73m(2)) the concentration ratios of cystatin C/creatinine, ß2-microglobulin/creatinine, and beta-trace protein/creatinine were significantly higher in patients with eGFRcystatin C ≤ 60% of eGFRcreatinine than in patients with eGFRcystatin C ≈ eGFRcreatinine for all GFR intervals. Similar results were obtained when the population without pregnant women was studied as well as the subpopulations of men or of non-pregnant women. Populations of pre-eclamptic women and pregnant women in the third trimester display similar results. Since the production of these four proteins with sizes similar to that of cystatin C is not co-regulated, the most likely explanation for the simultaneous increase of their creatinine-ratios in patients with eGFRcystatin C ≤ 60% of eGFRcreatinine is that their elimination by glomerular filtration is decreased. We suggest that this is due to a reduction in pore diameter of the glomerular membrane and propose the designation 'Shrunken pore syndrome' for this pathophysiological state.

mTOR inhibition with temsirolimus causes acute increases in glomerular permeability, but inhibits the dynamic permeability actions of puromycin aminonucleoside.

Inhibitors of the mammalian target of rapamycin (mTORi) can produce de novo proteinuria in kidney transplant patients. On the other hand, mTORi has been shown to suppress disease progression in several animal models of kidney disease. In the present study, we investigated whether glomerular permeability can be acutely altered by the mTORi temsirolimus and whether mTORi can affect acute puromycin aminonucleoside (PAN) or angiotensin II (ANG II)-induced glomerular hyperpermeability. In anesthetized Wistar rats, the left ureter was cannulated for urine collection, while simultaneously blood access was achieved. Temsirolimus was administered as a single intravenous dose 30 min before the start of the experiments in animals infused with PAN or ANG II or in nonexposed animals. Polydispersed FITC-Ficoll-70/400 (molecular radius 10-80 Å) and (51)Cr-EDTA infusion was given during the whole experiment. Measurements of Ficoll in plasma and urine were performed sequentially before the temsirolimus injection (baseline) and at 5, 15, 30, 60, and 120 min after the start of the experiments. Urine and plasma samples were analyzed by high-performance size-exclusion chromatography (HPSEC) to assess glomerular sieving coefficients (θ) for Ficoll10-80Å. Temsirolimus per se increased baseline glomerular permeability to Ficoll50-80Å 45 min after its administration, a reactive oxygen species (ROS)-dependent phenomenon. PAN caused a rapid and reversible increase in glomerular permeability, peaking at 5 min, and again at 60-120 min, which could be blocked by the ROS scavenger tempol. mTORi abrogated the second permeability peak induced by PAN. However, it had no effect on the immediate ANG II- or PAN-induced increases in glomerular permeability.

Dynamic, size-selective effects of protamine sulfate and hyaluronidase on the rat glomerular filtration barrier in vivo.

The proteinuric actions of protamine sulfate (PS) have classically been, at least partly, attributed to alterations of the negatively charged glomerular endothelial glycocalyx. To investigate whether the charge-selective properties of the glomerular filtration barrier (GFB) would be altered by PS, we assessed the glomerular sieving of conventional, uncharged, polydispersed Ficoll (n-Ficoll) compared with charge modified, conformationally intact, anionic (carboxymethylated) Ficoll (a-Ficoll) before and after systemic infusions of PS in rats. For comparison, we also investigated the impact of hyaluronidase (hyase), which partially degrades the glycocalyx, on GFB permeability. In anaesthetized Wistar rats, blood access was achieved, and the left ureter was cannulated for urine collection. Rats were infused with either n-Ficoll or a-Ficoll before and during systemic infusions with either PS or hyase. Plasma and urine samples were taken repeatedly and analyzed by high-performance size exclusion chromatography to assess glomerular sieving coefficients (θ) for Ficoll (radius 10-80 Å). The GFB showed a significant glomerular charge selectivity for Ficoll molecules of radius 20-35 Å. PS and hyase infusions reversibly increased θ for large Ficoll molecules (Ficoll molecules of radius 50-80 Å). Thus, for PS, θ for a-Ficoll molecules of radius 70 Å increased from 2.47 × 10(-5) ± 1.1(-5) to 7.25 × 10(-5) ± 1.1(-5) (P < 0.05) at 15 min. For hyase, changes in a-Ficoll molecules of radius 50-80 Å were, however, not statistically significant. Neither PS nor hyase had any effect on θ for n-Ficoll molecules of radius 20-45 Å or a-Ficoll molecules of radius 20-45 Å. It is concluded that systemically administered PS and hyase in moderate doses dynamically decreased the size selectivity of the rat GFB without affecting its charge selective properties.