In Vivo Peritoneal Chamber: Linking Structure and Transport Function

Objective

The present study aimed to develop an animal model of chronic peritoneal exposure that directly links transport with the tissue involved.

Methods

Daily, rats were intraperitoneally infused through subcutaneous ports with 20 mL of these solutions: isotonic Krebs (K), K + 2.5% mannitol (M), K + 2.5% N-acetylglucosamine (NAG). Controls included catheter-only (CC) and age-control rats (AC). After 2 months, each rat was anesthetized and a plastic chamber was affixed to the abdominal wall serosa to isolate a portion of the peritoneum for transport studies. In the first 90 minutes, a hypertonic solution (approximately 500 mosm/kg) containing 14C-mannitol was placed in chamber. The volume and 14C concentration were measured to determine the rate of osmotic flux (flow/Areachamber) into the chamber and the flux of mannitol from the chamber to the tissue. At 90 minutes, fluorescein isothiocyanate conjugate (FITC)–albumin was given intravenously. The rate of appearance of that substance in the chamber was measured over a period of 180 minutes and divided by Areachamber to determine the average flux. After the rat was humanely killed, the tissue under the chamber was collected for analysis of its hyaluronan concentration ([HA]).

Results

All data are given as mean ± standard error: Group Osmotic (μL/cm2) Flux per hour Mannitol (/cm2) Albumin (/cm2) [HA] (μg/g dry) AC 69.9±14.0 0.043±0.004 0.0114±0.0012 1247±136 CC 65.5±8.0 0.040±0.013 0.0105±0.0027 1360±157 K 47.0±11.5 0.067±0.015 0.0116±0.0027 1134±160 M 101±20 0.044±0.017 0.0300±0.0120 1146±157 NAG 72.6±11.4 0.052±0.006 0.0188±0.0144 1240±157

Conclusions

In the present pilot study, no significant correlations were observed, but the number of animals in each group was small ( n = 3 – 4). Nevertheless, the results demonstrate the ability of the chamber technique to determine transperitoneal transport of water, small solutes, and protein, and to link those values directly to the structure of the tissue lying below the chamber. Thus, chronic treatment can be directly correlated with peritoneal structure and transport function.

Changes in the Peritoneal Interstitium and Their Effect on Peritoneal Transport

Transperitoneal transport is a complicated process that includes diffusion and convection across the walls of blood microvessels into tissue interstitium, transport through the interstitium, and final passage across the peritoneum to the dialysis solution in the cavity. The purpose of this paper is to briefly review the normal physiology of this process and then to summarize the events that occur in response to inflammation within the cavity. These events begin with stimulation of macrophages, which in turn secrete cytokines. The cytokines stimulate mesothelial cells and fibroblasts in the tissue to synthesize and secrete other mediators. Those mediators initiate the complex events through which leukocytes migrate from blood vessel lumens through the interstitium and into the cavity. Much of the available data is from model in vitro systems, and therefore in vivo events must be deduced or hypothesized.

Effect of Oral Administration of Losartan, Prazosin, and Verapamil on Peritoneal Solute Transport in Continuous Ambulatory Peritoneal Dialysis Patients

Background

Several intraperitoneally administered drugs have been shown to modify transport of peritoneal solute and fluid. Fewer studies, however, have evaluated the effect of orally administered drugs. The present study was performed to evaluate the effects of oral losartan, prazosin, and verapamil on peritoneal membrane transport during a peritoneal equilibration test (PET), as well as the effects on creatinine clearance (CrCl), Kt/V urea, 24-hour protein in drained dialysate, and drained volume.

Methods

This was an open, controlled, crossover clinical trial performed in 20 patients on continuous ambulatory peritoneal dialysis. All subjects used four 2-L 1.5% glucose dialysis exchanges per day. After a 7-day washout period (without antihypertensives), they had a baseline standard PET and dialysis adequacy assessment performed. Subsequently, they were randomly allocated to receive the first of three study drugs (losartan, prazosin, and verapamil), which were administered orally for a 7-day period. Immediately after each drug period, patients had a new 3-day washout and subsequently started the next drug, until they had received each of the three drugs. On the last day of administration of each drug, patients were subjected to a new PET and adequacy of dialysis evaluation.

Results

None of the studied drugs significantly modified the peritoneal transport of creatinine, glucose, urea, sodium, potassium, or total protein as evaluated by PET. Verapamil significantly increased peritoneal CrCl [51.3 (44.3 – 53.3) vs baseline 45.8 (41.4 – 50.5) L/week/1.73 m2, p < 0.05], weekly Kt/V urea [1.75 (1.60 – 1.78) vs baseline 1.59 (1.54 – 1.73), p < 0.05], and drained dialysate volume [8.80 (8.30 – 8.96) vs baseline 8.44 (8.20 – 8.50) L/day, p < 0.05].

Conclusions

Oral administration of losartan, prazosin, and verapamil did not modify the peritoneal transport of solutes during a 4-hour PET. Oral verapamil significantly increased CrCl, Kt/V urea, and 24-hour drained dialysate volume. It is most likely that verapamil increases peritoneal (hydraulic) conductivity, and then net ultrafiltration volume and convective transport of urea, creatinine, and protein. Verapamil could be considered as an alternative in patients requiring increased dialysis dose and/or ultrafiltration.

Replacement of Glucose with N-Acetylglucosamine in Peritoneal Dialysis Fluid—Experimental Study in Rats

Background

Glucose is still used as an osmotic solute in peritoneal dialysis fluids, despite evidence of its local (peritoneal) and systemic toxicities. However a constant search is underway for a new, more biocompatible osmotic solute for peritoneal dialysis fluids.

Objective

The present study evaluated N-acetylglucosamine (NAG) in a concentration of 220 mmol/ L as an alternative to glucose for the osmotic solute in peritoneal dialysis fluid, during chronic peritoneal dialysis in rats.

Methods

For 8 weeks, male Wistar rats were infused with glucose-based or NAG-based dialysis fluid. Intraperitoneal inflammation and peritoneal permeability and morphology were evaluated in all rats during the study.

Results

Repeated intraperitoneal infusion of the NAG-based dialysis fluid resulted in a weaker intra-abdominal inflammatory reaction as compared with the reaction in rats infused with glucose-based dialysis solution. At the end of the study, the concentration of hyaluronan in the peritoneal interstitium obtained from NAG-treated rats was higher than that found in the interstitium taken from animals exposed to dialysis fluid containing glucose. Also, peritoneal permeability to total protein was lower in NAG-treated rats.

Conclusion

As an alternative to glucose, NAG used for the osmotic solute in peritoneal dialysis solution decreases the intraperitoneal inflammatory reaction induced by the process of peritoneal dialysis and, indirectly (owing to the increased hyaluronan content in the peritoneal interstitium), diminishes peritoneal permeability to protein.

Non Anticoagulant Effects of Heparin: Implications for Animal Models of Peritoneal Dialysis

Heparin is a glycosaminoglycan with well-known anticoagulant activity. That property is used in animal models of peritoneal dialysis to maintain catheter patency and to prevent the development of peritoneal adhesions. However, heparin has a host of biologic actions beyond its role as an anticoagulant.

Heparin modulates the activity of various inflammatory cells, affects the synthesis of extracellular matrix, has antiproliferative effects on several cell types, and influences neoangiogenesis. By virtue of those actions, intraperitoneally administered heparin may interfere with peritoneal membrane homeostasis. The potential side effects of heparin use in animal models of peritoneal dialysis should be recognized to permit correct interpretation of experimental studies conducted in those models.

Animal Models of Peritoneal Dialysis: Thirty Years of Our Own Experience

Experimental animal models improve our understanding of technical problems in peritoneal dialysis PD, and such studies contribute to solving crucial clinical problems. We established an acute and chronic PD model in nonuremic and uremic rats. We observed that kinetics of PD in rats change as the animals are aging, and this effect is due not only to an increasing peritoneal surface area, but also to changes in the permeability of the peritoneum. Changes of the peritoneal permeability seen during chronic PD in rats are comparable to results obtained in humans treated with PD. Effluent dialysate can be drained repeatedly to measure concentration of various bioactive molecules and to correlate the results with the peritoneal permeability. Additionally we can study in in vitro conditions properties of the effluent dialysate on cultured peritoneal mesothelial cells or fibroblasts. We can evaluate acute and chronic effect of various additives to the dialysis fluid on function and permeability of the peritoneum. Results from such study are even more relevant to the clinical scenario when experiments are performed in uremic rats. Our experimental animal PD model not only helps to understand the pathophysiology of PD but also can be used for testing biocompatibility of new PD fluids.

Glucose but not N-acetylglucosamine accelerates in vitro senescence of human peritoneal mesothelial cells

BACKGROUND

Preservation of the mesothelial cells (MCs) is crucial for longevity of the peritoneal dialysis membrane. Glucose accelerates aging of MC and we tested whether N-acetylglucosamine (NAG) has an identical effect.

METHODS

Replicative aging of MCs was studied during 10 passages performed every three days in cells cultured in standard medium or in medium supplemented with Glucose 30 mmol/L or NAG 30 mmol/L. Changes in population doubling time and ß-galactosidase activity were used as an index of aging and compared with other cellular parameters.

RESULTS

Repeated passages of MC cause their aging, as reflected by prolongation of the population doubling time, increased ß-galactosidase activity, oxidative stress and release of cytokines. Healing of injured mesothelial monolayer is impaired in senescent cells. Glucose accelerates in vitro aging of MC, whereas NAG does not cause this effect.

CONCLUSIONS

Replacement of glucose with NAG in the dialysis fluid can slow down aging of MC.

Hyaluronan--regulator and initiator of peritoneal inflammation and remodeling

Although previously described as an inert space filler, there is now compelling evidence to underscore the importance of hyaluronan in physiologic and pathologic processes. Despite its simple structure, hyaluronan plays essential roles in embryonic development, phenotypic changes, proliferation, wound healing, inflammation and angiogenesis. Hyaluronan is a major component of the glycocalyx that forms a protective barrier around mesothelial cells, and bestows upon the peritoneal membrane a slippery non-adhesive surface preventing abrasion, infection and tumor dissemination. Hyaluronan is associated with mesothelial-to-mesenchymal transdifferentiation, recruitment of leukocytes to sites of inflammation, and mediates the reparative process after tissue injury by initiating increased synthesis of growth factors. Serum and dialysate levels of hyaluronan are increased in patients maintained on peritoneal dialysis (PD), of which the levels are further increased during episodes of peritonitis. The level of hyaluronan in PD effluents is often used as a surrogate marker for peritoneal inflammation and can predict patient survival. This review will describe the multifaceted roles of hyaluronan in the peritoneum and how these roles are modulated during PD.

N-Acetylglucosamine Reduces Inflammatory Response during Acute Peritonitis in Uremic Rats

BACKGROUND

Peritoneal dialysis (PD) induces intraperitoneal inflammation and that process may be uremia. The goal of this study is to evaluate the effect of uremia on the kinetics of peritonitis and furthermore test the anti-inflammatory potential of N-acetylglucosamine (NAG) in a uremic environment.

METHODS

Experiments were performed on healthy Wistar rats and on animals with impaired renal function. Acute PD was performed in all animals with dialysis fluid containing either glucose (GLU) or NAG as osmotic solutes. Peritonitis was induced by addition of lipopolysaccharide from Escherichia coli (LPS) to the dialysis fluid. Transperitoneal transport of water and solutes as well as intraperitoneal and systemic inflammation were evaluated.

RESULTS

Uremia reduces peritoneal permeability to total protein during peritonitis (-33% vs. control, p < 0.001) and increases net ultrafiltration (+2.5 +/- 2.2 vs. -2.7 +/- 3.2 ml in control, p < 0.001). In uremic rats with peritonitis, reduced dialysate levels of the following inflammatory mediators were detected as compared to healthy animals: MCP-1 (-15%, p < 0.01); IL-1beta (-53%, p < 0.001), and elastase (-28%, p < 0.02). In the serum of uremic rats, the increase in TNFalpha and MCP-1 concentrations was smaller than in control rats: -44% (p < 0.02) and -39% (p < 0.001), respectively. NAG used as an osmotic solute in rats with preserved renal function decreases intraperitoneal and systemic inflammation during acute peritonitis. Drained dialysate volume was increased in the NAG group by 32% (p < 0.001) and transperitoneal loss of protein was reduced by 21% (p < 0.002). When NAG was used as the osmotic solute instead of GLU, intraperitoneal inflammation in uremic animals was further reduced: TNFalpha (-40%, p < 0.05); IL-1beta (-49%, p < 0.005); MCP-1 (-21%, p < 0.005). The presence of NAG also reduced the increased blood level of IL-1beta (-47%,p < 0.02) and MCP-1 (-36%, p < 0.02).

CONCLUSIONS

Intensity of acute peritonitis is reduced during uremia. NAG exerts a systemic and peritoneal anti-inflammatory action under conditions of uremia that confirms the potential use of this compound as an osmotic agent in the PD fluids that also decreases inflammation.

Correlating structure with solute and water transport in a chronic model of peritoneal inflammation

To study the process of chronic peritoneal inflammation from sterile solutions, we established an animal model to link structural changes with solute and water transport. Filtered solutions containing 4% N-acetylglucosamine (NAG) or 4% glucose (G) were injected intraperitoneally daily in 200- to 300-g rats and compared with controls (C). After 2 mo, each animal underwent transport studies using a chamber affixed to the parietal peritoneum to determine small-solute and protein mass transfer, osmotic filtration, and hydraulic flow. After euthanasia, parietal tissues were sampled for histological analysis, which demonstrated significant differences in peritoneal thickness (microm; C, 42.6 +/- 7.5; G, 80.4 +/- 22.3; NAG, 450 +/- 104; P < 0.05). Staining for VEGF correlated with CD-31 vessel counts (no./mm2: C, 53.1 +/- 16.1; G, 166 +/- 32; NAG, 183 +/- 32; P < 0.05). Tissue analysis showed treatment effects on tissue hyaluronan (micro/g: C, 962 +/- 73; G, 1,169 +/- 69; NAG, 1,428 +/- 69; P < 0.05) and collagen (microg/g: C, 56.9 +/- 12.0; G, 107 +/- 12; NAG, 97.6 +/- 11.4; P < 0.05) but not sulfated glycosaminoglycan. Transport experiments revealed no significant differences in mannitol transfer or osmotic flow. Changes were seen in hydrostatic pressure-driven flux (microl x min(-1) x cm(-2): C, 0.676 +/- 0.133; G, 0.317 +/- 0.124; NAG, 0.284 +/- 0.117; P < 0.05) and albumin transfer (microl x min(-1) x cm(-2): C, 0.331 +/- 0.028; G, 0.286 +/- 0.026; NAG, 0.229 +/- 0.025; P < 0.04). We conclude that alteration of the interstitial matrix correlates with diminished hydraulic conductivity and macromolecular transport.

Subchronic toxicity study of dietary N-acetylglucosamine in F344 rats

A subchronic toxicity study of N-acetylglucosamine (GlcNAc), a monomeric form of chitin, was conducted in groups of 10 male and 10 female F344 rats fed pelleted diets containing 0, 0.625, 1.25, 2.5 or 5% concentrations for 13 weeks. All animals survived until the end of the experiment. Slight, non-significant increase in body weights was observed in males receiving 0.625, 1.25 or 2.5% from week 4 until the end of the experiment, when significant elevation was found for the males receiving 0.625, 1.25 or 2.5% at the terminal sacrifice to result in decreased relative weights of many organs in these cases. However, there were no obvious indications of toxicity in any group receiving GlcNAc in terms of clinical signs, food intake, hematology, serum biochemistry, and histopathological findings. Thus, it was concluded that orally administered GlcNAc exerts no obvious toxicity to F344 rats at concentrations up to 5% in the diet for 13 weeks. Based on the present toxicity data, > or =5% was determined to be a no-observed-adverse-effect level, translated into 2476 and 2834 mg/kg/day for male and female rats, respectively.

A pilot study of N-acetyl glucosamine, a nutritional substrate for glycosaminoglycan synthesis, in paediatric chronic inflammatory bowel disease

BACKGROUND

The breakdown of glycosaminoglycans is an important consequence of inflammation at mucosal surfaces, and inhibition of metalloprotease activity may be effective in treating chronic inflammation.

AIM

To report an alternative approach, using the nutriceutical agent N-acetyl glucosamine (GlcNAc), an amino-sugar directly incorporated into glycosaminoglycans and glycoproteins, as a substrate for tissue repair mechanisms.

METHODS

GlcNAc (total daily dose 3-6 g) was administered orally as adjunct therapy to 12 children with severe treatment-resistant inflammatory bowel disease (10 Crohn's disease, 2 ulcerative colitis). Seven of these children suffered from symptomatic strictures. In addition, similar doses were administered rectally as sole therapy in nine children with distal ulcerative colitis or proctitis resistant to steroids and antibiotics. Where pre- and post-treatment biopsies were available (nine cases), histochemical assessment of epithelial and matrix glycosaminoglycans and GlcNAc residues was made.

FINDINGS

Eight of the children given oral GlcNAc showed clear improvement, while four required resection. Of the children with symptomatic Crohn's stricture, only 3 of 7 have required surgery over a mean follow-up of > 2.5 years, and endoscopic or radiological improvement was detected in the others. Rectal administration induced remission in two cases, clear improvement in three and no effect in two. In all cases biopsied there was evidence of histological improvement, and a significant increase in epithelial and lamina propria glycosaminoglycans and intracellular GlcNAc.

CONCLUSIONS

GlcNAc shows promise as an inexpensive and nontoxic treatment in chronic inflammatory bowel disease, with a mode of action which is distinct from conventional treatments. It may have the potential to be helpful in stricturing disease. However, controlled trials and an assessment of enteric-release preparations are required to confirm its efficacy and establish indications for use.

Effect of N-Acetylglucosamine on Function of Peritoneal Leukocytes

Objective

To compare effects of N-acetylglucosamine (NAG) -based and glucose-based dialysis fluids on the function of peritoneal leukocytes in conditions of peritoneal dialysis.

Design

In vitro experiments on ex vivo isolated rat peritonealleukocytes.

Materials

Peritoneal leukocytes were isolated from rats on chronic peritoneal dialysis. On alternate days, fluid exchanges were performed with NAG-based or glucosebased dialysis solutions. After a 4-hour dwell, dialysate was drained and peritoneal leukocytes were incubated in vitro :I= lipopolysaccharide (LPS). Production of nitrites (index of NO synthesis), tumor necrosis factor α (TNFα), interleukin-1 β (IL -1 β), and interferon gamma (IFN-y) by unstimulated or stimulated peritoneal leukocytes originating from NAG-based or glucose-based fluid was measured.

Results

Dialysate cell count was lower during exchanges with NAG-based fluid (2113 :I= 615 cells/μL) as compared to glucose-based fluid (3643 :I= 1108 cells/μL; p < 0.01). Differential cell count was similar in both studied groups. Unstimulated peritoneal leukocytes from NAGbased dialysate produced more NO (nitrites) (0.65 ± 0.07 μmol per 106 cells) than did cells from glucose-based dialysate (0.26 :I= 0.09 μmol per 106 cells, p < 0.01). Stimulated peritoneal leukocytes from NAG-based dialysate produced more cytokines than did cells from glucose-based dialysate: TNFα, 135.2 ± 37.0 pg versus 70.2 :I= 21.8 pg per 106 cells respectively, p < 0.01; IL -1 β, 143.2 :I= 60.9 pg versus 99.1 :I= 22.4 pg per 106 cells respectively, p < 0.05; IFN-y, 16.2:1= 12.5 pg versus 6.0:1= 1.8 pg per 106 cells respectively, p <0.01.

Conclusions

We demonstrated that rat peritonealleukocytes exposed in vivoto NAG-based dialysis fluid have better ability to produce inflammatory mediators than do peritoneal leukocytes from the same donor, but exposed in vivo to glucose-based dialysis solution.