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J Am Soc Nephrol 13:480-489, 2002
© 2002 American Society of Nephrology

Hemodynamic Effects of Peritoneal Dialysis Solutions on the Rat Peritoneal Membrane: Role of Acidity, Buffer Choice, Glucose Concentration, and Glucose Degradation Products

Siska Mortier*, An S. De Vriese*, Johan Van de Voorde{dagger}, Thomas P. Schaub{ddagger}, Jutta Passlick-Deetjen{ddagger} and Norbert H. Lameire*

*Renal Unit and {dagger}Department of Physiology, University Hospital, Gent, Belgium; and {ddagger}Fresenius Medical Care Deutschland GmbH, Bad Homburg, Germany.

Correspondence to Dr. Siska Mortier, Renal Unit, University Hospital, OK12, De Pintelaan 185, B-9000 Gent, Belgium. Phone: +32 9 2405301; Fax: +32 9 2404599; E-mail: siska.mortier{at}rug.ac.be


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
ABSTRACT. Conventional peritoneal dialysis fluids (PDF) are unphysiologic because of their hypertonicity, high glucose and lactate concentrations, acidic pH, and presence of glucose degradation products (GDP). Long-term exposure to conventional PDF may cause functional and structural alterations of the peritoneal membrane. New PDF have a neutral pH, a low GDP content, and contain bicarbonate or lactate as the buffer. Intravital microscopy was used to analyze the vasoactive effects of conventional and new PDF on the rat peritoneal membrane. A conventional, acidic pH, lactate-buffered 4.25% glucose PDF induced maximal vasodilation of mesenteric arteries, resulting in a doubling of the arteriolar flow and a 20% increase of the perfused capillary length per area. The hemodynamic effects of conventional PDF were similar after pH-adjustment with NaOH, indicating that acidity per se is not essential for the changes. Superfusion by a pH-neutral, lactate-buffered PDF with low GDP content caused only a transient arterial vasodilation despite continuous exposure, with a commensurate effect on arteriolar flow and capillary recruitment. Application of a pH-neutral, bicarbonate-buffered PDF with low GDP content did not affect the hemodynamic parameters. Resterilization of the bicarbonate solution increased GDP levels and completely restored the vasodilatory capacity. The corresponding 1.5% glucose PDF induced similar but less pronounced changes. Conventional PDF have important vasoactive effects on the peritoneal circulation, mainly because of the presence of GDP and transiently because of high lactate concentrations. Capillary recruitment may increase effective peritoneal vascular surface area. In addition, chronic vasodilation may induce structural adaptations in the blood vessel wall, contributing to vascular sclerosis. PDF with reduced GDP content induce no major hemodynamic effects and may thus have the potential to better preserve peritoneal vascular integrity.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Long-term peritoneal dialysis (PD) is associated with the development of functional and structural alterations of the peritoneal membrane (reviewed in references 1 and 2). Ultrafiltration capacity tends to decrease with time spent on dialysis, along with a progressive rise in small solute transport (3,4). Diverse morphologic changes have been observed in the peritoneum of PD patients, including reduplication of the basal lamina of the mesothelium and stromal blood vessels, interstitial fibrosis, presence of a hyalinizing vasculopathy, and neoangiogenesis (5–9). The prevalence of these alterations increases with time on PD, suggesting that chronic exposure of the peritoneum to the unphysiologic PD fluids (PDF) is an important causative factor.

Conventional PDF contains high glucose concentrations to create a transperitoneal osmotic gradient and high lactate concentrations as the buffer system. To curtail caramelization of glucose with formation of a variety of toxic glucose degradation products (GDP), conventional PDF are heat sterilized at a pH of approximately 5.5, but even at this low pH, considerable GDP formation occurs (10,11). The recognition that dialysate bioincompatibility has adverse effects on peritoneal structure and function has given impetus to the development of new PDF. A substantial reduction in GDP formation can be achieved by sterilizing glucose separately at a pH of approximately 3. The electrolytes and buffer are kept in another bag compartment at a pH of approximately 8. The contents of both chambers are mixed immediately before use, yielding a solution with neutral pH (12). Until recently, routine use of bicarbonate as the buffer anion has not been possible because of technical difficulties, including the precipitation of calcium and magnesium carbonate (13). Application of a similar double-chamber system allows the mixing of bicarbonate and divalent ions immediately before use. Because glucose is also sterilized separately at a low pH, formation of GDP is markedly reduced (12).

Conventional PDF have been reported to possess vasoactive properties (14–18), but the causative mechanisms are still unclear. Also, the pathophysiologic consequences of this PDF-induced vasodilation, in particular potential acute effects on fluid and solute transport and long-term changes of peritoneal function and structure, are incompletely understood. The aim of the study presented here was to evaluate the vasoactive effects of conventional and new PDF on the circulation of the rat peritoneal membrane by use of a well standardized intravital microscopy model (19–21). Attempts were made to separate the potential contribution of low pH, hyperosmolality, buffer system, glucose concentration, and presence of GDP in mediating the hemodynamic changes. Finally, the effect on effective peritoneal surface area was studied by directly measuring capillary recruitment after acute dialysate exposure.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Laboratory Animals and Dialysate Solutions
The studies were performed in 102 female Wistar rats (Iffa Credo, Brussels, Belgium), which received care in accordance with the national guidelines for animal protection. The following PDF (Fresenius Medical Care, Bad Homburg, Germany) were evaluated: (1) a conventional, single-chamber bag, acidic pH, lactate-buffered PDF, containing 1.5% glucose (continuous ambulatory PD [CAPD] 2) or 4.25% glucose (CAPD 3); (2) a conventional, single-chamber bag, acidic pH, lactate-buffered PDF, adjusted to pH 7.4 with NaOH, containing 1.5% glucose (CAPD 2 NaOH) or 4.25% glucose (CAPD 3 NaOH); (3) a new, double-chamber bag, pH-neutral, lactate-buffered PDF with low GDP content, containing 1.5% glucose (CAPD 2 Balance) or 4.25% glucose (CAPD 3 Balance); (4) a new, double-chamber bag, pH-neutral, bicarbonate-buffered PDF, containing 1.5% glucose (CAPD 20 Bicarbonate) or 4.25% glucose (CAPD 30 Bicarbonate); (5) a new double-chamber bag, pH-neutral, bicarbonate-buffered PDF, resterilized (second steam sterilization process) (22) to increase GDP content, containing 1.5% glucose (CAPD 20 Bicarbonate-R) or 4.25% glucose (CAPD 30 Bicarbonate-R). As control solution, we used Earle’s balanced salt solution (EBSS, Life Technologies, Paisley, Scotland), containing 5.6 mmol/L glucose, 26 mmol/L NaHCO3, 117 mmol/L NaCl, 1.8 mmol/L CaCl2, 5.3 mmol/L KCl, 0.8 mmol/L MgSO4, and 1 mmol/L NaH2PO4.

Analysis of GDP in PDF
Monofunctional aldehydes (formaldehyde, acetaldehyde, 5-hydroxymethylfurfural [5-HMF], 2-furaldehyde) were determined as 2,4-dinitrophenylhydrazine (DNPH) derivatives, as described previously (11). A total of 0.05 g DNPH was dissolved in 0.2 ml concentrated sulfuric acid and 0.3 ml water, then diluted with 10 ml acetonitrile. DNPH solution (0.1 ml) was added to 0.5 ml PDF, mixed, and allowed to stand at room temperature for 1 h. The determination was performed with a sample volume of 20 µl by reverse-phase HPLC at 40°C with gradient elution (flow 1 ml/min) and ultraviolet detection at a wavelength of 365 nm. The HPLC equipment we used was as follows: autosampler type GINA 50, gradient pump type M480, column oven type STH 585, detector type UVD 160 S, software Chromeleon (all from Dionex, formerly Gynkotek, Munich, Germany); column type Luna C18(2), 250 x 4 mm, 5 µm (Phenomenex, Aschaffenburg, Germany); and gradient elution (A, acetonitrile; B, phosphate buffer pH 7.5; gradient: given is percentage of A, addition to 100% is B: 30%/5 min -> 35%/17 min -> 40%/25 min -> 43%/42 min -> 60%/48 min -> 75%/50 min -> 75%/60 min -> 30%/60, 5 min -> 30%/65 min).

Bifunctional aldehydes (glyoxal, methylglyoxal, 3-deoxyglucosone) were analyzed as o-phenylenediamine derivatives. A total of 0.04 g o-phenylenediamine was dissolved in 10 ml water; 0.3 ml diamine solution was added to 0.5 ml PDF, mixed, and allowed to stand at room temperature for 2 h in the dark. HPLC was performed with the same chromatographic equipment (detection wavelength 235 nm) and isocratic elution (18% A, 82% B).

Intravital Microscopy
Rats were anesthetized with thiobutabarbital (Inactin; RBI, Natick, MA; 100 mg/kg administered subcutaneously). The trachea was intubated to facilitate breathing, a jugular vein was cannulated for continuous infusion of isotonic saline, and a carotid artery was cannulated for continuous monitoring of arterial BP. Cromoglycate (cromolyn sodium salt, 10 mg/kg administered intravenously; Sigma, St. Louis, MO) was administered 15 min before surgery to block degranulation of mast cells induced by the surgical manipulation.

A small midline abdominal incision was made, and a short segment of the small bowel was exteriorized, spread over a Plexiglas plate, and superfused continuously with EBSS maintained at 37°C. The surgeon carefully avoided stretching the tissue. The preparation was allowed to stabilize for 30 min after surgery was completed. Observations were made with an Axiotech Vario 100 HD microscope (Zeiss, Jena, Germany) with water immersion objectives (Achroplan x10, x40) and a nonimmersion objective (Plan-Neofluar x5). The microscopic stage was driven by a stepping motor control MCL-2 (Lang, Hüttenberg, Germany) operated by a joystick or a software program (Wincommander, Märzhäuser-Wetzlar, Wetzlar, Germany) via a RS-232 interface.

The tissue was transilluminated via a fiberoptic with a light source (KL 1500; Schott, Wiesbaden, Germany) equipped with a 150-W halogen lamp. The resulting image was displayed on a television monitor by a TK-1281 camera (Victor Company, Tokyo, Japan) or a high-speed video camera (Kodak Motioncorder Analyzer; Eastman Kodak, San Diego, CA) and recorded by a videorecorder (S-VHS Panasonic AG-7355; Matsushita, Osaka, Japan) for offline analysis. The video images were digitized with an IP-8/AT Matrox image processing board and analyzed with image analysis software (Cap-Image; Ingenieurbüro Zeintl, Heidelberg, Germany) as described previously (19,20).

Experimental Protocols
Mesenteric Arteries.
In each experimental animal, one mesenteric artery with a diameter of 250 to 350 µm was dissected from the surrounding tissue (Figure 1), and the luminal diameter was measured at different experimental conditions (19,20). A concentration-response curve was performed for acetylcholine (10-7 M to 10-5 M), nitroglycerin (10-6 M to 10-4 M), verapamil (10-6 M to 10-4 M), and papaverine (10-6 M to 10-4 M), dissolved in EBSS, and applied in randomized order (n = 6).



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  		Figure 1. Schematic drawing of the peritoneal circulation. Mesenteric arteries with a diameter of 250 to 350 µm were dissected from the surrounding tissue, and changes of luminal diameters in response to local application of vasodilators and dialysate were measured. In a separate series of experiments, arterioles with a diameter of 15 to 25 µm were selected, and flow changes in response to dialysate exposure were analyzed. In addition, the length of the perfused capillaries per area was measured before and after dialysate exposure to evaluate capillary recruitment.

 
For each of the 4.25% glucose PDF, mesenteric arterial reactivity was tested after superfusion with PDF, nitroglycerin 10-4 M dissolved in EBSS, and nitroglycerin 10-4 M dissolved in PDF (n = 6). The conditions were applied in randomized order to exclude time-dependent effects. Solutions containing bicarbonate were bubbled continuously with CO2 to maintain pH neutral and pCO2 and HCO3- concentration stable throughout the entire experiment.

Peritoneal Microcirculation.
In a separate group of experimental animals, the peritoneal microcirculation was studied. In each experimental animal, two arterioles with a diameter of 15 to 25 µm were selected for measurement of luminal diameter and red blood cell velocity (Figure 1). Blood flow rate was calculated by the following equation: blood flow rate = VRBC x {pi}D2/4, with VRBC indicating the red blood cell velocity and D indicating the luminal diameter (19,20). To evaluate perfused capillary length per area, the microscopic stage was driven through a meander consisting of two steps of 0.9 mm in the x-direction and three steps of 0.55 mm in the y-direction. The microscopic image was recorded at each of these 12 positions. Vessel length per area was determined for each microscopic image, and the average was calculated. Only vessels with active flow were included in the analysis (20,21).

For each of the 1.5 and 4.25% glucose PDF, luminal diameter, red blood cell velocity in the arterioles, and capillary recruitment were measured after exposure to EBSS for 10 min, PDF for 20 min, and reexposure to EBSS for 20 min (n = 6). Solutions containing bicarbonate were bubbled continuously with CO2 to maintain pH neutral and pCO2 and HCO3- concentration stable throughout the entire experiment.

Statistical Analyses
Dialysate-induced alterations were expressed as percentage changes versus the mean of two baseline observations. In univariate analysis, subgroups characterized by different PDF components were compared with respect to percentage changes in vasoactive outcomes according to t test. To evaluate the independent contributions of PDF components on these vasoactive outcomes after simultaneous adjustment in a multivariate framework, multiple regression analyses were performed. Model assumptions were checked by visualization of Pearson residuals. An a priori level of {alpha} = 0.05 was used to indicate statistical significance. All analyses were performed by SAS version 6.12 software (SAS, Cary, NC).


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
GDP Concentrations
GDP levels were higher in CAPD 2/3 than in CAPD 2/3 Balance and CAPD 20/30 Bicarbonate, except for 5-HMF (Table 1). Resterilization of CAPD 20/30 Bicarbonate increased GDP concentrations, although not to the same levels as in CAPD 2/3 (Table 1).


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  		Table 1. Concentrations of glucose degradation products in the different peritoneal dialysate fluids (µM)a
 
Mesenteric Arteries
Local application of acetylcholine, nitroglycerin, verapamil, and papaverine caused significant vasodilation of mesenteric arteries (Table 2) in the absence of any effects on systemic BP. Nitroglycerin 10-4 M was used for subsequent studies because it induced a maximal and rapidly reversible vasodilation and because it acts independently of the endothelium.


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  		Table 2. Percentage changes of luminal diameters of mesenteric arteries after local application of vasodilators
 
Superfusion with CAPD 3 caused a similar arterial vasodilation as nitroglycerin 10-4 M during the entire exposure period. No additive effects were seen when nitroglycerin 10-4 M and CAPD 3 were applied concomitantly (Figure 2A). Luminal diameters rapidly returned to baseline values after withdrawal of dialysate, nitroglycerin, or both.



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  		Figure 2. Percentage change of luminal diameters of mesenteric arteries after local application of continuous ambulatory peritoneal dialysis (CAPD) 3 (A, n = 6), CAPD 3 neutralized with NaOH (B, n = 6), CAPD 3 Balance (C, n = 6), CAPD 30 Bicarbonate (D, n = 6), and resterilized CAPD 30 Bicarbonate to increase the glucose degradation product levels (E, n = 6). The vasodilatory capacities of nitroglycerin 10-4 M dissolved in Earle’s balanced salt solution (EBSS) (NG), dialysate (D), and nitroglycerin 10-4 M dissolved in dialysate (D + NG) were compared. The three interventions were applied in random order. *P < 0.001 versus EBSS, #P < 0.05 versus EBSS, §P < 0.01 versus NG and D + NG, $P < 0.01 versus D + NG, &P < 0.01 versus D 2 min.

 
After adjustment of the pH to 7.4 with NaOH, CAPD 3 still induced a maximal vasodilation in the mesenteric arteries (Figure 2B). In contrast, application of CAPD 3 Balance induced a transient vasodilation that was maximal 2 min after exposure and decreased to values not significantly different from control after 20 min of ongoing superfusion (Figure 2C). Exposure to CAPD 30 Bicarbonate had no major effects on the diameters of the mesenteric arteries (Figure 2D). After resterilization, the vasodilatory capacity of CAPD 30 Bicarbonate was similar to that of CAPD 3 (Figure 2E). There was no apparent effect of the order in which the interventions were applied. None of the PDF caused significant changes in systemic BP (data not shown).

Peritoneal Microcirculation
Arteriolar flow increased up to 94.3 ± 12.2% after superfusion with CAPD 3 and recovered to baseline values after withdrawal of PDF and superfusion with EBSS. Application of CAPD 2 induced a less pronounced but still significant increase of the arteriolar flow up to 53.3 ± 5.5% (Figure 3A). The flow changes could entirely be attributed to a rise of the red blood cell velocity because the luminal diameter of the arterioles did not change significantly (data not shown). The elevated flow induced by CAPD 3 resulted in an increase of the perfused capillary length per area of up to 21.9 ± 4.4%. No capillary recruitment was observed after application of CAPD 2 (1.7 ± 2.5%) (Figure 3B). Adjustment of the pH to 7.4 with NaOH did not alter the hemodynamic effects of the CAPD 2/3 solutions: similar increases in arteriolar flow (up to 129.2 ± 10.1%) and perfused capillary length per area (up to 16.9 ± 2.2%) were observed as with the acidic solutions (Figure 4).



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  		Figure 3. (A) Percentage change of arteriolar flow or (B) perfused capillary length per area after superfusion with Earle’s balanced salt solution (EBSS) (open bars, n = 6), continuous ambulatory peritoneal dialysis (CAPD) 2 (hatched bars, n = 6), or CAPD 3 (solid bars, n = 6). Measurements were made before exposure to dialysate (EBSS0), 2 min after dialysate (D2), 10 min after dialysate (D10), 20 min after dialysate (D20), 10 min after withdrawal of dialysate (EBSS10), and 20 min after withdrawal of dialysate (EBSS20). (A) *P < 0.0001 versus EBSS0, #P < 0.0001 versus D20. (B) *P < 0.05 versus EBSS0, #P < 0.05 versus D20.

 


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  		Figure 4. Percentage change of (A) arteriolar flow or (B) perfused capillary length per area after superfusion with Earle’s balanced salt solution (EBSS) (open bars, n = 6), continuous ambulatory peritoneal dialysis (CAPD) 2 neutralized with NaOH (hatched bars, n = 6), or CAPD 3 neutralized with NaOH (solid bars, n = 6). Measurements were made before exposure to dialysate (EBSS0), 2 min after dialysate (D2), 10 min after dialysate (D10), 20 min after dialysate (D20), 10 min after withdrawal of dialysate (EBSS10), and 20 min after withdrawal of dialysate (EBSS20). (A) *P < 0.0001 versus EBSS0, #P < 0.0001 versus D20. (B) *P < 0.05 versus EBSS0, #P < 0.05 versus D20.

 
Superfusion of CAPD 3 Balance induced a transient rise in arteriolar flow (71.7 ± 13.6%), with a commensurate effect on capillary recruitment (8.8 ± 1.8%). Arteriolar flow and perfused capillary length per area returned to baseline values, despite ongoing exposure to PDF (Figure 5). No changes in arteriolar flow or capillary recruitment were observed after exposure to CAPD 20/30 Bicarbonate (Figure 6). In contrast, resterilized CAPD 20/30 Bicarbonate caused a similar increase in arteriolar flow (up to 63.9 ± 7.0% and 109.0 ± 9.3%, respectively) and perfused capillary length per area (up to 11.1 ± 2.5% and 28.5 ± 5.0%, respectively) as compared with CAPD 2/3 (Figure 7). None of the PDF had significant effects on mean arterial BP (data not shown).



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  		Figure 5. Percentage change of (A) arteriolar flow or (B) perfused capillary length per area after superfusion with Earle’s balanced salt solution (EBSS) (open bars, n = 6), continuous ambulatory peritoneal dialysis (CAPD) 2 balance (hatched bars, n = 6), or CAPD 3 balance (solid bars, n = 6). Measurements were made before exposure to dialysate (EBSS0), 2 min after dialysate (D2), 10 min after dialysate (D10), 20 min after dialysate (D20), 10 min after withdrawal of dialysate (EBSS10), and 20 min after withdrawal of dialysate (EBSS20). (A) *P < 0.001 versus EBSS0, #P < 0.01 versus D2. (B) *P < 0.05 versus EBSS0.

 


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  		Figure 6. Percentage change of (A) arteriolar flow or (B) perfused capillary length per area after superfusion with Earle’s balanced salt solution (EBSS) (open bars, n = 6), continuous ambulatory peritoneal dialysis (CAPD) 20 bicarbonate (hatched bars, n = 6), or CAPD 30 bicarbonate (solid bars, n = 6). Measurements were made before exposure to dialysate (EBSS0), 2 min after dialysate (D2), 10 min after dialysate (D10), 20 min after dialysate (D20), 10 min after withdrawal of dialysate (EBSS10), and 20 min after withdrawal of dialysate (EBSS20).

 


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  		Figure 7. Percentage change of (A) arteriolar flow or (B) perfused capillary length per area after superfusion with Earle’s balanced salt solution (EBSS) (open bars, n = 6), resterilized continuous ambulatory peritoneal dialysis (CAPD) 20 bicarbonate (hatched bars, n = 6), or resterilized CAPD 30 bicarbonate (solid bars, n = 6). Measurements were performed before exposure to dialysate (EBSS0), 2 min after dialysate (D2), 10 min after dialysate (D10), 20 min after dialysate (D20), 10 min after withdrawal of dialysate (EBSS10), and 20 min after withdrawal of dialysate (EBSS20). (A) *P < 0.00001 versus EBSS0, #P < 0.0001 versus D20. (B) *P < 0.01 versus EBSS0, #P < 0.05 versus D20.

 
Univariate and Multivariate Analyses
In both univariate and multivariate analysis, no significant effect of pH on hemodynamic variables was found (Tables 3 and 4). The use of lactate as the buffer anion was associated with a greater vasodilation and increase in flow only after 2 min of PDF exposure, but the effect on capillary recruitment was NS (Tables 3 and 4). The presence of high glucose concentrations and GDP was consistently associated with more pronounced hemodynamic effects (Tables 3 and 4).


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  		Table 3. Results of univariate analyses
 

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  		Table 4. Results of multivariate analyses
 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The results of the study presented here demonstrate that exposure to conventional PDF exerts important vasoactive effects in the circulation of the rat peritoneal membrane. An acidic pH, lactate-buffered 4.25% glucose PDF reversibly dilated mesenteric arteries by more than 20%. The extent of the PDF-induced vasodilation was similar to that of nitroglycerin 10-4 M, and no additive effects were observed when dialysate and nitroglycerin were applied simultaneously. In pilot experiments, nitroglycerin 10-4 M was found to have a maximal vasodilatory capacity. It thus follows that conventional PDF produce maximal vasodilation of mesenteric arteries. In contrast, the arterioles in the peritoneal membrane did not appear to respond directly to the vasodilatory stimuli because their luminal diameters remained unchanged during PDF exposure. However, the blood flow in these arterioles nearly doubled, indicating that they are passively conducting the rise in flow caused by the upstream vasodilation. The PDF-induced vasodilation resulted in capillary recruitment, increasing the number of perfused peritoneal capillaries by more than 20%.

The results presented here are in line with and extend previous observations of PDF-induced vasodilation in cremaster muscle arterioles and cecal arteries (14–17) and PDF-induced increments of celiac blood flow (18). The nature of the PDF components that are responsible for these hemodynamic effects is, however, incompletely understood. Conventional PDF is highly unphysiologic because of its acidic pH, high glucose and lactate concentrations, hyperosmolarity, and the more recently described presence of GDP (10,11). The vasodilatory effects of low pH (23,24), lactate (25,26), and hyperosmolality (27,28) are well recognized. As a consequence, PDF-induced vasodilation has generally been attributed to these factors, although little direct evidence supports this contention. In addition, there is a dearth of information on the potential hemodynamic effects of GDP in PDF. Exposure to heat-sterilized PDF was found to decrease the concentration of rolling leukocytes and to increase venular flow velocity as compared with filter-sterilized PDF (29), but no systematic assessment of the effects of GDP on peritoneal hemodynamics has been performed.

Further experiments were therefore conducted to attain a better understanding of the factors responsible for dialysate-induced vasoreactivity. Adjustment of the pH to 7.4 with NaOH did not affect the vasoreactivity, indicating that although low pH per se may cause vasodilation, it is not essential for the observed dialysate-induced hemodynamic effects. The results are in accordance with previous findings that pH adjustment does not alter the vasodilatory capacity and small solute clearances of conventional PDF (14). These observations are important because acidity is rapidly corrected after infusion of standard PDF in the abdominal cavity (30,31). Conventional PDF may thus maintain its vasodilatory potential during the entire dwell period.

PDF with low GDP content and high lactate concentrations induced only transient vasodilation and capillary recruitment despite ongoing exposure, whereas PDF with low GDP content and bicarbonate as the buffer anion was found to be entirely neutral with respect to hemodynamic parameters. The results thus suggest that lactate may only in part be responsible for the PDF-induced vasoreactivity, whereas GDP appear to exert major hemodynamic effects. As demonstrated previously (11), the concentrations of GDP in CAPD 2/3 Balance and CAPD 20/30 Bicarbonate were lower than in CAPD 2/3, except for 5-HMF. Although 5-HMF has been used as a parameter to assess PDF biocompatibility, several investigators have been unable to correlate its concentrations with toxicity (32,33), which is underlined by the results presented here. Resterilization of CAPD 20/30 Bicarbonate induced similar vasoreactivity as caused by CAPD 2/3. Because resterilization is expected to increase GDP levels without otherwise altering the chemical composition of the PDF, the results further support the causative role of GDP in peritoneal arterial vasodilation.

It is of note that although the levels of 5-HMF in the resterilized bicarbonate solutions exceeded those found in conventional dialysate, the concentrations of acetaldehyde, glyoxal, methylglyoxal, and 3-deoxyglucosone were actually lower. Nevertheless, the hemodynamic effects caused by resterilized CAPD 20/30 Bicarbonate were of the same magnitude as those of CAPD 2/3. These findings are in line with previous observations that application of individual GDP does not affect fibroblast and mesothelial cell viability and function to the same extent as heat-sterilized PDF (33,34). Taken together, the data suggest that other as yet unknown compounds are formed during heat sterilization and may exert toxic effects.

The results presented here do not support a role for hyperosmolality in PDF-induced vasoreactivity because the bicarbonate solutions were hemodynamically inert, even though their osmolality is identical to that of conventional PDF. In addition, the neutral effect of the bicarbonate solutions demonstrates that high glucose concentrations per se do not have hemodynamic effects. The more pronounced vasodilation induced by the 4.25% glucose than by the 1.5% glucose acidic lactate PDF most likely results from higher GDP levels (11). Collectively, the data indicate that the vasoactive properties of conventional PDF mainly result from the presence of GDP and to a minor extent from the high lactate concentrations.

The number of perfused peritoneal capillaries is not a static property. Under basal circumstances, only 25 to 50% of peritoneal capillaries are perfused (35). Additional capillaries can be recruited by a rise in splanchnic blood flow or during local inflammatory reactions. In the study presented here, local application of conventional acidic pH, lactate-buffered PDF induced a capillary recruitment of approximately 20%. Expansion of the effective peritoneal vascular surface area gives rise to an increased diffusive transport of small solutes (35). A rapid loss of glucose from the peritoneal cavity results in an early dissipation of the osmotic gradient and, consequently, in a decreased transcapillary ultrafiltration rate. It can thus be inferred that hemodynamically neutral PDF may improve ultrafiltration capacity in PD patients.

Several experimental studies have examined the effects of pH, different buffer anions, and the presence of GDP on solute transport and ultrafiltration capacity, yielding disparate results. pH adjustment of a conventional dialysate did not affect small solute clearances (14). No differences in ultrafiltration rate were observed in rabbits treated with a pH-neutral, bicarbonate-buffered PDF (without separate sterilization of glucose) or with a conventional dialysate (36). In contrast, higher ultrafiltration rates and lower glucose absorption were reported with a pH-neutral lactate-buffered PDF (37) and with a filter-sterilized bicarbonate-glycylglycine–buffered PDF (38), compared with standard dialysate. Another group found less glucose absorption and a better ultrafiltration profile with a pH-neutral filter-sterilized PDF than with an acidic heat-sterilized solution (39). Additional experiments showed that fluid and solute transport were not affected by either acidity or presence of GDP, only by the combination of the two factors (39).

Clinical studies with regard to the influence of GDP and buffer anion on solute and water transport show similarly conflicting results. Dialysis with a bicarbonate-buffered PDF significantly improved ultrafiltration rate (40). In contrast, two randomized multicenter trials found no difference in ultrafiltration between patients treated with a double-chamber bicarbonate solution or with conventional PDF (41,42). Computer simulations assuming a 40% difference in peritoneal surface area during the initial 70 min of a dwell with a conventional PDF or a pH-neutral, low-GDP solution predicted a net ultrafiltration gain of 100 ml after 4 h in favor of the pH-neutral, low-GDP solution (43). In the study presented here, conventional PDF induced a capillary recruitment of 20% during the entire exposure time, whereas CAPD Balance caused only a transient recruitment of approximately 10%, and the bicarbonate PDF had no effect on capillary perfusion at all. Taking into account rather large intraindividual and interindividual variations in drained volume, PDF-related differences in ultrafiltration rate may require large clinical trials to be studied validly.

Besides the potential acute effects on solute transport and ultrafiltration capacity, the PDF-induced vasoreactivity may have long-term effects on peritoneal vascular function and structure. Several authors have reported that the peritoneal vasculature of long-term PD patients is characterized by fibrosis and hyalinization of the media (6–9). The prevalence of this vasculopathy significantly increased with time spent on PD (9), suggesting that the continuous contact with the unphysiologic PDF is an important pathogenetic element. Chronic exposure to the high glucose concentrations in PDF with resultant advanced glycation end products formation and accumulation has been incriminated as a causative factor (7,8). The results presented here may provide an alternative explanation for the PD-related vascular changes. Long-term elevations in arteriolar blood flow and perfusion pressure may cause structural adaptations in the vascular wall, similar to those found in hypertensive vasculopathy, and might ultimately lead to vascular sclerosis. The possible pathophysiologic link of the chronic vascular alterations with the dialysate-induced vasoreactivity needs further exploration.

In conclusion, conventional PDF induces maximal vasodilation of the larger mesenteric arteries, resulting in an increased flow in the peritoneal microcirculation and an expansion of the effective peritoneal vascular surface area. Presence of GDP, and to a minor extent high lactate concentrations, may be responsible for these hemodynamic effects, whereas low pH, hyperosmolality, and high glucose concentrations do not appear to be essential. Although the pathophysiologic consequences of PDF-induced vasoreactivity remain to be fully determined, it can be speculated that chronic capillary recruitment may decrease ultrafiltration capacity. More importantly, continuous elevations of vascular flow may induce vascular remodeling and ultimately result in vascular sclerosis. New dialysates exert no major hemodynamic effects on the peritoneal circulation and thus may have the potential to better preserve vascular integrity.


   Acknowledgments
 
We thank Julien Dupont and Marc Gillis for their expert technical assistance and Dr. Michael Fünfrocken and Dr. Thomas Knerr for the measurement of GDP. SM is supported by a grant from Fresenius Medical Care–Germany, and ADV is supported by a grant from the Fund for Scientific Research–Flanders (N20/0).


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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Received for publication July 16, 2001. Accepted for publication September 27, 2001.




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