Resistance to intercompartmental mass transfer limits β2-microglobulin removal by post-dilution hemodiafiltration

Although clearance of beta(2)-microglobulin is greater with hemodiafiltration than with high-flux hemodialysis, beta(2)-microglobulin concentrations after long-term hemodiafiltration are only slightly less than those obtained with high-flux hemodialysis. Resistance to beta(2)-microglobulin transfer between body compartments could explain this observation. beta(2)-Microglobulin kinetics were determined in patients receiving on-line post-dilution hemodiafiltration for 4 h with 18 l of filtration. Plasma beta(2)-microglobulin concentrations were measured during and for 2 h following hemodiafiltration and immediately before the next treatment. The filter clearance of beta(2)-microglobulin was determined from arterial and venous concentrations. The beta(2)-microglobulin generation rate was calculated from the change in the plasma concentration between treatments. The intercompartmental clearance was obtained by fitting the observed concentrations to a two-compartment, variable volume model. The plasma clearance of beta(2)-microglobulin by the filter was 73 +/- 2 ml/min. Plasma beta(2)-microglobulin concentrations decreased by 68 +/- 2% from pre- to post-treatment (27.1 +/- 2.2-8.5 +/- 0.7 mg/l), but rebounded by 32+/-3% over the next 90 min. The generation rate of beta(2)-microglobulin was 0.136 +/- 0.008 mg/min. The model fit yielded an intercompartmental clearance of 82 +/- 7 ml/min and a volume of distribution of 10.2 +/- 0.6 l, corresponding to 14.3 +/- 0.7% of body weight. Hemodiafiltration provides a beta(2)-microglobulin clearance of similar magnitude to the intercompartmental clearance within the body. As a result, intercompartmental mass transfer limits beta(2)-microglobulin removal by hemodiafiltration. This finding suggests that alternative strategies, such as increased treatment times or frequency of treatment, are needed to further reduce plasma beta(2)-microglobulin concentrations.

Convective and adsorptive removal of beta2-microglobulin during predilutional and postdilutional hemofiltration

BACKGROUND

Beta(2)-microglobulin (beta2-m) removal in patients with end-stage renal disease (ESRD) is maximal with convective techniques, such as hemofiltration (HF) or hemodiafiltration (HDF). Although the infusion mode of the replacement solution (predilution or postdilution) is expected to influence the efficiency of HF, experimental data in this respect are scanty. We therefore investigated the impact of the fluid reinfusion mode on the efficiency of HF in 11 ESRD patients who underwent both treatments.

METHODS

The dialyzer (AK 200 ULTRA) was equipped with a 3-layer polyamide membrane (Poliflux 21 S, surface 2.1 m(2)) and blood flow was kept between 300 and 400 mL/min. beta2-m concentrations were measured in plasma water and ultrafiltrate at appropriate times during a 240-minute treatment. The following dialytic parameters were calculated: total amount of beta2-m removed (A(tot)), beta2-m removed by convection (A(con)) and by adsorption (A(ads)), percent reduction in beta2-m plasma water concentration (% Cpw(in)), total plasma water clearance (CLpw(tot)), convective plasma water clearance (CLpw(con)), adsorptive plasma water clearance (CLpw(ads)), and sieving coefficient (SC).

RESULTS

CLpw(tot), CLpw(ads), and% Cpw(in) were similar in pre- and postdilutional conditions, whereas CLpw(con) and SC were higher and CLpw(ads) was lower in postdilution than in predilution HF. Since a significant inverse correlation was found between A(ads) and SC, predilution probably determines greater protein fouling than postdilution.

CONCLUSION

The 2 techniques appear to be equivalent in terms of total beta2-m removal, although this final result is obtained by different contributions of convective and adsorptive elimination.

Kinetics of β2-Microglobulin and Phosphate during Hemodialysis: Effects of Treatment Frequency and Duration

Current understanding of beta2-microglobulin (beta2M) and phosphate (or inorganic phosphorus) kinetics during hemodialysis is reviewed. The postdialysis:predialysis concentration ratio for beta2M is determined by dialyzer clearance for beta2M, treatment time, patient body size (specifically, extracellular fluid volume), and total ultrafiltration volume during the treatment. Evaluation of these treatment parameters can be used to calculate dialyzer clearance for beta2M; however, such calculated values are only approximations, since they neglect intradialytic generation, nonrenal (nondialyzer) clearance, and postdialysis rebound of beta2M. The detailed kinetics of beta2M during hemodialysis are best described using a two-compartment model. Theoretical predictions from such two-compartment models suggest that the product of dialyzer clearance for beta2M and weekly treatment duration, independent of treatment frequency, is the main determinant of plasma beta2M concentrations. The kinetics of phosphate removal during hemodialysis are incompletely understood. Phosphate is removed from both extracellular and intracellular compartments during hemodialysis; the plasma phosphate concentration levels off after the first 1 or 2 hours of treatment and plasma concentrations can rebound even before therapy is complete. Increases in dialyzer clearance of phosphate have been previously achieved only by increasing dialysis membrane surface area or by the use of hemodiafiltration. A four-compartment model of phosphate kinetics proposed recently by Spalding et al. suggests that the major barrier to phosphate removal is limited transfer of phosphate between the intracellular and extracellular compartments, although other complex factors also play important roles. Theoretical predictions using the model of Spalding et al. suggest that increasing either treatment frequency or treatment duration can increase phosphate removal. The kinetics of beta2M are representative of middle molecules whose removal during hemodialysis is governed predominantly by clearance at the dialyzer. In contrast, phosphate removal is limited primarily by its sequestration in the intracellular compartment (and possibly other compartments), not by its clearance at the dialyzer. The kinetics of phosphate may therefore be representative of uremic toxins whose removal is limited by sequestration into compartments or by protein binding. Enhanced removal of both of these uremic toxins using a given therapy will require treatments of increased frequency and longer duration.

Ex vivo biocompatibility of a new beta2-microglobulin hemoperfusion polymer

Beta2-microglobulin (beta2-m) is an 11.8 kD protein that is excreted by the kidneys. In renal insufficiency, it accumulates in the body and can result in AB amyloidosis with bone and joint destruction. Four modifications of a new beta2-m adsorbent material were tested for biocompatibility with human whole blood. 500 ml of heparinized blood from healthy human donors was perfused ex vivo through minicolumns (adsorber beads: divinylbenzene with different biocompatible coatings) in the single-pass mode. Blood samples were taken from the antecubital vein before and at the column outlet during the 50 min test runs. Red and white cell counts remained virtually constant. No signs of hemolysis could be detected. Thrombogenicity of the columns was low as shown by the insignificant platelet loss, only slight platelet activation and moderate thrombin-antithrombin formation. There was no activation of leukocytes nor monocytes. Complement and bradykinin activation was minimal. Electrolyte concentrations and pH remained essentially constant. In conclusion, this new beta2-m adsorbent material exhibited favorable biocompatibility features in our ex vivo model and is thus a promising candidate for future clinical beta2-m hemoperfusion studies in patients.

Diacap alpha-polysulfone HI PS: a new dialysis membrane with optimum beta2-microglobulin elimination

BACKGROUND

Plasma concentration of beta2-microglobulin (beta2-m) in the case of renal insufficiency is about 20 to 30 times higher than normal. Beta2-m is associated with secondary amyloidosis, a late complication of regular dialysis therapy. To prevent the complications of secondary amyloidosis beta2-m should therefore be eliminated as efficiently as possible during dialysis treatment. This can be accomplished with dialysis membranes which guarantee sufficient clearance for this molecule. It is a matter of discussion whether removal of beta2-m by dialysis may be able to prevent secondary amyloidosis.

METHODS

The dialyzers Diacap HI PS 15 (B. Braun Melsungen) and F70 S (Fresenius Medical Care) were compared in five anuric dialysis patients. Arterial blood was taken at the start and at the end of dialysis. Dialysate samples were taken after 30 and 210 minutes and filtrate samples after 60 and 240 minutes from the start of dialysis. Beta2-m and total protein concentration were measured in plasma, filtrate and dialysate. SDS-PAGE of proteins in the filtrate was carried out and kinetics of beta2-m (Kt/V(beta2-m)) were calculated using the Stiller/Mann model.

RESULTS

In both dialyzers beta2-m is detectable at any time in the dialysate leaving the dialyzer. In the filtrate beta2-m concentration is about 10 times higher than in the dialysate. Protein pattern in filtrate of both dialyzers is similar and corresponds to that of the glomerulum filtrate. Beta2-m reduction ratio is slightly lower than urea reduction ratio. Using both dialyzers Kt/V(beta2-m) was 0.80, removing about 60% of the generated beta2-m.

CONCLUSIONS

In both dialyzers there is considerable removal of beta2-m. Examination of beta2-m kinetics showed an optimum of Kt/V(beta2) of 0.80 which can not be surpassed. Only 60% of generated beta2-m can be removed by three times per week hemodialysis therapy using high-flux dialyzers.

Impact of blood and dialysate flow and surface on performance of new polysulfone hemodialysis dialyzers

Optimization of hemodialysis treatment parameters and the characteristics of the dialyzer are crucial for short- and long-term outcome of end stage renal disease patients. The new high-flux membrane Helixone in the dialyzer of the FX series (Fresenius Medical Care, Germany) has interesting features, such as the relationship of membrane thickness and capillary diameter which increases middle molecule elimination by convection, as well as higher capillary packing and microondulation to improve the dialysate flow and distribution. Blood flow, dialysate flow and surface area are the main determinants of the performance of a dialyzer, however the impact of each parameter on small and middle molecule clearance in high flux dialysis has not been well explored. In order to find the best treatment condition for the new dialyzer series, we evaluated urea, creatinine, phosphate clearances and reduction rate of beta2-microglobulin in ten stable patients treated with different blood flows (effective Qb 280 and 360 ml/min), dialysate flow (Qd 300 or 500 ml/min) and dialyzer surfaces (1.4 and 2.2 m2, FX60 or FX100). KoA and Kt/V were also calculated. Blood flow, dialysate flow and surface area demonstrated a significant and independent effect on clearance of urea, creatinine and phosphate, as well as on Kt/V. Small solute clearance was stable over the treatment. In contrast to small solutes, reduction rate of beta2-microglobulin was related to increasing dialyzer surface only. The new dialyzer design of the FX series proves highly effective due to improved dialysate distribution and reduced diffusive resistance as shown by the small solute clearance. A high reduction rate of beta2-microglobulin is favored by improved fiber geometry and pore size distribution. These findings have potential long-term benefits for the patient.

Dissociation between clearances of small and middle molecules in incremental peritoneal dialysis

OBJECTIVE

To evaluate the peritoneal clearance of middle molecules in comparison with the peritoneal clearance of small molecules in incremental peritoneal dialysis (PD).

STUDY DESIGN

Peritoneal clearances of creatinine and beta2-microgloblulin (B2M) were compared in 57 continuous ambulatory PD patients on full dose of 4 exchanges, and 54 incremental PD patients with 2 or 3 exchanges over 24 hours. Clearances were also compared when there were changes in the PD regimen, such as in the number of exchanges and the duration of the dwell time.

SETTING

Tertiary-care university hospital.

RESULTS

Peritoneal creatinine clearance increased almost linearly with the increase in the number of exchanges. In contrast, peritoneal clearance of B2M was 9.1 +/- 3.6 L/week, 8.8 +/- 4.4 L/week, and 7.9 +/- 2.5 L/week with 2,3, and 4 exchanges, respectively, per day, amounts that were not different from each other. Peritoneal clearance of B2M did not change when there was an increase in the number of dialysate exchanges from 2 to 3 and from 3 to 4 over a period of 24 hours; whereas the peritoneal clearance of creatinine increased. Peritoneal clearance of B2M almost doubled, from 5.4 +/- 2.7 L/week with 2 exchanges over 12 hours per day, to 9.5 +/- 4.4 L/week with the same 2 exchanges over 24 hours. The creatinine clearance did not change.

CONCLUSION

In contrast to peritoneal clearance of small molecules, such as creatinine, which was dependent on the number of dialysate exchanges, peritoneal clearance of middle molecules, such as B2M, depended mainly on the total dwell hours of PD and not on the number of exchanges of peritoneal dialysate in incremental PD. This might be another advantage of incremental PD, since peritoneal clearance of middle molecules in incremental PD over 24 hours can be comparable to that in full dose PD.

[Interaction of the mechanisms of beta 2-microglobulin convection, diffusion, and adsorption in line hemodiafiltration]

The in vivo contribution of diffusion, convection ad adsorption to beta 2-microglobulin (beta 2-m) elimination by hemodiafiltration (HDF) was investigated. 11 patients (8M/3W), with a mean age of 59 +/- 10 years and weighing 62.7 +/- 8.7 kg were studied. A 1.89 m2 polysulphone membrane was used in 180 min postdilution HDF. Samples at blood inlet (bi), blood autlet (bo), dialysate outlet (do) and ultrafiltrate (uf) were taken to determine beta 2-m concentrations at 30 and 150 min. Rates of flow (Q, ml(min) prescribed were: infusion, Qinf = 103.6 +/- 12.3, Quf = 14.6 +/- 4.0 y Qb = 465 +/- 5.0. Effective Qbi was automatically measured by the machine and Qdo = 800 + Quf. The removed beta 2-m mass (M, mg/min) was obtained by multiplying rates of flow (Q, L/min) by beta 2-m concentrations (mg/L) at each sampling point. From mass balance, we calculated the mass of beta 2-m removed (mg/min) by adsorption 0.23 +/- 0.2, by convection 0.7 +/- 0.3 and by diffusion 1.0 +/- 0.4, at 30 min. At 150 min, the beta 2-m mass removed was -0.06 +/- 0.1 by adsorption 0.4 +/- 0.1 by convection and 0.3 +/- 0.1 by diffusion. In HDF, these beta 2-m eliminating mechanisms play a variable role throughout the session. The more significant conclusion is that diffusion of beta 2-m with a synthetic "open" membrane is an important method of removing beta 2-m, comparable to convection over the whole procedure. That result explain the relative efficacy of beta 2-m clearance by HDF convection, and also explain why isolated diffusion is an efficient mechanism for beta 2-m removal by high-flux hemodialysis.

Small and middle molecular weight solute clearance in nocturnal intermittent peritoneal dialysis

OBJECTIVES

To determine the dialysate-to-plasma (D/P) concentration ratios and peritoneal dialytic clearance (CI(D)) of substances with a wide range of molecular weights in subjects receiving a simulated nocturnal intermittent peritoneal dialysis (NIPD) session.

DESIGN

Open-label single-dose study.

SUBJECTS

Six end-stage renal disease patients undergoing peritoneal dialysis (PD).

SETTING

Clinical research center of a university-affiliated hospital.

INTERVENTIONS

Subjects received intravenous gentamicin and vancomycin on the first day of the study. Subjects received no PD until their return on the following day, when subjects underwent a simulated NIPD session utilizing four 2- to 2.5-L peritoneal dialysate dwells of 2 hours. Blood and dialysate samples were collected immediately before the session and after each dialysate dwell for determination of urea, creatinine, gentamicin, vancomycin, and beta2-microglobulin (beta2M) concentrations. Each solute's D/P concentration ratio and peritoneal CI(D) were calculated.

MEASUREMENTS AND MAIN RESULTS

The (mean +/- SD) 2-hour D/P concentration ratios were 0.78 +/- 0.05 (urea), 0.49 +/- 0.11 (creatinine), 0.38 +/- 0.08 (gentamicin), 0.11 +/- 0.06 (vancomycin), and 0.07 +/- 0.03 (beta2M). Peritoneal CI(D) values (mL/min of dialysis) were 19.0 +/- 2.8 (urea), 12.1 +/- 3.5 (creatinine), 8.4 +/- 2.8 (gentamicin), 2.7 +/- 1.5 (vancomycin), and 1.7 +/- 0.8 (beta2M). The D/P concentration ratios and peritoneal CI(D) values for urea, creatinine, and gentamicin were significantly different from vancomycin and beta2M (repeated measures ANOVA, p < 0.05). Beta2-microglobulin peritoneal CI(D) was strongly related to gentamicin peritoneal CI(D) (r = 0.96, p < 0.05).

CONCLUSION

Small molecular weight solutes have significantly greater D/P and peritoneal CI(D) than middle molecular weight solutes in NIPD. In NIPD, daily peritoneal CI(D) of beta2M is lower than that reported in continuous ambulatory PD. NIPD also results in lower drug CI(D) than that reported in continuous ambulatory PD studies.

In vitro and in vivo evaluation of a new polysulfone membrane for hemodialysis. Reference methodology and clinical results. (Part. 2: in vivo study)

Different high flux membranes have been recently developed. The present study is aimed at describing the technical features and the clinical performances of a new high flux polysulfone membrane (T-sulfone, Toray, Japan). The study has been carried out on two different dialyzers (surface area = 1.3 and 1.8 m2). The filters have been tested in vivo during hemodialysis and hemodiafiltration. The in vivo study was carried out on 12 ESRD patients on regular hemodialysis treatment. The protocol was reviewed and approved by the local ethical committee. The in vivo clearances (K) at 300 ml/min of blood flow are reported in the following Table: [Table in text]. Beta-2-m reduction ratio exceeded 50% in all sessions. Beta-2-m mass balance executed by collection of spent dialysate and elution from the used filters evidenced that removal is obtained mostly by filtration while absorption is negligible. Excellent tolerance and hemocompatibility was observed in all the studied sessions.

Kinetics of 131I-beta2 microglobulin in hemodialysis patients: assessment using total body counting

The kinetics of 131I-beta2-microglobulin (beta2-M) were studied using external total body gamma counting in a low noise chamber after administration of trace doses of radioactivity (4 microCi) in 14 uremic patients treated by either hemodialysis or hemofiltration. Data were collected over a 1 week period that included 3 dialysis sessions. The following artificial membranes were used: Cuprophan, polyacrylonitrile AN69, polysulfone, polymethylmethacrylate (PMMA), and polyamide. Radiolabeled beta2-M excretion by an extrarenal route was nearly nonexistent. The 131I-beta2-M half-life was between 2.4 and 8 days, shorter in patients with residual diuresis. A mean removal of 153+/-33 mg/L of beta2-M was obtained per dialysis session with a highly permeable membrane. A hemofiltration session (25 L exchange per session) was slightly more efficient in removing beta2-M than a 4 h hemodialysis session with the same AN69 highly permeable membrane. The amounts of 131I-beta2-M binding on the membranes, expressed as beta2-M equivalents, were 0, 16, 54, 58, and 59 mg/m2 for Cuprophan, polysulfone, polyacrylonitrile AN69, polyamide, and PMMA, respectively. In conclusion, the decrease of total body gamma counting directly reflected the beta2-M breakdown and removal in hemodialysis patients. Intact beta2-M was removed by convection with synthetic, highly permeable membranes. In addition, membrane adsorption accounted for 15% (polysulfone) to near 100% (PMMA) of the beta2-M removal per session. Adsorption was of the same magnitude regardless of the dialysis technique in use, indicating a membrane saturability process. None of the currently available dialysis procedures based on a 3 sessions per week schedule can balance beta2-M generation.

Synovial inflammatory cells captured 131I-beta 2-microglobulin in patients with dialysis related amyloidosis

Dialysis related amyloidosis (DRA) is a major complication of long term hemodialysis therapy. It is well recognized that scintigraphic study using radioisotope-labeled beta 2-microglobulin (beta 2M) as a tracer is a sensitive and specific technique to diagnose DRA non-invasively. The aim of this study is to clarify the mechanism of 131I-beta 2M accumulation around the amyloid tissue. Three dialysis patients with carpal tunnel syndromes were examined for consecutive 131I-beta 2M scintigraphies every 24 hours for 3 days till the carpal tunnel synovectomy. Removed synovial tissues were processed for histological study. The scintigraphic study demonstrated tracer accumulations in the joints involved with DRA and the intensity increased in a time dependent fashion. Microscopic observations revealed many inflammatory cells presenting CD68-monocytes/macrophages antigen infiltrated into the synovial tissues. 131I-beta 2M was evident in the cytoplasm of the infiltrating cells, while no radioactivity was detected above background in the amyloid tissues. In conclusion, the tracer accumulations observed in the 131I-beta 2M scintigraphic studies were the consequence of circulating beta 2M assimilated by the infiltrating monocytes/macrophages. Thus, the undetermined elimination pathway of circulating beta 2M in the dialysis patients was identified as the storage pool in those inflammatory cells. The inflammatory change may play a crucial role in the local progression of DRA through the accumulation of circulating beta 2M around the established amyloid tissues.

Adsorption of human recombinant erythropoietin on dialysis membranes in vitro

The adsorptive characteristics of 5 dialysis membranes for recombinant human erythropoietin (EPO) were studied in vitro in a closed circuit system. For 120 min, EPO added with bovine serum was significantly adsorbed by polymethylmethacrylate (PMMA) and polyacrylonitrile (PAN) membranes but not by Cuprophan, ethylene vinyl alcohol (EVAL), or polysulfone (PS) membranes. In addition the EPO adsorptive rate, as well as that of beta 2-microglobulin (beta 2-MG), was greater with a PMMA membrane than with a PAN membrane. EPO was not detected in the ultrafiltrate at 15 min with 5 membranes. These results indicate that EPO was eliminated by membrane adsorption only with some dialysis membranes.

Membrane adsorption of beta 2-microglobulin: equilibrium and kinetic characterization

Enhanced extracorporeal removal of beta 2-microglobulin (beta 2m) may prevent the development of dialysis-related amyloidosis (DRA). One mechanism of beta 2m removal is membrane adsorption. Therefore, we fundamentally characterized beta 2m adsorption to the highly permeable polyacrylonitrile (PAN) membrane. Porous and nonporous PAN fragments were incubated in buffer containing 125I-beta 2m. Over a concentration range of 8 to 60 mg/liter, the equilibrium adsorption isotherm was linear (r = 0.99) for porous PAN while the isotherm for nonporous PAN suggested either multilayer binding or adsorption of proteins with differing orientations. In kinetic analyses, the approach to equilibrium versus (time)1/2 was evaluated. For both porous and nonporous PAN, this relationship was linear (r = 0.99), consistent with a diffusion-controlled process. Adsorption reversibility was assessed by comparing the amount bound at varying residence times (0 to 4 hr) to the amount remaining adsorbed after a subsequent incubation in buffer. The fractions remaining bound at 60, 120, and 240 minutes (0.34 +/- 0.02, 0.36 +/- 0.06, and 0.44 +/- 0.03; mean +/- SEM) were significantly greater (P < 0.05) than the value at five minutes (0.23 +/- 0.01). This suggests membrane-induced conformational changes in adsorbed beta 2m. This investigation permits the comparison of beta 2m adsorptive properties of PAN to those of other membrane-based and nonmembrane-based therapies designed to prevent DRA.

Characterization of the beta 2-microglobulin endocytic pathway in rat proximal tubule cells

The uptake mechanism(s) of low-molecular-weight proteins by proximal tubule cells remains incompletely characterized. We utilized a biochemical and semiquantitative morphological approach to better characterize the endocytic pathway of an anionic protein, beta 2-microglobulin (beta 2M), in the rat proximal tubule. Indirect immunogold techniques revealed beta 2M was taken up via a classic receptor-mediated endocytic pathway. In vitro biochemical and morphological characterization of iodinated beta 2M and gold-conjugated beta 2M (gold-beta 2M) binding to isolated brush-border membrane vesicles (BBMV) documented specific and quantitatively similar binding interactions of the modified beta 2M with BBMV. Kinetic characterization of the in vivo endocytic pathway of gold-beta 2M was undertaken using microinfusion of individual tubules. beta 2M initially bound at the apical surface, was internalized into subapical coated vesicles and delivered to endosomal-like structures within 5 min, and, finally, was concentrated in lysosomal-like structures within 15 min. This uptake was inhibited by excess unconjugated beta 2M. In addition, we directly showed that uptake did not occur across the basolateral surface. Finally, by passing solubilized BBMV over beta 2M affinity columns we were able to isolate binding activity.

Beta 2-microglobulin kinetics in end-stage renal failure

The kinetics of beta 2-microglobulin (beta 2m) were studied in five anephric or anuric hemodialysis patients. Human beta 2m was isolated from peritoneal dialysate using ion-exchange and gel chromatography and radiolabeled with 125I. Patients were injected with 10 microCi labeled beta 2m. In one study (N = 4), plasma activity was measured over 72 hours. In a second study (N = 4), patients received low-flux dialysis 24 hours after injection and high-clearance dialysis (Bellco BL655) at 48 hours. Plasma activities were fitted to a three-compartment, variable volume model. Endogenous beta 2m levels (radioimmunoassay) were 56 +/- 6 mg/liter. The beta 2m distribution volume was 12.7 +/- 2.0 liter (0.20 +/- 0.03 liter/kg) and the non-renal clearance was 3.0 +/- 0.4 ml/min. The generation rate, 9.9 +/- 1.7 mg/hr (0.16 +/- 0.04 mg/kg/hr), was similar to that measured in subjects with normal renal function. The three compartment model derived from the turnover data gave an adequate fit of the arterial concentrations of endogenous and exogenous beta 2m during low-flux (nil beta 2m clearance) and high-clearance (beta 2m clearance of 19 ml/min) dialysis. Simulations based on this model indicate that extracorporeal treatment can at best remove about 50% of weekly production. These results suggest that beta 2m production is not increased in dialysis patients, that there is substantial non-renal beta 2m clearance, and that the amount of beta 2m that can be removed by extracorporeal therapy is therefore limited.

Transmembranous transport and adsorption of beta-2-microglobulin during hemodialysis using polysulfone, polyacrylonitrile, polymethylmethacrylate and cuprammonium rayon membranes

Beta-2-microglobulin (b2M) was identified as a causative agent of amyloidosis associated with long-term hemodialysis (HD). Therefore, we examined handling of b2M during a 4-hour hemodialysis session. We compared b2M adsoprtion and diffusive/convective elimination between high-flux membranes such as polysulfone (PS; F 60, Fresenius), polyacrylonitrile (AN 69; Filtral, Hospal) and polyacrylonitrile (PAN, PAN 12CX2, Asahi) and less permeable membranes such as cuprammonium rayon (CR; AM 160 H, Asahi) and polymethylmethacrylate (PMMA; BK-1.6 U, Toray). To calculate total elimination, arterio-venous differences of b2M were measured at 0, 5, 20, 60 and 240 minutes; dialysate concentration was analyzed to evaluate diffusive/convective transport. Differences between recovery in dialysate and total removal were regarded as amount removed by adsorption. Total elimination per 4-hour hemodialysis session and per m2 membrane surface was 154.7 +/- 12.3 mg for the PS, 137.8 +/- 28.4 mg for the AN 69, 179.8 +/- 47.5 mg for the PAN, 130.8 +/- 11.8 mg for the PMMA and 14.4 +/- 16.0 mg for the CR membrane. Diffusive/convective transport was 128.0 +/- 18.1 mg for PS, 54.7 +/- 8.1 mg for AN 69 and 106.5 +/- 20.8 mg for PAN and insignificant for PMMA and CR. Adsorption was 26.7 +/- 4.3 mg for PS, 83.1 +/- 29.0 mg for AN 69 and 59.8 +/- 17.2 mg for PAN. Besides transmembranous transport sorption is an important mode of elimination. Weekly endogenous generation rate is about twice as high as b2M elimination.

Clearance of beta-2-microglobulin using continuous ambulatory peritoneal dialysis

In the 9 continuous ambulatory peritoneal dialysis (CAPD) patients studied, the mean clearance of beta 2-microglobulin was significantly higher using hypertonic as opposed to isotonic exchanges (1 ml/min and 0.75 ml/min, respectively). Clearance of beta 2-microglobulin correlated with the clearance of albumin. The daily mass transfer of beta 2-microglobulin ranged from 19 to 62 mg. Although all the daily production of beta 2-microglobulin is not eliminated in patients on CAPD their serum beta 2-microglobulin levels would be expected to be lower than in patients on haemodialysis. Long-term prospective studies are needed to determine whether lower serum beta 2-microglobulin levels lead to a lower incidence of dialysis amyloid.

Tissue uptake of 125I-beta 2-microglobulin (beta 2-M) in anephric animals in the presence or absence of aluminium intoxication

In long-term haemodialysis patients a new type of amyloidosis composed of beta 2-microglobulin (beta 2-M) has recently been described. The amyloid deposition has a particular predilection for articular structures. In the pathogenesis of this complication markedly elevated plasma beta 2-M concentrations, such as those observed in anuric patients, have a role. However, other as yet ill-defined factors must also be implicated, possible candidates being aluminium intoxication and the widely used regenerated cellulose (cuprophan) membrane. In the present experimental study, we examined tissue distribution of exogenous beta 2-M after i.v. injection of 125I-beta 2-M to bilaterally nephrectomised rats. One hundred and twenty minutes after injection, most radioactivity remained in the vascular compartment. The accumulation in tissues was weak, and no predilection for a particular tissue became apparent. Interestingly, chronically aluminium-overloaded, acutely anephric rats accumulated a significantly greater amount of 125I-beta 2-M in their spleens than anephric rats without prior aluminium intoxication. We then attempted to induce beta 2-M amyloid deposition in rats and mice, some of whom had undergone chronic aluminium intoxication and subcutaneous implantation of regenerated cellulose fragments for various periods of time. They were subsequently made anephric to obtain high plasma beta 2-M concentrations. None of the animals developed beta 2-M amyloidosis in spleen, liver, skin and mechanically altered joint synovium. In conclusion, chronic aluminium intoxication enhances splenic accumulation of exogenous 125I-beta 2-M in anephric rats. The factors required to form beta 2-M-amyloidosis in vivo have still to be defined.

Effect of oxidation on the conformation of cell surface H-2 antigens detected by monoclonal antibodies

Periodate treatment of lymphoid cells is known to render them susceptible to nonspecific lysis by cytotoxic T lymphocytes. To detect alterations in cell surface molecules as a result of such treatment, we investigated the binding of antibodies to oxidized and nonoxidized lymphoid cells and found that oxidation selectively decreases the binding of some anti-H-2 and anti-beta 2m antibodies. This effect was not caused by loss of H-2 molecules from the cells, since the rate of exchange of cell surface beta 2m by exogenously added beta 2m was unchanged, and neither H-2 nor beta 2m molecules could be detected in the supernates of oxidized cells. Our findings suggest that oxidation alters the conformation of H-2 antigens in such a way as to reduce antibody binding. Such conformational changes may further affect the recognition of histocompatibility antigens by cytotoxic T cells.