title: Convective and diffusive losses of vitamin C during haemodiafiltration session: a contributive factor to oxidative stress in haemodialysis patients.
abstract: BACKGROUND
  Enhanced oxidative stress in haemodialysis (HD) patients may be considered as a risk factor for accelerated atherosclerosis. Reduced antioxidant defences include impairment in enzyme activities and decreased plasma levels of hydrophilic vitamin C (vit C), and cellular levels of lipophilic vitamin E (vit E).
  
  
  METHODS
  We investigated plasma levels of vit C in 19 patients undergoing regular haemodiafiltration (HDF) (mean age 62+/-7 years) and in 1846 healthy elderly subjects (HS) (mean age 69+/-5 years). The contribution of convection and diffusion was determined using paired filtration dialysis (PFD), a modified HDF technique which physically separates convective from diffusive fluxes. Blood samples were collected before and after the HDF session; in addition at 60 min of HDF, samples were drawn from arterial lines (AL) and venous lines (VL), dialysate (D) and ultrafiltrate (UF). Blood levels of total vit C were determined using an HPLC fluorescence method. Markers of oxidative stress were also assessed in both populations as follows: levels of malondialdehyde (MDA) were determined by fluorometric assay, measurements of advanced oxidation protein products (AOPP) and glutathione peroxidase (GSH-Px) activity were performed by spectrophotometric assay, and plasma vit E content was obtained by an HPLC procedure.
  
  
  RESULTS
  A significant reduction in plasma vit C level was observed in HDF patients when compared with HS (1.6+/-1.4 microg/ml in HDF vs 6.6+/-3.7 microg/ml in HS; P<0.01). The HDF session was associated with a dramatic reduction in vit C levels (1.87+/-1.57 microg/ml before HDF and 0.98+/-0.68 microg/ml after HDF); at 60 min of HDF, concentrations were as follows: AL=1.35+/-1.27 microg/ml; VL=0.37+/-0.31 microg/ml, D=0.40+/-0.34 microg/ml, UF=1.24+/-1.18 microg/ml; corresponding to a diffusive flux of 271 microg/min and a convective flux of 126 microg/min. Total loss of vit C could be assessed at 66 mg/session (8--230 mg/session). According to this loss of vit C, presence of an oxidative stress was demonstrated in HD population as shown by a significant increase in MDA (1.66+/-0.27 microM in HD vs 0.89+/-0.25 microM in HS; P<0.01) and AOPP (77.5+/-29.3 microM in HD vs 23.5+/-13.2 microM in HS; P<0.01) levels, and a decrease in GSH-Px activity (259.2+/-106.3 U/l in HD vs 661.2+/-92.2 U/l in HS; P<0.01). No change in plasma vit E between both populations (30.7+/-9.1 microM in HD vs 35.3+/-7.34 microM in HS) was observed.
  
  
  CONCLUSIONS
  These results suggest that HDF with highly permeable membranes is associated with a significant loss of vit C. Diffusive transport is responsible for two-thirds whereas convective phenomenon accounts for only one-third of this loss.
authors.affiliation: Department of Biochemistry, Lapeyronie Hospital, University of Montpellier I, F-34295 Montpellier, France.
authors.forename: Marion
authors.initials: M
authors.lastname: Morena
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Cristol
authors.forename: Jean-Yves
authors.initials: JY
authors.lastname: Bosc
authors.forename: Ciro
authors.initials: C
authors.lastname: Tetta
authors.forename: Gilles
authors.initials: G
authors.lastname: Forret
authors.forename: Claude-Louis
authors.initials: CL
authors.lastname: Leger
authors.forename: Cécile
authors.initials: C
authors.lastname: Delcourt
authors.forename: Laure
authors.initials: L
authors.lastname: Papoz
authors.forename: Bernard
authors.initials: B
authors.lastname: Descomps
authors.forename: Bernard
authors.initials: B
authors.lastname: Canaud
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001161:Arteriosclerosis / Q000209:etiology; D001205:Ascorbic Acid / Q000097:blood*; D015415:Biomarkers / Q000097:blood; D001798:Blood Proteins / Q000737:chemistry / Q000378:metabolism; D016022:Case-Control Studies; D018481:Convection; D004058:Diffusion; D005260:Female; D005979:Glutathione Peroxidase / Q000097:blood; D017583:Hemodiafiltration / Q000009:adverse effects*; D006801:Humans; D008297:Male; D008315:Malondialdehyde / Q000097:blood; D008875:Middle Aged; D018384:Oxidative Stress; D006435:Renal Dialysis / Q000009:adverse effects*; D012307:Risk Factors; D014810:Vitamin E / Q000097:blood
chemical_list: D015415:Biomarkers; D001798:Blood Proteins; D014810:Vitamin E; D008315:Malondialdehyde; D005979:Glutathione Peroxidase; D001205:Ascorbic Acid