Convective and diffusive losses of vitamin C during haemodiafiltration session: a contributive factor to oxidative stress in haemodialysis patients

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 mug/ml in HDF vs 6.6 +/- 3.7 mug/ml in HS; P<0.01). The HDF session was associated with a dramatic reduction in vit C levels (1.87 +/- 1.57 mu g/ml before HDF and 0.98 +/- 0.68 mu g/ml after HDF); at 60 min of HDF, concentrations were as follows: AL = 1.35 +/- 1.27 mu g/ml; VL = 0.37 +/- 0.31 mu g/ml, D = 0.40 +/- 0.34 mu g/ml, UF = 1.24 +/- 1.18 mu g/ml; corresponding, to a diffusive flux of 271 mu g/min and a convective flux of 126 mu g/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 mu M in HD vs 0.89 +/- 0.25 mu M in HS; P < 0.01) and AOPP (77.5 +/- 29.3 muM in HD vs 23.5 +/- 13.2 muM 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 muM in HD vs 35.3 +/- 7.34 muM 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.