A compensatory mechanism improving red cell membrane fluidity in hemodialysed patients

Effect of Hemodialysis on the Content of Fatty Acids in Monolayers of Erythrocyte Membranes in Patients with Chronic Renal Failure

Lipid metabolism disorders are found in patients with chronic renal failure (CRF). Changes in the content of fatty acids of the phospholipid fraction of erythrocyte membranes can lead to changes in the rheological properties. The objective of our study was to assess the effect of hemodialysis on the composition of fatty acids in two fractions of phospholipids: sphingomyelin (SPH, representative of the external monolayer) and phosphatidylethanolamine (PE, representative of the internal monolayer). Venous blood was drawn from patient with CRF before and after the HD procedure. Lipids from the erythrocyte stroma were extracted using the Rose and Oklander method and then were separated into phospholipid fractions using thin-layer chromatography (TLC). PE and SPH fractions were extracted, and the fatty acid profile was determined using gas chromatography (Perkin Elmer 8400; RTx 2330 column; length: 105 m). In the phospholipid fractions tested, a high content of saturated FA with a medium carbon chain (C 16:0 to C 18:2) and a long carbon chain such as C 24:0, C 24:1; C 22:6; and C 26:0 was found. The HD procedure affected the FA profile in the fractions tested. The proportion of saturated and unsaturated long-chain FA (above 18 C) increased in PE. However, the content of medium-chain FA C 16:0 to C 18:1 decreased. A significant decrease in the content of the majority of long-chain FA could be noted in SPH. The ratio of unsaturated (U) to saturated (S) fatty acids in the SPH fraction increased. Hemodialysis has a significant effect on the content of fatty acids in the PE and SPH fractions of erythrocyte membranes in patients with CRF.

CHARACTERIZATION OF CATIONIC AMINO ACID TRANSPORT SYSTEMS IN RAT ERYTHROCYTES: LACK OF EFFECT OF URAEMIA ON l-ARGININE INFLUX

1. Chronic renal failure (CRF) is associated with the abnormal regulation of nitric oxide (NO) synthesis at the systemic level. The transport of L-arginine, upregulated in blood cells from uraemic patients, modulates NO synthesis in this pathological condition. The model of partial nephrectomy in rats is widely accepted as a valid model of uraemia. Because there are no reports of L-arginine transport in blood cells from uraemic rats, the aim of the present study was to investigate L-arginine transport in red blood cells (RBCs) from these rats. 2. The kinetics of L-arginine transport in RBC and plasma and the amino acid profiles of RBC were investigated in control, sham-operated and subtotally nephrectomized rats. 3. L-Arginine transport was mediated via the cationic amino acid transport system y+ and a transport system with kinetics resembling the human system y+L. In control RBC, the apparent Ki for L-leucine inhibition of L-arginine transport via system y+L was 0.16 +/- 0.02 and 4.8 +/- 2 mmol/L in the presence of Li+ and Na+, respectively. 4. The Vmax values for L-arginine transport via system y+L and system y+ were similar in RBC from control sham-operated and uraemic rats. Moreover, L-arginine concentrations in plasma and RBC were not affected by uraemia. 5. The findings of the present study provide the first evidence that L-arginine transport in rat erythrocytes is mediated by two distinct cationic transport systems with characteristics of systems y+ and y+L, which accept neutral amino acids only in the presence of Li+. In contrast with previous studies in uraemic patients, plasma levels and maximal transport rates of L-arginine were not altered in this rat model of CRF.

Altered Na(+)-K+ ATPase activity in uraemic adolescents

The Na(+)-K+ ATPase enzyme plays an essential role in the regulation of cell composition and volume. Enzyme activity itself is regulated by substrate availability and several hormones. In adult uraemic patients red blood cell Na(+)-K+ ATPase activity is decreased. However, it is unknown if children with uraemia exhibit the same phenomenon. Therefore, in the present study we examined whether endogenous digoxin-like factors (EDLF) and physicochemical membrane properties play a role in the regulation of erythrocyte Na(+)-K+ ATPase activity in uraemic children and adolescents. Healthy age-matched children were used as controls. Enzyme activity was measured in detergent-pretreated red blood cells and erythrocyte ghosts. Na(+)-K+ ATPase activity (2204 +/- 538 nmol Pi ml erythrocyte-1 h-1 in detergent pretreated erythrocytes; 204 +/- 56 nmol Pi mg protein-1 h-1 in ghosts) in adolescents with uraemia was lower compared to controls (3245 +/- 362 nmol Pi ml erythrocyte-1 h-1; 266 +/- 37 nmol Pi mg protein-1 h-1, p < 0.001, p < 0.05, respectively). Plasma levels of EDLF were elevated in uraemic patients (0.30 +/- 0.05 versus 0.21 +/- 0.04 ng ml-1, p < 0.01). Furthermore, the membrane lipid component was decreased in patients with uraemia, while the cholesterol/phospholipid ratio and membrane fluidity were similar in both groups. No correlation was found between the decrease in Na(+)-K+ ATPase and the increase in EDLF concentration and altered membrane lipid components. Our results demonstrate, that similar to the findings of adults, the activity of Na(+)-K+ ATPase is diminished in uraemic adolescent patients, and that uraemia-associated elevation in EDLF and altered membrane components do not play a role in the down-regulation of Na(+)-K+ ATPase. Therefore other factors (presence of other inhibitors and/or reduced number of enzyme molecules) should contribute to the lower activity of the Na(+)-K+ pump.

Comparison of two fluorescent probes for the measurement of erythrocyte membrane fluidity in renal dialysis patients

Two fluorescent probes were used for the measurement of membrane fluidity in patients on haemodialysis and continuous ambulatory peritoneal dialysis. 1,6-Diphenyl-1,3,5-hexatriene (DPH) anisotropy gives an indication of lipid order and pyrene measures lateral diffusion through the membrane. Pyrene dimer/monomer ratio was significantly lower than controls in both pre-dialysis and post-dialysis samples but DPH anisotropy was unchanged. Both methods showed an increase in membrane fluidity across a 4 hour haemodialysis session. There was an increase in membrane fluidity in CAPD patient samples which was more marked using DPH than pyrene. These results suggest that the two probes give different but complementary information about changes in membrane fluidity and may be more informative when used together rather than singly.

Lipoperoxidation in plasma and red blood cells of patients undergoing haemodialysis: vitamins A, E, and iron status

In 14 patients undergoing haemodialysis, lipoperoxidation (LPO) processes were determined in plasma and red blood cells (RBC) before and after a dialysis session by determining (a) the direct substrate, polyunsaturated fatty acids (PUFA); (b) the end product of LPO, malondialdehyde (MDA); and (c) the hydrophobic antioxidant systems, vitamins A and E. In plasma before dialysis, linoleic and arachidonic acid, and the antioxidant vitamin E, were significantly lowered as compared to the healthy controls (p < 0.05). On the contrary, the free MDA level was enhanced (p < 0.05). These results were emphasized by a dialysis session. In RBC of these patients, no difference in linoleic acid, free MDA, or vitamin E level were observed before or after dialysis when compared to controls. However, only vitamin A was significantly higher in haemodialysis patients (before and after dialysis) and in renal failure patients (p < 0.05) than in the healthy control group. The present results suggest that increased RBC vitamin A may offer some degree of protection against oxidative stress in erythrocytes, but not in plasma where LPO is demonstrated.