ALTERED NA-K+ ATPASE ACTIVITY IN UREMIC ADOLESCENTS()

The Na+-K+ ATPase enzyme plays an essential role in the regulation of cell composition and volume. Enzyme activity itself is regulated by su bstrate availability and several hormones. In adult uraemic patients r ed blood cell Na+-K+ ATPase activity is decreased. However, it is unkn own if children with uraemia exhibit the same phenomenon. Therefore, i n the present study we examined whether endogenous digoxin-like factor s (EDLF) and physicochemical membrane properties play a role in the re gulation of erythrocyte Na+-K+ ATPase activity in uraemic children and adolescents. Healthy age-matched children were used as controls. Enzy me activity was measured in detergent-pretreated red blood cells and e rythrocyte ghosts. Na+-K+ ATPase activity (2204 +/- 538 nmol Pi ml ery throcyte(-1) h(-1) in detergent pretreated erythrocytes; 204 +/- 56 nm ol 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, respe ctively) 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 m embrane 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 l ipid components. Our results demonstrate, that similar to the findings of adults, the activity of Na+-K+ ATPase is diminished in uraemic ado lescent patients, and that uraemia-associated elevation in EDLF and al tered 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 lo wer activity of the Na+-K+ pump.