A kinetic study of cation transport in erythrocytes from uremic patients

A kinetic study of cation transport from uremic patients. We previously described in red blood cells (RBCs) from uremic patients on dialysis a reduction in sodium (Na) efflux through the Na, potassium (K) cotransport system (Na,K CoT) while Na efflux through the Na,K pump was normal. We then examined Na efflux in fresh cells and in cells loaded to obtain one level of intracellular sodium (Nai) concentration at about 25 mmol/liter cell. In the present study we used similar cation flux methodology to examine the kinetics of cation efflux through the Na,K pump and Na,K CoT in uremic patients on dialysis. RBCs were Na-loaded to attain five different levels of Nat concentration over a range of 5 to 50 mmol/liter cells using the ionophore nystatin. At each level of Na-loading, the Nai achieved was similar in RBCs from controls and patients. Ouabain–sensitive Na efflux through the Na,K pump showed no difference in rate between normals and dialysis patients. When the kinetic parameters of this transport pathway were considered, the apparent affinity (K0.5) for sodium was not significantly different between controls and patients (18.4 ± 2.3 vs. 20.0 ± 2.6 mmol/liter cell) and the maximal velocity of efflux (Vmax) was also not different between controls and patients (9.6 ± 0.7 vs. 8.5 ± 1.2 mmol/liter cell/hr). Comparison of Nai-activated Na versus K efflux rates through the Na,K CoT in normal subjects demonstrated similar saturation kinetics, (K0.5 15.8 ± 3.3 vs. 12.2 ± 2.8 mmol/liter cell, Vmax0.81 ± 0.1 vs. 0.78 ± 0.1 mmol/liter cell/hr) consistent with the known stoichiometric ratio of 1 Na:l K:2 C1 described for this mechanism. In dialysis patients Nai-activated, Na,K CoT-mediated Na efflux was markedly reduced. Analysis of the kinetic parameters of Na1-activated Na efflux showed that the reduced RBC Na,K CoT is due to reduction in Vmax and not to a change in K0.5 Maximum furosemide–sensitive K efflux rate was also reduced in dialysis patients. However, instead of exhibiting the anticipated saturation kinetics observed for Na, the K efflux rates were high at low levels of Nai and remained unchanged with increasing Nai concentrations. Ouabain- and furosemide-resistant Na and K effluxes were not significantly different between normals and dialysis patients. We conclude that Na efflux through RBC Na,K pump is intact over a wide range of Nai concentrations in dialysis patients. On the other hand, the furosemide–sensitive co-efflux of Na and K, which in normal RBCs displayed a typical 1 Na to 1 K transport characteristic, was quantitatively and qualitatively altered in dialysis patients. The maximum efflux rate of both Na and K was reduced and in addition, the usual stoichiometric ratio for Na and K exit through this furosemide–sensitive pathway was no longer observed