Volume-stimulated, Cl−-dependent K+ efflux is highly expressed in young human red cells containing normal hemoglobin or HbS

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Abstract
We report here that a Cl−-dependent K− (K∶Cl) efflux, which is stimulated by N-ethylmaleimide, (NEM) and by increased red cell volume, exists in young red cells of individuals with normal hemoglobin A (AA) and in those homozygous for hemoglobin S (SS). We have investigated this K∶Cl efflux in several density-defined red cell fractions obtained from Percoll-Stractan continuous density gradients. We found high activity of the NEM-stimulated K∶Cl transport in reticulocytes and young red cells from nine sickle cell (SS) patients (43±27 mean±sd mmol K+/liter of cells/hr=flux units (FU)) and in the young cell fraction of three AA individuals with high reticulocytosis recuperating from nutritional anemias (41.7±10 FU). In addition, we observed significant interindividual variation of this K∶Cl efflux in the discocyte fraction of SS blood. Cell swelling markedly stimulated the K∶Cl efflux, in SS whole blood (9.8±7.4 FU, in SS young cells (13±13 FU), and in AA young cells (21.4±11 FU). The activity of the Na−K−Cl cotransport, as estimated by the bumetanide sensitive K+ efflux was not found to be cell-age dependent in either AA or SS cells. Measurements of red cell density by isopycnic gradients indicated that 27% of the young cells reduce their volume by a Cl−-dependent process in hypotonic or low pH-induced swelling. The large volume-stimulated K∶Cl efflux in AA young cells raises the possibility that these fluxes may be involved in the maturation of erythropoietic precursors. The high activity in the red cells of sickle cell anemia patients and its interindividual variation may have pathophysiological consequences since it reverses the decrease in the intracellular concentration of hemoglobin which occurs in response to low pH or osmolarity, an unwelcome pro-sickling event.