Cell volume and K+ transport during differentiation of mouse erythroleukemia cells

In the present study, we evaluated the changes in cell volume, water content, and K+ transport in mouse erythroleukemia (MEL) cells during the transition from proerythroblast to young reticulocyte. When MEL cells were exposed to 1.8% dimethyl sulfoxide (DMSO) for a maximum of 7 days, they synthesized hemoglobin and reduced their volume by 66% while maintaining their water content. The total protein content decreased by 50%. We therefore concluded that the volume reduction was due to a loss of cellular material, water, and osmolytes. To evaluate the changes in pump and leak pathways, we performed 86Rb uptakes in the presence or absence of selected inhibitors. In undifferentiated cells, the uptake was mainly represented by the Na-K-2Cl cotransport (51%) and by the Na(+)-K+ pump (34%). A small portion of the uptake was mediated by barium- and quinidine-sensitive K+ channels (8%) and by the furosemide-sensitive K-Cl cotransporter (5%). After 4 days in DMSO, the 86Rb uptake was reduced by 57%, mainly due to a substantial (90%) decrease in Na-K-2Cl cotransport activity. The Na(+)-independent K-Cl cotransport activity also dramatically decreased by a factor of 10. In contrast, the Na(+)-K+ pump activity did not change after 4 days in DMSO. These results demonstrate a marked reduction in the activities of inorganic ion cotransport systems as red blood cells differentiate to reticulocytes. Our study also demonstrates that a strong correlation exists between cell volume reduction and a decrease in the main inward leak pathway for K+: the Na-K-2Cl cotransporter.