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 Rb-86 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 Rb-86 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.