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We have previously demonstrated that young normal (AA) and sickle cell anemia (SS) red blood cells are capable of a volume regulatory decrease response (VRD) driven by a K:CI cotransporter that is activated by low pH or hypotonic conditions. We now report on the characteristics of young SS cells (SS2, discocytes) capable of rapid increase in density in response to swelling. We have isolated cells with high VRD response (H-VRD) and low VRD response (L-VRD) cells by incubation and density-gradient centrifugation under hypotonic conditions. Comparison of these cells in patients homozygous for hemoglobin (Hb)S indicated that H-VRD cells have 91% more reticulocytes (P ≪ 9 × 10-9) than L-VRD cells, 25% less HbF (P ≪ 5.5 × 105), 106% more NEM (N-methylmaleimide)-stimulated K:CI cotransport activity (P <2 × 104), and 86% more volume-stimulated K:CI cotransport activity (P ≪ 1.8 × 10-3). H-VRD and L-VRD cells have similar G-6-PD and Na7H+antiport activity. In agreement with the reduced percent HbF in H-VRD cells, F cells (red blood cells that contain fetal Hb) are depleted from the H-VRD population; however, F reticulocytes are enriched in the H-VRD population to the same extent as non-F reticulocytes, which suggests that both F and non-F reticulocytes have a similar initial distribution of volume-sensitive K:CI cotransport activity but that it may be more rapidly inactivated in F than in S reticulocytes. We find that H-VRD cells consist of 20% reticulocytes (or 79% of all reticulocytes in SS2) and 80% more mature cells. This study demonstrates the role of K:CI cotransport in determining red blood cell density, the heterogeneity of K:CI cotransport activity in reticulocytes, and the capacity for rapid change in the density of reticulocytes with high K:CI cotransport activity. We speculate that the H-VRD population may be more susceptible to generation of dense and irreversibly sickled cells. © 1991 by The American Society of Hematology.