Selective loss of calcium permeability on maturation of reticulocytes

Calcium and sodium permeability of human reticulocytes have been studied and compared to mature erythrocytes. Mature erythrocytes had extremely low Ca2+ permeability which was less than 0.1% of values published for squid axon or HeLa cells. Calcium entry was markedly increased in reticulocyte-rich suspensions and the uptake was linearly related to the percentage of reticulocytes present. The data suggest that reticulocytes are 43-fold more permeable to Ca2+ than mature cells although their Ca2+ concentration is not increased. Sodium influx into reticulocyte-rich suspensions was also increased in direct proportion to the percent of reticulocytes present. Reticulocytes are sixfold more permeable to Na+ than mature cells so the ratio of Ca2+:Na+ permeability falls by sevenfold as the reticulocyte changes to an erythrocyte. [3H]Ouabain binding was increased in reticulocyte-rich cell suspensions and the correlation suggested a value of about 4,000 sites per reticulocyte compared with 362+/-69 per mature cell. Maturation of the human reticulocyte produces disproportionate changes in cation permeability and in particular a selective loss of Ca2+ permeability.