Cation transport and cell volume changes in maturing rat reticulocytes

During maturation, reticulocytes lose membrane material,including transporters, and this is accompanied by a loss of cell waterand volume. Here we determined a possible role of ion transport inadjusting cell volume during maturation. Reticulocytes and red bloodcells of different ages were prepared from erythropoietin-treated ratsby density gradient fractionation. Cell volume and ion transport weremeasured in freshly prepared cells and in reticulocytes during in vitromaturation. Reticulocytes had an increased K content and cell volume,whereas intracellular Na was decreased. All parameters approached wholeblood values after 2 days in culture. Na-K pump was elevated inreticulocytes and decreased during maturation. Na-K-2Cl cotransport(NKCC) activity was lower in reticulocytes and was activated 8- and20-fold by shrinkage and okadaic acid, respectively, whereasstimulation was barely detectable in high-buoyant density red bloodcells. The ouabain- and bumetanide-insensitive Na flux in reticulocytesdecreased on maturation. Most of it was inhibited by amiloride,indicating the presence of Na/proton exchange. Our results show that,although the Na-K-pump activity in reticulocytes is very muchincreased, the enhanced capacity of NKCC is essentially cryptic untilstimulated. Both types of capacities (activities) decrease duringmaturation, indicating a possible loss of transport protein. Thedecrease was constrained to the period of reticulocyte maturation. Lossof transport capacity appears to exceed the loss of membrane surfacearea. Reticulocyte age-related changes in the net electrochemicaldriving force indicate that the increasing NKCC activity mightcontribute to the reduction in cell water.