Cation transport and cell volume changes in maturing rat reticulocytes

During maturation, reticulocytes lose membrane material, including transpor ters, and this is accompanied by a loss of cell water and volume. Here we d etermined a possible role of ion transport in adjusting cell volume during maturation. Reticulocytes and red blood cells of different ages were prepar ed from erythropoietin-treated rats by density gradient fractionation. Cell volume and ion transport were measured in freshly prepared cells and in re ticulocytes during in vitro maturation. Reticulocytes had an increased K co ntent and cell volume, whereas intracellular Na was decreased. All paramete rs approached whole blood values after 2 days in culture. Na-K pump was ele vated in reticulocytes and decreased during maturation. Na-K-2Cl cotranspor t (NKCC) activity was lower in reticulocytes and was activated 8- and 20-fo ld by shrinkage and okadaic acid, respectively, whereas stimulation was bar ely detectable in high-buoyant density red blood cells. The ouabain- and bu metanide-insensitive Na flux in reticulocytes decreased on maturation. Most of it was inhibited by amiloride, indicating the presence of Na/proton exc hange. Our results show that, although the Na-K-pump activity in reticulocy tes is very much increased, the enhanced capacity of NKCC is essentially cr yptic until stimulated. Both types of capacities (activities) decrease duri ng maturation, indicating a possible loss of transport protein. The decreas e was constrained to the period of reticulocyte maturation. Loss of transpo rt capacity appears to exceed the loss of membrane surface area. Reticulocy te age-related changes in the net electrochemical driving force indicate th at the increasing NKCC activity might contribute to the reduction in cell w ater.