EVIDENCE FOR A DIRECT RETICULOCYTE ORIGIN OF DENSE RED-CELLS IN SICKLE-CELL-ANEMIA

To explore our hypothesis of a direct reticulocyte origin of irreversibly sickled cells (ISCs), we fractionated light, reticulocyte-rich, and discocyte-rich sickle cell anemia red cells on Stractan gradients, and examined the effects of deoxygenation-induced sickling, external Ca2+, acidification, and replacing external Na+ by impermeant N-methyl-D-glucamine (NMG+). Sickling permeabilized light reticulocyte-rich cells to cations (NA+, K+, and Ca2+) more than discocytes; without external Ca2+, Na+ influx matched K+ efflux, with stable cell volume; with Ca2+, many light, low hemoglobin (Hb) F reticulocytes dehydrated rapidly (preventable by quinine, a Ca2+-dependent K+ channel inhibitor). Acidification of oxygenated discocytes (high mean Hb F) and reticulocyte-rich fractions yielded denser, reticulocyte-enriched cells with lower Hb F (as in light reticulocyte or dense ISC-rich fractions). Light cells shrank when NMG+ replace Na+, supporting predictions of a Na+-dependent volume control system. Demonstration of sickling-induced, Ca2+-dependent dehydration of Hb F-free reticulocytes, and conservation of acid-stimulated K:Cl cotransport among low Hb F, reticulocyte-enriched cells in discocyte fractions support the hypothesis. Ancillary new findings included heparin stimulation of sickling-induced Na+ and K+ permeabilizations, and Ca2+ inhbiition of the Na+ leak.