We developed a mathematical model of the reticulocyte, seeking to explain how a cell with similar volume but much higher ionic traffic than the mature red cell (RBC) regulates its volume, pH, and ion content in physiological and abnormal conditions. Analysis of the fluxbalance required by reticulocytes to conserve volume and composition predicted the existence of previously unsuspected Na '-dependent Cl -entry mechanisms.

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

To explore basic properties of the sickling-induced cation permeability pathway, the Ca 2 ϩ component (P sickle-Ca ) was studied in density-fractionated sickle cell anemia (SS) discocytes through its effects on the activity of the cells' Ca 2 ϩ -sensitive K ϩ -channels (K Ca ). The instant state of K Ca channel activation was monitored during continuous or cyclic deoxygenation of the cells using a novel thiocyanate-densecell formation method. Each deoxy pulse caused a reversible, sustained P sickle-Ca , which activated K Ca channels in only 10-45% of cells at physiological [Ca

SUMMARY1. Our findings of a low total magnesium content in the dense fraction (over 1418 g ml-1) of sickle cell anaemia (SS) red cells seemed inconsistent with the low Mg2+ permeability and outward Mg2+ gradient seen in normal red cells, and prompted studies of the Mg2+ permeability and equilibria in the SS cells.2 5. The above findings suggested that the documented low sodium pump fluxes in dense SS cells may result from an increased Mg2+:ATP ratio, which is known to inhibit Na+-K+ exchange fluxes through the sodium pump. If so, deoxygenation, by increasing the Mg2+: ATP ratio, should inhibit the pump further, whereas increasing ATP should relieve the inhibition. Experiments designed to test this possibility showed that in these dense SS cells, the ouabain-sensitive K(86Rb) influx was low in oxygenated cells, was reduced further by deoxygenation, but was substantially increased after treatment with inosine, pyruvate and phosphate to increase their organic phosphate pool. These results were thus consistent with such a mechanism for Na+ pump inhibition in the dense SS cells.

Experiments were performed to test specific predictions of an integrated red cell model developed by Lew and Bookchin [Lew, V.L., Bookchin, R.M. J. Membrane Biol. 92:57-74 (1986)], that K-permeabilized human red cells suspended in low-K media would dehydrate and lose an alkaline, hypertonic fluid with excess K over accompanying anions, and that cell dehydration would precede medium alkalinization. Red cells were suspended at about 30% hematocrit in an initially K-free Na-saline and permeabilized to K by the addition of valinomycin. The results showed that by the time a quasi-steady state had been reached the cells had lost the equivalent of a hypertonic fluid containing about 180 mM KCl (SCN) and 10 mM KOH, and that cell dehydration did precede alkalinization of the medium, in good agreement with the theoretical predictions. Since these experiments critically test the interaction between transport, pH and volume regulatory functions in the human red cell, the observed agreement validates the basic assumptions and structure of the integrated model. The functional implications of these results are discussed.