Determinants of red cell sickling. Effects of varying pH and of increasing intracellular hemoglobin concentration by osmotic shrinkage.

The effects of varying pH and of increasing intracellular hemoglobin (Hb) concentration on red cell sickling and oxygen affinity were studied in whole blood from persons with sickle cell anemia (SS) and sickle cell trait (SA). Small increases in SS blood pH inhibited sickling, and small reductions in both SS and SA blood pH promoted sickling far more than accounted for by the Bohr effect. Sickling behavior correlated with minimum gelling concentrations (MGC) of deoxygenated hemolysates without 2,3-diphosphoglycerate. MGC values fell sharply when pH was lowered from 7.25 to 7.15 for HbS and from 7.15 to 6.90 for SA hemolysates, suggesting effects on specific ionic interactions involved in Hb gelation. Possible clinical counterparts are acute metabolic acidosis and alkalosis (prior to change in red cell 2,3-diphosphoglycerate), where the Bohr effect and oxygen affinity-independent effects of pH alterations on sickling would be additive. Osmotic shrinkage of HbS-containing red cells produced a large fall in oxygen affinity and a marked increase in sickling independent of that fall. The oxygen affinity and sickling properties of SA cells whose MCHC was raised to 40 per cent resembled those of unaltered SS cells, supporting a relationship between molecular aggregation of Hb and low oxygen affinity. Sickling of aerated SS cells in hypertonic saline depended upon partial Hb desaturation due to lowered oxygen affinity. Thus osmotic shrinkage of HbS-containing cells acts synergistically with partial deoxygenation to promote sickling. These conditions are present in the renal medulla, but may occur elsewhere in severe hyperosmolar states.