Dehydration response of sickle cells to sickling-induced Ca++ permeabilization

Interaction of hemoglobin S polymers with the red blood cell (RBC) membrane induces a reversible increase in permeability ("P-sickle") to (at least) Na+, K+, Ca2+, and Mg2+. Resulting changes in [Ca2+] and [H+] in susceptible cells activate 2 transporters involved in sickle cell dehydration, the Ca2+-sensitive K+ ("Gardos") channel (K-ca) and the acid- and volume-sensitive K:Cl cotransport. We investigated the distribution Of P-sickle expression among deoxygenated sickle cell anemia (SS) RBCs using new experimental designs in which the RBC Ca2+ pumps were partially inhibited by vanadate, and the cells' dehydration rates were detected as progressive changes in the profiles of osmotic fragility curves and correlated with flow cytometric measurements. The results exposed marked variations in (sickling plus Ca2+)-induced dehydration rates within populations of deoxygenated SS cells, with complex distributions, reflecting a broad heterogeneity of their P-sickle values. P-sickle-mediated dehydration was inhibited by clotrimazole, verifying the role of K-ca, and also by elevated [Ca2+](o), above 2 mM. Very high P-sickle values occurred with some SS discocytes, which had a wide initial density (osmotic resistance) distribution. Together with its previously shown stochastic nature, the irregular distribution of P-sickle documented here in discocytes is consistent with a mechanism involving low-probability, reversible Interactions between sickle polymers and membrane or cytoskeletal components, affecting only a fraction of the RBCs during each deoxygenation event and a small number of activated pathways per RBC. A higher participation of SS reticulocytes In P-sickle- triggered dehydration suggests that they form these pathways more efficiently than discocytes despite their lower cell hemoglobin concentrations. (C) 2002 by The American Society of Hematology.