Ca2+ accumulation and loss by aberrant endocytic vesicles in sickle erythrocytes

Sickle cells contain internal vesicles which accumulate Ca2+. As shown here, the membrane enclosing the vesicles contains the plasma membrane Ca(2+)-ATPase, or Ca2+ pump, as judged by staining with an antibody directed against the protein. Moreover, the number of cells containing such vesicles increases upon deoxygenation. These findings argue strongly that the vesicles arise by endocytosis from the plasma membrane, and explain how they accumulate Ca2+. When sickle cells are depleted of ATP, Ca2+ is lost from the vesicles, as judged by the disappearance of staining with the Ca2+/membrane probe chlortetracycline (CTC), without a corresponding loss of antibody staining. This loss of Ca2+ can be inhibited by nitrendipine, a Ca2+ channel blocker. These results suggest that the vesicle membrane allows outward passage of Ca2+ by a nitrendipine-sensitive pathway, which can be overcome by the inward-directed activity of the Ca2+ pump of the vesicle membrane. If so, the Ca2+ which vesicles contain is in dynamic equilibrium with the cytoplasm of the sickle erythrocyte.