Staining of sickle cells with the fluorescent probes chlortetracycline (a Ca2+ probe) and diindocarbocyanine (a general membrane probe) revealed the presence of Ca2+-containing vesicles which are not found in normal erythrocytes. These vesicles increase in number upon deoxygenation, and are apparently formed by endocytosis, as judged by the use of the extracellular fluorescent probe lucifer yellow. The presence of vesicles is not restricted to any particular morphological or density class of cells in the general population.
One of the secondary effects of sickle cell disease is an abnormally high level of calcium in erythrocytes, which increases when sickle cells are deoxygenated (Palek 1973(Palek ,1976(Palek ,1977;; Eaton et al., 1973); however, the mechanism by which Ca2+ becomes elevated is not known. Recently, Bookchin et al. (1981, 1984) found that the rate of K + leakage from sickle cells is lower than predicted from the measured levels of Ca2+ present in them. They therefore suggested that the Ca2+ in sickle cells was compartmentalized. We report here experiments demonstrating that this suggestion is correct. We provide evidence that sickle cells, unlike normal erythrocytes, contain internal membrane-bounded vesicles formed by endocytosis upon deoxygenation, within which Ca2+ is sequestered.