Erythrocyte membrane abnormalities in sickle cell disease

A large fraction of circulating sickle red cells contain one or more large vesicles which are not found in normal erythrocytes. These vesicles contain very high levels of Ca2+, and probably account for the long-known elevation of cellular Ca2+ in sickle cells. These vesicles contain the plasma membrane CaATPase and leak Ca2+ by a nitrendipine-sensitive pathway. The question of whether an abnormal endocytic process occurs in sickle cells which could give rise to these vesicles was examined using the nonspecific endocytic marker Lucifer yellow (LY). The kinetics of formation of LY-labeled endocytic vesicles in sickle cells includes a slow component which is not found in normal (or sickle) reticulocytes. In addition, the number of sickle cells in which endocytosis can be demonstrated with this nonspecific marker consistently exceeds the number which can be labeled with markers of the receptor-mediated endocytic process. The results suggest that a slow, abnormal endocytosis takes place in sickle cells which may be the source of the Ca2(+)-containing vesicles, and therefore of the elevated Ca2+ levels characteristic of the circulating cells in this disease.