Compartmentalization of sickle-cell calcium in endocytic inside-out vesicles

Abstract

Much recent interest in the mechanism of dehydration of the dense subpopulation of sickle-cell anaemia (SS) red cells, including the ‘irreversibly sickled cells’ (ISCs), stems from the view that these relatively rigid cells have a major role in the two main clinical features of the disease, namely haemolytic anaemia and microvascular occlusion¹. The discovery that SS red cells have an elevated calcium content and accumulate Ca²⁺ during deoxygenation-induced sickling^2–4 suggested a working hypothesis of wide appeal for the mechanism of cell dehydration: retained calcium would activate the red cell Ca²⁺-sensitive K⁺ channels⁵, causing progressive net loss of KCl and water^6–8. However, retained calcium, which seemed as weakly bound to cytoplasmic buffers as in normal red cells⁹, failed to show any measurable activation of K⁺ channels or Ca²⁺ pumps in metabolically normal SS cells, despite the apparent functional normality or near-normality of these transport systems^9–17. We now offer a possible explanation for this failure. We show that, contrary to the traditional views, SS cells, and to a lesser extent normal human red cells, possess intracellular vesicles with ATP-dependent Ca²⁺-accumulating capacity, and that nearly all the measurable calcium of fresh SS cells is contained within such vesicles, probably in the form of precipitates with inorganic or organic phosphates.