Cation channels trigger apoptotic death of erythrocytes

Erythrocytes are devoid of mitochondria and nuclei and were considered unable to undergo apoptosis. As shown recently, however, the Ca2+-ionophore lonomycin triggers breakdown of phosphatidylserine asymmetry (leading to annexin binding), membrane blebbing and shrinkage of erythrocytes, features typical for apoptosis in nucleated cells. In the present study, the effects of osmotic shrinkage and oxidative stress, well-known triggers of apoptosis in nucleated cells, were studied. Exposure to 850 mOsm for 24 h, to tert-butylhydroperoxide (1 mM) for 15 min, or to glucose-free medium for 48 h, all elicit erythrocyte shrinkage and annexin binding, both sequelae being blunted by removal of extracellular Ca2+ and mimicked by lonomycin (1 muM). Osmotic shrinkage and oxidative stress activate Ca2+-permeable cation channels and increase cytosolic Ca2+ concentration. The channels are inhibited by amiloride (1 mM), which further blunts annexin binding following osmotic shock, oxidative stress and glucose depletion. In conclusion, osmotic and oxidative stress open Ca2+-permeable cation channels in erythrocytes, thus increasing cytosolic Ca2+ activity and triggering erythrocyte apoptosis.