Inhibition of Erythrocyte Cation Channels by Erythropoietin

Abstract

Abstract. Recombinant human erythropoietin therapy is used to counteract anemia that is the result of renal insufficiency. It stimulates the formation of peripheral blood erythrocytes by inhibiting apoptosis of erythrocyte precursor cells. Mature erythrocytes have similarly been shown to undergo apoptosis. Hyperosmotic shock and Cl Ϫ removal activate a Ca 2ϩ -permeable, ethylisopropylamiloride-inhibitable cation channel. The subsequent increase of cytosolic Ca 2ϩ activates a scramblase that breaks down cell membrane phosphatidylserine asymmetry, leading to annexin binding. Studied was whether channel activity and erythrocyte cell death are regulated by erythropoietin. Scatchard plot analysis disclosed low-abundance, highaffinity binding of 125 I-erythropoietin to erythrocytes. Whole cell patch clamp experiments revealed significant inhibition of the ethylisopropylamiloride-sensitive current by 1 U/ml erythropoietin. Cl Ϫ removal triggered annexin binding, an effect abrogated by erythropoietin (1 U/ml) but not by GM-CSF (10 ng/ml). Osmotic shock (700 mOsm) stimulated annexin binding within 24 h in the majority of the erythrocytes, an effect blunted by erythropoietin (1 U/ml) but not by GM-CSF (10 ng/ml). In the nominal absence of Ca 2ϩ , the effect of osmotic shock was blunted and the effect of erythropoietin abolished. In hemodialysis patients, intravenous administration of erythropoietin (50 IU/kg) within 4 h decreased the number of annexin binding circulating erythrocytes. Erythropoietin binds to erythrocytes and inhibits volume-sensitive erythrocyte cation channels and thus the breakdown of phosphatidylserine asymmetry after activation of this channel. The effect could prolong the erythrocyte lifespan and may contribute to the enhancement of the erythrocyte number during erythropoietin therapy in dialysis patients.

Erythrocyte programmed cell death

“…The Ca 21 -permeable cation channels have been shown in one study to be inhibited by erythropoietin (98). Thus, the hormone does not only inhibit apoptosis of erythrocytic progenitor cells (99,100), but similarly blunts the suicidal death of mature erythrocytes.…”

Anemia in Critical Illness

“…The essential factors for erythropoiesis include iron, zinc, folate, and vitamin B 12 , under the influence of erythropoietin (EPO), thyroxine, androgens, cortisol, and catecholamines. RBC formation occurs at a basal rate of 15-20 ml/day under steady-state conditions, and upward of 200 ml/day after hemolysis or heavy blood loss in iron-replete healthy persons (9).…”

“…Eryptosis is thought to be partially triggered by excessive oxidant RBC injury, among other stressors, and is inhibited by EPO, which thus extends the life span of circulating RBCs (12). This apoptosis-like process is characterized by a cascade of biochemical and biomechanical changes, leading to cell shrinkage, dysregulation of normal membrane asymmetry with exposure of normally sequestered phosphatidylserine on the outer membrane leaflet, and the formation of membrane blebs and microparticles.…”

Erythrocyte: Programmed Cell Death

“…In a preliminary study, mature erythrocytes from patients with chronic renal insufficiency exhibited higher annexin binding when compared with red blood cells from healthy individuals. Moreover, the number of cells expressing phosphatidylserine externalization decreased after dialysis only when patients received erythropoietin immediately before dialysis (Myssina et al, 2003). Irrespective the erythropoietin mechanism it seems that the hormone does not only inhibit apoptosis of erythroid progenitor cells, but blunts the suicidal death of mature erythrocytes.…”

“…Later, other works were performed to elucidate mechanisms of action of erythropoietin upon mature erythrocytes. Myssina et al (2003) reported that erythropoietin inhibited cell death through a direct effect via erythropoietin receptor on mature erythrocytes. Unlike the general knowledge of the absence of erythropoietin receptors in reticulocytes, the authors detected the expression of about six erythropoietin binding sites per mature red blood cell.…”