Inhibition of erythrocyte "apoptosis" by catecholamines

Osmotic shock, oxidative stress and Cl- removal activate a non-selective Ca2+-permeable cation conductance in human erythrocytes. The entry of Ca2+ leads to activation of a scramblase with subsequent exposure of phosphatidylserine at the cell surface. Phosphatidylserine mediates binding to phosphatidylserine receptors on macrophages which engulf and degrade phosphatidylserine exposing cells. Moreover, phosphatidylserine exposure may lead to adherence of erythrocytes to the vascular wall. In the present study, we explored whether activation of the non-selective cation conductance and subsequent phosphatidylserine exposure might be influenced by catecholamines. Phosphatidylserine exposure has been determined by FITC-annexin V binding while cell volume was estimated from forward scatter in FACS analysis. Removal of Cl- enhanced annexin binding and decreased forward scatter, an effect significantly blunted by the beta agonist isoproterenol (IC50 approx. 1 microM). Fluo-3 fluorescence measurements revealed an increase of cytosolic Ca2+ activity following Cl- removal, an effect again significantly blunted by isoproterenol exposure (10 microM). Whole-cell patch-clamp experiments performed in Cl- free bath solution indeed disclosed a time-dependent inactivation of a non-selective cation conductance following isoproterenol exposure (10 microM). Phenylephrine (IC50<10 microM), dobutamine (IC50 approx. 1 microM) and dopamine (IC50 approx. 3 microM) similarly inhibited the effect of Cl- removal on annexin binding and forward scatter. In conclusion, several catecholamines inhibit the Cl- removal-activated Ca2+ entry into erythrocytes, thus preventing increase of cytosolic Ca2+ activity, subsequent cell shrinkage and activation of erythrocyte scramblase. The catecholamines thus counteract erythrocyte phosphatidylserine exposure and subsequent clearance of erythrocytes from circulating blood.