Red blood cells (RBCs) contain appreciable concentrations of ATP generated via a glycolytic pathway. In response to a number of stimuli, including reductions in oxygen tension as well as mechanical deformation, human RBCs release ATP in amounts adequate for activation of endothelial P2y purinergic receptors. We suggested a hypothesis that the stress-induced release of ATP from RBCs has, an autocrine effect on RBCs by acting on the purinergic receptors of erythrocytes, thereby changing their deformability.
We have examined this hypothesis by analyzing the effects of external ATP and ADP on RBC deformability through measurements spontaneous cell membrane fluctuations (CMF) based on Point Dark Field Microscopy. The measurement of CMF was carried at four points along the periphery of 8-10 RBCs in each experiment. RBCs were perfused with extracellular ATP and ADP in the concentration range of 1-1000 µM. Incubation of RBCs with 100 µM ATP led to immediate shape changes. These shape changes involved transformation from the normal discocyte shaple into a stomatocyte one. A similar phenomenon was observed also following incubation with ADP at a ten fold higher concentration of 1 mM. All the observed shape changes were irrevesible. At lower concentrations no major shape change was observed. It may be suggested that these effects are mediated via a specific class of plasma membrane receptors called purinergic P2 receptors which include two subfamilies: G protein-coupled (P2Y) and ligand-gated ion channels (P2X). Once in the pericellular environment, ATP can serve as a ligand for P2 receptors or be quickly hydrolyzed by powerful ubiquitous ecto-ATPases and ectonucleotidases. The existence of of P2 receptors in erythrocytes was previousely shown. It may be suggested that monovalent ion fluxes (Na+ and K+ fluxes) underly the observed shape chnages in view of a recent study suggesting that ATP-induced efflux and influx of 86Rb+ and 22Na+ were mediated by the P2X7 receptor. These ADP and ATP induced transformation into echynocytes were accompanied by attenuation of CMF, reflecting decrease of RBC deformability. At a lower ATP concentration of 10 µM the level of CMF increased by 23% (p=0.03). These findings demonstrate that ATP, at specific range of concentrations is a modulator of RBC deformability.
