Canine erythrocytes express the P2X7 receptor:: greatly increased function compared with human erythrocytes

Over three decades ago, Parker and Snow ( Am J Physiol 223: 888 - 893, 1972) demonstrated that canine erythrocytes undergo an increase in cation permeability when incubated with extracellular ATP. In this study we examined the expression and function of the channel/ pore- forming P2X(7) receptor on canine erythrocytes. P2X(7) receptors were detected on canine erythrocytes by immunocytochemistry and immunoblotting. Extracellular ATP induced Rb-86(+) ( K+) efflux from canine erythrocytes that was 20 times greater than that from human erythrocytes. The P2X(7) agonist 2'( 3')- O-( 4- benzoylbenzoyl) adenosine 5'- trisphosphate ( BzATP) was more potent than ATP, and both stimulated Rb-86(+) efflux from erythrocytes in a dose- dependent fashion with EC50 values of similar to 7 and similar to 309 mu M, respectively. 2- Methylthioadenosine 5'- triphosphate and adenosine 5'- O-( 3- thiotriphosphate) induced a smaller Rb-86(+) efflux from erythrocytes, whereas ADP, AMP, UTP, or adenosine had no effect. ATP- induced Rb-86(+) efflux from erythrocytes was inhibited by oxidized ATP, KN-62, and Brilliant blue G, known P2X(7) antagonists. ATP also induced uptake of choline(+) into canine erythrocytes that was 60 times greater than that into human erythrocytes. Overnight incubation of canine erythrocytes with ATP and BzATP induced phosphatidylserine exposure in > 80% of cells and caused up to 20% hemolysis. In contrast, < 30% of human erythrocytes showed phosphatidylserine exposure after overnight incubation with ATP and BzATP, and hemolysis was negligible. Flow cytometric measurements of ATP- induced ethidium(+) uptake showed that P2X(7) function was three times lower in canine monocytes than in human monocytes. These data show that the massive cation permeability increase induced by extracellular ATP in canine erythrocytes results from activation and opening of the P2X(7) receptor channel/ pore.