Quantifying Ca2+ Current and Permeability in ATP-gated P2X7 Receptors

ATP-gated P2X7 receptors are prominently expressed in inflammatory cells and play a key role in the immune response. A major consequence of receptor activation is the regulated influx of Ca²⁺ through the self-contained cation non-selective channel. Although the physiological importance of the resulting rise in intracellular Ca²⁺ is universally acknowledged, the biophysics of the Ca²⁺ flux responsible for the effects are poorly understood, largely because traditional methods of measuring Ca²⁺ permeability are difficult to apply to P2X7 receptors. Here we use an alternative approach, called dye-overload patch-clamp photometry, to quantify the agonist-gated Ca²⁺ flux of recombinant P2X7 receptors of dog, guinea pig, human, monkey, mouse, rat, and zebrafish. We find that the magnitude of the Ca²⁺ component of the ATP-gated current depends on the species of origin, the splice variant, and the concentration of the purinergic agonist. We also measured a significant contribution of Ca²⁺ to the agonist-gated current of the native P2X7Rs of mouse and human immune cells. Our results provide cross-species quantitative measures of the Ca²⁺ current of the P2X7 receptor for the first time, and suggest that the cytoplasmic N terminus plays a meaningful role in regulating the flow of Ca²⁺ through the channel.