Quantitative Measure of Ca2+ Current and Permeability in ATP-Gated P2X7 Receptors

Abstract

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+. Although the physiological importance of the resulting rise in [Ca2+]i is universally acknowledged, the biophysics of the Ca2+ current responsible for the effect are poorly understood, largely because traditional methods of measuring Ca2+ permeability cannot be applied to P2X7 receptors. Here we use an alternative approach, called patch-clamp photometry, to quantify the agonist-gated Ca2+ current of recombinant P2X7 receptors of dog, guinea-pig, human, monkey, mouse, rat, and zebrafish. We find that the magnitude of the Ca2+ current depends on the species of origin, the splice variant, and the concentration of the purinergic agonist. Surprisingly, murine P2X7kRs transduce smaller Ca2+ currents than do their P2X7aR counterparts, despite having identical pore-forming domains. We also measured a significant contribution of Ca2+ to the agonist-gated current of the native P2X7Rs of mouse and human macrophages. Our results provide cross-species quantitative measures of the Ca2+ currents of P2X7 receptors for the first time, and suggest that the cytoplasmic domains play a meaningful role in regulating the flow of Ca2+ through the channel.