Kinetics of P2X7 Receptor-Operated Single Channels Currents

Human P2X7 receptors were expressed in Xenopus laevis oocytes and single channels were recorded using the patch-clamp technique in the outside-out configuration. ATP4− evoked two types of P2X7 receptor-mediated single channel currents characterized by short-lived and long-lived openings. The short- and long-lasting open states had mean open times of ∼5 and ∼20 ms and slope conductances near −60 mV of 9 and 13 pS, respectively. The open probabilities of the short and long openings were strongly [ATP4−]-dependent with EC50 values of ∼0.3 mM and ∼0.1 mM ATP4−, respectively. The channel kinetics did not change significantly during sustained P2X7 receptor activation for several minutes, as was also observed in recordings in the cell-attached patch-clamp configuration. Activation and deactivation of the short openings followed exponential time courses with time constants in the range of 20 ms, and displayed a shallow [ATP4−] dependence of the activation process. The kinetics of the short channel openings at negative membrane potentials fitted well to a linear C-C-C-O model with two ATP4− binding steps at equal binding sites with a dissociation constant Kd of 139 μM.