Kinetics of P2X7 receptor-operated single channels currents

Human P2X(7) receptors were expressed in Xenopus laevis oocytes and single channels were recorded using the patch-clamp technique in the outside- out configuration. ATP(4-) evoked two types of P2X(7) 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 similar to 5 and similar to 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 [ ATP(4-)]- dependent with EC50 values of similar to 0.3 mM and similar to 0.1 mM ATP(4-), respectively. The channel kinetics did not change significantly during sustained P2X(7) 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 [ ATP(4-)] 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 ATP(4-) binding steps at equal binding sites with a dissociation constant K-d of 139 mu M.