Species- and agonist-dependent differences in the deactivation-kinetics ofP2X(7) receptors

In this study we have expressed recombinant P2X(7) receptors in HEK293 cell s and examined the reasons for the species- and agonist-dependent differenc es in the time taken for the closure of the P2X(7) receptor ion-channels af ter agonist removal. Channel closure times, measured in electrophysiological studies or by measu ring cellular permeability to ethidium cations, were slower at rat than at human or mouse P2X(7) channels following washout of the P2X(7) agonist 2'- and 3'-O-(4-benzoylbenzoyl)-ATP (BzATP). In contrast, there were no species differences in channel closure times when ATP was the agonist. BzATP was m ore potent than ATP at the three species homologues and exhibited highest p otency for rat P2X(7) receptors suggesting that channel closure time was re lated to agonist potency. Furthermore, BzATP potency for the P2X(7) recepto r could be modified by changing extracellular ionic concentrations or by mu tating the receptor and modifications which increased agonist potency also increased the time taken for channel closure. The dependence of channel closure time on agonist potency suggests it refle cts agonist dissociation from the P2X(7) receptor rather being an intrinsic property of the ion-channel. Consistent with this, our previous studies ha ve shown that agonist potency increases after repeated agonist applications and in this study channel closure time at rat P2X(7) receptors increased a fter repeated agonist applications. Overall these results suggest that the species differences in channel closure times reflect differences in agonist dissociation rates which arise as a consequence of the marked species diff erences in agonist potency.