Apparent species differences in the kinetic properties of P2X7 receptors

Abstract

1. Apparent species differences in the responses of recombinant P2X7 receptors to repeated application of 2'- and 3'-O-(4-benzoylbenzoyl)-ATP (BzATP) have been investigated. 2. Repeated application of 100 μM BzATP resulted in a progressive increase in current magnitude (current growth) at mouse and human, but not rat P2X7 receptors. 3. Current growth was thought to reflect progressive dilation of the P2X7 ion-channel to a pore permeable to large molecules (MW < 900), suggesting that channel dilation was not occurring at the rat P2X7 receptor. However, 100 μM BzATP produced a rapid influx of YO-PRO-1 (MW375) in cells expressing rat or human P2X7 receptors. 4. There were, however, species differences in agonist potency such that 100 μM BzATP was a supra-maximal concentration at rat, but not human or mouse, P2X7 receptors. Importantly, when sub-maximal concentrations of BzATP or ATP were examined, current growth occurred at rat P2X7 receptors. 5. The rate of current growth and YO-PRO-1 accumulation increased with agonist concentration and appeared more rapid at rat and human, than at mouse P2X7 receptors. 6. The potency of BzATP and ATP was 1.5-10 fold lower in naive cells than in cells repeatedly exposed to ATP. 7. This study demonstrates that current growth occurs at mouse, rat and human P2X7 receptors but only when using sub-maximal concentrations of agonist. Previously, current growth was thought to reflect the progressive increase in pore diameter of the P2X7 receptor ion channel, however, the results of this study suggest a progressive increase in agonist potency may also contribute.