Kinetic and Pharmacological Properties of P2X3 and P2X2/3 Receptors

Abstract

P2X receptors are ligand-gated ion channels activated by extracellular ATP. They are permeable to small monovalent cations, some having significant divalent or anion permeability. The P2X2 and P2X3 subunits are predominantly expressed in primary sensory neurons and have been proposed to play a role in thermal sensation, taste and pain. They form functional hetero- or homotrimers which are activated by αβ-methylene ATP (αβmeATP). The stoichiometry of P2X2/3 heteromers appears to be dependent on the relative abundance of the two subunits. A mixture of P2X2 and P2X3 homomers as well as P2X2/3 heteromers are likely to exist, which can be distinguished through their biophysical and pharmacological properties. The receptors open in response to an increase in extracellular ATP which occurs under pathological conditions such as tissue damage. The resulting depolarization leads to propagation of the pain signal. Due to its role in nociception and pain signaling, these receptors are important targets for pain management. Here we present data collected on automated patch clamp (APC) systems showing activation and inhibition of P2X2/3 and P2X3 receptors expressed in CHO cells with rapid and brief application of ligand. αβmeATP or ATP activated P2X2/3 receptors in a concentration-dependent manner with an EC50 of 8.3 µM (n = 190) and 1.1 µM (n = 191), respectively. The kinetics of currents mediated by homomeric P2X3 receptors were distinctly faster than those of heteromeric P2X2/3 receptors. Currents mediated by P2X3 receptors were activated by ATP and αβmeATP in a concentration-dependent manner with an EC50 of 2.0 µM (n = 372) and 1.4 µM (n = 311). Suramin blocked homomeric P2X2 or heteromeric P2X2/3 receptors with an IC50 of approximately 17 µM. The compound A-317491 also blocked P2X3-mediated currents activated by αβmeATP with an IC50 of 115 nM (n = 318).