Calcium permeability and block at homomeric and heteromeric P2X2 and P2X3 receptors and P2X receptors in rat nodose neurones

摘要

class="enumerated" style="list-style-type:decimal">Whole-cell recordings were made from HEK 293 (human embryonic kidney) cells stably transfected with cDNAs encoding P2X2, P2X3 or both receptors (P2X2/3) and from cultured rat nodose neurones. Nodose neurones all showed immunoreactivity for both P2X2 and P2X3, but not P2X1, receptors.Reversal potentials were measured in extracellular sodium, N-methyl-D-glucamine (NMDG) and NMDG containing 5 mM Ca²⁺; the values were used to compute relative permeabilities (PNMDG/PNa and PCa/PNa). PNMDG/PNa was not different for P2X2, P2X2/3 and nodose neurones (0.03) but was significantly higher (0.07) for P2X3 receptors. PCa/P_Na was not different among P2X₃, P2X_2/3 and nodose neurones (1.2-1.5) but was significantly higher (2.5) for P2X₂ receptors.External Ca²⁺ inhibited purinoceptor currents with half-maximal concentrations of 5 mM at the P2X₂ receptor, 89 mM at the P2X₃ receptor and 15 mM at both the P2X_2/3 heteromeric receptor and nodose neurones. In each case, the inhibition was voltage independent and was overcome by increasing concentrations of agonist.These results may indicate that Ca²⁺ permeability of the heteromeric (P2X_2/3) channel is dominated by that of the P2X₃ subunit, while Ca²⁺ block of the receptor involves both P2X₂ and P2X₃ subunits. The correspondence in properties between P2X_2/3 receptors and nodose ganglion neurones further supports the conclusion that the native α,β-methylene ATP-sensitive receptor is a P2X_2/3 heteromultimer.