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

1. Whole-cell recordings were made from HEK 293 (human embryonic kidney) cells stalely transfected with cDNAs encoding P2X(2), P2X(3) or both receptors (P2X(2/3)) and from cultured rat nodose neurones. Nodose neurones all showed immunoreactivity for both P2X(2) and P2X(3), but not P2X(1), receptors. 2. 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 (P-NMDG/P-Na and P-Ca/P-Na). P-NMDG/P-Na was not different for P2X(2), P2X(2/3) and nodose neurones (0.03) but was significantly higher (0.07) for P2X(3) receptors. P-Ca/P-Na was not different among P2X(3), P2X(2/3) and nodose neurones (1.2-1.5) but was significantly higher (2.5) for P2X(2) receptors. 3. External Ca2+ inhibited purinoceptor currents with half-maximal concentrations of 5 mM at the P2X(2) receptor, 89 mM at the P2X(3) receptor ana 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. 4. These results may indicate that Ca2+ permeability of the heteromeric (P2X(2/3)) channel is dominated by that of the P2X(3) subunit, while Ca2+ block of the receptor involves both P2X(2) and P2X(3) subunits. The correspondence in properties between P2X(2/3) receptors and nodose ganglion neurones further supports the conclusion that the native alpha,beta-metllylene ATP-sensitive receptor is a P2X(2/3) heteromultimer.