CALCIUM PERMEABILITY AND BLOCK AT HOMOMERIC AND HETEROMERIC P2X(2) AND P2X(3) RECEPTORS, AND P2X RECEPTORS IN RAT NODOSE NEURONS

1. Whole-cell recordings were made from HEK 293 (human embryonic kidne y) cells stalely transfected with cDNAs encoding P2X(2), P2X(3) or bot h receptors (P2X(2/3)) and from cultured rat nodose neurones. Nodose n eurones all showed immunoreactivity for both P2X(2) and P2X(3), but no t P2X(1), receptors. 2. Reversal potentials were measured in extracell ular 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) an d 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 no dose 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 P2 X(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 res ults 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 correspo ndence in properties between P2X(2/3) receptors and nodose ganglion ne urones further supports the conclusion that the native alpha,beta-metl lylene ATP-sensitive receptor is a P2X(2/3) heteromultimer.