Modulatory activity of extracellular H+ and Zn2+ on ATP-responses at rP2X(1) and rP2X(3) receptors

1 The modulatory activity of extracellular H+ and Zn2+ was examined on ATP- responses at rat P2X(1) (rP2X(1)) and rat P2X(3) (rP2X(3)) receptors expres sed in Xenopus oocytes and studied under voltage-clamp conditions. 2 Superfused ATP (0.03-30 mu M, at pH 7.5) evoked inward currents at rP2X(1 ) receptors (EC50 value, 300+/-7 nM). ATP potency was reduced 2 fold at pH 6.5, and 6 fold at pH 5.5, without altering the maximum ATP effect. Alkalin e conditions (pH 8.0) did not alter. ATP activity. 3 Superfused ATP (0.01-300 mu M, at pH 7.5) evoked inward currents at rP2X( 3) receptors (EC50 value. 1.8+/-0.3 mu M). ATP activity was affected only a t pH 5.5, reducing agonist potency 15 fold without altering the maximum ATP effect. 4 Extracellular Zn2+ inhibited ATP-responses at rP2X(1) receptors in a time -dependent manner, a 20 min pre-incubation being optimal (IC50 value, 1.0+/ -0.2 mu M). However, the Zn2+ effect was pH-independent, suggesting Zn2+- a nd H+-inhibition of ATP-responses occur through independent processes. 5 Extracellular Zn2+ weakly potentiated ATP-responses at rP2X(3) receptors (EC50, value, 11+/-1 mu M). The Zn2+ effect was dependent on pre-incubation time and, with 20 min pre-incubation periods, Zn2+ potentiated then inhibi ted ATP-responses in a concentration-dependent, but pH-independent, manner. 6 In summary, ATP activity at rP2X(1) receptors was decreased by both extra cellular H+ and Zn2+ and their effects were additive. ATP activity at rP2X( 3) receptors was less sensitive to H+-inhibition and, in contrast, was pote ntiated by Zn2+ in a pH-independent manner. These differential effects may help distinguish P2X(1), and P2X(3), receptors in whole tissues.