Proton potentiation of ATP-gated ion channel responses to ATP and Zn2+ in rat nodose ganglion neurons

1. The modulation by protons of ATP-gated ion channel responses to ATP and Zn2+ was studied in freshly isolated rat nodose ganglion neurons using the whole cell parch-clamp technique. 2. Reduced external pH enhanced, whereas elevated external pH suppressed, current activated by 10 mu M ATP. The pH producing the half-maximal effect (EC(50)) at this ATP concentration was 7.1. 3. Acidification shifted the ATP concentration-response curve to the left, decreasing the EC(50) for ATP, and alkalinization shifted the ATP concentration-response curve to the right, increasing the EC(50) for ATP. Fitting the data to a single-site pH model yielded an apparent pk(a) of the site on the ATP-gated ion channel of 7.6. Between PH 6.8 and 7.8, a change of 0.1 DH unit was calculated to change the ATP EC(50) by 4.03 mu M. Changing pH did not alter the maximal response to ATP. 4. The potentiating effect of protons appeared to be due to a direct action on the ATP-gated channel, as it could not be explained by an increase in the concentration of one or more species of ATP. 5. Lowering PH also increased the potency of Zn2+ for enhancement of ATP-activated current without altering its maximal response. Changing the pH from 7.3 to 6.8 changed the Zn2+ EC(50) from 12 to 1.7 mu M. 6. The potentiation of ATP-activated current by protons could not be attributed solely to an increase in the affinity of the receptor for Zn2+, as the Zn2+ chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine did not alter the effect of protons. 7. Protons and Zn2+ do not appear to act at the same site on ATP-gated channels. as responses to maximally effective concentrations of Zn2+ were enhanced further by protons and vice verse. 8. These results suggest that protons regulate the function of P2X purinoceptors in rat nodose ganglion neurons by modulating the affinity of the binding sites for ATP and Zn2+ on these receptor channels.