Proton Potentiation of ATP-Gated Ion Channel Responses to ATP and Zn 2+ in Rat Nodose Ganglion Neurons. LI, CHAOYING, ROBERT W. PEOPLES, AND FORREST F. WEIGHT. Laboratory of Molecular and Cellular Neurobiology, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Bethesda, Maryland 20892-8205.
APStracts 3:0149N, 1996.
1. The modulation by protons of ATP-gated ion channel responses to ATP and Zn 2+ was studied in freshly-isolated rat nodose ganglion neurons using the whole-cell patch-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 p K 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 pH 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 Zn 2+ for enhancement of ATP-activated current without altering its maximal response. Changing the pH from 7.3 to 6.8 changed the Zn 2+ 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 Zn 2+ , as the Zn 2+ chelator N,N,N',N' -tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) did not alter the effect of protons. 7. Protons and Zn 2+ do not appear to act at the same site on ATP-gated channels, as responses to maximally- effective concentrations of Zn 2+ were enhanced further by protons, and vice- versa. 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 Zn 2+ on these receptor-channels.

Received 10 April 1996; accepted in final form 24 June 1996.
APS Manuscript Number J295-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 25 July 1996