Mechanisms Involved in an Acidic pH‐conditioned NOXmediated Chloride Conductance in Nodose Sensory Neurons

Abstract

We recently reported a novel prolonged low pH‐conditioned current in nodose sensory neurons following brief exposures to acidic extracellular pH. This current is similar to the Swell‐ Activated Chloride current, and mediated by NADPH oxidase (NOX) dependent hydrogen peroxide. Since NOX is known to be activated by intracellular (IC) alkaline pH, we hypothesized that low extracellular pH conditioning activates NOX by inducing IC alkalinity. We first buffered the IC pipette solution with 40mM HEPES, and found that the pH‐conditioned current was inhibited from 11.8±2.0 pA/pF to 3.4±0.7 pA/pF (n=6, p<0.01), indicating that an IC pH change is required for the current activation. The pH sensitive fluorescence dye 2′‐7′‐bis(carboxyethyl)‐5(6)‐ carboxyfluorescein showed significant but transient decreases in IC pH during the brief exposures to extracellular low pH (n=12). However, sustained reductions of IC pH to 7.0 and 6.0 did not elicit the current. Instead, increases in IC pH to 7.5, 7.6 and 8.0 dose‐dependently induced a similar current averaging 9.2±1.8, 16.3±4.1 and 30.6±9.7 pA/pF. Finally, inhibition of proton extrusion by blocking the Na+‐H+ exchanger with amiloride and proton channels with Zn2+ inhibited the low pH‐conditioned current (n=6). We therefore conclude that transient extracellular pH reductions can be transmitted intracellularly and activate Na+‐ H+ exchanger and proton channels. Our data suggest that the resulting activation of NOX and increase in chloride conductance involve IC alkalinization (NIH HL 14388).