Modulation of acid-sensing ion channel currents, acid-induced increase of intracellular Ca2+, and acidosis-mediated neuronal injury by intracellular pH

Acid-sensing ion channels (ASICs), activated by lowering extracellular pH (pH(o)), play an important role in normal synaptic transmission in brain and in the pathology of brain ischemia. Like pHo, intracellular pH (pH(i)) changes dramatically in both physiological and pathological conditions. Although it is known that a drop in pHo activates the ASICs, it is not clear whether alterations of pHi have an effect on these channels. Here we demonstrate that the overall activities of ASICs, including channel activation, inactivation, and recovery from desensitization, are tightly regulated by pHi. In cultured mouse cortical neurons, bath perfusion of the intracellular alkalizing agent quinine increased the amplitude of the ASIC current by similar to 50%. In contrast, intracellular acidification by withdrawal of NH4Cl or perfusion of propionate inhibited the current. Increasing pH buffering capacity in the pipette solution with 40 mM HEPES attenuated the effects of quinine and NH4Cl. The effects of intracellular alkalizing/ acidifying agents were mimicked by using intracellular solutions with pH directly buffered at high/low values. Increasing pHi induced a shift in H+ dose-response curve toward less acidic pH but a shift in the steady state inactivation curve toward more acidic pH. In addition, alkalizing pHi induced an increase in the recovery rate of ASICs from desensitization. Consistent with its effect on the ASIC current, changing pHi has a significant influence on the acid-induced increase of intracellular Ca2+, membrane depolarization, and acidosis-mediated neuronal injury. Our findings suggest that changes in pHi may play an important role in determining the overall function of ASICs in both physiological and pathological conditions.