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 pH_o, intracellular pH (pH_i) changes dramatically in both physiological and pathological conditions. Although it is known that a drop in pH_o activates the ASICs, it is not clear whether alterations of pH_i 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 pH_i. In cultured mouse cortical neurons, bath perfusion of the intracellular alkalizing agent quinine increased the amplitude of the ASIC current by ∼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 pH_i 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 pH_i induced an increase in the recovery rate of ASICs from desensitization. Consistent with its effect on the ASIC current, changing pH_i 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 pH _i may play an important role in determining the overall function of ASICs in both physiological and pathological conditions.