TRPM2 is an intracellular acidic pH sensor. (A) Effects of low pH_i on single-channel currents of TRPM2 obtained from inside-out patches. Single-channel currents were recorded at +80 mV, with the pipette solution containing 2 mM [Ca²⁺]_o (external solution) and the bath solution (internal solution) containing 100 nM [Ca²⁺]_i and 200 µM [ADPR]_i. Intracellular low pH blocked TRPM2 channel openings (+80 mV), and the effect was partially reversed by pH_i 8.0. (B–D) Single-channel currents recorded at the indicated pH_is. Current amplitude histograms illustrate the number of open channels (right). (B) At pH_i 7.2, there were >10 open channels. (C) At pH_i 6.5, the number of open channels decreased to two. Average current amplitude (6.13 ± 0.02 pA; n = 10) estimated a single-channel conductance of 76.6 pS. (D) Channel opening was almost eliminated at pH_i 5.5. Single-channel conductance estimated from the mean current amplitude (6.06 ± 0.02 pA; n = 10) was 75.7 pS. (E) Concentration-dependent effects of internal protons on TRPM2. Macroscopic currents of TRPM2 were obtained in inside-out patches. A best fit of the normalized current amplitude yielded an IC₅₀ of 6.7 ± 0.02 pH units (n_H = 3.0 ± 0.3; n = 10 for each indicated pH). Analysis using currents recorded at +80 mV or at −80 mV produced identical IC₅₀s.