Effects of internal protons on [Ca²⁺]_i- and [ADPR]_i-induced TRPM2 gating. (A) Concentration-dependent effects of internal protons on TRPM2 at various concentrations of internal [Ca²⁺]_i. TRPM2 currents were elicited by voltage ramps (−100 to +100 mV; HP = 0 mV) under inside-out configuration, and the macroscopic current amplitude was measured at +100 mV. The intracellular solution (bath solution) contained 200 µM ADPR with 100 µM or 100 nM Ca²⁺; the external solution (pipette solution) was normal Tyrode's solution containing 2 mM Ca²⁺. Best fit of normalized current yielded an IC₅₀ of 6.7 ± 0.02 (n = 10) at 100 nM [Ca²⁺]_i and 6.3 ± 0.02 (n = 11; P < 0.01) at 100 uM [Ca²⁺]_i. (B) Effects of internal low pH_i on [ADPR]_i-induced TRPM2 gating. Macroscopic currents were recorded using inside-out patches by voltage ramps (−100 to +100 mV; HP = 0 mV) in the nominal Ca²⁺-free (∼10 µM) internal solutions at pH_i 7.2 or 6.7 with various concentrations of ADPR. The EC₅₀s of ADPR were 2.7 ± 0.9 µM (n = 5) at pH_i 7.2 and 154.2 ± 27 µM at pH_i 6.7 (n = 5).