Lysosomal TMEM175 currents with luminal pH 4.0. Whole lysosomal patch recordings were from TMEM175-transfected HEK293T cells. I-Ψ relationships were obtained with a ramp protocol from −120 to +120 mV in 1 s, Ψ_h = 0 mV. Pipette and bath solutions had pH 4.0 and pH 7.2, respectively. (A and B) Currents are recorded with bath solutions containing 0 mM (NMDG-containing) or 150 mM K⁺ and pipette solutions containing 0 mM (A) or 50 mM (B) K⁺. For each pipette condition, representative traces recorded with NMDG-containing and 150 mM K⁺-containing bath from the same lysosome are shown on the left. Summaries of the inward (at −100 mV) and outward (at +100 mV) currents are on the right. Each circle is a response to the respective bath from a lysosomal patch, n = 5. (C) Average reversal potentials obtained with 150 mM K⁺-containing bath and pipette solutions containing 0 or 50 mM K⁺, n = 4 and 5, respectively. The corresponding calculated P_H/P_K for pH 4.0/NMDG and 150 mM K⁺ bath is ∼85 and P_H/P_K for pH 4.0/50K and 150 mM K⁺ bath is ∼63. Error bars are SEM.