K_Ca2.2 and K_Ca2.3 channel P(o) was not affected by extracellular [H⁺]_o. Nonstationary noise analysis of K_Ca2.2 and K_Ca2.3 channel currents in extracellular pH 7.4 or 5.5 was used to estimate channel P(o). (A) Representative traces from inside-out macropatches containing K_Ca2.2 or K_Ca2.3 channels in different external pHs held at −50 mV. Patches were bathed in 60 nM Ca²⁺, and current was activated by rapid application of 1 µM Ca²⁺ for 500 ms. (B) Plots of variance (σ²) against mean current (I) were fit with Eq. 2 to give estimates of single-channel amplitude (i), which were converted to conductance (γ) using γ = i/V. Channel P(o) was calculated from I_max/Ni. Plots are data reduced for clarity, and curve-fitting procedures were performed on full datasets. (C) Nonstationary noise analysis estimates for γ showed a significant reduction evoked by pH 5.5 for K_Ca2.2 (n = 3; P < 0.05) and K_Ca2.3 (n = 3; P < 0.03). (D) Neither K_Ca2.2 nor K_Ca2.3 channel P(o) was changed by extracellular pH 5.5.