Extracellular acidosis reduced single K_Ca2.2 and K_Ca2.3 channel conductance. (A, i) Single K_Ca2.2 channel activity at extracellular pH 7.4 in the presence of 200 nM of intracellular Ca²⁺ at −50, −70, and −90 mV. (Left) Current–voltage relationship showing that K_Ca2.2 channels exhibited two conductance levels of ∼15 and 11 pS. (A, ii) Amplitude histogram of channel openings at −50 mV, with mean amplitudes of 0.58 and 0.39 pA. (B, i) Acidification of the pipette (external) solution to pH 5.9 resulted in lower conductance openings. (Right) K_Ca2.2 channels exhibited only one conductance level in pH 5.9 of ∼9.63 pS. (B, ii) Amplitude histogram of channel openings at −50 mV, with a mean amplitude of 0.45 pA. (C, i) Single K_Ca2.2(H337N) channel openings at extracellular pH 7.4 of low amplitude. (Right) Current–voltage relationship showing that the mutant channel exhibited only low conductance openings of 10.7 pS. (C, ii) Amplitude histogram of channel openings at −50 mV, with a mean amplitude of 0.47 pA. (D, i) Single K_Ca2.3 channel openings at extracellular pH 7.4 activated by 200 nM of intracellular Ca²⁺ at −50, −70, and −90 mV. (Left) K_Ca2.3 channels exhibited two conductance levels of ∼16 and 8 pS. (D, ii) Amplitude histogram of channel openings at −50 mV, with mean amplitudes of 0.68 and 0.41 pA. (E, i) K_Ca2.3 channels only exhibited low conductance openings in pH 6.6. (Right) Current–voltage relationship showing that conductance was ∼6.5 pS in extracellular pH 6.6. (E, ii) Amplitude histogram of channel openings at −50 mV, with a mean amplitude of 0.37 pA.