The second H residue within the outer pore region of K_Ca2.3 conferred extra sensitivity to [H⁺]_o. (A) Ramp currents of wild-type K_Ca2.2 and the mutant K_Ca2.2(N368H) to mimic the outer pore sequence of K_Ca2.3 (in extracellular pH 7.4) were normalized to the maximum current at pH 7.4. The addition of extracellular pH 6.6 to wild-type K_Ca2.2 and the mutant K_Ca2.2(N368H) evoked no inhibition of wild-type current, but significant block of the mutant. (B) Mutation of the second H residue in K_Ca2.3 produced a channel current that was poorly sensitive to extracellular pH 6.6 when compared with the wild-type channel current. (C) The concentration–inhibition relationship for wild-type K_Ca2.2 and K_Ca2.3 currents, and mutants K_Ca2.2(N368H) and K_Ca2.3(H522N) currents by extracellular acidosis. Channel currents were measured at −60 mV and fitted with the Hill equation to yield the following: K_Ca2.2(N368H), IC₅₀ = 160 ± 33 nM and pIC₅₀ = 6.8; n = −2.13 ± 0.08 (reflecting the increased sensitivity to [H⁺]_o compared with K_Ca2.2, pIC₅₀ = 6.16); and K_Ca2.3(H522N), IC₅₀ = 688 ± 28 nM and pIC₅₀ = 6.43; n = 2.08 ± 0.16 fit (showing the reduced sensitivity to [H⁺]_o when compared with K_Ca2.3, pIC₅₀ = 6.8) (n = 3–5).