Figure 5. The amplitude of acid-evoked Ca²⁺ responses varies with [HOAc] but not with [H⁺] (i.e. pH).

Acetic acid solutions of varying pH and acid concentration were focally applied to the taste pore in the lingual slice preparation. Each data point is the mean ±s.e.m. (n = 3–22). A, responses evoked by acetic acid solutions of a constant [H⁺] (pH 5) but varying [HOAc] from 3 to 100 mm. B, responses obtained when [HOAc] was maintained at 64 mm but [H⁺] was varied from pH 5–3. (Note that this situation contrasts with the experiment shown in Fig. 4. In Fig. 4 the proportion of the protonated species in the stimulus, HOAc, was not maintained at a constant value but allowed to increase as the pH dropped, as per Henderson–Hasselbalch relations, see Methods.) The deviation of the slope from zero in B is not significant (P < 0.23) (r²= 0.20). These data indicate that acid taste responses vary with the concentration of the membrane-permeant acid (HOAc) but not with the membrane-impermeant proton (H⁺), supporting the interpretation that intracellular not extracellular acidification evokes sour taste.