Figure 3.

TRPA1 channels are blocked by extracellular protons. A, Whole-cell currents in HEK-293 cells expressing TRPA1 were activated by CO₂ in HEPES-buffered saline, pH 5.3, but not by extracellular acidification alone, pH 5.3. The small inward current in response to the pH 5.3 solution represents the endogenous ASIC current. Both the internal and external solution were Ca²⁺-free. B, Average peak outward current (at 80 mV) from experiments as in A. Data are represented by the mean ± SEM. **p < 0.01 (two-tailed Student's t test). C, TRPA1 currents activated by Cin (100 μm) were blocked in a dose-dependent manner by extracellular acidification. Ca²⁺ was omitted from the extracellular solution to prevent inactivation. D, Dose dependence of proton block of TRPA1 currents from experiments as shown in A (n = 7). For currents at V _m = 80 mV, the data were fit was with a Hill equation with V _max = 1, K_1/2 = 140 μm, n = 0.62. For currents at V _m = −80 mV, the data were fit with V _max = 1, K_1/2 = 8.04 μm, n = 0.91.