Figure 4.
pH modulates TRPM8 activation at an intracellular site. A, Although icilin could still evoke inward currents after a 2 min exposure to pHo 5.4, intracellular acidification reduced the current amplitude evoked by 1 μm icilin with an estimated half-maximal inhibition of pH 7.2 (n = 5-9). Inward currents evoked by 1 mm menthol were smaller than currents evoked by 1 μm icilin and not affected by intracellular pH (n = 12-16). B, The latency before icilin-evoked currents developed was increased by intracellular acidification. Data points are averages of cells responding within 120 sec (n = 5-9 cells, except for pHi 6.5, where the only two responding cells are included individually); three of five cells recorded at pHi 6.5 and two of seven cells recorded at pHi 6.9 failed to respond during 120 sec. C, Intracellular acidification did not affect the time course of icilin-evoked currents. The rise time (time from 10 to 90% of current developed) and the τ (inactivation) was similar at all pHi tested. Because only two cells responded at pHi 6.5, the individual data points are shown (n = 5-9 for the other pHi values). D, Representative recordings illustrating how pHi affects icilin-evoked currents. As shown in A and B, the current amplitude is depressed and the latency is increased by intracellular acidification. The box demonstrates that inward currents evoked by menthol were unaffected by pHi 6.5 (compare with Fig. 1 E). E, Extracellular acidification reduced the pHi in a time- and pH-dependent manner. Cells were incubated in pHo 7.3 (weakly buffered with 1 mm HEPES). After 22 sec, the pHo was changed from 7.3 to the indicated values by adding a three times larger volume of strongly buffered solution (10 mm HEPES or MES). Error bars have been omitted for clarity.