TASK channels are members of the two-pore domain K⁺ channel family, and are sensitive to the extracellular pH. In TASK1 and -3, the amino acid responsible for pH sensing has been reported as histidine 98 (Kim et al. 2000; O’Connell et al. 2001). In TASK2, however, the equivalent residue is an asparagine (N103), perhaps explaining its lowered pH sensitivity. If this residue were able to play a similar role in pH sensing in TASK2, mutation to histidine should result in increased pH sensitivity.

Capped cRNA encoding wild-type (WT, 50 ng), or N103H (100 ng) was injected into stage V or VI Xenopus oocytes. Currents were measured using two-electrode voltage clamp. Bath solutions contained (mM): 100 KCl, 1 MgCl₂, 1 CaCl₂, 5 Hepes and 5 Pipes, titrated to between pH 4 and 10 with KOH or HCl. Cells were held at 0 mV and test potentials from +20 to -100 mV, in 20 mV steps, of 400 ms duration applied. The interpulse interval was 2 s. TASK2 currents were independent of clamp voltage, and the values given were determined from currents at -100 mV. Results are given as means ± S.E.M. and statistical analysis was performed with ANOVA, with significance assumed at the 5 % level.

WT currents were maximally inhibited at pH 5 and maximally stimulated at pH 10. The dissociation constant (K_D) for WT was 5.76 ± 1.23 nM H⁺ (pH 8.3; n = 7). The K_D for N103H was 11.3 ± 3.13 nM H⁺ (pH 8.0; n = 7). These values were not significantly different from each other. However, there was a significant difference in the Hill coefficients: 1.00 ± 0.05 for WT (n = 7) and 0.67 ± 0.03 for N103H (n = 7).

Thus the N103H mutant is less sensitive to pH than WT-TASK2, which contrasts with the increased sensitivity of such an amino acid configuration in TASK1 and -3. We conclude that the pH-sensing mechanism in TASK2 is fundamentally different from that of TASK1 and -3.

The financial support of the MRC is gratefully acknowledged. The TASK2 clone was kindly supplied by Dr H.J. Meadows and Dr C.D. Benham of SmithKline Beecham Pharmaceuticals.