pHi modulates the MgATP dependence of CFTR channel gating. A, relationship between [MgATP] and Po of wild-type CFTR at the indicated pHi values. The continuous lines are Michaelis-Menten fits to mean data. B and C, effects of pHi on the Po of wild-type (WT), D572N-, and D1370N-CFTR in the presence of ATP (3 mm in B or 1 mm in C). In B, wild-type CFTR data were acquired in the presence (circles) and absence (columns) of Mg2+ (3 mm), whereas in C, wild-type, D572N-, and D1370N-CFTR data were acquired in the continuous presence of Mg2+ (3 mm). All data are means ± S.E. (n = 5–7); asterisks indicate values that are significantly different from the control value (p < 0.05).