The G551D mutation alters the time course of current decay upon washout of ATP. (A and B) Biphasic changes of macroscopic currents upon ATP removal in the presence of VX-770 for G551D-CFTR (A) and WT-CFTR (B). (A) In the continuous presence of VX-770, addition of PKA and ATP slowly activated macroscopic G551D-CFTR currents to a steady-state. Subsequent removal of PKA and ATP results in a biphasic change of the current. The red line represents curve fit using a single exponential function. The inset shows expanded time course of current decrease upon the addition of Inh-172 (green box). (B) A real-time current trace showing different results with WT-CFTR under the same experimental protocol as in A. The red line marks curve fit using a double exponential function. (C) The current decay phase upon ATP removal in a patch containing fewer WT-CFTR. The red-circled areas are expanded as indicated.