HEK 293 coexpression of SLC26A9 with wtCFTR increases forskolin-stimulated chloride currents over cells expressing CFTR alone. (A) Coexpressers (gray) consistently had larger currents than CFTR alone (black) as assessed at four measurement points: the basal currents before stimulation (60 s), the transient peak in response to 10 µM forskolin (150 s on average), the current plateau in the continued presence of forskolin (240 s), and the poststimulation current observed 3 min after forskolin washout (420 s). The poststimulation current in the coexpressers did not diminish during extended recordings (extra 8 min, this case). Representative tracings of at least three experiments each are shown. I-V measurements were performed at the indicated intervals (solid diamonds). (B) Cells expressing CFTR alone displayed the classical linear I-V in response to forskolin, with minimal current before stimulation and after forskolin washout. Curves are for black trace shown in A. (C) Cells expressing SLC26A9 + CFTR also displayed a linear I-V in response to forskolin; however, the magnitudes were larger and included the constitutive and enhanced poststimulation currents (note scale doubled from B). Curves are for gray trace shown in A. (D) Summary of the responses for SLC26A9 and CFTR expressed alone and together. Representative tracings are shown in A and Fig. 2 A. Fixed measurement points are identified in A. Peak values were measured at the actual peak of each tracing, with average peak times occurring at 150 ± 7 s for SLC26A9 + wtCFTR coexpressors and 158 ± 6 s for wtCFTR alone. The value for SLC26A9 alone was measured at 150 s. The SLC26A9 + wtCFTR coexpressor currents exceed the sum of the wtCFTR and SLC26A9 individual currents during and after forskolin stimulation (see Results). All current values were normalized to cell capacitance; V_H = −40 mV. *, P < 0.05 compared with CFTR alone.