Dependency of c-NKCC2-mediated Tl+ influx on external Cl- in c-NKCC2-expressing LLC-PK1 cells. A) Tl+ influx assays were conducted in assay buffer with varying concentrations of Cl-, in which Cl- was replaced by gluconate. Representative raw signal traces after 3, 30, 70, 135 mM Cl- addition were reported. B) The initial rate of Tl+ influx at the different Cl- concentrations was calculated and plotted against 3, 10, 30, 70, 100, 135 mM [Cl-]o. Values are means ± SE of 3 independent experiments and the curve represents the best fit of data to a model of activation at a single site. Michaelis-Menten constant Km was deduced.