(A) Initial ³⁶Cl⁻ influx as a function of concentration. Cells (sul1) were grown overnight in LCAPG and then incubated in fresh medium at either pH 4.0 or pH 7. Media were buffered with 5 mM K-citrate (initial pH 4.5, final pH 4) or 5 mM K-citrate, 10 mM bistris, 10 mM Tris (initial pH 7.6, final pH 6.8). The initial (1 min) influx of ³⁶Cl⁻ was measured at 30°C in LCAPG with the indicated concentration of total Cl⁻ (0.005 mM nonradioactive Cl⁻ plus various added amounts of ³⁶Cl⁻). The solid curves through the data are rectangular hyperbolae with K_1/2 = 0.018 mM, J_Max = 0.092 mEq/L cell water-min (pH 4), and K_1/2 = 0.30 mM, J_Max = 0.261 mEq/L cell water-min (pH 7). (B) Very slow efflux of ³⁶Cl⁻ following resuspension of ³⁶Cl⁻-loaded cells (sul1) in LCAPG medium. Overnight culture of sul1 cells in LCAPG was incubated in fresh LCAPG, 5 mM citrate, pH 4.5, and 8 μM ³⁶Cl⁻. After 2 h at 30°C, cells were centrifuged and resuspended in fresh medium containing no ³⁶Cl⁻ and incubated at 30°C. (C) Rapid efflux of ³⁶Cl⁻ following addition of extracellular Cl⁻. Overnight culture of sul1 cells in LCAPG was prepared and incubated as in B, except that 2 mM KCl was added at the arrow. Vertical axis is logarithmic; the curve through the data represents the sum of two exponentials with 70% of the tracer efflux with rate constant 0.41/min and the remaining 30% with rate constant 0.050/min.