Figure 6.

Pendred thyrocytes quickly reach equilibrium for iodide uptake and progressively accumulate intracellular iodide. A–D, Intracellular iodide uptake by thyrocytes incubated with different NaI concentrations, at different times of incubation. Although at short incubation times (30 sec, A), the curves were similar for both lines, at 5 min (B), T-PS2 thyrocytes reached equilibrium with K_m = 22 ± 4.8 μm, similar to the Michaelis-Menten constant at equilibrium for iodide uptake by NIS (K_m around 30–40 μm). In contrast, NT showed K_m = 81 ± 4.2 μm, far above equilibrium. Similar behavior was maintained at 30 min (C), and at 1 h (D), NT thyrocytes finally reached equilibrium (K_m = 40.3 ± 4.4 μm). Except at very short incubation times (30 sec), when Vmax was higher in NT cells, Vmax was about twice as high in T-PS2 thyrocytes at any given time. This result suggests that initially only the NIS transporter is working in NT, as in T-PS2, but that after a few seconds pendrin starts to work in NT, and equilibrium is reached later.