Figure 5. Sodium dependence of 5-HT uptake.

A, initial velocity profile of 5-[³H]HT (0.1 μm) uptake as a function of [Na⁺]_o. External [Na⁺] was changed by isotonic replacement with Li⁺. A Na⁺-independent component of transport was evident at 0 mm [Na⁺]_o. Removing this Na⁺-independent component gives a velocity profile characterized with unitary Hill coefficient. B, relationship between 1/K_m,obs and [Na⁺]_o. The linearity of this relationship suggests that model 3 is most consistent with experimental findings. The K_m,obs values were attained from kinetic analyses (using 50 nm to 10 mm 5-[³H]HT) conducted with 100, 33.3 and 0 % of the physiological [Na⁺]_o level. Lineweaver-Burk analyses of the results of such kinetic analyses revealed invariant V_max,obs but changing K_m,obs as a function of [Na⁺]_o. Each point is the mean ±s.d. of six measurements.