Figure 6. Chloride dependence of 5-HT uptake.

A, initial velocity profile of 5-[³H]HT (0.1 μm) uptake as a function of [Cl⁻]_o. External [Cl⁻] was changed by isotonic replacement with isethionate. No Cl⁻-independent component of transport was evident, and the velocity profile was characterized with a unitary Hill coefficient. B, relationship between 1/K_m,obs and [Cl⁻]_o. The hyperbolic nature of this relationship and its intersection with the origin suggests that model 2 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, 37.9 and 6.8 % of the physiological [Cl⁻]_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 [Cl⁻]_o. Each point is the mean ±s.d. of six analyses.