Internal 5-HT binding as a function of intracellular K⁺ concentrations. Scheme: Representation of the recording conditions indicating the concentrations of intracellular and extracellular substrate and cosubstrate ions. (A–C) Top: Original traces of representative experiments at intracellular K⁺ concentrations of 140 (A), 70 (B), and 35 mM (C). Bottom: Corresponding quantification of steady-state current amplitudes induced by 10 µM 5-HT. Data are means ± SD (n = 5–11). The solid lines were drawn by fitting the data to a rectangular hyperbola.