Peak current recovery as a function of extracellular Na⁺ concentration. (A) Top: Schematic rendering of the recording conditions indicating the concentrations of intracellular and extracellular substrate and cosubstrate ions and reaction scheme of a Na⁺-coupled transporter. Bottom: Original traces of representative experiments using 20, 70, or 140 mM extracellular Na⁺. (B–F) Time-dependent recovery of 5-HT–induced (10 µM) peak current amplitude after application of 30 µM PAL-287 (B; n = 5), 30 µM PAL-1046 (C; n = 5), 30 µM PAL-1045 (D; n = 5), 10 µM 5-HT (E; n = 5), and 30 µM p-chloroamphetamine (F; n = 5). Data are means ± SD. The data points were fitted by monoexponential functions in the case of 5-HT and the PAL substrates. A biexponential function was used to fit the recovery data of p-chloroamphetamine. The insets in B and C show a magnified view of the data points and fits near to the respective time constants, which are indicated by the dashed lines. (G) Relaxation rates were obtained by the fits from B–F. The rates shown for p-chloroamphetamine are the k_fast values obtained from the biexponential fit. The k_slow values are (±SEM) 10 mM Na⁺_e: 0.35 ± 0.23 s⁻¹; 70 mM Na⁺_e: 0.10 ± 0.12 s⁻¹; 140 mM Na⁺_e: 0.49 ± 0.15 s⁻¹. Data are means ± 95% confidence intervals of the fits. The points marked in blue were taken from a previously published study (Bhat et al., 2017). Statistical significance was determined by one-way ANOVA followed by multiple comparisons using the Fisher’s LSD method. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001; n.s., not significant.