Fig. 5: TadNaC2 conducts biphasic currents in vitro that are relatively insensitive to Ca²⁺ block.

a Sample current recordings of the TadNaC2 channel at pH 4 and 3.5, revealing a biphasic current with a fast transient component (i.e., early current), and a slower, sustained (late current) component. The biphasic current becomes more evident at pH 3.5. b Sample proton-activated TadNaC2 currents recorded at different voltages (voltage protocol on top), under bi-ionic conditions of equimolar intracellular Na⁺ and extracellular Na⁺ (Na⁺ ext.) or K⁺ (K⁺ ext.). The star and square symbols denote regions of the currents that were measured to determine reversal potentials. c Plot of average reversal potential data (± standard deviation) for the bi-ionic reversal potential experiments, revealing a leftward shift for both the early and late current components of bimodal currents in the presence of external K⁺ (n = 6–7) compared to external Na⁺ (n = 7) and Li⁺ (n = 4). d Box plot of average reversal potential data, showing statistically significant differences for both the early and late currents when extracellular Na⁺ was replaced with K⁺. The denoted p values are from Tukey post hoc tests after one-way ANOVA (p < 1E−30, F = 2569). e Na⁺/K⁺ permeability ratios calculated using the bi-ionic reversal potential data, revealing that the late current exhibits higher Na⁺ selectivity compared to the early current (p value is for a two-sample t-test). f Sample sequential TadNaC2 currents elicited by pH 4.5 solutions bearing increasing Ca²⁺ concentrations. g Average Ca²⁺ dose–response curve (n = 12) revealing a similar decline in normalized peak inward current with increasing Ca²⁺ concentration compared to tachyphylaxis (in constant 2 mM Ca²⁺). h Sample sequential paired currents elicited by pH 4.5 solutions bearing either 0 mM or 10 mM Ca²⁺ ions. i Plot of average percent block of peak inward current (e.g., 1 - P2/P1 from (h) ×100%) after switching from 0 mM Ca²⁺ to either 0 mM or 10 mM Ca²⁺(n = 13 and 10, respectively). The denoted p value is for a two-sample T-test.