Figure 4

Cation, pH and voltage dependence of SbMATE-mediated transport in X. oocytes. (A) Mean current to voltage (I/V) relationship for SbMATE-expressing oocytes recorded in different ionic (bath solution) conditions, where the reduction in NaCl in the bath media from 96 to 1 mM NaCl, pH 4.5, resulted in a decrease in the magnitude of SbMATE-mediated inward currents and a positive shift in the E_rev (the potential at which the net current is zero). (B) Representative currents recorded under voltage clamp and corresponding mean current to voltage (I/V) relationships from oocytes expressing SbMATE, where the 96 mM NaCl in the bath solution (left set of traces) was replaced by 96 mM KCl (right set of traces). Current and time scales are given at the bottom left margin. Currents were recorded in control cells (not expressing SbMATE) under identical sets of conditions (traces not shown) are depicted by triangle symbols with the same color scheme. (C) I/V curves for currents recorded from oocytes expressing SbMATE (left panel) or control (right panel) cells in 1 mM Na⁺ bath solution at the different extracellular pHs as indicated by the different colored symbols. (D) SbMATE current magnitudes as a function of external H⁺. The K_m ^H values for each voltage were determined from the Michaelis-Menten function fitting of the SbMATE current magnitudes as a function of external H⁺ concentrations at a given voltage. Steady currents were normalized to the current elicited at −160 mV at pH 4.5. (E) Effect of voltage on the apparent affinity constants for protons (K_m ^H) derived from (D). The curve shown represents a single exponential function ([H] = [H₀] exp (V/τ₀), with H and V being substrate and voltage, respectively, and extrapolated. Fitting parameters were H₀ = 1 ± 0.1 µM and τ₀ = 30 ± 2 mV. All electrophysiological recordings were performed in the absence of exogenous intracellular citrate or ethidium bromide (i.e. cell which were not microinjected with any substrate prior to the recordings).