Ionic contribution to membrane potential changes detected by the FMP dye.

<p>(<b>A</b>) HEK cells over-expressing the Na⁺-selective channel TRPM8 were co-labelled with both FMP and SBFI, and treated with 500 µM menthol in HBSS. Cells were monitored for changes in membrane potential and intracellular Na⁺ using FMP and SBFI dyes, respectively (left panels). Shown are FMP (ΔF/F₀) and SBFI (F₃₄₀/F₃₈₀) signals of individual cells. Upon overlaying average FMP (black) and SBFI (green) signals, similar responses are seen (right panel). (<b>B</b>) FMP signals induced by the Na⁺ ionophore SQI-Pr. Two different concentrations of SQI-Pr (5 µM and 10 µM) were tested. Mock applications (0 µM) were performed as controls. (<b>C</b>) FMP responses to high K⁺ steps in the absence (0 µM) and presence (2 µM) of the K⁺ ionophore valinomycin (VAL). Experiments in (B) and (C) were carried out with HEK cells in BS. Average traces are shown in bold and individual traces are in grey; arrows indicate the time point of mock, SQI-Pr and valinomycin applications.