Inhibition by furosemide. (A) Single-channel recordings with solution A in the bath and pipette. Seal resistance equal to 490 GΩ; imposed V_m as indicated. (B) Single-channel recordings with indicated furosemide concentrations added to the bath and pipette. V_m equal to -100 mV. Opening events (downward deflections) are less frequent as furosemide concentration increases, but their amplitudes are not affected. (C) Whole-cell recordings with 0 μM or25 μM furosemide in identical bath and pipette solutions of 115 mM NaCl, 10 mM MgCl₂, 5 mM CaCl₂, 20 mM Na-HEPES, pH 7.4. In each group of traces, 50-ms steps from V_m of 0 mV to values between -100 mV and +100 in 20-mV increments are superimposed. Vertical scale bar represents 2 pA (A-B) and 713 pA (C); horizontal bar is 50 ms (A), 100 ms (B), and 18.6 ms (C). (D) Furosemide dose-responses determined for PSAC single-channel open probabilities (▴, mean ± standard error of the mean [SEM]), whole-cell currents (□), osmotic lysis in sorbitol (○, calculated from the data in Wagner et al¹³ ), and ¹⁴C-lactate^- accumulation (•), each normalized to 1.0 at 0 μM furosemide to allow comparison. Solid curve represents a least-squares fit to y = K_m/(K_m + x) with a K_m of 2.7 μM.