Single PSAC recordings at different [Cl^-]. (A) Single-channel recordings with bath and pipette solutions of 115 mM NaCl, 10 mM MgCl₂, 5 mM CaCl₂, 20 mM Na-HEPES, and 185 mM (top trace), 595 mM (middle), or 1000 mM (bottom) choline chloride, pH 7.4. V_m equal to -100 mV; scale bar represents 2 pA/200 ms. The nominal Cl concentration in each experiment, as indicated in mM, determines the single-channel amplitude; openings are reflected by downward transitions. (B) Single-channel chord conductances, calculated between V_m of -100 mV and 0 mV, for the 3 solutions in panel A. Cl^- activities (ordinate) were measured with a Cl^- electrode. Solid line represents the best fit to y = a*x/(x + b). Unambiguous single PSAC measurements at physiologic [Cl^-] of 145 mM are not possible because their predicted amplitudes are comparable to that of an optimally minimized baseline noise.