Fig. 1. Calibration and application of ion-selective electrodes. (A) K⁺-selective electrode. (B) Cl^-selective electrode. (a) Representative traces of electrical potentials from each ion-selective electrode in each of three different calibration solutions of known ionic activities (labeled below each segment of the trace). (b) Representative traces of electrical potentials from each ion-selective electrode inserted in situ first into the cytosol and then into the CV. Ionic activities in the cytosol and the CV fluid were estimated from the calibration plots shown in c. (c) Plots of the electrical potentials estimated from the traces in (a) against their respective corresponding ion activities in the calibration solutions. (C) Photographs showing the CV and the double-barreled ion-selective electrode (stars) in the cytosol (left) and the CV (right, broken line).