Potassium-specific ion-exchanger microelectrodes to measure K + activity in the renal distal tubule.

The recent introduction of miniature liquid ion-exchanger electrodes provides the possibility of overcoming previously encountered limitations on the measurement of specific ionic activities in biological fluids. Hydrogen ion and sodium ion activities have been stuccessfully determined by means of glass-membrane electrodes(l-5). However, in the presence of sodium, glass electrodes are usually inadequate for measuring potassium, divalent cation or anion activities(4). The adaptation of the liquid ion-exchanger principle for use with microelectrodes, permits measurement of K+ and Clactivities on botlh sides of the cell membrane(6-8). We hlave constructed potassium-selective liquid ion-exchanger microelectrodes to measure K+ activity in rat distal tubules. Because this technique is still in a relatively early stage of development, it may be useful to review some details of the theoretical basis for the method and some of the practical considerations relevant to its application. Our experience has been with K+ ion-exchanger electrodes but the same teclhniques apply to anion-sensitive electrodes. Liquid ion exchangers are solutions in an organic solvent of a charged organic compound(9). Such ion-exchanger solutions were originally developed for industrial purposes; in the early 1960's they were increasingly studied as model membrane systems lhaving many of the properties of carrier-medicated ion permeation(9-11). A Ca selective macroelectrode was described by Ross(12), and