Saline-filled Micro-electrodes in Relation to Membrane Potential Measurement in Freshwater Protozoa

Abstract

THE measurement of membrane potentials in fresh-water protozoa is complicated by the presence of junction potentials existing between the micro-electrode and the medium. These seem to be associated with the fine dimensions of the electrode, coupled with a steep ionic gradient between the contents of the glass electrode and the medium. Adrian¹ was the first to consider in detail the problem of electrode tip potentials and its relationship to membrane potentials. He noted that potentials in frog muscle cells are often increased when the tips of saline-filled micro-electrodes break. Bingley² describes a technique for reducing electrode resistance and tip potential before recording membrane potentials in amoebae. It is necessary to pre-treat micro-electrodes filled with 3.0 M potassium chloride by passing them through agar gel. Potentials were only successfully recorded from amoebae with micro-electrodes which had resistances less than 15.0 M ohm but more than 4.0 M ohm. Above the upper limit, negative tip potentials were too high and no potential was recorded; below, the amoebae sustained damage with a lowering of membrane potential.