Direct measurement of potassium leak from single 3 M KCl microelectrodes

Potassium loss was measured from individual microelectrodes filled with 3 M KCl. The potassium content of a distilled-water droplet into which a microelectrode was inserted for a known time interval was analyzed by helium-glow photometry. The rate of potassium loss from single microelectrodes (18-97 M omega) was within the detectable limits of this method. The rate of potassium loss appeared to decrease with increasing input resistance (determined in 0.16 M NaCl) for electrodes having a low resistance (18-50 M omega). Higher resistance electrodes (greater than 50 M omega) appeared to achieve a uniform rate of potassium loss of 3.5 fmol X s-1. Estimates of the rate of potassium loss calculated from the rate of swelling of impaled cells were identical to those measured directly on the same group of electrodes. We conclude the electrolyte loss not only occurs into a distilled-water drop but also during intracellular recording. An alternative method for fabrication of microelectrodes was tested. Of these electrodes, the ones having an input resistance greater than 50 M omega lost potassium at rates below 1 fmol X s-1.