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Laboratory of Experimental Optometry, University of Wales College of Cardiff.

1. When rabbit corneal endothelial cells are impaled with 3 M-KCl-filled microelectrodes (Rt = 20-70 M omega) a stable membrane potential of -28.7 +/- 4.8 mV (mean +/- S.D., n = 400) is measured. 2. Varying the [KCl] of the filling solution causes a change in measured membrane potential; 154 mM gives typically -2 mV, 10 mM typically +37 mV. 3. Variation in membrane potential with different [KCl] cannot be ascribed to tip potential. Double-barrelled microelectrodes containing a different [KCl] in each barrel both give the same membrane potential when inserted into a cell. 4. Microelectrodes filled with a reference liquid ion exchanger (RLIE) give a membrane potential of +2 mV. 5. Impaling a cell with a double-barrelled microelectrode, one barrel containing KCl and the other RLIE, showed that the ion exchanger is not having a toxic effect on the cell and does not affect membrane potential measured by the KCl-filled barrel. 6. We suggest that microelectrodes containing non-isotonic concentrations of KCl generate a significant and artifactual change in membrane potential of corneal endothelial cells caused by the movement of excess KCl (originating from the microelectrode tip) across the plasma membrane where a liquid-junction potential is generated. 7. We further suggest that the physiological membrane potential of corneal endothelial cells is around zero. This could result from a solution of the constant field equation where: 0.9 PNa congruent to PK congruent to 3.2 PCl.