Apparent Change in Ion Selectivity Caused by Changes in Intracellular K + during Whole-Cell Recording

In whole-cell recordings from HEK293 cells stably transfected with the delayed rectifier K + channel Kv2.1, long depolarizations produce current-dependent changes in [K +] i that mimic inactivation and changes in ion selectivity. With 10 mM K o + or K i +, and 140–160 mM Na i,o +, long depolarizations shifted the reversal potential ( V R) toward E Na. However, similar shifts in V R were observed when Na i,o + was replaced with N-methyl- d-glucamine (NMG +) i, o. In that condition, [K +] o did not change significantly, but the results could be quantitatively explained by changes in [K +] i. For example, a mean outward K + current of 1 nA for 2 s could decrease [K +] i from 10 mM to 3 mM in a 10 pF cell. Dialysis by the recording pipette reduced but did not fully prevent changes in [K +] i. With 10 mM K i,o +, 150 mM Na i +, and 140 mM NMG o +, steps to +20 mV produced a positive shift in V R, as expected from depletion of K i +, but opposite to the shift expected from a decreased K +/Na + selectivity. Long steps to V R caused inactivation, but no change in V R. We conclude that current-dependent changes in [K +] i need to be carefully evaluated when studying large K + currents in small cells.