Fig. 1. Measurement of the time course of whole cell solution exchange. Endogenous potassium current in an HEK 293 cell was activated by a depolarizing voltage step from −60 to −20 mV. During this step, the extracellular solution bathing the cell was exchanged for a solution higher in potassium concentration. The rate of change in the outward current produced by the reduced potassium driving force was used as a measure of the rate of solution exchange over the surface of the cell. For comparison, the bottom two traces show the open-tip junction current obtained after discontinuing the recording.