Steep concentration gradients of monovalent cations (250 mM versus 12.5 mM in pipette and bath, respectively) were imposed to improve the resolution of the selectivity measurements. Therefore, it seemed appropriate to start this study by measuring steady state current voltage relations in the physiological voltage range. To this end, the clamped voltage was stepped in 20 mV increments to various potentials ranging from − 100 to + 120 mV (well below the threshold for field induced pore formation; [8]) for 1.5 s. Fig.