Large Na currents carried by nystatin channels have similar kinetics to those carried by veratridine-modified Na channels. (A) 50 µM nystatin was applied in the absence of Na, and NMDG was then replaced by Na, generating an initial steady-state current of ∼1 nA. The outward current activated by removing Na amounts to 2 nA, and peak inward current on reapplying Na was 3 nA with current decay amounting to approximately one half of peak current. The mean τ for current decay was 20.2 s, the peak conductance was 90 nS, and the access resistance was 90 nS. We estimate the mixing volume to be 9.3 pL and the steady-state cellular Na concentration to be 43 mM. (B) Current–voltage relations and access resistance changes during the same protocol. Access resistance rises and falls with a time course that is similar to the Na current, therefore revealing no longitudinal diffusion delays. Note that inward current decay is not a simple exponential function, possibly reflecting a change of the nystatin conductance. Reversal potentials (right) shift negatively as Na accumulates, and the calculated cytoplasmic Na concentration (inset) approach a steady state of 32 mM monotonically with a τ of 22 s, similar to the mean τ for outward current decay (21.2 s).