Rapid pipette perfusion ( Hilgemann and Lu, 1998 ) to determine the time course of cytoplasmic Na exchange with the pipette tip. Experimental conditions were as described previously (Lu and Hilgemann, 2017), and myocytes were made permeable to monovalent cations with veratridine (3 µM). Using Na as the sole permeable cation, 100 mM Na was exchanged for 100 mM NMDG in the pipette tip. The veratridine-activated outward current decays with a time constant of 17 s upon removing Na from the pipette tip and is restored upon reintroduction of Na with a time constant of 17 s. The fact that current decays somewhat below baseline upon removing Na reflects the existence of a small myocyte Na conductance in the absence of veratridine. Experiments reveal no slow components of Na exchange or evidence for local, subsarcolemmal Na depletion/accumulation.