Figure 3. Quantitative analysis of pump current decay in relation to predictions for cytoplasmic Na depletion.
(A) The initial rate of decline of current should be predictable from the current magnitude, the number of Na moved per charge moved (Na/e = 3), the slope of the Na-current relation ('Hill slope', ~2), the cytoplasmic volume in which Na mixes, and the cytoplasmic Na concentration. Alternatively, the apparent cell volume can be calculated from the other equation parameters. (B) Pump currents at 37 and 23°C with 25 mM Na and 90 mM cytoplasmic K with current decay fitted to single exponential functions. (C) Composite results from experiments with 25 mM cytoplasmic Na at 37°C and 23°C with 90 mM K Na, at 37°C without K, and at 37°C with 40 mM Li and no K. Pump currents are four-fold smaller at 23°C, but the fractional decay is similar; currents are increased by removal of K, but fractional current decay is decreased; and currents are small with a high Li concentrations, while fractional current decay is increased. All results are inconsistent simple depletion models.