The time course of filling the membrane pool of porous ghosts with ATP. Ghosts were loaded with ATP according to the standard procedure described in Materials and methods. However, the protocols for the timed sequences of sampling were different for the two experiments shown. In A, ghosts were added to a large batch of loading medium and then sampled for the determination of E_Na-P at time zero, 10, 20, and 30 min. Samples were processed according to the usual procedure after exposure to the stop solution. In B, four different sets of tubes were prepared to contain the ATP-loading medium with ghosts added at zero time and at 10-min intervals up to 30 min. All samples were then processed together for the determination of the ΔNa⁺ component. It is evident that halftime (T_1/2) is 10–15 min, essentially the same for the two protocols.