The Na⁺ and Ca²⁺ pumps use the same pool of ATP. In the results shown here, the pools of porous ghosts were filled with ATP, as described in Materials and methods. Whereas in the previously presented results the pool was emptied by incubation with Na⁺ plus K⁺ (and Mg²⁺), here the ghost pool of ATP was emptied by incubation (for 30 min at 37°C) in the presence of Ca²⁺ (and Mg²⁺) before assay for E_Na-P and E_Ca-P. The solution used for this second incubation (the ghosts being filled with ATP in the first incubation) was (in mM): 0.05 CaCl₂, 2.0 MgCl₂, 40 NMDG, 0.2 EDTA, and 10 Tris Cl, pH 7.3 at 37°C. The results of two experiments (hatched and shaded bars) are shown. Comparison between their respective counterparts on the left-hand and right-hand sides indicates that both types of pumps share the same pool of ATP. Values are ±SEM, where n = 5.