IOVs were prepared and the fluxes were performed as described in Materials and methods. The ATP pool of IOVs was manipulated as indicated in separate incubations. After the final washing, the fluxes were measured over a 2-min period at 25°C by suspending the IOVs in a solution that contained, when present, (in mM): 8.0 NaCl, 0.5 KCl, 1 µM valinomycin, 1 µM strophanthidin, and 2.5 glycylglycine, pH 7.4, ± 0.1 CaCl₂. It is evident in experiments A, B, and C that the uptake of ²²Na⁺ is stimulated when ATP is present in the pool together with K⁺. The pool-driven uptake of ²²Na⁺ is also inhibited by the cardiotonic aglycone, strophanthidin. These results, as indicated in Results, are similar to those presented by Mercer and Dunham (1981). Importantly, as shown in experiment D, pool ATP alone stimulates the active transport of ⁴⁵Ca⁺⁺ across the plasma membrane. Values are ±SEM, where n = 3–4.