Diagram of excitation-induced exchange of Na⁺ and K⁺ between ECV and ICV. See Table 2. Excitation (left) is induced by a rapid influx of Na⁺ (11.2 µmol/g wet wt) from ECV into ICV, followed by an equivalent efflux of K⁺ (11.0 µmol/g wet wt) from ICV into the ECV. This takes place while the muscles are in air, and during the following washout in ice-cold Na⁺-free Tris-sucrose, 11 µmol/g wet wt of K⁺ is removed from the ECV as can be seen from the loss of 11 µmol/g wet wt while the Na⁺ taken up into ICV stays in the ICV, leading to a reduction in total K⁺ content and a similar increase in total Na⁺. As shown in Table 2, the Na⁺,K⁺ contents are restored during the 600-s incubation without buffer by stimulation of the Na⁺,K⁺ pumps. When the muscles are incubated in air, these movements of Na⁺ and K⁺ may be followed undisturbed of exchange with a surrounding incubation medium.