The muscles were mounted in force transducers for measurement of isometric contractions, adjusted to optimal length, and equilibrated for 30 min at 30°C during gassing with a mixture of 95% O₂ and 5% CO₂. Then all muscles were moved to empty tubes, and the last drips of the buffer adhering to the muscle surface were carefully removed. One group of three muscles was left resting for 60 s, another group of six muscles was stimulated for 60 s at 60 Hz using 0.2-ms pulses at 10 V, and a third group of three muscles was stimulated for 60 s at 60 Hz using 0.2-ms pulses at 10 V and then left resting for 600 s in the empty tubes, allowing reaccumulation of the K⁺ lost to the extracellular space into the muscle cells (see Fig. 1). Finally, all muscles were washed four times for 15 min in ice-cold Na⁺-free Tris-sucrose buffer, blotted, weighed, and taken for flame photometric determination of Na⁺ and K⁺ contents. All values are given as means ± SEM, with the number of muscles in parentheses.

P < 0.001 compared with resting muscles.

P < 0.01 compared with resting muscles.

P < 0.001 compared with 60-Hz stimulation, 60 s.