When scientists zap a rat's calf muscle with electric pulses at a high speed for a short time, the muscle gets much better at pumping out sodium—up to 18 times better—without needing more of the special pumps that usually do this job.

Scientists used a special dye to count how many sodium-pumping machines are working in rat leg muscle, and when they turned the muscle on, the amount of sodium it pushed out matched what they expected based on the dye count — so the dye must be counting almost all the working pumps.

When scientists zap rat muscle with electricity for a couple of hours, it tears a little bit of the muscle’s outer layer (which you can tell by a chemical leaking out), but it doesn’t break the muscle’s internal pumps that keep it working properly.

When insulin is added to rat leg muscle, it makes the muscle’s sodium-potassium pump work 23% harder, which lowers the sodium inside the cells by 27%—but it doesn’t create more pumps to do this job.