In rat muscles, a specific protein called Na⁺–K⁺-ATPase α2 is mostly found in tiny tunnels inside the muscle cells, and when scientists try to track it using a special dye, they can’t tell if it’s moving between the surface and the tunnels because the dye picks up both places at once.

When insulin is present, certain parts of a pump in rat muscle cells move to the surface of the cell—these parts work together like a team that responds to insulin.

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.