When insulin is added to muscle cells in a lab dish at normal body levels, it makes a specific pump in the cells work 60% harder to move potassium-like molecules in, but doesn’t change how easily those molecules can pass through the cell membrane.

When insulin is added to these special muscle cells in a lab dish, it makes a specific pump that moves sodium and hydrogen ions work twice as fast—and it does so in the exact same way it boosts another similar pump in the cell.

When insulin is added to these lab-grown muscle cells, the amount of sodium inside the cells goes up by 80%, and this change happens at the same rate and with the same insulin doses as when the cell’s sodium-pumping machine gets activated.

In muscle cells grown in a lab, if you block a specific sodium swap system or lower the sodium outside the cells, insulin can no longer boost the activity of a key pump that moves sodium and potassium in and out of the cell.

In muscle cells grown in a lab, a chemical called monensin makes sodium enter the cells the same way insulin does—and just like insulin, it turns up the activity of a cellular pump that moves sodium and potassium around.