How insulin shifts potassium into cells
Insulin lowers the potassium level in the blood by stimulating Na-K-ATPase, a cell membrane pump, to move potassium into cells.
Remember
Insulin shifts potassium into cells through Na-K-ATPase within minutes, rather than removing it from the body, so the serum level needs rechecking afterwards.
About 98% of the body's potassium sits inside cells, so even a small shift across the cell membrane changes the serum level (the potassium in the blood) a lot. Moving potassium into and out of cells is the internal balance, which acts within minutes, whereas the kidneys take hours.
Insulin increases the activity of Na-K-ATPase, a pump in the cell membrane that uses energy to push potassium into the cell and sodium out. This happens particularly in muscle and liver cells, so potassium leaves the blood and enters the cells in a transcellular shift: potassium moves from the blood into the cells without leaving the body.
The fall in serum potassium is fast, with insulin acting within minutes and the effect peaking at about one hour, and it does not rely on the kidneys. It is a redistribution, not a loss: total body potassium is unchanged and the effect lasts several hours, so potassium needs rechecking afterwards.
Because insulin shifts potassium into cells quickly, it is used to treat hyperkalaemia (a high blood potassium). Dextrose (a glucose solution) is given with it to prevent hypoglycaemia (a dangerously low blood glucose). The body does a similar job naturally, because a rise in potassium in splanchnic blood (the blood draining the gut) stimulates insulin release, which helps to stop the level climbing after a meal.
Key facts
- Mechanism
- Insulin increases Na-K-ATPase activity in muscle and liver cells, so potassium moves into cells.
- Speed
- Insulin acts within minutes and the effect on serum potassium peaks at about one hour, whereas kidney excretion takes hours.
- Shift, not removal
- Total body potassium is unchanged, so serum potassium needs rechecking afterwards.
- Insulin deficiency
- Low insulin lets potassium move out of cells, so people with untreated type 1 diabetes tend to run a high serum potassium.
- Acidosis
- Acidosis (excess hydrogen ions in the blood) reduces potassium uptake by cells and tends to raise serum potassium.
- Other hormones
- Aldosterone (a hormone released when the blood potassium is raised) and catecholamines (adrenaline-like hormones) also stimulate the pump.
Exam traps
- Closing potassium channels (the channels that let potassium leak passively out of cells) is not insulin's main effect, because its main effect is active uptake by the Na-K-ATPase pump.
- A potassium-binding resin such as calcium Resonium (a resin that binds potassium in the bowel) does remove potassium from the body, but its onset is delayed, whereas insulin works by shifting potassium into cells rather than by acting in the gut.
- Calcium gluconate (an emergency drug that protects the heart from the effects of potassium) does not itself move potassium into cells, and its effect is short-lived.
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