The kidneys and potassium
In order to maintain potassium content in the body, the kidneys must excrete the 1-2 mmol/kg of potassium ingested each day.
Control of potassium excretion takes place primarily in the cortical collecting duct. This is carried out by the principal cell.
Aldosterone stimulates sythesis of the epithelial sodium channel (ENaC). This effect is blocked by amiloride. If reabsorption of sodium is in excess of that of chloride this leads to creation of an electrical gradient which augments the net secretion of potassium. Increased sodium in the principal cell also enhances the activity of the Na+K+ ATPase, bringing more potassium into the cell. The result is a [K+] in the luminal fluid 10 times higher than in the ECF. Aldosterone also has a direct effect on the synthesis of Na+K+ ATPase.