Two hours after starting an insulin infusion in a patient with diabetic ketoacidosis, serum potassium falls from 4.8 to 3.1 mEq/L. The mechanism of this fall is:

• A Insulin-mediated renal tubular secretion of potassium
• B Insulin-driven shift of potassium into cells via stimulation of Na-K ATPase ✓
• C Osmotic diuresis caused directly by insulin
• D Insulin inhibition of the sodium-potassium-chloride cotransporter

Explanation

Insulin binds its receptor and activates the Na-K ATPase pump, driving potassium from extracellular fluid into cells. In DKA, total body potassium is depleted even when serum levels look normal, so insulin therapy routinely unmasks hypokalemia, and potassium replacement is started once levels permit. Renal secretion contributes little acutely, insulin itself does not cause osmotic diuresis (glucose does), and NKCC2 inhibition is the action of loop diuretics.

Reference: Ganong's Review of Medical Physiology, 27th ed.

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