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Chapter 8: Disorders of Magnesium Balance: Hypomagnesemia and Hypermagnesemia

A 35-year-old man receives a course of cisplatinum for testicular cancer. One month after his last dose of cisplatinum, he presents to his primary physician complaining of palpitations and is noted to have frequent premature ventricular complexes. His laboratory tests reveal: serum potassium 3 mEq/L, serum magnesium 1 mg/dL, 24-hour urine potassium 60 mEq/day. Despite treatment with eight Slow Mag pills/day and 80 mEq of potassium chloride/day for 1 month, his serum magnesium remains low at 1.2 mg/dL and his serum potassium increases only slightly to 3.2 mEq/L.

Which one of the following medications is effective as an adjuvant therapy to correct the chronic hypomagnesemia?

A. Chlorthalidone

B. Acetazolamide

C. Amiloride

D. Furosemide

E. Omeprazole

The answer is C. Amiloride is used as an adjuvant treatment of hypomagnesemia. By inhibiting sodium transport via ENaC, it establishes a negative membrane potential in the distal convoluted tubule cells and therefore favors Mg2+ reabsorption.

What is the most likely mechanism of the hypomagnesemia-induced hypokalemia in the above patient?

A. Hypomagnesemia inhibits the NKCC2 transporter in the thick ascending limb of the loop of Henle.

B. Hypomagnesemia results in potassium to shift from the extracellular to the intracellular compartment.

C. Hypomagnesemia impairs normal function of the renal outer medullary potassium (ROMK) channels.

D. Hypomagnesemia causes secondary hyperaldosteronism leading to renal potassium secretion.

E. Hypomagnesemia impairs intestinal absorption of potassium.

The answer is C. Normal intracellular magnesium levels will inhibit K+ efflux via the ROMK channel. Low intracellular magnesium levels are believed to relieve this inhibition, thereby causing ROMK-mediated potassium secretion.

A 64-year-old woman has been receiving weekly cetuximab infusions for the past 3 months to treat her metastatic colon cancer. She had a witnessed grand mal seizure. Imaging of her brain was normal. Her serum magnesium was low at 0.4 mg/dL. Which of following is the most likely explanation for the hypomagnesemia?

A. Competitive inhibition of epidermal growth factor receptor (EGFR)

B. Laxative abuse

C. Impaired Na+-K+-ATPase activity due to decreased HNF1B expression

D. Impaired paracellular Mg2+ transport via claudin-16

E. Transcellular shifting of magnesium ...

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