A 65 year old man with mild hea failure is treated with a loop diuretic. A few days later the man complains of muscle weakness. Laboratory results are shown below. Aerial PCO2: 48 mm Hg Aerial pH: 7.49 Plasma HCO3-: 35 mEq/L Which of the following is most likely decreased in this man?

The data shown in the table indicate that the man has developed metabolic alkalosis (increased PCO2, pH, and HCO3-), which occurs commonly with overuse of diuretics (thiazides and loop diuretics). The overuse of a loop diuretic increases the excretion of sodium and potassium by the kidneys. the increase in potassium excretion leads to a decrease in plasma potassium levels. The sodium depletion stimulates renin secretion, which in turn raises angiotensin II levels in the plasma (which also stimulates aldosterone secretion). Ref: Michel T., Hoffman B.B. (2011). Chapter 27. Treatment of Myocardial Ischemia and Hypeension. In B.C. Knollmann (Ed), Goodman & Gilman's The Pharmacological Basis of Therapeutics, 12e.