A patient has been referred to you because of recent-onset ventricular ectopy, second degree AV nodal block, chromatopsia, and other extracardiac signs and symptoms of digoxin intoxication; He is treated with furosemide and digoxin for his heart failure. Blood tests show that serum digoxin levels are well within a normal range. We believe the problems are diuretic-induced. Which of the following does the diuretic most likely do to account for the digoxin toxicity?