Effect of diuretics on urinary excretion of phosphate, calcium, and magnesium in thyroparathyroidectomized dogs

Studies were performed to investigate the effect of furosemide, ethacrynic acid, chlormerodrin, mercaptomerin, chlorothiazide, and acetazolamide on the urinary excretion of calcium, magnesium, and phosphate. To avoid the effects of extracellular fluid volume contraction and parathormone stimulation, thyroparathyroidectomized dogs, whose urinary losses were continuously replaced, were utilized. The administration of all diuretic agents resulted in a progressive increase in the fractional clearance of calcium, magnesium, and phosphate as the clearance of sodium increased. Phosphate reabsorptiou occurs mainly in the proximal tubule; therefore, the increase in phosphate clearance induced by all the diuretics studied suggests that each of these agents exerts some effect on the proximal tubule. The rise in fractional clearance of phosphate induced by acetazolamide and chlorothiazide was greater than that of the other diuretics; however, the rise in calcium and magnesium produced by these two agents was the smallest. This may be attributed to the reabsorption of sodium, calcium, and magnesium in the loop of Henle where acetazolamide and chlorothiazide have little or no effect.