Diuretics are drugs that can increase the amount of water and sodium that is excreted in the urine. There are several classes of diuretics, each with a different mechanism of action. The main classes of diuretics and their mechanisms are described below.
These are the most effective type of diuretic, leading to the excretion of up to 20% of the filtered salt (sodium chloride) and water. Usually, only around 0.4% of the filtered sodium passes into the urine during renal reabsorption. The agents prevent the body from reabsorbing sodium at the ascending loop of Henle in the nephron. This sodium is then excreted, along with the water that follows it. Examples of drugs in this class include furosemide, ethacrynic acid, torsemide and bumetanide.
Drugs of this class inhibit the sodium-chloride symporter, preventing the retention of water as well as sodium. Thiazides are used as antihypertensive drugs due to their ability to decrease preload and therefore blood pressure. One example of a thiazide diuretic is hydrochlorothiazide.
Carbonic Anhydrase Inhibitors
These drugs inhibit the enzyme carbonic anhydrase which has the effect of decreasing the reabsorption of bicarbonate in the proximal tubule. This leads to retention of bicarbonates and potassium in the urine and decreased sodium absorption.
Both thiazides and loop diuretics cause the body to lose potassium, depleting the body’s stores of this important electrolyte. The potassium-sparing diuretics promote diuresis without causing the loss of potassium. Drugs in this class include:
Osmotic diuretics inhibit the reabsorption of sodium and water, increasing the osmolarity of the blood and the renal filtrate. Examples of these agents include isosorbide and mannitol, which may be used for the following clinical purposes: