DIURETICS are used to reduce fluid in the body by increasing the excretion of electrolytes by the kidney — so increasing urine production. They have an extensive use. Reducing oedema is, in itself, of benefit in some disorders, and diuretics may be used in acute pulmonary oedema, congestive heart failure, some liver and kidney disease, glaucoma and in certain electrolyte disturbances, such as hypercalcaemia and hyperkalcaemia. The commonest use of diuretics is in antihypertensive therapy, where their action of reducing oedema is of value in reducing the load on the heart, which then — over some days or weeks — gives way to a beneficial reduction in blood pressure (that seems associated with vasodilator action). See ANTIHYPERTENSIVE AGENTS.

In relation to their specific actions and uses, diuretics can be divided into a number of distinct classes.

Osmotic diuretics (e.g. mannitol, urea) are inert compounds that are secreted into the proximal tubules of the kidney, and are not reabsorbed, so carry salts and water with them into the urine.

Loop diuretics (e.g. ethacrynic acid, bumetanide, frusemide) have a vigorous action on the ascending tubules of the loop of Henle (inhibiting resorption of sodium and water, and also some potassium), and are used for short periods, especially in heart failure. See ATPASE INHIBITORS.

Thiazide and related diuretics (e.g. bendrofluazide, benzthiazide, chlorothiazide, chlorthalidone, clopamide, cyclopenthiazide, hydrochlorothiazide, hydroflumethiazide, indapamide, mefruside, metolazone, polythiazide and xipamide) are the most used and have a moderate action in inhibiting sodium reabsorption at the distal tubule of the kidney, allowing their prolonged use as antihypertensives, but they may cause potassium loss from the blood to the urine which needs correction. See ATPASE INHIBITORS.

Potassium-sparing diuretics (e.g. amiloride, spironolactone and triamterene) have a weak action on the distal tubule of the kidney, which, as the name suggests, cause retention of potassium; making them suitable for combination with some of the other diuretic classes, and for some specific conditions. See ATPASE INHIBITORS.

ALDOSTERONE ANTAGONISTS (e.g. potassium canrenoate and spironolactone) work by blocking the action of the hormone aldosterone (a MINERALOCORTICOID) , and this makes them suitable for treating oedema associated with aldosteronism, liver failure and certain heart conditions.

CARBONIC ANHYDRASE INHIBITORS (e.g. acetazolamide and dichlorphenamide) are weak diuretics, but are now rarely used to treat systemic oedema, though useful in reducing fluid in the anterior chamber of the eye where it is causing raised intraocular pressure (glaucoma).

In the treatment of hypertension, diuretics are commonly used in combination with other classes of drugs, usually beta-blockers (see beta-adrenoceptor antagonists).