Molecular actions of diuretics.

The criteria upon which diuretics are classified is based upon their site of action within the nephron. Carboanhydrase inhibitors act in the proximal tubule, high-ceiling diuretics in the ascending loop of Henle, the thiazides in the early distal tubule and the potassium-sparing diuretics in the late distal tubule and in the collecting duct. On the molecular level diuretics do not inhibit Na+-K+-ATPase but interfere with the permeability of the tubule membranes or transport systems for certain ions and thus also influence the potential differences in the different parts of the nephron. Since carboanhydrase is located in the proximal tubule cells, not only in the cytosol but also in the brushborders and in the peritubular membranes, acetazolamide and other carboanhydrase inhibitors act on three different sites in these cells. The loop diuretics inhibit the secondary active chloride reabsorption. The receptors in this part of the nephron are stereospecific. Only the levorotatory isomere of ozolinone has active diuretic properties whereas the dextrorotatory isomere does not. Perfusion experiments of the loop of Henle with different lectins give evidence that glycoproteins containing alpha-1-fucose are involved in the reabsorption of Na+ and Cl-. Experiments on the isolated stripped rabbit colon under the condition of chloride secretion reveal striking similarities between the receptors for chloride reabsorption in the luminal cell membranes of the ascending loop of Henle and in the serosal cell membranes of the colon. The potassium-sparing diuretics amiloride and triamterene act by blocking sodium channels in the distal parts of the nephron. Thus the lumen negative potential difference decreases and (passive) potassium secretion is diminished.