Convoluted tubules proximal, diuretics acting

Thiazide diuretics act in the distal convoluted tubule, where they block Na -Cl cotransport (Fig. 21.4). The Na" -Cl cotransport takes place on the luminal surface of distal convoluted tubules. Thus, to exert their diuretic action, the thiazides must reach the luminal fluid. Since the thiazide diuretics are largely bound to plasma proteins and therefore are not readily filtered across the glomeruli, access to the luminal fluid is accomplished by the proximal tubule organic acid secretory system. The drugs then travel along the nephron, presumably being concentrated as fluid is abstracted, until they reach their site of inhibitory action in the distal convoluted tubule. 

Figure 10.1 Sites and mechanisms of action of diuretics. The location of each cell type along the nephron is indicated by the shading patterns. Spironoiactone (not shown) is a competitive aldosterone antagonist and acts primarily in the collecting duct. PT, proximal tubule LH, loop of Henie TAL, thick ascending limb DT, distal tubule DCT, distal convoluted tubule CD, collecting duct PC, principal cell CA, carbonic anhydrase CAI, carbonic anhydrase inhibitors , primary active transport. (Adapted with permission from Ellison D H 1991 The physiologic basis of diuretic synergism its role in treating diuretic resistance. Annals of Internal Medicine 114 886-894.)...

Spironolactone acts in the collecting tubule, not the proximal convoluted tubule. This drug is not usually capable of causing a 40% sodium diuresis. Bumetanide, a loop diuretic, can produce a 30 0% increase in sodium excretion. Metolazone, a thiazide-like drug, acts in the distal convoluted tubule, not in the collecting tubule. The answer is (A). 

Efficacy is determined, in part, by the site of action of the diuretic. Drugs (e g., carbonic anhydrase inhibitors) that act primarily on the proximal convoluted tubule to induce diuresis are weak diuretics because of the ability of the nephron to reabsorb a significant portion of the luminal contents in latter portions of the nephron. Likewise dmgs (potassium-spann diureti ... 

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