Convoluted tubules distal, diuretics acting

Potassium-Sparing Diuretics. Potassium-sparing diuretics act on the aldosterone-sensitive portion of cortical collecting tubules, and partially in the distal convoluted tubules of the nephron. The commonly used potassium-sparing diuretics are triamterene, amiloride, and spironolactone (Table 3). Spironolactone is a competitive aldosterone receptor antagonist, whereas triamterene and amiloride are not (44,45). 

Lasilix containing furosemide is classified as a loop diuretic and acts by inhibiting re-absorption from the ascending part of the loop of Henle. Thiazide diuretics, such as bendroflumethiazide, act by inhibiting re-absorption at the beginning of the distal convoluted tubule. 

Thiazide diuretics act on the beginning of the distal convoluted tubule by inhibiting sodium re-absorption. Thiazide diuretics are indicated in hypertension, and at higher doses to relieve oedema caused by heart failure. Thiazide diuretics lead to hyponatraemia and hypokalaemia. They may cause hypercalcaemia and are therefore avoided in patients with this condition. 

In distal convoluted tubules, calcium is transported by an active transport mechanism through rather than between cells. Moreover, in distal convoluted tubules there is a reciprocal relation between the direction and magnitude of calcium on Na+ transport. As Na+ absorption increases, calcium decreases, and conversely, reductions of Na+ absorption are accompanied by elevated calcium reabsorption. This interaction has important implications for diuretics acting in the distal convoluted tubule. 

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. 

Thiazides. Diuretics that act on the distal convoluted tubule and inhibit the sodium chloride symporter, leading to retention of water in the urine. An example is hydrochlorothiazide. 

Amiloride is a potassium-sparing diuretic that acts in the distal convoluted tubule independently of the action of aldosterone, inhibiting sodium channels. It is a relatively safe drug with few reported adverse effects. 

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). 

Thiazide diuretic prototype acts in distal convoluted tubule (DCT) to block Na+/Cl transporter used in HTN, CHF, nephrolithiasis. Tox a sulfonamide increased serum lipids, uric acid, glucose K wasting. 

Na /Cl transport in the distal convoluted tubule. Spironolactone exerts its dimetic effect by blocking the binding of the hormone aldosterone to its receptors. The osmotic diuretics, such as mannitol, act by increasing the osmolality of the filtrate, and hence increasing urine volume. 

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