Diuretics notes

Principles of Diuretic Action

By definition, diuretics are drugs that increase the rate of urine flow; however, clinically useful diuretics also increase the rate of Na⁺ excretion (natriuresis) and of an accompanying anion, usually Cl^–. NaCl in the body is the major determinant of extracellular fluid volume, and most clinical applications of diuretics are directed toward reducing extracellular fluid volume by decreasing total-body NaCl content. A sustained imbalance between dietary Na⁺ intake and Na⁺ loss is incompatible with life. A net positive Na⁺ balance would result in volume overload with pulmonary edema, and a net negative Na⁺ balance would result in volume depletion and cardiovascular collapse. Although continued diuretic administration causes a sustained net deficit in total-body Na⁺, the time course of natriuresis is finite because renal compensatory mechanisms bring Na⁺ excretion in line with Na⁺ intake, a phenomenon known as diuretic braking. These compensatory, or braking, mechanisms include activation of the sympathetic nervous system, activation of the rennin–angiotensin–aldosterone axis, decreased arterial blood pressure (which reduces pressure natriuresis), renal epithelial cell hypertrophy, increased renal epithelial transporter expression, and perhaps alterations in natriuretic hormones such as atrial natriuretic peptide (Ellison, 1999). This is shown in Figure 25–5.

Historically, the classification of diuretics was based on a mosaic of ideas such as site of action (loop diuretics), efficacy (high-ceiling diuretics), chemical structure (thiazide diuretics), similarity of action with other diuretics (thiazide-like diuretics), and effects on K⁺ excretion (K⁺-sparing diuretics). However, since the mechanism of action of each of the major classes of diuretics is now well understood, a classification scheme based on mechanism of action is used in this chapter.

Diuretics not only alter the excretion of Na⁺ but also may modify renal handling of other cations (e.g., K⁺, H⁺, Ca²⁺, and Mg²⁺), anions (e.g., Cl^–, HCO₃^–, and H₂PO₄^–), and uric acid. In addition, diuretics may alter renal hemodynamics indirectly. Table 25–1 gives a comparison of the general effects of the major diuretic classes.

Diuretic Mechanism CATIONS ANIONS URIC ACID RENAL HEMODYNAMICS (Primary site of action) Na+   K+   H+b   Ca2+   Mg2+   Cl–   HCO3–   H2PO4–   Acute  Chronic  RBF GFR FF TGF Inhibitors of CA (proximal tubule) + ++ – NC V (+) ++ ++ I – – – NC + Osmotic diuretics (loop of Henle) ++ + I + ++ + + + + I + NC – I Inhibitors of Na+-K+-2Cl– symport (thick ascending limb)   ++ ++ + ++ ++ ++ +c   +c   + – V(+) NC V(–) – Inhibitors of Na+-Cl– symport (distal convoluted tubule)   + ++ + V(–) V(+) + +c   +c   + – NC V(–) V(–) NC Inhibitors of renal epithelial Na+ channels (late distal tubule, collecting duct)   + – – – – + (+) NC I – NC NC NC NC Antagonists of mineralocorticoid receptors (late distal tubule, collecting duct) + – – I – + (+) I I – NC NC NC NC

aExcept for uric acid, changes are for acute effects of diuretics in the absence of significant volume depletion, which would trigger complex physiological adjustments. bH+, titratable acid and NH4+. cIn general, these effects are restricted to those individual agents that inhibit carbonic anhydrase. However, there are notable exceptions in which symport inhibitors increase bicarbonate and phosphate (e.g., metolazone, bumetanide). ++, +, (+),–, NC, V, V(+), V(–) and I indicate marked increase, mild to moderate increase, slight increase, decrease, no change, variable effect, variable increase, variable decrease, and insufficient data, respectively. For cations and anions, the indicated effects refer to absolute changes in fractional excretion. RBF, renal blood flow; GFR, glomerular filtration rate; FF, filtration fraction; TGF, tubuloglomerular feedback; CA, carbonic anhydrase.

Inhibitors of Carbonic Anhydrase

Acetazolamide (DIAMOX, others) is the prototype of a class of agents that have limited usefulness as diuretics but have played a major role in the development of fundamental concepts of renal physiology and pharmacology.

Mechanism and Site of Action

Proximal tubular epithelial cells are richly endowed with the zinc metalloenzyme carbonic anhydrase, which is found in the luminal and basolateral membranes (type IV carbonic anhydrase, an enzyme tethered to the membrane by a glycosylphosphatidylinositol linkage), as well as in the cytoplasm (type II carbonic anhydrase). Carbonic anhydrase plays a key role in NaHCO3 reabsorption and acid secretion.

In the proximal tubule, the free energy in the Na⁺ gradient established by the basolateral Na⁺ pump is used by an Na⁺-H⁺ antiporter (also referred to as an Na⁺-H⁺ exchanger [NHE]) in the luminal membrane to transport H⁺ into the tubular lumen in exchange for Na⁺ (Figure 25–6). In the lumen, H⁺ reacts with filtered HCO₃^– to form H₂CO₃, which decomposes rapidly to CO₂ and water in the presence of carbonic anhydrase in the brush border. Normally, the reaction occurs slowly, but carbonic anhydrase reversibly accelerates this reaction several thousand times. CO₂ is lipophilic and rapidly diffuses across the luminal membrane into the epithelial cell, where it reacts with water to form H₂CO₃, a reaction catalyzed by cytoplasmic carbonic anhydrase (Figure 25–6). Continued operation of the Na⁺-H⁺ antiporter maintains a low proton concentration in the cell, so H₂CO₃ ionizes spontaneously to form H⁺ and HCO₃^–, creating an electrochemical gradient for HCO₃^– across the basolateral membrane. The electrochemical gradient for HCO₃^– is used by an Na⁺-HCO₃^– symporter (also referred to as the Na⁺-HCO₃^– co-transporter [NBC]) in the basolateral membrane to transport NaHCO3 into the interstitial space. The net effect of this process is transport of NaHCO₃ from the tubular lumen to the interstitial space, followed by movement of water (isotonic reabsorption). Removal of water concentrates Cl^– in the tubular lumen, and consequently, Cl^– diffuses down its concentration gradient into the interstitium by the paracellular pathway

Other Actions

Carbonic anhydrase is present in a number of extrarenal tissues, including the eye, gastric mucosa, pancreas, central nervous system (CNS), and erythrocytes. Carbonic anhydrase in the ciliary processes of the eye mediates formation of large amounts of HCO₃^– in aqueous humor. Inhibition of carbonic anhydrase decreases the rate of formation of aqueous humor and consequently reduces intraocular pressure. Acetazolamide frequently causes paresthesias and somnolence, suggesting an action of carbonic anhydrase inhibitors in the CNS. The efficacy of acetazolamide in epilepsy is due in part to the production of metabolic acidosis; however, direct actions of acetazolamide in the CNS also contribute to its anticonvulsant action. Owing to interference with carbonic anhydrase activity in erythrocytes, carbonic anhydrase inhibitors increase CO₂ levels in peripheral tissues and decrease CO₂ levels in expired gas. Large doses of carbonic anhydrase inhibitors reduce gastric acid secretion, but this has no therapeutic application. Acetazolamide causes vasodilation by opening vascular Ca²⁺-activated K⁺ channels; however, the clinical significance of this effect is unclear.

Absorption and Elimination

Carbonic anhydrase inhibitors are avidly bound by carbonic anhydrase, and accordingly, tissues rich in this enzyme will have higher concentrations of carbonic anhydrase inhibitors following systemic administration. See Table 25–2 for structures and pharmacokinetic data.

Therapeutic Uses

Although acetazolamide is used for treatment of edema, the efficacy of carbonic anhydrase inhibitors as single agents is low, and carbonic anhydrase inhibitors are not employed widely in this regard. The combination of acetazolamide with diuretics that block Na⁺ reabsorption at more distal sites in the nephron causes a marked natriuretic response in patients with low basal fractional excretion of Na⁺ (<0.2%) who are resistant to diuretic monotherapy (Knauf and Mutschler, 1997). Even so, the long-term usefulness of carbonic anhydrase inhibitors often is compromised by the development of metabolic acidosis.

The major indication for carbonic anhydrase inhibitors is open-angle glaucoma. Two products developed specifically for this use are dorzolamide (TRUSOPT, others) and brinzolamide (AZOPT), which are available only as ophthalmic drops. Carbonic anhydrase inhibitors also may be employed for secondary glaucoma and preoperatively in acute angle-closure glaucoma to lower intraocular pressure before surgery (Chapter 64). Acetazolamide also is used for the treatment of epilepsy (Chapter 21). The rapid development of tolerance, however, may limit the usefulness of carbonic anhydrase inhibitors for epilepsy.

Acetazolamide can provide symptomatic relief in patients with high-altitude illness or mountain sickness; however, it is more appropriate to give acetazolamide as a prophylactic measure. Acetazolamide also is useful in patients with familial periodic paralysis. The mechanism for the beneficial effects of acetazolamide in altitude sickness and familial periodic paralysis is not clear, but it may be related to the induction of a metabolic acidosis. Other off-label clinical uses include the treatment of dural ectasia in individuals with Marfan syndrome, of sleep apnea, and of idiopathic intracranial hypertension. Finally, carbonic anhydrase inhibitors can be useful for correcting a metabolic alkalosis, especially one caused by diuretic-induced increases in H⁺ excretion.

Osmotic Diuretics

Osmotic diuretics are agents that are freely filtered at the glomerulus, undergo limited reabsorption by the renal tubule, and are relatively inert pharmacologically. Osmotic diuretics are administered in doses large enough to increase significantly the osmolality of plasma and tubular fluid. Table 25–3 lists four osmotic diuretics—glycerin (OSMOGLYN), isosorbide, mannitol (OSMITROL, others), and urea (currently not available in the U.S.).

Mechanism and Site of Action

For many years it was thought that osmotic diuretics act primarily in the proximal tubule as nonreabsorbable solutes that limit the osmosis of water into the interstitial space and thereby reduce luminal Na⁺ concentration to the point that net Na⁺ reabsorption ceases. Although early micropuncture studies supported this concept, subsequent studies suggested that this mechanism, while operative, may be of only secondary importance and that the major site of action of osmotic diuretics is the loop of Henle.

By extracting water from intracellular compartments, osmotic diuretics expand extracellular fluid volume, decrease blood viscosity, and inhibit renin release. These effects increase RBF, and the increase in renal medullary blood flow removes NaCl and urea from the renal medulla, thus reducing medullary tonicity. Under some circumstances, prostaglandins may contribute to the renal vasodilation and medullary washout induced by osmotic diuretics. A reduction in medullary tonicity causes a decrease in the extraction of water from the DTL, which in turn limits the concentration of NaCl in the tubular fluid entering the ATL. This latter effect diminishes the passive reabsorption of NaCl in the ATL. In addition, the marked ability of osmotic diuretics to inhibit Mg²⁺ reabsorption, a cation that is reabsorbed mainly in the thick ascending limb, suggests that osmotic diuretics also interfere with transport processes in the thick ascending limb. The mechanism of this effect is unknown.

In summary, osmotic diuretics act both in proximal tubule and loop of Henle, with the latter being the primary site of action. Also, osmotic diuretics probably act by an osmotic effect in the tubules and by reducing medullary tonicity.

Effects on Urinary Excretion

Osmotic diuretics increase urinary excretion of nearly all electrolytes, including Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^–, HCO₃^–, and phosphate

Therapeutic Uses

One use for mannitol is in the treatment of dialysis disequilibrium syndrome. Too rapid a removal of solutes from the extracellular fluid by hemodialysis results in a reduction in the osmolality of extracellular fluid. Consequently, water moves from the extracellular compartment into the intracellular compartment, causing hypotension and CNS symptoms (headache, nausea, muscle cramps, restlessness, CNS depression, and convulsions). Osmotic diuretics increase the osmolality of the extracellular fluid compartment and thereby shift water back into the extracellular compartment.

By increasing the osmotic pressure of plasma, osmotic diuretics extract water from the eye and brain. All osmotic diuretics are used to control intraocular pressure during acute attacks of glaucoma and for short-term reductions in intraocular pressure both preoperatively and postoperatively in patients who require ocular surgery. Also, mannitol and urea are used to reduce cerebral edema and brain mass before and after neurosurgery.

Inhibitors of Na⁺-K⁺-2Cl^– Symport (Loop Diuretics, High-Ceiling Diuretics)

Drugs in this group of diuretics inhibit activity of the Na⁺-K⁺-2Cl^– symporter in the thick ascending limb of the loop of Henle; hence these diuretics also are referred to as loop diuretics. Although the proximal tubule reabsorbs ~65% of filtered Na⁺, diuretics acting only in the proximal tubule have limited efficacy because the thick ascending limb has a great reabsorptive capacity and reabsorbs most of the rejectate from the proximal tubule. Diuretics acting predominantly at sites past the thick ascending limb also have limited efficacy because only a small percentage of the filtered Na⁺ load reaches these more distal sites. In contrast, inhibitors of Na⁺-K⁺-2Cl^– symport in thick ascending limb are highly efficacious, and for this reason, they sometimes are called high-ceiling diuretics. The efficacy of inhibitors of Na⁺-K⁺-2Cl^– symport in the thick ascending limb of the loop of Henle is due to a combination of two factors: (1) approximately 25% of the filtered Na⁺ load normally is reabsorbed by the thick ascending limb, and (2) nephron segments past the thick ascending limb do not possess the reabsorptive capacity to rescue the flood of rejectate exiting the thick ascending limb.

Mechanism and Site of Action

Inhibitors of Na⁺-K⁺-2Cl^– symport act primarily in the thick ascending limb. Micropuncture of the DCT demonstrates that loop diuretics increase delivery of solutes out of the loop of Henle. Also, in situ microperfusion of the loop of Henle and in vitro microperfusion of CTAL indicate inhibition of transport by low concentrations of furosemide in the perfusate. Some inhibitors of Na⁺-K⁺-2Cl^– symport may have additional effects in the proximal tubule; however, the significance of these effects is unclear.

It was thought initially that Cl^– was transported by a primary active electrogenic transporter in the luminal membrane independent of Na⁺. Discovery of furosemide-sensitive Na⁺-K⁺-2Cl^– symport in other tissues prompted a more careful investigation of the Na⁺ dependence of Cl^– transport in isolated perfused rabbit CTAL. Scrupulous removal of Na⁺ from the luminal perfusate demonstrated the dependence of Cl^– transport on Na⁺.

It is now well accepted that flux of Na⁺, K⁺, and Cl^– from the lumen into epithelial cells in thick ascending limb is mediated by an Na⁺-K⁺-2Cl^– symporter (Figure 25–7). This symporter captures free energy in the Na⁺ electrochemical gradient established by the basolateral Na⁺ pump and provides for "uphill" transport of K⁺ and Cl^– into the cell. K⁺ channels in the luminal membrane (called ROMK) provide a conductive pathway for the apical recycling of this cation, and basolateral Cl^– channels (called CLC-Kb) provide a basolateral exit mechanism for Cl^–. Luminal membranes of epithelial cells in thick ascending limb have a large conductive pathway (channels) for K⁺; therefore, apical membrane voltage is determined by the equilibrium potential for K⁺ (E_K) and is hyperpolarized. In contrast, the basolateral membrane has a large conductive pathway (channels) for Cl^–, so the basolateral membrane voltage is less negative than E_K; that is, conductance for Cl^– depolarizes the basolateral membrane. Hyperpolarization of the luminal membrane and depolarization of the basolateral membrane result in a transepithelial potential difference of ~10 mV, with the lumen positive with respect to the interstitial space. This lumen-positive potential difference repels cations (Na⁺, Ca²⁺, and Mg²⁺) and thereby provides an important driving force for the paracellular flux of these cations into the interstitial space.

Figure 25–7.

NaCl reabsorption in thick ascending limb and mechanism of diuretic action of Na+-K+-2Cl symport inhibitors. Numbers in parentheses indicate stoichiometry. Designated voltages are the potential differences across the indicated membrane or cell. The mechanisms illustrated here apply to the medullary, cortical, and postmacular segments of the thick ascending limb. S, symporter; CH, ion channel; BL, basolateral membrane; LM, luminal membrane.

Inhibitors of Na⁺-K⁺-2Cl^– symport bind to the Na⁺-K⁺-2Cl^– symporter in the thick ascending limb and block its function, bringing salt transport in this segment of the nephron to a virtual standstill. The molecular mechanism by which this class of drugs blocks the Na⁺-K⁺-2Cl^– symporter is unknown, but evidence suggests that these drugs attach to the Cl^– binding site located in the symporter's transmembrane domain (Isenring and Forbush, 1997). Inhibitors of Na⁺-K⁺-2Cl^– symport also inhibit Ca²⁺ and Mg²⁺ reabsorption in the thick ascending limb by abolishing the transepithelial potential difference that is the dominant driving force for reabsorption of these cations.

Na⁺-K⁺-2Cl^– symporters are an important family of transport molecules found in many secretory and absorbing epithelia. The rectal gland of the dogfish shark is a particularly rich source of the protein, and a cDNA encoding an Na⁺-K⁺-2Cl^– symporter was isolated from a cDNA library obtained from the dogfish shark rectal gland by screening with antibodies to the shark symporter (Xu et al., 1994). Molecular cloning revealed a deduced amino acid sequence of 1191 residues containing 12 putative membrane-spanning domains flanked by long N and C termini in the cytoplasm. Expression of this protein resulted in Na⁺-K⁺-2Cl^– symport that was sensitive to bumetanide. The shark rectal gland Na⁺-K⁺-2Cl^– symporter cDNA was used subsequently to screen a human colonic cDNA library, and this provided Na⁺-K⁺-2Cl^– symporter cDNA probes from this tissue. These latter probes were used to screen rabbit renal cortical and renal medullary libraries, which allowed cloning of the rabbit renal Na⁺-K⁺-2Cl^– symporter (Payne and Forbush, 1994). This symporter is 1099 amino acids in length, is 61% identical to the dogfish shark secretory Na⁺-K⁺-2Cl^– symporter, has 12 predicted transmembrane helices, and contains large N- and C-terminal cytoplasmic regions. Subsequent studies demonstrated that Na⁺-K⁺-2Cl^– symporters are of two varieties (Kaplan et al., 1996). The "absorptive" symporter (called ENCC2, NKCC2, or BSCl) is expressed only in the kidney, is localized to the apical membrane and subapical intracellular vesicles of the thick ascending limb, and is regulated by cyclic AMP/PKA (Obermüller et al., 1996; Plata et al., 1999). At least six different isoforms of the absorptive symporter are generated by alternative mRNA splicing (Mount et al., 1999), and alternative splicing of the absorptive symporter determines the dependency of transport on K⁺ (Plata et al., 2001). The "secretory" symporter (called ENCC3, NKCCl, or BSC2) is a "housekeeping" protein that is expressed widely and, in epithelial cells, is localized to the basolateral membrane. The affinity of loop diuretics for the secretory symporter is somewhat less than for the absorptive symporter (e.g., 4-fold difference for bumetanide). A model of Na⁺-K⁺-2Cl^– symport has been proposed based on ordered binding of ions to the symporter (Lytle et al., 1998). Mutations in genes coding for the absorptive Na⁺-K⁺-2Cl^– symporter, the apical K⁺ channel, the basolateral Cl^– channel, or the chloride channel subunit Barttin are causes of Bartter syndrome (inherited hypokalemic alkalosis with salt wasting and hypotension) (Simon and Lifton, 1998).

Other Actions

Loop diuretics may cause direct vascular effects. Loop diuretics, particularly furosemide, acutely increase systemic venous capacitance and thereby decrease left ventricular filling pressure. This effect, which may be mediated by prostaglandins and requires intact kidneys, benefits patients with pulmonary edema even before diuresis ensues. Furosemide and ethacrynic acid can inhibit Na⁺,K⁺-ATPase, glycolysis, mitochondrial respiration, the microsomal Ca²⁺ pump, adenylyl cyclase, phosphodiesterase, and prostaglandin dehydrogenase; however, these effects do not have therapeutic implications. In vitro, high doses of inhibitors of Na⁺-K⁺-2Cl^– symport can inhibit electrolyte transport in many tissues. Only in the inner ear, where alterations in the electrolyte composition of endolymph may contribute to drug-induced ototoxicity, is this effect important clinically. Irreversible ototoxicity is more common at high doses, with rapid IV administration, and during concommitant therapy with other drugs known to be ototoxic (e.g., aminoglycoside antibiotics, cisplatin, vancomycin).

Therapeutic Uses

All loop diuretics except torsemide are available as oral and injectable formulations. Bumetanide is labeled for once daily administration and may be used in patients allergic to furosemide (the bumetamide:furosemide conversion ratio is approximatly 1:40). A major use of loop diuretics is in the treatment of acute pulmonary edema. A rapid increase in venous capacitance in conjunction with a brisk natriuresis reduces left ventricular filling pressures and thereby rapidly relieves pulmonary edema. Loop diuretics also are used widely for treatment of chronic congestive heart failure when diminution of extracellular fluid volume is desirable to minimize venous and pulmonary congestion (Chapter 28). In this regard, a meta-analysis of randomized clinical trials demonstrates that diuretics cause a significant reduction in mortality and the risk of worsening heart failure, as well as an improvement in exercise capacity (Faris et al., 2002).

Diuretics are used widely for treatment of hypertension (Chapter 28), and controlled clinical trials demonstrating reduced morbidity and mortality have been conducted with Na⁺-Cl^– symport (thiazides and thiazide-like diuretics) but not Na⁺-K⁺-2Cl^– symport inhibitors. Nonetheless, Na⁺-K⁺-2Cl^– symport inhibitors appear to lower blood pressure as effectively as Na⁺-Cl^– symport inhibitors while causing smaller perturbations in the lipid profile. However, the relative potency and short elimination half-lives of loop diuretics render them less useful for hypertension than thiazide-type diuretics.

The edema of nephrotic syndrome often is refractory to less potent diuretics, and loop diuretics often are the only drugs capable of reducing the massive edema associated with this renal disease. Loop diuretics also are employed in the treatment of edema and ascites of liver cirrhosis; however, care must be taken not to induce volume contraction. In patients with a drug overdose, loop diuretics can be used to induce a forced diuresis to facilitate more rapid renal elimination of the offending drug. Loop diuretics, combined with isotonic saline administration to prevent volume depletion, are used to treat hypercalcemia. Loop diuretics interfere with the kidney's capacity to produce a concentrated urine. Consequently, loop diuretics combined with hypertonic saline are useful for the treatment of life-threatening hyponatremia.

Loop diuretics also are used to treat edema associated with chronic kidney disease. Higher doses of loop diuretics are often required in patients with chronic kidney disease. The dose-response curve is shifted to the right (Figure 25–8). Animal studies have demonstrated that loop diuretics increase P_GC by activating the renin-angiotensin system, an effect that could accelerate renal injury (Lane et al., 1998). Most patients with ARF receive a trial dose of a loop diuretic in an attempt to convert oliguric ARF to nonoliguric ARF. However, there is no evidence that loop diuretics prevent ATN or improve outcome in patients with ARF, and as mentioned earlier, the appearance of undesired effects increases with dose.

Inhibitors of Na⁺-Cl^– Symport (Thiazide and Thiazide-Like Diuretics)

Benzothiadiazides were synthesized in an effort to enhance the potency of inhibitors of carbonic anhydrase. However, unlike carbonic anhydrase inhibitors (that primarily increase NaHCO3 excretion), benzothia diazides predominantly increase NaCl excretion, an effect shown to be independent of carbonic anhydrase inhibition.

Mechanism and Site of Action

Some studies using split-droplet and stationary-microperfusion techniques described reductions in proximal tubule reabsorption by thiazide diuretics; however, free-flow micropuncture studies have not consistently demonstrated increased solute delivery out of the proximal tubule following administration of thiazides. In contrast, micropuncture and in situ microperfusion studies clearly indicate that thiazide diuretics inhibit NaCl transport in DCT. The DCT expresses thiazide binding sites and is accepted as the primary site of action of thiazide diuretics; the proximal tubule may represent a secondary site of action.

Figure 25–9 illustrates the current model of electrolyte transport in DCT. As with other nephron segments, transport is powered by a Na⁺ pump in the basolateral membrane. Free energy in the electrochemical gradient for Na⁺ is harnessed by an Na⁺-Cl^– symporter in the luminal membrane that moves Cl^– into the epithelial cell against its electrochemical gradient. Cl^– then exits the basolateral membrane passively by a Cl^– channel. Thiazide diuretics inhibit the Na⁺-Cl^– symporter. In this regard, Na⁺ or Cl^– binding to the Na⁺-Cl^– symporter modifies thiazide-induced inhibition of the symporter, suggesting that the thiazide-binding site is shared or altered by both Na⁺ and Cl^–.

Figure 25–9.

NaCl reabsorption in distal convoluted tubule and mechanism of diuretic action of Na+-Cl– symport inhibitors. Numbers in parentheses indicate stoichiometry. S, symporter; CH, ion channel; BL, basolateral membrane; LM, luminal membrane.

Using a functional expression strategy (Cl^–-dependent Na⁺ uptake in Xenopus oocytes), Gamba and colleagues (1993) isolated a cDNA clone from the urinary bladder of the winter flounder that codes for an Na⁺-Cl^– symporter. This Na⁺-Cl^– symporter is inhibited by a number of thiazide diuretics (but not by furosemide, acetazolamide, or an amiloride derivative) and has 12 putative membrane-spanning domains, and its sequence is 47% identical to the cloned dogfish shark rectal gland Na⁺-K⁺-2Cl^– symporter. Subsequently, the rat and human Na⁺-Cl^– symporters were cloned. The human Na⁺-Cl^– symporter has a predicted sequence of 1021 amino acids, 12 transmembrane domains, and 2 intracellular hydrophilic amino and carboxyl termini and maps to chromosome 16q13. The Na⁺-Cl^– symporter (called ENCCl or TSC) is expressed predominantly in kidney and is localized to the apical membrane of DCT epithelial cells (Bachmann et al., 1995). Expression of the Na⁺-Cl^– symporter is regulated by aldosterone (Velázquez et al., 1996). Mutations in the Na⁺-Cl^– symporter cause a form of inherited hypokalemic alkalosis called Gitelman syndrome (Simon and Lifton, 1998).

Effects on Urinary Excretion

As would be expected from their mechanism of action, inhibitors of Na⁺-Cl^– symport increase Na⁺ and Cl^– excretion. However, thiazides are only moderately efficacious (i.e., maximum excretion of filtered Na⁺ load is only 5%) because ~90% of the filtered Na⁺ load is reabsorbed before reaching the DCT. Some thiazide diuretics also are weak inhibitors of carbonic anhydrase, an effect that increases HCO₃^– and phosphate excretion and probably accounts for their weak proximal tubular effects. Like inhibitors of Na⁺-K⁺-2Cl^– symport, inhibitors of Na⁺-Cl^– symport increase K⁺ and titratable acid excretion by the same mechanisms discussed for loop diuresis. Acute thiazide administration increases uric acid excretion. However, uric acid excretion is reduced following chronic administration by the same mechanisms discussed for loop diuretics. The acute effects of inhibitors of Na⁺-Cl^– symport on Ca²⁺ excretion are variable; when administered chronically, thiazide diuretics decrease Ca²⁺ excretion. The mechanism involves increased proximal reabsorption owing to volume depletion, as well as direct effects of thiazides to increase Ca²⁺ reabsorption in the DCT. Thiazide diuretics may cause a mild magnesuria by a poorly understood mechanism, and there is increasing awareness that long-term use of thiazide diuretics may cause magnesium deficiency, particularly in the elderly. Since inhibitors of Na⁺-Cl^– symport inhibit transport in the cortical diluting segment, thiazide diuretics attenuate the kidney's ability to excrete dilute urine during water diuresis. However, since the DCT is not involved in the mechanism that generates a hypertonic medullary interstitium, thiazide diuretics do not alter the kidney's ability to concentrate urine during hydropenia.

Other Actions

Thiazide diuretics may inhibit cyclic nucleotide phosphodiesterases, mitochondrial O₂ consumption, and renal uptake of fatty acids; however, these effects are not clinically significant.

Absorption and Elimination

Pharmacokinetic parameters of Na⁺-Cl^– symport inhibitors are listed in Table 25–5.

Note the wide range of half-lives for this class of drugs. Sulfonamides are organic acids and therefore are secreted into the proximal tubule by the organic acid secretory pathway. Since thiazides must gain access to the tubular lumen to inhibit the Na⁺-Cl^– symporter, drugs such as probenecid can attenuate the diuretic response to thiazides by competing for transport into proximal tubule. However, plasma protein binding varies considerably among thiazide diuretics, and this parameter determines the contribution that filtration makes to tubular delivery of a specific thiazide.

Toxicity, Adverse Effects, Contraindications, Drug Interactions

Thiazide diuretics rarely cause CNS (e.g., vertigo, headache, paresthesias, xanthopsia, and weakness), GI (e.g., anorexia, nausea, vomiting, cramping, diarrhea, constipation, cholecystitis, and pancreatitis), hematological (e.g., blood dyscrasias), and dermatological (e.g., photosensitivity and skin rashes) disorders. The incidence of erectile dysfunction is greater with Na⁺-Cl^– symport inhibitors than with several other antihypertensive agents (e.g., adrenergic receptor antagonists, Ca²⁺-channel blockers, or angiotensin converting enzyme inhibitors) (Grimm et al., 1997), but usually is tolerable. As with loop diuretics, most serious adverse effects of thiazides are related to abnormalities of fluid and electrolyte balance. These adverse effects include extracellular volume depletion, hypotension, hypokalemia, hyponatremia, hypochloremia, metabolic alkalosis, hypomagnesemia, hypercalcemia, and hyperuricemia. Thiazide diuretics have caused fatal or near-fatal hyponatremia, and some patients are at recurrent risk of hyponatremia when rechallenged with thiazides.

Thiazide diuretics also decrease glucose tolerance, and latent diabetes mellitus may be unmasked during therapy. Recent concerns have also been raised in randomized prospective blood-pressure lowering trials regarding an increased incidence of type II diabetes mellitus compared to other antihypertensive agents such as angiotensin-converting enzyme inhibitors and angiotensin receptor blockers. The mechanism of impaired glucose tolerance is not completely understood but appears to involve reduced insulin secretion and alterations in glucose metabolism. Hyperglycemia may be related in some way to K⁺ depletion, in that hyperglycemia is reduced when K⁺ is given along with the diuretic. In addition to contributing to hyperglycemia, thiazide-induced hypokalemia impairs its antihypertensive effect and cardiovascular protection (Franse et al., 2000) afforded by thiazides in patients with hypertension. Thiazide diuretics also may increase plasma levels of LDL cholesterol, total cholesterol, and total triglycerides. Thiazide diuretics are contraindicated in individuals who are hypersensitive to sulfonamides.

With regard to drug interactions, thiazide diuretics may diminish the effects of anticoagulants, uricosuric agents used to treat gout, sulfonylureas, and insulin and may increase the effects of anesthetics, diazoxide, digitalis glycosides, lithium, loop diuretics, and vitamin D. The effectiveness of thiazide diuretics may be reduced by NSAIDs, nonselective or selective COX-2 inhibitors, and bile acid sequestrants (reduced absorption of thiazides). Amphotericin B and corticosteroids increase the risk of hypokalemia induced by thiazide diuretics.

A potentially lethal drug interaction warranting special emphasis is that involving thiazide diuretics and quinidine. Prolongation of the QT interval by quinidine can lead to the development of polymorphic ventricular tachycardia (torsades de pointes) owing to triggered activity originating from early after-depolarizations (Chapter 29). Torsades de pointes may deteriorate into fatal ventricular fibrillation. Hypokalemia increases the risk of quinidine-induced torsades de pointes, and thiazide diuretics cause hypokalemia. Thiazide diuretic–induced K⁺ depletion may account for many cases of quinidine-induced torsades de pointes. In addition, alkalinization of the urine by thiazides increases the systemic exposure to quinidine by reducing its elimination.

Therapeutic Uses

Thiazide diuretics are used for the treatment of edema associated with heart (congestive heart failure), liver (hepatic cirrhosis), and renal (nephrotic syndrome, chronic renal failure, and acute glomerulonephritis) disease. With the possible exceptions of metolazone and indapamide, most thiazide diuretics are ineffective when the GFR is <30-40 mL/min.

Thiazide diuretics decrease blood pressure in hypertensive patients by increasing the slope of the renal pressure-natriuresis relationship (Figure 26–7), and thiazide diuretics are used widely for the treatment of hypertension either alone or in combination with other antihypertensive drugs (Chapter 28). In this regard, thiazide diuretics are inexpensive, as efficacious as other classes of antihypertensive agents, and well tolerated. Thiazides can be administered once daily, do not require dose titration, and have few contraindications. Moreover, thiazides have additive or synergistic effects when combined with other classes of antihypertensive agents. A common dose for hypertension is 25 mg/day of hydrochlorothiazide or the dose equivalent of another thiazide. The ALLHAT study (ALLHAT Officers and Coordinators for the ALLHAT Collaborative Research Group, 2002) provides strong evidence that thiazide diuretics are the best initial therapy for uncomplicated hypertension, a conclusion endorsed by the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (Chobanian et al., 2003) (Chapter 27). Studies also suggest that the antihypertensive response to thiazides is influenced by polymorphisms in the angiotensin-converting enzyme and -adducin genes (Sciarrone et al., 2003).

Thiazide diuretics, which reduce urinary Ca²⁺ excretion, sometimes are employed to treat Ca²⁺ nephrolithiasis and may be useful for treatment of osteoporosis (Chapter 44). Thiazide diuretics also are the mainstay for treatment of nephrogenic diabetes insipidus, reducing urine volume by up to 50%. Although it may seem counterintuitive to treat a disorder of increased urine volume with a diuretic, thiazides reduce the kidney's ability to excrete free water. They do so by increasing proximal tubular water reabsorption (secondary to volume contraction) and by blocking the ability of the distal convoluted tubule to form dilute urine. This latter effect results in an increase in urine osmolality. Since other halides are excreted by renal processes similar to those for Cl^–, thiazide diuretics may be useful for the management of Br^– intoxication.

Inhibitors of Renal Epithelial Na⁺ Channels (K⁺-Sparing Diuretics)

Triamterene (DYRENIUM) and amiloride (MIDAMOR, others) are the only two drugs of this class in clinical use. Both drugs cause small increases in NaCl excretion and usually are employed for their antikaliuretic actions to offset the effects of other diuretics that increase K⁺ excretion. Consequently, triamterene and amiloride, along with spironolactone (described in the next section), often are classified as potassium (K⁺)-sparing diuretics.

Principal cells in the late distal tubule and collecting duct have, in their luminal membranes, epithelial Na⁺ channels that provide a conductive pathway for Na⁺ entry into the cell down the electrochemical gradient created by the basolateral Na⁺ pump. The higher permeability of the luminal membrane for Na⁺ depolarizes the luminal membrane but not the basolateral membrane, creating a lumen-negative transepithelial potential difference. This transepithelial voltage provides an important driving force for the secretion of K⁺ into the lumen by K⁺ channels (ROMK) in the luminal membrane. Carbonic anhydrase inhibitors, loop diuretics, and thiazide diuretics increase Na⁺ delivery to the late distal tubule and collecting duct, a situation that often is associated with increased K⁺ and H⁺ excretion. It is likely that the elevation in luminal Na⁺ concentration in distal nephron induced by such diuretics augments depolarization of the luminal membrane and thereby enhances the lumen-negative V_T, which facilitates K⁺ excretion. In addition to principal cells, the collecting duct also contains type A intercalated cells that mediate H⁺ secretion into the tubular lumen. Tubular acidification is driven by a luminal H⁺-ATPase (proton pump), and this pump is aided by partial depolarization of the luminal membrane. The luminal H⁺-ATPase is of the vacuolar-type and is distinct from the gastric H⁺-K⁺-ATPase that is inhibited by drugs such as omeprazole. However, increased distal Na⁺ delivery is not the only mechanism by which diuretics increase K⁺ and H⁺ excretion. Activation of the renin-angiotensin-aldosterone axis by diuretics also contributes to diuretic-induced K⁺ and H⁺ excretion, as discussed later in the section on mineralocorticoid antagonists.

Considerable evidence indicates that amiloride blocks epithelial Na⁺ channels in the luminal membrane of principal cells in late distal tubule and collecting duct. The amiloride-sensitive Na⁺ channel (called ENaC) consists of three subunits (, , and ) (Kleyman et al., 1999). Although the subunit is sufficient for channel activity, maximal Na⁺ permeability is induced when all three subunits are coexpressed in the same cell, probably forming a tetrameric structure consisting of two subunits, one subunit, and one subunit. Studies in Xenopus oocytes expressing ENaC suggest that triamterene and amiloride bind ENaC by similar mechanisms. The K_i of amiloride for ENaC is submicromolar, and molecular studies identified critical domains in ENaC that participate in amiloride binding (Kleyman et al., 1999). Liddle syndrome is an autosomal dominant form of low-renin, volume-expanded hypertension that is due to mutations in the or subunits, leading to increased basal ENaC activity

Toxicity, Adverse Effects, Contraindications, Drug Interactions

The most dangerous adverse effect of renal Na⁺-channel inhibitors is hyperkalemia, which can be life-threatening. Consequently, amiloride and triamterene are contraindicated in patients with hyperkalemia, as well as in patients at increased risk of developing hyperkalemia (e.g., patients with renal failure, patients receiving other K⁺-sparing diuretics, patients taking angiotensin-converting enzyme inhibitors, or patients taking K⁺ supplements). Even NSAIDs can increase the likelihood of hyperkalemia in patients receiving Na⁺-channel inhibitors.

Pentamidine and high-dose trimethoprim are used often to treat Pneumocystis jirovecii pneumonia in patients with acquired immune deficiency syndrome (AIDS). Because these compounds are weak inhibitors of ENaC, they too may cause hyperkalemia. Risk with trimethoprim is related to both the dosage employed and the underlying level of renal function. It was first reported with high-dose therapy (200 mg/kg/day) in 1983. Subsequent reports of hyperkalemia occurred in HIV-infected patients with normal renal function on high doses of the drug. In one series of 30 such patients, 50% developed a serum K⁺ concentration >5.0 mEq/L and 10% >6.0 mEq/L. In a prospective study of otherwise healthy outpatients, the frequency was lower with a serum K⁺ concentration >5.5 mEq/L in only 6%. Within 3-10 days after onset of treatment, 10-21% of patients develop a K⁺ concentration >5.5 mEq/L (Perazella, 2000). Risk factors for severe hyperkalemia are older age, high-dose therapy, renal impairment, hypoaldosteronism, and treatment with other drugs that impair renal K⁺ excretion (e.g., NSAIDs and ACE inhibitors). Serum K⁺ concentration should be monitored after 3-4 days of trimethoprim treatment especially in those at increased risk. Likewise, routine monitoring of the serum K⁺ level is essential in patients receiving K⁺-sparing diuretics. Cirrhotic patients are prone to megaloblastosis because of folic acid deficiency, and triamterene, a weak folic acid antagonist, may increase the likelihood of this adverse event. Triamterene also can reduce glucose tolerance and induce photosensitization and has been associated with interstitial nephritis and renal stones. Both drugs can cause CNS, GI, musculoskeletal, dermatological, and hematological adverse effects. The most common adverse effects of amiloride are nausea, vomiting, diarrhea, and headache; those of triamterene are nausea, vomiting, leg cramps, and dizziness.

Therapeutic Uses

Because of the mild natriuresis induced by Na⁺-channel inhibitors, these drugs seldom are used as sole agents in treatment of edema or hypertension. Rather, their major utility is in combination with other diuretics; indeed, each is marketed in a fixed-dose combination with a thiazide: triamterene/hydrochlorothiazide (DYAZIDE, MAXZIDE, others), amiloride/hydrochlorothiazide (generic).

Co-administration of an Na⁺-channel inhibitor augments the diuretic and antihypertensive response to thiazide and loop diuretics. More important, the ability of Na⁺-channel inhibitors to reduce K⁺ excretion tends to offset the kaliuretic effects of thiazide and loop diuretics; consequently, the combination of an Na⁺-channel inhibitor with a thiazide or loop diuretic tends to result in normal plasma K⁺ values. Liddle syndrome can be treated effectively with Na⁺-channel inhibitors. Approximately 5% of people of African origin carry a T594M polymorphism in the subunit of ENaC, and amiloride is particularly effective in lowering blood pressure in patients with hypertension who carry this polymorphism (Baker et al., 2002). Aerosolized amiloride has been shown to improve mucociliary clearance in patients with cystic fibrosis. By inhibiting Na⁺ absorption from the surfaces of airway epithelial cells, amiloride augments hydration of respiratory secretions and thereby improves mucociliary clearance. Amiloride also is useful for lithium-induced nephrogenic diabetes insipidus because it blocks Li⁺ transport into collecting tubule cells.

Antagonists of Mineralocorticoid Receptors (Aldosterone Antagonists, K⁺-Sparing Diuretics)

Mineralocorticoids cause salt and water retention and increase K⁺ and H⁺ excretion by binding to specific mineralocorticoid receptors. Early studies indicated that some spirolactones block the effects of mineralocorticoids; this finding led to the synthesis of specific antagonists for the mineralocorticoid receptor (MR). Currently, two MR antagonists are available in the U.S., spironolactone (a 17-spirolactone; ALDACTONE, others) and eplerenone (INSPRA, others); two others are available elsewhere (Table 25–7).

Mechanism and Site of Action

Epithelial cells in late distal tubule and collecting duct contain cytosolic MRs with a high aldosterone affinity. MRs are members of the superfamily of receptors for steroid hormones, thyroid hormones, vitamin D, and retinoids. Aldosterone enters the epithelial cell from the basolateral membrane and binds to MRs; the MR-aldosterone complex translocates to the nucleus, where it binds to specific sequences of DNA (hormone-responsive elements) and thereby regulates the expression of multiple gene products called aldosterone-induced proteins (AIPs). Consequently, transepithelial NaCl transport is enhanced, and the lumen-negative transepithelial voltage is increased. The latter effect increases the driving force for K⁺ and H⁺ secretion into the tubular lumen.

The discovery of gene mutation responsible for rare monogenic diseases that cause hypertension such as Liddle syndrome and apparent mineralocorticoid excess helped clarify how aldosterone regulates Na⁺ transport in the distal nephron (Figure 25–11). Mutations in the carboxy-terminal PY motif of either the or subunits of ENaC are associated with Liddle syndrome. The PY motif is an area involved in protein-protein interaction. The PY motif of ENaC interacts with the ubiquitin ligase Nedd4-2, a protein that ubiquitinates ENaC. This then results in internalization of ENaC and proteasome-mediated degradation. Subsequent studies revealed that Nedd4-2 is phosphorylated and inactivated by SGK1 (serum and glucocorticoid-stimulated kinase) and that SGK1 is up-regulated after ~30 minutes by aldosterone. By tracing back the pathophysiologic mechanism of Liddle syndrome, one of the mechanisms whereby aldosterone acts in the collecting duct was identified. Aldosterone up-regulates SGK1, which phosphorylates and inactivates Nedd4-2. As a result, ENaC is not ubiquitinated and removed from the membrane, thereby increasing Na⁺ reabsorption. When the mineralocorticoid receptor was cloned and tested in vitro it was noted to have equal affinity for mineralocorticoid and glucocorticoids. It had been assumed that the mineralocorticoid receptor would have specificity for mineralocorticoids, but surprisingly this was not the case. Given that glucocorticoids circulate at 100- to 1000-fold higher concentration than mineralocorticoids, it was unclear how mineralocorticoids would ever bind to their receptor. The mineralocorticoid receptor would be predominantly occupied by glucocorticoids. This mystery was solved with the cloning of the enzyme type II 11--hydroxysteroid dehydrogenase (HSD). Mineralocorticoid target tissues expresses type II 11--HSD, which converts cortisol to the inactive cortisone. This allows mineralocorticoids to bind to the receptor. The type II 11--HSD enzyme is genetically absent in the inherited disorder of apparent mineralocorticoid excess.

Figure 25–11.

Effects of aldosterone on late distal tubule and collecting duct and diuretic mechanism of aldosterone antagonists. A. Cortisol also has affinity for the mineralocorticoid receptor (MR), but is inactivated in the cell by 11--hydroxysteroid dehydrogenase (HSD) type II. B. Serum and glucorticoid-regulated kinase (SGK)-1 is upregulated after ~30 minutes by aldosterone. SGK-1 phosphorylates and inactivates Nedd4-2 a ubiquitin-protein ligase that acts on ENaC, leading to its degradation. Phosphorylated Nedd4-2 no longer interacts with the PY motif of ENaC. As a result, the protein is not ubiquitinated and remains in the membrane, the end result of which is increased Na+ entry into the cell. 1. Activation of membrane-bound Na+ channels. 2. Na+ channel (ENaC) removal from the membrane is inhibited. 3. De novo synthesis of Na+ channels. 4. Activation of membrane-bound Na+,K+- ATPase. 5. Redistribution of Na+,K+-ATPase from cytosol to membrane. 6. De novo synthesis of Na+,K+-ATPase. 7. Changes in permeability of tight junctions. 8. Increased mitochondrial production of ATP. AIP, aldosterone-induced proteins; ALDO, aldosterone; MR, mineralocorticoid receptor; CH, ion channel; BL, basolateral membrane; LM, luminal membrane.

Drugs such as spironolactone and eplerenone competitively inhibit the binding of aldosterone to the MR. Unlike the MR-aldosterone complex, the MR-spironolactone complex is not able to induce the synthesis of AIPs. Since spironolactone and eplerenone block biological effects of aldosterone, these agents also are referred to as aldosterone antagonists.

MOther Actions

Spironolactone has some affinity toward progesterone and androgen receptors and thereby induces side effects such as gynecomastia, impotence, and menstrual irregularities. Owing to the 9,11-epoxide group, eplerenone has very low affinity for progesterone and androgen receptors (<1% and <0.1%, respectively) compared with spironolactone. High spironolactone concentrations were reported to interfere with steroid biosynthesis by inhibiting steroid hydroxylases (e.g., CYPs 11A1, 11B1, 11B2, and C21; see Chapters 40 and 41). These effects have limited clinical relevance.

R antagonists are the only diuretics that do not require access to the tubular lumen to induce diuresis.

Toxicity, Adverse Effects, Contraindications, Drug Interactions

As with other K⁺-sparing diuretics, MR antagonists may cause life-threatening hyperkalemia. Indeed, hyperkalemia is the principal risk of MR antagonists. Therefore, these drugs are contraindicated in patients with hyperkalemia and in those at increased risk of developing hyperkalemia either because of disease or administration of other medications. MR antagonists also can induce metabolic acidosis in cirrhotic patients.

Salicylates may reduce the tubular secretion of canrenone and decrease diuretic efficacy of spironolactone, and spironolactone may alter the clearance of digitalis glycosides. Owing to its affinity for other steroid receptors, spironolactone may cause gynecomastia, impotence, decreased libido, hirsutism, deepening of the voice, and menstrual irregularities. Spironolactone also may induce diarrhea, gastritis, gastric bleeding, and peptic ulcers (the drug is contraindicated in patients with peptic ulcers). CNS adverse effects include drowsiness, lethargy, ataxia, confusion, and headache. Spironolactone may cause skin rashes and, rarely, blood dyscrasias. Breast cancer has occurred in patients taking spironolactone chronically (cause and effect not established), and high doses of spironolactone are associated with malignant tumors in rats. Whether or not therapeutic spironolactone doses can induce malignancies remains an open question. Strong inhibitors of CYP3A4 may increase plasma levels of eplerenone, and such drugs should not be administered to patients taking eplerenone, and vice versa. Other than hyperkalemia and GI disorders, the rate of adverse events for eplerenone is similar to that of placebo (Pitt et al., 2003).

Therapeutic Uses

As with other K⁺-sparing diuretics, spironolactone often is coadministered with thiazide or loop diuretics in the treatment of edema and hypertension, and spironolactone in combination with hydrochlorothiazide (ALDACTAZIDE, others) is marketed. Such combinations result in increased mobilization of edema fluid while causing lesser perturbations of K⁺ homeostasis. Spironolactone is particularly useful in the treatment of resistant hypertension due to primary hyperaldosteronism (adrenal adenomas or bilateral adrenal hyperplasia) and of refractory edema associated with secondary aldosteronism (cardiac failure, hepatic cirrhosis, nephrotic syndrome, and severe ascites). Spironolactone is considered the diuretic of choice in patients with hepatic cirrhosis. Spironolactone, added to standard therapy, substantially reduces morbidity and mortality (Pitt et al., 1999) and ventricular arrhythmias (Ramires et al., 2000) in patients with heart failure (Chapter 28).

Clinical experience with eplerenone is less than that with spironolactone. Eplerenone appears to be a safe and effective antihypertensive drug (Ouzan et al., 2002). It is somewhat more specific for the MR and therefore the incidience of progesterone-related adverse effects (e.g., gynecomastia) is lower than with spironolactone. In patients with acute myocardial infarction complicated by left ventricular systolic dysfunction, addition of eplerenone to optimal medical therapy significantly reduces morbidity and mortality (Pitt et al., 2003).