English Reading Materials Chapter 23:Diuretic Agents INTRODUCTION Abnormalities in fluid volume and electrolyte composition are common and important clinical disorders.Drugs that block specific transport functions of the renal tubules are valuable clinical tools in the treatment of these disorders.Although various agents that increase urine volume (diuretics)have been described since antiquity,it was not until 1957 that a practical and powerful diuretic agent (chlorothiazide)became available for widespread use. Technically,a "diuretic"is an agent that increases urine volume,while a "natriuretic" causes an increase in renal sodium excretion.Because natriuretics almost always also increase water excretion,they are usually called diuretics. The nephron is divided structurally and functionally into several segments(Figure 1), which are discussed in the first part of this chapter.Many diuretics exert their effects on specific membrane transport proteins in renal tubular epithelial cells.Other diuretics exert osmotic effects that prevent water reabsorption (mannitol),inhibit enzymes (acetazolamide),or interfere with hormone receptors in renal epithelial cells (aldosterone receptor blockers).These effects are discussed in the second part of the chapter.The physiology of each segment is closely linked to the pharmacology of the drugs acting there. 1
1 English Reading Materials Chapter 23: Diuretic Agents INTRODUCTION Abnormalities in fluid volume and electrolyte composition are common and important clinical disorders. Drugs that block specific transport functions of the renal tubules are valuable clinical tools in the treatment of these disorders. Although various agents that increase urine volume (diuretics) have been described since antiquity, it was not until 1957 that a practical and powerful diuretic agent (chlorothiazide) became available for widespread use. Technically, a "diuretic" is an agent that increases urine volume, while a "natriuretic" causes an increase in renal sodium excretion. Because natriuretics almost always also increase water excretion, they are usually called diuretics. The nephron is divided structurally and functionally into several segments (Figure 1), which are discussed in the first part of this chapter. Many diuretics exert their effects on specific membrane transport proteins in renal tubular epithelial cells. Other diuretics exert osmotic effects that prevent water reabsorption (mannitol), inhibit enzymes (acetazolamide), or interfere with hormone receptors in renal epithelial cells (aldosterone receptor blockers). These effects are discussed in the second part of the chapter. The physiology of each segment is closely linked to the pharmacology of the drugs acting there
Proxamal NaHCO NaCl NaCl Distal convoluted CO nvoluted (+PTH) ④ tubule tubule Proximal straight tubule K'H,0 Thin ⑥ADH antagonists ascending limb Loop of Henle Inner medulla Figure I.Tubule transport systems and sites of action of diuretics. I.RENAL TUBULE TRANSPORT MECHANISMS PROXIMAL TUBULE Sodium bicarbonate (NaHCO3),sodium chloride (NaCl),glucose,amino acids,and other organic solutes are reabsorbed via specific transport systems in the early proximal tubule (proximal convoluted tubule,PCT).Potassium ions (K)are reabsorbed via the paracellular pathway.Water is reabsorbed passively,maintaining the osmolality of proximal tubular fluid at a nearly constant level.As tubule fluid is processed along the length of the proximal tubule,the luminal concentrations of these solutes decrease relative to the concentration of inulin,a marker that is filtered but neither secreted nor absorbed by renal tubules.Approximately 66%of total sodium ions (Na",but 85%of the filtered NaHCO3),65%of the K",60%of the water,and virtually all of the filtered glucose and amino acids are reabsorbed in the proximal tubule. Of the various solutes reabsorbed in the proximal tubule,the most relevant to diuretic action are NaHCO3 and NaCl.Of the currently available diuretics,only one group (carbonic anhydrase inhibitors,which block NaHCO3 reabsorption)acts predominantly in the PCT.In view of the large quantity of NaCl absorbed in this 2
2 Figure 1. Tubule transport systems and sites of action of diuretics. I. RENAL TUBULE TRANSPORT MECHANISMS PROXIMAL TUBULE Sodium bicarbonate (NaHCO3), sodium chloride (NaCl), glucose, amino acids, and other organic solutes are reabsorbed via specific transport systems in the early proximal tubule (proximal convoluted tubule, PCT). Potassium ions (K+ ) are reabsorbed via the paracellular pathway. Water is reabsorbed passively, maintaining the osmolality of proximal tubular fluid at a nearly constant level. As tubule fluid is processed along the length of the proximal tubule, the luminal concentrations of these solutes decrease relative to the concentration of inulin, a marker that is filtered but neither secreted nor absorbed by renal tubules. Approximately 66% of total sodium ions (Na+ , but 85% of the filtered NaHCO3), 65% of the K+ , 60% of the water, and virtually all of the filtered glucose and amino acids are reabsorbed in the proximal tubule. Of the various solutes reabsorbed in the proximal tubule, the most relevant to diuretic action are NaHCO3 and NaCl. Of the currently available diuretics, only one group (carbonic anhydrase inhibitors, which block NaHCO3 reabsorption) acts predominantly in the PCT. In view of the large quantity of NaCl absorbed in this
segment,a drug that specifically blocked proximal tubular absorption of NaCl would be a particularly powerful diuretic.No such drug is currently available Sodium bicarbonate reabsorption by the PCT is initiated by the action of a Na/H exchanger(NHE3)located in the luminal membrane of the proximal tubule epithelial cell (Figure 2).This transport system allows Na"to enter the cell from the tubular lumen in exchange for a proton (H)from inside the cell.As in all portions of the nephron,Na/K*ATPase in the basolateral membrane pumps the reabsorbed Na*into the interstitium so as to maintain a low intracellular Na'concentration.The H secreted into the lumen combines with bicarbonate(HCO3)to form H2CO3(carbonic acid),which is rapidly dehydrated to CO2 and H2O by carbonic anhydrase.Carbon dioxide produced by dehydration of H2CO3 enters the proximal tubule cell by simple diffusion where it is then rehydrated back to H2CO3,facilitated by intracellular carbonic anhydrase.After dissociation of H2CO3,the H is available for transport by the Na/H exchanger,and the HCO3 is transported out of the cell by a basolateral membrane transporter(Figure 2).Bicarbonate reabsorption by the proximal tubule is thus dependent on carbonic anhydrase.This enzyme can be inhibited by acetazolamide and related agents. In the late proximal tubule,as HCO3 and organic solutes have been largely removed from the tubular fluid,the residual luminal fluid contains predominantly NaCl.Under these conditions,Na reabsorption continues,but the H secreted by the Na/H exchanger can no longer bind to HCO3.Free H causes luminal pH to fall,activating a still poorly defined Cl/base exchanger(Figure 2).The net effect of parallel Na/H exchange and Cl/base exchange is NaCl reabsorption.As yet,there are no diuretic agents that are known to act on this conjoint process. Because water is reabsorbed in direct proportion to salt reabsorption in the proximal tubule,luminal fluid osmolality remains nearly constant along its length and an impermeant solute like inulin rises in concentration as water is reabsorbed.If large amounts of an impermeant solute such as mannitol (an osmotic diuretic,see below) are present in the tubular fluid,water reabsorption causes the concentration of the solute and osmolality of tubular fluid to rise,eventually preventing further water reabsorption. Organic acid secretory systems are located in the middle third of the straight part of the proximal tubule(S2 segment).These systems secrete a variety of organic acids (uric acid,nonsteroidal anti-inflammatory drugs [NSAIDs],diuretics,antibiotics,etc) into the luminal fluid from the blood.These systems thus help deliver diuretics to the luminal side of the tubule,where most of them act.Organic base secretory systems (creatinine,choline,etc)are also present,in the early (S1)and middle(S2)segments of the proximal tubule. 3
3 segment, a drug that specifically blocked proximal tubular absorption of NaCl would be a particularly powerful diuretic. No such drug is currently available. Sodium bicarbonate reabsorption by the PCT is initiated by the action of a Na+ /H+ exchanger (NHE3) located in the luminal membrane of the proximal tubule epithelial cell (Figure 2). This transport system allows Na+ to enter the cell from the tubular lumen in exchange for a proton (H+ ) from inside the cell. As in all portions of the nephron, Na+ /K+ ATPase in the basolateral membrane pumps the reabsorbed Na+ into the interstitium so as to maintain a low intracellular Na+ concentration. The H+ secreted into the lumen combines with bicarbonate (HCO3 - ) to form H2CO3 (carbonic acid), which is rapidly dehydrated to CO2 and H2O by carbonic anhydrase. Carbon dioxide produced by dehydration of H2CO3 enters the proximal tubule cell by simple diffusion where it is then rehydrated back to H2CO3, facilitated by intracellular carbonic anhydrase. After dissociation of H2CO3, the H+ is available for transport by the Na+ /H+ exchanger, and the HCO3 - is transported out of the cell by a basolateral membrane transporter (Figure 2). Bicarbonate reabsorption by the proximal tubule is thus dependent on carbonic anhydrase. This enzyme can be inhibited by acetazolamide and related agents. In the late proximal tubule, as HCO3 - and organic solutes have been largely removed from the tubular fluid, the residual luminal fluid contains predominantly NaCl. Under these conditions, Na+ reabsorption continues, but the H+ secreted by the Na+ /H+ exchanger can no longer bind to HCO3 - . Free H+ causes luminal pH to fall, activating a still poorly defined Cl- /base exchanger (Figure 2). The net effect of parallel Na+ /H+ exchange and Cl- /base exchange is NaCl reabsorption. As yet, there are no diuretic agents that are known to act on this conjoint process. Because water is reabsorbed in direct proportion to salt reabsorption in the proximal tubule, luminal fluid osmolality remains nearly constant along its length and an impermeant solute like inulin rises in concentration as water is reabsorbed. If large amounts of an impermeant solute such as mannitol (an osmotic diuretic, see below) are present in the tubular fluid, water reabsorption causes the concentration of the solute and osmolality of tubular fluid to rise, eventually preventing further water reabsorption. Organic acid secretory systems are located in the middle third of the straight part of the proximal tubule (S2 segment). These systems secrete a variety of organic acids (uric acid, nonsteroidal anti-inflammatory drugs [NSAIDs], diuretics, antibiotics, etc) into the luminal fluid from the blood. These systems thus help deliver diuretics to the luminal side of the tubule, where most of them act. Organic base secretory systems (creatinine, choline, etc) are also present, in the early (S1) and middle (S2) segments of the proximal tubule
Proximal Lumen- convoluted Interstitium- urine tubule blood Na NHE3 A Na" HCOs+H+ H*+HCO H2C03 H2C03 CA H20+C02 CO2+H2O C Base Figure 2.Apical membrane Na/H exchange (via NHE3)and bicarbonate reabsorption in the proximal convoluted tubule cell.Na/KATPase is present in the basolateral membrane to maintain intracellular sodium and potassium levels within the normal range.Because of rapid equilibration,concentrations of the solutes are approximately equal in the interstitial fluid and the blood.Carbonic anhydrase(CA)is found in other locations in addition to the brush border of the luminal membrane. LOOP OF HENLE At the boundary between the inner and outer stripes of the outer medulla,the proximal tubule empties into the thin descending limb of Henle's loop.Water is extracted from the descending limb of this loop by osmotic forces found in the hypertonic medullary interstitium.As in the proximal tubule,impermeant luminal solutes such as mannitol oppose this water extraction.The thin ascending limb is relatively water-impermeable. The thick ascending limb(TAL)of the loop of Henle actively reabsorbs NaCl from the lumen(about 25%of the filtered sodium),but unlike the proximal tubule and the thin limb of Henle's loop,it is nearly impermeable to water.Salt reabsorption in the TAL therefore dilutes the tubular fluid,and it is called a "diluting segment." Medullary portions of the thick ascending limb contribute to medullary hypertonicity and thereby also play an important role in concentration of urine by the collecting duct. 4
4 Figure 2. Apical membrane Na+ /H+ exchange (via NHE3) and bicarbonate reabsorption in the proximal convoluted tubule cell. Na+ /K+ ATPase is present in the basolateral membrane to maintain intracellular sodium and potassium levels within the normal range. Because of rapid equilibration, concentrations of the solutes are approximately equal in the interstitial fluid and the blood. Carbonic anhydrase (CA) is found in other locations in addition to the brush border of the luminal membrane. LOOP OF HENLE At the boundary between the inner and outer stripes of the outer medulla, the proximal tubule empties into the thin descending limb of Henle's loop. Water is extracted from the descending limb of this loop by osmotic forces found in the hypertonic medullary interstitium. As in the proximal tubule, impermeant luminal solutes such as mannitol oppose this water extraction. The thin ascending limb is relatively water-impermeable. The thick ascending limb (TAL) of the loop of Henle actively reabsorbs NaCl from the lumen (about 25% of the filtered sodium), but unlike the proximal tubule and the thin limb of Henle's loop, it is nearly impermeable to water. Salt reabsorption in the TAL therefore dilutes the tubular fluid, and it is called a "diluting segment." Medullary portions of the thick ascending limb contribute to medullary hypertonicity and thereby also play an important role in concentration of urine by the collecting duct
The NaCl transport system in the luminal membrane of the TAL is a Na/K/2CI cotransporter(called NKCC2 or NK2CL)(Figure 3).This transporter is selectively blocked by diuretic agents known as "loop"diuretics (see below).Although the Na"/K/2CI transporter is itself electrically neutral (two cations and two anions are cotransported),the action of the transporter contributes to excess Kaccumulation within the cell.Back diffusion of this K into the tubular lumen causes a lumen-positive electrical potential that provides the driving force for reabsorption of cations including magnesium and calcium via the paracellular pathway.Thus, inhibition of salt transport in the thick ascending limb by loop diuretics,which reduces the lumen-positive potential,causes an increase in urinary excretion of divalent cations in addition to NaCl. Thick ascending Lumen- limb Interstitium- urine blood NKCC2 Na Na" K 2CI (+)Potential Mg2*,Ca2* Figure 3.Ion transport pathways across the luminal and basolateral membranes of the thick ascending limb cell.The lumen positive electrical potential created by K back diffusion drives divalent (and monovalent)cation reabsorption via the paracellular pathway.NKCC2 is the primary transporter in the luminal membrane. DISTAL CONVOLUTED TUBULE Only about 10%of the filtered NaCl is reabsorbed in the distal convoluted tubule (DCT).Like the thick ascending limb of Henle's loop,this segment is relatively impermeable to water and NaCl reabsorption further dilutes the tubular fluid.The mechanism of NaCl transport in the DCT is an electrically neutral thiazide-sensitive Na"and CI cotransporter(NCC,Figure 4). Because K does not recycle across the apical membrane of the DCT as it does in the TAL,there is no lumen-positive potential in this segment,and Caand Mg2are not driven out of the tubular lumen by electrical forces.Instead,Ca2 is actively 5
5 The NaCl transport system in the luminal membrane of the TAL is a Na+ /K+ /2Clcotransporter (called NKCC2 or NK2CL) (Figure 3). This transporter is selectively blocked by diuretic agents known as "loop" diuretics (see below). Although the Na+ /K+ /2Cl- transporter is itself electrically neutral (two cations and two anions are cotransported), the action of the transporter contributes to excess K+ accumulation within the cell. Back diffusion of this K+ into the tubular lumen causes a lumen-positive electrical potential that provides the driving force for reabsorption of cations including magnesium and calcium via the paracellular pathway. Thus, inhibition of salt transport in the thick ascending limb by loop diuretics, which reduces the lumen-positive potential, causes an increase in urinary excretion of divalent cations in addition to NaCl. Figure 3. Ion transport pathways across the luminal and basolateral membranes of the thick ascending limb cell. The lumen positive electrical potential created by K+ back diffusion drives divalent (and monovalent) cation reabsorption via the paracellular pathway. NKCC2 is the primary transporter in the luminal membrane. DISTAL CONVOLUTED TUBULE Only about 10% of the filtered NaCl is reabsorbed in the distal convoluted tubule (DCT). Like the thick ascending limb of Henle's loop, this segment is relatively impermeable to water and NaCl reabsorption further dilutes the tubular fluid. The mechanism of NaCl transport in the DCT is an electrically neutral thiazide-sensitive Na+ and Cl- cotransporter (NCC, Figure 4). Because K+ does not recycle across the apical membrane of the DCT as it does in the TAL, there is no lumen-positive potential in this segment, and Ca2+ and Mg2+ are not driven out of the tubular lumen by electrical forces. Instead, Ca2+ is actively
reabsorbed by the DCT epithelial cell via an apical Ca2 channel and basolateral Na'/Ca2+exchanger(Figure 4).This process is regulated by parathyroid hormone. Distal convoluted Lumen- tubule Interstitium- urine blood NCC Na Na ATP Ca2+ Ca Na Ca2 Figure 4.Ion transport pathways across the luminal and basolateral membranes of the distal convoluted tubule cell.As in all tubular cells,Na"/K ATPase is present in the basolateral membrane.NCC is the primary sodium and chloride transporter in the luminal membrane.(R,parathyroid hormone [PTH]receptor.) COLLECTING TUBULE The collecting tubule(CCT)is responsible for only 2-5%of NaCl reabsorption by the kidney.Despite this small contribution,the CCT plays an important role in renal physiology and in diuretic action.As the final site of NaCl reabsorption,the collecting tubule is responsible for tight regulation of body fluid volume and for determining the final Na"concentration of the urine.Furthermore,the collecting tubule is a site at which mineralocorticoids exert a significant influence.Lastly,the collecting tubule is the most important site of K*secretion by the kidney and the site at which virtually all diuretic-induced changes in K*balance occur. The mechanism of NaCl reabsorption in the CCT is distinct from the mechanisms found in other tubule segments.The principal cells are the major sites of Na',K, 6
6 reabsorbed by the DCT epithelial cell via an apical Ca2+ channel and basolateral Na+ /Ca2+ exchanger (Figure 4). This process is regulated by parathyroid hormone. Figure 4. Ion transport pathways across the luminal and basolateral membranes of the distal convoluted tubule cell. As in all tubular cells, Na+ /K+ ATPase is present in the basolateral membrane. NCC is the primary sodium and chloride transporter in the luminal membrane. (R, parathyroid hormone [PTH] receptor.) COLLECTING TUBULE The collecting tubule (CCT) is responsible for only 2-5% of NaCl reabsorption by the kidney. Despite this small contribution, the CCT plays an important role in renal physiology and in diuretic action. As the final site of NaCl reabsorption, the collecting tubule is responsible for tight regulation of body fluid volume and for determining the final Na+ concentration of the urine. Furthermore, the collecting tubule is a site at which mineralocorticoids exert a significant influence. Lastly, the collecting tubule is the most important site of K+ secretion by the kidney and the site at which virtually all diuretic-induced changes in K+ balance occur. The mechanism of NaCl reabsorption in the CCT is distinct from the mechanisms found in other tubule segments. The principal cells are the major sites of Na+ , K+
and water transport(Figure 5),and the intercalated cells are the primary sites of H secretion.Unlike cells in other nephron segments,the principal cells do not contain cotransport systems for Na*and other ions in their apical membranes.Principal cell membranes exhibit separate ion channels for Na and K*.Since these channels exclude anions,transport of Na*or K*leads to a net movement of charge across the membrane.Because Na"entry into the principal cell predominates over K"secretion, a 10-50 mV lumen-negative electrical potential develops.Na*that enters the principal cell from the tubular fluid is then transported back to the blood via the basolateral Na/K*ATPase(Figure 5).The 10-50 mV lumen-negative electrical potential drives the transport of Cl back to the blood via the paracellular pathway and draws K out of cells through the apical membrane K channel.Thus,there is an important relationship between Na'delivery to the CCT and the resulting secretion of K" Diuretics that act upstream of the CCT will increase Na'delivery to this site and will enhance K secretion.If the Na'is delivered with an anion that cannot be reabsorbed as readily as Cl(eg,HCO3),the lumen-negative potential is increased,and K" secretion will be enhanced.This mechanism,combined with enhanced aldosterone secretion due to volume depletion,is the basis for most diuretic-induced K'wasting. Reabsorption of Na*via the epithelial Na channel(ENaC)and its coupled secretion of Kis regulated by aldosterone.This steroid hormone,through its actions on gene transcription,increases the activity of both apical membrane channels and the basolateral Na/K*ATPase.This leads to an increase in the transepithelial electrical potential and a dramatic increase in both Na'reabsorption and K*secretion The collecting tubule is also the site at which the final urine concentration is determined.Antidiuretic hormone (ADH,also called arginine vasopressin,AVP) controls the permeability of this segment to water by regulating the insertion of preformed water channels (aquaporin-2,AQP2)into the apical membrane via a G protein-coupled cAMP-mediated process (Figure 6).In the absence of ADH,the collecting tubule (and duct)is impermeable to water and dilute urine is produced. ADH markedly increases water permeability and this leads to the formation of a more concentrated final urine.ADH also stimulates the insertion of urea transporter UT1 molecules into the apical membranes of medullary collecting tubule cells.Urea concentration in the medulla plays an important role maintaining the high osmolarity of the medulla and in the concentration of urine.ADH secretion is regulated by serum osmolality and by volume status. 1
7 and water transport (Figure 5), and the intercalated cells are the primary sites of H+ secretion. Unlike cells in other nephron segments, the principal cells do not contain cotransport systems for Na+ and other ions in their apical membranes. Principal cell membranes exhibit separate ion channels for Na+ and K+ . Since these channels exclude anions, transport of Na+ or K+ leads to a net movement of charge across the membrane. Because Na+ entry into the principal cell predominates over K+ secretion, a 10-50 mV lumen-negative electrical potential develops. Na+ that enters the principal cell from the tubular fluid is then transported back to the blood via the basolateral Na+ /K+ ATPase (Figure 5). The 10-50 mV lumen-negative electrical potential drives the transport of Cl- back to the blood via the paracellular pathway and draws K+ out of cells through the apical membrane K+ channel. Thus, there is an important relationship between Na+ delivery to the CCT and the resulting secretion of K+ . Diuretics that act upstream of the CCT will increase Na+ delivery to this site and will enhance K+ secretion. If the Na+ is delivered with an anion that cannot be reabsorbed as readily as Cl- (eg, HCO3 - ), the lumen-negative potential is increased, and K+ secretion will be enhanced. This mechanism, combined with enhanced aldosterone secretion due to volume depletion, is the basis for most diuretic-induced K+ wasting. Reabsorption of Na+ via the epithelial Na channel (ENaC) and its coupled secretion of K+ is regulated by aldosterone. This steroid hormone, through its actions on gene transcription, increases the activity of both apical membrane channels and the basolateral Na+ /K+ ATPase. This leads to an increase in the transepithelial electrical potential and a dramatic increase in both Na+ reabsorption and K+ secretion. The collecting tubule is also the site at which the final urine concentration is determined. Antidiuretic hormone (ADH, also called arginine vasopressin, AVP) controls the permeability of this segment to water by regulating the insertion of preformed water channels (aquaporin-2, AQP2) into the apical membrane via a G protein-coupled cAMP-mediated process (Figure 6). In the absence of ADH, the collecting tubule (and duct) is impermeable to water and dilute urine is produced. ADH markedly increases water permeability and this leads to the formation of a more concentrated final urine. ADH also stimulates the insertion of urea transporter UT1 molecules into the apical membranes of medullary collecting tubule cells. Urea concentration in the medulla plays an important role maintaining the high osmolarity of the medulla and in the concentration of urine. ADH secretion is regulated by serum osmolality and by volume status
Lumen- Collecting Interstitium- urine tubule blood Principal cell ENaC ⊕ R Aldosterone Na' Na* ATP K Intercalated cell HCO3 ATP H Figure 5.Ion transport pathways across the luminal and basolateral membranes of collecting tubule and collecting duct cells.Inward diffusion of Na"via the epithelial sodium channel(ENaC)leaves a lumen-negative potential,which drives reabsorption of CI'and efflux of K.(R,aldosterone receptor;ADH,antidiuretic hormone.) 8
8 Figure 5. Ion transport pathways across the luminal and basolateral membranes of collecting tubule and collecting duct cells. Inward diffusion of Na+ via the epithelial sodium channel (ENaC) leaves a lumen-negative potential, which drives reabsorption of Cl- and efflux of K+ . (R, aldosterone receptor; ADH, antidiuretic hormone.)
Lumen- Collecting Interstitium- urine tubule blood AQP2 H2O CAMP R —ADH H20 AQP2 AQP3,4 H20 H20 Figure 6.Water transport across the luminal and basolateral membranes of collecting duct cells.Above,low water permeability exists in the absence of antidiuretic hormone(ADH).Below,in the presence of ADH,aquaporins are inserted into the apical membrane,greatly increasing water permeability.(V2,vasopressin V2 receptor, AQP2,apical aquaporin water channels;AQP3,4,basolateral aquaporin water channels.) II.BASIC PHARMACOLOGY OF DIURETIC AGENTS CARBONIC ANHYDRASE INHIBITORS Introduction Carbonic anhydrase is present in many nephron sites,but the predominant location of this enzyme is the luminal membrane of the PCT(Figure 2),where it catalyzes the dehydration of H2CO3 as described above.By blocking carbonic anhydrase,inhibitors block NaHCO3 reabsorption and cause diuresis. Carbonic anhydrase inhibitors were the forerunners of modern diuretics.They were 9
9 Figure 6. Water transport across the luminal and basolateral membranes of collecting duct cells. Above, low water permeability exists in the absence of antidiuretic hormone (ADH). Below, in the presence of ADH, aquaporins are inserted into the apical membrane, greatly increasing water permeability. (V2, vasopressin V2 receptor; AQP2, apical aquaporin water channels; AQP3, 4, basolateral aquaporin water channels.) II. BASIC PHARMACOLOGY OF DIURETIC AGENTS CARBONIC ANHYDRASE INHIBITORS Introduction Carbonic anhydrase is present in many nephron sites, but the predominant location of this enzyme is the luminal membrane of the PCT (Figure 2), where it catalyzes the dehydration of H2CO3 as described above. By blocking carbonic anhydrase, inhibitors block NaHCO3 reabsorption and cause diuresis. Carbonic anhydrase inhibitors were the forerunners of modern diuretics. They were
discovered when it was found that bacteriostatic sulfonamides caused an alkaline diuresis and hyperchloremic metabolic acidosis.With the development of newer agents,carbonic anhydrase inhibitors are now rarely used as diuretics,but they still have several specific applications that are discussed below.The prototypical carbonic anhydrase inhibitor is acetazolamide. Pharmacokinetics The carbonic anhydrase inhibitors are well absorbed after oral administration.An increase in urine pH from the HCO3 diuresis is apparent within 30 minutes,maximal at 2 hours,and persists for 12 hours after a single dose.Excretion of the drug is by secretion in the proximal tubule S2 segment.Therefore,dosing must be reduced in renal insufficiency Pharmacodynamics Inhibition of carbonic anhydrase activity profoundly depresses HCO3 reabsorption in the PCT.At its maximal safely administered dosage,85%of the HCO3 reabsorptive capacity of the superficial PCT is inhibited.Some HCO3'can still be absorbed at other nephron sites by carbonic anhydrase-independent mechanisms,so the overall effect of maximal acetazolamide dosage is only about 45%inhibition of whole kidney HCO3" reabsorption.Nevertheless,carbonic anhydrase inhibition causes significant HCO3" losses and hyperchloremic metabolic acidosis.Because of reduced HCO3 in the glomerular filtrate and the fact that HCO;depletion leads to enhanced NaCl reabsorption by the remainder of the nephron,the diuretic efficacy of acetazolamide decreases significantly with use over several days. At present,the major clinical applications of acetazolamide involve carbonic anhydrase-dependent HCO3 and fluid transport at sites other than the kidney.The ciliary body of the eye secretes HCO3 from the blood into the aqueous humor. Likewise,formation of cerebrospinal fluid by the choroid plexus involves HCO3" secretion.Although these processes remove HCO3 from the blood (the direction opposite to that in the proximal tubule),they are similarly inhibited by carbonic anhydrase inhibitors. Clinical Indications Dosage A.GLAUCOMA The reduction of aqueous humor formation by carbonic anhydrase inhibitors decreases the intraocular pressure.This effect is valuable in the management of glaucoma,making it the most common indication for use of carbonic anhydrase inhibitors.Topically active carbonic anhydrase inhibitors(dorzolamide,brinzolamide) are also available.These topical compounds reduce intraocular pressure,but plasma levels are undetectable.Thus,diuretic and systemic metabolic effects are eliminated 10
10 discovered when it was found that bacteriostatic sulfonamides caused an alkaline diuresis and hyperchloremic metabolic acidosis. With the development of newer agents, carbonic anhydrase inhibitors are now rarely used as diuretics, but they still have several specific applications that are discussed below. The prototypical carbonic anhydrase inhibitor is acetazolamide. Pharmacokinetics The carbonic anhydrase inhibitors are well absorbed after oral administration. An increase in urine pH from the HCO3 - diuresis is apparent within 30 minutes, maximal at 2 hours, and persists for 12 hours after a single dose. Excretion of the drug is by secretion in the proximal tubule S2 segment. Therefore, dosing must be reduced in renal insufficiency. Pharmacodynamics Inhibition of carbonic anhydrase activity profoundly depresses HCO3 - reabsorption in the PCT. At its maximal safely administered dosage, 85% of the HCO3 - reabsorptive capacity of the superficial PCT is inhibited. Some HCO3 - can still be absorbed at other nephron sites by carbonic anhydrase-independent mechanisms, so the overall effect of maximal acetazolamide dosage is only about 45% inhibition of whole kidney HCO3 - reabsorption. Nevertheless, carbonic anhydrase inhibition causes significant HCO3 - losses and hyperchloremic metabolic acidosis. Because of reduced HCO3 - in the glomerular filtrate and the fact that HCO3 - depletion leads to enhanced NaCl reabsorption by the remainder of the nephron, the diuretic efficacy of acetazolamide decreases significantly with use over several days. At present, the major clinical applications of acetazolamide involve carbonic anhydrase-dependent HCO3 - and fluid transport at sites other than the kidney. The ciliary body of the eye secretes HCO3 - from the blood into the aqueous humor. Likewise, formation of cerebrospinal fluid by the choroid plexus involves HCO3 - secretion. Although these processes remove HCO3 - from the blood (the direction opposite to that in the proximal tubule), they are similarly inhibited by carbonic anhydrase inhibitors. Clinical Indications Dosage A. GLAUCOMA The reduction of aqueous humor formation by carbonic anhydrase inhibitors decreases the intraocular pressure. This effect is valuable in the management of glaucoma, making it the most common indication for use of carbonic anhydrase inhibitors. Topically active carbonic anhydrase inhibitors (dorzolamide, brinzolamide) are also available. These topical compounds reduce intraocular pressure, but plasma levels are undetectable. Thus, diuretic and systemic metabolic effects are eliminated
