Pharmacology of Diuretic Drugs
Instructor: Lee, Hon-Cheung
1. In this and the next lecture, we will be concerned with diuretic drugs. They are one of the most
commonly prescribed drugs. These are the drugs that are used to reduce the abnormal accumulation
of excess fluid in the body. These diuretics can do this by eliminating the excess fluid through
increasing the volume of urine excretion. The organ responsible for urine formation is the kidney.
It is not surprising that all diuretics act on the kidney. We will therefore be concerned with how the
kidney produces urine. Diuretics, in fact, act to modulate different processes to reduce sodium and
water reabsorption, resulting in increased water excretion. In these two lectures I will be talking
about the mechanisms of how these drugs can reduce sodium and water reabsorption by the
kidneys and what is the relationship between reduction in reabsorption can result in increase urine
excretion.
2. There are six different classes of diuretic drugs. They vary widely in structure, ranging from simple
compounds such as glycerin, which is a three-carbon compound, to highly complicated compounds
such as spironolactone, containing five carbon rings. Why all these different compounds have the
same diuretic effect? This is the kind of questions we would like to answer in these two lectures.
Pharmacology is a science. It is not simply a compilation of drugs. Pharmacology is not simply
reading a handbook and knowing about appropriate dosages or various side effects of drugs.
Pharmacology is about understanding the mechanism of drug action. We want to know why a
specific drug has a specific effect. This understanding is of utmost importance since it can lead to
development of new and better drugs. A simple answer to the question why these diverse
compounds all have diuretic effect is that they act on different targets. Each class of these diuretic
drugs acts on a different and specific target. Why modulating different targets can all effect
diuresis? To answer this kind of questions we have to understand the physiology of diuresis. We
need to understand the kind of physiological processes that are responsible for producing urine and
how altering these processes can increase or decrease urine formation. Therefore, in the first lecture
we will concentrate on the physiology of fluid handling in the body and in the next lecture we will
discuss specific mechanisms of action of these six classes of diuretic drugs.
3. First, we want to know how the body handles fluid. When we say the body retains excess fluid,
where is the fluid retained? There are three major fluid compartments in the body. The blood
compartment can be further divided into arterial and venous compartments. About half of the blood
volume is plasma and the rest is blood cells. The total plasma water is about 3.5 L, which amounts
to about 5% of body weight. The largest fluid compartment in the body is the intracellular
compartment, which has a total volume of about 28L amounting to about 40% of the body weight.
In between the intracellular compartment and the blood compartment is the interstitial
compartment, which also includes the lymphatic compartment. The total volume of this
compartment is about 10.5 L amounting to about 15% of the body weight. When the body retains
excess fluid, the fluid is mainly accumulated in the interstitial compartment. The diuretic drugs are
very effective in reducing the excess fluid in the blood and interstitial compartments by stimulating
the excretion of the excess fluid in the form of urine. Generally, water can move freely between
these compartments. The two main driving forces of water movement are the osmolarity
differences between these compartments and the blood pressure. The composition of the fluid of
the blood and interstitial compartments are essentially the same, and the major ionic component is
NaCl. The composition of the fluid in the intracellular compartment is, however, quite different.
The major ionic component is K+ instead. When excess fluid is accumulated in the interstitial
compartment, it leads to edema.
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4. Therefore, a major clinical use of diuretics is to reduce edema. There are three most common uses
of diuretics. First is to reduce edema: the abnormal accumulation of interstitial fluid, resulting in
large expansion of the extracellular space. If this occurs in the lungs, pulmonary edema would
result, which can greatly reduce the gas exchange function of the lungs. Diuretic drugs are also
used to treat hypertension, which is abnormally high systemic arterial pressure. Diuretic-induced
excretion of sodium and water reduces the body content of these substances, which can bring about
arteriolar dilation and a lowering of the blood pressure.
5. Another major use of diuretics is to treat congestive heart failure. Blood normally is pumped out by
right ventricle to the lungs where it is oxygenated and returned to the left heart compartments. It is
then pumped out to the rest of the body and finally returns after circulating through the tissues via
veins to the right heart compartment. When the pumping capacity of the heart is reduced or when
one ventricle fails to pump blood to the same extent as the other, the result is congestion of blood
in the veins. The increase in venous blood pressure force fluid out of the veins and into the
interstitial space, resulting in edema. The congestion of blood in the ventricle of the heart can
stretch the heart muscles so much that their ability to pump blood out is further compromised,
increasing edema and starting a vicious cycle that can lead to eventual heart failure. Again, by
increasing the excretion of excess body fluid as urine, diuretics can relieve the edema and prevent
the fatal condition of congestive heart failure. Therefore, a major factor that controls fluid
movement is blood pressure.
6. The other main factor that controls the movement of fluid between the intracellular and the
interstitial compartments is the osmolarity difference. In the body fluid, [Na+]o = 140 mM and
[K+]o = 5 mM. Inside cells, [Na+]c = 5 mM and [K+]c = 140 mM. So the osmolarity of the body
fluid is determined mainly by the concentration of Na+. If Na+ concentration increases significantly
above the normal value of 140 mM, water will move out of the cell and it will shrink. On the other
hand, if the Na+ concentration falls, the osmolarity of the body fluid decreases correspondingly and
water moves into the cell and it swells. In fact, if the Na+ concentration is too far below normal,
cell lysis will occur. So it is quite obvious that maintaining Na+ concentrations at a normal value is
of crucial importance to body functions. Abnormal accumulation of Na+ in the interstitial
compartment can result in extraction of water from the intracellular compartment, which then
accumulates in the interstitial space resulting in edema. If this occurs in the lungs, pulmonary
edema results and the gas-exchange is greatly impaired. It is therefore obvious that it would be
good to maintain the osmolarity of the body fluid relatively constant so that there would not be any
net movement of fluid between compartments.
Mechanisms of Renal Handling of Sodium and Water.
7. The organ in the body that is responsible for maintaining the water and sodium balance is the
kidney. In order to understand the mechanism of action of diuretics, it is important to first
understand the how the kidneys handle sodium and water. The main functions of the kidneys are to
balance the electrolytes, such as sodium, potassium etc., of the body fluid and to excrete waste and
excess water in the form of urine.
8. The kidneys accomplish these important functions by acting basically as a very efficient filtering
unit. As the blood goes through the kidney, a portion of the protein-free plasma is filtered into the
kidney. Blood cells and proteins are retained and only small molecules are filtered. In the kidney,
the composition of the filtrate is modified. Useful substances such as water, ions and nutrients such
as glucose are delivered back to the blood. The wastes such as urea are concentrated and excreted as
urine.
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9. In order for the kidney to function efficiently, a large quantity of blood has to circulate through it.
Indeed, the weight of the kidneys is only about 0.5% of the body weight and yet they receive 2025% of the cardiac output. For a normal person, the rate the heart pumps blood, or the cardiac
output, is about 6 liters/min. Of this, 20-25% goes to the kidneys, which means 1.2-1.5 liter of blood
circulates through the kidneys every minute. The entire blood volume in the body is about 6 liters,
which means that the entire blood volume passes through the kidneys in 5 min, or 300 times a day.
During each passage, a portion of the protein-free plasma is filtered into the kidneys, where the
composition of the electrolyte is readjusted. This is basically how the kidney works.
10. How fast is the protein-free plasma filtered into the kidneys? This can be calculated very simply.
About 45% of the blood volume is composed of blood cells and the rest, 55%, is plasma. So the
renal plasma flow = renal blood flow x 55%=1200 ml/min X 55% = 660 ml/min. Of all this plasma
that flows through the kidney about 20% is filtered into the kidney. Multiplying the renal plasma
flow of 660 ml/min by 20% gives 130 ml/min, which is equivalent to 180 liter/day. This rate is
called the Glomerular Filtration Rate (GFR). The entire plasma volume is about 3 liters. If one
divides the plasma volume (3000 ml) by GFR (130 ml/min) one gets a value of 23 min. This means
that the entire plasma volume in the body is filtered by the kidneys every 23 min. The value for
GFR is a good number to remember because it gives us an idea of the magnitude and the importance
of renal functions. What would happen to the all this volume filtered into the kidneys? The total
plasma volume would be lost as urine in 23 min if the kidneys do not reabsorb it. Indeed, for all
useful substances, the reabsorption process is as fast as the filtration.
11. In general,
Rate of Excretion = Rate of Filtration - Rate of Reabsorption + Rate of Secretion
For sodium and water, there is no secretion. Only filtration and reabsorption need to be considered.
If the rate of reabsorption of water or sodium is higher than the rate of filtration, then it will not
appear in the urine. The filtration rate for water is the GFR = 180 liter/day and over 99% of the
filtered water, or about 178.5 L is reabsorbed by the kidney and delivered back to the blood supply.
The difference between filtration and reabsorption is the rate of excretion, or the rate of urine
formation, which is about 1.5 liter/day.
Anatomy of the kidney.
12. What parts of the kidney are responsible for filtration and reabsorption? The kidney is a bean shape
organ. It is composed of two regions. The outer region is called the cortex. The inner region is
called the medulla. Blood comes into the kidney through the renal artery, which enters at the middle
portion of the kidney. Within the kidney, a portion of the blood is filtered. The useful substances are
returned to the blood, which comes out of the kidney through the renal vein. The wastes such as
urea are concentrated and excreted out through the ureter in the form of urine. If one magnified a
section of the kidney, one finds that the kidney is made up of a large number of individual units
called nephrons, which have a tubular structure. There are about a million of nephrons in each
kidney. Each of these nephrons functions similarly and the overall function of the entire kidney is in
fact the summation of all these individual nephrons. The blood supply comes in through the renal
artery and branches off into very fine arterioles.
13. The arterioles then come into close contact with the nephron in a region called the glomerulus. The
arteriole that enters the glomerulus is called the afferent arteriole and that which comes out is called
the efferent arteriole. The efferent arteriole then branches out into a network of capillaries, which
wrap around the rest of the tubular portion of the nephron before they recombine into the renal vein.
Filtration occurs as the blood enters the glomerulus. The blood comes into the glomerulus through
the afferent arteriole. Filtration occurs during the passage through the glomerular capillaries. The
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protein-free filtrate goes into the Bowman's capsule, which is the balloon-like structure surrounding
the glomerular capillaries. The filtrate then flows into the proximal tubule. The segment of the
nephron that connects to the proximal tubule is called the thin descending limb of the loop of Henle.
The diameter of the thin descending limb is smaller than that of the proximal tubule. The tubule of
the nephron then makes a U-turn and the portion that goes up toward the cortex is called the thick
ascending limb. The segment that connects the nephron to the connecting tubule is called the distal
tubule. Finally, many different nephrons are connected to a common collecting duct which goes all
the way from the cortex to deep into the medulla. The reason why different segments of the nephron
have different names is because they are not only different structurally but they also perform
different functions. The six different classes of diuretics each acts on a different segment of the
tubule and modulate a specific function. It is therefore clear that different class of diuretics should
have different a structure, since that act different target of the tubule.
14. The details of the glomerulus are shown. Filtration occurs as the blood gets into the capillaries. A
portion of the protein-free plasma is filtered into the Bowman' s capsule, which is the thin layer of
cells that surrounds the glomerulus. The diameter of the glomerulus in humans is about 200 µ.
Filtration occurs from the capillaries to the Bowman's capsule. The interface between them is the
filtration barrier. It is composed of three components: the endothelial cells of the capillary, a thick
protein matrix called the basement membrane and a layer of specialized cells called podocytes. The
function of the filtration barrier is to retain blood cells and proteins in the capillary but allow small
molecules such as water, electrolytes etc. to go through.
Water Reabsorption.
15. The main function of the glomerulus is filtration. It is where the fluid enters the kidneys. The
reabsorption process occurs in the rest of the nephron. The filtrate flows from the Bowman's capsule
to the proximal tubule. During the passage through the proximal tubule sodium and water are
reabsorbed. The two sides of the epithelial cells are functionally different. The surface facing the
lumen of the proximal tubule is called the apical surface. Na+ passively enters the proximal tubule
cells through this surface. The other side of the cell is called the basolateral surface. On this surface
there is an enzyme called an ATPase. This enzyme uses the chemical energy of ATP to transport Na+
out of the cell in exchange for K+ into the cell. This enzyme is also called the Na+-pump because its
function is to pump Na+ actively out of the cells. The epithelial cells are therefore asymmetric, that
is, the two sides of the cells have different function. The epithelial cells are tied together by some
special protein molecules forming a structure called the tight-junction. The tight-junction is also
responsible for maintaining the asymmetry of the cell.
The overall transport of Na+ across the epithelium of the proximal tubule is therefore an active
process, since the Na+-pump requires direct use of metabolic energy. Because of this active transport
of Na+, the osmolarity of the lumenal regions adjacent to the apical side of the cell decreases, while
the local concentration of Na+ in regions that surround the basolateral surfaces of the cells increases.
The cell membrane is, in general, very permeable to water and so water moves from low osmolarity,
which is the lumen, to regions having higher osmolarity, which is the basolateral side. As a result, the
hydrostatic pressure in the region increases, pushing water and NaCl out of the region and eventually
to the capillaries. The movement of water is therefore purely passive. It just goes to regions of higher
osmolarity generated by the active transport of Na+. Renal water reabsorption is thus driven by the
active transport of Na+. It is important to know that the Na+ and water movements are tightly
coupled. Any treatments that impair renal Na+ transport of Na+ also inhibit water reabsorption.
Indeed, the majority of the diuretic drugs are inhibitors of renal Na+ transport.
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16. How water and Na+ are reabsorbed in the rest of the nephron is shown next. The filtration occurs in
the glomerulus. The composition of the filtrate is the same as that of the plasma except it is free of
proteins. The filtrate first enters the Bowman's capsule. There is no reasbsorption in the Bowman's
capsule. From the Bowman's capsule, the filtrate then enters the proximal tubule. Na+ is actively
transported out from the lumen of the tubule to the interstitial space and water passively follows the
Na+. The composition of the filtrate coming out of the proximal tubule is still basically very similar
to that of the plasma. The osmolarity is about 300 mOs, the same as in the plasma. The situation is
totally different as the filtrate passes through the rest of the nephron. The composition of the filtrate
is drastically modified. Essentially all of the Na+ and water is reabsorbed and the waste, such as urea
is greatly concentrated.
One of the most important factors that governs reabsorption of water and sodium in the rest of the
nephron is the existence of the medullary osmolarity gradient. It turns out that the osmolarity of the
fluid in the kidney that surrounds the nephron is not uniform. In the cortical region, the osmolarity is
about 300 mOs which is essentially the same as the plasma. In the deepest part of the medulla, the
osmolarity of the interstitial fluid is 1,200 mOs.
From the proximal tubule the filtrate enters the thin descending limb of the loop of Henle. In the thin
descending limb, Na+ permeability is zero and there is no active transport. The water permeability is
very high. As the filtrate goes down the thin descending limb, water moves out of the tubule because
the osmolarity of the filtrate is lower than outside of the tubule, which high at 1200 mOs.
From the thin descending limb the filtrate goes into the thin ascending limb. The Na+ permeability is
high and there is no active transport. The NaCl is high in the filtrate but is low on the outside. So
NaCl is passively reabsorbed out of the tubule. The permeability for water is zero, so there is very
little movement of water.
From the thin ascending limb the filtrate goes into the thick ascending limb. There is active transport
in the thick ascending limb, which pumps out essentially all of the NaCl remaining in the filtrate.
Water permeability is zero; therefore there is no movement of water. The active Na+ transport is very
powerful and is responsible for reabsorbing about 25% of all the Na+ filtered into the nephron. The
Na+ pumped out raises the osmolarity in the interstitial fluid surrounding the nephron and is
responsible for generating and maintaining the osmolarity gradient between the cortex and medulla.
The thick ascending limp is thus extremely important in for water reabsorption. Indeed, the most
powerful diuretic drugs are those that target this segment of the nephron. If you block the Na+
transport in the thick ascending limp, every little reabsorption of water will occur and all the water
that filter into kidneys will be lost as urine.
The collecting duct is normally very permeable to water. The osmolarity of the filtrate is low and the
outside is high at 1,200 mOs. Water passively moves out of the collecting duct. At the end of the
collecting duct, the volume of the filtrate is reduced 180 times and over 99% of water and Na+ are
reabsorbed.
The geometry of the nephron is therefore also very important for water and Na+ reabsorption. The
filtrate goes from the cortex to the medulla twice and each time water is extracted out. This is
because the osmolarity in the medulla region is higher than the cortical region. When the filtrate
flows in the opposite direction, from the medulla to the cortex, Na+ is actively pumped out mainly in
the thick ascending limb, and also in the collecting duct.
Finally, the differential permeability of different segments of the nephron is also important factor
for water and Na+ reabsorption. So the descending limb and the collecting duct have high water
permeability allowing water to move out.
17. Summarizing the important factors that affect the renal water reabsorption, the primary driving force
is the active transport of Na+. Water simply follows Na+ passively. So any treatments that interfere
with the Na+ transport would also inhibit water reabsorption. The second important factor is the
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medullary osmolarity gradient. This allows water to be extracted out of the nephron every time the
filtrate goes into the medullary regions. The osmolarity is generated and maintained by the active
Na+ transport in the thick ascending limb. The U-shape geometry of the nephron is also important.
This directs the filtrate to go into the medulla regions, allowing water to be extracted twice. Finally,
the differential water and Na+ permeability of the various segments of the nephron is also important.
18. Summary of the three processes that determine urine formation. The first process is filtration. This
occurs in the glomerulus at a rate of about 180 L/day. The driving force is the blood pressure in the
glomerular capillary. The composition of the filtrate is essentially the same as plasma. The
osmolarity of both is about 300 mOs, containing mostly NaCl. One major difference is that the
filtrate is mostly protein free.
Essentially all of the 180 L of filtrate that goes into the kidneys is reabsorbed and delivered back to
the blood. Therefore, the next important process is renal reabsorption. This occurs throughout the
tubule of the nephrons. Na+ is actively transported by the Na+-pump out of the nephron and
reabsorbed. Water passively follows the osmolarity gradient generated by the Na+ -transport. More
than 99% of Na+ and water in the filtrate is reabsorbed. The balance is excreted as urine, which is
about 1.5 L/day. The osmolarity (1200 mOs) of urine is 4 times higher than plasma and about half
of the solutes in the urine is urea.
Mechanisms of Action of Diuretic Drugs.
19. Now we understand the physiology of the kidney and how urine is formed, we are in a position to
understand the specific actions of various diuretic drugs. Diuretics can be classified into six
different types based on mechanism of action. All of them interfere with renal Na+-reabsorption at
various segments of the nephron. The first class of diuretics is inhibitors of carbonic anhydrase, an
enzyme in the proximal tubule. The second class is called osmotic diuretics and their primary site
of action is the loop of Henle. The third class is inhibitors of the Na+/K+/2Cl- symport of the thick
ascending limb. The thick ascending limb is a major site for renal reabsorption and is also
responsible for generating and maintaining the medullary osmolarity gradient. This class of
diuretics, by interfering with the Na+ -transport of the thick ascending limb, is the most effective
class of diuretics. The forth class are inhibitors of the Na+/Cl- symport in the distal tubule. The fifth
class are inhibitors of the Na+-channels in the late distal tubule. The last class is antagonists of the
aldosterone receptors in the collecting duct. Aldosterone is a hormone, which is involved in Na+transport in the collecting duct. It is clear that diuretics act on various types of Na+-transport
mechanisms. As you recall, different class of diuretics has entirely different structure. This is
because there are different class of proteins responsible for Na+-transport. Therefore, we need to
know a more detailed understanding of exactly how sodium is transported.
20. Ion transport can be divided into two classes. Active transport is the movement of a substance
against its electrochemical gradient. This type of transport requires energy input. The second class
is passive transport. The movement, in this case, follows the electrochemical gradient and therefore
does not require energy input. Active transport can be further divided into primary and secondary
active transport. An example of primary active transport is the Na+-pump. [Na+] outside the cell is
normally at about 140 mM and is about 5 mM inside a cell. Movement of Na+ into a cell through a
Na+-channel is a passive process since Na+ is moving down its concentration gradient. The
concentration gradient is also called chemical gradient. The movement of Na+ into the cell is also
down its electrical gradient since most cell have a membrane potential, inside being more negative
than outside. Na+ is a positively charged ion so it likes to move toward the negatively charged
region. In other words, movement of Na+ into a cell is down its electrochemical gradient and
therefore, does not require input of energy.
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Movement of Na+ out of the cell is thus against its electrochemical gradient and requires energy
input. The Na+-pump hydrolyzes ATP to ADP and phosphate. The chemical energy of the
hydrolysis is used to pump 3 Na+ out of the cell and 2 K+ into the cell. This is an example of a
primary active process since the ion movement is directly coupled to the use of the chemical
energy of ATP.
Secondary active transport involves various types of carriers. One example is the Na+/H+ antiport.
This carrier transports Na+ in exchange for a H+. H+ is a positive ion, and its movement out of the
cell to against its electrical gradient since outside is more positive than inside. It is also against its
chemical gradient since outside is more acidic than inside. So the movement of H+ is an active
process requiring energy. This energy comes from the downhill movement of the Na+ into the cell.
Since metabolic energy such as ATP is not directly involved, it is a secondary active transport
process. Another example of secondary active transport is Na+/Cl- symport. This carrier transports
both Na+ and Cl- in the same direction, into the cell. The movement of Cl- into the cell is active and
the energy comes from the downhill movement of Na+. Another example of symport is the
Na+/K+/Cl- carrier. It transports all three ions simultaneously into the cell. All these various
transport mechanisms are involved in the renal reabsorption of Na+ and each is targeted by a
different type of diuretics.
1. Inhibitors of carbonic anhydrase.
21. The site of action is at the proximal tubule. Carbonic anhydrase is an enzyme that catalyzes the
interconversion between CO2 and carbonic acid. CO2 is gas and it has limited solubility in water.
Normally the reaction between CO2 and water occurs slowly, but carbonic anhydrase reversibly
accelerates this reaction several thousand-fold. What this enzyme does is to convert the limited
amounts of CO2 desolved in water to carbonic acid, which is very soluble in water. In this way
CO2 can be continuously desolved into water. The enzyme catalyzes the reaction in both
directions. If there is too much carbonic acid in the medium, the enzyme will also convert the
excess to CO2.
Two examples of this class is: Acetazolamide and dichlophenamide, the latter is about 30 times
more potent than the former. Acetaszolamide is generally taken orally and is well absorbed.
Plasma half-life is about 6-9 hours. Its elimination is mainly through urinary excretion by the
kidneys.
22. A reminder of how Na+ and water are reabsorbed in the proximal tubule. Na+ enters the cells
passively through the apical surface from the lumen. It is then actively pumped out in the
basolateral surface. In the proximal tubule, the energy in the Na+ gradient is established by the Na+pump in the basolateral surface. Water passively follows the Na+ movement. How does Na+ enter
the cell? Na+ is an ion, it cannot cross the cell membrane by itself.
23. Na+ is, in fact, carried by the Na+/H+ antiport that is present in the apical surface of the proximal
tubule cells. The movement of H+ out of the cell is a secondary
active process. The energy comes
+
from the downhill movement of Na+. In the lumen, H reacts with filtered HCO3- to form H2CO3,
which rapidly decomposes to CO2 and water. This is catalyzed by a carbonic anhydrase associated
with the apical membrane of the cell. CO2 is lipophilic and rapidly diffuses across the luminal
membrane into the epithelial cell. In the cytosol of the cells, there is a cytosolic carbonic anhydrase
which converts it to H2CO3. The carbonic acid then ionizes to H+ and bicarbonate spontaneously.
This increases the concentration of bicarbonate inside the cell. Its movement out of the cells is
down its concentration gradient. It is also down its electrical gradient since it is negatively charged
ion and is attracted by the relatively positive potential outside the cell. Its outward movement is
carried by a Na+-HCO3- symporter in the basolateral membrane. The net effect of this process is
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transport of NaHCO3 from the tubular lumen to the interstitial space followed by movement of
water (isotonic reabsorption).
Carbonic anhydrase inhibitors potently inhibit (IC50 for acetazolamide is 10 nM) both the
membrane-bound and cytoplasmic forms of carbonic anhydrase and stops the interconvertion of
CO2 and carbonic acid. The result is nearly complete abolition of NaHCO3 reabsorption in the
proximal tubule.
24. The mechanism of carbonic anhydrase inhibitors is summarized. This class of diuretics is also
called proximal tubule diuretics because of their sites of action. They act by inhibiting the carbonic
anhydrase. This limits the interconversion of CO2 and carbonic acid, which spontaneously ionizes
to bicarbonate. As a result the Na+-bicarbonate reabsorption in the proximal tubule is inhibited. The
overall unidirectional movement of Na+ -bicarbonate from the lumen to the blood side requires that
different transporters are present in apical surface as compared with the basolateral side. This
functional asymmetry is maintained by the tight junction. The movement of Na+ into the cell via
the Na+/H+ antiport is passive. The efflux of Na+ from the basolateral surface by the Na+-pump is
active.
2. Osmotic Diuretics.
25. Two examples: Glycerin and mannitol. The primary site of action is the loop of Henle. Glycerin is
well absorbed orally but mannitol is not. The later is normally administered intravenously. Plasma
half-life is 0.5-0.75 hours for glycerin and 0.25-1.7 hours for mannitol. Glycerin is eliminated by
metabolism, while mannitol is excreted by the kidneys.
26. Osmotic diuretics have two main mechanisms of action. They are small molecules that are freely
filtered at the glomerulus. They are also quite inert and are not reabsorbed or transported out of the
tubule. Let's take the case of manintol. Once administered intravenously, it is filtered into the
nephrons. Once it gets into the nephron it will stay there. In the proximal tubule, Na+ is actively
reabsorbed and water follows. The reabsorption of water results in a decrease in volume of the
filtrate. Mannitol is trapped and cannot move out of the tubule; its concentration in the filtrate
therefore increases. This increase in manitol concentration increases the osmolarity of the filtrate.
Water movement is driven by the osmolarity difference between the lumen and the interstitial
space. This difference is reduced because of the osmolarity of the trapped manitol. As a result less
water is reabsorbed and more water is excreted. Therefore, osmotic diuretics generally are
administered in large enough doses to increase significantly the osmolality of plasma and tubular
fluid.
27. A second major action of osmotic diuretics such as mannitol is due to the increase in plasma
osmolarity. This increased osmolarity extracts water from the intracellular compartment to the
blood compartment. This decreases the blood viscocity and increases renal blood flow. The blood
flow washes out the medullary osmolarity gradient by removing NaCl and urea from the renal
medulla. As I mentioned earlier, the medullary osmolarity gradient is one of the major
determinants of renal water reabsorption. The degraded osmolarity gradient reduces water
reabsorption. The final result is diuresis.
28. Therefore, osmotic diuretics act by increasing osmolarity in blood as well as in the renal filtrate.
They do not work by inhibiting enzymes as is the case for proximal tubule diuretics. They are in
fact inert substances that act by altering the driving force of renal water reabsorption, which is the
osmolarity.
3. Inhibitors of Na+-K+-2Cl- symport (loop diuretics; high ceiling diuretics).
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29. This class of diuretics is also called the loop diuretics or high ceiling diuretics. The site of action is
the thick ascending limb. Two common examples are furosemide and bumetanide, the latter is 40
times more potent than the former. Both are administered orally and are readily absorbed. Plasma
half-life is in the order of an hour or so. They are partly eliminated through metabolism and partly
eliminated by renal excretion.
30. Inhibitors of Na+-K+-2Cl- symport act primarily at the thick ascending limb. In there, flux of Na+,
K+, and Cl- from the lumen into the epithelial cell is mediated by a Na+-K+-2Cl- symporter. This
symporter captures the 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 provide a conductive pathway for the apical recycling of this cation, and
basolateral Cl- channels provide a basolateral exit mechanism for Cl-. In addition, a Na+-Clsymporter in the basolateral membrane permits cotransport of Cl- down an electrochemical
gradient with concomitant transport of Na+ against an electrochemical gradient. The overall
process is the transport of NaCl from the lumen to the medullary space. It is this NaCl transport
that is responsible for the generation and maintenance of the medullary osmolarity gradient. The
Inhibitors of Na+-K+-2Cl- symport bind to the Na+-K+-2Cl- symporter in the thick ascending limb
and block its function. In doing so, the diuretics destroy the osmolarity gradient, bringing salt
transport in this segment of the nephron to a virtual standstill and causing a profound increase in
the urinary excretion of Na+ and Cl-. This could result in inhibiting the reasbsorption of up to 25%
of the filtered load of Na+.
31. The inhibitors of Na+/K+/2Cl symport are also called high ceiling diuretics because of their high
efficacy. The reasons for the high efficacy are three fold. The active Na+ reabsorption in the TAL is
responsible for generating and maintaining the medullary osmolarity gradient, which is critically
important for water reabsorption. Inhibiting the Na+-transport destroys the osmolarity gradient and
greatly impairs water reabsorption. The TAL is responsible for reabsorbing about 25% of the
filtered Na+. The segments after the TAL have limited capacity for Na+ reabsorption. So once the
TAL is inhibited, the Na+ comes out of the TAL would directly come out in the urine.
4. Inhibitors of Na+-Cl- symport (thiazide and thiazide-like diuretics).
32. They are also called thiazide or thiazide-like diuretics or called diluting segment diuretics. The site
of action is the distal tubule. Two common examples are hydrochlorothiazide and metolazone, the
latter is 10 times more potent than the former. Both are commonly administered orally and are
readily absorbed. Plasma half-life is in the order of 2-3 hours. Both are mainly eliminated by the
kidneys.
33. As with other nephron segments, transport is powered by a Na+ pump in the basolateral membrane.
The energy in the electrochemical gradient for Na+ is harnessed by a Na+-Cl- symporter in the
luminal membrane which moves Cl- into the epithelial cell against its electrochemical gradient.
Although movement of Cl- into the cell is down its concentration gradient, it is against its electrical
gradient since membrane potential is negative inside the cell. The overall movement is therefore is
active transport. Cl- then passively exits the basolateral membrane via a Cl- channel. As we have
already seen, this is a recurring theme. The Na+ entry in the apical side is passive, while the+ exit in
the basolateral is primary active driven by the Na+-pump. Thiazide diuretics inhibit the Na -Clsymporter, perhaps by competing for the Cl- binding site.
34. As would be expected from their mechanism of action, inhibitors of Na+-Cl- symport decrease
Na+ and Cl- reabsorption and therefore increase their excretion. However, thiazides are only
moderately efficacious (i.e., maximum excretion of filtered load of Na+ is only 5%), since
approximately 90% of the filtered load is reabsorbed before reaching the distal tubule.
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5. Inhibitors of renal epithelial Na+ channels (K+-sparing diuretics).
35. This class of diuretics are also called K+-sparing diuretics. The site of action is in the late distal
tubule and the collecting duct. Two examples are triamterene and amiloride, the latter is 10 times
more potent than the former. Both are normally administered orally with acceptable absorption
rate. Plasma half-life is in the order of 2-3 hours. Amiloride is eliminated by renal excretion while
triamterene is eliminated mainly by metabolism.
36. The cells in the late distal tubule and collecting duct have in their luminal membranes a Na+
channel that provides a conductive pathway for the entry of Na+ into the cell down the
electrochemical gradient created by the basolateral Na+ pump. The Na+ influx also provides an
important driving force for the secretion of K+ into the lumen via K+ channels in the luminal
membrane. Amiloride blocks the Na+ channels in the luminal membrane and inhibits Na+
reabsorption in these cells. Blockage of the Na+ channels also reduces the electrical driving force of
the K+ movement out of the cells and prevent unwanted loss of the ion. These types of diuretics not
only inhibit Na+ reabsorption but also promote retention of K+ and they are thus called K+-sparing
diuretics. Amiloride, at concentrations higher than needed to elicit therapeutic effects, also blocks
the Na+-H+ exchanger of the proximal tubule and the Na+ pump.
6. Antagonists of mineralocorticoid receptors (aldosterone).
37. Aldosterone is a hormone secreted by the cortical region of the adrenal glands, which are located
on top of the kidneys. These diuretics are also K+-sparing. An example is spironolactone. It is well
absorbed by oral administration. Plasma half-life is 1.6 hour and is eliminated by metabolism.
38. Aldosterone is a hormone secreted by the adrenal cortex. Its effect on the kidney is to increase
renal reabsorption of sodium. Epithelial cells in the late distal tubule and collecting duct contain the
cytoplasmic receptor for the hormone. Aldosterone enters the epithelial cell from the basolateral
membrane and binds to the receptors; the receptor-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). AIP
can activates Na+ channels and Na+ pumps that preexist in the cell membrane but are nonfunctional. Consequently, transepithelial NaCl transport is enhanced, and the lumen-negative
transepithelial voltage is increased. The latter effect increases the driving force for secretion of K+
into the tubular lumen. Drugs such as spironolactone competitively inhibit the binding of
aldosterone to the receptor; it is thus referred to as an aldosterone antagonist. Unlike the receptoraldosterone complex, the recepter-spironolactone complex is not able to induce the synthesis of
AIPs resulting in blockage of the biological effects of aldosterone. By antagonizing the action of
aldosterone, spironolactone reduces Na+ reabsorption as well as preventing excessive K+ loss.
Therefore the action and the results of these types of diuretics are similar to the Na+-channel
inhibitors described earlier.
39. Both Na+-channel inhibitors and aldosterone antagonists reduce Na+-channel activity. Both reduce
the loss of K+ and are called K+-sparing diuretics. Both are mildly efficacious, increasing NaCl
excretion by ~2% of the filtered load. Both are mainly used in combination with loop diuretics (e.g.
furosamide) to reduce excessive loss of K+. Both can produce the life-threatening effect of
hyperkalemia, the retention of excessive K+.
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Practice questions:
Answers: 1d, 2b, 3e, 4d, 5a, 6c, 7b, 8d
1. Which of the following is the main driving force for glomerular filtration?
a. The sodium pump (Na+/K+-ATPase).
b. Medullary osmolarity gradient.
c. Blood sodium concentration.
d. Blood pressure.
e. None of the above.
2. Sodium is not reabsorbed by which of the following parts of the nephron?
a. Thick ascending limb.
b. Bowman’s capsule.
c. Collecting duct.
d. Proximal tubule.
e. Distal tubule.
3. Which of the following statements is incorrect?
a. Water moves out of a cell if its external osmolarity is higher than inside.
b. Edema occurring in congestive heart failure is due to increase in venous pressure.
c. Increasing sodium concentration in the interstitial space would extract water from the
intracellular compartment.
d. Increasing blood pressure would drive water from the blood compartment into the interstitial
space.
e. Increasing the osmolarity of the filtrate in the nephron would increase renal reabsorption of
water.
4. Which of the following processes is active?
a. Sodium entry to the proximal tubular cells at the apical surface.
b. Water reabsorption at the thin descending limb of loop of Henle.
c. Sodium entry to the distal tubular cells at the apical surface.
d. Sodium efflux from the proximal tubular cells at the basolateral surface.
e. None of the above.
5. Which the following classes of diuretic drug targets mainly the proximal tubule?
a. Inhibitors of carbonic anhydrase.
b. Osmotic diuretics.
c. Inhibitors of Na+/K+/2Cl- symport.
d. Inhibitors of Na+/Cl- symport.
e. Inhibitors of Na+ channels.
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6. Which of the following diuretic drugs has the highest efficacy?
a. Matolazone.
b. Spironolactone.
c. Furosemide.
d. Hydrochlorothiazide.
e. Triamterene.
7. Which of the following components of the nephron is targeted by acetazolamide?
a. Thin descending limb.
b. Proximal tubule.
c. Distal tubule.
d. Collecting duct.
e. Thick ascending limb.
8. Which of the following class of diuretic drugs has K+-sparing effect?
a. Inhibitors of the carbonic anhydrase.
b. Inhibitors of the Na+/Cl- symporter.
c. Inhibitors of the Na+/K+/2Cl- symporter.
d. Inhibitors of the aldosterone receptor.
e. Osmotic diuretics.
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