Diuretics

Question 1

What are diuretics?

Answer 1

Drugs that act on the renal tubule to promote excretion of Na+, Cl- and H2O

Question 2

What percentage of filtered fluid is reabsorbed in the proximal tubule?

Answer 2

• 65-70%

Question 3

1) How does water move into the epithelial cells from the lumen in the proximal tubule?
2) What other force is present, within the interstitium, that helps draw water in from the tubule?

Answer 3

1)

• Osmosis – it will follow the diffusion of Na+ into the cell

2)

• Constant oncotic pressure between the blood and the PCT because the glomerulus filters the blood but cannot filter out the protein, this remains in the blood and sets up this osmotic gradient as it attracts lots of water back to the blood

Question 4

What important protein is present on the basolateral membrane of epithelial cells along most of the tubule and is responsible for maintaining the concentration gradient that allows sodium reabsorption?

Answer 4

• Na+/K+ ATPase
• So this pumps out Na+ out of the epithelial cell into the blood in order to feed it into the blood as a form of reabsorption
• This Na+ efflux also maintains this concentration gradient set up in regards there being a lower concentration of sodium in the epithelial cells and high concentration in the lumen of the PCT in order to facilitate the movement of sodium into the epithelial cell

Question 5

Other than through the cell (transcellular pathway), what other route is there for the movement of ions and water at the PCT and what is this route dependent on?

Answer 5

• Paracellular pathway - i.e. through the gap junctions that are in between the epithelial cells of the PCT

Question 6

What two other molecules in the filtrate are reabsorbed in the proximal tubule and are coupled with Na+ reabsorption?

Answer 6

• Glucose
• Amino acids

Question 7

Explain the actions of carbonic annhydrase, various transporters and the transport of bicarbonate and hydrogen at the PCT and mention also how this is linked to sodium reabsorption here

Answer 7

• HCO₃^- and H⁺ are converted by carbonic annhydrase on the apical membrane of the epithelial cells in the PCT into H₂O and CO₂ which can freely diffuse across the membrane (as you’ve gone from polar to non-polar)
• Carbonic annhydrase also converts the H₂O and CO₂ now within the cell back into HCO₃^- and H⁺
• The now intracellular H⁺ is useful as its exchanged at an antiporter on the apical surface of the epithelial cell membrane with Na⁺ (so H⁺ is effluxed and Na⁺ is influxed to the cell)
• Glucose and Amino acids transport is also coupled to this transporter in order for influx to the epithelial cell
• HCO₃^- is then symported at the basolateral membrane with Na⁺ into the blood to complete the way in which this whole thing is coupled to Na+ reabsorption

Question 8

How are exogenous agents such as drugs removed in the kidneys?

Answer 8

• Drugs are removed by transport proteins that pick up drugs as they pass through the kidneys and transport them into the lumen

Question 9

Describe the permeability of the loop of Henle to water

Answer 9

• The descending limb is freely permeable to water but not to ions
• The ascending limb is impermeable to water but is permeable to ions

Question 10

What is the main channel present on the apical membrane of the epithelial cells of the ascending limb of the loop of Henle?

Answer 10

• Na+/K+/2Cl- cotransporter (symporter)

Question 11

What are the channels that are present on the basolateral membrane of the epithelial cells of the ascending limb of the loop of Henle?

Answer 11

• Na+/K+ ATPase
• K+/Cl- cotransporter

Question 12

Summarise the movement of ions across the ascending loop of Henle, mentioning the transporters involved here for transcellular movement, and mention another type of movement

Answer 12

• There is a sodium / potassium / chloride symporter on the apical membrane of the epithelial cells of the ascending loop of Henle so these all enter the cell
• There is a Na⁺ / K⁺ ATPase on the basolateral membrane so Na⁺ is exported into the blood for reabsorption and there is K⁺ is imported into the epithelial cell in exchange
• There is a K⁺ / Cl^- symporter on the basolateral membrane also which exports the K⁺ and Cl^- together out into the blood for reabsorption
• There is also paracellular movement of some ions such as Cl^- but this is a minor phenomenon within the ALOH

Question 13

Describe how the counter-current system is established and then say what the point of it all is

Answer 13

• The descending loop is permeable to water and impermeable to Na⁺
• The filtrate travels down the loop of Henle and meanwhile, Na+ moves from the ascending limb tubule to the interstitium thus making the interstitium hypertonic and the tubular fluid hypotonic
• Then, more fluid will come down the descending limb (permeable to water) and the hypertonic interstitium will attract water which leaves the descending limb as it is water permeable, making the tubular fluid more concentrated
• By increasing the concentration of the tubular fluid, even more Na+ will be reabsorbed and move into the interstitium. Remember, the ascending limb is impermeable to water so this prevents the destruction of this concentration set-up
• This occurs repetitively and you end up with a very hypertonic interstitium and hypotonic tubular fluid leaving the loop of Henle
• AKA the point is you’re trying to make the interstitium as concentrated as possible so by the time you get to the collecting duct, you’ll get lots of water reabsorption by osmosis as mediated by VP

Question 14

What are the main channels on the apical membrane of epithelial cells of the distal tubule?

Answer 14

• Na+/Cl- cotransporter
• Aldosterone dependent sodium channels

Question 15

Which channels are found on the basolateral membrane of the epithelial cells of the distal tubule?

Answer 15

• Na+/K+ ATPase
• K+/Cl- cotransporter (symporter which effluxes them both into the blood - reabsorption)

Question 16

Which aquaporin molecules are found in epithelial cells of the distal tubule?

Answer 16

• AQP2 – apical membrane
• AQP3/AQP4 – basolateral membrane

Question 17

Which vasopressin receptors are present on collecting duct cells?

Answer 17

• V2 receptors

Question 18

Describe the effect of aldosterone on collecting duct cells

Answer 18

• Aldosterone stimulates the production of Na+ channels on the apical membrane and the production of Na+/K+ ATPases
• Thereby greatly increasing the reabsorption of Na⁺

Question 19

Describe the effect of vasopressin on collecting duct cells

Answer 19

• Vasopressin stimulates the production and assembly of AQP2 molecules on the apical membrane, thus increasing the ability of the collecting duct to reabsorb water

Question 20

List the five groups of diuretic

Answer 20

1. Osmotic Diuretics
2. Carbonic Anhydrase Inhibitors
3. Loop Diuretics
4. Thiazide Diuretics
5. Potassium Sparing Diuretics

Question 21

Give an example of an osmotic diuretic

Answer 21

Mannitol

Question 22

Describe the mechanism of action of osmotic diuretics

Answer 22

• This is a pharmacologically inert chemical that can increase plasma and urine osmolarity
• It is filtered by the glomerulus but not reabsorbed
• Increasing the osmolarity of the filtrate means that less water leaves the lumen and is reabsorbed

Question 23

What are osmotic diuretics used for?

Answer 23

• They are mainly used for their effect in increasing plasma osmolarity –they draw out fluid from cells and tissues (e.g. in oedema)

Question 24

Give an example of a carbonic anhydrase inhibitor

Answer 24

• Acetazolamide

Question 25

Describe the mechanism of action of carbonic anhydrase inhibitors

Answer 25

* Inhibition of carbonic anhydrase reduces HCO3- reabsorption into the blood * It also reduces the amount of H+ available within epithelial cells to drive the Na+/H+ exchanger and allow Na+ reabsorption * Less Na⁺ reabsorption means less water reabsorption so more water in the urine to be excreted

Question 26

Give an example of a loop diuretic

Answer 26

* Frusemide (furosemide)

Question 27

What is the target of loop diuretics?

Answer 27

* Na+/K+/2Cl- cotransporter on the ascending limb of the loop of Henle

Question 28

Explain how loop diuretics exert their diuretic effect

Answer 28

* They block the triple transporter (Na⁺ / K⁺ / Cl^- symporter) thus reducing the reabsorption of Na+ in the ascending tubule * This increases the tubular fluid osmolarity thus reducing water reabsorption from the tubular fluid so the urine fluid volume increases

Question 29

Explain why loop diuretics cause an increase in urinary excretion of Mg2+ and Ca2+

Answer 29

* Potassium recycling, under normal conditions, means that there is a certain amount of K+ in the tubular fluid that can maintain the positive lumen potential and drive other positively charged ions (Mg2+ and Ca2+) into the interstitium via the paracellular pathway * Loop diuretics cause the loss of potassium recycling meaning that there is insufficient K+ in the lumen to drive the other positive ions through the paracellular pathway so you get increased urinary excretion of Mg2+ and Ca2+ instead

Question 30

Why do loop diuretics cause an increase in K+ loss?

Answer 30

* Loop diuretics increase the concentration of Na+ in the tubular fluid that is reaching the distal tubule * This means that there is increased Na+/K+ exchange is the distal tubule --\> increased K+ loss - hypokalaemia

Question 31

What is the main use of loop diuretics?

Answer 31

* Reducing oedema

Question 32

What are the unwanted effects of loop diuretics?

Answer 32

* Hypovolaemia * Hypotension * Hypokalaemia * Metabolic Alkalosis

Question 33

Give an example of a thiazide diuretic

Answer 33

* Bendrofluazide (bendroflumethiazide)

Question 34

Ho do thiazide diuretics work, where do they act and what do they act on?

Answer 34

* They act in the distal tubule * They bind to and inhibit the Na+/Cl- cotransporter * Thereby reducing reabsorption of these ions and threreby reducing water reabsorption

Question 35

What effect do thiazide diuretics have on Mg2+ and Ca2+?

Answer 35

* Increase in Mg2+ and Ca2+ reabsorption (unknown mechanism)

Question 36

What are the uses of thiazide diuretics?

Answer 36

* Hypertension * Heart failure * Nephrogenic diabetes insipidus * Idiopathic hypercalciuria

Question 37

What are the unwanted effects of thiazide diuretics?

Answer 37

* K+ loss – metabolic alkalosis * Inhibits insulin secretion (bad in diabetes mellitus)

Question 38

What effect do loop diuretics have on the macula densa cells?

Answer 38

* Macula densa cells have the same Na+/K+/2Cl- cotransporter that is present in the ascending limb of the loop of Henle and is targeted by loop diuretics * This means that loop diuretics prevent the entry of sodium into macula densa cells

Question 39

Where are macula densa cells found?

Answer 39

* At the top of the ascending limb of the loop of Henle * The top of the ascending limb comes very close to the afferent arteriole

Question 40

1) Explain the counter-productive effects of loop and thiazide diuretics on the renin-angiotensin system and why this is more severe with loop diuretics than with thiazide diuretics 2) What measure can be taken to prevent this from happening?

Answer 40

1) * Given that they cause a loss of Na+ in the urine, loop and thiazide diuretics will eventually cause reduced Na+ in the blood meaning that less Na+ is filtered in the glomerulus and hence less Na+ will reach the macula densa cells * Loop diuretics have this effect even more because they directly inhibit the Na⁺ / K⁺ / Cl⁺ which is also found on the macula densa cells in the DCT as well as on the PCT (PCT being the intended target), thereby reducing further the entry of sodium to the macula densa cells * A reduction in the Na+ reaching the macula densa is a stimulus for renin secretion * This leads to aldosterone production, which promotes sodium reabsorption (hence counterproductive to the effects we are trying to achieve with diuretics) 2) * Give ACE inhibitors with the diuretics

Question 41

What are the two classes of potassium sparing diuretic? Give an example of a drug that falls into each class

Answer 41

1. Aldosterone receptors antagonist – spironolactone 2. Inhibitors of aldosterone-sensitive sodium channels – amiloride * Basically both of them inhibit the actions of aldosterone but in different ways

Question 42

Why do potassium-sparing diuretics not have much effect on fluid loss i.e. weak diuretic effect?

Answer 42

* Because they act on the late DCT and CD mainly where there is a low [Na⁺] already so will not have much effect as they won't be able to manipulate the sodium as much

Question 43

Describe the effects of potassium-sparing diuretics

Answer 43

* They reduce sodium reabsorption in the late DCT, which leads to increased tubular osmolarity * This will result in reduced water reabsorption from the tubular fluid in the collecting duct * They also lead to increased H+ retention (because of reduced Na+/H+ exchange)

Question 44

What is the main use of amiloride?

Answer 44

* It is given with K+ losing diuretics

Question 45

What are the main uses of spironolactone?

Answer 45

* Hypertension/heart failure * Hyperaldosteronism

Question 46

State some unwanted effects of K+ sparing diuretics

Answer 46

* Hyperkalaemia – metabolic acidosis * Spironolactone (very non-specific action) – gynaecomastia, menstrual irregularities