Diuretics

Question 1

What is the function o the Na+/K+ ATPase in the kidney

Answer 1

To maintain the ceoncetration gradient of sodium

From the filtrate into the cell

(kidney wants to reabsorb as much sodium as possible) `

Question 2

How is water absorbed in the kidney

Answer 2

Due to sodium (osmosis)

Due to oncotic pressure of the proteins in the blood moves H2O from filtrate to the blood

Question 3

As well as transcellular movement what is the pathway, and what can move via this pathway

Answer 3

Paracellular

H2O, Na+, Cl-, HCO3-

Question 4

Outline physiology in the PCT

Why is CA necessary for bicarbonate absorption

Answer 4

Present in the cell and on the apical membrane

HCO3- in the filtrate reacts with H+ (which is exchanging with Na+, AA and glucose being pumped in) to form H2CO3… this is broken down into CO2 and H2O by CA, and then these are absorbed into the cell

Now within the cell, Carbonic anhydrase does the following to the CO2 and H2O:

CO2 + H2O–> H+ + HCO3-

Bicarbonate absorbed into blood (symport with Na+)

Hydrogen goes out with Na+/H+ antiporter (H+ out and Na+ in)

Then Glucose and amino acids enter via symport with Na+

Big gaps so more paracellular transport here

Why is CA necessary for bicarb absorption:
there are no HCO3- channels, so it cannot diffuse in

Question 5

As well as reabsorption of nutrients, what is another important function of the PCT

Answer 5

To remove exogenous angents across from the capillaries into the PCT and then into the filtrate for excretion

There are special transporters on the basolateral membrane which recognise side chains and functional groups revealed in phase I metabolism, and the kidney then moves these molecules into the filtrate for excretion

Question 6

What proportion of each substance is reabsorbed in the PCT

Answer 6

Almost 100% of glucose

70% of Na+ and HCO3-

Question 7

What happens in descending limb of the nephron

Answer 7

Water moves across (permeable)

Not permeable to Na+

No protein transport

Hypertonic in the interstitium with respect to the lumen so water moves by osmosis out of the tubule

The filtrate is isotonic (300mOsm)

Question 8

Characterise the ascending limb

Answer 8

IMPERMEABLE to water

Permable to Na+, Cl- and K+ due to the triple transporter Na+/K+/2Cl-…. k+ then flows back out into the lumen

Reduced paracellular route (Na+ flows through though, and a small amount of water, but ALOH is essentially impermeable to water)

NOTE: THE TRIPLE TRANSPORTER IS BRINING IN Na+ and Cl- FROM THE LUMEN INTO THE ALOH CELLS. THEN THE BASOLATERAL PROTEINS BELOW WILL DUMP IT INTO THE MEDULLARY INTERSITIUM TO INCREASE OSMOLARITY OF THE INTERSTITIUM

Basolateral proteins are the Na+/K+ and then K+/Cl- symporter

Question 9

How is the countercurrent effect achieved

Answer 9

Na+ moves out of the tubule, but the water cannot follow

So reduced concentration in ascending limb,
but higher concentrated in the interstitium

This then causes water to moves out of the descending limb to equilibrate with the surrounding interstitium

So descending limb increases in osmolarity

Question 10

How is the countercurrent exchange mechanism cyclic

Answer 10

Because after the first round has taken place, new filtrate will come into the nephron.

This will mean the more concentrated filtrate in the DLOH will be pushed around to the ALOH and some isotonic filtrate will enter the top of the DLOH

Now, more Na+ can be pumped out of the concentrated fluid that just entered the ALOH and, equilibration can occur again

A difference of 200mOsm can be maintained between the ALOH cell and the filtrate

Question 11

Why is the countercurrent system important

Answer 11

Because, WHEN THE COLLECTING DUCT BECOMES PERMEABLE TO WATER (when aqp inserted)

Then water will move into the interstitium if it is concentrated

Question 12

Where is aldosterone present

Answer 12

Only the late part of the distal tubule

Question 13

What is present in the early DCT

Answer 13

Apical membrane:
Na+/Cl- symporter

BL membrane:
Na+/K+ ATPase
K/Cl- symporter

Aldosterone not effective here

Also no aquaporins and not ADH responsive

Question 14

What is important in the end of the DCT/collecting duct

Answer 14

Aldosterone diffuses into cell and binds to MR

Increases number of Na+ channels and the number of Na+/K+ ATPase in the BL membrane

ADH binds to V2 receptors increases number of AQP 2 in the apical membrane

Question 15

T/F the DCT and collecting duct are permeable to water

Answer 15

F…. only with aquaporins. There are tight junctions between cells

Question 16

What is present in the collecting duct

Answer 16

No aldosterone, just ADH

Question 17

Differentiate principal cells and intercalated cells in the late DCT/ CD

Answer 17

Principal cells are the main Na+ reabsorbing cells and the site of action of aldosterone, K+-sparing diuretics, and spironolactone

Type A interacalated secrete acid and absorb HCO3

Type B intercalated secrete HCO3- and absorb H+

Question 18

How do diuretics work

Answer 18

Inhibit the reabsorption of Na+ and Cl-
i.e. inrease excretion

Increase osmolarity of tubular fluid, so there is less difference between blood and filtrate osmolarity (reduce osmotic gradient across epithelia)

Question 19

What are the 5 classes of diuretic, and site of action for first 2

Answer 19

1. Osmotic diuretics
   - whole kidney tubule e.g. mannitol (these are filtered but NOT reabsorbed, increase osmolarity of the filtrate, leading to increased reabsorption at PCT, DLoH and collecting duct ). Pharmacologically inert
2. Carbonic anhydrase inhibitors
   e. g. acetazolamide (works in PCT)

ABOVE NOT USED FOR DIURETIC EFFECT

3. Loop diuretics
   e. g. frusemide (furosemide)
4. Thiazides
   e. g. bendrofluazide (bendroflumethiazide)
5. Potassium sparing diuretics
   e. g. amiloride, spironolactone.

Question 20

What is the action of carbonic anhydrase inhibitors

Why do CA inhibitors increase K+ loss

Answer 20

CA is inhibited on the membrane and intracellularly.

Less H+ produced, which means less antiporting with sodium. Also less absorption of HCO3- as this cannot combine with the H+.

Less water reabsorption in the DLoH because of the increased Na+ and HCO3- in filtrate.

K+ excretion increased in attempt to reabsorb some Na+ at the collecting duct.

Therefore ↑ urine volume and ↑K+, ↑Na+ and ↑HCO3- excretion+

USED IN GLAUCOMA

Question 21

What is the target of frusemide

Answer 21

LOOP DIURETIC

Work on the ALOH.

Blocks the Na+/2Cl-/K+ channel that moves ions from the filtrate into the cell (these ions are then moved into the interstitium by basolateral proteins)

Question 22

Why is Na+, Ca2+ and Mg2+ lost with loop diuretics

Answer 22

Because of K+ handling

Normally when K+ is absorbed form the filtrate into the ALOH cell, it leaks back into the filtrate. This is potassium recycling

Because the 2Cl- that was reasorbed with the Na+ and K+ is moved into the interstitium via K+/Cl- symport and Cl- basolateral channels,

2 negative ions are moving out of the tubule, but and 1 positive ion is moving back in, causing a POSITIVE LUMEN POTENTIAL

But if the triple transporter is blocked, you can’t bring K+ into the cell, thus it can’t leak back into the tubule lumen

you get reduced positive lumen potential

Positive lumen potential is linked to Na+, Mg2+ and Ca2+ reabsorption via the paracellular route due to the positive charged repelling this cations through the paracellular route into the blood stream.

This is lost with loop diuretics

Question 23

Effect of loop diuretics on

Na+ reabsoption

H2O reabsoprtion

Other effects

Answer 23

Action on Na+ reabsorption: Inhibit Na+ and Cl- reabsorption in ascending limb – 30% (as the other 70% of the Na+ was reabsorbed in the PCT). Na+ reabsorption also reduced because of lack of positive lumen potential

Action on H20 reabsorption: increases tubular fluid osmolarity/ reduce osmolarity of medullary interstitium = reduced H2O reabsorption in the collecting duct.

Question 24

Other effects of loop diuretics not related to Na+ or H2o

Answer 24

Other effects: increase deliery of Na+ to distal tubule so K+ loss increases (aldosterone increases Na+/K+ transporter)… in common with thiazides.

Ca2+ and Mg2+ loss (due to the K+ effect), whereas with thiazides, Ca2+ absorption increased and only Mg2+ lost (unknown)

Question 25

How does bendrofluazide work Effects on Na+ reabsorption Effects on H2O reabsoprtion

Answer 25

Thiazide! Blocks Na+/Cl- in DCT Action on Na+ reabsorption: Inhibit Na+ and Cl- reabsorption in early distal tubule – 5-10% (most has been absorbed by the PCT and the ALOH by this point) Action on H20 reabsorption: increases tubular fluid osmolarity = reduced H2O reabsorption in the collecting duct. *Note that the DCT is impermeable to water so this is referring to medulary interstitium concentratin when aqauporins open :D*

Question 26

Side effects of thiazides

Answer 26

Increase delivery of Na+ to distal tubule increased K+ loss (increased Na+/K+ exchange) – in common with loops Increased Mg2+ loss and increased Ca2+ reabsorption (unknown why.... not to do with positive lumen potential though as this is unaffected by thiazaide diuretics)

Question 27

Why are thiazides less powerful than loop diurects.

Answer 27

* The further along the kidney you get, the less powerful the diuretic effect becomes * This is because there is less fluid and less sodium to impact * So with thiazides, max 5-10% Na+ loss and water loss

Question 28

What is the major problem encountered with thiazides and loop diurectics How can this be improved

Answer 28

Because increased excretion of Na+ which occurs with diurectics will reduce blood Na+ levels The reduced Na+ concentration of the filtrate is sensed by the macula densa cells which causes renin release and eventually aldosterone This will increase reabsorption of Na+ and thus water via principal cells. This will render the diurects less useful Improve by giving ACE inhibitor so that the increased angiotensin I produced by renin (angiotensinogen --> angiotensin I) cannot be converted to angiotensin II and thus cannot increase aldosterone release

Question 29

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Answer 29

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Question 30

Why does renin increase with diuretics

Answer 30

Because increased excretion of Na+ which occurs with diurectics will reduce blood Na+ levels Over time, the reduced blood Na+ will be reflected in reduced Na+ concentration of the filtrate (I know that diuretics are increasing Na+, but this is the long term effect of hyponatraemia) Hyponatreamia is sensed by the macula densa in the JGA in the DCT cells which causes renin release and eventually aldosterone

Question 31

Which diuretic has the biggest effect on renin secretion and why

Answer 31

The same triple channel that is blocked by loop diuretics in the ascending limb is the one responsible for moving the Na+ into the macula densa cells, and thus the sensing capability of these cells the macula densa cells will think there is no Na+ as the triple trnaporter is blocked by the loop diuretic. There would be huge release of renin, in addition to the indirect effect of reduced Na+ in the filtrate due to the hyponatraemia (long term)

Question 32

What are the targets of amiloride and spironolactone

Answer 32

Amiloride and spironolactone both POTASSIUM SPARING DIURECTICS Aldosterone receptor antagonists e.g. spironolactone (I.E. block the intracellular MR) Inhibitors of aldosterone-sensitive Na+ channels e.g. amiloride (i..e block the apical Na+ channels on principal cells responsible for the increased Na+ reabsorption mediated by aldosterone)

Question 33

Summary of action of potassiunm sparing diurectics Why are they referred to as potassium sparing

Answer 33

Action on Na+ reabsorption: Inhibit Na+ reabsorption (and concomitant K+ secretion) in early distil tubule – 5% Action on H20 reabsorption: increases tubular fluid osmolarity = reduced H2O reabsorption in the collecting duct.

Question 34

What are the other effects of potassium sparing diuretic

Answer 34

reduced reabsorption of Na+ in distal tubule increases H+ retention (reduces Na+/H+ exchange, which is antiporter) You can think about it that with all the other diuretics, the increased Na+ can attempt to be reabsorbed in the DCT in exchange for K+, which increases K+ loss. However, with potassium sparing, you have affected the machinery so that the Na+ and Na+/K+ are impaired so that you can't absorb Na+, nor can you excrete K+ with spironolactone so they are potassium sparing.

Question 35

Common side effects and why

Answer 35

Loop and thiazides: 1. Hypokalaemia 2. Hyponatraemia 3. Hypovolaemia (wanted) 4. Metabolic alkalosis (due to Cl- loss) 5. Hyperuricaemia K+ sparing diuretics: 1. hyperkalaemia (less Na+/K+ exchange)

Question 36

Why is there hyperuricaemia in loop and thiazide diurectics

Answer 36

To have an effect, the diuretic must get from the blood to the lumen, and must use a transporter. The transporter for loops/thiazide is called the ORGANIC ANION TRANSPORTER It is shared by uric acid transporter so uric acid cannot get from the blood into the filtrate

Question 37

How are thiazides used clinically

Answer 37

HTN: First line in most countries but Not used unless 'salt sensitive HTN' in UK (over 55 or afrocaribbean) because they cause you to go to the toilet a lot and patients don't like it Similarly effective, if not more effective than ACEi or CCB for lowering BP

Question 38

Why are thiazides used over other diuretics

Answer 38

Loop diuretics do cause more water loss (as more of the Na+ is still in the filtrate) But diuretics don't only lower BP due to reducing plasma volume (if they did, loop diuretics would be better) plasma volume is restored after 4-6 weeks due to renin Why thiazides then Chronic thiazide use doesn't just reduce blood volume, but BUT Thiazides also vasodilate, so they can reduce TPR long term after the blood volume has been restored

Question 39

Outline chronic thiazide use in HTN

Answer 39

Activation of eNOS (endothelium) , Ca2+ channel antagonism, opening of KCa channel (smooth muscle)

Question 40

As well as HTN, what else is diuretics used for

Answer 40

HF

Question 41

What is the impact of HF

Answer 41

Reduced CO output SNS will increase Increased RAS Increased Ang II... vasoconstrictor (increases TPR, afterload) Increased ang II and aldosterone affects heart remodelling Aldosterone increases Na+ and water, so increased volume, increased pressure on heart

Question 42

T/F SNS is helpful when activated in HF

Answer 42

F

Question 43

Which diuretics are used in HF

Answer 43

Loop diuretics (affect 30% of Na+ load) acute reduction in congestion massive fluid loss Less venous return/pressure on failing heart

Question 44

Why can loop diuretics not be used long term in HF

Answer 44

Loop diuretic will increase renin secretion (RAS) which will reduce the fluid loss effects this can also exacerbate HF because of the angiotensin and aldosterone in cardiac remodelling

Question 45

What else can be used for HF relating to diuretics

Answer 45

You can use the loop diuretic in combination with a potassium sparing diuretic This will prevent aldosterone effects due to RAS activation due to the loop diuretics

Question 46

What is the effect of spironolactone with loop diuretics

Answer 46

Spironolactone is a potassium sparing diuretic 30% reduction in risk of death compared to ACEi/loop diuretic