21. Diuretics

Question 1

outline the functions of the proximal convoluted tubule

Answer 1

• permeable to water
• permeable to sodium
• Na-K-ATPase channels on the basal side work to retain the concentration gradient for sodium
• large gap junctions between endothelial cells resulting in the movement of water/electrolytes via the paracellular route

Question 2

why is carbonic anhydrase important?

Answer 2

• important enzyme in the PCT on the apical surface
• converts CO2 and water into hydrogen and bicarbonate, allowing substances to be moved from one side of the cell to the other

Question 3

how does carbonic anhydrase act on the surface of proximal tubule cells?

Answer 3

1. H+ is exported from the proximal tubule cell (actively pumped out by H+-ATPase or coupled with Na+ entry by an antiporter) at the lumen side
2. bicarbonate comes in through the tubular fluid and reacts with H+ to form carbonic acid in the lumen
3. carbonic anhydrase breaks H2CO3 into CO2 and water which enters the cell
4. inside the cell CO2 and water recombine to form carbonic acid
5. this then dissociates back into HCO3- and H+ in the cell
6. H+ is exported out the cell (step 1) and HCO3- is exported out of the cell at the interstitial (blood) side cotransported with Na+ or exchanged for Cl- by an antiporter

Question 4

what is glucose and amino acid transport always coupled to?

Answer 4

sodium movement

Question 5

what do certain special transporters in the proximal tubule do?

Answer 5

• recognise side chains and functional groups revealed in phase I metabolism
• if the side chain/group is present it is recognised by the kidney which moves the drug molecule into the lumen of the kidney for excretion in the urine

Question 6

what occurs in the descending limb of the loop of Henle?

Answer 6

• very permeable to water
• the tubule lumen side is more isotonic and the interstitium is more hypertonic so water travels transcellularly and paracellularly from the lumen to the interstitium (apical –> basal)

Question 7

what occurs in the ascending limb of the loop of Henle?

Answer 7

• apical membrane is impermeable to water
• a very small amount can move through the paracellular route
• sodium-chloride-potassium triple transporters on the apical membrane allow Na, Cl and K to move in to the cell
• the Na-K ATPase on the basal membrane maintains a sodium gradient

Question 8

what are the main features of the descending and ascending limbs of Henle?

Answer 8

DLOH: water is let out

• loose tight junctions
• not many mitochondria
• don’t pump ions

ALOH: Na and Cl pumped out

• very tight junctions
• lots of mitochondria
• very high metabolic activity

Question 9

describe the counter current mechanism that occurs in the loop of henle

Answer 9

• water leaves the descending limb
• in the ascending limb, water cannot leave but Na, Cl and K can
• as fluid moves up the ascending limb sodium is removed from the filtrate but water doesn’t follow
• the space between the 2 limbs becomes more hypotonic (concentrated with ions) because Na is added without the addition of water

Question 10

why do we need a counter current mechanism?

Answer 10

• to promote water movement from the collecting duct
• the process continues until there is a large concentration of sodium in the interstitium
• this acts as an osmotic gradient for water to move out of the collecting duct into the interstitium and then into the blood

Question 11

what occurs in the distal convoluted tubule?

Answer 11

• there is a Na-Cl cotransporter on the tubular side (apical membrane)
• on the basal membrane there is Na-K-ATPase (ensures the concentration gradient is maintained and that Na is reabsorbed into the blood)
• K and Cl transporters are also present on the basal side and contransport K and Cl into the interstitium

Question 12

what do vasopressin and aldosterone do in the collecting duct?

Answer 12

ALDOSTERONE

• mineralocorticoid which binds to the MR receptor and influences nuclear transcription
• increases transcription of Na channels and Na-K-ATPase –> cells reabsorb more sodium

VASOPRESSIN
- interacts with the V2 receptor and sticks aquaporin channels into the apical membrane –> allows water movement across the cell

Question 13

how does aldosterone work?

Answer 13

steroid hormone –> diffuses into cell and binds with steroid hormone intracellular receptors bound to chaperone proteins –> chaperone protein released from receptor –> dimerization of steroid hormone-receptor complexes –> enters nucleus –> transcription of gene you want

Question 14

what is liddle’s syndrome?

Answer 14

• an inherited disease of high BP
• mutation in the aldosterone activated Na channel
• channel is always ‘on’ –> Na retention –> hypertension

Question 15

how do diuretics work?

Answer 15

• by inhibiting the reabsorption of Na and Cl
• losing more ions to the urine means water follows
• they also increase the osmolarity of the tubular fluid

Question 16

what are the 5 major classes of diuretics? give examples of each

Answer 16

1. osmotic diuretics (e.g. mannitol)
2. carbonic anhydrase inhibitors (e.g. acetazolamide)
3. loop diuretics (e.g. frusemide)
4. thiazides (e.g. bendrofluazide)
5. potassium sparing diuretics (e.g. spironolactone)

Question 17

where do osmotic diuretics act?

Answer 17

throughout the kidney

Question 18

how do osmotic diuretics work?

Answer 18

• they are filtered by the glomerulus but not reabsorbed
• they increase the osmolarity of the tubular fluid
• the increased osmolarity is maintained throughout
• if there is a higher osmolarity in the tubule, less Na and Cl leave so less water leaves the tubule as there is less of a concentration gradient/osmotic force
• there is a decrease in water reabsorption where the nephron is freely permeable to water

Question 19

what are the clinical uses of osmotic diuretics?

Answer 19

pulmonary oedema, cerebral oedema

Question 20

how do carbonic anhydrase inhibitors work?

Answer 20

• act in the proximal tubule
• inhibits both versions of the carbonic anhydrase enzyme so the bicarbonate and hydrogen aren’t converted to CO2 and H2O effectively
• the little CO2 and H2O that gets in won’t be converted back to bicarbonate and hydrogen ions
• less hydrogen ions means there is less Na-H exchange at the apical membrane
• less Na enters cells and is reabsorbed, so less water follows via osmosis
• water follows Na, so if Na progresses through the tubule so does water

Question 21

what do carbonic anhydrase inhibitors do to K+?

Answer 21

there is loss of K+ recycling

Question 22

how do loop diuretics work?

Answer 22

• act at the ascending loop of Henle, targeting to triple transporter
• prevent Na from moving across the cell and into the interstitium, impacting the counter current effect
• Na reabsorption is massively impaired
• tubular fluid osmolarity increases and the medullary interstitium osmolarity decreases so there is less water reabsorption in the collecting duct

Question 23

what is potassium recycling?

Answer 23

• occurs in the PCT and in the LOH
• the constant movement of potassium replenishes the positive charge to the lumen (which contributes to the positive lumen potential)
• this leads to repulsion within the tubule
• there is a paracellular route between cells that allows Ca, Na and Mg to move across into the interstitium

Question 24

what happens if potassium recycling is interfered with?

Answer 24

if potassium recycling is interfered with potassium movement is reduced and the excess positive charge is diminished so less Ca, Mg and Na moves through the paracellular route

Question 25

what are the other effects of loop diuretics?

Answer 25

- increased delivery of Na+ to the distal tubule which promotes K+ loss (Na+/K+ exchange) - decreased reabsorption of Ca2+ and Mg2+ as a result of reduced K+ recycling

Question 26

what is the main use of loop diuretics and why?

Answer 26

oedema loop diuretics cause a large increase in urine volume and Na, Cl and K loss (and Ca and Mg)

Question 27

what are the unwanted effects of loop diuretics?

Answer 27

- hypokalaemia - hypovolaemia and hypotension - K+, Ca2+ and Mg2+ loss - metabolic alkalosis

Question 28

how do thiazide diuretics work?

Answer 28

- act on the Na-Cl transporter in the distal tubule (on the apical side) - has its action as it is being excreted - inhibits Na+ and Cl- reabsorption in the early distal tubule - increases the tubular fluid osmolarity leading to reduced water reabsorption in the collecting duct

Question 29

what are the other effects of thiazide diuretics?

Answer 29

- increased delivery of Na+ to the distal tubule - increases K+ loss (increased Na+/K+ exchange downstream) - increased Mg2+ loss - increased Ca2+ reabsorption

Question 30

what is the major problem with thiazide and loop diuretics?

Answer 30

- they have an effect on renin secretion - over time patients suffer a degree of hyponatraemia - low Na concentration stimulates renin secretion - renin increases aldosterone which promotes reabsorption

Question 31

what are diuretics often given with to reduce their effect on renin secretion?

Answer 31

ACE inhibitors

Question 32

what are the 2 classes of potassium-sparing drugs?

Answer 32

1. aldosterone receptor antagonists (e.g. spironolactone) | 2. inhibitors of aldosterone-sensitive Na+ channels (e.g. amiloride)

Question 33

what does spironolactone do?

Answer 33

- it is an MR inhibitor | - it decreases the ability of aldosterone to produce Na channels and Na-K-ATPase

Question 34

what does amiloride do?

Answer 34

prevents Na from getting into the cell

Question 35

what do potassium sparing diuretics do in general?

Answer 35

- inhibit Na+ reabsorption in the late distal tubule - increases tubular fluid osmolarity leading to a decrease in water reabsorption in the collecting duct - increased H+ retention (because of reduced Na+/H+ exchange) - increased loss of uric acid

Question 36

why are some diuretics not potassium-sparing?

Answer 36

they cause an increase in the concentration of Na+ reaching the collecting duct this leads to increased Na+/K+ exchange in the collecting duct and so K+ is lost to the urine

Question 37

why is hyperuricaemia common with loop and thiazide diuretics?

Answer 37

- they directly affect the transporter that moves uric acid into the lumen - there is a transporter on the basal side of kidney cells that exchanges organic ions with uric acid - less exchange occurs, leading to less uric acid moving into the cell and tubule from the blood - uric acid builds up in the blood, which can be associated with gout

Question 38

what are the clinical uses of diuretics? why?

Answer 38

first line treatments for hypertension in afro-caribbean people and anyone over 55 people over 55 and afro-caribbeans have salt-sensitive hypertension which is associated with low renin, so there is no point giving them ACE inhibitors

Question 39

what can chronic thiazide use result in?

Answer 39

vasodilatory effect decreased TPR, activation of eNOS (endothelium), Ca2+ channel antagonism, opening of K channel (smooth muscle) --> reduced BP

Question 40

why are diuretics used to treat heart failure?

Answer 40

- in HF there is less cardiac output - this leads to activation of the renin-angiotensin system - this eventually leads to Na and water retention - loop diuretics promote Na and water loss which decreases the work that the heart has to do - potassium sparing diuretics try to interfere further with the renin-angiotensin system so can be given with a loop diuretic