7. Diuretics Flashcards
What are the roles of the kidney?
Mnemonic REEM
R- regulatory: fluid balance, acid base balance, electrolytes balance
E- excretory: waste products, drug elimination
E- endocrine: renin, erythropoietin, prostaglandins, 1 alpha calcidiol
M- metabolic: vitamin D, insulin, morphine, paracetamol
Name the classes of drugs acting on the renal tubule?
COAL TAP
C: carbonic anhydrase inhibitors O: osmotic diuretics A: Aldosterone antagonists L: Loop diuretics T: Thiazide diuretics A: ADH antagonists P: Potassium sparing diuretics
Name 2 drinks which have a diuretic action and explain why
Alcohol - inhibits the release of ADH and therefore get a diuresis - urinate more than liquid consumed
Caffeine - increases the GFR and decreases sodium reabsorption. If you drink it regularly has no effect
How do diuretics reach their site of action?
- if given orally get absorbed through the gut into blood
- if given IV straight into blood
- can bind to albumin and be delivered to the peritubular capillaries which are next to the PCT
- they travel by OAT 1/3 transporters into PCT epithelial cells
- then travel via OAT 4 transporters to the lumen of the PCT
- from there they can make their way through the nephron to their site of action
In which kinds of patients, will diuretics be difficult to get to the site of action?
- nephrotic syndrome: these patients have an oedematous gut (as well as face, lungs and legs) making it difficult to absorb the diuretic in the gut
- heart failure: for diuretics to br absorbed need to have a good blood supply to the kidneys and gut which isn’t happening
- hypoalbuminaemia: if have low albumin then cannot transport the diuretic to the kidney tubules
Explain normal autoregulation i.e. when there is normal renal perfusion what do we see?
- if there is normal perfusion
- then intrarenal prostacyclin levels will be low
- prostacyclins are prostaglandins which cause afferent vasodilation
- GFR will be normal
- the level of circulating vasoconstrictors will be low
Explain what we would see in terms of autoregulation if there was reduced renal perfusion
- renal artery stenosis or hypovolaemia can lead to reduced renal perfusion
- this will lead to high levels of intrrenal prostacyclins which will lead to afferent vasodilation
- this will increase blood flow into the glomerulus
- this will maintain GFR - i.e. will be normal
- simultaneously there will be activation of RAAS and this will increase the levels of ATII and aldosterone which are circulating vasoconstrictors
- This will cause efferent vasoconstriction
How do NSAIDS and ACE inhibitors affect renal perfusion?
Particularly if someone has a disease state where there is reduced renal perfusion e.g. diabetes, hypertension, kidney disease etc…
- NSAIDs: block the action of prostacyclins (which are prostaglandins) so can prevent the afferent arteriole from vasodilating - hence can get a fall in GFR
- ACE inhibitors: reduce the activity of the RAAS system, so will get less ATII and aldosterone produced and thus reduced levels of circulating vasoconstrictors and hence less efferent arteriole vasoconstriction -hence can get a fall in GFR
In essence they are preventing normal renal autoregulation from occuring
What is the mechanism of action of carbonic anhydrase inhibitors e.g. acetazolamide?
- they inhibit the action of carbonic anhydrase in the PCT
- this prevents the reabsorption of sodium and bicarbonate ions in the PCT
Why would we use Carbonic anhydrase inhibitors e.g. acetazolamide?
- not used as diuretic - this is because any sodium not absorbed in the PCT would be reabsorbed later on
- mainly use for glaucoma and altitude sickness
Name some side effects of Carbonic anhydrase inhibitors
- metabolic acidosis (because not reabsorbing bicarbonate)
- renal stones
How do osmotic diuretics e.g mannitol work?
- osmolarity is the concentration of solute particles per litre
- osmotic diuretics increase the osmolarity of blood and renal filtrate
- this prevents the reabsorption of water along the nephron and so it is lost in urine (diuresis)
What is mannitol (osmotic diuretic) used for?
High intracerebral pressure
Name a side effect of mannitol (osmotic diuretic)
Allergic reaction
What is the mechanism of action of loop diuretics e.g. furosemide?
- they act on the TAL of loop of henle
- on the NKCC2 (sodium, potassium, 2 chloride)
- reduce reabsorption of sodium into the medullary interstitium
- this reduces the osmolarity of the interstitium
- therefore there is less of an osmotic gradient for water to be reabsorbed through aquaporin channels in the collecting duct
- also reduce potassium ion reabsorption
