Diuretics, ACE Inhibitors & RAAS

• Study notes on diuretics, ACE inhibitors, and related topics, covering mechanisms, uses, and more

Learning Outcomes

• Mechanisms of action for diuretics,ACE inhibitors, ARBs, mineralocorticoid receptor antagonists, natriuretic peptides, and neprilysin inhibitors should be understood
• Understand the role of immune modulators in kidney disease
• Understand the composition and uses of intravenous fluids

Role of Sodium

• High plasma sodium levels increases osmotic pressure
• High plasma sodium levels increases resistance through remodeling of small blood vessels and large arteries
• Increase in volume and resistance leads to endothelial dysfunction
• Endothelial dysfunction results from reduced production of nitric oxide
• The sympathetic nervous system tone is increased

Renin-Angiotensin-Aldosterone System (RAAS)

• Angiotensinogen, a 452 amino acid peptide, is synthesized in the liver
• Angiotensin I (Ang1-10) consists of the first 10 amino acids cleaved by renin
• Angiotensin I is inactive
• Angiotensin-converting enzyme (ACE) removes two terminal amino acids from Angiotensin I, forming Angiotensin II (Ang1-8)
• Angiotensin II can be further cleaved into bioactive peptides

Angiotensinogen

• Ang 1-7 acts as a vasodilator and increases nitric oxide levels, counterbalancing Ang II
• COVID virus binds to ACE 2 and reduces its proper function
• Des(Ang I)AGT is 11-452 and makes up 98% of the protein and has no known function

Angiotensin Receptors

• Angiotensin II interacts with angiotensin II receptors
• The two types of Angiotensin II receptors are AT1 and AT2
• AT1 and AT2 are G-protein-coupled receptors, specifically Gaq-linked
• Ang II binding to AT1 primarily causes vasopressor effects
• Vasopressor effects include vasoconstriction, elevated noradrenaline levels and increased sodium reabsorption

Aldosterone

• Aldosterone is a steroid hormone
• End goal of aldosterone is to promote sodium reabsorption
• Increases blood pressure

Diuretics

• Kidneys filter ions from plasma, drawing water along
• A portion of filtered ions get reabsorbed
• Increase urine production
• Classes of diuretics:
  • Loop diuretics
  • Thiazide diuretics
  • Potassium-sparing diuretics
  • Carbonic anhydrase inhibitors
  • Osmotic diuretics
  • Vasopressin receptor antagonists

Thiazide Diuretics

• Thiazide diuretics are based on benzothiadiazine
• Bendroflumethiazide is commonly used in the UK but not in the US
• Hydrochlorothiazide is seldom used in the UK, but is common in the US
• Block sodium reabsorption to increase water loss
• They block the Na/Cl channel in the proximal segment of the distal convoluted tubule
• Thiazide-like diuretics share similar mechanisms of action and structural differences, such as Indapamide (mainly in UK, seldom in US) and Chlorthalidone

Thiazide Diuretics - Adverse Effects

• Can cause hyponatremia due to overall water content being higher
• Compensation happens due to higher sodium in urnine
• Na+/Ca2+ exchange increases, which may lead to hypercalcemia
• Na+/K+ exchange increases, which leads to increase in K+ secretion
• In turn leads to hypokalemia, metabolic alkalosis, and hyperglycaemia
• Can also cause hyperuricemia due to increase in urate absorption, leading to gout

Loop Diuretics

• Loop diuretics have high plasma protein binding
• The drugs are Furosemide, bumetanide, and torsemide
• Organic anion transporters (OAT) transport loop diuretics into cells in the proximal convoluted tubule
• The multidrug resistance-associated protein 4 (MRAP4) excretes them into the urine
• They then inhibits a sodium potassium chloride co-transporter in the ascending limb of the loop of Henle
• Blocks re-absorption of Na+, K+, and Cl-
• Can result in renal dysfunction, hyponatraemia, hypokalaemia, hypochloraemia, and metabolic alkalosis

Potassium-Sparing Diuretics

• Hypokalemia is a common adverse effect with diuretics
• Epithelial Na channel (ENaC) blockers may be beneficial, they include Amiloride and Triamterene
• Drugs that cause diuresis without hypokalemia are very useful, such as Epithelial Na channel (ENaC) blockers
• Mineralocorticoid receptor antagonists:
  • Spironolactone
  • Eplerenone
  • Finerenone (non-steroid)
• ACE inhibitors and angiotensin receptor blockers are potassium-sparing but are not diuretics

Loss of Potassium

• Hypotension causes RAAS and aldosterone secretion activation
• Aldosterone promotes sodium reabsorption in the collecting tubule
• Which results in increased potassium secretion
• Preventing sodium reabsorption in collecting tubule prevents hypokalemia
• As a consequence, they can cause hyperkalaemia
• Can be co-administered with thiazide or loop diuretics to prevent hypokalemia

Potassium-Sparing Diuretics - MOA

• Blocks ENaC, which prevents Na+ reabsorption from urine
• Prevents the exchange of Na+ for K+ in the collecting tubules, which can prevent hypokalemia
• Aldosterone is a steroid that binds to the mineralocorticoid receptor
• Upregulates ENaC and Na+/K+ATPase

Carbonic Anhydrase

• Carbonic anhydrase catalyzes the interconversion of bicarbonate and carbon dioxide/water
• Formula: HCO3- + H+ ⇌ CO2 + H2O
• Distributed vastly throughout the body
• Aids with reabsorption of Na+, H+ and H2O from the urine in the kidneys
• Inhibitors increase excretion of Na+, H2O and HCO3-, for example Acetazolamide
• Diuresis is generally weak following its usage
• Beneficial in patients with oedema and metabolic alkalosis as the loss of HCO3- causes an acidosis which corrects the alkalosis

Effects of Acetazolamide

• In normal urine, pH is low, H2CO3 becomes H2O + CO2
• H2O + CO2 enter the cells at the proximal tubule
• Increases pH, H2O + CO2 become H+ + HCO3-
• NHE3 is a Na+/H+ exchanger
• kNBC1 is a Na+/HCO3 transporter

Effects of Acetazolamide - Treated

• AZM stays in the urine: H2CO3 <>x H2O + CO2
• HCO3- gets trapped in the urine
• H+ HCO3- are not raised in the cell
• NHE3 does not reuptake Na+
• KNBC1 cannot transport Na/HCO3 and gets stuck in blood

Osmotic Diuretics - Mannitol

• Poorly absorbed alcohol sugar
• It can be administered through IV
• Retains water in urine by increasing osmotic pressure
• Creates a water diuresis with little impact on sodium levels
• If the drug is still in the blood then it could increase blood volume

Sodium-Glucose-Linked Transport Inhibitors

• They are in the kidney
• Secondary active transporter
  • Na+/K+/ATPase pumps out 3 Na+ in exchange for 2 K+
• Then glucose is imported into the cell
• SGLT-2 recovers glucose from the urine
• Blocking SGLT-2 increases glucose in urine, which in turn acts as an osmotic diuretic

Vasopressin/Anti-Diuretic Hormone

• Anti-diuretic hormone (ADH, vasopressin) is a nonapeptide that is synthesized in the hypothalamus
• Acts on V receptors
• V1 are Gaq-linked receptors on blood vessels and lead to increase vascular tone
• V2 receptors Gas-linked and are found in the nephron, leads to increased expression of aquaporin 2 (AQP2)
• AQP2 is a water channel; increases water absorption

Vasopressin Receptor Antagonists

• Vasopressin receptor antagonists (vaptans)
  • Tolvaptan (V2 selective)
  • Conivaptan (non-selective)
• Increase water-loss
• Elevated plasma sodium levels
• Can decrease blood pressure
• Vaptans cannot be administered to patients with liver disease because they are hepatotoxic
• Cannot be used for more than 30 days

Natriuretic Peptides

• Family of peptides that cause Sodium excretion and include:
  • Atrial (ANP)
  • Brain (BNP)
  • C-type (CNP)
• Act by binding to specific receptors
  • NPR-A, -B & -C
• Metabolized by neprilysin
• Sacubitril is a neprilysin inhibitor
• Often administered with valsartan, an angiotensin receptor antagonist (ARNI)
• Increases sodium and water excretion
• Is used in cases of heart failure

Effects of ARNI's

• Neprilysin breaks down NPs.
• Inhibitors will protect these NPs and thus may cause green outcomes in patients
• Further, ARNI's reduce Neprilysin induced breaking down of AngII
• With that they have red outcomes
• ARBs prevent the red outcome

Clinical Use of Diuretics

• For hypertension
• They increase Na+ excretion which reduces blood pressure
  • Primarily thiazide diuretics
  • Loop diuretics (2nd line)
• Heart failure is associated with oedema
  • Loop blockers
  • Thiazides
  • Potassium-sparing to supplement other diuretics
• For treating Glaucoma
  • Use carbonic anhydrase inhibitors
• And to reduce Raised intracranial pressure
  • Use osmotic diuretics

Intravenous Fluids - Fluid Resuscitation

• Can treat blood loss by targeting hypovolemia
• Can correct electrolyte imbalance
• Hyponatremia and hypernatremia
• Multiple fluids or a choice between 2 types
• Crystalloids involve using water with electrolytes with possible addition of isotonic or hypotonic solutions
  • Normal saline
  • Ringer's
  • Lactated Ringer's (Hartmann's)
• Colloids – a suspension of insoluble particles
  • Albumin
  • Dextrans and starches

Crystalloids

• Standard fluid is normal (isotonic) saline (0.9% NaCl) and is unbuffered, having a higher NaCl concentration than plasma
• Increased NaCl concentration may cause hyperchloremic acidosis
• Buffered crystalloid (balanced solutions) have better outcomes
• Reduced NaCl with a buffer such as bicarbonate or lactate
  • Lactate Ringers
  • PlasmaLyte
• There is no clear reason to support a normal saline vs a balanced crystalloid
• Normal saline is used as an initial resuscitation fluid
• Balanced crystalloid as a second option of the case of large amount of fluid needing to be administered

Colloids

• Colloids consist of albumin or dextrans in suspension
• Hyperoncotic albumin can be used for volume expansion
• Crystalloid are more beneficial and hyperoncotic starch has been shown to increase cases of acute kidney injury and is not recommended for managing hypovolemia

Renal Failure

• Requires a patient to undergo dialysis
• Far from an ideal treatment option and is limited
• Most effective treatment is a kidney transplant, with a compatible donor
• The biggest challenge is graft rejection
• Immune suppressants are administered

Role of IL-2

• Is 15.5 Kda in protein mass
• Serves as an important stimulator of cells in the immune system
• Achieved by targeting Interlukin-2
• Also plays a major role in renal transplant rejection (which helps it serve as a target in renal pharmacotherapy)
• Achieved by:
  • Decrease IL-2 levels
  • Block IL-2 receptors
  • Block IL-2 signalling

Calcineurin Inhibitors

• Two major types:
  • Cyclosporine (CsA) and Tacrolimus (FK506)

Cyclosporine (CSA)

• Cyclic undecapeptide, an metabolite from fungus Tolypocladium inflatum
• Inhibits the (MLR) lymphocyte interaction and is not cytotoxic
• Inhibits anti-body production of T-dependent antigens

Tacrolimus

• Macrolide antibiotic isolated from Streptomyces tsukubaensis
• Has an affinity to inhibit MLR (identified in 1980s)
• Structurally un-related to Cyclosporine
• Shares similar qualities to Cyclosporine and can be 10 to 100x more potent

Mechanism of Action of Calcineurin Inhibitors

• Both Tacrolimus and CsA can inhibit gene transcription
  • IL, 3 and 4
  • GM - CSF
  • TNF(a)
  • INF(y)
• Show specific affinity with intracellular components for Ca2+ increase

Immunophilins

• Receptors within cells, that have the ability to bind drugs
• Immunophilin consist of about 0.1/0.4% of cells by way of protein
• They are isomers consisting of "proyl-peptidyl cis-trans"
• Involved in the early folding stages of proteins
  • Csa is often bound to "cylcophillin (CyP)"
• FK506 must be bound to "FK506-binding protein (FKBP)"

Calcineurin

• Has immunosupressive activity that is indepent of rotamase activity
• Drug/Immunopihlin complexes attach, and inhibit calcineruim
• Causing is to then become a theronine phophatase
• A substrate in N-FAT is needed to activate this T-cell, using nitrogen from the substance, as a transcription, and IL2 promotor

Rapamycin

• Has the form factor the same FK or "Sirolimus"
• It can already have the ability to supress the immune system through biotic factors
• Has "Streptomcyes Hygro", making it unique to FK506
• Does have some qualities that make it similar FK506
• Only binds to FKBP
• While has qualities that make it different

Target of Ramamycin (TOR)

• Complexes of ramacyin -FKBP, work without calcineurin
• Works by inhibiting "II-2" in relation to the receptor for induction
• As well as reduce epxression for molecule
  • And cause the cell cycle to halt at G

Other Immune Supressants

• Mycophenolate mofetil is one of the alternative options
• It converts to "mycophenolic acid" instead
  • which helps stop "IMPNDH (inosine monophosphate dehydrogenase)
• Its an immunosuppressant that does the opposite of proliferation
• Is combined with cases of renal transpalnts

Basiliximab

• A drug used as a anti body to -IL2
  • Is approved for those undergoing a transplant
• Used as induction method