Cardiovascular System and Diuretics

Learning Outcomes

• Mechanisms of action for diuretics should be understood
• Need to understand mechanisms of action for ACE inhibitors and ARBs
• Mechanisms of action for mineralocorticoid receptor antagonists is key
• Understand the mechanisms of action for natriuretic peptides and neprilysin inhibitors
• The role of immune modulators in kidney disease should be understood
• Need to be able to compare the composition and uses of intravenous fluids

Importance of Sodium

• Sodium is important in the increase of plasma volume by increasing osmotic pressure
• Sodium increases resistance by remodelling small resistance vessels and large elastic arteries
• Sodium increases endothelial dysfunction by reduction in NO production
• Sodium increases the sympathetic tone

Renin-Angiotensin-Aldosterone System (RAAS)

• Angiotensinogen is a 452 amino acid peptide produced in the liver
• Angiotensin I (Ang1-10) is the first 10 amino acids that are cleaved by renin
• Angiotensin I is the inactive form of renin
• Angiotensin II (Ang1-8) is produced when the terminal two amino acids are removed by angiotensin-converting enzyme (ACE) and its sequence is Asp-Arg-Val-Tyr-Ile-His-Pro-Phe
• Angiotensin II is further processed into bioactive peptides

Angiotensinogen and COVID Virus

• Ang 1-7 is a vasodilator that raises NO, and it is a counterweight to Ang II
• COVID virus binds to ACE 2, reducing its function
• des(Ang I)AGT is 11-452 and accounts for 98% of the protein with no known function

Angiotensin II Receptors

• Ang II functions on angiotensin II receptors
• Both AT1 and AT2 receptors exist
• G protein-coupled receptors are Gαq-linked
• Ang II binding to AT1 is primarily responsible for vasopressor effects like vasoconstriction, increased NAd production, and increased sodium reabsorption

Aldosterone Functions

• Aldosterone is a steroid hormone
• Promotes sodium reabsorption
• Increases blood pressure

Diuretics Overview

• Kidneys filter ions from plasma with water
• Some of the filtered ions get reabsorbed
• Production of urine is increased by diuretics
• Loop diuretics, thiazide diuretics, and potassium-sparing diuretics are classes of diuretics
• Carbonic anhydrase inhibitors, osmotic diuretics, and vasopressin receptor antagonists are classes of diuretics

Thiazide Diuretics

• Benzothiadiazine is the basis for thiazide diuretics
• Bendroflumethiazide is common in the UK but not in the US
• Hydrochlorothiazide is seldom used in the UK, but is common in the US
• Some diuretics like indapamide (mainly in UK) and chlorthalidone are structurally different but have similar MOA
• Sodium reabsorption is blocked, increasing water loss
• The Na/Cl channel in the proximal segment of the distal convoluted tubule is blocked

Thiazide Diuretics: Adverse Effects

• May cause hyponatremia
• Compensation occurs because urine Na+ is increased
• Hypercalcemia occurs due to increased Na+/Ca2+ exchange
• hypokalemia, metabolic alkalosis and hyperglycaemia occur due to Na+/K+ exchange

Loop Diuretics

• Highly plasma protein bound, such as furosemide, bumetanide, and torsemide
• Transported into cells by organic anion transporters (OAT) in the proximal convoluted tubule
• Excreted into the urine by multidrug resistance-associated protein 4 (MRAP4)
• Sodium, potassium, and chloride re-absorption are blocked
• Inhibit the Na+/K+/Cl- co-transporter in the ascending limb of the Loop of Henle
• May cause renal dysfunction, hyponatremia, hypokalemia, hypochloraemia, and metabolic alkalosis

Potassium-Sparing Diuretics

• Hypokalemia is a common adverse effect of diuretics
• Drugs that cause diuresis without hypokalemia are very useful
• Amiloride and Triamterene are the epithelial Na channel (ENaC) blockers
• Spironolactone, Eplerenone and Finerenone are the mineralocorticoid receptor antagonists
• ACE inhibitors and angiotensin receptor blockers are potassium-sparing but not diuretics

Potassium Loss Management

• RAAS and aldosterone secretion are activated by Hypotension
• Collecting tubule Na+ reabsorption is stimulated by Aldosterone
• K+ secretion increases as a result
• Preventing Na+ reabsorption in the collecting tubule prevents hypokalemia
• Hyperkalemia can result as a consequence
• Can be used with thiazide or loop diuretics to prevent hypokalemia, but not very powerful diuresis

Potassium-Sparing Diuretics MOA

• Na+ reabsorption from urine is blocked by blocking ENaC
• Hypokalemia is caused by preventing the exchange of Na+ for K+ in collecting tubules
• Mineralocorticoid receptor agonist is Aldosterone, a steroid
• ENaC and Na+/K+ATPase are upregulated

Carbonic Anhydrase Enzymes

• Carbonic anhydrase catalyze the interconversion of bicarbonate and carbon dioxide/water
• Occurs due to the equation HCO3- + H+ ⇌ CO2 + H2O
• It is widely distributed
• Reabsorption of Na+, H+ and H2O from urine is facilitated by in the kidney
• Excretion of Na+, H2O and HCO3- is increased by inhibitors of the enzyme and this can be induced by - Acetazolamide
• Diuresis is weak
• Useful in patients with oedema and metabolic alkalosis as acidosis which corrects alkalosis occurs from HCO3- loss

Osmotic Diuretics

• This alcohol sugar is Poorly absorbed
• It is Given IV
• Osmotic pressure is increased and it retains water in the urine
• Minimal impact on sodium and creates a water diuresis
• May initially act to increase blood volume once in the blood

Sodium-Glucose-Linked Transport

• Found in the kidney
• Na+/K+/ATPase pumps out 3 Na+ in exchange for 2 K+
• Secondary active transporters are used
• Cell then imports glucose
• Urine glucose is recovered by SGLT-2
• Acts as an osmotic diuretic and increases glucose in the urine when SGLT-2 is blocked

Vasopressin and ADH

• Anti-diuretic hormone (ADH, vasopressin) is a nonapeptide from the hypothalamus
• Acts on V receptors
• Vascular tone increases as V1 are Gαq-linked receptors on blood vessels
• Aquaporin 2 (AQP2) surface expression is increased in the nephron as V2 receptors Gαs-linked are activated
• AQP2 is a water channel and is important for water absorption

Vasopressin Receptor Antagonists

• These are called vaptans
• Examples include Tolvaptan (V2 selective) and Conivaptan (non-selective)
• Increases water loss
• Increases plasma Na+ concentration
• Can decrease blood pressure
• Should not be used for more than 30 days
• Patients with liver disease should not use Vaptans

Natriuretic Peptides Details

• Sodium excretion is increased by these peptides of a certain family
• Atrial (ANP), Brain (BNP), C-type (CNP) are examples of peptides
• NPR-A, -B & -C is how they act by binding to certain receptors
• Metabolized by neprilysin
• Is inhibited by Sacubitril
• With valsartan, an angiotensin receptor antagonist, it is used in conjunction
• This is an Angiotensin receptor/ neprilysin inhibitor (ARNI)
• Sodium and water excretion increases
• Used in heart failure

Clinical Use of Diuretics

• For hypertension
• Na+ excretion increase reduces blood pressure
• Thiazide diuretics as the primary option
• Loop diuretics as a 2nd like
• Heart failure associated with edema is also treated with diuretics
• Use loop blockers
• Use Thiazides
• Potassium-sparing to supplement other diuretics
• Glaucoma
• Carbonic anhydrase inhibitors
• Raised intracranial pressure
• Osmotic diuretics

Intravenous Fluids

• Treats blood loss (hypovolemia)
• Corrects electrolyte imbalance (hyponatremia & hypernatremia)
• Used to provide a supply of glucose
• Usually is a mixture of multiple fluids
• Can use crystalloids (water and electrolytes) which can either be isotonic or hypotonic
• Normal Saline, Ringer's, and Lactated Ringer's (Hartmann's) are examples
• Colloids are suspensions of insoluble particles
• Albumin, Dextrans and starches are examples

Crystalloids Details

• Commonly used
• Has a higher NaCl concentration than plasma and is unbuffered (0.9% NaCl) and is unbuffered
• May cause hyperchloremic acidosis
• Buffered crystalloid (balanced solutions)
• NaCl will be buffered with bicarbonate
• Lactate Ringers
• PlasmaLyte
• No clear evidence to support normal saline vs balanced crystalloid
• Normal saline is used as initial resuscitation fluid
• If a large volume of fluid is to be administered it is recommended balanced crystalloid as second line

Details on Colloids

• Contain albumin or dextrans in suspension
• Hyperoncotic albumin has more effective volume expansion
• But very expensive and clinical data does not show the benefit compared with crystalloid
• Can cause acute kidney injury
• Hyperoncotic starch is not recommended

Basic information regarding Renal Failure

• Requires a patient to undergo dialysis
• Not an ideal treatment option
• Kidney transplant from a compatible donor is the best option
• Biggest challenge is graft rejection
• Treated with immune suppressants

Role of IL-2

• It is a 15.5 kDa protein
• IL-2 acts on IL-2 receptors
• It is an important stimulator of cell-mediated immune system
• IL-2 is a key target in pharmacotherapy
• IL-2 plays an important role in renal graft rejection
• Options for treatment are as follows
  • Decrease IL-2 levels
  • Block IL-2 receptors
  • Block IL-2 signalling

Calcineurin Inhibitors

• Cyclosporine (CsA)
• Tolypocladium inflatum is a fungal metabolite
• Cyclic undecapeptide
• Inhibits mixed lymphocyte reaction (MLR) but not cytotoxic
• Inhibits antibody production against T-cell-dependent antigens
• However not for T-Cell Independant antigens
• Tacrolimus (FK506)
• Macrolide antibiotic isolated from Streptomyces tsukubaensis
• Inhibits MLR
• Not similar to cyclosporine
• More potent similar activity to cyclosporine

Calcineurin inhibitors Actions

-Tacrolimus & CsA inhibit gene transcription such as

• Interleukin-2, 3&4
• GM-CSF
• TNF-α
• IFN-γ
• Specific for pathways involving increase in intracellular Ca2+ (e.g., T-cell receptor)

Immunophilins

• Both drugs bind to a family of intracellular receptors known as immunophilins
• 0.1-0.4% of cell protein are from immunophilins
• They are prolyl-peptidyl cis-trans isomerases (rotamase)
• In protein folding
• CsA binds to cyclophilin (CyP)
• FK506 binds to FK506-binding protein (FKBP)

Details on Calcineurin

• Immunosuppressive activity is independent of rotamase activity
• Drug-immunophilin complexes bind to and inhibit calcineurin
• Calcineurin is a serine-threonine phosphatase
• Calcineurin and NFACT
• Nuclear factor of activated T- cells (NF-AT) is a substrate for calcineurin
• NF-AT is also a transcription factor and It binds to the IL-2 promoter

Rapamycin (Sirolimus) Drug

• Structural homolog of FK506
• Existing antibiotic shown to have immunosuppressant activity since 1989
• Isolated from Streptomyces hygroscopicus
• Similar profile to FK506
• Binds to FKBP and works by a different mechanism

Targets of Rapamycin

• Rapamycin-FKBP complex does not inhibit calcineurin or gene induction
• Rapamycin inhibits signal transduction by IL-2 receptor
• Rapamycin-FKBP binds to targets of rapamycin (mTOR 1) a phosphatidylinositol 3-kinase
• Leads to reduced expression of adhesion molecules like sphingosine-1-phosphate receptor
• Causes cell cycle arrest at G1

Other Immune Suppressants

• Mycophenolate Mofetil is converted to active metabolite mycophenolic acid
• Inhibits inosine monophosphate dehydrogenase (IMPDH)
• IMPDH is essential for guanine nucleotide synthesis
• It is both immunosuppressive and anti-proliferative
• Basiliximab is an Anti-IL-2 monoclonal antibody
• Recommended for renal transplants as prophylaxis with calcineurin inhibitors
• Approved for renal transplant
• Used in induction therapy