Renin-Angiotensin-Aldosterone System (RAAS)

• Renin-Angiotensin-Aldosterone System (RAAS) is a module in the Renal System course (MED 204).
• Course lecturers are Dr. Stephen Keely & Dr. Patrick Walsh and the lecture is scheduled for February 2025.

Learning Objectives

• Understanding the factors controlling the distribution of body water.
• Defining normal plasma osmolarity and its contribution to sodium/water balance.
• Explaining the RAAS and its main functions.
• Defining the mechanisms that control the release of renin and aldosterone.
• Outlining the functions of renin and aldosterone.
• Discussing the causes and differences between primary and secondary hyperaldosteronism.

Water Distribution in the Body

• Total body water is distributed between intracellular fluid (ICF) and extracellular fluid (ECF) compartments.
• Total body water in a 70kg person is about 42L.
• Intracellular fluid (ICF) comprises 2/3 of TBW, so approximately 28L. 65% of Body fluid
• Extracellular fluid (ECF) comprises 1/3 of TBW, about 14L. 35% of Body fluid
• Interstitial fluid (IF) accounts for 10.5L of ECF.
• Plasma is about 3L.
• Transcellular fluid equates to 0.5L

Fluid movement

• Between ICF and IF is driven by osmotic pressure (ionic gradients).
• Between IF and Plasma is driven by hydrostatic pressure.
• Water ingestion is between 1-2 L.
• Water excretion is between 1-2 L (Urine, feces, sweat).

ECF Osmolarity

• Maintaining a constant concentration of solutes and electrolytes in the ECF is important for normal cellular function.
• Osmolarity equals the sum of solute + electrolytes divided by the volume of ECF.
• Normal ECF osmolarity is approximately 300 mOsm/L.
• Plasma osmolarity= 2 x [Na+] + 2 x [K+] + [Glucose] + [Urea]

Principle function of the renin-angiotensin system

• Control fluid volume and blood pressure

• Guyton and Hall 19.10 outline the pressure compensating effect of RAAS after severe haemorrhage

• Low blood pressure reduces oxygen perfusion

• A symptom is a lack of concentration, dizziness, nausea, fainting etc.

• RAAS responds to lowering of BP (over long and short terms).

Angiotensin in the renin-angiotensin system

• Angiotensinogen comes from the liver
• Renin is produced in the kidney.
• Angiotensin I is converted through endopeptidase.
• Angiotensin I becomes Angiotensin II (located in endothelial cells).
• Angiotensin II causes tubular reabsorption, aldosterone secretion, vasoconstriction, and ADH release.
• Through aminopeptidase, it can become Angiotensin III-IV
• This increases BP through tubular reabsorption, aldosterone secretion, vasoconstriction, and ADH release.

Renin

• Synthesized in juxtaglomerular cells in the kidney.
• Preprorenin (406 AA's).
• Prorenin (383 AA's).
• Active form is Renin (340 AA's)
• Stored in secretory granules.
• Released in response to decreased blood pressure.
• Half-life in the bloodstream is ≈80 minutes.
• Primary function is cleavage of the precursor protein Angiotensinogen to release Angiotensin I.
• Renin is the rate liming step
• Renin's location on Angiotensinogen is at Asp Arg Val Tyr Ile His Pro Phe His Leu Val Ile His Ser -439 aa

RAAS Activity

• Is measured clinically as PRA (Plasma Renin Activity) which is used in the diagnosis of patients with hypertension or hypotension.

Juxtaglomerular apparatus

• A specialized structure near the glomerulus is formed by the distal convoluted tubule and the glomerular afferent arteriole.
• Consists of macula densa, juxtaglomerular cells, and extraglomerular mesangial cells.
• Macula densa are modified epithelial cells of the DCT close in proximity to juxtaglomerular cells, Renin is synthesized at this site
• Special smooth muscle cells in afferent arterioles on entry to glomeruli

Renin Secretion

• Stimulated by decreased BP in the afferent arteriole.
• Sensed by baroreceptors in the Juxtaglomerular cells
• Stimulated by Decreased NaCl reabsorption by Macula Densa cells.
• MD cells stimulate release of renin from JG cells via prostaglandins
• Increased adrenergic activity is influenced via sympathetic renal nerves and circulatory catecholamines which induce renin release via Beta-adrenoreceptors
• Inhibited by Increased NaCl absorption across macula densa
• Inhibited by Increased afferent arteriolar pressure
• Inhibited by Decreased sympathetic activity
• Inhibited by Angiotensin II (negative feedback inhibition)
• Inhibited by GqPCRs and Ca2+ inhibit renin release
• GsPCRs and cAMP induce renin release

Increased renin secretion

• Conditions that increase renin secretion can be categorized into low blood pressure or fight or flight.
• Sodium depletion
• Taking Diuretics
• Haemorrhage
• Dehydration
• Heart failure
• Cirrhosis of liver
• Renin inhibitors (e.g. Aliskirin) can be used clinically to treat high blood pressure
• Sympathetic Nervous system activation or a surge of plasma adrenaline
• Guyton and Hall 19.10 is source that outlines pressure compensating effect of RAAS after severe haemorhage

Angiotensin II

• Derived from Angiotensinogen.
• Glycoprotein with 453 amino acid residues occurs in the liver.
• Present in high amounts in the bloodstream.
• Renin cleaves Leu-Val bond of angiotensinogen to form Angiotensin I.
• Angiotensin I is usually physiologically inactive.
• ACE (Angiotensin Converting Enzyme) is primarily located in endothelial cells in the lung.
• ACE is a carboxypeptidase enzyme, and it removes His and Leu from the carboxy terminus of Angiotensin I to form Angiotensin II.
• Angiotensin II is the main effector of the RAAS.
• ACE inhibitors are important drugs for reducing blood pressure (e.g., captopril).

Angiotensin Peptides Processing

• Angiotensinogen is processed to Angiotensin I by renin, which is the rate-limiting step.
• Angiotensin I is converted to Angiotensin II by ACE.
• Angiotensin II: Converted to Angiotensin III by AP , or by ACE2.
• Angiotensin 1-7: Is converted to Angiotensin III by AP (aminopeptidase).
• Location of Angiotensinogen peptides on chain are at positions is at Asp Arg Val Tyr Ile His Pro Phe His Leu Val Ile His Ser -439 aa

Angiontensin II receptors

• AT1R (primarily mediates cardiovascular effects of ANG II).
  • AT1A principally occur in blood vessels
  • AT1B occur in adrenal cortex and anterior pituitary
• AT2R more plentiful occur in foetus and neonate.
• Functions are unclear – activated by angiotensin III.
• AT1Rs are GPCR linked to PLC and elevations in Ca2+.
• AT1R receptor blockers (ARBs) is used for hypertension and cardiac failure (e.g. candesartan, eprosartan).

Biological Actions of Angiotensin II

• Includes Tubular Na+ reabsorption, Vasoconstriction, Aldosterone release, ADH Release and happens through Angiotensin II and AR1T
• Leads to inhibition of Renin Release from JG cells with feedback inhibition

Tubular Na Reabsorption

• Mediated by AT₁ receptors
• Angiotensin II stimulates sodium reabsorption in the PCT.
• Occurs via activation of AT1 receptors.
• Stimulates NHE across the luminal membrane
• Stimulates the Na+/K+ ATPase pump on the basolateral membrane
• Stimulates Na+/HCO3¯ co-transport across the basolateral membrane
• Leads to a Net increase in Na+ absorption to drive water reabsorption

Vasocontriction

• Increases overall systemic blood pressure.
• In the kidney, constriction of arterioles reduces the filtration rate and urine production. With reduced pressure in peritubular capillaries, resorption also increases

Aldosterone synthesis

• Synthesis of Aldosterone occurs in zona glomerulosa of the Adrenal Cortex - mediated by AT₁
• Compared to the zona fasciculata and zona reticularis regions, zona glomerulosa lacks 17ɑ hydroxylase activity and processes 'Aldosterone synthase' activity.

Aldosterone

• Is a Mineralocorticoid, it will act at the nuclear Mineralocortocoid Receptor (MR)
• Its plasma levels are very low (0.17nmol/L)
• T1/2 is about 20 minutes
• Metabolised principally by the liver
• It's principal function is to increase enhanced reabsorption of Na+ from distal convoluted tubules of kidney

Efftect of aldosterone - sodium

• on P cell of distal nephron will lead to Transcription, Translation and protein synthesis
• Ultimately this increases ENaC and Na/K ATPase, which leads to greater fluid abosrption

Aldosterone Secretion

• Juxtaglomerular apparatus interacts with Inhibits Angiotensinogen to increase Renin
• Angiotensin and converting enzyme to increase angiotensin II
• Adrenal Cortex interacts with and aldosterone
• Feedback mechanism regulating its secretion through with with renal arterial mean pressure as well as the a decrease in the discharge of renal nerves

Regulation of aldosterone

• Increased levels of extracellular fluid volume
• Decreased Na+ (and water) excretion

Hyperaldosteronism

• Too much aldosterone is produced by the adrenal glands.
• Incidence is about 5-10% of essential/primary hypertension.
• Common symptoms are hypertension, +/- hypokalaemia, adrenal adenoma and adrenal hyperplasia.
• Diagnosed by a decrease serum renin and an increase increase in serum aldosterone (renin-aldosterone ratio drops).
• It is treatable with aldosterone antagonists (e.g., spironolactone).

Causes of hyperaldosteronism

• Primary is caused by Adrenal adenoma (Conn's syndrome) and idiopathic adrenal hyperplasia
• Secondary is caused by decreased renal perfusion such as heart failure, Nephrotic syndrome, Cirrhosis of liver and Renal artery constriction (stenosis)

Hypoaldosteronism

• Too little aldosterone produced by adrenal glands.
• Primary is caused by a defective aldosterone release from the adrenals (e.g., Addison's Disease) and autoimmune destruction of Adrenal gland:
  • Affects about 1/10,000
  • Autoimmune destruction of Adrenal gland: • Hypovolaemia with urinary Na release and hyperkalemia • Postural hypotension • Dehydration • Diagnosis: decrease serum aldosterone, increase serum renin
• It can show defects from a variety of causes
• Is treatable with mineralocorticoids such as hydrocortisone, fludrocortisone as well as a salt diet

Secondary Hypoaldosteronism causes

• Occurs due to renal injury, leading to decreased renin production
  • Includes diabetes, can be from a variety of causes
  • obstructive nephropathy
  • chronic nephritis
• Diagnosis: ↓ serum aldosterone and ↓ serum renin

Anti-Diuretic Hormones

• Biological actions occur via stimulation of Anti-diuretic hormone secretion which is mediated by the AT1R receptor
• This is a peptide hormone of about 9aa long
• This hormone is synthesized at the pituatary glands
• These will act as GPCRs
• T1/2 minutes of about 20 minutes
• Primary functions of Vasoconstriction (Ca release) and Increasing water absorption in the kidney
• Ultimately will result to AVP2/cAMP production

Anti-Diuretic hormone and Aquaporin

• Increases water absorption in collecting ducts, which is due to increased luminal expression of Aquaporin 2
• Promotes the expression of urea transporters, which facilitates the absorption of urea into the medullary interstitium that ultimately drive H2O absorption

RAAS Summary

• Overall the reactions lead to:
  • Water and salt retention
  • Effective circulation within the volume increases
  • Perfusion of the juxtaglomerular apparatus increases

Clinical relevance of RAAS

• RAAS controls blood volume and arteriolar tone (in short and long term).
• When inappropriately activated, this can lead to hypertension.
• Pharmacologically, the RAAS can be targeted in the management of heart failure, hypertension, and acute myocardial infarction.
• Drugs used include, ACE inhibitors, Angiotensin receptor blockers, Aldosterone antagonists, and Renin inhibitors
  • These will decrease solute and water reabsorption, blood volume, and arteriolar tone.
• RAAS modulators are also used management of diabetes mellitus:
  • excess glucose damages the glomerulus, where it is exacerbated by high BP
  • RAAS will inhibit reduce glomerular pressure, thereby preventing diabetic nephropathy

Reading material

Principals of Medical Biochemistry. Meisenberg & Simmonds (2nd ed.) Clinical Biochemistry Gaw, Cowan, O'Reilly, Stewart, Shepard (2nd ed). Textbook of Biochemistry 3rd edition. Thomas Devlin Sparks et al 2014 review of RAAS in kidney physiology https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4137912/

RAAS Videos

General overview of RAAS: http://www.youtube.com/watch?v=BVUeCLt68lk J-G apparatus: http://www.youtube.com/watch?v=AtlhAhONHyM Aldosterone: http://www.youtube.com/watch?v=64_ZfIemRGM