L23: Countercurrent Multiplier + ADH

Question 1

Benefits of counter-current exchange

Answer 1

Maximum amount transfer can be achieved since there is always a concentration gradient across
—> systems do not reach equilibrium
—> exchange takes place along entire length

Question 2

Loop of Henle

Answer 2

Descending limb:
Permeable to water, impermeable to salt

Ascending limb:
Pump out salt, impermeable to water
- Na/K-ATPase (basolateral membrane) pump out Na, create electrochemical gradient
—> allow NKCC (bumetanide sensitive) to reabsorb Na, K, Cl (Apical membrane) from tubular fluid

Question 3

***Countercurrent mechanism in LoH (Juxtamedullary nephron only, long LoH)

Answer 3

Salt is pumped out (ascending limb)
—> ↑ interstitial fluid osmolality
—> Water leaves through osmosis (descending limb)
—> ↑ osmolality of filtrate in descending limb
—> ↑ osmolality of filtrate in ascending limb
—> salt is pumped out (ascending limb)

Overall:

• lower into medullary region, more concentrated is filtrate in tubules
• longer the loop create greater the osmotic gradient

Question 4

Location of ADH synthesis (Hypothalamo-pituitary axis)

Answer 4

Magnocellular neurones in Paraventricular nucleus (PVN) + Supraoptic nucleus (SON) in Hypothalamus
—> Transported through axon to axon terminals in Posterior Pituitary gland
—> Stored in secretory granule (Herring bodies) inside magnocellular neurones
—> released in response to stimuli detected at cell body of SON + PVN
—> magnocellular neurones depolarised
—> propagate action potential
—> ↑ intracellular Ca in axon terminal
—> exocytosis of ADH-containing vesicle into fenestrated capillary

Question 5

Osmolality of body fluid

Answer 5

Controlled purely by changing amount of free water in body

Little change in Osmolality (1%) can trigger change in plasma ADH level significantly

Question 6

Detection of osmolality in body

Answer 6

Osmoreceptor (mechanosensitive receptor: Transient Receptor Potential Vanilloid TRPV, Cation channel)

Located in:

1. Organum Vasculosum of Lamina Terminalis (Circumventricular organ)
2. Subfornical Organ (Circumventricular organ)
3. Hypothalamic PVN + SON

Circumventricular organ: lack BBB, therefore can sense ionic + hormonal environment of systemic circulation

Question 7

Hyper-osmolality and Hypo-osmolality

Answer 7

Hyper-osmolality / Dehydration
—> cell shrinkage
—> conformational change in osmoreceptor
—> opening of pore loop
—> influx of Cation
—> depolarisation —> ADH release

Hypo-osmolality
—> cell stretched
—> suppress osmoreceptor
—> reduce Cation conductance
—> Hyperpolaristion
—> inhibit action potential
—> inhibit ADH release

Question 8

Formation of dilute urine in collecting duct

Answer 8

Simply allow dilute filtrate in LoH to pass to collecting duct

No AQP2 inserted —> no water reabsorbed —> large volume of dilute urine

Question 10

***Formation of concentrated urine in collecting duct

Answer 10

ADH bind to Vasopressin type 2 receptor (V2R)
—> phosphorylation of AQP2 at Ser256
—> trafficking of AQP2 intracellular vesicles to apical membrane
—> depolymerisation of apical actin network —> AQP2 sorting

Insertion of Aquaporin 2 (AQP2) on Apical membrane of Principal cells of Collecting duct
—> water diffuses out by osmosis (due to osmotic gradient in medulla created by LoH)
—> small volume of concentrated urine

Basolateral membrane: 本身已有 AQP3, AQP4

Question 11

Role of urea in urine concentration

Answer 11

Inner medullary collecting duct is permeable to urea (upper part of collecting duct is not permeable) via

• UTA1 (apical)
• UTA3 (basolateral)

As water is reabsorbed in collecting duct
—> urea concentration increases downwards
—> urea diffuses down concentration gradient (via UTA1, UTA3 in IMCD)
—> accumulation of urea in interstitium
—> contribute more to osmotic driving force to draw water from CD

ADH also increases urea permeability by phosphorylation —> ↑ in UTA1 (apical)

Question 12

Constituent of osmotic gradient in outer and inner medulla

Answer 12

Outer medulla: NaCl

Inner medulla: NaCl + urea

Question 13

Use of Vasa Recta in countercurrent mechanism

Answer 13

Vasa recta is freely permeable to water and solute

Vasa recta prevent osmotic gradient from being dissipated by
1. taking in the water coming out from descending limb
2. keep salt pumped out from ascending limb remain in interstitium
—> maintain osmolality of interstitium along cortico-medullary axis

Vasa recta has slow blood flow —> allow more time for equilibration between vasa recta and interstitium