AIM Concept Integration
4th Year MBBS
Endocrine + Reproduction
4th Year MBBS
Endocrine + Reproduction
Disorders of Water Balance: Diabetes Insipidus, SIADH and Vasopressin Pharmacology
Connect normal vasopressin physiology with diabetes insipidus, SIADH and the pharmacological actions of desmopressin for rapid clinical revision. :contentReference[oaicite:0]{index=0}
1. THE TOPIC IN ONE CONNECTED FLOW
Vasopressin determines whether the collecting duct conserves or excretes water. Too little vasopressin action produces diabetes insipidus, while inappropriate excess action produces SIADH. The same V2 receptor–aquaporin pathway therefore explains the physiology, diagnostic findings and major treatment principles.
Physiological Trigger
↑ plasma osmolality or ↓ effective circulating volume
↑ plasma osmolality or ↓ effective circulating volume
→
Vasopressin Release
Hypothalamic synthesis → posterior-pituitary release
Hypothalamic synthesis → posterior-pituitary release
→
Renal V2 Action
Gs → cAMP → aquaporin-2 insertion
Gs → cAMP → aquaporin-2 insertion
→
Water Reabsorption
Collecting duct becomes more water-permeable → urine concentrates
Collecting duct becomes more water-permeable → urine concentrates
Too little effective vasopressin action → Diabetes Insipidus
Central deficiency or renal resistance
→
inadequate aquaporin-mediated water reabsorption
→
polyuria + dilute urine
→
polydipsia and possible hypernatremic dehydration.
→
inadequate aquaporin-mediated water reabsorption
→
polyuria + dilute urine
→
polydipsia and possible hypernatremic dehydration.
Excess inappropriate vasopressin action → SIADH
Persistent V2 stimulation
→
excessive free-water reabsorption
→
dilutional hyponatremia + low plasma osmolality
→
urine remains inappropriately concentrated.
→
excessive free-water reabsorption
→
dilutional hyponatremia + low plasma osmolality
→
urine remains inappropriately concentrated.
Central DI Treatment
Desmopressin → V2 stimulation → restores renal water conservation
Desmopressin → V2 stimulation → restores renal water conservation
↔
Nephrogenic DI Treatment
Correct cause + thiazide / amiloride / selected NSAID → reduce polyuria
Correct cause + thiazide / amiloride / selected NSAID → reduce polyuria
2. KEY CLINICAL CONNECTIONS
Polyuria: Central or Nephrogenic?
Water deprivation
→
urine remains dilute
→
desmopressin given.
→
urine remains dilute
→
desmopressin given.
Urine concentrates
→
central DI;
little response
→
nephrogenic DI.
→
central DI;
little response
→
nephrogenic DI.
Desmopressin: Benefit and Risk
V2 receptor activation
→
aquaporin-2 insertion
→
↓ urine volume in central DI.
→
aquaporin-2 insertion
→
↓ urine volume in central DI.
Excess antidiuresis + high water intake
→
water retention
→
dilutional hyponatremia.
→
water retention
→
dilutional hyponatremia.
Nephrogenic DI: Match Drug to Mechanism
Thiazide
→
mild volume contraction
→
↑ proximal water reabsorption
→
↓ urine volume.
→
mild volume contraction
→
↑ proximal water reabsorption
→
↓ urine volume.
Lithium-induced DI
→
amiloride blocks ENaC
→
↓ lithium entry into principal cells.
→
amiloride blocks ENaC
→
↓ lithium entry into principal cells.
3. AIM HIGH-YIELD INTEGRATION REVIEW
⭐ V2 receptor activation
→ cAMP
→ aquaporin-2 insertion
→ increased collecting-duct water reabsorption and concentrated urine.
→ cAMP
→ aquaporin-2 insertion
→ increased collecting-duct water reabsorption and concentrated urine.
Central DI
→ deficient vasopressin
→ dilute polyuria
→ urine concentration improves after desmopressin.
→ deficient vasopressin
→ dilute polyuria
→ urine concentration improves after desmopressin.
⭐ Nephrogenic DI
→ renal resistance despite available vasopressin
→ little desmopressin response
→ treat the cause and reduce urine output pharmacologically.
→ renal resistance despite available vasopressin
→ little desmopressin response
→ treat the cause and reduce urine output pharmacologically.
Lithium exposure
→ principal-cell concentrating defect
→ nephrogenic DI
→ amiloride reduces lithium entry through ENaC.
→ principal-cell concentrating defect
→ nephrogenic DI
→ amiloride reduces lithium entry through ENaC.
⭐ SIADH
→ inappropriate V2 activity
→ free-water retention
→ low plasma osmolality + hyponatremia with inappropriately concentrated urine.
→ inappropriate V2 activity
→ free-water retention
→ low plasma osmolality + hyponatremia with inappropriately concentrated urine.
Desmopressin
→ preferential V2 effect
→ strong antidiuresis with less V1 vasoconstriction
→ excessive effect may cause water intoxication and hyponatremia.
→ preferential V2 effect
→ strong antidiuresis with less V1 vasoconstriction
→ excessive effect may cause water intoxication and hyponatremia.
V1a receptor activation
→ increased vascular smooth-muscle calcium
→ vasoconstriction; this distinguishes vascular vasopressin action from renal V2-mediated antidiuresis.
→ increased vascular smooth-muscle calcium
→ vasoconstriction; this distinguishes vascular vasopressin action from renal V2-mediated antidiuresis.
AIM Exam Trap:
Diabetes insipidus and SIADH involve opposite water-balance errors:
DI → free-water loss with dilute urine,
whereas
SIADH → free-water retention with inappropriately concentrated urine.
Diabetes insipidus and SIADH involve opposite water-balance errors:
DI → free-water loss with dilute urine,
whereas
SIADH → free-water retention with inappropriately concentrated urine.