Pharmacology · Cardiovascular

Vasopressin Pharmacology and Vaptans

Receptor subtypes, SIADH, vaptan comparison, and desmopressin uses

Vasopressin acts through two receptor subtypes with distinct tissue distributions and effects: V1 receptors on vascular smooth muscle produce vasoconstriction without tachycardia, making vasopressin useful as an adjunct vasopressor in catecholamine-refractory septic shock; V2 receptors on renal collecting duct principal cells insert aquaporin-2 water channels, producing concentrated urine — the primary antidiuretic mechanism.
SIADH results from vasopressin secretion despite normal or low plasma osmolality, causing free water retention and dilutional euvolemic hyponatremia; the key diagnostic findings are urine osmolality above 100 mOsm/kg despite low plasma osmolality and urine sodium above 40 mEq/L in a euvolemic patient — distinguishing it from the hyponatremia of heart failure, cirrhosis, and hypovolemia.
The most critical rule in hyponatremia management is the correction rate limit: chronic hyponatremia must not be corrected faster than 8 to 10 mEq/L per 24 hours; overcorrection causes osmotic demyelination syndrome — a largely irreversible neurological injury due to rapid osmotic shifts damaging myelin sheaths in the pons and extrapontine structures.
Vaptans produce aquaresis (electrolyte-free water excretion) by blocking V2 receptors: tolvaptan is oral and selective for V2, approved for euvolemic and hypervolemic hyponatremia, but carries a black box warning for hepatotoxicity and is limited to 30 days of use with hospital initiation; conivaptan blocks both V1 and V2 and is given intravenously in hospital only — its V1 blockade can worsen hypotension.
Desmopressin is a synthetic vasopressin analog with selective V2 agonism and no V1 activity, used in central diabetes insipidus (replacing deficient vasopressin to concentrate urine), nocturnal enuresis, and mild hemophilia A or type 1 von Willebrand disease — where V2 activation on vascular endothelial cells triggers Weibel-Palade body exocytosis, releasing stored von Willebrand factor and factor VIII; tachyphylaxis develops with repeated doses within 24 to 48 hours.

Feature	Detail
Mechanism	Vasopressin secreted despite low osmolality → free water retained → dilutional hyponatremia
Volume status	Euvolemic (distinguishes from heart failure, cirrhosis)
Key lab finding	Urine osmolality >100 mOsm/kg despite low plasma osmolality; urine Na >40 mEq/L
Common causes	CNS disorders, pneumonia, small cell lung cancer, drugs (SSRIs, carbamazepine, thiazides)
Treatment ladder	Fluid restriction → vaptans or salt tablets → hypertonic saline (severe/symptomatic)

Indication	Mechanism and Notes
Central diabetes insipidus	Replaces deficient vasopressin; V2 activation concentrates urine. Ineffective in nephrogenic diabetes insipidus (receptor absent or blocked)
Nocturnal enuresis	Reduces overnight urine volume; fluid restriction required after dosing to prevent hyponatremia
Mild hemophilia A / Type 1 von Willebrand disease	V2 activation on endothelial cells → Weibel-Palade body exocytosis → von Willebrand factor and factor VIII release. Tachyphylaxis with repeated doses within 24–48 hours

Suggested References

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