Vasopressin Decreases Urine Output Through Antidiuretic Effects
Vasopressin decreases urine output by promoting water reabsorption in the kidney through V2 receptor activation, which increases aquaporin-2 water channels in the collecting duct, allowing water to move from the tubule lumen into the hypertonic medullary interstitium and concentrate the urine. 1
Mechanism of Antidiuretic Action
Vasopressin's primary physiological role is to reduce free water excretion through the following cascade 1:
- V2 receptor activation on the basolateral membrane of collecting duct cells triggers adenylyl cyclase and cAMP production 1
- This leads to insertion of aquaporin-2 water channels into the apical membrane, dramatically increasing water permeability 1
- Water flows from dilute urine into the concentrated medullary interstitium, producing concentrated urine and reducing urine volume 1
- Vasopressin also promotes sodium reabsorption via ENaC in the distal nephron, which maintains the corticomedullary osmotic gradient necessary for maximal water reabsorption 2
Clinical Context: The Paradox in Shock States
However, in vasodilatory shock, vasopressin paradoxically increases urine output despite its antidiuretic properties 1, 3:
- At low doses (0.01-0.04 units/min), vasopressin preferentially constricts efferent arterioles, increasing glomerular filtration pressure 1, 3
- This results in greater urine output and creatinine clearance compared to norepinephrine at equivalent blood pressures 1, 3
- The improved renal perfusion and glomerular filtration overcomes the antidiuretic V2 receptor effects in this specific clinical scenario 4
- Multiple studies in septic shock demonstrate 79% increase in urine output at 4 hours after vasopressin initiation 5
Post-Discontinuation Effects
After stopping vasopressin, patients may develop reversible diabetes insipidus with polyuria, dilute urine, and hypernatremia 6:
- This occurs because the kidneys lose their ability to concentrate urine when vasopressin is withdrawn 6
- Monitor serum electrolytes, fluid status, and urine output after discontinuation 6
- Some patients require readministration of vasopressin or desmopressin to correct fluid and electrolyte shifts 6
Dose-Response Relationship
The antidiuretic effect follows a dose-dependent curve 7:
- Infusions as low as 1 pg/min/kg begin decreasing urine flow within 2 hours 7
- 5 pg/min/kg produces pronounced antidiuresis, reducing urine output from 14.4 to 0.9 ml/min 7
- Maximal urine concentration occurs at 25 pg/min/kg 7
- In shock states, doses above 0.04 U/min are not more effective and may cause cardiac arrest 1, 5
Critical Caveat
Avoid vasopressin in nephrogenic diabetes insipidus, where the kidneys cannot respond to vasopressin due to V2 receptor or aquaporin-2 dysfunction 1. In these patients, vasopressin will not decrease urine output and may worsen certain conditions like ADPKD 3.