Vasopressin and Its Analogues: Pharmacology for the ICU

Overview and Endogenous Physiology

Vasopressin (arginine vasopressin, AVP; also called antidiuretic hormone, ADH) is a nonapeptide synthesised in the hypothalamus and stored in and secreted from the posterior pituitary gland. In all mammals except swine, the neurohypophyseal peptide is 8-arginine vasopressin - the terms vasopressin, AVP, and ADH are used interchangeably.

The hormone serves two principal physiological roles that are concentration-dependent:

1. At low plasma concentrations: Antidiuresis - acting on V2 receptors in the distal renal tubule and collecting duct to increase water reabsorption
2. At high plasma concentrations: Vasoconstriction - acting on V1A receptors on vascular smooth muscle

This dose-response relationship is critical to understanding both the physiology of shock and the pharmacological rationale for vasopressin use in the ICU.

Vasopressin also participates in stimulating ACTH secretion from the anterior pituitary, providing sustained activation of the hypothalamic-pituitary-adrenal (HPA) axis during chronic stress. This is mediated via V1B (V3) receptors on pituitary corticotrophs and may be particularly relevant in septic shock, where relative adrenal insufficiency is common.

Vasopressin Receptor Subtypes

Understanding receptor subtypes is fundamental to understanding both endogenous effects and the pharmacology of analogues.

V1A Receptor Mechanism (Vasoconstriction)

$$V_{1A} \rightarrow G_q \rightarrow PLC \rightarrow IP_3 + DAG \rightarrow \uparrow [Ca^{2+}]_i \rightarrow \text{smooth muscle contraction}$$

V2 Receptor Mechanism (Antidiuresis)

$$V_2 \rightarrow G_s \rightarrow \uparrow cAMP \rightarrow PKA \rightarrow AQP2 \text{ insertion into apical membrane} \rightarrow \uparrow H_2O \text{ reabsorption}$$

Clinically Available Vasopressin Agonists

The available agents span a spectrum from the non-selective endogenous peptide to highly V2-selective synthetic analogues.

The antidiuretic-to-vasopressor ratio is the key pharmacological metric distinguishing these agents: - Vasopressin: ratio = 1 (reference standard) - Desmopressin: ratio approximately 3,000× greater than vasopressin (near-pure V2 agonist) - Deamino [Val4, D-Arg8]AVP: ratio approximately 11,000× greater than vasopressin

This selectivity makes desmopressin the preferred agent for central diabetes insipidus, where pressor effects are undesirable.

Structural Basis of Receptor Selectivity

All vasopressin-like peptides are nonapeptides. Structural modifications confer receptor selectivity:

• Desmopressin: Deamination of the N-terminal cysteine + substitution of D-arginine at position 8 → eliminates most V1A activity, prolongs duration of action
• Terlipressin: Pro-drug consisting of three glycine residues added to lysine-vasopressin; cleaved by tissue peptidases to release active lysine-vasopressin → prolonged V1A-mediated vasoconstriction
• V1A and V2 receptor selectivity is structurally achievable; V1B selectivity is more difficult to engineer
• Oxytocin is [Ile³, Leu⁸]AVP - the structural similarity means high-dose vasopressin agonists may cross-react with oxytocin receptors, causing uterine contraction and GI smooth muscle contraction

Pharmacokinetics

The short half-life of endogenous vasopressin necessitates continuous infusion for haemodynamic indications in the ICU.

Non-Renal and Non-Vascular Effects

Several non-renal effects of vasopressin are clinically relevant in the ICU:

Coagulation Effects (V2-Mediated)

• V2 receptor activation (by both vasopressin and desmopressin) increases circulating levels of Factor VIII and von Willebrand factor
• Mechanism: stimulates release from storage sites in vascular endothelium (intermediate factors likely involved, as direct application to isolated vessels does not replicate this)
• This forms the pharmacological basis for desmopressin use in:
• Haemophilia A (mild-to-moderate)
• von Willebrand disease Type 1
• Uremic platelet dysfunction

Platelet Aggregation (V1A-Mediated)

• Vasopressin is stored in platelets
• V1A receptor activation on platelets stimulates aggregation

Hepatic Effects (V1A-Mediated)

• V1A receptor activation on hepatocytes stimulates glycogenolysis
• Potentially relevant to hyperglycaemia management in septic shock patients receiving vasopressin infusions

HPA Axis (V1B-Mediated)

• Vasopressin acts synergistically with CRF to promote ACTH release
• Provides sustained HPA activation during prolonged critical illness/stress
• Not the principal corticotropin-releasing factor, but maintains axis activation when CRF stimulus wanes

GI and Uterine Smooth Muscle

• At high concentrations, vasopressin causes GI tract smooth muscle contraction via V1A receptors
• Uterine contraction occurs via oxytocin receptor cross-reactivity
• Relevant: high-dose vasopressin infusions may cause intestinal ischaemia

Disease States: Diabetes Insipidus

Diabetes insipidus (DI) is a syndrome of impaired renal water conservation, clinically manifesting as: - Urine output >30 mL/kg/day - Dilute urine <200 mOsm/kg - Polydipsia (if thirst mechanism intact - may be absent in critically ill/sedated patients)

Pregnancy can accentuate or reveal DI by increasing plasma vasopressinase levels and reducing renal sensitivity to vasopressin.

Desmopressin is the treatment of choice for central DI due to its V2 selectivity, long duration of action, and resistance to vasopressinase degradation (due to deamination).

ICU Relevance

Vasopressin in Vasodilatory (Distributive) Shock

Vasopressin has become a cornerstone of vasopressor management in septic shock and other distributive states:

Rationale for use in septic shock: - Endogenous vasopressin stores are depleted in prolonged septic shock - a state of "relative vasopressin deficiency" - Vasopressin is a catecholamine-sparing vasopressor: it acts via a distinct mechanism (V1A → Gq, not adrenoceptors), restoring vascular tone independent of catecholamine sensitivity - Synergism with noradrenaline allows lower doses of both agents

Typical ICU dosing:

• Vasopressin is typically used as a fixed-dose infusion (not titrated to response like noradrenaline), reflecting its role as a hormone replacement rather than a pressor titration
• Doses above 0.04 units/min are associated with increased risk of digital, mesenteric, and coronary ischaemia - reflecting unopposed V1A-mediated vasoconstriction

Central DI in the ICU

Central DI is common in: - Traumatic brain injury - Post-neurosurgery (especially pituitary/hypothalamic surgery) - Brain death - Hyponatraemia overcorrection syndromes

Recognition clues in the ICU: - Sudden onset of large-volume dilute urine (>200-300 mL/hr) - Rising serum sodium and osmolality - Urine osmolality inappropriately low (<200 mOsm/kg) despite rising serum osmolality

Management: - Desmopressin: typically 1-4 mcg IV 8-12 hourly (titrated to urine output and sodium) - In brain-dead organ donors: careful management of DI is essential to maintain haemodynamic stability and organ viability - Risk of iatrogenic hyponatraemia with over-treatment - regular serum sodium monitoring mandatory

Desmopressin for Haemostasis in the ICU

• Consider in uremic platelet dysfunction (e.g., AKI/CKD patients with bleeding)
• Useful adjunct in mild haemophilia A or Type 1 vWD patients presenting with bleeding or requiring procedures
• Dose: typically 0.3 mcg/kg IV over 15-30 minutes
• Tachyphylaxis occurs with repeated dosing (depletion of endothelial vWF/Factor VIII stores) - generally limit to 1-2 doses within 24-48 hours
• Hyponatraemia risk: V2 stimulation causes free water retention - monitor sodium, especially in paediatric patients and those receiving hypotonic fluids

Variceal Haemorrhage

• Vasopressin and terlipressin reduce portal pressure via V1A-mediated splanchnic vasoconstriction
• Terlipressin is preferred in many centres due to its longer duration of action (4-6 hours), allowing bolus dosing
• Used as a bridge to definitive endoscopic therapy (sclerotherapy, banding)
• Relevant in ICU patients with decompensated liver disease/portal hypertension

Dose Adjustments in Organ Failure

Monitoring Parameters During Vasopressin Infusion in ICU

Key Examination Points

• Vasopressin is a non-selective V1/V2 agonist; its pressor effect is V1A-mediated; its antidiuretic effect is V2-mediated
• Desmopressin is ~3,000× more selective for V2 over V1A compared to vasopressin - hence used for DI without pressor effect
• V1B (V3) receptors mediate ACTH release - vasopressin supports HPA axis activation in critical illness
• All vasopressin agonists are nonapeptides; structural modifications alter receptor selectivity and duration of action
• Terlipressin is a prodrug - enzymatically converted to active lysine-vasopressin in tissues
• Desmopressin increases Factor VIII and vWF via extrarenal V2 receptors - important for haemostatic indications
• In septic shock, vasopressin is used at fixed low dose (0.03-0.04 units/min) as catecholamine-sparing therapy, not titrated as the primary vasopressor