Antidiuretics are medicines that help the body conserve water by reducing urine formation. They act on the kidneys to increase water reabsorption, especially in situations where the body loses too much water or cannot control fluid balance properly. The main natural antidiuretic hormone in the body is vasopressin, also known as antidiuretic hormone (ADH).
What Is Vasopressin?
Vasopressin is a hormone produced in the hypothalamus and stored in the posterior pituitary gland. It is released when the body needs to save water, such as in dehydration or low blood pressure. Vasopressin works by acting on specific receptors in the kidneys and blood vessels.
Types of Vasopressin Receptors
- V1 receptors – found in blood vessels; cause constriction and raise blood pressure
- V2 receptors – found in kidney tubules; help reabsorb water and reduce urine output
Because natural vasopressin is a peptide, it cannot be taken orally. It is given through IV, IM, SC, or intranasal routes.
Vasopressin Analogues
Several synthetic versions of vasopressin are used in clinical practice. They provide longer action and more selective effects.
1. Desmopressin (DDAVP)
Desmopressin is a selective V2 receptor agonist. It has strong antidiuretic action but almost no effect on blood vessels, making it safer than vasopressin.
Uses
- Central (neurogenic) diabetes insipidus – drug of choice
- Bedwetting (nocturnal enuresis)
- Bleeding disorders like haemophilia A and von Willebrand disease (helps release clotting factors)
Routes
Can be given orally, intranasally, or by injection.
2. Lypressin
Acts on both V1 and V2 receptors. It is longer acting than vasopressin but less commonly used today. Given parenterally.
3. Terlipressin
A prodrug that acts mainly on V1 receptors.
Uses
- Emergency treatment of bleeding esophageal varices – reduces blood flow to portal circulation and helps stop bleeding
Uses of Antidiuretics
Antidiuretics are used in several important medical conditions:
1. Diabetes Insipidus (DI)
DI is a condition where the body produces large amounts of dilute urine. There are two main types:
- Central DI – low ADH secretion from the brain; treated with desmopressin
- Nephrogenic DI – kidneys do not respond to ADH; desmopressin is ineffective
2. Bleeding Disorders
Desmopressin helps release clotting factors and is used in haemophilia A and von Willebrand disease.
3. Bedwetting in Children
Desmopressin reduces urine production at night, helping control nocturnal enuresis.
4. Bleeding Esophageal Varices
Terlipressin is preferred because it constricts blood vessels supplying the portal system.
Adverse Effects
Although useful, antidiuretics may cause some side effects:
- Nausea, vomiting, abdominal cramps
- Headache and dizziness
- Uterine cramps and backache (due to V1 action)
- Chest pain or worsening angina (vasopressin constricts coronary arteries)
- Local irritation with intranasal desmopressin
- Fluid retention and hyponatraemia – most serious risk
They should be avoided in patients with uncontrolled hypertension, coronary artery disease, or acute renal failure.
SIADH (Syndrome of Inappropriate ADH Secretion)
SIADH occurs when the body produces ADH in excess even when it is not needed. This leads to water retention, low sodium levels, and low plasma osmolality.
Causes
- Head injury
- Meningitis
- Brain tumors
- Severe lung diseases
Symptoms
- Headache
- Confusion
- Low sodium levels (hyponatraemia)
- Seizures in severe cases
Treatment
- Fluid restriction
- Demeclocycline (induces nephrogenic DI)
- ADH antagonists when required
Detailed Notes:
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