Antidiuretic Hormone (ADH)

ADH's full form in medical biology stands for an antidiuretic hormone, which refers to a hormone that helps blood vessels constrict and the kidneys control or balance the amount of water and salt in the body.
ADH (means against diuresis), most often spelt as Vasopressin, Pitressin, or Arginine Vasopressin (AVP), is a small peptide, i.e., a nonapeptide (nine amino acid peptide) hormone.

It is one of the two neurohormones synthesized by the neurosecretory cells present in the hypothalamus and carried to the neurohypophysis along the nerve fibers, where it is stored along with Oxytocin. It is released into the blood when required.

In the human body, this naturally occurring hormone controls several physical processes, preventing many life-risking fatal conditions such as bleeding abnormalities and septic shocks etc.

ADH Hormone: Synthesis and Storage

Although the posterior pituitary lobe, or neurohypophysis, does not synthesize hormones, it does store and secretes two peptide hormones, namely oxytocin and vasopressin.

ADH synthesis occurs in the supraoptic and paraventricular nuclei present in the hypothalamus. Specialized neurons of the hypothalamus with the ability to secrete known as “neurosecretory cells,” synthesize these hormones, which are then transported axonally via neurohypophysial capillaries to the posterior lobe.

Thus, the antidiuretic hormone (ADH or Vasopressin hormone) is secreted into the bloodstream by the posterior lobe of the pituitary gland present at the base of the brain.

Factors Affecting the Regulation of ADH

The release of ADH into the bloodstream is regulated by various factors. These include:

• A drop in the blood volume or blood pressure is detected by the large blood vessels, and the receptors in the heart activate the release of ADH.

• The rise in the salt concentration in the bloodstream also controls the secretion of ADH, which is identified by special neurosecretory cells in the hypothalamus.

• Alcohol consumption also prevents ADH from being released, which results in dehydration and increased urine production.

• The kidney in the body retains water if the levels of ADH are higher. When excess ADH is released when it is not needed, the blood gets diluted due to excess water retention.

• As a result, the salt level in the blood drops. High levels of ADH might result from pharmacological adverse effects, pulmonary conditions, etc. Leukemia, lymphoma, bladder cancer, brain cancer, etc. are all linked to elevated ADH.

Vasopressin Hormone

Vasopressin hormone (also known as ADH or antidiuretic hormone) is a natural hormone. This helps in the management of several life-threatening conditions and controls various bodily functions.

Vasopressin Hormone Functions

• Vasopressin Hormone causes vasoconstriction and clot formation.

• Vasopressin plays a significant role in regulating the biological clock.

• This hormone maintains the body's homeostasis, blood volume & the smooth and proper flow of urine from the kidneys.

• It also maintains an appropriate volume of water in the spaces surrounding the cells inside the body.

• It aids in smooth cellular functions.

Vasopressin Mechanism of Action

Vasopressin mechanism of action can be explained as follows-

Vasopressin exerts an antidiuretic action by enhancing the reabsorption of water from the kidneys back into circulation. Vasopressin affects V1 and V2 receptors to produce its effects.

It raises cyclic AMPs, which in return increase the water permeability of the tubules of the kidney. This is mediated by V2 receptors.

Additionally, it encourages smooth muscle contraction in the cardiovascular system and kidneys, which is accomplished through the activation of V1 receptors.

ADH Function

1. ADH causes constriction of blood vessels and raises blood volume or blood pressure, which is why it is called vasopressin.

2. It acts on the proximal renal tubule and promotes the absorption of water from glomerular filtrate in the uriniferous tubules of the kidney. Thus, it reduces the amount of water in the urine and hence concentrates it.

3. It is responsible for the contraction of smooth muscles of the intestine, gallbladder, urinary bladder, and blood vessels.

4. It is mainly responsible for body homeostasis. It also maintains the proper cellular functions.

5. ADH or Vasopressin plays a vital role in regulating the circadian rhythm.

Risk Factors and Levels of ADH

• A low level of ADH in the blood results in diabetes insipidus, where the reabsorption of water is reduced, resulting in excessive water loss through dilute urine. This often lowers blood pressure and increases urine volume.

• Hyposecretion of ADH also causes damage to the hypothalamus and pituitary gland.

• A high ADH level causes the blood to dilute and the plasma to be low in sodium.

• Hypersecretion of ADH can result in acute conditions with symptoms such as nausea, headache, dehydration, vomiting etc.

ADH Hormonal Disorders

A hyper or hypo secretion of ADH often results in several disorders.

These are as follows-

• Insomnia

• Head injuries

• Infertility

• Delayed puberty

• Frequent urination

• Autoimmune disorders

• Change in appetite etc.

Antidiuretics and Antidiuretic Drugs

Antidiuretics refer to those substances which control the fluid balance in the body by reducing urination. As the name suggests, these are “anti” to diuresis.

Hence, antidiuretic drugs are those drugs with antidiuretic properties which reduce urine volume, especially used in conditions such as diabetes insipidus (DI).

Examples include anti-diuretic hormones (ADH, vasopressin), Lypressin, Terlipressin, Thiazide diuretics, Amiloride, Indomethacin, Chlorpropamide etc.