When it comes to keeping the body’s normal blood pressure and blood volume, there is one hormone in the body that is responsible for such tasks. This is none other than arginine vasopressin, which is also called the antidiuretic hormone. So, what are its functions and what are the related diseases when there is an over or underproduction?
What is Arginine Vasopressin?
Arginine vasopressin or AVP is a peptide hormone that is formed in the hypothalamus. It is then sent to the posterior pituitary with the help of axons. Afterward, it is released into the bloodstream.
AVP acts on the blood vessels and the kidneys which, in turn, control blood pressure. Its most vital function is to preserve the body’s fluid volume by reducing the quantity of water excreted in the urine. As mentioned, there are two primary sites of action where AVP is involved – the kidneys and the blood vessels.
- In the kidneys, AVP acts upon the collecting ducts through the V2 receptors. This increases water permeability which, in turn, leads to a decrease in urine production. Also, this action increases arterial pressure, cardiac output, and blood volume.
- In the blood vessels, AVP attaches to the V1 receptors in the smooth muscles. This causes vasoconstriction that results in an increase in arterial pressure.
Factors Affecting Release
Numerous mechanisms affect the release of AVP. Such factors include the following.
- Angiotensin II receptors found in the hypothalamus control the release of AVP.
- Hypothalamic osmoreceptors when triggered by extracellular osmolarities, such as dehydration, release AVP.
- Hypotension results in a heightened sympathetic activity which results in the release of high amounts of AVP.
- Hypovolemia, which occurs during dehydration or hemorrhage, causes a drop in arterial pressure. This results in cardiopulmonary baroreceptors, which are specialized stretch receptors found in the large veins and arterial walls, to enter the atria and decrease its firing rate. Afterward, it leads to an increase in AVP being released.
Healthcare professionals use an AVP test to diagnose the cause of electrolyte and fluid imbalance, or the cause of the overproduction of the antidiuretic hormone. In the test, a blood sample is taken. However, before the examination, the individual is advised to avoid consuming water or other beverages for at least four to six hours.
In addition to the AVP blood test, it is accompanied by an electrolyte test, urine test, and a physical examination. The doctor may also require that the laboratory test the blood for sodium levels and plasma osmolality.
A healthy, low, or high results from an AVP test helps doctors come up with a diagnosis. The standard and healthy amount of AVP is approximately 4.3 picograms per milliliter for adults. So, what happens if the amount is high or low?
Overproduction of AVP
When there is an overproduction or high amounts of AVP in the body, it leads to a condition known as Syndrome of Inappropriate Anti-Diuretic Hormone secretion. Also known as SIADH, it is a type of hyponatremia. This means there is an excess of AVP released into the bloodstream even when it is not needed.
With excessive water retention, it dilutes the blood and results in abnormal concentration levels of salt. An overproduction of AVP is likely a result of prescription medications as well as diseases involving the pituitary gland, hypothalamus, chest wall, or lungs. Moreover, certain tumors, especially those in lung cancer, produces AVP. Other types of cancer that result in high AVP are as follows.
- Brain Cancer
- Bladder Cancer
- Pancreatic Cancer
Furthermore, high levels of AVP results in a fluid imbalance that may lead to cerebral edema or seizures. As for moderate to high amounts of AVP in the bloodstream, it is likely caused by the following conditions.
- Multiple Sclerosis
- Guillain-Barre Syndrome
- Cystic Fibrosis
- Acute Intermittent Porphyria
Underproduction of AVP
On the other hand, when there is underproduction or low level of AVP, it causes the kidneys to remove excessive amounts of water. This, in turn, results in a blood pressure drop and dehydration. Furthermore, a low level of AVP likely indicates damage to the pituitary gland or hypothalamus. It may also indicate a condition known as primary polydipsia.
In primary polydipsia, which is also known as excessive or compulsive water drinking, the low levels of AVP indicate the body’s effort to remove excessive amounts of water. Another condition that may lead to low levels of AVP is diabetes insipidus, which is commonly associated with an increase in urine production and thirst.
Another cause of low AVP is due to low serum osmolality. This is a condition where the sodium levels in the body are deficient. Certain medications may also cause low levels of AVP. Such drugs include ethanol, phenytoin, and lithium.
Effects on Body
While it is commonly produced by the body, AVP can also be injected into the body, especially in cases where healthcare practitioners want to conserve water in the body of a patient. An antidiuretic agent injected into the body helps lessen the formation of urine and reduces the amount of water excreted via the urine as well.
As for its effect on the vascular system, antidiuretic hormones help constrict the arteries which leads to an increase of arterial pressure. This helps thwart possible damages caused by bleeding.
One of the common diseases associated with the antidiuretic hormone is none other than diabetes insipidus. Such a condition is a result of two possible situations.
Nephrogenic Diabetes Insipidus
This occurs when the kidneys are incapable of responding to AVP. Nephrogenic diabetes insipidus is likely caused by a form of renal disease. However, recent studies suggest that it may be due to a mutation in the gene encoding aquaporin-2 or the ADH receptor gene.
Hypothalamic Diabetes Insipidus
Also called central diabetes insipidus, this condition is due to a shortage of AVP secreted from the posterior pituitary. It is a result of either a tumor or infection in the hypothalamus or trauma to the head.
As mentioned above, one of the significant symptoms of diabetes insipidus is excessive thirst and urine production. Individuals diagnosed with this condition may produce more than 16 liters of urine in a day. The disease is rarely life-threatening, especially if there is enough water consumed. Moreover, treatment involves an exogenous antidiuretic hormone.