angiotensin

Introduction 

Angiotensin is a hormone of the human body that is responsible for increasing blood pressure by vasoconstriction of vessels. This hormone is protein-based. 

The liver plays an important role in its formation. The liver secretes angiotensinogen, the inactive form of the hormone. The enzyme renin, secreted by the kidney, activates angiotensinogen to Angiotensin. The hormone flows in the bloodstream, and there it converts into Angiotensin II, which is responsible for controlling blood pressure. The Angiotensin-converting enzyme (ACE) is responsible for this conversion. 

Thus, when BP is low, the renin enzyme activates angiotensinogen, which eventually increases blood pressure. In the case of high blood pressure, drugs that inhibit its production are used.

Definition of Angiotensin

Angiotensin is a protein hormone that constricts blood vessels thus increasing blood pressure. It is a peptide hormone and part of renin- angiotensin system.

Angiotensinogen 

Angiotensinogen is the precursor of the active hormone, i.e., Angiotensin. It is a member of the serpin family of proteins. However, it is quite different from its other family members. For instance, other members have a generalised crystalline structure, whereas angiotensinogen has an elongated N-terminus.

renin cleaves angiotensinogen at the N-terminus to release AT1 protein. Angiotensinogen levels increase with the increase in plasma corticosteroids, estrogen, and thyroid hormones. 

Angiotensin Classification

Angiotensin exists in its inactive form, and the renin enzyme activates it. After activation, it converts into Angiotensin I and then into Angiotensin II in the bloodstream. 

Angiotensin classification has the following four categories:

1. Angiotensin I (AT1)- Angiotensin I is also referred to as proangiotensin. The renin enzyme acts on angiotensinogen to activate it. It breaks the peptide bond between the protein chain of leucine and valine. As the name suggests, renin is secreted by the kidneys. However, this type is not very significant except that it acts as a precursor for Angiotensin II.
2. Angiotensin II (AT2)- Angiotensin-converting enzyme (ACE) is responsible for the conversion of the precursor into its 2nd form. Angiotensin II is the functional protein that acts on the central nervous system. It enables the CNS to secrete more vasopressin for raising blood pressure.

It is also responsible for increasing the secretion of aldosterone, a corticosteroid hormone. Therefore, it can also be referred to as an endocrine hormone due to its activity on endocrine secretion. 

In case of high BP, it is necessary to inhibit the secretion of this hormone. Thus, several receptor blockers are available to block its activity.

1. Angiotensin III (AT3)- It is also a protein formed by removing an amino acid from the Angiotensin II by the Aminopeptidase A. It has less pressor activity than the II type. Angiotensin III triggers the activity of natriuresis; a process of excretion of sodium in urine by the activity of kidneys. 
2. Angiotensin IV (AT4)- It has a major role in the human central nervous system (CNS). However, it is not as active as the AT2 type. The exact activities of AT4 are not determined. It may play a major role as an insulin regulator or may be associated with memory acquisition and recall. 

Examples of AT2 Receptor Blockers

•     Irbesartan (Avapro)
•     Azilsartan (Edarbi)
•     Candesartan (Atacand)
•     Eprosartan
•     Losartan (Cozaar)
•     Telmisartan (Micardis)
•     Valsartan (Diovan)
•     Olmesartan (Benicar)

Side Effects of AT2 Receptor Blockers

Although there are not many side effects, AT2 receptor blockers should only be taken after prescription. Some of the side effects you may observe on intake of these drugs are as follows:

• Dizziness
• Higher potassium content in the body and lower sodium content
• Swelling in the skin
• Intestinal issues
• Diarrhoea
• Fatal for pregnant women.

Activity of Angiotensin 

Angiotensin binds with receptors in the body for starting its task. It constricts the blood vessels, thus increasing the blood pressure of the body. If there is low blood pressure in the body, the liver will secrete the renin enzyme to activate angiotensinogen. Thus, renin and Angiotensin act side by side in controlling or maintaining blood pressure. 

It triggers the thirst sensation too. It is also responsible for the secretion of aldosterone from the adrenal glands and the antidiuretic hormone from the pituitary. Aldosterone is a corticosteroid hormone that maintains sodium concentration in blood. Thus, Angiotensin also increases sodium absorption in blood by the secretion of the aldosterone hormone. 

High Level of Angiotensin in Blood

If the body starts producing an excess of this protein (specifically AT2), it may be fatal. The excess production may be due to hypertension that increases blood pressure. High blood pressure due to excess production of this protein can sometimes cause heart failure. Therefore, in case of high BP, the doctors can suggest some drugs to lower the secretion. The drugs contain Angiotensin-converting enzyme inhibitors to stop the conversion of the protein into its active form. 

Low Levels of Angiotensin in Blood

Low levels of AT2 can also cause serious health issues. It can cause low BP, and also increases the potassium content in blood and decreases the sodium content. This can lead to an increase in urine formation. Thus, it is necessary to have an adequate amount of AT2 in blood. 

Conclusion 

Angiotensin is a proteinaceous hormone that is responsible for increasing blood pressure in the human body. This is of great importance as it maintains the blood pressure and also the sodium-potassium content. It is present in an inactive form as angiotensinogen, which is activated by the action of the renin enzyme. Angiotensin has four categories. However, AT2 is more important than others as it performs many functions in the body.