What is angiotensin?
Angiotensin is a naturally produced hormone that forms part of a complex interaction of hormones, known as the renin-angiotensin hormone system that regulates blood volume, body fluid levels and blood pressure. Angiotensin is produced by the liver as angiotensinogen, which is converted to angiotensin I by the action of renin, released by the kidney when blood pressure and volume is low. Angiotensin I is the inactive precursor for Angiotensin II, which is produced by the action of Angiotensin Converting Enzyme (ACE) mainly in the lungs, but also in the kidney and endothelial cells lining the blood vessels.
Angiotensin II is a highly vasoactive hormone that has potent effects on blood pressure and fluid balance. It stimulates vascular smooth muscle to contract, which causes blood vessels to constrict (narrow). This increases blood volume while at the same time reducing blood vessel volume, which increases blood pressure. Angiotensin II also stimulates the production of the hormone aldosterone from the adrenal gland. Aldosterone stimulates the tubule cells of the kidney to reabsorb salt (sodium and potassium) and water from the blood, which also increase blood volume and pressure.
ACE also degrades the enzyme bradykinin, which is a potent vasoldilator, and this action of ACE further increases blood pressure.
Angiotensin II receptor
The binding of Angiotensin II to the Angiotensin receptor (AR) activates the receptor and this mediates all the actions of Angiotensin II via a series of trans-membrane and intracellular signal transduction systems, including activation or inhibition of enzymes (activation of phospholipases C and A2 and tyrosine kinases, inhibition of adenylate cyclase) and opening of calcium channels.
The Angiotensin II receptor is widely distributed and is found on the cells of many organs and tissues of the body, including the heart, blood vessel walls, kidney, adrenal gland, lung and brain. The consequences of activating the AR depend on which tissue the receptor is located in. In the cortex of the adrenal gland, activation of the AR by Angiotensin II stimulates the release of aldosterone, which in turn stimulates the reuptake of salt and water in the kidney. In the heart muscle, it causes cardiac hypertrophy or growth, which is a pathological condition associated with high blood pressure. In the vascular smooth muscle of blood vessel walls it causes vasoconstriction and narrowing of the blood vessels, which contributes to increased blood pressure.
Angiotensin II receptor blockers for hypertension and heart failure
The angiotensin receptor (AR) is a target for antihypertensive drugs since blocking the receptor also blocks all the actions of Angiotensin II that happen as a result of activation of the AR. Angiotensin receptor blockers (ARBs) cause vasodilation or widening of the blood vessels, which reduces blood pressure. They also reduce the production of aldosterone, which reduces uptake of salt and water by the kidneys, blood volume and fluid retention in the tissues, and therefore, ARBs are suitable for treating congestive heart failure.
Unlike ACE inhibitors, ARBs do not prevent the degradation of bradykinin, which further lowers blood pressure but also causes persistent dry cough as a side effect due to bradykinin. Therefore, ARBs are often used for those intolerant to ACE inhibitors.