Angiotensin receptor antagonists, better known as angiotensin II receptor antagonists, or angiotensin II receptor blockers (ARBs), are chemicals that constitute a pharmaceutical group named for their ability to suppress angiotensin. This is a peptide, or organic compound formed out of two or more amino acids, which constricts blood vessels. Angiotensin receptor antagonists are used to monitor the activity of the renin-angiotensin system (RAS). Also known as the renin-angiotensin-aldosterone system, this hormone system features angiotensin as a major participant and works to manage blood pressure and balance fluids.
The particular type of angiotensin that ARBs work against is angiotensin II. This version of the peptide is created when angiotensin I, which is formed out of the reaction of angiotensinogen with the enzyme renin, or angiotensinogenase, has two of its terminal residues removed by the enzyme angiotensin-converting enzyme (ACE). This substance is also known as the angiotensin I-converting enzyme, or kinase.
Angiotensin II does two things, both of which can affect blood pressure. First, it is responsible for contracting the muscles surrounding blood vessels. Second, it is responsible for causing the adrenal cortex to release the hormone aldosterone.
When the muscles tighten, the blood vessels are narrowed as well; this is known as vasoconstriction. That limits the transportation of blood and drives up the blood pressure, known as hypertension. Aldosterone also contributes to high blood pressure because it increases the blood’s reabsorption of sodium ions and water from the kidneys, which increases the volume of the body’s fluid.
The development of angiotensin receptor antagonists began toward the end of the 19th century, when a pair of physiologists discovered that rabbits developed hypertension when injected with renin. It was not until four decades later, however, that further research revealed that renin, acting on its own, does not cause high blood pressure, but catalyzes the creation of the peptides responsible for this condition. By the 1970s, it was well established that angiotensin II, as part of the renin-angiotensin system, causes harm to the heart and kidneys, consequently leading to the debut of angiotensin converting enzyme (ACE) inhibitors to suppress not angiotensin II itself, but the enzyme that forms it.
By the end of the century, however, several angiotensin receptor antagonists had hit the pharmaceutical market. Losartan, with the trade name Cozaar, is reputed as the first one to be marketed. Other angiotensin receptor antagonists include irbesartan, whose trade name is Avapro; candesartan, trade name Atacand; and Valsartann, trade Diovan. Besides controlling hypertension, ARBs are used to prevent kidney failure, congestive heart failure, diabetes and stroke, and physicians typically turn to them when patients cannot tolerate ACE inhibitors. Complications to be aware of with angiotensin receptor antagonists include low blood pressure, or hypotension; hyperkalemia, or heightened potassium levels in the blood; headache; drowsiness; sexual dysfunction; and dizziness.
