Angiotensin is a peptide hormone that causes vasoconstriction and a subsequent increase in blood pressure. It is part of the renin-angiotensin system, which is a major target for drugs that lower blood pressure. Angiotensin also stimulates the release of aldosterone, another hormone, from the adrenal cortex. Aldosterone promotes sodium retention in the distal nephron, in the kidney, which also drives blood pressure up.
Angiotensin is an oligopeptide and is a hormone and a powerful dipsogen. It is derived from the precursor molecule angiotensinogen, a serum globulin produced in the liver. It plays an important role in the renin-angiotensin system. Angiotensin was independently isolated in Indianapolis and Argentina in the late 1930s (as ‘angiotonin’ and ‘hypertensin’, respectively) and subsequently characterised and synthesized by groups at the Cleveland Clinic and Ciba laboratories in Basel, Switzerland.[1]
Angiotensinogen is an α-2-globulin produced constitutively and released into the circulation mainly by the liver. It is a member of the serpin family, although it is not known to inhibit other enzymes, unlike most serpins. Plasma angiotensinogen levels are increased by plasma corticosteroid, estrogen, thyroid hormone, and angiotensin II levels.
Angiotensinogen is also known as renin substrate. Human angiotensinogen is 453 amino acids long, but other species have angiotensinogen of varying sizes. The first 12 amino acids are the most important for activity.
Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu-Val-Ile-… Angiotensin I (CAS# 11128-99-7) is formed by the action of renin on angiotensinogen. Renin cleaves the peptide bond between the leucine (Leu) and valine (Val) residues on angiotensinogen, creating the ten-amino acid peptide (des-Asp) angiotensin I. Renin is produced in the kidneys in response to renal sympathetic activity, decreased intrarenal blood pressure ( Angiotensin I appears to have no biological activity and exists solely as a precursor to angiotensin II.
Angiotensin I is converted to angiotensin II (AII) through removal of two C-terminal residues by the enzyme angiotensin-converting enzyme (ACE), primarily through ACE within the lung (but also present in endothelial cells and kidney epithelial cells). ACE found in other tissues of the body has no physiological role (ACE has a high density in the lung, but activation here promotes no vasoconstriction, angiotensin II is below physiological levels of action). Angiotensin II acts as an endocrine, autocrine/paracrine, and intracrine hormone.
ACE is a target for inactivation by ACE inhibitor drugs, which decrease the rate of AII production. Angiotensin II increases blood pressure by stimulating the Gq protein in vascular smooth muscle cells (which in turn activates an IP3-dependent mechanism leading to a rise in intracellular calcium levels and ultimately causing contraction). In addition, angiotensin II acts at the Na/H exchanger in the proximal tubules of the kidney to stimulate Na reabsorption and H excretion which is coupled to bicarbonate reabsorption. This ultimately results in an increase in blood volume, pressure, and pH.[3] Hence, ACE inhibitors are major anti-hypertensive drugs. Other cleavage products of ACE, seven or 9 amino acids long, are also known; they have differential affinity for angiotensin receptors, although their exact role is still unclear. The action of AII itself is targeted by angiotensin II receptor antagonists, which directly block angiotensin II AT1 receptors.
Angiotensin II is degraded to angiotensin III by angiotensinases located in red blood cells and the vascular beds of most tissues. It has a half-life in circulation of around 30 seconds, whereas, in tissue, it may be as long as 15–30 minutes.
Angiotensin III has 40% of the pressor activity of angiotensin II, but 100% of the aldosterone-producing activity. Increases mean arterial pressure.
Angiotensin IV is a hexapeptide that, like angiotensin III, has some lesser activity. The sequence of angiotensin I: Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu. The sequence of angiotensin II: Asp-Arg-Val-Tyr-Ile-His-Pro-Phe. The sequence of angiotensin III: Arg-Val-Tyr-Ile-His-Pro-Phe. The sequence of angiotensin IV: Val-Tyr-Ile-His-Pro-Phe.
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Reference [1] Basso N, Terragno NA; Terragno (December 2001). “History about the discovery of the renin-angiotensin system”. Hypertension 38 (6): 1246–9. [2] Le, Tao (2012). First Aid for the Basic Sciences. Organ Systems. McGraw-Hill. p. 625. [3] Yvan-Charvet L, Quignard-Boulangé A (2011). “Role of adipose tissue renin-angiotensin system in metabolic and inflammatory diseases associated with obesity”. Kidney Int. 79 (2): 162–8.
