Angiotensin

Angiotensinogen

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

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 (<90mmHg systolic blood pressure ) at the juxtaglomerular cells, or decreased delivery of Na+ and Cl- to the macula densa. If a reduced NaCl concentration in the distal tubule is sensed by the macula densa, renin release by juxtaglomerular cells is increased. This sensing mechanism for macula densa-mediated renin secretion appears to have a specific dependency on chloride ions rather than sodium ions. Studies using isolated preparations of thick ascending limb with glomerulus attached in low NaCl perfusate were unable to inhibit renin secretion when various sodium salts were added but could inhibit renin secretion with the addition of chloride salts. This, and similar findings obtained in vivo, has led some to believe that perhaps "the initiating signal for MD control of renin secretion is a change in the rate of NaCl uptake predominantly via a luminal Na,K,2Cl co-transporter whose physiological activity is determined by a change in luminal Cl concentration."

Angiotensin I appears to have no biological activity and exists solely as a precursor to angiotensin II.

Angiotensin II

Asp-Arg-Val-Tyr-Ile-His-Pro-Phe | His-Leu

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 Angiotensin II 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. 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

Asp | Arg-Val-Tyr-Ile-His-Pro-Phe

Angiotensin III has 40% of the pressor activity of angiotensin II, but 100% of the aldosterone-producing activity. Increases mean arterial pressure.

Angiotensin IV

Arg | Val-Tyr-Ile-His-Pro-Phe

Angiotensin IV is a hexapeptide that, like angiotensin III, has some lesser activity.

Effects

See also Renin-angiotensin system#Effects

Angiotensins II, III and IV have a number of effects throughout the body:

Adipic

Angiotensins "modulate fat mass expansion through upregulation of adipose tissue lipogenesis ... and downregulation of lipolysis "

Cardiovascular

They are potent direct vasoconstrictors, constricting arteries and veins and increasing blood pressure. This effect is achieved through activation of the GPCR AT1, which signals through a Gq protein to activate Phospholipase C, and subsequently increase intracellular calcium.

Angiotensin II has prothrombotic potential through adhesion and aggregation of platelets and stimulation of PAI-1 and PAI-2.

When cardiac cell growth is stimulated, a local (autocrine-paracrine) renin-angiotensin system is activated in the cardiac myocyte, which stimulates cardiac cell growth through protein kinase C. The same system can be activated in smooth muscle cells in conditions of hypertension, atherosclerosis, or endothelial damage. Angiotensin II is the most important Gq stimulator of the heart during hypertrophy, compared to endothelin-1 and α1 adrenoreceptors.

Neural

Angiotensin II increases thirst sensation (dipsogen) through the subfornical organ of the brain, decreases the response of the baroreceptor reflex, and increases the desire for salt. It increases secretion of ADH in the posterior pituitary and secretion of ACTH in the anterior pituitary. It also potentiates the release of norepinephrine by direct action on postganglionic sympathetic fibers.

Adrenal

Angiotensin II acts on the adrenal cortex, causing it to release aldosterone, a hormone that causes the kidneys to retain sodium and lose potassium. Elevated plasma angiotensin II levels are responsible for the elevated aldosterone levels present during the luteal phase of the menstrual cycle.

Renal

Angiotensin II has a direct effect on the proximal tubules to increase Na⁺ reabsorption. It has a complex and variable effect on glomerular filtration and renal blood flow depending on the setting. Increases in systemic blood pressure will maintain renal perfusion pressure; however, constriction of the afferent and efferent glomerular arterioles will tend to restrict renal blood flow. The effect on the efferent arteriolar resistance is, however, markedly greater, in part due to its smaller basal diameter; this tends to increase glomerular capillary hydrostatic pressure and maintain glomerular filtration rate. A number of other mechanisms can affect renal blood flow and GFR. High concentrations of Angiotensin II can constrict the glomerular mesangium, reducing the area for glomerular filtration. Angiotensin II is a sensitizer to tubuloglomerular feedback, preventing an excessive rise in GFR. Angiotensin II causes the local release of prostaglandins, which, in turn, antagonize renal vasoconstriction. The net effect of these competing mechanisms on glomerular filtration will vary with the physiological and pharmacological environment.