Renin–angiotensin system: Difference between revisions
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# Renin cleaves an inactive [[peptide]] called ''[[angiotensinogen]]'', converting it into ''[[angiotensin|angiotensin I]]''. |
# Renin cleaves an inactive [[peptide]] called ''[[angiotensinogen]]'', converting it into ''[[angiotensin|angiotensin I]]''. |
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# Angiotensin I is then converted to ''[[angiotensin|angiotensin II]]'' by [[angiotensin-converting enzyme]] (ACE)<ref>{{cite journal |author=Paul M, Poyan Mehr A, Kreutz R |title=Physiology of local renin-angiotensin systems |journal=Physiol. Rev. |volume=86 |issue=3 |pages=747–803 |year=2006 |month=July |pmid=16816138 |doi=10.1152/physrev.00036.2005 |url=http://physrev.physiology.org/cgi/content/full/86/3/747}}</ref> which is found mainly in [[lung]] [[capillaries]]. |
# Angiotensin I is then converted to ''[[angiotensin|angiotensin II]]'' by [[angiotensin-converting enzyme]] (ACE)<ref>{{cite journal |author=Paul M, Poyan Mehr A, Kreutz R |title=Physiology of local renin-angiotensin systems |journal=Physiol. Rev. |volume=86 |issue=3 |pages=747–803 |year=2006 |month=July |pmid=16816138 |doi=10.1152/physrev.00036.2005 |url=http://physrev.physiology.org/cgi/content/full/86/3/747}}</ref> which is found mainly in [[lung]] [[capillaries]]. |
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# Angiotensin II is the major bioactive product of the renin-angiotensin system which binds to [[extraglomerular mesangial cells]], causing them to contract along with the blood vessels surrounding them. Angiotensin II acts as an [[endocrine system |endocrine]], [[autocrine signalling |autocrine]]/[[paracrine signalling |paracrine]], and [[intracrine]] hormone. |
# Angiotensin II is the major bioactive product of the renin-angiotensin system which binds to [[extraglomerular mesangial cells]], causing them to contract along with the blood vessels surrounding them and causes the release of [[aldosterone]] from the [[adrenal cortex]]. Angiotensin II acts as an [[endocrine system |endocrine]], [[autocrine signalling |autocrine]]/[[paracrine signalling |paracrine]], and [[intracrine]] hormone. |
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# Patil Jaspal et al. have shown local synthesis of Angiotensin II in neurons of sympathetic ganglia.<ref>{{cite journal |author=Patil J, Heiniger E, Schaffner T, Mühlemann O, Imboden H |title=Angiotensinergic neurons in sympathetic coeliac ganglia innervating rat and human mesenteric resistance blood vessels |journal=Regul. Pept. |volume=147 |issue=1-3 |pages=82–7 |year=2008 |month=April |pmid=18308407 |doi=10.1016/j.regpep.2008.01.006 |url=}}</ref> |
# Patil Jaspal et al. have shown local synthesis of Angiotensin II in neurons of sympathetic ganglia.<ref>{{cite journal |author=Patil J, Heiniger E, Schaffner T, Mühlemann O, Imboden H |title=Angiotensinergic neurons in sympathetic coeliac ganglia innervating rat and human mesenteric resistance blood vessels |journal=Regul. Pept. |volume=147 |issue=1-3 |pages=82–7 |year=2008 |month=April |pmid=18308407 |doi=10.1016/j.regpep.2008.01.006 |url=}}</ref> |
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Revision as of 03:06, 17 April 2009
The renin-angiotensin system (RAS) or the renin-angiotensin-aldosterone system (RAAS) is a hormone system that regulates blood pressure and water (fluid) balance.
When blood volume is low, the kidneys secrete renin. Renin stimulates the production of angiotensin. Angiotensin causes blood vessels to constrict resulting in increased blood pressure. Angiotensin also stimulates the secretion of the hormone aldosterone from the adrenal cortex. Aldosterone causes the tubules of the kidneys to retain sodium and water. This increases the volume of fluid in the body, which also increases blood pressure.
If the renin-angiotensin-aldosterone system is too active, blood pressure will be too high. There are many drugs which interrupt different steps in this system to lower blood pressure. These drugs are one of the main ways to control high blood pressure (hypertension), heart failure, kidney failure, and harmful effects of diabetes.[1][2]
Activation
The system can be activated when there is a loss of blood volume or a drop in blood pressure (such as in hemorrhage).
- If the perfusion of the juxtaglomerular apparatus in the kidney's macula densa decreases, then the juxtaglomerular cells release the enzyme renin.
- Renin cleaves an inactive peptide called angiotensinogen, converting it into angiotensin I.
- Angiotensin I is then converted to angiotensin II by angiotensin-converting enzyme (ACE)[3] which is found mainly in lung capillaries.
- Angiotensin II is the major bioactive product of the renin-angiotensin system which binds to extraglomerular mesangial cells, causing them to contract along with the blood vessels surrounding them and causes the release of aldosterone from the adrenal cortex. Angiotensin II acts as an endocrine, autocrine/paracrine, and intracrine hormone.
- Patil Jaspal et al. have shown local synthesis of Angiotensin II in neurons of sympathetic ganglia.[4]
Effects
- Further reading: Angiotensin#Effects and Aldosterone#Function
It is believed that Angiotensin I may have some minor activity, but angiotensin II is the major bio-active product. Angiotensin II has a variety of effects on the body:
- Throughout the body, it is a potent vasoconstrictor of arterioles.
- In the kidneys, it constricts glomerular arterioles, having a greater effect on efferent arterioles than afferent. As with most other capillary beds in the body, the constriction of afferent arterioles increases the arteriolar resistance, raising systemic arterial blood pressure and decreasing the blood flow. However, the kidneys must continue to filter enough blood despite this drop in blood flow, necessitating mechanisms to keep glomerular blood pressure up. To do this, Angiotensin II constricts efferent arterioles, which forces blood to build up in the glomerulus, increasing glomerular pressure. The glomerular filtration rate (GFR) is thus maintained, and blood filtration can continue despite lowered overall kidney blood flow. Because the filtration fraction has increased, there is less plasma fluid in the downstream peritubular capillaries. This in turn leads to a decreased hydrostatic pressure and increased osmotic pressure (due to unfiltered plasma proteins) in the peritubular capillaries. The effect of decreased hydrostatic pressure and increased osmotic pressure in the peritubular capillaries will facilitate increased reabsorption of tubular fluid.
- Angiotensin II decreases medullary blood flow through the vasa recta. This decreases the washout of NaCl and urea in the kidney medullary space. Thus, higher concentrations of NaCl and urea in the medulla facilitate increased absorption of tubular fluid. Furthermore, increased reabsorption of fluid into the medulla will increase passive reabsorption of sodium along the thin ascending limb of the loop of Henle.
- Angiotensin II stimulates Na+/H+ exchangers located on the apical membranes (faces the tubular lumen) of cells in the proximal tubule and thick ascending limb of the loop of Henle in addition to Na+ channels in the collecting ducts. This will ultimately lead to increased sodium reabsorption
- Angiotensin II stimulates the hypertrophy of renal tubule cells, leading to further sodium reabsorption.
- In the adrenal cortex, it acts to cause the release of aldosterone. Aldosterone acts on the tubules (e.g the distal convoluted tubules and the cortical collecting ducts) in the kidneys, causing them to reabsorb more sodium and water from the urine. Potassium is secreted into the tubules in exchange for the sodium, which is excreted. Aldosterone also acts on the central nervous system to increase an individual's appetite for salt, and to stimulate the sensation of thirst.
- Release of Anti-Diuretic Hormone (ADH), also called vasopressin -- ADH is made in the hypothalamus and released from the posterior pituitary gland. As its name suggests, it also exhibits vaso-constrictive properties, but its main course of action is to stimulate reabsorption of water in the kidneys.
These effects directly act in concert to increase blood pressure.
Clinical significance
The renin-angiotensin system is often manipulated clinically to treat high blood pressure.
- Inhibitors of angiotensin-converting enzyme (ACE inhibitors) are often used to reduce the formation of the more potent angiotensin II. Captopril is an example of an ACE inhibitor.
- Alternatively, angiotensin receptor blockers (ARBs) can be used to prevent angiotensin II from acting on angiotensin receptors.
- Direct renin receptor inhibitors are aliskiren[5] and the investigational remikiren[6].
- As of November 2008[update], a vaccination against angiotensin II, codenamed CYT006-AngQb, is undergoing clinical trials.[7]
Other uses of ACE
Interestingly, ACE cleaves a number of other peptides, and in this capacity is an important regulator of the kinin-kallikrein system.
Fetal renin-angiotensin system
In the fetus, the renin-angiotensin system is predominantly a sodium-losing system, as angiotensin II has little or no effect on aldosterone levels. Renin levels are high in the fetus, while angiotensin II levels are significantly lower — this is due to the limited pulmonary blood flow, preventing ACE (found predominantly in the pulmonary circulation) from having its maximum effect.
See also
- ACE inhibitor
- Angiotensin II receptor antagonist
- Discovery and development of angiotensin receptor blockers
References
- ^ "High Blood Pressure: Heart and Blood Vessel Disorders". Merck Manual Home Edition.
- ^ Solomon, Scott D (2005). "A Brief Overview of Inhibition of the Renin-Angiotensin System: Emphasis on Blockade of the Angiotensin II Type-1 Receptor". Medscape Cardiology. 9 (2).
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Paul M, Poyan Mehr A, Kreutz R (2006). "Physiology of local renin-angiotensin systems". Physiol. Rev. 86 (3): 747–803. doi:10.1152/physrev.00036.2005. PMID 16816138.
{{cite journal}}: Unknown parameter|month=ignored (help)CS1 maint: multiple names: authors list (link) - ^ Patil J, Heiniger E, Schaffner T, Mühlemann O, Imboden H (2008). "Angiotensinergic neurons in sympathetic coeliac ganglia innervating rat and human mesenteric resistance blood vessels". Regul. Pept. 147 (1–3): 82–7. doi:10.1016/j.regpep.2008.01.006. PMID 18308407.
{{cite journal}}: Unknown parameter|month=ignored (help)CS1 maint: multiple names: authors list (link) - ^ Gradman A, Schmieder R, Lins R, Nussberger J, Chiang Y, Bedigian M (2005). "Aliskiren, a novel orally effective renin inhibitor, provides dose-dependent antihypertensive efficacy and placebo-like tolerability in hypertensive patients". Circulation. 111 (8): 1012–8. doi:10.1161/01.CIR.0000156466.02908.ED. PMID 15723979.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Richter WF, Whitby BR, Chou RC (1996). "Distribution of remikiren, a potent orally active inhibitor of human renin, in laboratory animals". Xenobiotica. 26 (3): 243–54. PMID 8730917.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Tissot, AC (2008). "Effect of immunisation against angiotensin II with CYT006-AngQb on ambulatory blood pressure: a double-blind, randomised, placebo-controlled phase IIa study". 371. The Lancet: 821–827.
{{cite journal}}: Cite journal requires|journal=(help)
- Banic A, Sigurdsson GH, Wheatley AM (1993). "Influence of age on the cardiovascular response during graded haemorrhage in anaesthetized rats". Res Exp Med (Berl). 193 (5): 315–21. doi:10.1007/BF02576239. PMID 8278677.
{{cite journal}}: CS1 maint: multiple names: authors list (link)
External links
- Renin-Angiotensin+System at the U.S. National Library of Medicine Medical Subject Headings (MeSH)