Aldosterone

Aldosterone is the main mineralocorticoid steroid hormone produced by the zona glomerulosa of the adrenal cortex in the adrenal gland.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> It is essential for sodium conservation in the kidney, salivary glands, sweat glands, and colon.<ref name=":0">Template:Cite book</ref> It plays a central role in the homeostatic regulation of blood pressure, plasma sodium (Na⁺), and potassium (K⁺) levels. It does so primarily by acting on the mineralocorticoid receptors in the distal tubules and collecting ducts of the nephron.<ref name=":0" /> It influences the reabsorption of sodium and excretion of potassium (from and into the tubular fluids, respectively) of the kidney, thereby indirectly influencing water retention or loss, blood pressure, and blood volume.<ref name="Marieb">Marieb Human Anatomy & Physiology 9th edition, chapter:16, page:629, question number:14</ref> When dysregulated, aldosterone is pathogenic and contributes to the development and progression of cardiovascular and kidney disease.<ref>Template:Cite journal</ref> Aldosterone has exactly the opposite function of the atrial natriuretic hormone secreted by the heart.<ref name="Marieb"/>

Aldosterone is part of the renin–angiotensin–aldosterone system. It has a plasma half-life of less than 20 minutes.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Drugs that interfere with the secretion or action of aldosterone are in use as antihypertensives, like lisinopril, which lowers blood pressure by blocking the angiotensin-converting enzyme (ACE), leading to lower aldosterone secretion. The net effect of these drugs is to reduce sodium and water retention but increase the retention of potassium. In other words, these drugs stimulate the excretion of sodium and water in urine, while they block the excretion of potassium.

Another example is spironolactone, a potassium-sparing diuretic of the steroidal spirolactone group, which interferes with the aldosterone receptor (among others) leading to lower blood pressure by the mechanism described above.

Aldosterone was first isolated by Sylvia Tait (Simpson) and Jim Tait in 1953; in collaboration with Tadeusz Reichstein.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Biosynthesis

Template:More citations needed section The corticosteroids are synthesized from cholesterol within the zona glomerulosa and zona fasciculata of adrenal cortex. Most steroidogenic reactions are catalysed by enzymes of the cytochrome P450 family. They are located within the mitochondria and require adrenodoxin as a cofactor (except 21-hydroxylase and 17α-hydroxylase).

Aldosterone and corticosterone share the first part of their biosynthetic pathways. The last parts are mediated either by the aldosterone synthase (for aldosterone) or by the 11β-hydroxylase (for corticosterone). These enzymes are nearly identical (they share 11β-hydroxylation and 18-hydroxylation functions), but aldosterone synthase is also able to perform an 18-oxidation. Moreover, aldosterone synthase is found within the zona glomerulosa at the outer edge of the adrenal cortex; 11β-hydroxylase is found in the zona glomerulosa and zona fasciculata.

File:Steroidogenesis.svg

Steroidogenesis, showing aldosterone synthesis at upper-right corner.<ref name="HäggströmRichfield2014">Template:Cite journal</ref>

Aldosterone synthase is normally absent in other sections of the adrenal gland.<ref>Template:Cite book</ref>

Stimulation

Aldosterone synthesis is stimulated by several factors:

• increase in the plasma concentration of angiotensin III, a metabolite of angiotensin II
• increase in plasma angiotensin II, ACTH, or potassium levels, which are present in proportion to plasma sodium deficiencies. (The increased potassium level works to regulate aldosterone synthesis by depolarizing the cells in the zona glomerulosa, which opens the voltage-dependent calcium channels.) The level of angiotensin II is regulated by angiotensin I, which is in turn regulated by renin, a hormone secreted in the kidneys.
• Serum potassium concentrations are the most potent stimulator of aldosterone secretion.
• the ACTH stimulation test, which is sometimes used to stimulate the production of aldosterone along with cortisol to determine whether primary or secondary adrenal insufficiency is present. However, ACTH has only a minor role in regulating aldosterone production; with hypopituitarism there is no atrophy of the zona glomerulosa.
• plasma acidosis
• the stretch receptors located in the atria of the heart. If decreased blood pressure is detected, the adrenal gland is stimulated by these stretch receptors to release aldosterone, which increases sodium reabsorption from the urine, sweat, and the gut. This causes increased osmolarity in the extracellular fluid, which will eventually return blood pressure toward normal.
• adrenoglomerulotropin, a lipid factor, obtained from pineal extracts. It selectively stimulates secretion of aldosterone.<ref>Template:Cite journal</ref>

The secretion of aldosterone has a diurnal rhythm.<ref>Template:Cite journal</ref>

Biological function

Aldosterone is the primary of several endogenous members of the class of mineralocorticoids in humans.Template:Citation needed Deoxycorticosterone is another important member of this class. Aldosterone tends to promote Na⁺ and water retention, and lower plasma K⁺ concentration by the following mechanisms:

1. Acting on the nuclear mineralocorticoid receptors (MR) within the principal cells of the distal tubule and the collecting duct of the kidney nephron, it upregulates and activates the basolateral Na⁺/K⁺ pumps, which pumps three sodium ions out of the cell, into the interstitial fluid and two potassium ions into the cell from the interstitial fluid. This creates a concentration gradient which results in reabsorption of sodium (Na⁺) ions and water (which follows sodium) into the blood, and secreting potassium (K⁺) ions into the urine (lumen of collecting duct).
2. Aldosterone upregulates epithelial sodium channels (ENaCs) in the collecting duct and the colon, increasing apical membrane permeability for Na⁺ and thus absorption.
3. Cl⁻ is reabsorbed in conjunction with sodium cations to maintain the system's electrochemical balance.
4. Aldosterone stimulates the secretion of K⁺ into the tubular lumen.<ref name="pmid10760062">Template:Cite journal</ref>
5. Aldosterone stimulates Na⁺ and water reabsorption from the gut, salivary and sweat glands in exchange for K⁺.
6. Aldosterone stimulates secretion of H⁺ via the H+/ATPase in the intercalated cells of the cortical collecting tubules
7. Aldosterone upregulates expression of NCC in the distal convoluted tubule chronically and its activity acutely.<ref>Template:Cite journal</ref>

Aldosterone is responsible for the reabsorption of about 2% of filtered sodium in the kidneys, which is nearly equal to the entire sodium content in human blood under normal glomerular filtration rates.<ref>Template:Cite book</ref>

Aldosterone, probably acting through mineralocorticoid receptors, may positively influence neurogenesis in the dentate gyrus.<ref>Template:Cite journal</ref>

Mineralocorticoid receptors

Steroid receptors are intracellular since steroid hormones are able to cross the cell membrane without a specific transporter. The aldosterone mineralocorticoid receptor (MR) complex binds on the DNA to specific hormone response element, which leads to gene specific transcription. Some of the transcribed genes are crucial for transepithelial sodium transport, including the three subunits of the epithelial sodium channel (ENaC), the Na⁺/K⁺ pumps and their regulatory proteins serum and glucocorticoid-induced kinase, and channel-inducing factor, respectively.

The MR is stimulated by both aldosterone and cortisol, but a mechanism protects the body from excess aldosterone receptor stimulation by glucocorticoids (such as cortisol), which happen to be present at much higher concentrations than mineralocorticoids in the healthy individual. The mechanism consists of an enzyme called 11β-Hydroxysteroid dehydrogenase (11β-HSD). This enzyme co-localizes with intracellular adrenal steroid receptors and converts cortisol into cortisone, a relatively inactive metabolite with little affinity for the MR. Liquorice, which contains glycyrrhetinic acid, can inhibit 11β-HSD and lead to a mineralocorticoid excess syndrome.

Control of aldosterone release from the adrenal cortex

File:Renin-angiotensin system in man shadow.svg

The renin–angiotensin system, showing role of aldosterone between the adrenal glands and the kidneys<ref>Page 866-867 (Integration of Salt and Water Balance) and 1059 (The Adrenal Gland) in: Template:Cite book</ref>

Major regulators

The role of the renin–angiotensin system

Angiotensin is involved in regulating aldosterone and is the core regulation.<ref>Template:Cite journal</ref> Angiotensin II acts synergistically with potassium, and the potassium feedback is virtually inoperative when no angiotensin II is present.<ref>Template:Cite journal</ref> A small portion of the regulation resulting from angiotensin II must take place indirectly from decreased blood flow through the liver due to constriction of capillaries.<ref>Template:Cite journal</ref> When the blood flow decreases so does the destruction of aldosterone by liver enzymes.

Although sustained production of aldosterone requires persistent calcium entry through low-voltage-activated Ca²⁺ channels, isolated zona glomerulosa cells are considered nonexcitable, with recorded membrane voltages that are too hyperpolarized to permit Ca²⁺ channels entry.<ref name="pmid22546854">Template:Cite journal</ref> However, mouse zona glomerulosa cells within adrenal slices spontaneously generate membrane potential oscillations of low periodicity; this innate electrical excitability of zona glomerulosa cells provides a platform for the production of a recurrent Ca²⁺ channels signal that can be controlled by angiotensin II and extracellular potassium, the 2 major regulators of aldosterone production.<ref name="pmid22546854"/> Voltage-gated Ca²⁺ channels have been detected in the zona glomerulosa of the human adrenal, which suggests that Ca²⁺ channel blockers may directly influence the adrenocortical biosynthesis of aldosterone in vivo.<ref name= "Felizola">Template:Cite journal</ref>

The plasma concentration of potassium

The amount of plasma renin secreted is an indirect function of the serum potassium<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> as probably determined by sensors in the carotid artery.<ref name="pmid13896654" /><ref>Template:Cite journal</ref>

Adrenocorticotropic hormone

Adrenocorticotropic hormone (ACTH), a pituitary peptide, also has some stimulating effect on aldosterone, probably by stimulating the formation of deoxycorticosterone, a precursor of aldosterone.<ref>Template:Cite journal</ref> Aldosterone is increased by blood loss,<ref>Ruch TC Fulton JF 1960 Medical Physiology and Biophysics. W.B. Saunders and Co., Phijl & London. On p1099.</ref> pregnancy,<ref name="pmid13590935" /> and possibly by further circumstances such as physical exertion, endotoxin shock, and burns.<ref name=Glaz>Template:Cite book</ref><ref>Template:Cite journal on 529</ref>

Miscellaneous regulators

The role of sympathetic nerves

The aldosterone production is also affected to one extent or another by nervous control, which integrates the inverse of carotid artery pressure,<ref name="pmid13896654">Gann DS Mills IH Bartter 1960 On the hemodynamic parameter mediating increase in aldosterone secretion in the dog. Fed. Proceedings 19; 605–610.</ref> pain, posture,<ref name="pmid13590935">Template:Cite journal</ref> and probably emotion (anxiety, fear, and hostility)<ref name="pmid13449153">Template:Cite journal</ref> (including surgical stress).<ref>Template:Cite journal</ref> Anxiety increases aldosterone,<ref name="pmid13449153" /> which must have evolved because of the time delay involved in migration of aldosterone into the cell nucleus.<ref>Sharp GUG Leaf A 1966 in; Recent Progress in Hormone Research. (Pincus G, ed.</ref> Thus, there is an advantage to an animal's anticipating a future need from interaction with a predator, since too high a serum content of potassium has very adverse effects on nervous transmission.

The role of baroreceptors

Pressure-sensitive baroreceptors are found in the vessel walls of nearly all large arteries in the thorax and neck, but are particularly plentiful in the sinuses of the carotid arteries and in the arch of the aorta. These specialized receptors are sensitive to changes in mean arterial pressure. An increase in sensed pressure results in an increased rate of firing by the baroreceptors and a negative feedback response, lowering systemic arterial pressure. Aldosterone release causes sodium and water retention, which causes increased blood volume, and a subsequent increase in blood pressure, which is sensed by the baroreceptors.<ref>Copstead, E. C. & Banasik, J. L. (2010.) Pathophysiology. (4th ed.). St. Louis, Mo: Saunders Elsevier.</ref> To maintain normal homeostasis these receptors also detect low blood pressure or low blood volume, causing aldosterone to be released. This results in sodium retention in the kidney, leading to water retention and increased blood volume.<ref>Marieb, E. N. (2004) Human anatomy and physiology (6th ed) San Francisco: Pearson Benjamin Cummings.</ref>

The plasma concentration of sodium

Aldosterone levels vary as an inverse function of sodium intake as sensed via osmotic pressure.<ref>Template:Cite journal</ref> The slope of the response of aldosterone to serum potassium is almost independent of sodium intake.<ref>Template:Cite journal</ref> Aldosterone is increased at low sodium intakes, but the rate of increase of plasma aldosterone as potassium rises in the serum is not much lower at high sodium intakes than it is at low. Thus, potassium is strongly regulated at all sodium intakes by aldosterone when the supply of potassium is adequate, which it usually is in hunter-gatherer diets.

Aldosterone feedback

Feedback by aldosterone concentration itself is of a nonmorphological character (that is, other than changes in the cells' number or structure) and is poor, so the electrolyte feedbacks predominate, short term.<ref name=Glaz/>

Associated clinical conditions

Hyperaldosteronism is abnormally increased levels of aldosterone, while hypoaldosteronism is abnormally decreased levels of aldosterone.

A measurement of aldosterone in blood may be termed a plasma aldosterone concentration (PAC), which may be compared to plasma renin activity (PRA) as an aldosterone-to-renin ratio.

Hyperaldosteronism

Primary aldosteronism, also known as primary hyperaldosteronism, is characterized by the overproduction of aldosterone by the adrenal glands,<ref name=conn>Template:Cite journal</ref> when not a result of excessive renin secretion. It leads to arterial hypertension (high blood pressure) associated with hypokalemia, usually a diagnostic clue. Secondary hyperaldosteronism, on the other hand, is due to overactivity of the renin–angiotensin system.

Conn's syndrome is primary hyperaldosteronism caused by an aldosterone-producing adenoma.

Depending on cause and other factors, hyperaldosteronism can be treated by surgery and/or medically, such as by aldosterone antagonists.

The ratio of renin to aldosterone is an effective screening test to screen for primary hyperaldosteronism related to adrenal adenomas.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> It is the most sensitive serum blood test to differentiate primary from secondary causes of hyperaldosteronism.<ref name="renin">Template:Cite journal</ref> Blood obtained when the patient has been standing for more than 2 hours are more sensitive than those from when the patient is lying down. Before the test, individuals should not restrict salt and low potassium should be corrected before the test because it can suppress aldosterone secretion.<ref name="renin"/>

Hypoaldosteronism

An ACTH stimulation test for aldosterone can help in determining the cause of hypoaldosteronism, with a low aldosterone response indicating a primary hypoaldosteronism of the adrenals, while a large response indicating a secondary hypoaldosteronism. The most common cause of this condition (and related symptoms) is Addison's disease; it is typically treated by fludrocortisone, which has a much longer persistence (1 day) in the bloodstream.

Additional images

• Corticosteroid-biosynthetic-pathway-rat.png

  Corticosteroid biosynthetic pathway in rat

• Corticosterone.svg

  Corticosterone