Aldosterone: Secretion, Functions, and Control

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Aldosterone

Aldosterone is a mineralocorticoid hormone. It is essential to life because it regulates the amounts of electrolytes in the body.

Secretion

Aldosterone is secreted by the adrenal cortex. The adrenal cortex, where aldosterone is produced, is part of the adrenal gland.

The adrenal cortex is the outer layer of the adrenal gland. The interior of the adrenal gland is known as the adrenal medulla or simply medulla. The medulla and the cortex perform very different functions, and each is critical to healthy life.

Using cholesterol, the adrenal cortex manufactures a number of compounds with a variety of uses, many of which play a role in metabolism and blood chemistry.

The adrenal glands are located on top of the kidneys. The cortex is yellow in healthy individuals. There are three distinct layers in the adrenal cortex, each of which is responsible for manufacturing different chemicals for use by the body.

The cells in each layer have slightly different structures, reflecting their different functions.

On the outside of the adrenal cortex, the zona glomerulosa makes mineralcorticoids such as aldosterone. The next layer, the zona fasciculata, makes glucocorticoids like cortisol, while the inner layer, known as the zona reticularis, makes androgens such as testosterone.

Factors that Stimulate Aldosterone synthesis

1. Increase in the plasma concentration of Angiotensin II, a metabolite of Angiotensin I.

Increased 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 the hormone renin. Potassium levels are the most sensitive stimulator of aldosterone.

However, ACTH has only a minor role in regulating aldosterone production; with hypopituitarism there is no atrophy of the zona glomerulosa.

2. Plasma acidosis stimulates the aldosterone synthesis.

3. Stretch receptors stimulates the aldosterone synthesis. 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.

Finally, it is important to note that the secretion of aldosterone has a diurnal rhythm.

Functions of Aldosterone

Aldosterone is the primary of several endogenous mineralocorticoids in human.

Deoxycorticosterone is another important mineralocorticoid. At the late distal tubule and collecting duct, aldosterone has three main actions:

1. Acting on the nuclear mineralocorticoid receptors within the principal cells of the distal tubule and the collecting duct of the kidney nephron, it increases the permeability of the apical membrane to potassium and sodium and activates the basolateral Na+/K+ pumps, stimulating ATP hydrolysis leading to phosphorylation of the pump and a conformational change in the pump exposes the Na+ ions to the outside. The phosphorylated form of the pump has a low affinity for Na+ ions, hence reabsorbing sodium (Na+) ions and water into the blood, and secreting potassium (K+) ions into the urine.

2. Aldosterone stimulates H+ secretion by intercalated cells in the collecting duct, regulating plasma bicarbonate levels and its acid/base balance.

3. Aldosterone may act on the central nervous system via the posterior pituitary gland to release vasopressin (ADH) which serves to conserve water by direct actions on renal tubular reabsorption.

4. 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 rate (GFR).

Regulation/Control of Aldosterone release

Control of aldosterone release from the Adrenal Cortex:

1. The role of the renin-angiotensin system:

Angiotensin is involved in regulating aldosterone and is the core regulation. Angiotensin II acts synergistically with potassium, and the potassium feedback is virtually inoperative when angiotensin II is absent.

2. 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, pain, posture, anxiety, fear, hostility and stress, etc. Anxiety increases aldosterone, which must have evolved because of the time delay involved in migration of aldosterone into the cell nucleus.

3. The role of baroreceptors:

Pressure in the carotid artery decreases aldosterone.

4. The role of the juxtaglomerular apparatus:

The amount of aldosterone secreted is a direct function of the serum potassium as probably determined by sensors in the carotid artery.

5. The plasma concentration of sodium:

Aldosterone is a function of the inverse of the sodium intake as sensed via osmotic pressure. The slope of the response of aldosterone to serum potassium is almost independent of sodium intake. Aldosterone is much 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.

6. Adrenocorticotropic hormone (ACTH) regulations:

The pituitary peptide ACTH, also has some stimulating effect on aldosterone probably by stimulating deoxycorticosterone formation which is a precursor of aldosterone. Aldosterone is increased by blood loss, pregnancy, and possibly by other circumstances such as physical exertion, endotoxin shock, and burns.

7. Aldosterone feedback:

Feedback by aldosterone concentration itself is of a non morphological character (that is other than changes in the cells’ number or structure) and is poor so the electrolyte feedbacks predominate short term.

There is the need to maintain optimal levels of aldosterone in the bloodstream. If aldosterone levels in the body are out of sync, symptoms can result.

Abnormally high levels of aldosterone can cause high blood pressure, muscle cramps and weakness. On the other hand, abnormally low levels may indicate disease, such as diabetes.

Usually, aldosterone levels vary between the sexes and may be affected by the amount of sodium in a person’s diet. Women often have significantly higher levels of aldosterone when pregnant.

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