3. Physiology
        3.12. Renal
            3.12.7. Control of sodium and water excretion
                3.12.7.4. Control of Sodium balance
3.12.7.4.2. Aldosterone

Aldosterone

[Ref: WG21:Chp20; AV6:p116-118]

 

Basics

Synthesis

Produced in zona glomerulosa

Zona glomerulosa lacks 17alpha-hydroxylase activity, but is the only zone that contains aldosterone synthase.

Steps of production

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Cholesterol converted to Pregnenolone

--> Progesterone

--> 11-Deoxycorticosterone

--> Corticosterone

--> 18-Hydroxycorticosterone

--> Aldosterone

NB:

Halflife

Bound to protein slightly

Halftime is short: about 20 minutes

Metabolism

Most converted to tetrahydroglucuronide derivative in liver

Some converted to 18-glucuronide in liver and kidney
* Reverts back to free aldosterone at pH 1
* i.e. acid-labile conjugate

Less than 1% excreted in urine in free form.

Mineralocorticoid receptors

[WG21:p379]

11beta-hydroxysteroid dehydrogenase type 2

Mineralocorticoid receptors actually have higher affinity for glucocorticoids than glucocorticoid receptors do
* i.e. glucocorticoids bind to mineralocorticoid receptor better than to glucocorticoid receptors

However,

In kidney and other mineralocorticoid-sensitive tissues, enzyme (11beta-hydroxysteroid dehydrogenase type 2) is present that converts cortisol and corticosterone into steroid forms that do not bind to the mineralocorticoid receptors.

If this enzyme is absent or inhibited (e.g. by prolonged ingestion of licorice)
--> Cortisol has marked mineralocorticoid effect
--> Apparent mineralocorticoid excess syndrome

Apparent mineralocorticoid syndrome

Actions

[WG21:p378]

Actions

Main Action:
* Sodium resorption

Other actions:
* Increase urinary K+ loss
* Increase urinary H+ loss

NB:

Sodium resorption

Mechanism of action

[AV6:p117]

Cross freely into principle cells

--> Combine with mineralocorticoid receptors in the cytoplasm

--> Receptor acts as transcription factor

--> synthesis of specific mRNA and protein

--> Increase activity/number of apical sodium channel and basolateral Na-K ATPase pump

NB:

Significance of sodium retention effect

[AV6:p117]

Complete absence of aldosterone
--> 2% of filtered sodium would be excreted

Maximal aldosterone level
--> Almost no sodium excreted

Adrenal insufficiency

[WG21:p379]

Loss of mineralocorticosteroid leads to

Effect of excess mineralocorticoid

[WG21:p380]

Regulation

Factors affecting aldosterone secretion

[WG21:p381]

3 primary factors stimulating aldosterone release

Effect of ACTH
Role of angiotensin II

Angiotensin II binds to AT1 receptors in the zona glomerulosa

--> Act via G protein

--> Activating phospholipase C

--> In increased protein kinase C

Thus,

NB:

Effect of K+

Need to increase only by 1meq/L to stimulate aldosterone secretion

Acts by depolarising the cells

--> Opening up voltage-gated Ca2+ channels

--> Increase intracellular Ca2+

Increases sensitivity of zona glomerulosa to angiotensin II

Factors stimulating secretion of both aldosterone and glucocorticoids

Factors stimulating aldosterone secretion without effect on glucocorticoids

Standing

Aldosterone release is increased in standing position because

ANP inhibiting aldosterone release

[AV6:p117]

[WG21:p383]

Interaction with ANP

In mineralocorticoid excess,

Once ECF expansion passes a certain point

--> Na+ excretion is increased in spite of actions of excess mineralocorticoids

--> Escape phenomenon

Probably due to increased secretion of ANP

Others

[KB-acidbase:7.2] Primary hyperaldosteronism often cause metabolic alkalosis with increase ECF, hypokalaemia and hypochloraemia.

Other steroids affecting Na+ excretion

 



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