Aldosterone
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BACK TO ELECTROLYTE BALANCE
Overview
- Steroid hormone
- Secreted from the zona glomerulosa of the adrenal cortex of the adrenal gland
- Mineralocorticoid
- Most important regulator of plasma pottassium
- Stimulated directly by increased plasma pottassium
- Also stimulated as part of the Renin-Angiotensin-Aldosterone System (RAAS)
Release
- Release is stimulated by 3 things
- Corticotropin (ACTH)
- Angiotensin 2
- K+
- Its release is inhibited by Atrial Natiuretic Peptide
- Most increases in the concentration of aldosterone however can be explained by increases in the Renin-Angiotensin-Aldosterone System and therefore angiotensin 2 and/or by increases in K+ concentration
- Only in severe fluid loss does ACTH significantly stimulate the release of aldosterone
- ANP is secreted in response to sodium/water loading and therefore inhibits aldosterone secretion
Action
- Diffuses across the cell membrane - lipophillic (essentially steroidal)
- Of the principal cells of distal tubule
- Binds to cytoplasmic receptors
- Works by altering gene transcription and increases synthesis of proteins
- Affects ATP levels
Sodium
- Affects sodium entry and transport
- Increases number of apical sodium channels, NaCl co-transporters and Na+K+ATPase
- Increases membrane permeability
- Increases sodium pump activity
- Total quantity of sodium is conserved not the actual plasma concentration
- This is because water follows sodium so the volume is altered according to the amount of sodium
- Angiotensin 2 and aldosterone also affect ECF so only quantity affected not concentration
- ADH and thirst response also work together to dilute the ECF so although there is more NaCl the actual concentration is constant.
Pottassium
- In cases of increased K+
- Increased sodium pump activity increases the amount of K+ in cells to reduce plasa K+
- Generally not excreted
- However if plasma K+ is still high aldosterone is stimulated
- Causes pottassium secretion
- Pottassium via apical leak channels in the principal cells
- Very tightly regulated system
- Allows large increase in K+ to have a miniscule effect on plasma K+
Hydrogen
- Hydrogen by proton secretory proteins