Aldosterone

Introduction

Aldosterone is a steroid hormone which is secreted from the zona glomerulosa of the adrenal gland. It has a mineralocorticoid activity and is the most important regulator of plasma potassium. When plasma potassium increases increased stimulation of aldosterone occurs directly and as a result of Renin-Angiotensin-Aldosterone System (RAAS). It is also the most important regulator of sodium excretion.

Release

• Release is stimulated by 3 things

1. Corticotropin (ACTH)
2. Angiotensin 2
3. K⁺

• Its release is inhibited by Atrial Natriuretic 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 and collecting duct
• 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 activity of the hydrogen sodium exchanger in the apical membrane
• Increases membrane permeability
• Increases sodium pump activity
• Total quantity of sodium is conserved not the actual plasma concentration
  • When sodium is reabsorbed water follows it so the volume of the plasma is altered rather than the concentration of sodium changing
  • Angiotensin 2 and Aldosterone affect sodium but they also affect ECF volume so only quantity affected not concentration
  • ADH and thirst response also work together to dilute the ECF if concentrations of sodium are high so although there is more NaCl the actual concentration is not really changed.
• If there was no secretion of aldosterone a 20kg dog would excrete 15g per 24 hours
• At maximal secretion no significant amount of sodium would be excreted

Potassium

• In cases of increased K⁺
• Increased Na⁺ / K⁺ ATPase pump activity increases the amount of K⁺ in cells to reduce plasma K⁺
• Generally not excreted
• However if plasma K⁺ is still high aldosterone is stimulated
• Causes potassium secretion
  • Stimulates Na⁺ / K⁺ ATPases in the basolateral membrane of the principal cells
  • Increased potassium in the cells
  • Potassium leaves via apical leak channels
  • Thanks to electro-chemical gradient
• Very tightly regulated system
  • Allows large increase in K⁺ to have a miniscule effect on plasma K⁺

Hydrogen

• Increases hydrogen secretion by increasing Hydrogen ATPases in the apical membrane of the intercalated cells
• Increases hydrogen secretion by increasing sodium hydrogen exchanger in the apical membrane of the principal cells

Revision

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