Aldosterone

()Map KIDNEY ELECTROLYTE BALANCE (Map)

Overview

Aldosterone is a steroid hormone which is secreted from the zona glomerulosa of the adrenal gland. It has a mineralocorticoid activitiy and is the most important regulator of plasma pottasium. When plasma pottasium 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 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 Na⁺ / K⁺ ATPase 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
  • Stimulates Na⁺ / K⁺ ATPases in the basolateral membrane of the principal cells
  • Increased pottasium in the cells
  • Pottassium 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

• • Hydrogen by proton secretory proteins