Aldosterone

Aldosterone is synthesised and secreted by the zona glomerulosa of the adrenal cortex. Aldosterone is classed as a mineralocorticoid & is the primary mineralocorticoid of the human body. Aldosterone functions to regulate salt and water homeostasis.

Factors that stimulate aldosterone production

• Angiotensin II
• High potassium (K⁺)
• Fluid loss as detected by atrial stretch receptors
• ACTH (minor stimulant)

Mechanism of action
Aldosterone acts mainly at the renal distal convoluted tubule (DCT) and collecting ducts to cause sodium retention and potassium loss. Aldosterone acts via protein synthesis so its effects take hours to work

Aldosterone's main effects are though its actions on two cell types:

• Principle cells of the distal tubule
• Intercalated cells of the collecting duct

Firstly on the principle cells of the renal tubules aldosterone increases expression of:

• Na⁺/K⁺ pump (Na⁺/K⁺ ATPase pump) on the basolateral membrane
• ENaC sodium ion channels on the apical membrane

The increased intracellular potassium concentration as a result of the Na⁺/K⁺ pump causes K⁺ ions to diffuse out of apical K⁺ channels (ROMK channels). The overall effect of aldosterone here is Na⁺ ion reabsorption which in turn causes water reabsorption & K⁺ ion excretion.


Aldosterone acts on alpha intercalated cells of the collecting duct to increase expression of the following:

• K⁺/H⁺ ATPase on the apical membrane
• H⁺ ATPase on the apical membrane

Both of these pumps drive H⁺ ions into the lumen for excretion. Aldosterone's overall effect on intercalated cells is to drive H⁺ excretion (alkalosis) and K⁺ reabsorption. The potassium reabsorbed here partially offsets the potassium ion excretion aldosterone promotes from principal cells.