- Aldosterone is a steroid hormone and is the primary physiological mineralocorticoid secreted by the adrenal cortex. Aldosterone plays a variety of roles in regulating the transport of ions, particularly sodium and potassium, especially in the kidneys and as such is an important physiological regulator of salt and water balance.
- Aldosterone is synthesized by cells of the zona glomerulosa in the adrenal cortex. The basic biochemistry of aldosterone synthesis is described in Adrenocortical Hormone Biosynthesis.
- Like all adrenocortical hormones, synthesis of aldosterone requires the presence of ACTH (See: Adrenocortical Hormone Regulation). However, very low levels of ACTH are sufficient for maintaining aldosterone synthesis and thus modulation of ACTH is not the principal mechanism of regulating this hormone. Rather, synthesis of aldosterone is stimulated by elevated levels of blood potassium as well as Angiotensin II, a product of the renin-angiotensin-aldosterone system released in scenarios of low renal perfusion. Potassium and Angiotensin II appear to induce expression of Aldosterone Synthase, likely the rate-limiting enzyme of aldosterone synthesis (See: Adrenocortical Hormone Biosynthesis).
- Molecular Mechanism
- Like all steroid hormones, aldosterone mediates its effect in responsive cells by binding cellular nuclear receptors which are then stimulated to transcribe a variety of specific target genes. The most important aldosterone target genes are a NaK ATPase which is typically directed toward the basolateral surface of the responsive cell, and a potassium channel, which is typically directed toward the luminal surface of the responsive cell. Additionally, aldosterone does appear to mildly enhance the synthesis of a H+ ATPase which is directed toward the luminal surface of the responsive cell.
- Physiological Effects
- The most important physiological effect of aldosterone is stimulation of sodium resorption and potassium secretion by principal cells of the late distal tubule and collecting duct. These effects are discussed in more detail in ECF Volume Regulation and external potassium balance. While the effects on tubular transport represent the major physiological contribution of aldosterone, increased levels of this hormone also slightly increase acid excretion by Intercalated Cells and mildly increase sodium resorption by enterocytes as discussed in GI Water and Electrolyte Absorption.