Renal Hormonal Regulation Lecture

Key Hormones

• Total of 5 renal hormones: Angiotensin II, Aldosterone, Antidiuretic Hormone (ADH), Atrial Natriuretic Peptide (ANP), Parathyroid Hormone (PTH)

Renin–Angiotensin–Aldosterone System (RAAS)

• Trigger: ↓ blood volume / pressure → ↓ stretch of afferent arteriole
• Juxtaglomerular cells release renin → \text{Angiotensinogen} \to \text{Ang I} \xrightarrow{ACE} \text{Ang II}
• Angiotensin II actions ("3 C’s")
  • Constricts afferent arteriole → ↓ GFR
  • Counters Na⁺/H⁺ antiporter in PCT ↑ → ↑ obligatory Na⁺ & H₂O reabsorption
  • Calls for Aldosterone from adrenal cortex
• Aldosterone (principal cells, collecting duct)
  • ↑ Na⁺ reabsorption / K⁺ secretion via Na⁺-K⁺ pumps & Na⁺ channels
  • Water follows (obligatory) → ↑ blood volume & pressure

Antidiuretic Hormone (ADH / Vasopressin)

• Main stimulus: ↑ plasma / interstitial osmolarity (primary); severe hypovolemia or drugs (secondary)
• Site of release: Posterior pituitary
• Action: Inserts aquaporins in late DCT & collecting-duct principal cells → ↑ facultative H₂O reabsorption
• Feedback: Osmoreceptors detect as little as 1\% ↑ osmolarity; process ends once \approx 300\,\text{mOsm} restored
• Lack of ADH activity → Diabetes insipidus (urine output up to 20\,\text{L/day} vs normal 1.5-2\,\text{L/day})

Atrial Natriuretic Peptide (ANP)

• Trigger: ↑ atrial stretch from ↑ blood volume
• Actions
  • Inhibits aldosterone & ADH release
  • ↓ Na⁺ & H₂O reabsorption (PCT, collecting duct) → ↑ urinary Na⁺ & H₂O loss
  • Net: ↓ blood volume & pressure (only hormone in set that lowers them)

Parathyroid Hormone (PTH)

• Trigger: ↓ plasma Ca²⁺
• Actions
  • Early DCT: ↑ Ca²⁺ reabsorption → restores blood Ca²⁺
  • PCT: ↓ phosphate reabsorption → ↑ phosphate excretion

Quick Comparison

• Obligatory reabsorption: Ang II & Aldosterone (Na⁺ + H₂O)
• Facultative reabsorption: ADH (H₂O only)
• Opposes reabsorption: ANP (Na⁺ + H₂O)
• Targets calcium/phosphate: PTH

Net Effects on Blood Volume / Pressure

• ↑ Volume/Pressure: Ang II, Aldosterone, ADH
• ↓ Volume/Pressure: ANP
• No direct effect on volume: PTH (focuses on Ca²⁺ balance)