Renal Hormonal Regulation Lecture

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

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

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 $1.5-2\,\text{L/day}$ )

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)

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