Hormonal Regulation of Lipid, Glucose & Stress Metabolism – Detailed Study Notes

Melatonin & Fat Metabolism

• Highly variable hormone; secretion influenced by light/dark cycle, rest quality, age, species, sex, physiological state, environmental cues.
• Poor sleep or chronic alertness (e.g.a0predation pressure) a0↑ tissue catabolism to fuel wakefulness.
• Primary metabolic roles
  • Reduces lipid storage during night "fast" ("breaka0fast" = break the fast).
  • Promotes lipolysis: frees fatty acids (FFA) to supply brain/organ energy while eating is suspended.
  • Overall effect aligns lipid mobilization with diurnal rhythm.
• Variability
  • Can also stimulate lipogenesis under certain sex/species/weather conditions.
  • Human supplement response ranges: some people sleep better, others unaffected.

Lipoproteins & Lipid Transport

• Blood lipids transported as proteinlipid complexes; apolipoproteins (labelled "APO-X") provide tissue recognition.
• Two tissues synthesize apolipoproteins
  • Liver (re-packages dietary & endogenous lipids).
  • Intestine/gut (absorbs dietary fats; forms chylomicrons for delivery to liver).
• Relative particle size & protein density
  • HDL: smallest, highest protein density; carries least lipid.
  • LDL: larger; moderate protein.
  • VLDL: very large "oil drops"; very low density lipoproteins, few apoproteins.

Adipocyte Lipid Dynamics

• Arrival of lipoprotein at cell surface
  • $\text{LPL}$ (lipoprotein-lipase) on plasma membrane cleaves TGs → FFAs, cholesterol, cholesterol esters.
  • Specialized FFA transporters import liberated lipids.
• Intracellular processing
  • FFAs + glycerol → triglycerides (TG) for storage in lipid droplet.
  • Adipocytes exhibit hypertrophy: droplet & cell enlarge dramatically.
• Mobilization (lipolysis)
  • Hormone-sensitive lipase (HSL) breaks TG → glycerol + FFA.
  • FFAs released to blood; liver oxidizes via $\beta$-oxidation.

Adrenal Gland Anatomy

• Sits atop kidneys; steroidogenic.
• Layers
  • Capsule: epithelial "coat"; non-secretory.
  • Cortex (3 "floors")
  1. Zona glomerulosa (outer) – mineralocorticoids.
  2. Zona fasciculata (middle, thickest) – glucocorticoids.
  3. Zona reticularis (inner) – androgens.
  • Medulla (core) – chromaffin cells; sympathetic innervation.
• Regulatory axis: Hypothalamus $\xrightarrow{\text{CRH}}$ Pituitary $\xrightarrow{\text{ACTH}}$ Adrenal cortex.

Cortical Hormones & Actions

1. Mineralocorticoids (e.g.a0aldosterone)
   • ↑ Na⁺ reabsorption → "Where sodium goes, water flows".
   • ↑ water retention, blood volume, and blood pressure.
2. Glucocorticoids (cortisol/corticosterone)
   • Potent stress hormones; released by ACTH.
   • Suppress immune function; clinical use: dexamethasone, prednisone (must taper to avoid adrenal shutdown/Cushing).
   • Catabolic: mobilize AA, lipid, glucose; induce hepatic gluconeogenesis.
   • ↑ catecholamine sensitivity; stimulate appetite → post-stress hyperphagia.
3. Androgens (reticularis)
   • Anabolic; promote muscle, bone growth; masculinizing.
   • Used in livestock implants & some OTC supplements.

Medullary Hormones

• Chromaffin cells secrete catecholamines under direct sympathetic signal (fight/flight):
  • Epinephrine (adrenaline).
  • Norepinephrine.
• Effects: rapid tissue catabolism, nutrient redirection to muscle/heart/brain, ↑ HR, pupil dilation, bronchodilation.

Stress Physiology in Livestock Examples

• Shipping/transport → acute stress → cortisol spike → immunosuppression → bacterial bloom (e.g.a0shipping‐fever pneumonia).
• Manager presence, handling, confinement also elevate cortisol/catecholamines, reducing growth efficiency (nutrients spent on survival, not accretion).

Pancreas – Dual Function

Exocrine Portion (Acinar System)

• Acinar cells: digestive enzymes (proteases, lipases, amylases) secreted as zymogens.
• Duct cells: secrete $\text{NaHCO}_3$ buffer; neutralizes gastric acid entering duodenum.
• Premature enzyme activation → pancreatitis (self-digestion).

Endocrine Portion (Islets of Langerhans)

Thyroid Hormones (T3 & T4)

• Secretion ratio: $\text{T4} : \text{T3} \approx 4:1$; T3 ≈ 3–4× more potent.
• Functions
  • Sets basal metabolic rate (BMR); age, diet, disease alter set-point.
  • ↑ mitochondrial size & number → higher ATP output.
  • ↑ sensitivity to catecholamines; synergizes with fight/flight.
  • Regulates concurrent protein synthesis & degradation rates.
• Clinical/production notes
  • Starvation/anorexia lowers BMR; later re-feeding → rapid fat accrual.
  • Older animals: lower thyroid output → lean loss despite same diet.
  • Poultry industry sometimes manipulates T3/T4 to synchronize molting/feather regrowth.

Mitochondrial Adaptations

• Under high T3/T4
  • Bean-shaped organelles enlarge; cristae surface area ↑.
  • Alternatively, cells generate more spherical mitochondria.
• Functional outcome: sustained endurance capacity (e.g.a0marathon training) via amplified oxidative phosphorylation.

Key Terminology & Concepts

• Lipogenesis: TG synthesis & storage.
• Lipolysis: TG breakdown → FFA + glycerol.
• Gluconeogenesis: hepatic formation of glucose (e.g.a0$2\,\text{pyruvate} \rightarrow \text{glucose}$) stimulated by glucagon/cortisol.
• Catabolism vs Anabolism
  • Catabolic hormones: cortisol, catecholamines → breakdown.
  • Anabolic hormones: insulin, androgens, (to some extent T3 in protein synthesis) → building.
• "Redistribution of nutrients" = hormonal reprioritization of metabolic currency toward survival or growth.